Tool Steels
Tool Steels
Tools used for working steels and other metals are to be stronger and harder than the steels or the materials they cut or form. Normally tool steels are known for their distinctive toughness, resistance to abrasion, their ability to hold a cutting edge, and / or their resistance to deformation at high temperatures (red hardness). The significance of tool steel goes far beyond what is normally perceived. Nearly all of the objects which people are surrounded by and encounter on a daily basis are manufactured with the help of tool steels.
Tool steels are iron-based alloys with properties, which are different from those of carbon steels because of the presence of the alloying elements. These steels belong to a large group of steels which, upon heat treatment show high strength, high hardness, and high wear resistance relative to other steel types. Moreover, several tool steel types have good microstructural stability at high temperature, and they retain their properties to an appreciable depth in the material.
The term tool steel is a generic description for those steels which have been developed specifically for tooling (cutting and forming) applications. These steels are used for making cutting tools which cut other materials, punches, as well as moulds and dies which shape and form materials etc. Some of the operations for which the tool steels are used include drawing, blanking, mould inserts, stamping, metal slitting, forming and embossing, die casting, extrusion, and forging although their use is not limited to just these areas.
Tool steels are high-quality steels made to controlled chemical composition and processed to develop properties useful for working and shaping of other materials. The carbon content in tool steels can range from as low as 0.1 % to as high as more than 1.6 % and several tool steels are alloyed with alloying elements such as chromium, molybdenum, and vanadium. Alloy design, the manufacturing route of the tool steel, and quality heat treatment are the key factors in order to develop or parts with the improved properties which only tool steel can offer.
Within tool steels, there are several classifications, such as air-hardening tool steels, high-carbon high-chromium tool steels, water-hardening tool steels, oil-hardening tool steels, hot-work tool steels, and shock-resisting tool steels etc. Appropriate steel selection is necessary for the functionality and manufacturability of a tool. There is a growing challenge facing cutting tool producers, tool users, and those who provide surface engineering solutions to improve tool performance.
Tool steels are very different from steels used in consumer goods. They are made on a smaller scale with stringent quality requirements, and are designed to perform in specific applications. Different applications are made possible by adding particular alloying element along with the appropriate amount of carbon. The alloy element combines with the carbon to improve the steel’s wear, strength, or tools toughness characteristics. These alloy elements also contribute to the steel’s ability to resist thermal and mechanical stresses.
The metallurgical characteristics of different compositions of tool steels are extremely complex. There are hundreds of different types of tool steels available and each can have a specific composition and end use. Tool steels are mainly medium to high carbon steels with specific alloying elements added in different quantities to provide it special characteristics. The carbon in the tool steel is provided to help harden the steel to higher hardness for cutting and wear resistance while alloying elements are added to the tool steel for providing it higher toughness or strength. In some cases, alloying elements are added to retain the size and shape of the tool during its heat treat hardening operation or to make the hardening operation safer and to provide red hardness to it so that the tool retains its hardness and strength when it becomes extremely hot. Different alloying elements in addition to carbon are chromium, cobalt, manganese, molybdenum, nickel, tungsten, and vanadium.
Tool steel is normally delivered in the soft annealed condition. This makes the material easy to machine with cutting tools and it provides a microstructure suitable for hardening. The soft annealed microstructure consists of a soft matrix in which carbides are embedded (Fig 1). In carbon steel, these carbides are iron carbides, while in alloyed steel these are chromium, tungsten, molybdenum, or vanadium carbides, depending on the composition of the steel. Carbides are compounds of carbon and alloying elements and are characterized by very high hardness. Higher carbide content means a higher resistance to wear. Also, non-carbide forming alloying elements are used in tool steel, such as cobalt and nickel which are dissolved in the matrix. Cobalt is normally used to improve red hardness in high-speed steels, while nickel is used to improve through-hardening properties and also increase the toughness in the hardened conditions.
Fig 1 Microstructure of tool steel
Tool steels are alloyed with carbide forming elements such as vanadium, tungsten, molybdenum, and chromium. Some tool steel types contain cobalt, which raises the temperatures at which martensite transformation initiates and at which the transformation is complete. Addition of alloying elements serves primarily two purpose (i) to improve the hardenability, and (ii) to provide harder and thermally more stable carbides than cementite. The effects of the alloying elements on different properties of the tool steel are described below.
Chromium – It results into deeper hardness penetration during heat treatment and contributes wear resistance and toughness to the tool steel.
Cobalt – Cobalt is normally used in high-speed steels and it increases the red hardness so that these steels can be used at higher operating temperatures.
Manganese – Manganese, when used in small quantities, aids in making tool steel sound and further additions help tool steel to harden deeper and more quickly in heat treatment. It also helps to lower the quenching temperature necessary to harden steels. Larger quantities of manganese in the range of 1.20 % to 1.60 % allow tool steels to be oil quenched rather than water quenched.
Molybdenum – Molybdenum, in tool steels, increases the hardness penetration during heat treatment and reduces quenching temperatures. It also helps increase red hardness and wear resistance.
Nickel – Nickel adds toughness and wear-resistance to the tool steels. It is used in conjunction with hardening elements.
Tungsten – Tungsten is added to tool steel to increase its wear resistance. It also provides red hardness characteristics. Around 1.5 % of tungsten increases wear resistance and around 4 % of tungsten in combination with high carbon considerably increases wear resistance of tool steel. Tungsten in large quantities with chromium provides for red hardness.
Vanadium – Vanadium in small quantities increases the toughness and reduces grain size in tool steels. Vanadium in quantities higher than 1 % provides extreme wear resistance especially to the high-speed steels. Smaller quantities of vanadium in conjunction with chromium and tungsten help in increasing red hardness properties in tool steels.
AISI designations
Tool steels refer to a variety of carbon and alloy steels which are well-suited and widely used to make tools primarily used for perforating and fabrication. Tool steels are made to a number of grades for different forming and fabrication applications. The most common designations used to identify various grades of tool steel are the AISI / SAE (American Iron and Steel Institute / Society of Automotive Engineers) designations. These designations of tool steels are (i) ‘W’ series for work hardening tool steels, (ii) ‘S’ series for shock resisting tool steels, (iii) ‘O’ series for oil hardening type cold work tool steels, (iv) ‘A’ series for medium alloy steel and air hardening type cold work tool steels, (v) ‘D’ series for high carbon, high chromium type cold work tool steel, (vi) ‘L’ series for low alloy tool steels, (vii) ‘F’ series for carbon-tungsten tool steels, (viii) ‘P’ series for low carbon and other mould tool steels, (ix) ‘H’ series for chromium base, tungsten base, and molybdenum base hot work tool steels, (x) ‘T’ series for tungsten base high-speed tool steels, and (xi) ‘M’ series for molybdenum base high-speed tool steels. Fig 2 gives types of tool steels.
Fig 2 Types of tool steels
The tool steels are designed to have compositions which are compatible for their special purpose use. Certain tool steels are made for producing die blocks while some are made for making moulds. Some tool steels are made for the purpose of hot working while some others are made for high-speed cutting applications. Tool steels are classified into the following types.
High-speed tool steels – High-speed tool steels are used for cutting tools where strength and hardness are to be retained at temperatures up to 760 deg C. These steels include all molybdenum-based and tungsten-based steels. High-speed tools steels can be hardened to 62 HRC (Rockwell hardness ‘C’ scale) to 67 HRC and can maintain this hardness in high service temperatures. This makes these steels very useful in high-speed machines. Typical applications are end milling machines, drills, lathe tools, planar tools, punches, reamers, routers, taps, saws, broaches, chasers, and hobs.
Cold work tool steels – These tool steels have high strength, hardenability, impact toughness, and wear resistance. These steels can be oil hardening type, air hardening medium alloy type, or high carbon high chromium type. These steels are used in those working conditions where operating temperatures are typically less than 200 deg C. These tool steels are used on larger parts or those parts which need minimal distortion during hardening. The use of oil quenching and air hardening helps reduce distortion as opposed to higher stress caused by quicker water quenching. More alloying elements are used in these steels, as compared to water-hardening grades. The alloys in these steels increase the steel’s hardenability, and hence need a less severe quenching process. These steels are also less likely to crack. Typical applications include cold working operations such as stamping dies, draw dies, burnishing tools, coining tools, and shear blades.
Hot work tool steels – These tool steels have several of the properties of cold working steels, with the added advantage that these properties are retained at high operating temperatures where the tool literally becomes red-hot. Hot work tool steels include chromium, tungsten, and molybdenum alloying elements. They are typically used for forging, die casting, heading, piercing, trim, extrusion, and hot shear and punching blades.
Shock resistant tool steels – These tool steels are designed to resist shock at both low and high temperatures. Low carbon content is needed in these steels for the necessary toughness (around 0.5 % carbon). Carbide forming alloying elements provides the necessary abrasion resistance, hardenability, and hot-working characteristics in these steels. This family of steels displays very high impact toughness and relatively low abrasion resistance. These steels can attain relatively high hardness (58 HRC / 60 HRC). These steels are among the toughest of the tool steels, and are typically used for jack-hammer bits, screw driver, shear blades, chisels, knockout pins, punches, and riveting tools.
Water hardening tool steels – These tool steels do not retain hardness at high temperatures but they have high resistance to surface wear. They begin to soften above 150 deg C to a noticeable degree. These steels are essentially high carbon steels and can attain high hardness (66 HRC). These steels are brittle compared to other tool steels. These types of tool steels are the most commonly used tool steels because of their low cost compared to other tool steels. They work well for small parts and for those applications where high temperatures are not encountered. Typical applications include blanking dies, set screws, files, drills, taps, counter-sinks, hammers, sledges, reamers, shear blades, razor blades, lathe tools, jewellery dies, and cold striking dies etc.
Mould steels – Mould steels include all low carbon and one type medium carbon tool steels. They are typically used for compression and injection moulds for plastics, and die casting dies.Special purpose tool steels – These steels include low alloy tool steels. They are normally quenched, which makes them relatively tough and easily machinable. They are typically used for arbors, punches, taps, wrenches, drills, and brake forming dies.
Properties of tool steels
Besides chemical composition, the properties affect the performance of tool steels. The most important properties of tool steels are bending strength, hardness, and wear resistance at working temperatures as well as resistance to rapid, cyclic changes in temperature (thermal fatigue). These properties are achieved by the proper selection of chemical composition and heat treatment conditions. By changing the microstructure through heat treatment, it is possible to achieve a needed combination of high hardness, mechanical strength, and good ductility. As the phase transformations occurring during tempering are controlled by diffusion, for a steel with a given chemical composition, the major role in getting the needed properties is played by the tempering parameters, such as temperature and time. However, all the desired characteristics as well as the requirements of the job are to be considered when making the selection of tool steel type. Some of the properties of tool steels are described below.
Hardness – It is a measure of steel’s resistance to deformation. Hardness in tool steels is normally measured using the Rockwell C test. Hardness of the hardened cold work tool steels range normally from 58 HRC to 64 HRC depending on the grade. However, majority of the values are in the range of 60 HRC to 62 HRC, although some tool steels are occasionally used up to around 66 HRC hardness. Tools which plastically deform in service possess insufficient hardness. Permanent bending of cutting edges, mushrooming of punch faces, or indenting of die surfaces (peening) all indicate insufficient hardness.
Toughness – Toughness of tool steel is defined as the relative resistance to breakage, chipping, or cracking under impact or stress. Tool steel toughness tends to decrease as the alloy content increases. Toughness is also affected by the production process of the steel. Toughness can be thought of as the opposite of brittleness. Majority of the tool steels are notch sensitive, meaning that any small notch present in the sample permits it to fracture at a much lower energy. In addition to inherent material properties, the impact resistance of tool components is considerably impaired by notches, undercuts, geometry changes, and other common features of tools and dies.The PM (particle metallurgy) production process can improve the toughness of the steel grade because of the uniformity of its microstructure. Hardness also affects toughness. A grade of tool steel has higher toughness at a lower hardness. However, the lower hardness can have a negative effect on other characteristics necessary to achieve acceptable tool life.
In service, wear failures are normally preferable to toughness failures (breakage). Breakage failures can be unpredictable, catastrophic, interruptive to production, and perhaps even a safety concern. Toughness data is useful to predict which steels can be more or less prone to chipping or breakage than other steels, but toughness data cannot predict the performance life of tools.
Wear resistance – Wear resistance is the ability of the tool steel to resist being abraded or eroded by contact with the work material, other tools, or outside influences such as scale, and grit, etc. Wear resistance is provided by both the hardness level and the chemistry of the tool. Wear tests are quite specific to the circumstances creating the wear and the application of the tool. Majority of the wear tests involve creating a moving contact between the surface of a sample and some destructive medium.
There are two basic types of wear damage in tool steels namely (i) abrasive, and (ii) adhesive. Abrasive wear involves erosion or breaking down the cutting edge. Wear involving erosion or rounding of edges, as from scale or oxide, is the abrasive wear. Abrasive wear does not require high pressures. Abrasive wear testing can involve sand, sandpaper, or different slurries or powders.
Adhesive wear is experienced when the work piece material adheres to the punch point, reducing the coefficient of friction, which increases the perforating pressure. Wear from intimate contact between two relatively smooth surfaces, such as steel on steel, carbide on steel etc., causes adhesive wear. Adhesive wear can involve actual tearing of the material at points of high pressure contact because of the friction.
Increased alloy content typically means increased wear resistance since more carbides are present in the steel. Carbides are hard particles which provide wear resistance. The size and dispersion of the majority of carbides are formed when alloys, such as vanadium, tungsten, molybdenum, and chromium combine with carbon as the liquid steel begins to solidify. Higher quantities of carbide improve wear resistance, but reduce toughness.
It is frequently expected that a harder tool resist wear better than a softer tool. However, different grades, used at the same hardness, provide varying wear resistance. In fact, in some situations, lower hardness and high alloy steel grades can outwear higher hardness and lower alloy steel grades. Hence, factors other than hardness contribute to wear properties of the tool steels.
Compressive strength – Compressive strength is a little known and frequently being overlooked characteristic of tool steels. It is a measurement of the maximum load an item can withstand before deforming or before a catastrophic failure occurs. Two factors affect compressive strength. They are content of the alloying element and tool steel hardness. Alloying elements such as molybdenum and tungsten contribute to the compressive strength. Higher hardness also improves compressive strength.
Heat treatment of tool steels
Heat treatment involves a number of processes which are used to alter the physical and mechanical properties of the tool steel. Heat treatment, which includes both the heating and cooling of the material, is an efficient method for manipulating the properties of the steel to achieve the desired results. Each grade of tool steel has specific heat-treating guidelines which are to be followed to achieve optimum results for a given application.
Unlike cutting tools, the nature of the stamping operation places a high demand on toughness. Hence, a specific steel grade used as a tool steel for stamping is to be heat treated differently than one used in a cutting tool. Tool steel heat treatment processes include material segregation, fixturing, pre-heating, soaking, quenching, and tempering. Certain steels need different timing, preheating, and soaking temperatures, and number of tempers.
Material segregation and fixturing – Segregation by size is extremely important since different individual sizes need different rates in preheating, soaking, and quenching. Fixturing ensures even support and uniform exposure during heating and cooling.
Pre-heating and soaking – During the pre-heating, both cold work and high-speed tool steels are evenly heated to prevent distortion and cracking. Soaking (austenitizing) is done for a specific time to force some of the alloying elements into the matrix of the steel.
Hardening and tempering – When a tool is hardened, several factors influence the result. In soft annealed condition, majority of the carbide-forming alloying elements are bound up with carbon in carbides. When the steel is heated up to hardening temperature, the matrix is transformed from ferrite to austenite. This means that the iron atoms change their position in the atomic lattice and generate a new lattice with different crystallinity. Austenite has a higher solubility limit for carbon and alloying elements, and the carbides dissolve into the matrix to some extent. In this way the matrix acquires an alloying content of carbide-forming elements which gives the hardening effect, without becoming coarse grained.
If the steel is quenched sufficiently rapidly in the hardening process, the carbon atoms do not have the time to reposition themselves to allow the reforming of ferrite from austenite, as in for example annealing. Instead, they are fixed in positions where they really do not have enough space, and the result is high micro-stresses which contribute to increased hardness. This hard structure is called martensite. Hence, martensite can be seen as a forced solution of carbon in ferrite.
When the steel is hardened, the matrix is not completely converted into martensite. There is always some austenite which remains in the structure and it is called retained austenite. The quantity increases with increasing alloying content, higher hardening temperature, longer soaking times, and slower quenching. After quenching, the steel has a microstructure consisting of martensite, retained austenite, and carbides. This structure contains inherent stresses which can easily cause cracking. But this can be prevented by reheating the steel to a certain temperature, reducing the stresses, and transforming the retained austenite to an extent which depends upon the reheating temperature. This reheating after hardening is called tempering. Hardening of tool steel is always to be followed immediately by tempering.
It is to be noted that tempering at low temperatures only affects the martensite, while tempering at high temperature also affects the retained austenite. After one tempering at a high temperature the microstructure consists of tempered martensite, newly formed martensite, some retained austenite, and carbides. Precipitated secondary (newly formed) carbides and newly formed martensite can increase hardness during high temperature tempering. As an example, Typical of this is the so-called secondary hardening of high-speed steels and high alloyed tool steels.
Normally a certain hardness level is needed for each individual application of the steel, and hence heat treatment parameters are chosen to some extent in order to achieve the desired hardness. It is very important to have in mind that hardness is the result of several different factors, such as the quantity of carbon in the martensitic matrix, the micro-stresses contained in the material, the quantity of retained austenite, and the precipitated carbides during tempering.
It is possible to make use of different combinations of these factors which result in the same hardness level. Each of these combinations corresponds to a different heat treatment cycle, but certain hardness does not guarantee any specific set of properties of the material. Fig 1 shows the hardened structure of tool steel. The material properties are determined by its microstructure and this depends on the heat treatment cycle, and not on the achieved hardness. Fig 3 shows the influence of different factors on the secondary hardening.
Fig 3 Influence of different factors on the secondary hardening
Proper heat treatment delivers not only desired hardness but also optimized properties of the material for the chosen application. Tool steels are always to be at least double tempered. The second tempering takes care of the newly formed martensite during cooling after the first tempering. Three number of temperings are desired in several cases such as (i) high speed steel with high carbon content, (ii) complex hot work tools, especially in the case of die casting dies, (iii) big moulds for plastic applications, and (iv) when high dimension stability is a requirement (such as in the case of gauges or tools for integrated circuits).
Stress relieving – Distortion because of the hardening is to be taken into account when a tool is rough machined. Rough machining causes thermal and mechanical stresses which remains embedded in the material. This may not be significant on a symmetrical part of simple design, but can be of high importance in an asymmetrical and complex machining, as in the case of one half of a die casting die. Here, stress-relieving heat treatment is always desired. This treatment is done after rough machining and before hardening and involves heating in the range of 550 deg C to 700 deg C. The material is to be heated until it has achieved a uniform temperature all the way through, where it remains 2 hours to 3 hours and then cooled slowly, for example in a furnace. The reason for a necessary slow cooling is to avoid new stresses of thermal origin in the stress-free material. Fig 4 shows residual stresses of thermal origin.
Fig 4 Residual stresses of thermal origin
The idea behind stress relieving is that the yield strength of the material at high temperatures is so low that the material cannot resist the stresses contained in it. The yield strength is exceeded and these stresses are released, resulting in a higher or lesser degree of plastic deformation. The excuse that stress relieving takes too much time is hardly valid when the potential consequences are considered.
Rectifying a part during semi-finish machining is with few exceptions cheaper than making dimensional adjustments during finish machining of a hardened tool. The proper work sequence before hardening operation is rough machining, stress relieving. and semi-finish machining.
Heating to hardening temperature – As has already been explained, stresses contained in the material produces distortion during heat treatment. For this reason, thermal stresses during heating are to be avoided. The fundamental rule for heating to hardening temperature is hence, that it is to take place slowly, increasing just a few degrees per minute. In every heat treatment, the heating process is named ramping. The ramping for hardening is to be made in different steps, stopping the process at intermediate temperatures, normally named preheating steps. The reason for this is to equalize the temperatures between the surface and the centre of the part. Typically, chosen preheating temperatures are 600 deg C to 650 deg C and 800 deg C to 850 deg C. In the case of big tools with complex geometry, a third preheating step close to the fully austenitic region is desired.
Holding time at hardening temperature – It is not possible to briefly state exact requirements to cover all heating situations. Factors such as furnace type, hardening temperature, the weight of the charge in relation to the size of the furnace, the geometry of the different parts in the charge, etc., are to be taken into consideration in each case. The use of thermocouples permits an overview of the temperature in the different areas of the various tools in the charge. The ramping step finishes when the core of the parts in the furnace reaches the chosen temperature. Then the temperature is maintained constant for a certain time. This is called holding time. For the holding time, there is a general rule applicable in the majority of the cases. Once the tool has been heated through, it is to be held for at least for two hours at full temperature each time.
In case of high-speed steel, the holding time is shorter when the hardening temperature is higher than 1,100 deg C. If the holding time is prolonged, micro-structural issues like grain growth can arise. The use of thermocouples gives an overview of the temperature in different areas during heat treatment.
Quenching – Quenching is the sudden cooling of the parts from the austenitizing temperature through the martensite transfer range. The steel is transformed from austenite to martensite, resulting in hardened parts.
The choice between a fast and a slow quenching rate is normally a compromise. For getting the best microstructure and tool performance, the quenching rate is to be rapid. To minimize distortion, a slow quenching rate is desired. Slow quenching results in less temperature difference between the surface and the core of a part, and sections of different thickness have a more uniform cooling rate. This is of high importance when quenching through the martensite range, below the Ms temperature. Fig 5 gives different diagrams for the quenching of tool steel.
Fig 5 Quenching of tool steel
Martensite formation leads to an increase in volume and stresses in the material. This is also the reason why quenching is to be interrupted before room temperature has been reached, normally at 50 deg C to 70 deg C. However, if the quenching rate is very slow, especially in case of heavier cross-sections, undesirable transformations in the micro-structure can take place, risking a poor tool performance.
Quenching media used for alloy steel nowadays are hardening oil, polymer solutions, air, and inert gas. It is still possible to find some heat treatment shops which use salt baths, but this technique is now disappearing because of the environmental aspects. Oil and polymer solutions are normally utilized for low alloy steel and for tool steel with low carbon contents. Air hardening is used for steel with high hardenability, which in majority of the cases is because of the combined presence of manganese, chromium, and molybdenum.
Risk of distortion and hardening cracks can be reduced by means of step quenching or martempering. In this process the material is quenched in two steps. In the first step, it is cooled from the hardening temperature until the temperature at the surface is just above the Ms temperature. Then it is held there until the temperature has been equalized between the surface and the core. After this, the cooling process continues. This method permits the core and the surface to transform into martensite at more or less the same time and diminishes thermal stresses. Step quenching is also a possibility when quenching in vacuum furnaces. The maximum cooling rate which can be achieved in a part depends on the heat conductivity of the steel, the cooling capacity of the quenching media, and the cross-section of the part.
Cooling rates for different media – A poor quenching rate leads to carbide precipitation at the grain boundaries in the core of the part, and this is very detrimental to the mechanical properties of the steel. Also, the achieved hardness at the surface of larger parts can be lower for tools with bigger cross-sections than that for smaller parts, as the high quantity of heat which has to be transported from the core through the surface produces a self-tempering effect. Some heat treatment shops satill use salt baths, but this technique is disappearing because of the environmental aspects. Oil and polymer solutions are normally utilized for low alloyed steel and for tool steel with low carbon contents.
Some practical issues – At high temperature, steel is very likely to suffer oxidation and variations in the carbon content (carburization or decarburization). Protected atmospheres and vacuum technology help in these regards. Decarburization results in low surface hardness and a risk of cracking. Carburization, on the other hand, can result in two different issues, The first and easiest to identify is the formation of a harder surface layer, which can have negative effects and the second possible issue is the retained austenite at the surface. Retained austenite can be in several cases confused with ferrite when observing it through the optical microscope. These two phases also have similar hardness, and hence, what at first sight can be identified as a decarburization can in some cases be the completely opposite issue. For these reasons, it is very important that the environment in which the heat treatment takes place does not affect the carbon content of the part. Wrapping in a hermetically closed stainless-steel foil also provides some protection when heating in a muffle furnace. The steel foil is required to be removed before quenching.
Vacuum technology – A vacuum furnace is used to heat the metals to very high temperatures and allow high consistency and low contamination in the process. Vacuum technology is the most used technology nowadays for hardening of high alloy steel. Vacuum heat treatment is a clean process, so the parts do not need to be cleaned afterwards. It also offers a reliable process control with high automation, low maintenance, and environmental friendliness. All these factors make vacuum technology especially attractive for high-quality parts.
The vacuum furnace process starts by removing the atmosphere and creating a vacuum and electrically heating the parts in the hot zone. The different steps in the functioning of a vacuum furnace can schematically be listed as ((i) when the furnace is closed after charging operation, air is pumped out from the heating chamber in order to avoid oxidation, (ii) an inert gas (normally nitrogen) is injected into the heating chamber until a pressure of around 0.1 MPa to 0.15 MPa is reached, (iii) the heating system is started.
The inert gas is used as a means of conducting heat away from the parts. A large turbine blower forces room temperature nitrogen across the parts, cooling (quenching) them through the martensite transfer range. Hot nitrogen leaves the hot zone through gates at the front and rear of the chamber. The nitrogen circulates through a heat exchanger where it is cooled. The cooled nitrogen is recirculated over the parts until they reach room temperature.
The presence of the inert gas makes possible the heat transfer process through convection mechanisms. This is the most efficient way to heat up the furnace to a temperature of around 850 deg C. When the furnace reaches a temperature of around 850 deg C, the effect of radiation heating mechanisms overshadows that of the convection ones in the heat transfer process. Hence, the nitrogen pressure is lowered, in order to optimize the effects of radiation and convection heating mechanisms are negligible under these new physical conditions. The new value of the nitrogen pressure is around 0.7 kilopascals. The reason for having this remaining pressure is to avoid sublimation of the alloying elements, i.e., to avoid the loss of alloying elements in the vacuum. This low-pressure condition is maintained invariant during the last part of the heating process, as well as during the holding time at the chosen hardening temperature.
The cooling down is carried out by a huge injection of inert gas (normally nitrogen) into the heating chamber in alternating directions and reaching the over-pressure which has been earlier chosen when programming of the furnace has been done. The maximum over-pressure is a nominal characteristic of each furnace and it gives an idea of its cooling capacity.
Tempering – Untempered martensitic steel is very hard, but too brittle for the majority of the applications. Tempering is heating the steel to a lower-than-critical temperature to improve toughness. Tool steels are typically tempered at temperatures between 200 deg C to 540 deg C.
The tool steel material is required to be tempered immediately after quenching. Quenching is to be stopped at a temperature range of 50 deg C to 70 deg C and tempering is to be done at once. If this is not possible, the material is to be kept warm, e.g., in a special ‘hot cabinet’ awaiting tempering. The stresses contained in the as-quenched material can result in the breakage of the crystalline structure and the formation of cracks if the tempering is not done immediately after the quenching process. This breakage of the crystalline structure can take place in a violent way. Hence, the importance of tempering as soon possible is not only to safeguard the part from cracks, but it is also a matter of personal safety.
It is necessary that the correct tempering temperature is chosen. The first priority when choosing the tempering temperature is the mechanical properties, since some small dimensional adjustments can be made in a last fine machining step. The mechanical and physical properties achieved after tempering depend heavily on the chosen tempering temperature. High-temperature tempering results in a lower content of retained austenite than low-temperature tempering. The material hence has higher compressive strength and improved dimensional stability (in service and at surface coating). When tempering at high temperature, other differences in properties are also noticeable, like higher heat conductivity.
Precipitation of secondary carbides occurs when tempering high alloyed steel at a high temperature. This is detrimental to its corrosion resistance but gives to it somewhat higher wear resistance. If the tool is to be electrical discharge machined (EDM) or coated, high-temperature tempering is necessary.
Two tempers are normally desired for tool steel, except in the cases of large cross-sections, parts with complex geometries, or very high demands on dimensional stability. In these cases, a third tempering is normally needed. The basic rule of quenching is to interrupt in the range of 50 deg C to 70 deg C. Hence, a certain quantity of austenite remains untransformed when the material is ready to be tempered. When the material cools after tempering, majority of the austenite is transformed to newly formed martensite (untempered). A second tempering gives the material optimum toughness at the chosen hardness level.
Holding times in connection with tempering – Here there is also a general rule, applicable in majority of the cases. This rule says that once the tool has been heated through, it is to be held for at least two hours at full temperature each time. Fig 6 shows the evolution of the phase content along different steps of heat treatment.
Fig 6 Evolution of the phase content along different steps of heat treatment
Dimensional and shape stability – Distortion takes place during hardening and tempering of tool steel. When a piece of tool steel is hardened and tempered, some warpage or distortion normally occurs. This is well known and it is normal practice to leave some machining allowance on the tool prior to hardening, making it possible to adjust the tool to the correct dimensions after hardening and tempering by grinding. The cause of distortion is the stresses in the material. These stresses can be divided into (i) machining stresses, (ii) thermal stresses, and (iii) transformation stresses.
Machining stresses are generated during machining operations such as turning, milling, and , or any type of cold working. If stresses have built up in a part, they are released during heating. Heating reduces strength, while releasing stresses through local distortion. This can lead to overall distortion. In order to reduce distortion while heating during the hardening process, a stress relieving operation is carried out prior to the hardening operation. It is desired to stress relieve the material after rough machining. Any distortion can then be adjusted during semi-finish machining prior to hardening operation.
Thermal stresses arise every time there is a temperature gradient in the material, i.e., when the temperature is not even all over the part. Thermal stresses grow with increasing heating rate. Uneven heating can result in local variations in volume because of uneven dilatation rates and this also contributes to the arising of stresses and distortion. In order to tackle this issue, it is a normal practice to heat up the material in steps, in order to equalize the temperature between the surface and the centre.
An attempt is always to be made to heat slowly enough so that the temperature remains virtually equal throughout the workpiece. What is applicable during heating, also applies also to cooling. Very powerful stresses arise during quenching. As a general rule, the slower quenching can be done, the less distortion occurs because of the thermal stresses. But a faster quenching result in better mechanical properties. It is important that the quenching medium is applied as uniformly as possible. This is especially valid when forced air or protective gas atmosphere (as in vacuum furnaces) is used. Otherwise, temperature differences in the tool can lead to considerable distortion.
Transformation stresses arise when the microstructure of the steel is transformed. This is because the three phases in question, namely ferrite, austenite and martensite, have different densities, i.e., volumes. Out of all the microstructural changes which take place during heat treatment, the biggest contribution to transformation stresses is caused by the transformation of austenite into martensite. This causes a volume increase. Excessively rapid and uneven quenching can also cause local martensite formation and thereby volume increases locally in a piece and gives rise to stresses in this section. These stresses can lead to distortion and, in some cases, hardening cracks. Fig 7 shows effect of temperature on tool steel thermal stability.
Fig 7 Effect of temperature on tool steel thermal stability
Distortion can be minimized by (i) keeping the design simple and symmetrical, (ii) eliminating machining stresses by stress relieving after rough machining, (iii) heating up slowly to hardening temperature, (iv) using a suitable grade of steel (v) quenching the piece as slowly as possible, but fast enough to achieve a correct microstructure in the steel, (vi) using martempering or step quenching, (vii) tempering at a suitable temperature.
Sub-zero treatment – It is also known as cryogenics. Cryogenics is a process which aids in transformation of austenite to martensite, ensuring higher hardness and reducing internal stresses. This process takes place at temperatures between -80 deg C to -196 deg C and varies in duration, depending on the size of the parts.
Retained austenite in a tool can transform into martensite during service. This leads to local distortion and embrittlement of the tool because of the presence of untempered martensite. Hence, the requirement of maximum dimensional stability in service has an implied demand for very low or no retained austenite content. This can be achieved by using sub-zero treatment after quenching or by high temperature tempering.
The sub-zero treatment leads to a reduction of retained austenite content by exposing the tool or part to very low temperatures. This, in turn, results in a hardness increase of up to 1 HRC to 2 HRC, in comparison to non-sub-zero treated tools, if low temperature tempering is used. For high temperature tempered tools there is little or no hardness increase.
Tools which are high temperature tempered, even without a sub-zero treatment, normally have a low retained austenite content and in majority of the cases, have a sufficient dimensional stability. However, for high demands on dimensional stability in service, it is also desired to use a sub-zero treatment in combination with high temperature tempering. For the highest requirements on dimensional stability, sub-zero treatment in liquid nitrogen is desired after quenching and after each tempering. It is also desired to finish always with a tempering as last operation, in order to avoid the existence of untempered martensite in the part.
Surface treatment
Nitriding – Nitriding is performed by exposing the parts to some media rich in nitrogen under certain physical conditions which results in the diffusion of nitrogen atoms into the steel and the formation of nitrides. The part surface is then be harder and has a higher wear resistance in its outer layer.
In the case of corrosion resistant steel with high-chromium content, it is very important to take into consideration the fact that nitriding has a detrimental effect on the corrosion resistance of the material. In other cases, nitriding can have a positive effect on the corrosion resistance. Appropriate steel to be nitrided are normally medium-carbon steel with nitride-forming elements such as chromium, aluminum, molybdenum, and vanadium.
The core is to act as a stable substrate regarding mechanical properties and microstructure. This means that for hardened material, it is necessary to temper above the nitriding temperature in order to avoid softening of the core during the nitriding process. However, a nitrided surface cannot be machined with cutting tools and can only be ground with difficulty.
A nitrided surface causes problems in weld repairing as well. There are several technologies available in the field of nitriding. The main ones are gas nitriding, high pressure nitriding (carried out in vacuum furnaces), and plasma nitriding. Two common issues of conventional nitriding technologies are possible over-tempering of the substrate material and thickening of the nitrided layer in the sharp corners.
Pulsed plasma nitriding technology diminishes the possibility of over tempering by applying the plasma intermittently on the part. This provides a better control over the local temperatures during the process. Active screen plasma nitriding is also a development of plasma nitriding technology. This technology promises a uniform thickness of the nitride layer independently of its geometry
Nitro-carburizing – Nitro-carburizing is a process in which the parts are to be enriched in nitrogen and also in carbon, the enrichment is carried out by exposure to an environment rich in these two elements. A mixture of ammonia gas and carbon monoxide or carbon dioxide is an example of a suitable environment for this purpose. The temperature range for this process is 550 deg C to 580 deg C and the time of exposure is between 30 minutes and 5 hours. After the exposure, the part is to be cooled down rapidly.
Case hardening – Case hardening is a process in which a finished part is exposed to a carburizing environment and high temperature simultaneously. The temperature range is 850 deg C to 950 deg C. This exposure generates a layer with higher carbon content, normally 0.1 mm to 1.5 mm thick. After the layer has been formed, the part is to be quenched in order for the layer to transform into martensite with higher carbon content, and hence, it has a higher hardness. Tempering of the part is to follow.
Thermal diffusion – Thermal diffusion is a process in which vanadium diffuses into the material and reacts with existing carbon, to form a vanadium carbide layer. The steel needs to have a minimum of 0.3 % carbon. This surface treatment provides a very high level of abrasive wear resistance.
Surface coating – Surface coating of tool steel has become a common practice. The general aim for these kinds of processes is to generate an outer layer with a very high hardness and low friction which results in good wear resistance, minimizing the risk for adhesion and sticking. To be able to use these properties in an optimal way a tool steel of high quality is to be chosen. The normally used coating methods are (i) physical vapour deposition coating (PVD coating), (ii) chemical vapour deposition coating (CVD coating). Chemical vapour deposition coating can also be carried out with a plasma assisted technology (PACVD) for this purpose. The temperature range for this process is 550 deg C to 580 deg C and the time of exposure is between 30 minutes and 5 hours. After the exposure the part is to be cooled down rapidly.
Plating – Chromium and nickel metallic platings are normally used for a variety of tooling applications, like plastic injection moulds. Platings can be deposited over majority of the steel grades. Plating prevents seizing and galling, reduces friction, increases surface hardness, and prevents or reduces corrosion of the substrate’s surface.
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