Gray Iron

Gray Iron

Gray iron (also called grey iron) is a type of cast iron that has a graphitic microstructure. It is named after the grey color of the fracture it forms, which is due to the presence of graphite. It is the most common cast iron and the most widely used cast material.

Gray iron is one of the oldest cast ferrous products. In spite of competition from newer materials and their energetic promotion, gray iron is still used for those applications where its properties have proved it to be the most suitable material available.

MacKenziein his 1944 Howe memorial lecture referred to gray cast iron as ‘steel plus graphite’. Although this simple definition still applies, the properties of gray iron are affected by the amount of graphite present as well as the shape, size, and distribution of the graphite flakes.

Composition and effect of composition on properties

 Gray iron is commercially produced over a wide range of compositions. The range of compositions which one may find in gray iron castings is given below.

  • Carbon (C) – 2.75 % to 4.00 %
  • Silicon (Si) – 0.75 % to 3.00 %
  • Manganese (Mn) – 0.25 % to 1.50 %
  • Sulfur (S) – 0.02 % to 0.20 %
  • Phosphorus (P) – 0.02 % to 0.75 %

One or more of the alloying elements namely molybdenum, copper, nickel, vanadium, titanium, tin, antimony, and chromium may be present in varying amounts. Nitrogen is generally present in the range of 20 to 92 ppm. Si is important for the gray iron since it is a graphite stabilizing element in cast iron, which means it helps the iron to produce graphite instead of iron carbides. Another factor affecting graphitization is the solidification rate. The slower is the rate, the greater is the tendency for graphite to form.

The graphite takes on the shape of a three dimensional flake. In two dimensions, as seen as a polished surface under a microscope, the graphite flakes appear as fine lines. The graphites have no appreciable strength, so they can be treated as voids. The tips of the flakes act as preexisting notches making the gray iron brittle.  Microscopically, all gray irons contain flake graphite dispersed in a silicon iron matrix.

The basic strength and hardness of the iron is provided by the metallic matrix in which the graphite occurs. The properties of the metallic matrix can range from those of soft, low carbon steel to those of hardened, high carbon steel. The matrix can be entirely ferrite for maximum machinability but such iron  has reduced wear resistance and strength. An entirely pearlitic matrix is characteristic of high strength gray irons, and many castings are produced with a matrix microstructure of both ferrite and pearlite to obtain intermediate hardness and strength. Alloy additions and/or heat treatment can be used to produce gray iron with very fine pearlite or with an acicular matrix structure. Typical microstructure of gray iron is shown in Fig 1.

Micro structure of gray iron

Fig 1 Typical microstructure of gray iron

Graphite has little strength or hardness. It decreases these properties of the metallic matrix; however, the presence of the graphite provides several valuable characteristics to cast iron. These include the following.

  • The ability to produce sound castings economically in complex shapes such as water cooled engine blocks.
  • Good machinability even at wear resisting hardness levels and without burring.
  • Dimensional stability under differential heating such as in brake drums and disks.
  • High vibration damping as in power transmission cases.
  • Borderline lubrication retention as in internal combustion engine cylinders.

The properties of gray iron are primarily dependent on its composition. The lower strength grades of gray iron can be consistently produced by simply selecting the proper melting stock. Iron castings in the higher strength grades require close control of their processing as well as their composition. The majority of the carbon in gray iron is present as graphite which has little strength or hardness.

Increasing amounts of graphite result into with increasing amount of total carbon content in the gray iron. This decreases the strength and hardness of the iron, but increases other desirable characteristics which are listed above. Appreciable silicon content is also necessary in gray iron because this element causes the precipitation of the graphite in the iron. The silicon also contributes to the distinctive properties of gray iron. It maintains a moderate hardness level even in the fully annealed condition and thus assures excellent machinability. Silicon also imparts corrosion and elevated temperature oxidization resistance to gray iron. The effects of carbon, silicon and phosphorous on the tensile properties of gray iron are combined into a number called the carbon equivalent (CE) which is determined by the following equations.

CE = % C + 0.33 (% Si) + 0.33 (% P) – 0.027 (% Mn) + 0.4 (% S)

CE = % C + 0.33 (% Si)

CE = % C + 0.33 (% Si + % P)

The CE is used to determine if the alloy is hypoeutectic, eutectic or hypereutectic. The CE equations indicate the relative amount of the eutectic that forms during solidification of the iron with 4.3 % C as 100 % eutectic composition. These equations also, in effect, indicate that on the basis of percent by weight, the silicon and phosphorous contents of gray iron also influence its tensile properties in the same manner as its total carbon content but only by one third as much. Irons with a carbon equivalent of more than 4.3 are hypereutectic and usually contain coarse graphite. They are of lower strength, but are excellent in thermal shock applications and for vibration damping. Gray irons with less than 4.3 carbon equivalent are hypoeutectic and of higher strength because the amount and size of the graphite flakes decrease with the CE value. Also, there is less shrinkage as the CE increases.

The effect of higher carbon equivalent is to reduce strength because of the formation of larger amounts of coarser graphite and, commonly, more ferrite. Manganese, sulfur and phosphorus are present in plain gray irons and influence the tensile strength to some extent. Sulfur is a very significant element because it exerts marked effects on the solidification behavior of iron. For this reason, the sulfur content in iron is usually controlled within limits and with a selected ratio to the manganese content since sulfur combines chemically with manganese to form manganese sulfide. The minimum manganese content in iron is generally 1.7 times the sulfur content plus 0.12 % manganese. This assures sufficient manganese so that all of the sulfur is combined with manganese rather than with iron. Manganese in excess of this amount is a mild carbide stabilizer, refining the pearlite and increasing the hardness and tensile strength. An excess of manganese or phosphorus can cause dispersed internal porosity in heavier sections such as bosses. For this reason, phosphorous is kept as low as practical except for special purpose irons. Increasing phosphorus provides a somewhat higher tensile strength, but contents over 0.20 % reduce machinability particularly in drilling operations.

The chemical composition of gray iron is not normally specified since it does not assure obtaining specific mechanical properties. However, for special applications some aspect of chemical composition may be specified to assure the suitability of the iron for a specific need. For example, an alloy content range may be specified to assure an adequate response to heat treatment or to provide strength or oxidization resistance in service at a red heat. Minimum carbon content may be specified to provide adequate thermal shock resistance.

Gray irons can be alloyed to increase their strength and hardness as cast or to increase their response to hardening by heat treatment. Gray iron can be annealed to a low hardness to increase machinability. Of course, this also decreases its strength.

The tensile strength of gray iron is influenced by both the normal elements present in plain irons such as carbon, silicon, phosphorus, sulfur and manganese, and the presence of alloying additions and trace elements.

Effect of thickness on properties of gray iron

A very important influence on the properties of gray iron is the effective thickness of the section in which it is cast. The thicker the metal in the casting and the more compact the casting, the slower the liquid metal will solidify and cool in the mould. As with all metals, slower solidification causes a larger grain size to form during solidification. In gray iron, slower solidification produces a larger graphite flake size. The cooling of a casting from red heat is in effect a heat treatment. A slower cooling of the casting will produce a lower hardness in the metallic matrix.

Alternately, iron that is cast into a section that is too thin will solidify very rapidly and can be file hard. A casting with separate sections that are appreciably different in thickness can have differences in graphite size and matrix hardness between the thick and thin sections even though the entire casting was poured with the same iron. These differences in structure produce differences in mechanical properties.

Properties of gray iron

Gray irons are commonly classified by their minimum tensile strength. A class 220 gray iron indicates that it has a nominal tensile strength of 220 newtons per square millimeters. A class designation may be used to indicate a grade of iron even when tensile strength is not an important consideration and may not be specified or tested. However, when the class designation is used in conjunction with a standard specification that requires a minimum tensile strength, then actual tensile tests are made to determine if the metal meets this requirement.

In a particular size or type of casting, gray irons can also be satisfactorily designated by their Brinell hardness. This designation has the advantage of using a nondestructive test that can be applied in routine inspection. These methods of designation are satisfactory for most applications because the common engineering properties of gray iron can be related to its strength and hardness.

Hardness is the most commonly determined property of metal because it is a simple test and many of the useful properties of metal are directly related to its hardness. Within a class or type of gray iron, hardness is a good indicator of its engineering properties, but this relation is not useful between types of gray iron because differences in graphite structure have more of an effect on tensile properties than on hardness. Compression strength does correlate very well with hardness for all types of iron because hardness is essentially a compression test. Hardness usually gives a good indication of tool life in machining, however, the presence of free carbides in the microstructure reduces the machinability much more than it increases the hardness.

Metals which are subjected to repeated or fluctuating loads, such as alternating between tension and compression, can break after a large number of loading cycles even though the maximum stress was well below the static strength of the metal. This type of fracture is called a fatigue failure, although the rate of load application or the lengths of time over which the cycles occur are not significant. The occurrence of a fatigue crack is directly influenced by the maximum unit stress and the cumulative number of times it is applied.

A fatigue crack starts in an area of high stress concentration after a large number of loading cycles. It is always a brittle type of fracture even when occurring in ductile metals. As the crack progresses, it increases the stress concentration, and the rate of propagation under the cyclic loading increases. When the cross section of the remaining metal becomes insufficient to support the maximum load, complete failure occurs as it would under an excessive steady stress.

The number of stress applications that will induce a fatigue failure is less at higher maximum stress values, and conversely a larger number of stress cycles can occur at a lower maximum stress level before a fatigue crack is initiated. When the number of cycles without failure exceeds 10 million, the endurance life is considered infinite for body centered cubic (bcc) ferrous metals. The maximum stress that allows this number of cycles is established as the endurance limit, or the fatigue strength or fatigue limit.

The relative ability of a material to absorb vibration is evaluated as its damping capacity. The quelling of vibration by converting the mechanical energy into heat can be very important in structures and in devices with moving parts. Components made of materials with a high damping capacity can reduce noise such as chatter, ringing and squealing, and also minimize the level of applied stresses. Vibration can be critical in machinery and can cause unsatisfactory operation or even failure.

An accumulation of vibrational energy without adequate dissipation can result in increasing amplitude of vibration. Excessive vibration can result in inaccuracy in precision machinery and in excessive wear on gear teeth and bearings. Mating surfaces normally considered in steady contact can be caused to fret by vibration.

The exceptionally high damping capacity of gray cast iron is one of the most valuable qualities of this material. For this reason it is ideally suited for machine bases and supports, engine cylinder blocks and brake components. The damping capacity of gray iron is considerably greater than that of steel or other kinds of iron. This behavior is attributed to the flake graphite structure of the gray iron, along with its unique stress-strain characteristics.

Damping capacity decreases with increasing strength since the larger amount of graphite present in the lower strength irons increases the energy absorbed. Larger cast section thicknesses increase damping capacity and inoculation usually decreases it. Heat treating can also have an appreciable effect on damping capacity.

The fracture toughness of a material is a measure of the work required to fracture it. This required work is related to the material’s resistance to crack initiation and growth. The work or energy dissipated in fracturing a material is associated with the elastic and plastic deformation of the material and/or crazing (micro cracking) that precedes final fracture. The fracture toughness will generally vary with temperature, state of stress and strain-rate, all of which influence the amount of deformation which precedes fracture.

Heat treatment                   

Although the majority of gray iron castings are used in the as-cast condition, gray iron is heat treated for a variety of reasons, such as to relieve residual stresses, improve machinability, increase the hardness of the surface either through induction or flame hardening, or harden the entire section through an oil quench and draw treatment. The graphite structure cannot be changed by heat treatment, although the graphite may increase in volume if a pearlitic iron is completely converted into ferrite, in which case, the graphite is usually deposited on the flakes originally present. The matrix however is quite responsive to heat treatment just as in the case of steel.

Stress relief heat treatments are generally made in the temperature range of  540 deg C to 590 deg C. Below 510 deg C the relief of stresses proceeds rather slowly, while at temperatures above 540 deg C, some loss of strength may be experienced in some softer gray irons. Stress relief heat treatments may be given to improve the dimensional stability of machined castings and are required for pressure containing parts operating at over 230 deg C and up to 340 deg C made from gray iron castings for pressure containing parts. Heating and cooling rates for such a heat treatment are generally limited to 200 deg C/hr per 25 mm of thickness. This is particularly important on heating as the residual stresses in the casting may be increased as a result of thermal expansion of various parts of the casting.

Annealing for improved machinability is carried out in two temperature ranges. If the principal purpose is merely to reduce hardness to some lower level and no carbides are present, temperatures of 680 deg C to 790 deg C are generally employed depending on how much reduction in hardness is desired. If the castings have cementite or carbides, it is necessary to heat to a 900 deg C to 940 deg C range to break down such carbides.

Gray iron can successfully be hardened by either flame or induction heating. The matrix of the iron should be pearlitic. It is also desirable to keep silicon at the lowest feasible level, generally below l.75 percent. As the silicon content of gray iron is raised, not only is the Ac3 temperature increased, but a two-phase field of ferrite and austenite is encountered. Satisfactory hardness will not be obtained when the iron is heated in this temperature range. The higher austenitizing temperatures required for the higher silicon irons also increase the possibility of cracking during the quenching cycle.

Machinability of gray iron

 The presence of graphite flakes makes the gray Iron easily machinable as they tend to crack easily across the graphite flakes. Of the widely used ferrous materials for construction purposes, gray iron for a given hardness level is one of the most readily machinable. Gray iron is free cutting in that the chips are small and easily removed from the cutting area. Furthermore, there is little difficulty with the chips marring the finished surface. The free cutting behavior is a result of the randomly distributed graphite flakes which interrupt the continuity of the matrix. Although gray iron is very successfully machined without coolants, they may be found necessary if high machining rates and close tolerances are desired. The coolant not only helps in chip removal but also controls the temperature of the casting, which is necessary for close tolerance work.

In spite of the good machinability of gray iron, various machining problems are encountered such as hard edges, reduced tool life, inability to obtain a satisfactorily smooth surface, and difficulty with maintaining the desired dimensional tolerances. Some of these problems are a result of selection of the wrong grade of iron, shortcomings in design of the casting, or incorrect machining procedures.

Application of gray iron

Gray iron also has very good damping capacity and hence it is mostly used as the base for machine tool mountings. For parts such as cylinder liners, through hardening by austenitizing and oil quenching followed by a draw to yield the desired hardness greatly improves the performance of the liner. There are many applications for which this type of heat treatment is more suitable than flame or induction hardening.

The excellent performance of gray iron in applications involving sliding surfaces is well known. Gray iron is also known for its resistance to galling and seizing. It is seldom possible to obtain perfect fits, and, ordinarily, high spots in mating metal surfaces may result in high unit pressures causing seizing.

Gray iron is a common engineering material because of its relatively low cost and good machinability, which results from the graphite lubricating the cut and breaking up the chips. It also has good galling and wear-resistance because the graphite flakes self lubricate. The graphite also gives gray iron an excellent damping capacity because it absorbs the energy.

Gray iron also experiences less solidification shrinkage than other cast irons that do not form a graphite microstructure. The silicon promotes good corrosion resistance and increase fluidity when casting.  Gray iron is generally considered easy to weld. Compared to the more modern iron alloys, gray iron has a low tensile strength and ductility; therefore, its impact and shock resistance is almost nonexistent.