Glossary of technical terms for the use of metallurgical engineers Terms starting with alphabet ‘G’
Glossary of technical terms for the use of metallurgical engineers
Terms starting with alphabet ‘G’
Gabbro – It is a dark, coarse-grained igneous rock.
Gable – It is the normally triangular portion of a wall between the edges of intersecting roof pitches. The shape of the gable and how it is detailed depends on the structural system used, which reflects climate, material availability, and aesthetic concerns.
Gable wall, gable end – It normally refers to the entire wall, including the gable and the wall below it.
Gadolinium-iron garnet (GdIG) – It belongs to the family ferro-magnetic rare–earth garnets which are assigned to cubic structure (space group Ia3d) with every cell containing eight R3Fe5O12 molecules. Rare–earth ion R3+ cannot occupy the octahedral and tetrahedral sites because of its large ion radius, so R3+ ion can only occupy dodecahedral sites which have larger space.
Galena – It is lead sulphide which is the most common ore mineral of lead.
Gall – It means damage to the surface of a powder metallurgy compact or die part, which is caused by adhesion of powder to the die cavity wall or a punch surface.
Galling – It is a condition whereby excessive friction between high spots results in localized welding with subsequent spalling and a further roughening of the rubbing surfaces of one or both of two mating parts. It is also a severe form of scuffing associated with gross damage to the surfaces or failure. Galling has been used in several ways in tribology, hence, each time it is encountered its meaning is to be ascertained from the specific context of the usage.
Gallium – It is a chemical element which occurs in very low concentrations in the crust of the Earth and virtually all primary gallium is recovered as a by-product, principally from the processing of bauxite to alumina. Majority of the gallium applications need very high purity levels, and the metal is to be refined before use until it contains no more than 1 parts per million (ppm) total impurities. Elemental gallium is a relatively soft, silvery metal at standard temperature and pressure. In its liquid state, it becomes silvery white. If enough force is applied, solid gallium may fracture conchoidally. Gallium has limited commercial applications in its metallic form. Its principal use is in the manufacture of semi-conducting compounds, mainly gallium arsenides (GaA) and gallium phosphide (GaP). Majority of the gallium consumed is used for optoelectronic devices and integrated circuits (ICs). Optoelectronic devices, light-emitting diodes (LEDs), laser diodes, photodiodes, and solar (photovoltaic) cells, take advantage of the ability of GaAs to convert electrical energy into optical energy and vice versa.
Galvalume steel – It is the steel sheet with a unique coating of 55 % Al and 45 % Zn which resists corrosion. The coating is applied in a continuous hot-dipped process, which improves the weather resistance of the steel.
Galvanic anode – It is a metal which, because of its relative position in the galvanic series, provides sacrificial protection to metals which are more noble in the series, when coupled in an electrolyte. Galvanic cell – It is a cell in which chemical change is the source of electrical energy. It normally consists of two dissimilar conductors which are in contact with each other and with an electrolyte, or of two similar conductors in contact with each other and with dissimilar electrolytes. It is also a cell or system in which a spontaneous oxidation-reduction reaction occurs, the resulting flow of electrons being conducted in an external part of the circuit.
Galvanic corrosion – It refers to corrosion damage where two dissimilar metals have an electrically conducting connection and are in contact with a common corrosive electrolyte. In the electro-chemical model of corrosion, one of the two partial reactions (anodic metal dissolution and cathodic oxygen reduction) takes place almost exclusively on one metal. Normally, the less noble metal is dissolved (anodic metal dissolution), whereas the more noble part is not attacked by corrosion (serves only as the cathode for oxygen reduction). Where galvanic corrosion takes place, the rate of corrosion of the less noble metal is higher than it would be in a free corroding environment without contact to another metal. Using thermodynamic data and taking common experience gained in typical applications into account, it is possible to predict which material combinations are affected by galvanic corrosion. A positive example of active utilization of the galvanic corrosion phenomenon described here is the way zinc protects carbon steels and low alloy steels. Zinc is the less noble metal which actively protects steel by being
Galvanic couple – It is a pair of dissimilar conductors, normally metals, in electrical contact.
Galvanic couple potential – it is the potential of a sample (or samples in a galvanic couple) when two or more electro-chemical reactions are occurring.
Galvanic current – It is the electric current which flows between metals or conductive non-metals in a galvanic couple.
Galvanic series – It consists of a list of metals and alloys arranged according to their relative corrosion potentials in a given environment.
Galvanize – It means coating a metal surface with zinc using any of various processes.
Galvanized coating – Galvanized coating on the steel sheet consists of the steel core, with an inter-metallic alloy layer and outer zinc layer on both surfaces. Besides the outer layer of zinc (eta layer), it contains gamma, delta and zeta layers. The hardness of gamma, delta and zeta layers of the zinc coating, as expressed in diamond pyramid number (DPN), is higher than the underlying steel. Because of this higher hardness, these layers provide excellent protection against coating damage through abrasion.
Galvanized iron (GI) sheets – These are basically steel sheets which have been coated with zinc. These sheets include a range of hot dip galvanized and electro-galvanized steel sheet. The zinc coating provides a continuous barrier which does not allow moisture and oxygen to reach the steel. It reacts with the atmosphere to provide the base steel a protection.
Galvanized steels – These are steels coated with a thin layer of zinc to provide corrosion resistance. These are normally used in under body auto parts, garbage cans, storage tanks, or fencing wire. Sheet steels are normally cold rolled prior to the galvanizing stage. Galvanized steels are produced either by hot dipping process or by electro-galvanizing process.
Galvanized steel reinforcement bars – These are the normal reinforcement steel bars which are coated with a protective layer of zinc metal. Zinc coating is normally carried out by hot dip galvanizing process. The zinc coating serves as a barrier to the corrosive environment which the rebars are exposed to when embedded in concrete. In addition to the barrier protection, zinc also provides cathodic protection where zinc corrodes preferentially when in contact with unprotected steel. This means that in case of any gap in zinc coating the surface of bare steel, the reinforcement bar is protected by the surrounding zinc.
Galvanizing – It is the process of applying a protective zinc coating to steel.
Galvanizing alloy – Galvanizing baths are alloyed with small amounts of other metals such as aluminum, nickel, or lead to improve the fluidity and resistance to oxidation of the zinc.
Galvanizing temperature – It is the temperature at which the molten zinc bath is kept in order to react with the steel. Typically, this temperature is between 443 deg C and 454 deg C.
Galvanneal – It means to produce a zinc-iron alloy coating on iron or steel by keeping the coating molten after hot dip galvanizing until the zinc alloys completely with the basis metal.
Galvannealed steel – In this steel an extra tight coat of galvanizing metal (Zn) applied to a soft steel sheet, after which the sheet is passed through an oven at around 650 deg C. The resulting coat is dull gray without spangle especially suited for subsequent painting.
Galvanometer – It is an instrument for detecting small electric currents.
Galvanostatic – It is an experimental technique whereby an electrode is maintained at a constant current in an electrolyte.
Gamma distribution– The gamma distribution includes as special cases the chi-square distribution and the exponential distribution. It has several important applications. In Bayesian inference, for example, it is sometimes used as the a priori distribution for the parameter (mean) of a Poisson distribution.
Gamma iron – It is the face-centered cubic form of pure iron. It is stable from 910 deg C to 1,400 deg C.
Gamma layer – It is the zinc-iron alloy layer closest to the surface of the steel in the galvanized coating. It is the first layer of zinc iron alloy growth from the base steel formed during the galvanizing process. The chemical composition of this layer is around 75 % zinc and 25 % iron. Gamma layer is the hardest layer in the coating and has a diamond pyramid number (DPN) of 250 compared to the DPN of 159 for the base steel.
Gamma-phase iron – It is also known as austenite. It is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, it exists above the critical eutectoid temperature of 727 deg C. Other alloys of steel have different eutectoid temperatures.
Gamma radiation – It consists of very high-energy electro-magnetic rays produced during radioactive decay. These are similar to visible light and X-rays but significantly more energetic than the latter.
Gamma ray – It is short wave-length electro-magnetic radiation, similar to x-rays but of nuclear origin, with a range of wave-length from around 0.0005 nanometers to 0.14 nanometers.
Gamma-ray spectrometry – It is the most powerful tool in the field of radio-nuclide analysis. It is an instrument which measures the distribution of the intensity of gamma radiation against the energy of each photon. It is used for the qualitative and quantitative determination of radio-nuclides which emit gamma radiation. Majority of the radio-nuclides send out gamma radiation during their transformations to stable decay products.
Gamma-ray spectroscopy – It is the qualitative study of the energy spectra of gamma-ray sources, such as in the nuclear industry, geochemical investigation, and astrophysics. It is used for the determination of the energy distribution of gamma-rays emitted by a nucleus.
Gamma structure – It consists of structurally analogous phases or electron compounds which are having ratios of 21 valence electrons to 13 atoms. This is generally a large, complex cubic structure.
Gangman – A gangman is part of rail / cable maintenance / engineering crew (called ‘Gang’) who are responsible for the upkeep of the track / cable conditions.
Gang milling – It consists of milling with several cutters mounted on the same arbor or with work-pieces similarly positioned for cutting either simultaneously or consecutively during a single set-up.
Gang slitter – It is a machine with a number of pairs of rotary cutters spaced on two parallel shafts. It is used for slitting metal into strips or for trimming the edges of sheets.
Gangue – It is the worthless portion of an ore which is separated from the desired part before smelting is commenced.
Gantry cranes – These cranes are essentially the same as the regular overhead travelling cranes except that the bridge for carrying the trolley or trolleys is rigidly supported on two or more legs running on fixed rails or another runway. These ‘legs’ eliminate the supporting runway and column system and connect to end trucks which run on a rail either embedded in, or laid on top of, the floor. The crane frame is supported on a gantry system with equalized beams and wheels that run on the gantry rail, normally perpendicular to the trolley travel direction. This crane comes in all sizes, and some can move very heavy loads.
Gantt chart – It is a type of bar chart which shows a project schedule. This chart lists the tasks to be performed on the vertical axis, and time intervals on the horizontal axis. The width of the horizontal bars in the graph shows the duration of each activity. Gantt charts show the start and finish dates of the terminal elements and summary elements of a project. Terminal elements and summary elements constitute the work breakdown structure of the project. Modern Gantt charts also show the dependency (i.e., precedence network) relationships between activities. Gantt charts can be used to show current schedule status using percent-complete shadings and a vertical ‘TODAY’ line. Gantt charts are sometimes equated with bar charts.
Gap – It is a non-standard term for joint clearance and root opening. In rolling mills, it refers to the space between the rolls in a roll stand in which the forming of the rolled material depending on the roll gap force takes place. The setting of this gap is crucial for the roll gap force, determining the thickness of the rolled metal (e.g. rolled steel sheets, rolled flat bar or rolled steel tube). Controlling the height of the roll gap, frequently supported by precise measurements using laser technology, allows direct control over the material thickness and speed.
Gap control in the rolling mill – It consists of controlling the height of the roll gap, frequently supported by precise measurements using laser technology. It allows direct control over the rolled product thickness and speed.
Gap-frame press -It is a general classification of press in which the uprights or housings are made in the form of a letter ‘C’, hence, making three sides of the die space accessible.
Gap measurement – It is needed for precise monitoring or quality control in order to satisfy technical or visual requirements. Single point sensors can be used to monitor the gap distance between rollers, and sheets etc. Laser scanners can be used to profile the gap of a seam, object, or weld.
Gap statistic – It is a standard method for determining the number of clusters in a set of data. The Gap statistic standardizes the graph of log(Wk), where Wk is the within-cluster dispersion, by comparing it to its expectation under an appropriate null reference distribution of the data.
Garnet – It is a generic name for a related group of mineral silicates which have the general chemical formula A3B2(SiO4)3, where ‘A’ can be calcium, magnesium, manganese, or ferrous iron, and ‘B’ can be aluminum, ferric iron, chromium, or titanium. Garnet is used for coating abrasive paper or cloth, for bearing pivots in watches, for electronics, and the finer specimens for gemstones. The hardness of garnet varies from Mohs 6 to 8 (1,360 Knoop), the latter being used for abrasive applications.
Gas – It is one of the four fundamental states of matter. The others are solid, liquid, and plasma. A pure gas can be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g., oxygen), or compound molecules made from a variety of atoms (e.g., carbon di-oxide). A gas mixture, such as air, contains a variety of pure gases. What distinguishes gases from liquids and solids is the vast separation of the individual gas particles. This separation normally makes a colourless gas invisible to the human observer.
Gas analysis – It is the determination of the constituents of a gaseous mixture.
Gas atomization – It is an atomization process whereby molten metal is broken up into particles by a rapidly moving inert gas stream.
Gas bearing – It is a journal or thrust bearing which is lubricated with gas.
Gas brazing – It is a non-standard term for torch brazing.
Gas burner – It is a device which converts gaseous fuel (liquefied petroleum gas, natural gas, coke oven gas, converter gas, blast furnace gas, and mixed gas etc.) into heat energy by means of a chemical reaction called combustion, i.e., air and gaseous fuel are mixed in the right proportion through a premixing device to make it burn fully. Gas burners have the advantages of easy ignition, complete combustion, stable flame, good spreading, no tempering, no de-flaming, large adjustment range, low noise, long life, safety and reliability because of the reasonable structural design.
Gas chromatograph – It is an analytical instrument which is designed to separate organic species for subsequent detection by an analyte specific detector.
Gas chromatography (GC) – It is a common type of chromatography used in analytical chemistry for separating and analyzing compounds which can be vapourized without decomposition. It is an analytical technique which applicable to gas, liquid, and solid samples (components which are vapourized by heat). If a mixture of compounds is analyzed using gas chromatography, each compound can be separated and quantified. Typical uses of gas chromatography include testing the purity of a particular substance, or separating the different components of a mixture.
Gas chromatography-infrared (GC-IR) spectroscopy – It combines the high-efficiency separation ability of gas chromatography and the molecular structure identification ability of infrared spectroscopy, and is an effective method for the analysis of complex mixtures.
Gas chromatography–mass spectrometry (GC-MS) – It is used to identify the molecular weight, elemental constitution, and molecular structure of the compounds present in the sample. It is an analytical method which combines the features of gas-chromatography and mass spectrometry to identify different substances within a test sample. It comprises two very distinct analytical instruments namely gas chromatography, and mass spectrometry.
Gas classification – It is the separation of a powder into its particle size fractions by means of a gas stream of controlled velocity flowing counter-stream to the gravity-induced fall of the particles. The method is used to classify sub-mesh-size particles.
Gas classifier – It is a device for gas classification. It can be of laboratory size for quality control testing or of industrial capacity for accommodating powder production requirement.
Gas cleaning plant – The main function of the gas cleaning plant is to remove particulate matter from the exhaust gas of a furnace The exhaust is cleaned in gas cleaning plant normally in two stages namely primary gas cleaning stage and secondary gas cleaning stage. The gas cleaning plant can be a wet type plant or a dry type plant.
Gas compressor – It is a mechanical device which increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor. Several compressors can be staged, i.e., the gas is compressed several times in steps or stages, to increase discharge pressure. Frequently, the second stage is physically smaller than the first (main) stage, to accommodate the already compressed gas without reducing its pressure. Each stage further compresses the gas and increases its pressure and also its temperature (if inter cooling between stages is not used).
Gas condensation, coke oven – As the coke oven gas is cooled, water, tar and naphthalene condense out. The condensate collects in the primary cooler system and is discharged to the tar and liquor plant. As the raw coke oven gas is cooled, tar vapour condenses and forms aerosols which are carried along with the gas flow.
Gas constant – It is the constant of proportionality appearing in the equation of state of an ideal gas, equal to the pressure of the gas multiplied by its molar volume divided by its temperature. It is also known as universal gas constant.
Gas-cooled reactor – It is the broad / generic expression which describes a nuclear reactor where gas is used as the coolant.
Gas cutter – It is a non-standard term for oxygen cutter.
Gas cutting – It is a non-standard term for oxygen cutting.
Gas cyaniding – It is a misnomer for carbo-nitriding.
Gas cylinder – It is a portable container which is used forthe transportation and storage of a compressed gas.
Gaseous corrosion – It is the corrosion with gas as the only corrosive agent and without any aqueous phase on the surface of the metal. It is also called dry corrosion.
Gaseous lubricants – Common gaseous lubricants are air, helium, and carbon dioxide etc.
Gaseous reduction – It is the reaction of a metal compound with a reducing gas to produce the metal. It is also the conversion of metal compounds to metallic particles by the use of a reducing gas.
Gas-filled tube – It is an electron tube device which relies on the presence of gas for operation, normally at less than atmospheric pressure.
Gas gouging – It is a non-standard term for oxygen gouging.
Gas holes -These are holes in castings or welds which are formed by gas escaping from molten metal as it solidifies. Gas holes can occur individually, in clusters, or throughout the solidified metal.
Gasification of coal – It is a conversion technology which converts any carbon containing material, such as coal, into synthesis gas (syngas). It is a high temperature process with temperature reaching typically 1,225 deg C. The temperature is optimized to produce a fuel gas with a minimum of liquid and solids. This process consists of heating the feed material coal in a vessel with or without the addition of oxygen. Carbon reacts with water in the form of steam and oxygen at relatively high pressure typically higher that 3 MPa and produce raw syngas, a mixture composed mainly of carbon mono-oxide and hydrogen and some minor byproducts. The byproducts are removed to produce a clean syngas which can be used (i) as a fuel to generate power or steam, (ii) as a basic chemical building block for a large number of uses in the petrochemical and refining industries, and (ii) for the production of hydrogen.
Gasifiers – Gasifiers are the equipment in which the complete gasification reactions are carried out. Gasifiers are required to be operated at certain temperature in order to drive certain endothermic carbon – steam and carbon – carbon di-oxide reactions. The required temperature is maintained by heat evolved from exothermic reaction between oxygen and coal. Depending upon the medium of gasification, gasifiers are classified into two categories namely (i) air blown, and (ii)oxygen blown. In air blown gasifiers, air is used as gasification medium while in oxygen blown gasifiers pure oxygen is used as gasification medium. When air is used as gasification medium, the nitrogen is simultaneously brought into the process which results in the product gas dilution. As a result, product gas has a lower calorific value. Depending upon the contact between gas and fuel, there are four types of gasifiers. These are namely (i) moving or fixed bed gasifier, (ii) fluidized bed gasifier, (iii) entrained bed gasifier, and (iv) transport flow gasifier. All the four types of gasifiers are based on partial oxidation (gasification) of a carbonaceous (carbon containing) feed material (coal). While each of these can make an acceptable reducing gas, the fixed bed and fluidized bed gasifiers are the preferred choice for high ash coals.
Gasket – It is a compressible material, or a combination of materials, which when clamped between two stationary members, it prevents the passage of the media across those members. The gasket material selected is required to be capable of sealing mating surfaces, resistant to the medium being sealed, and able to withstand the application temperatures and pressures. Normally, gaskets are called upon to affect a seal across the faces of contact with the flanges. Permeation of the media through the body of the gasket is also a possibility depending on material, confined media, and acceptable leakage rate.
Gasket lip expansion – This is a phenomenon which occurs due to edge swelling when the gasket is affected by confined fluid. This causes the gasket material to swell and increase the interaction of the gasket against the flange faces.
Gas laser – It is a laser in which the lasing medium is a gas.
Gas lubrication – It is a system of lubrication in which the shape and relative motion of the sliding surfaces cause the formation of a gas film having sufficient pressure to separate the surfaces.
Gas masks – These consist of a full-mask face piece, which covers the eyes, nose and mouth, connected either directly or through a non-kink flexible hose to a canister containing a granular sorbent.
Gas mass spectrometry – It is an analytical technique which provides quantitative analysis of gas mixtures through the complete range of elemental and molecular gases.
Gas metal arc cutting (GMAC) – It is an arc cutting process which is used to cut metals by melting them with the heat of an arc between a continuous metal (consumable) electrode and the work. Shielding is achieved entirely from an externally supplied gas or gas mixture.
Gas metal arc welding (GMAW) – It is an arc welding in which coalescence of metals is produced by heating them with an arc between a continuous filler metal electrode and the work-pieces. Shielding is achieved entirely from an externally supplied gas.
Gas metal arc welding (GMAW) process – This process is an arc welding process which joins metals together by heating them with an electric arc which is established between a consumable electrode (wire) and the work piece. An externally supplied gas or gas mixture acts to shield the arc and molten weld pool. In the gas metal arc welding process, an arc is established between a continuously fed electrode of filler metal and the work piece. After proper settings are made by the operator, the arc length is maintained at the set value, despite the reasonable changes which are to be expected in the gun-to-work distance during normal operation. This automatic arc regulation is achieved in one of the two ways. The most common method is to utilize a constant-speed (but adjustable) electrode feed unit with a variable-current (constant-voltage) power source. As the gun-to-work relationship changes, which instantaneously alters the arc length, the power source delivers either more current (if the arc length is decreased) or less current (if the arc length is increased). This changes in current causes a corresponding change in the electrode melt-off rate, thus maintaining the desired arc length. The characteristics of the gas metal arc welding process are best described by reviewing the three basic means by which metal is transferred from the electrode to the work namely (i) short-circuiting transfer, (ii) globular transfer, or (iii) spray transfer. The type of transfer is determined by a number of factors. The gas metal arc welding process can be operated in semi-automatic and automatic modes. All commercially important metals, such as carbon steel, high-strength low-alloy steel, stainless steel, and aluminum, copper, and nickel alloys can be welded in all positions by this process if appropriate shielding gases, electrodes, and welding parameters are chosen. The advantages make the process particularly well suited to high production and automated welding applications.
Gas pocket – It is a cavity caused by entrapped gas.
Gas porosity – It consists of fine holes or pores within a metal which are caused by entrapped gas or by the evolution of dissolved gas during solidification.
Gas pressure regulator – It is a spring loaded, dead weighted or pressure balanced device which maintains the gas pressure to the burner supply line.
Gas quenching – In gas quenching, the quenching medium remains gaseous throughout, and there are no phase transitions as with liquid quenchants. The only cooling mechanism is convection.
Gas regulator – It is a device for controlling the delivery of gas at some substantially constant pressure.
Gas separator – It is a pressure vessel which separates gas from gas-liquid mixed fluid.
Gas shielded arc welding – It is also an electric fusion welding process. In this process, the weld pool is produced by the effects of an electric arc. The arc is quite visible as it burns between the electrode and the work piece. The electrode, arc, and weld pool are protected against the atmosphere by an inert or active shield gas which is constantly fed into the weld area. The gas shield arc welding processes are classified according to the type of electrodes and the gas used. These are normally divided into two main categories. The categories are (i) gas tungsten arc welding (GTAW) namely TIG (tungsten inert gas), and THG (tungsten hydrogen gas) arc welding, and (ii) gas metal arc welding (GMAW) namely MIG (metal inert gas), and MAG (metal active gas) welding.
Gassing – It is the absorption of gas by a metal. It is also evolution of gas from a metal during melting operations or upon solidification. Gassing is also evolution of gas from an electrode during electrolysis.
Gas shut off valve, blast furnace – This valve is used for separation of a gas burner from the hot blast stove under ‘on-blast’ operation of the stove. It facilitates fast and safe changing of stove from ‘gas’ to ‘blast’. It is normally a water-cooled valve. The valve opens or closes quite fast (in around 10 seconds) to complete the stove changing cycle, and incorporates electro-mechanical actuator. The valve has normally steel fabricated construction, is self-supporting, and needs small space.
Gas spring – In this spring, the spring action is provided by a volume of gas which is compressed.
Gas thermometer – It is a thermometer which measures temperature by the variation in volume or pressure of a gas. This thermometer functions by Charles’s law which states that when the temperature of a gas increases, so does the volume.
Gas torch – It is a torch used for cutting and welding of metals. It is also known as cutting torch (arc), cutting torch (oxy-fuel gas), welding torch (arc), and welding torch (oxy-fuel gas).
Gas tracer method (technique) – It is used for the precise measurement of volumetric flow rates through small ducts or pipes. The procedure entails spiking the effluent with a known quantity of a ‘tracer gas’ and measuring the concentration of the tracer compound downstream of the injection point after allowing for complete mixing. The tracer compound is selected based on knowledge of the process. The tracer compound is required to be a stable, non-reactive gas which is not otherwise found in the native effluent.
Gas transmission rate – It is a measure of the permeability of a packaging film to gases by measuring the movement of a gas through the film under specified conditions.
Gas tungsten arc cutting (GTAC) – It is an arc-cutting process in which metals are cut by melting them with an arc between a single tungsten (non-consumable) electrode and the work-piece. Shielding is achieved from a gas or gas mixture.
Gas tungsten arc welding (GTAW) – It is an arc welding coalescence of metals is produced by heating them with an arc between a tungsten (non-consumable) electrode and the work. Shielding is achieved from a gas or gas mixture. Pressure and filler metal may or may not be used.
Gas tungsten arc welding (GTAW) process – This process is also known as HeliArc, tungsten inert gas (TIG), and tungsten arc welding. The melting temperature necessary to weld materials in the gas tungsten arc welding process is achieved by maintaining an arc between a tungsten alloy electrode and the work-piece. Weld pool temperatures can approach 2,500 deg C. An inert gas sustains the arc and protects the molten metal from atmospheric contamination. The inert gas is normally argon, helium, or a mixture of helium and argon. This process is used extensively for welding stainless steel, aluminum, magnesium, copper, and reactive materials (e.g., titanium and tantalum). The process can also be used to join carbon and alloy steels. In carbon steels, it is mainly used for root-pass welding with the application of consumable inserts or open-root techniques on pipe. The materials welded range from 0.05 mm to several millimeters in thickness. The gas tungsten arc welding process is applicable when the highest weld quality is needed.
Gas turbine, gas turbine engine – It is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and in the direction of flow are a rotating gas compressor, a combustor, and a compressor-driving turbine. Additional components have to be added to the gas generator to suit its application.
Gas welding – Gas welding joins metals by heating the materials to be joined so they can melt and fuse. It is the process of joining two metal items together by melting and cooling parts of both. This is achieved by creating a very hot flame using oxygen and a fuel gas. There are a few main types of gas welding which use different gases such as acetylene, gasoline, natural gas, MAPP (methylacetylene-propadiene propane), butane, propane, or hydrogen. Gas welding equipment includes a welding torch, control valves for fuel and oxygen and cylinders for both gases.
Gate – It is the portion of the runner in a mould through which molten metal enters the mould cavity. The generic term is sometimes applied to the entire network of connecting channels which conduct metal into the mould cavity.
Gate driver – It is a power amplifier which accepts a low-power input from a controller integrated circuit (IC) and produces a high-current drive input for the gate of a high-power transistor such as an insulated gate bi-polar transistor (IGBT) or power metal-oxide-semiconductor field-effect transistor (MOSFET). Gate drivers can be provided either on-chip or as a discrete module.
Gated pattern – In foundry practice, it is a pattern which includes not only the contours of the part to be cast but also the gates.
Gated system – It is the complete assembly of sprues, runners, and gates in a mould through which metal flows to enter the casting cavity. This term also applies to equivalent portions of the pattern.
Gating system – Gating is the term used to describe all of the passages leading to the casting cavity. When molten metal is poured into a mould, it is poured into the pouring basin or cup. It travels down the sprue through the runner into the feeder or riser then through the gate into the casting cavity. The gate is the breaking point at the casting from which the gating system is separated from the casting.
Gate turn-off thyristor (GTO) – It is a four-layer power semi-conductor device which can be turned on and off by signals at a control (gate) terminal.
Gate valve – A gate valve is a linear motion valve used to start or stop fluid flow. However, it does not regulate or throttle flow. The name gate is derived from the appearance of the disk in the flow stream. The disk of a gate valve is completely removed from the flow stream when the valve is fully open. This characteristic offers virtually no resistance to flow when the valve is open. Hence, there is little pressure drop across an open gate valve. When the valve is fully closed, a disk-to-seal ring contact surface exists for 360-degree, and good sealing is provided. With the proper mating of a disk to the seal ring, very little or no leakage occurs across the disk when the gate valve is closed. Gate valve is a straight-through pattern valve whose closure element is a wedge or parallel-sided slab, situated between two fixed seating surfaces, with means to move it in or out of the flow stream in a direction perpendicular to the pipeline axis.
Gathering – It is a forging operation which increases the cross section of part of the stock. It is normally a preliminary operation.
Gathering stock – It is an operation whereby the cross section of a portion of the forging stock is increased beyond its original size.
Gating system – It consists of the complete assembly of sprues, runners, and gates in a mould through which metal flows to enter casting cavity. The term is also applied to equivalent portions of the pattern.
Gatorizing – This method consists of (i) pre-conditioning the stock under controlled conditions to secure a temporary condition of low strength and high ductility, (ii) hot working to the desired shape while maintaining those attributes, and (iii) restoring normal properties to the work-piece through heat treatment. With gatorizing, hard-to-work nickel alloys have been forged for the first time, and the higher strength which can be got from these alloys resulted in higher strength-to-weight ratios in aircraft components such as jet engine disks. This process also led to development of techniques for isothermal forging of integrally bladed engine rotors (disks of super-alloys forged integrally with ceramic blades).
Gauge – It is the thickness of sheet or the diameter of wire. The different standards are arbitrary and differ with regard to ferrous and non-ferrous products as well as sheet and wire. It is also an aid for visual
inspection which enables an inspector to determine more reliably whether the size or contour of a formed part meets dimensional requirements. Gauge is also an instrument which used to measure thickness or length.
Gauge cock – It is a valve attached to a water column or drum for checking water level.
Gauge control – It is a technique to control strip thickness in the cold rolling mills, enabling tighter control of thickness by providing faster and more accurate control of the roll gap. It is normally done through automatic gauge control (AGC).
Gauge glass – It is the transparent part of a water gauge assembly connected directly or through a water column to the boiler, below and above the water line, to indicate the water level in a boiler.
Gauge length – It is the original length of that portion of the test sample over which strain, change of length and other characteristics are measured.
Gauge pressure – It is the pressure measured with respect to the atmospheric pressure and is normally expressed, for example, in ‘kPa g’. Gauge pressure varies with atmospheric pressure, which in turn varies with the altitude above the mean sea level and the weather conditions. Gauge pressure is the commonly used unit. A gauge pressure instrument indicates zero pressure when vented to the atmosphere.
Gaussian distribution – The gaussian distribution is another name for the normal distribution.
Gauss’s law – It is a mathematical relation between the electric flux passing through a surface and the charge contained within that surface.
Geiger counter – It is a detection instrument which is used to detect particles of ionizing radiation namely alpha particles, beta particles, or gamma radiation. This instrument is also used to measure the radio-activity which emanates from certain minerals by means of a Geiger-Mueller tube.
Gear – It is a rotating circular machine part having cut or inserted teeth which mesh with another compatible toothed part to transmit torque and speed. Each gear tooth essentially functions as a lever with its fulcrum at the centre of the gear. Gear is typically used to transmit rotational motion and / or torque by means of a series of teeth which engage with compatible teeth of another gear or other part. The teeth can be integral saliences or cavities machined on the part, or separate pegs inserted into it.
Gear box – It is frequently called a transmission. Gear boxes simply refer to a set of gears and their casing. Gear box is a mechanical device which uses a gear set, two or more gears working together, to change the speed, direction of rotation, or torque multiplication / reduction in a machine. Gear box allows the machinery to operate efficiently and even aid in slowing and shutting down machinery.
Gear coupling – It is a mechanical device which is used for transmitting torque between two shafts which are not collinear. It consists of a flexible joint fixed to each shaft. The two joints are connected by a third shaft, called the spindle.
Gear cutting – It consists of producing tooth profiles of equal spacing on the periphery, internal surface, or face of a work-piece by means of an alternate shear gear-form cutter or a gear generator.
Geared press – It is a press whose main crank or eccentric shaft is connected by gears to the driving source.
Gear (form) grinding – It is the removal of material to get correct gear tooth form by grinding. This is one of the more exact methods of finishing gears.
Gear hobbing – It consists of gear cutting by use of a tool resembling a worm gear in appearance, and having helically spaced cutting teeth. In a single-thread hob, the rows of teeth advance exactly one pitch as the hob makes one revolution. With only one hob, it is possible to cut inter-changeable gears of a given pitch of any number of teeth within the range of the hobbing machine.
Gear milling – It consists of gear cutting with a milling cutter which has been formed to the shape of the tooth space to be cut. The tooth spaces are machined one at a time.
Gear ratio – It is the ratio of the pitch circles of mating gears which defines the speed ratio and the mechanical advantage of the gear set.
Gear shaping – It consists of gear cutting with a reciprocating gear-shaped cutter rotating in mesh with the work blank.
Gear train, gear set – It is a machine element of a mechanical system formed by mounting two or more gears on a frame such that the teeth of the gears engage.
Gear train, planetary – It provides high gear reduction in a compact package. A planetary gear train is a gear reduction assembly consisting of two gears mounted so that the center of one gear (the ‘planet’) revolves around the centre of the other (the ‘sun’). A carrier connects the centre of the two gears and rotates, to carry the planet gear(s) around the sun gear. The planet and sun gears mesh so that their pitch circles roll without slip. If the sun gear is held fixed, then a point on the pitch circle of the planet gear traces an epicycloid curve.
Gel – It is the initial jelly-like solid phase which develops during the formation of a resin from a liquid. It is a semi-solid system consisting of a network of solid aggregates in which liquid is held.
Gelatin replica – It is a reproduction of a surface prepared in a film composed of gelatin.
Gelation – It is the point in a resin cure when the resin viscosity has increased to a point such that it barely moves when probed with a sharp instrument.
Gelation time – It is that interval of time, in connection with the use of synthetic thermosetting resins, which is extending from the introduction of a catalyst into a liquid adhesive system until the start of gel formation. It is also the time under application of load for a resin to reach a solid state.
Gel coat – It is a quick-setting resin applied to the surface of a mould and gelled before lay-up. The gel coat becomes an integral part of the finished laminate, and is normally used to improve surface appearance and bonding.
Gelling agent – It is a solid material dispersed in a liquid lubricant to produce a grease. Silica, clays, and metallic soaps are widely used as gelling agents.
Gel-permeation chromatography (GPC) – It is a liquid chromatography method which separates molecules on the basis of their physical size. The polymer molecules are separated by their ability or inability to penetrate the material in the separation column. This technique is very frequently used in the analysis of polymers.
Gel point – It is the stage at which a liquid begins to show pseudo-elastic properties. This stage can be conveniently observed from the inflection point on a viscosity time plot.
General arrangement (GA) drawing – It depicts the physical relationship of significant items using appropriate projections or perspective views. Reference dimensions are to be included in this drawing. This drawing does not establish item identification. It is prepared to convey a general description of the configuration and location of significant items. It is not normally used to control design. This drawing normally includes (i) sufficient views so that a general understanding of the configuration and location of significant items is conveyed, (ii) overall, locating, and other general dimensions necessary to describe the configuration, (iii) identities of significant items, and (iv) reference to applicable documents for further details.
General conditions of the contract – These are those commercial conditions which are mostly common to all the types of contracts. General conditions of the contract are normally annexed to the contract.
General corrosion – It is a form of deterioration which is distributed more or less uniformly over a surface. It is also the corrosion dominated by uniform thinning which proceeds without appreciable localized attack.
General exploration – General exploration involves the initial delineation of an identified deposit. Methods used include surface mapping, widely spaced sampling, trenching, and drilling for preliminary evaluation of mineral quantity and quality (including mineralogical tests on laboratory scale if needed), and limited interpolation based on indirect methods of investigation. The objective is to establish the main geological features of a deposit, giving a reasonable indication of continuity and providing an initial estimate of size, shape, structure and grade. The degree of accuracy is to be sufficient for deciding whether a pre-feasibility study and detailed exploration are warranted.
General layout drawing – General layout drawing is normally prepared in plan-view. It describes (i) entry gates and plant boundary, (ii) plant approach roads, (iii) location of the plant, equipment and facilities, (iv) provides the linkages between the plant equipment and facilities, (v) road and rail movements, (iv) conveyors and pipeline routings, (vi) critical and regulatory clearances if needed, (vii) location of chimneys and stacks, and (viii) green belts. The drawing is normally prepared to the scale with building dimensions.
General precipitate – It is a precipitate which is dispersed throughout the matrix.
General purpose polystyrene (GPPS) – It is a clear polymer which shows high stiffness, good dimensional stability, low specific gravity and excellent electrical properties. It offers several advantages over other polymers because of its clarity and ease of processing, both of which are because of its amorphous nature.
Generator – It is a device which uses mechanical energy got from external sources to produce electricity. Multiple energy sources are used to turn the generator.
Generic specifications – These are specifications which apply to the classification of products of a resource project using United Nations Framework Classification (UNFC).
Geochemistry – It means the study of the chemical properties of rocks.
Geodesic – It is the shortest distance between two points on a surface.
Geodesic isotensoid – It is the constant stress level in any given filament at all points in its path.
Geodesic-isotensoid contour – In filament-wound reinforced plastic pressure vessels, it is a dome contour in which the filaments are placed on geodesic paths so that the filaments show uniform tensions throughout their length under pressure loading.
Geodesic ovaloid – It is a contour for end domes, the fibres forming a geodesic line. It is the shortest distance between two points on a surface of revolution. The forces exerted by the filaments are proportioned to meet hoop and meridional stresses at any point.
Geology – It is the science which is concerned with the study of the rocks which compose the Earth.
Geographical information system – It captures, stores, analyzes, manages, and presents data which are linked to location.
Geological disposal facility (GDF) – It is a long-term nuclear waste management option involving the disposal of waste in an engineered underground facility, where the geology provides a barrier against the escape of radioactivity and where the depth protects the waste from disturbances rising at the surface. Depth in this context can refer to both horizontal as well as vertical depth, e.g., if the disposal facility is built into the side of a mountain.
Geological study – A geological study is an initial evaluation of economic viability. This is got by applying meaningful cut-off values for grade, thickness, depth and costs, estimated from comparable mining operations. Economic viability categories, however, cannot in general be defined from the geological study because of the lack of detail necessary for an economic viability evaluation. The resource quantities estimated can indicate that the deposit is of intrinsic economic interest, i.e., in the range of economic to potentially economic. A geological study is normally carried out in the four main stages namely (i) reconnaissance, (ii) prospecting, (iii) general exploration, and (iv) detailed exploration. The purpose of the geological study is to identify mineralization, to establish continuity, quantity, and quality of a mineral deposit, and thereby define an investment opportunity.
Geometric adaptive control (GAC) – It is a type of adaptive control, in which the process is controlled using on-line measurements to maintain desired product geometry (for example, dimensional accuracy or surface roughness).
Geometrically close-packed – In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density. i.e., the highest fraction of space occupied by spheres, which can be achieved by a lattice packing is pi/3.root 2 = around 0.74048π32≈0.74048. The same packing density can also be achieved by alternate stackings of the same close-packed planes of spheres, including structures that are aperiodic in the stacking direction.
Geometric dimensioning and tolerancing (GD&T) – It is a comprehensive system of standards for the location of features on an engineering drawing and describing their tolerances with respect to form, profile, orientation, and run-out.
Geometric mean – It is a mean or average which indicates a central tendency of a finite set of positive real numbers by using the product of their values (as opposed to the arithmetic mean which uses their sum).
Geophysical exploration – Airborne geophysical surveys are normally used as the first step in geophysical exploration. Large areas can be effectively covered in a short period of time. The most common aero geophysical maps are magnetometer maps which record the variations in the earth’s magnetic field with high degree of accuracy. The optimal selection of altitude and spacing as well as choice of instrumentation are important in an airborne geophysical survey. On surface, different geophysical methods are used to explore subsurface formations, based on the physical properties of rock and iron bearing minerals such as magnetism, gravity, electrical conductivity, radioactivity, and sound velocity. Two or more methods are frequently combined in one survey for acquiring more reliable data. Results from the surveys are compiled, and matched with geological information from surface and chips or core samples from any previous core drilling, to decide if it is worthwhile to proceed with further exploration. In case the survey results points towards further exploration then the information form basis for the drilling activities. As geophysical survey is normally conducted from the air to begin with, information from the surface surveys is compared and added to the airborne mapping.
Geophysical survey – It is a scientific method of prospecting which measures the physical properties of rock formations. Common properties which are investigated include magnetism, specific gravity, electrical conductivity and radioactivity.
Geophysical techniques – These are used in the search for iron ores as in most geophysical mapping. These are based upon the presence of measurable contrasts of physical properties between the ore minerals and the surrounding rocks. The physical properties used principally are magnetism (both permanent and induced) and density. Electrical methods (including polarization and electromagnetism) and seismic studies are used sometimes in conjunction with magnetic or gravity surveys to obtain better definition of the ore bodies.
Geophysics – It is the study of the physical properties of rocks and minerals. Geophysics, as applied to iron ore explorations, is mainly a reconnaissance tool which provides information that must subsequently be complemented by geological mapping, petrographic studies, drilling and the evaluation of ore analyses and treatment tests.
Geotechnical engineering – It is also known as geotechnics. It is concerned with the engineering behaviour of earth materials. It uses the principles of soil mechanics and rock mechanics to solve its engineering problems. It also relies on knowledge of geology, hydrology, geophysics, and other related sciences.
Geothermal – It pertains to the heat of the interior of the Earth.
Germanium (Ge) – It is a semiconducting metalloid element. Although it looks like a metal, it is fragile like glass. Its electrical resistivity is about midway between that of metallic conductors and that of good electrical insulators. It has interesting electrical properties. Its first significant use has been in solid-state electronics, and with it the transistor has been invented. Indeed, the entire modern field of semi-conductors owes its development to the early successful use of germanium. Germanium is still used in the field of electronics, but its use in the field of infrared optics has surpassed its electronic applications. Germanium has also found widespread use in the fields of gamma ray spectroscopy, catalysis, and fibre optics.
Getter – It is a substance which is used in a sintering furnace for the purpose of absorbing or chemically binding elements or compounds from the sintering atmosphere which are damaging to the final product.
Gettering box – It is a container for the getter substance which is readily accessible to the atmosphere and prevents contamination of sintered product by direct contact
Ghost lines – These are lines running parallel to the rolling direction which appear in a sheet metal panel when it is stretched. These lines may not be evident unless the panel has been sanded or painted.
Gibs – These are guides or shoes which ensure the proper parallelism, squareness, and sliding fit between metal-forming press components such as the slide and the frame. They are normally adjustable to compensate for wear and to establish operating clearance.
Gibbs free energy – It is a thermodynamic potential which can be used to calculate the maximum amount of work, other than pressure-volume work, which can be performed by a thermodynamically closed system at constant temperature and pressure It is the thermodynamic function delta G = delta H – T delta S, ‘H’ is enthalpy, ‘T’ is absolute temperature, and ‘S’ is entropy.
Gibbs triangle – It is an equilateral triangle which is used for plotting composition in a ternary system.
Gimbal system of charging – Gimbal system of charging facilitates controlled distribution of charge material into the blast furnace through a Gimbal type oscillating chute.
Glacial drift – It is the sedimentary material which has been transported by glaciers.
Glacial striations – These are the lines or scratches on a smooth rock surface caused by glacial abrasion.
Glancing angle – It is the angle (normally small) between an incident x-ray beam and the surface of the sample.
Gland (or gland bushing) – It is that part of a valve which retains or compresses the stem packing in a stuffing box (where used) or retains a stem O-ring, lip seal, or stem O-ring bushing.
Gland plate – It is the plate in a valve which retains the gland, gland bushing, or stem seals, and sometimes guides the stem.
Glass – It is an inorganic solid material which is normally transparent or translucent as well as hard, brittle, and impervious to the natural elements. It is made from natural and abundant raw materials (sand, soda ash and limestone) which are melted at high temperature to form a new material, i.e., glass. It is also a term which is sometimes used for porcelain enamel or frit.
Glass ceramics – It is a family of fine-grained crystalline materials which are made by a process of controlled crystallization from special glass compositions containing nucleating agents.
Glass cloth – It is the conventionally woven glass fibre material.
Glass electrode – It is a half cell in which the potential measurements are made through a glass membrane.
Glass fibre – It is a fibre spun from an inorganic product of fusion which has cooled to a rigid condition without crystallizing. It is a material consisting of several extremely fine fibre of glass. It is formed when thin strands of silica-based or other formulation glass are extruded into several fibres with small diameters suitable for textile processing. Glass fibre has roughly comparable mechanical properties to other fibres such as polymers and carbon fibre. Although not as rigid as carbon fibre, it is much cheaper and considerably less brittle when used in composites. Glass fibre reinforced composites are used in marine industry and piping industries because of good environmental resistance, better damage tolerance for impact loading, high specific strength and stiffness.
Glass fibre reinforced plastic – It is a composite material which consists of a polymer matrix and glass fibres. The polymer matrix is normally an epoxy, vinyl-ester, or polyester thermo-setting resin. The resin which brings the environmental and chemical resistance to the product, is the binder for the fibres in the structural laminate and defines the form of a glass fibre reinforced plastic part. The glass fibres add strength to the composite. They can be randomly arranged, or conveniently oriented. The most common type of glass fibre used for glass fibre reinforced plastic is E-glass (electric), which is alumino-boro-silicate glass. E-CR-glass (electrical / chemical resistance) is also normally used in applications which need particularly high protection against acidic corrosion.
Glass filament – It is a form of glass which has been drawn to a small diameter and extreme length. Majority of the filaments are less than 0.15 millimeters in diameter.
Glass filament bushing – It is the unit through which molten glass is drawn in making glass filaments.
Glass finish – It is a material applied to the surface of a glass reinforcement for improving the bond between the glass and the plastic resin matrix.
Glass flake – It is a thin, irregularly shaped flake of glass, which is typically made by shattering a thin- walled tube of glass.
Glass former – It is an oxide which forms a glass easily. It is also the one which contributes to the network of silica glass when added to it.
Glass mat thermo-plastic (GMT) – It is a semi-finished resin-fibre combination supplied as blanks for compression moulding.
Glass, percent by volume – It is the product of the specific gravity of a laminate and the percent glass by weight, divided by the specific gravity of the glass.
Glass reinforced plastic (GRP) – It is made up of a combination of glass fibre and polymer or plastic. It has several desirable properties which include high strength to weight ratio, excellent durability, light weight, electrically non-conductive, radar / radio wave transparent, and non-corroding. Glass reinforced plastic products are made from glass fibre reinforced polymers, typically with a polyester or vinyl-ester thermo-set resin matrix. Thermo-set polymers are formed by a chemical reaction, initiated by adding a catalyst, which causes an irreversible hardening of the resin. This is coupled with reinforcement achieved by the incorporation of glass fibres during the production process. The fibres can be in the form of fine long strands, chopped stands or woven mats. The production techniques by which this is achieved are varied and can range from a simple manual process to one which is highly automated, utilizing robotic machinery.
Glass stress – In a filament-wound part, normally a pressure vessel, it is the stress calculated using the load and the cross-sectional area of the reinforcement only.
Glass transition – It is the reversible change in an amorphous polymer or in amorphous regions of a partially crystalline polymer from, or to, a viscous or rubbery condition to, or from, a hard and relatively brittle one.
Glass transition temperature (Tg) – It is the temperature at which an amorphous polymer (or the amorphous regions in a partially crystalline polymer) changes from a hard and relatively brittle condition to a viscous or rubbery condition. In this temperature region, several physical properties, such as hardness, brittleness, thermal expansion, and specific heat, undergo considerable, rapid changes. It is also the approximate mid-point of the temperature range over which the glass transition takes place. Glass and silica fibre show a phase change at around 955 deg C and carbon / graphite fibres at
2,205 deg C to 2,760 deg C. It is also the temperature at which increased molecular mobility results in considerable changes in the properties of a cured resin system. Also, it is the inflection point on a plot of modulus against temperature. The measured value of Tg depends to some extent on the method of test.
Glassware – It consists of a variety of equipment traditionally made of glass which are used for chemical analysis. Glass can be blown, bent, cut, moulded, or formed into several sizes and shapes. It is normally used in analytical laboratories.
Glass wool – It is an insulating material made from glass fibre arranged using a binder into a texture similar to wool. The process traps many small pockets of air between the glass, and these small air pockets result in high thermal insulation properties.
Glassy phase – It is an amorphous phase, i.e., there are no crystals. Since blast furnace granulated slag gets lesser time for crystallization. It contains around 90 % of amorphous glassy phase. Slags differ in their glass content, depending upon their cooling pattern.
Glaze – It is a ceramic coating matured to the glassy state on a formed ceramic article, or the material or mixture from which the coating is made. In tribology, It is a ceramic or other hard, smooth surface film produced by sliding.
Glazing – It consists of dulling of the abrasive grains in the cutting face of a wheel during grinding.
Glide – It is same as slip. It is also a non-crystallographic shearing movement, such as of one grain over another.
Globe valve – A globe valve is a type of valve used for regulating flow in a pipeline, consisting of a movable disk-type element and a stationary ring seat in a generally spherical body. The valve can have a stem or a cage, similar to ball valves, which moves the plug into and out of the globe. The fluid’s flow characteristics can be controlled by the design of the plug being used in the valve. A seal is used to stop leakage through the valve. Globe valves are designed for easy maintenance. They normally have a top which can be easily removed, exposing the plug and seal. Globe valves are good for on, off, and accurate throttling purposes but especially for situations when noise and cavitation are factors. It is a valve with a linear motion closure member, one or more ports, and a body distinguished by a globular shaped cavity around the port region. Globe valves can be further classified as two-way single-ported, two-way double-ported angle-style, or three-way.
Globular transfer – In consumable-electrode arc welding, it is a type of metal transfer in which molten filler metal passes across the arc as large droplets.
Glory hole – It is an open pit from which ore is extracted, especially where broken ore is passed to underground workings before being hoisted.
Gloss – It is the shine or luster of a porcelain enamel or a painted surface. It is an optical property which indicates how well a surface reflects light in a specular (mirror-like) direction. It is one of the important parameters which are used to describe the visual appearance of an object.
Gloves – Gloves are used for the safety of hands. They are normally made from a wide variety of materials and are designed for several types of workplace hazards. In general, gloves fall into four groups namely (i) gloves made of leather, synthetic fibers or metal meshes, (ii) gloves made of fabric and coated fabric, (iii) gloves for chemical protection, and (iv) gloves made of insulating rubber gloves. Leather, synthetic fibre or metal mesh gloves are sturdy and provide protection against cuts and burns. Leather or canvas gloves also protect against sustained heat. They protect against sparks, moderate heat, blows, chips and rough objects. These gloves can be used for tasks such as welding. Aluminized gloves provide radiant heat protection by reflection and insulate/reduce heat conduction with a liner or insert.
Glow discharge mass spectrometry (GDMS) – It is a powerful analytical method used for direct trace analysis of solids, liquids, and conducting or nonconducting materials. It involves ionizing neutral species in a glow discharge plasma and separating positive ions based on their mass-to-charge ratios for detection.
Glow-discharge optical emission spectroscopy (GDOES) – It is a spectroscopic method for the quantitative analysis of metals and other non-metallic solids. Ordinary atomic spectroscopy can be used to determine the surface of a material, but not its layered structure. In contrast, glow-discharge optical emission spectroscopy gradually ablates the layers of the sample, revealing the deeper structure. Glow-discharge optical emission spectroscopy can be used for the quantitative and qualitative determination of elements and is hence a method of analytical chemistry.
Glycol – It is an organic compound belonging to the alcohol family. It is more commonly referred to as antifreeze, and it is known to have a sweet taste. However, it can be toxic and is normally fluorescent dyed or clear and slightly oily in terms of consistency. in the molecule of a glycol, two hydroxyl (-OH) groups are attached to different carbon atoms. The term is frequently applied to the simplest member of the class, ethylene glycol.
Gneiss – It is a layered or banded crystalline metamorphic rock, the grains of which are aligned or elongated into a roughly parallel arrangement.
Goals – Goals are strategic objectives which the organizational management establishes to outline expected outcomes and guide employees’ efforts toward the achievement of those outcomes.
Goethite – It is a mineral of the diaspore group, consisting of iron (III) oxide-hydroxide, specifically the alpha-polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite is an iron oxyhydroxide containing ferric iron. It is the main component of rust and bog iron ore. Goethite’s hardness ranges from 5.0 to 5.5 on the Mohs Scale, and its specific gravity varies from 3.3 to 4.3. The mineral forms prismatic needle-like crystals (needle ironstone) but is more typically massive.
Goggles – Goggles are safety equipment used for eye protection. They are tight-fitting eye protection which completely cover the eyes, eye sockets and the facial area immediately surrounding the eyes and provide protection from impact, dust, mists, vapours and splashes. Goggles with direct ventilation typically are used for impact hazards and dusts, not for protection against chemical splashes or vapours. Goggles with indirect ventilation are used for protection from dusts and splash hazards. Goggles with no ventilation provide protection from dusts, splashes, mists and vapours. Goggles with foam or cloth padding are not to be used for chemical splash protection. Some goggles fit over corrective lenses.
Gold – It is a chemical element, has symbol Au, and atomic number 79. In its pure form, it is a bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal, a group 11 element, and one of the noble metals.
Gold filled – It means covered on one or more surfaces with a layer of gold alloy to form a clad or composite material. Gold-filled dental restorations are an example of such materials.
Gold lacquer – It is a protective coating which has a yellow colour. It can be of lacquer or baked enamel.
Goniometer – It is an instrument devised for measuring the angle through which a sample is rotated or for orienting a sample (e.g., a single crystal) in a specific way.
Go / no-go decision – It is a type of decision where an absolute lower (or upper) limit has been established as necessary for the functioning of the design. All materials or components which do not exceed the lower limit specification are rejected (no-go), while those which do are accepted and subjected to further screening.
Good manufacturing practices – It means a system for ensuring that products are consistently produced and controlled as per the quality standards. These practices conform to the guidelines recommended by standards / regulatory agencies.
Goodness of fit – Goodness of fit describes a class of statistics used to assess the fit of a model to observed data. There are several measures of goodness of fit which include the coefficient of determination, the F-test, the chi-square test for frequency data, and numerous other measures. It is to be noted that goodness can refer to the fit of a statistical model to data used for estimation, or data used for validation.
Goose-neck – In die casting, it is a spout connecting a molten metal holding pot, or chamber, with a nozzle or sprue hole in the die and containing a passage through which molten metal is forced on its way to the die. It is the metal injection mechanism in a hot chamber machine.
Goose-neck, coke ovens – In a coke oven, a goose-neck is a curved pipe or duct that connects the top of the coke oven to the gas collection system. Its main functions are to collect and channel the volatile gases released during the coking process, regulate gas pressure, and prevent leaks.
Gossan – It is the rust-coloured capping or staining of a mineral deposit, normally formed by the oxidation or alteration of iron sulphides.
Gouge – It is fine, putty-like material composed of ground-up rock found along a fault. It is also a gross scratch.
Gouge, rolled in – It is a more localized gross rolled-in scratch.
Gouging – In welding practice, it consists of the forming of a bevel or groove by material removal. Gouging is also surface tearing found on the inner surface of seamless (extruded) pipes and it is caused by excessive friction between the mandrel and the inner surface of the pipe.
Gouging abrasion – It is a form of high-stress abrasion in which easily observable grooves or gouges are created on the surface.
Governor – It is a speed regulator for a machine such as a generator. It is an early important feedback control cybernetic system.
G-P zone – It means a Guinier-Preston zone. A G-P zone is a fine-scale metallurgical phenomenon, involving early-stage precipitation. G-P zones are associated with the phenomenon of age hardening, whereby room-temperature reactions continue to occur within a material through time, resulting in changing physical properties.
Grab sample – It is a sample from a rock outcrop which is assayed to determine if valuable elements are contained in the rock. A grab sample is not intended to be representative of the deposit, and normally the best-looking material is selected.
Graben – It is a down-faulted block of rock.
Grade – It is the product classification as per the quality, based on standard specifications. Grade is also the category or rank given to entities having the same functional use but different requirements for quality. Grade reflects a planned or recognized difference in requirement for quality. The emphasis is on the functional use and cost relationship. A high-grade entity can be of unsatisfactory quality and vice versa. Where grade is denoted numerically, the highest grade is normally designated as 1, with lower grades extending to 2, 3, 4, etc. Where grade is denoted by a point score, such as a number of star symbols, the lowest grade normally has the least points or stars.
Graded abrasive – It is an abrasive powder in which the sizes of the individual particles are confined to certain specified limits.
Graded coating – It is a thermal spray coating consisting of several successive layers of different materials, e.g., starting with 100 % metal, followed by one or more layers of metal-ceramic mixtures, and finishing with 100 % ceramic.
Gradient elution – It is a technique for improving the efficiency of separations achieved by liquid chromatography. It refers to a step-wise or continuous change with time in the mobile phase composition.
Graduations – These are marks on a container to show fluid levels of contents on a scale of full to empty.
Grain – It is an individual crystal in a polycrystalline material. It may or may not contain twinned regions and sub-grains.
Grain boundary – It is a narrow zone in a metal or ceramic corresponding to the transition from one crystallographic orientation to another, hence separating one grain from another. The atoms in each grain are arranged in an orderly pattern.
Grain-boundary corrosion – It is the corrosion which occurring preferentially at grain boundaries, normally with slight or negligible attack on the adjacent grains.
Grain-boundary denudation – It is a non-equilibrium condition in which there is a solute composition gradient of a solute from the grain boundary to the grain interior. The condition is frequently created when a phase rich in the solute forms in the grain boundary.
Grain boundary diffusion – It is one of the diffusion mechanisms in sintering. It is characterized by a very high diffusion rate because of an abundance of imperfections in the grain boundaries.
Grain-boundary etching – In metallography, it consists of the development of intersections of grain faces with the polished surface. Because of severe, localized crystal deformation, grain boundaries have higher dissolution potential than grains themselves. Accumulation of impurities in grain boundaries increases this effect.
Grain-boundary liquation – It is an advanced stage of overheating in which material in the region of austenitic grain boundaries melts. It is also termed as burning.
Grain-boundary sulphide precipitation – It is an intermediate state of overheating of metals in which sulphide inclusions are redistributed to the austenitic grain boundaries by partial solution at the overheating temperature and reprecipitation during subsequent cooling.
Grain coarsening – It is a heat treatment which produces excessively large austenitic grains in metals.
Grain-contrast etching – In metallography, it consists of the development of grain surfaces lying in the polished surface of the micro-section. These become visible through differences in reflectivity caused by reaction products on the surface or by differences in roughness.
Grain fineness number – It is a weighted average grain size of a granular material. The American Foundrymen’s Society grain fineness number is calculated with prescribed weighting factors from the standard screen analysis.
Grain flow – It is fibre-like lines on polished and etched sections of forgings which are caused by orientation of the constituents of the metal in the direction of working during forging. Grain flow produced by proper die design can improve the needed mechanical properties of forgings.
Grain growth – It is an increase in the average size of the grains in poly-crystalline material, normally as a result of heating at high temperature. In poly-crystalline materials, it is a phenomenon which is occurring fairly close below the melting point in which the larger grains grow still larger while the smallest ones gradually diminish and disappear.
Graining – It is the process of vigorously stirring or agitating a partially solidified material to develop large grains having a thin oxide coating.
Grain refinement – It is the manipulation of the solidification process to cause more (and hence smaller) grains to be formed and / or to cause the grains to form in specific shapes. The term refinement is normally used to denote a chemical addition to the metal but can refer to control of the cooling rate.
Grain refiner – It is a material added to a molten metal to induce a finer-than-normal grain size in the final structure in the subsequent casting.
Grain refining – It is heating from some temperature below the transformation range to a suitable temperature above that range followed by cooling at a suitable rate.
Grain size – It is a measure of the areas or volumes of grains in a poly-crystalline metal or alloy, normally expressed as an average when the individual sizes are fairly uniform. In metals containing two or more phases, the grain size refers to that of the matrix unless otherwise specified. Grain size is reported in terms of number of grains per unit area or volume, average diameter, or as a number derived from area measurements. For grinding wheels, it is the nominal size of abrasive particles in a grinding wheel, corresponding to the number of openings per linear unit length in a screen through which the particles can pass. In sintered metals, it is a measure of the areas or volume of the grains. It is not to be mistaken for the particles of the original powder which have not yet dissolved in the structure.
Grain size distribution – It means measures of the characteristic grain or crystallite dimensions (normally, diameters) in a poly-crystalline solid, or of their populations by size increments from minimum to maximum. It is normally determined by microscopy.
Gram-equivalent weight – It is the mass in grams of a reactant which contains or reacts with Avogadro’s number of hydrogen atoms.
Gram-molecular weight – It is the mass of a compound in grams equal to its molecular weight.
Granite – It is a coarse-grained intrusive igneous rock consisting of quartz, feldspar and mica.
Granular fracture – It is a type of irregular surface produced when metal is broken. It is characterized by a rough, grain-like appearance, rather than a smooth or fibrous one. It can be sub-classified as trans-granular fracture or inter-granular fracture. This type of fracture is frequently called crystalline fracture. However, the inference that the metal broke since it ‘crystallized’ is not justified, because all the metals are crystalline in the solid state.
Granular material – It is a conglomeration of discrete solid, macroscopic particles which are characterized by a loss of energy whenever the particles interact (the most common example is the friction when grains collide). The constituents which compose granular material are large enough such that they are not subject to thermal motion fluctuations.
Granular powder – It is a powder having equi-dimensional but non-spherical particles.
Granulated blast furnace slag – Granulated blast furnace slag is produced by quickly quenching (chilling) the liquid slag to produce a glassy, granular product. The most common process is quenching with water, but air or a combination of air and water can be used. When the liquid slag is cooled and solidified by rapid water quenching to a glassy state, little or no crystallization takes place. This process results in the formation of sand size (or frit-like) fragments, normally with some friable clinker like material. Granulated blast furnace slag is a glassy granular material which normally varies in size from coarse popcorn like friable structure higher than 4.75 millimeters (No. 4 sieve) in diameter to dense sand size grains passing a 4.75 millimeters sieve.
Granulated iron – Granulated iron has good chemical and physical properties like pig iron and can be used as a prime raw material for the purpose of steelmaking. It has a chemical composition identical to the liquid iron which is being granulated. There is no oxidation or slag entrapment in the granulated iron and there is high metallic content.
Granulated metal – It consists of small pellets produced by pouring liquid metal through a screen or by dropping it onto a revolving disk, and, in both the cases, chilling with water.
Granulated pig iron – It is the main product of the granulation of hot metal.
Granulating – It is the production of coarse metal particles by pouring molten metal through a screen into water or by agitating the molten metal violently while it is solidifying.
Granulation – It is the process of quenching liquid slag which breaks up the material into small particles and then solidifies as granules like coarse sand aggregates. It is also the process of forming grains or granules from a powdery or solid substance, producing a granular material. It is applied in several technological processes in the chemical industry.
Granulation of liquid Iron (hot metal) – Granulation of liquid iron is a method of handling of excess production of hot metal in a blast furnace, which cannot be consumed by steelmaking in the steel melting shop of an integrated iron and steel plant. It is a cost-effective method of producing a solid product which is known as granulated iron. The four basic steps of the granulation process for liquid iron are (i) control of flow of liquid iron to the granulator, (ii) granulation by forming of droplets of liquid iron and their rapid quenching in water in a granulator, (iii) discharge of solidified and cooled granulated iron normally by air water ejector, and (iv) dewatering of granulated iron and transport to storage location.
Graphical diagram – It is the short-hand system of the industry and is normally preferred for design and troubleshooting purpose in pneumatic and hydraulic systems. In these diagrams, Simple geometric symbols represent the components and their controls and connections.
Graphite – It is a form of carbon which is used in nuclear fission reactors to slow down (moderate) neutrons. It is normally constructed in the form of blocks or sleeves.
Graphite carbon block, partial – It is made of calcined anthracite under high temperature and graphite. Medium pitch acts as binder. Main processes are extruding, baking and processing. Typical properties are with good thermal conductivity and anti-alkaline. Partial graphite carbon block is used in bottom of the blast furnace.
Graphite carbon block, ultra-thermal conductivity – It is made of low ash petroleum-coke with pitch acting as a binder. It is made by extruding, baking, impregnation, graphitization and processing. The product is with higher thermal conductivity which is helpful in reducing the temperature, cooling and slowing the erosion of the blast furnace bottom. Ultra-thermal conductivity graphite block is used in the hearth and bottom of blast furnace.
Graphite fibre – It is a fibre made from a precursor by an oxidation, carbonization, and graphitization process, which provides a graphitic structure.
Graphite furnace atomic absorption spectrometry (GFAAS) – It is also known as electro-thermal atomic absorption spectrometry (ETAAS). It is a type of spectrometry which uses a graphite-coated furnace to vapourize the sample. Briefly, the technique is based on the fact that free atoms absorb light at frequencies or wave-lengths characteristic of the element of interest (hence the name atomic absorption spectrometry). Graphite furnace atomic absorption spectrometry is an established technology for quantifying elements at trace and ultra-trace levels (down to low micrograms per litre) while using only small sample volumes (normally less than 100 micro-litres).
Graphite material – The term, graphite, also called synthetic, artificial or electro-graphite, refers to a carbon product that has been further heat treated at a temperature between 2,400 deg C and 3,000 deg C. This process of graphitization changes the crystallographic structure of carbon and also changes the physical and chemical properties. Graphite is also found in nature in flake form, and, if used in a refractory product, usually forms part of a mixture of ceramic materials for the binder. This ceramic bonded, natural graphite containing refractory is considered a ceramic product.
Graphite shapes, cast iron – There are four types of the graphite shapes in cast irons. These are (i) lamellar (flake) graphite (FG), (ii) spheroidal (nodular) graphite (SG), (iii) compacted (vermicular) graphite (CG), and (iv) temper graphite (TG). Temper graphite results from a solid-state reaction (malleabilization).
Graphitic carbon – It is the free carbon in steel or cast iron.
Graphitic corrosion – It is the corrosion of gray iron in which the iron matrix is selectively leached away, leaving a porous mass of graphite behind. It occurs in relatively mild aqueous solutions and on buried pipe and fittings.
Graphitic steel – It is an alloy steel made so that part of the carbon is present as graphite.
Graphitization – It is the process of heating amorphous carbon for a prolonged period of time, rearranging the atomic structure to achieve an ordered crystalline structure which is typical of solids. During graphitization, carbon atoms are rearranged to fill atom vacancies and improve atom layout. The graphitization process needs heating up to temperatures of 3,000 deg C.
Graphitization, iron and steel – It is the formation of graphite in iron or steel. Where graphite is formed during solidification, the phenomenon is termed primary graphitization and where it is formed later by heat treatment, it is termed as secondary graphitization.
Graphitizing – It consists of annealing a ferrous alloy such that some or all the carbon precipitates as graphite.
Grate kiln process – In the grate kiln process, the traveling grate is used to dry and preheat the pellets. Material moves on straight travelling grate till it attains the temperature in the range of 800 deg C to 1,000 deg C. After that, the material is transferred to refractory lined rotary kiln for induration where the temperature is further raised in the range of 1,250 deg C to 1,300 deg C. At 800 deg C, the ferrous oxide (FeO) of the magnetite iron ore gets converted into ferric oxide (Fe2O3) in an exothermic reaction. The liberated heat hardens the green balls which is helpful to withstand the tumbling impact because of the rotation of the rotary kiln. A circular cooler is used for cooling of the fired pellets.
Grating – It is any regularly spaced collection of essentially identical, parallel, elongated elements. Gratings normally consist of a single set of elongated elements, but can consist of two sets, in which case the second set is normally perpendicular to the first. When the two sets are perpendicular, this is also known as a grid or a mesh.
Gravel – It is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentary and erosive geological processes. It is also produced in large quantities commercially as crushed stone. Gravel is classified by particle size range and includes size classes from granule- to boulder-sized fragments.
Gravitational constant (G) – It is an empirical physical constant involved in the calculation of gravitational effects in the Newton’s law of universal gravitation and in Einstein’s general theory of relativity. In Newton’s law, it is the proportionality constant connecting the gravitational force between two bodies with the product of their masses and the inverse square of their distance. In the Einstein field equations, it quantifies the relation between the geometry of spacetime and the energy–momentum tensor.
Gravity – It is a natural phenomenon by which all things with mass or energy are attracted to one another. On Earth, gravity gives weight to physical objects. Gravity has an infinite range, although its effects become weaker as objects get further away.
Gravity-based structure (GBS) – It is a support structure held in place by gravity. These structures are often constructed in fjords (a long narrow piece of sea between cliffs) because of their protected area and sufficient depth.
Gravity concentration – It is widely being used in the beneficiation of hematite iron ores. This technology is used to suspend and transport lighter gangue away from the heavier valuable mineral. This separation process is based mainly on differences in the specific gravities of the materials and the size of the particles being separated. Value fractions can be removed along with the gangue material (tailings) despite differences in density if the particle sizes vary. Because of this potential problem, particle sizes are to be kept uniform with the use of classifiers (such as screens and hydro cyclones).
Gravity die-casting – In gravity die casting, there is a metal, graphite or ceramic mould (other than an ingot mould) which is repeatedly used for the production of several castings of the same form. Liquid metal is poured in by gravity.
Gravity drop-hammers – They are the oldest type of forging equipment available. The principle of operation is that the moving die block is raised by a lifting mechanism and then released, so that it falls onto the fixed die attached to the anvil. The quantity of deformation which can be carried out is determined by the potential energy of the moving die block at its maximum height. This potential energy is converted into kinetic energy as the die block falls and is then dissipated in deformation of the work piece. Different in lifting mechanisms are used, including frictional means with boards, band brakes or belts, or a lifting cylinder employing steam, compressed air, or hydraulic fluid. These machines are available a range of blow energies from 0.6 kilo newton metre to 400 kilo newton metre.
Gravity flow – It is a flow of a liquid fluid such as water from a higher elevation to a lower elevation because of the force of gravity, and not by the energy provided by a pump. Gravity flow also refers to the movement of materials such as landslides, pyroclastic flows, and aeolian sediment under water because of the force of gravity, even if they originated on land.
Gravity hammer – It is a class of forging hammer in which energy for forging is achieved by the mass and velocity of a freely falling ram and the attached upper die. Examples are the board hammer and air-lift hammer.
Gravity meter, gravimeter – It is an instrument for measuring the gravitational attraction of the earth. Gravitational attraction varies with the density of the rocks in the vicinity.
Gravity segregation – It means variable composition of a casting or ingot caused by settling out of heavy constituents, or rising of light constituents, before or during solidification.
Gravity separation – It is an industrial method of separating two components, either a suspension, or dry granular mixture where separating the components with gravity is sufficiently practical: i.e. the components of the mixture have different specific weight. Every gravitational method uses gravity as the primary force for separation. One type of gravity separator lifts the material by vacuum over an inclined vibrating screen covered deck. This results in the material being suspended in air while the heavier impurities are left behind on the screen and are discharged from the stone outlet. Gravity separation is used in a wide variety of industries, and can be most simply differentiated by the characteristics of the mixture to be separated – principally that of ‘wet’ i.e., a suspension against ‘dry’, a mixture of granular product. Gravity separation technique is used in iron ore beneficiation, where iron bearing minerals are free from associated gangue materials. The specific gravity of iron bearing minerals is normally higher than the specific gravity of gangue materials. Effectiveness efficiency of the gravity separation depends largely on to proper crushing and sizing of the ore so as to ensure a proper size feed to the gravity separation equipment and also removal of slime from the equipment. A large variety of equipments / processes functioning on gravity separation principle are available. Three gravity separation methods have historically been used for iron ore namely (i) washing, (ii) jigging, and (iii) heavy-media separators. Spirals and Reichert cones are other two methods for gravity separation.
Gray (Gy) – It is a unit of measurement for the absorbed dose. The absorbed dose has been formerly measured in rads and 1 gray = 100 rads. When it comes into contact with matter, ionizing radiation collides with the atoms comprising it. During these interactions, it releases a part or all of its energy. The absorbed dose (expressed in Gray) is defined by the ratio of this released energy over the mass of the matter. A Gray corresponds to one Joule of energy released in one kilogram of matter.
Gray cast iron, gray iron – It is a cast iron which shows a gray fracture surface consisting of ferrite and dispersed graphite flakes. When the composition of the iron and the cooling rate at solidification are suitable, a substantial portion of the carbon content separates out of the liquid to form flakes of graphite. The fracture path of such iron follows the graphite flakes. The fracture surface of this iron appears gray because of the predominance of exposed graphite. Gray cast iron is the result of stable solidification produced at slow cooling. It has several unique properties which are derived from the existence of flake graphite in the micro-structure. It possesses high compressing strength, fatigue resistance, and wear resistance. It is relatively soft and can be machined and welded easily. It has hardness conducive to good wear resistance. It resists galling under boundary-lubrication conditions. It has very good properties for use in vibration damping or moderate thermal shock applications. It is used for engine cylinder blocks, gears, flywheels, water pipes, brake discs, and machine tool structures etc.
Gray coatings – Some steels produce a matt gray galvanized coating. These coatings are 100 % alloy layer and contain no free zinc. They tend to be thicker than standard shinier galvanized coatings.
Grease – It is a semi solid lubricant. It is a mixture consisting of natural or synthetic oil base combined with thickeners and additives. It generally consists of a soap emulsified with mineral or vegetable oil. The National Lubricating Grease Institute (NLGI) defines grease as ‘a solid to semi-solid product of dispersion of a thickening agent in a liquid lubricant. Additives imparting special properties may be included’.
Grease-less compounds – These are a variety of buffing compounds in which the abrasive is blended with water and gelatin glue rather than tallow, wax, or oil.
Grease streak – It is a narrow discontinuous streak caused by rolling over an area containing grossly excessive lubricant drippage.
Greek letters – In statistics, Greek letters are used for the parameters of the population and for a few other things.
Green – It is that powder compact which is not sintered.
Green compact – It is an unsintered powder metallurgy or ceramic compact.
Green density – It is the density of a green compact.
Green design – It is a design methodology in which environmental factors are considered of equal importance to performance factors
Green energy – It is an energy type which is generated from natural resources, such as sunlight, wind or water. It frequently comes from renewable energy sources although there are some differences between renewable and green energy. The key with these energy resources is that they do not harm the environment through factors such as releasing green-house gases into the atmosphere.
Greenfield project – It is that project (e.g., steel plant) which lacks constraints imposed by prior work, i.e., the project is constructed on unused land where there is no need to remodel or demolish an existing structure. The analogy is to that of construction on greenfield land where there is no need to work within the constraints of existing buildings or infrastructure.
Greenhouse gases – Gases which trap heat in the atmosphere are called greenhouse gases. Several chemical compounds in the atmosphere act as greenhouse gases. These gases allow sunlight (shortwave radiation) to freely pass through the atmosphere and heat the land and oceans. Greenhouse gases consist of three or more atoms. This molecular structure makes it possible for these gases to trap heat in the atmosphere and then transfer it to the surface of the Earth which further warms the Earth. The main greenhouse gases are carbon di-oxide, methane, nitrous oxide, and industrial gases, including hydro-fluoro-carbons, per-fluoro-carbons, and sulphur hexa-fluoride. Carbon di-oxide, methane, nitrous oxide, and certain manufactured gases called halogenated gases (gases which contain chlorine, fluorine, or bromine). Greenhouse gases become well mixed throughout the global atmosphere because of their long lifetimes and because of transport by winds.
Green hydrogen – It is defined as hydrogen produced from water electrolysis with zero-carbon electric power, can have considerable potential in helping countries transition their economies to meet climate goals. It is produced by using renewable energy. It is the most suitable one for a fully sustainable energy transition. The most established technology option for producing green hydrogen is water electrolysis fuelled by renewable electricity. The electrolysis process splits water molecules into hydrogen and oxygen. Electrolysis needs energy. This energy comes from lower-cost renewable sources and this makes this form of hydrogen ‘green’.
Green liquor – It is the liquor resulting from dissolving molten smelt from the kraft recovery furnace in water.
Green pellets or balls – Green pellets with a size range of 8 millimeters to 16 millimeters are prepared in a balling drum or in a disc pelletizer. Disc pelletizer is preferred for the production of the quality green pellets since in the disc pelletizer it is easy to control the operation with minimum of foot space. The pre wetted mix is fed into the disc at a controlled rate. In the disc, the material is coagulated and because of the continuous rotary motion gets formed into nodules/ pellets. Ore fines are lifted upwards until the friction is overcome by gravity and the material rolls down to the bottom of the disc. This rolling action first forms small granules called seeds. Growth occurs in the subsequent revolutions of the disc by the addition of more fresh feeds and by collision between small pellets. Surface tension of water, capillary action of water, and the gravitational force in the balling disc are the forces which act on the ore particles. Hence, they get coalesce together and form nuclei which grow in size and into ball shape.
Green rot – It is a form of high-temperature attack on stainless steels, nickel-chromium alloys, and nickel-chromium-iron alloys which are subjected to simultaneous oxidation and carburization. Basically, attack occurs first by precipitation of chromium as chromium carbide, then by oxidation of the carbide particles.
Green sand -It is a naturally bonded sand, or a compounded moulding sand mixture, which has been ‘tempered’ with water and which is used while it is still moist.
Green sand core – It is a core made of green sand and used as-rammed. It is also a sand core which is used in the unbaked condition.
Green sand mould – It is a casting mould which is composed of moist prepared moulding sand.
Green sand moulding – The mould is composed of a prepared mixture of sand, clay, sea coal, and moisture for use while still in the damp condition. The mould is not cured or dried and hence is known as a green (uncured) sand mould.
Green steelmaking – It consists of the use of those processes which result into reduction in emissions of carbon di-oxide. Development work for the green steelmaking processes is being done in several countries. For the development of the technologies for the green steelmaking, five key directions are being explored. These directions are (i) technologies involving coal usage, (ii) technologies involving use of hydrogen, (iii) technologies involving electrons, (iv) technologies involving use of biomass, and (v) technologies involving carbon capture, use, and / or storage (CCUS). There are several pathways which are being explored for the break-through technologies for cutting of the carbon di-oxide emissions from the ore-based steel production routes.
Green strength – It is the strength of a tempered foundry sand mixture at room temperature. It is also the ability of a green compact to maintain its size and shape during handling and storage prior to sintering. It is also the tensile or compressive strength of a green compact.
Greenstone belt – It is an area underlain by metamorphosed volcanic and sedimentary rocks, normally in a continental shield.
Grey hydrogen – The most common process for grey hydrogen is to use either natural gas or coal as feedstock which reacts with steam at high temperatures and pressures to produce synthesis gas, which consists primarily of hydrogen and carbon mono-oxide. The process is known as steam methane reforming (SMR). The synthesis gas is then reacted with additional water to produce pure hydrogen and carbon di-oxide. These are well-established processes, but they generate considerable carbon di-oxide emissions, which is why the resulting element is termed ‘grey hydrogen’. The use of grey hydrogen emits substantial carbon di-oxide emissions, which makes these hydrogen technologies unsuitable for a route towards net-zero emissions.
Grid, electrical – An electrical grid is an interconnected network for electricity delivery from producers to consumers. Electrical grids consist of power stations, electrical substations to step voltage up or down, electric power transmission to carry power over long distances, and finally electric power distribution to customers.
Grid-tie inverter – It is a power inverter which allows synchronization with the electrical grid for export of energy surplus to the facility’s needs.
Greige, gray goods – It is a fabric before finishing, as well as any yarn or fibre before bleaching or dyeing, hence, it is a fabric with no finish or size.
Grindability – It is the relative ease of grinding. It is analogous to machinability. It is a value which represents the efficiency of the grinding process. Grindability = (G-ratio) / (specific energy), where G-ratio = (volume of material removed) / (volume of the grinding wheel used) and specific energy = (grinding power) / (material removal rate).
Grindability index – It is a measure of the grindability of a material under specified grinding conditions. It is expressed in terms of volume of material removed per unit volume of wheel wear.
Grinding – It is removing material from a work-piece with a grinding wheel or abrasive belt. In case of ores and minerals, grinding is done after initial crushing. It reduces the ore particles to the consistency of fine powder (325 mesh, 0.44 micrometers). The choice of grinding circuit is based on the density and hardness of the ore to be ground. Although use of the rod mill or ball mill grinding is quite common, a few facilities use autogenous or semi autogenous grinding systems. Autogenous grinding uses coarse pieces of the ore itself as the grinding media in the grinding mill. Semi autogenous operations use metallic balls and / or rods to supplement the grinding action of the ore pieces. Autogenous grinding is best suited to weakly cemented ores containing some hard material. The benefit of autogenous grinding is that it is less capital and labour intensive. Semi autogenous grinding eliminates the need for a secondary crushing circuit. Rod and ball wear, the principal maintenance cost of traditional grinders, is also eliminated with this method.
Grinding burn – It consists of getting the work hot enough to cause discolouration or to change the micro-structure by tempering or hardening.
Grinding cracks – These are the cracks which are formed in the surfaces of relatively hard materials because of excessive grinding heat or the high sensitivity of the material. Grinding cracks develop at locations where there is a localized heating of the base metal and they are normally shallow and at right angle to the grinding direction. Such cracks can be caused by the use of glazed wheels, inadequate coolant, excessive feed, or grinding depth.
Grinding fluid – It is an oil-based or water-based fluid introduced into grinding operations to (i) reduce and transfer heat during grinding, (ii) lubricate during chip formation, (iii) wash loose chips or swarf from the grinding belt or wheel, and (iv) chemically aid the grinding action or machine maintenance.
Grinding mills – These are size reductions machines which frequently often follow crushers in the processes where finer products are desired after crushing. Different grinding machines are normally named as mills, for example rod mills, ball mills, and attrition mills. The grinding mills are categorized in three groups, namely (i) tumbling mills, (ii) roller mills, and (iii) very fine grinding mills which include (a) high speed pulverizing or hammer mill, (b) vibrating mill, (c) pin mill, (d) turbo mill, (e) fluid energy mill, (f) stirred media mill.
Grinding oil – It is an oil-type grinding fluid, it can contain additives, but not water.
Grinding relief – It is a groove or recess located at the boundary of a surface to permit the corner of the wheel to overhang during grinding.
Grinding sensitivity – It is the susceptibility of a material to surface damage such as grinding cracks. It can be affected by such factors as hardness, micro-structure, hydrogen content, and residual stress.
Grinding stress – It is the residual stress, generated by grinding, in the surface layer of the work. It can be tensile or compressive, or both.
Grinding wheel – It is a cutting tool of circular shape which is made of abrasive grains bonded together.
Grip – It means to seize or hold firmly.
Gripper dies -These are the lateral or clamping dies which are used in a forging machine or mechanical upsetter.
Grip length – It is the length of the unthreaded portion of the fastener (i.e. shank) measured axially from the underside of the bearing surface to the starting thread.
Gripping lifters – They use either friction or indentation-causing pressure to hold a load. Tong grabs or clamps utilize a scissor-type action to grip a load. Coil grabs grasp the outer diameter of a coil via tongs or gripping mechanisms to lift or turn it.
Grit – It is the crushed ferrous or synthetic abrasive material in different mesh sizes which is used in abrasive blasting equipment to clean castings.
Grit blasting – It is the abrasive blasting with small irregular pieces of steel, malleable cast iron, or hard non-metallic materials.
Grit size – It is the nominal size of abrasive particles in a grinding wheel, corresponding to the number of openings per linear unit length in a screen through which the particles can pass. It is also the particle size of an abrasive powder, such as carborundum, corundum, silicon carbide, or diamond used in cutting and machining operation.
Grizzly (or mantle) – It is a grating, normally constructed of steel rails, placed over the top of a chute or ore pass for the purpose of stopping large pieces of rock or ore which can hang up in the pass.
Groove – It is a long and narrow indentation built into a material, normally for the purpose of allowing another material or part to move within the groove and be guided by it. Examples include (i) a canal cut in a hard material, normally metal. This canal can be round, oval or an arc in order to receive another component such as a boss, a tongue or a gasket. It can also be on the circumference of a dowel, a bolt, an axle or on the outside or inside of a tube or pipe etc. This canal can receive a circlip, an O-ring, or a gasket, (ii) a depression on the entire circumference of a cast or machined wheel, a pulley or sheave. This depression can receive a cable, a rope or a belt, and (iii) a longitudinal channel formed in a hot rolled rail profile such as a grooved rail. This groove is for the flange on a train wheel. In thermal spraying, it is a method of surface roughening in which grooves are made and the original surface roughened and spread.
Groove angle – It is the total included angle of the groove between the work-pieces.
Grooved rolls – These rolls are used in section rolling mills. They are for rolling of work-pieces and section material. Bodies of these rolls have so called grooves (passes). They track profile of section metal. These grooves have a definition, they are called passes. Passes of two rolls with distance between them are called a groove.
Groove face – It is that surface of a joint member which is included in the groove.
Groove radius – It is the radius which is used to form the shape of a J-groove or U-groove weld.
Groove weld – It is a weld which is made in a groove between the work-pieces.
Groove weld size – It is the joint penetration of a groove weld.
Groove weld throat – It is a non-standard term for groove weld size.
Grossmann chart – It is a chart describing the ability of a quenching medium to extract heat from a hot steel work-piece in comparison to still water.
Grossmann number (H) – It is a ratio describing the ability of a quenching medium to extract heat from a hot steel-work-piece in comparison to still water defined by the equation H = h/2k, where ‘h’ is the heat transfer coefficient and ‘k’ is the conductivity of the metal.
Gross porosity – In weld metal or in a casting, it consists of pores, gas holes, or globular voids which are larger and in much higher numbers than those achieved in good practice.
Gross sample – It is one or more increments of material taken from a larger quantity (lot) of material for assay or record purposes. It is also termed as a bulk sample or a lot sample.
Ground – It is a reference point for electrical potential, frequently connected to the Earth.
Ground and neutral – It is protective and circuit return conductors in a wiring system.
Ground-bed – It is a buried item, such as junk steel or graphite rods, which serves as the anode for the cathodic protection of pipelines or other buried structures.
Ground coat – It is a porcelain enamel which is applied directly to the base metal to function as an intermediate layer between the metal and the cover coat. It is also a porcelain enamel coating on sheet steel containing adherence-promoting agents which can be used either as an intermediate layer between the metal and the cover coat or as a single coat over the base metal.
Ground connection – In arc welding, it is a device which is used for attaching the work lead (ground cable) to the work. It is an electrical connection of the welding machine frame to the earth for safety.
Ground fault circuit interrupter (GFCI) – It is an electrical safety device which interrupts an electrical circuit when the current passing through a conductor is not equal and opposite in both directions, hence indicating leakage current to ground or current flowing to another powered conductor. The purpose of the device is to reduce the severity of injury caused by an electric shock.
Ground granulated slag – The ground granulated slag is used in composite cements and as a cementitious component of concrete. The use of ground granulated slag as a separately ground material added at the concrete mixer together with Portland cement has gained acceptance. In some countries the term ‘slag cement’ is used for pure ground granulated slag. Practically, there are no concrete, mortar or grout applications which preclude the use of an appropriate quantity of ground granulated slag.
Ground lead – It is a non-standard term for workpiece lead.
Ground-level power supply – It is a system for providing powers for electric trams without overhead wires and without a permanently energized third rail.
Group – It consists of number of samples tested at one time, or consecutively, at one stress level. A group can comprise one or more samples.
Group technology – It is an approach to design and manufacturing which seeks to reduce manufacturing system information content by identifying and exploiting the sameness or similarity of parts based on their geometrical shape and / or similarities in their production process.
Grout – It is a dense fluid which hardens upon application and is used to fill gaps or as reinforcement in existing structures. Grout is normally a mixture of water, cement, and sand, and is used in pressure grouting, embedding rebar in masonry walls, connecting sections of precast concrete, filling voids, and sealing joints such as those between tiles.
Grouting – It is the process of sealing off a water flow in rocks by forcing a thin slurry of cement or other chemicals into the crevices. It is normally done through a diamond drill hole.
Growler – It is a test instrument that is used to diagnose some faults with alternating current motors.
Growth – It is an increase in compact or part size as a result of excessive pore formation during sintering.
Growth (cast iron) – It is a permanent increase in the dimensions of cast iron resulting from repeated or prolonged heating at temperatures above 480 deg C because of either to graphitizing of carbides or oxidation.
Growth-curve modelling (also known as the linear mixed model) – It is a regression analysis in which the response variable is the trajectory of change over time in a quantitative study end point. Interest centres on describing the average trajectory of change, as well as what subject characteristics lead to different trajectories of change for different types of subjects.
Growth twin – It is a twin formed in a crystal or grain during recrystallization or, rarely, during solidification.
Guarding – It is use of any device or combination of devices designed to keep any part of a worker’s body out of danger zone of a machine during its operating cycle.
Guards – These are used for preventing the bending of the rolling stock upwards or downwards during its exit from the rolls and / or collaring on the roll. The placing of the guards is important depending on whether the mill stand works with top or bottom pressure, i.e. whether the rolling stock shows a tendency towards collaring on the bottom or top roll.
Guerin process – It is a rubber-pad forming process for forming sheet metal. The principal tools are the rubber pad and form block, or punch.
Guide – It consists of the parts of a drop hammer or press which guide the up-and-down motion of the ram in a true vertical direction.
Guide bearing – It is a bearing which is used for positioning a slide, or for axial alignment of a long rotating shaft.
Guided bend – It is the bend got by use of a plunger to force the sample into a die in order to produce the desired contour of the outside and inside surfaces of the sample.
Guided bend test – It is a test in which the sample is bent to a definite shape by means of a punch (mandrel) and a bottom block.
Guided iteration – It is a general problem-solving methodology applicable to engineering design. The steps in this process are namely formulation of the problem, generation of alternative solutions, and evaluation of the alternatives. If none of the alternatives is acceptable, the process continues with redesign, guided by the results of the evaluation.
Guide mill – It is a small hand mill with several stands in a train and with guides for the work at the entrance to the rolls.
Guide pin bushings – These are the bushings, pressed into a die shoe, which allow the guide pins to enter in order to maintain punch-to-die alignment.
Guide pins – These are hardened, ground pins or posts which maintain alignment between punch and die during die fabrication, set-up, operation, and storage. If the press slide in out of alignment, the guide pins cannot make the necessary correction unless heel plates are engaged before the pins enter the bushings.
Guides – The purpose of the guides is to correctly guide the rolling stock into the groove of work rolls at the entry side of the roll stand and its safe exit to the run-out table of the roll stand. Guide equipments guide the rolling stock at the entry and the exit of the roll pass so as to have smooth rolling of the rolling stock. The guiding equipments are to be sturdy, accurate and stable. They play a major role in ensuring the surface quality of the rolled product. The guides are to be designed for the wide variety of sizes and shapes of rolling stock which are normally encountered in the long product rolling.
Guillotine shear – It has a moving blade which runs on straight slides. The moving blade is almost parallel to the fixed blade during the entire stroke. The guillotine design uses a drive system to power the moving blade down. The guillotine shear needs a gibbing system to keep the blade beams in the proper position as they pass each other.
Guinier-Preston (G-P) zone – It is a small precipitation domain in a super-saturated metallic solid solution. A G-P zone has no well-defined crystalline structure of its own and contains an abnormally high concentration of solute atoms. The formation of G-P zones constitutes the first stage of precipitation and is normally accompanied by a change in properties of the solid solution in which they occur.
Gum – In lubrication, it is a rubber-like, sticky deposit, black or dark brown in colour, which results from the oxidation and / or polymerization of fuels and lubricating oils. Harder deposit are described as lacquers or varnishes.
Gun – It is a device used in cutting and welding, e.g., air cutting gun, arc welding gun, electron beam gun, resistance welding gun, soldering gun, and thermal spraying gun.
Gun drill – It is a drill, normally with one or more flutes and with coolant passages through the drill body, used for deep hole drilling.
Gunn diode – It is a two-terminal solid-state device which hat is used in micro-wave oscillators.
Guniting – It is a process used for the hot repair of the refractory lining which get damaged during the operation of the furnace. It is an important process which is frequently used for enhancing the refractory lining life of the furnace. Sometimes it is also called shotcreting. It is a process or technique which involves pumping of refractory guniting refractory mixes under high pressure through a hose to a nozzle (spray-gun) and then spraying them at high velocity onto a surface either by dry-mix or by wet-mix process. Guniting makes it possible to repair refractory linings in horizontal, vertical, and overhead positions or of irregular shapes. It does not need any forms.
Guniting equipment – The main equipment for guniting of the refractory material is a guniting machine. The guniting machines are mounted on wheels for portability. A valve is provided at the nozzle for the regulated flow of water which ensures very efficient and controlled hydration of the guniting refractory mix. For avoiding manual work, one lever operation is generally provided for charging the guniting material into working chamber. This also makes the operation easy and safe. Auxiliary equipment upstream includes the necessary pumps and pressure regulators, pre-wetting equipment, and the solid feed regulating system where the solids flow rate is determined. Manifolds are provided in airline to ensure minimum air pressure loss for the optimum utilization of air. This results into saving in air consumption. The upper and bottom vessels in the guniting machine can be unbolted for inspection and cleaning. A check valve is normally provided before manifold to prevent back flow of material into valves and air motor. The guniting machines are simple and safe to operate and can be operated even by an inexperienced person.
Guniting refractories – These refractories are monolithic refractories which are installed by guniting process. Guniting refractories consist of graded refractory aggregate and a bonding compound, and can contain plasticizing agent to increase their stickiness when pneumatically placed onto a furnace wall. Typically, guniting refractories are supplied dry. To use, they are pre-damped in a batch mixer, then continuously fed into a guniting machine. Water is added to the guniting mix as required by the guniting process to reach the proper consistency. Guniting refractories include siliceous, fireclay, high alumina, and dead burned magnesite and chrome types. Guniting refractories are granular refractory mixtures designed for application with air placement guns. A variety of air guns are used to spray the mixes at high velocity and pressure to form homogeneous compact lining essentially free from lamination and cracks. These refractories are either air-setting or heat-setting and some allow repairs to furnace linings without greatly reducing the furnace temperature. Light weight guniting refractories are used for insulation, while the denser guniting refractories are used in the more severe applications. Some compositions combine relatively low heat losses with good strength. Although guniting refractories need more skills than pouring castables, it can place a higher volume of material in less time than any other method. The properties of guniting refractories vary considerably, and hence encompass a wide range of applications. The guniting refractory mixes perform very well in both original linings and maintenance applications within a service range of 850 deg C to 1,900 deg C.
Gunning mixes – Gunning mixes consist of graded refractory aggregate and a bonding compound, and can contain plasticizing agent to increase their stickiness when pneumatically placed onto a furnace wall. They are granular refractory materials sprayed on application area using a variety of air placement guns. These are heat setting and are used for patching and maintenance works for kilns and furnaces. Typically, gunning mixes are supplied dry. To use, they are pre-damped in a batch mixer, then continuously fed into a gun. Water is added to the mix at the nozzle to reach the proper consistency.
Gutter – It is a depression around the periphery of a forging die impression outside the flash pan which allows space for the excess metal. It surrounds the finishing impression and provides room for the excess metal used to ensure a sound forging. It is a shallow impression outside the parting line.
Gutter-way – In bearings, it is a special type of spreader in which the groove is adjacent to the joint.
Gypsum – It is a sedimentary rock consisting of hydrated calcium sulphate.
Gyratory crusher – It is similar in basic concept to a jaw crusher, consisting of a concave surface and a conical head with both the surfaces are typically lined with manganese steel liners. The inner cone has a slight circular movement, but it does not rotate. The movement is generated by an eccentric arrangement. The crushing action is caused by the closing of the gap between the mantle line (movable) mounted on the central vertical spindle and the concave liners (fixed) mounted on the main frame of the crusher. The gap is opened and closed by an eccentric on the bottom of the spindle that causes the central vertical spindle to gyrate. The vertical spindle is free to rotate around its own axis. The material travels downward between the two surfaces being progressively crushed until it is small enough to fall out through the gap between the two surfaces. A gyratory crusher is used both for primary or secondary crushing. Typically, it has a higher capacity than a jaw crusher.
Gyrotron – It is a high-power vacuum tube oscillator which can produce micro-wave frequencies up to hundreds of giga-hertz at power levels up to mega-watts.
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