Gray Iron

Gray Iron 

Gray iron (also written as grey iron) is a type of cast iron which has a graphitic micro-structure. It is named after the grey colour of the fracture it forms, which is because of the presence of graphite. It is the most common type of cast irons and the most widely used cast material. Cast irons are alloys of iron, carbon, and silicon in which more carbon is present than can be retained in solid solution in austenite at the eutectic temperature. In gray cast iron, the carbon which exceeds the solubility in austenite precipitates in the form of flake graphite.

American standard ASTM 48M, European standard EN 1561, and the International Organization for Standardization (ISO) standard ISO 185 are the international standards for gray iron. ASTM 48M classify gray cast iron into a number of grades based on the minimum tensile strength in MPa. EN 1561 specifies six grades of gray cast iron as per the tensile strength and Brinell hardness. ISO 185 specifies eight grades of gray cast iron as per the tensile strength and six grades of gray cast iron as per the Brinell hardness.

Casting is a very common and old method to produce metallic parts and it has gained its popularity from its simplicity. It is produced by letting the liquid metal poured into a cavity of desired shape. This leads to a geometry which is very close to the final form of the part that is to be manufactured. Cast parts are widely used in the industry where they are used for their functionality and low manufacturing costs. Among cast materials, gray cast iron and ductile iron makes up about three quarters of all cast parts. This makes the manufacturing and machining of gray cast iron an important economical factor for the industry.

Gray iron is a common engineering alloy because of its relatively low cost and good machinability, which results because of the graphite lubricating the cut and breaking up the chips. It also has good galling (a form of wear caused by adhesion between sliding surfaces), and wear resistance properties since the graphite flakes self-lubricate. The graphite also gives gray iron a very good damping capacity since it absorbs the energy and converts it into heat.

Gray iron has a number of desirable characteristics which are not possessed by any other metal and yet it is among the cheapest of ferrous materials available to the engineers. Gray iron castings are readily available in nearly all industrial areas and can be produced in foundries representing comparatively modest investments. It is used for the body of the machinery and the housing for the rotating machines where the stiffness of the component is more important than its tensile strength.

Gray iron is one of the oldest ferrous cast 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. MacKenzie in his 1944 Howe memorial lecture referred to gray iron as ‘steel plus graphite’. Although this simple definition still applies, the properties of gray iron are affected by the quantity of graphite present as well as the shape, size, and distribution of the graphite flakes.

The fluidity of liquid gray iron, and its expansion during solidification because of the formation of graphite, has made this metal ideal for the production of shrinkage-free intricate castings. The flake-like shape of graphite in gray iron exerts a dominant influence on its mechanical properties. The graphite flakes act as stress raisers, and initiate fracture in the matrix at higher stresses. Hence, gray iron shows no elastic behaviour, but has very good damping characteristics, and fails in tension without significant plastic deformation. Both major and minor elements have a direct influence on the morphology of flake graphite.

The strength of gray cast iron depends mainly on the matrix in which the graphite flakes are embedded. Slow cooling rates and high carbon (C) and silicon (Si) contents promote full graphitization, and the majority of the carbon dissolved in the iron (Fe) at high temperatures is deposited as graphite on the existing flakes during cooling. The micro-structure of gray iron then consists of graphite flakes in a ferrite matrix. This gray iron is referred to as ferritic gray cast iron. Ferritic gray cast iron is normally soft and weak. If graphitization of the carbon dissolved in the iron at high temperatures is prevented during cooling, iron carbide precipitates out and the matrix is pearlitic. This gray iron is referred to as pearlitic gray cast iron. Fig 1 shows micro-structures of two types of gray cast iron.

Fig 1 Micro-structures of gray cast iron.

Composition of gray iron and effect of composition on properties of gray iron – The composition of gray iron is to be selected in such a way as to satisfy three basic micro-structural requirements which are (i) the required graphite shape and distribution, (ii) the carbide-free (chill-free) structure, and (iii) the required matrix. Gray iron is commercially produced over a wide range of compositions. The range of compositions of gray iron has 2.5 % to 4 % carbon, 0.75 % to 3 % silicon, and additions of manganese (Mn), depending on the desired micro-structure (as low as 0.1 % Mn in ferritic gray iron and as high as 1.2 % in pearlitic gray iron). Sulphur (S) and phosphorus (P) are also present in small quantities as residual impurities. Sulphur is present normally in gray iron in the range of 0.02 % to 0.2 %, while phosphorus is present normally in the range of 0.02 % to 0.75 %.

For common cast iron, the main elements of the chemical composition are carbon and silicon. High carbon content increases the quantity of graphite or cementite (Fe3C). High carbon and silicon contents increase the graphitization potential of the iron as well as its castability. 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 equations (i) CE = % C + 0.33 (% Si) + 0.33 (% P) – 0.027 (% Mn) + 0.4 (% S), (ii) CE = % C + 0.33 (% Si), and (iii) CE = % C + 0.33 (% Si + % P).

The carbon equivalent value (CEV) is used to determine if the iron alloy is hypo-eutectic, eutectic, or hyper-eutectic. The carbon equivalent equations indicate the relative quantity of the eutectic which is formed during the solidification of the iron with 4.3 % carbon as 100 % eutectic composition. These equations also, in effect, indicate that on the basis of percent, 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. Iron with a carbon equivalent value of more than 4.3 is hyper-eutectic and normally contains coarse graphite. It is of lower strength, but is very good in thermal shock applications and for vibration damping. Gray irons with carbon equivalent value of less than 4.3 are hypo-eutectic and of higher strength since the quantity and size of the graphite flakes decrease with the carbon equivalent value. Also, there is less shrinkage as the carbon equivalent value increases.

The effect of higher carbon equivalent value is to reduce strength because of the formation of larger quantities of coarser graphite and, normally more ferrite. Manganese, sulphur, and phosphorus are present in plain gray irons and influence the tensile strength to some extent. Sulphur is a very significant element since it exerts marked effects on the solidification behaviour of gray iron. For this reason, the sulphur content in gray iron is normally controlled within limits and with a selected ratio to the manganese content since sulphur combines chemically with manganese to form manganese sulphide (MnS).

Although increasing the carbon and silicon contents in gray iron improves the graphitization potential and hence decreases the chilling tendency, the strength is adversely affected. This is because of the ferrite promotion and the coarsening of pearlite. The manganese content varies as a function of the desired matrix. Typically, it can be as low as 0.1 % for ferritic gray irons and as high as 1.2 % for pearlitic gray irons. The higher manganese content in pearlitic gray iron is because of manganese being a strong pearlite promoter.

The effect of sulphur is to be balanced by the effect of manganese. Without manganese in the iron, undesired iron sulphide (FeS) forms at the grain boundaries. If the sulphur content is balanced by manganese, manganese sulphide forms, which is harmless since it is distributed within the grains. The optimum ratio between manganese and sulphur for an iron sulphide free micro-structure and maximum quantity of ferrite is given by the equation % Mn = 1.7 (% S) + 0.15.

Sulphur modifies the length and distribution of graphite in gray cast iron and promotes type ‘A’ graphite. Decreasing the sulphur contents below 0.02 % produces type ‘D’ inter-dendritic graphite distribution because of a high increase in the undercooling. Type ‘A’ graphite and type ‘D’ graphite are explained in later part of the article. When increasing the sulphur content, sulphide particles are formed, which act as nucleation sites for graphite precipitation.

The minimum manganese content in gray iron is normally 1.7 times the sulphur content plus 0.12 % manganese. This assures sufficient manganese to ensure that all of the sulphur is combined with manganese rather than with iron. Manganese in excess of this quantity is a mild carbide stabilizer, and it refines the pearlite and increases 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 gray irons. Increasing phosphorus provides a somewhat higher tensile strength, but phosphorus contents of over 0.20 % reduce machinability particularly in the drilling operations.

The properties of gray iron can be changed by adding different alloying elements. Alloying elements are used in grey cast iron for improving or altering the material properties in a desired way. Alloying a gray cast iron with phosphorus improves the fluidity of liquid metal and prolongs the solidification time. This facilitates the casting procedure by allowing the liquid metal to reach all spaces in the mould. Phosphorus also lowers the toughness of gray cast iron, which can be a desired feature since it improves the chip breaking during machining and reduces adhesion of work-piece material on the cutting tool edge.

Silicon is the most important alloying element since it helps in forming free graphite. A low percentage of silicon allows carbon to remain in solution forming iron carbide and the production of white cast iron. A high percent of silicon forces the carbon out of solution forming graphite and the production of gray iron.

Other alloying elements are manganese, chromium (Cr), molybdenum (Mo), titanium (Ti), and vanadium (V) etc. which counteracts silicon, promotes the retention of carbon, and the formation of carbides in micro structure, hence improving tensile strength, impact strength, wear and heat resistance, and corrosion resistance etc. Nickel (Ni) and copper (Cu) increase strength, and machinability, but do not change the quantity of graphite formed.

Molybdenum is used to increase the strength at higher temperatures and to reduce the risk of thermal fatigue. The same features as for molybdenum applies for chromium, however, this alloying element increases the risk of so-called white iron, which is hard and brittle and undesired and forms in corners and sharp edges of the cast part. For countering this, addition of copper is common when chromium is used, since it can neutralize the carbide stabilization of chromium.

Aluminum (Al), titanium, vanadium, and niobium (Nb) act as grain refiners which improves the toughness of gray cast iron. This is most frequently not desired since it reduces the machinability. Hence, these elements are not added intentionally but can still be present since impurities and trace elements frequently are unavoidable in the liquid metal. The exception is aluminum, which can be used as a nucleoid.

Calcium (Ca) and manganese act as sulphide formers when used as alloying elements. Sulphides are soft and lubricates the cutting process, hence increase the machinability. Since manganese sulphide (MnS) inclusions have a higher thermal expansion coefficient than the iron matrix in gray cast iron, high tensions around the inclusions are at higher temperatures. The tension enables the material to crack in the cutting zone during machining, which improves chip breaking and increases machinability. The lubricating features of the manganese sulphide inclusions reduce friction and heat generation at the tool-chip interface, which reduces tool wear, it can also act as a nucleoid during the solidification process.

One or more of the alloying elements namely molybdenum, copper, nickel, vanadium, titanium, tin (Sn), antimony (Sb), and chromium can be present in varying quantities. Nitrogen (N2) is normally present in the range of 20 ppm (parts per million) to 92 ppm. Silicon 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 which affects the graphitization is the solidification rate. The slower is the rate, the higher is the tendency for graphite to form. Other minor elements, such as aluminum, antimony, arsenic (As), bismuth (Bi), lead (Pb), magnesium (Mg), cerium (Ce), and calcium can considerably alter both the graphite morphology and the micro-structure of the matrix.

In general, alloying elements can be classified into three categories. Silicon and aluminum increase the graphitization potential for both the eutectic and eutectoid transformations and increase the number of graphite particles. They form colloid solutions in the matrix. Since they increase the ferrite / pearlite ratio, they lower strength and hardness.

Nickel, copper, and tin increase the graphitization potential during the eutectic transformation, but decrease it during the eutectoid transformation, hence they raise the pearlite / ferrite ratio. This second effect is because of the retardation of carbon diffusion. These elements form solid solution in the matrix. Since they increase the quantity of pearlite, they raise strength and hardness of the gray iron.

Chromium, molybdenum, tungsten (W), and vanadium decrease the graphitization potential at both the stages. Hence, they increase the quantity of carbides and pearlite. They concentrate in principle in the carbides, forming ‘(FeX)nC’ type carbides, but also alloy the alpha iron solid solution. As long as carbide formation does not occur, these elements increase strength and hardness of the gray iron. Above a certain level, any of these elements determine the solidification of a micro-structure with cementite (mottled structure), which has lower strength but higher hardness.

The chemical composition of gray iron is not normally specified since it does not assure achievement of specific mechanical properties. However, for special applications some aspect of chemical composition can be specified for assuring the suitability of the gray iron for a specific need. For example, an alloy content range can be specified for assuring an adequate response to heat treatment or for providing strength, or oxidization resistance in service at a red heat. Minimum carbon content can 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 the hardening by heat treatment. Gray iron can be annealed to a low hardness for increasing the machinability. Of course, this also decreases its strength. The tensile strength of gray iron is influenced by both the normal elements present in plain gray iron such as carbon, silicon, phosphorus, sulphur and manganese, and the presence of alloying additions and trace elements.

Phases and micro-structure – In metallurgy, the term phase refers to a homogeneous state of matter. The phases differ from each other in chemical composition, type of atomic bonding and arrangement of elements. Also, which phases, and in what quantities they are present in a metal, highly impacts the material properties. Iron-carbon alloys have four different phases namely alpha-ferrite, austenite, cementite, and delta-ferrite.

Phases can exist simultaneously in gray cast iron. The micro-structure which is called pearlite consists of the ferrite and cementite phases, with each phase having its own material properties. In iron-alloys, the ferrite-phase is soft and ductile and the cementite phase is hard and brittle. In pearlite, the two phases act synergetic and results in material properties which are both harder than the ferrite phase and also more ductile than the cementite phase. The quantity of carbon in an iron alloy is important in determining what quantities the phases are going to be present. The pearlite micro-structure is formed when the cooling process of the liquid metal is slow. Gray iron is, among other things, characterized by the presence of free forming carbon embedded in the micro-structure, this carbon forms as graphite flakes in the metallic matrix.

The basic strength and hardness of the gray 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 gray iron has reduced wear resistance and strength. An entirely pearlitic matrix is characteristic of high strength gray irons, and several castings are produced with a matrix micro-structure of both ferrite and pearlite to get 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.

The normal micro-structure of gray iron consists of flake (lamellar) graphite dispersed throughout a ferrite matrix or pearlite matrix. The mechanical and physical properties of gray iron depend on the length and distribution of the graphite flakes and on the ferrite-pearlite ratio. The graphite flakes increase the wear resistance of the gray iron but decrease the toughness. The graphite structure, as for the micro-structure, has a heavy impact on the material properties.

Foundry practice can be varied so that nucleation and growth of graphite flakes occur in a pattern which improves the desired properties. The quantity, size, and distribution of graphite are important. Cooling which is too fast can produce so-called chilled iron, in which the excess carbon is found in the form of massive carbides. Cooling at intermediate rates can produce mottled iron, in which carbon is present in the form of both primary cementite (iron carbide) and graphite.

The graphite flakes, which have a three-dimensional shape, has a decisive impact on the properties and machinability of grey iron alloys. It increases the machinability by acting as a chip breaker and lubricator in the cutting process. The lubricating effect of the graphite flakes is also beneficial in some applications where the products are subjected to a high degree of wear. Mechanically the graphite flakes act as a stress raiser, which can initiate fracture in the matrix at high stresses, hence grey iron has little elastic behaviour and fails under tension with almost no plastic deformation.

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 the gray cast iron. These include the (i) the ability to produce sound castings economically in complex shapes such as water-cooled engine blocks, (ii) good machinability even at wear resisting hardness levels and without burring, (iii) dimensional stability under differential heating such as in brake drums and disks, (iv) high vibration damping as in power transmission cases, and (v) border-line lubrication retention as in internal combustion engine cylinders.

Increasing quantities of graphite result into with increasing quantity of total carbon content in the gray iron. This decreases the strength and hardness of the gray iron, but increases other desirable characteristics. Appreciable silicon content is also necessary in gray iron since 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 hence assures very good machinability. Silicon also imparts corrosion resistance and elevated temperature oxidization resistance to the gray iron.

The graphite flakes give gray iron a high damping capacity. Several factors affect the graphite structure, including carbon content, alloying elements, and cooling time. Depending upon character and distribution of graphite flakes, gray cast iron is classified into five types as given below.

Type A with uniform distribution and random orientation – Type A is a random distribution of flakes of uniform size. These flakes break the continuity of steel matrix but are least damaging. A high degree of nucleation which promotes eutectic solidification close to the equilibrium graphite is necessary for the formation of A-type graphite.

Type B with rosette pattern and random orientation – In this type of gray iron, the eutectic cell size is large because of the low degree of nucleation. Fine flakes form at the centre of the rosette since eutectic solidification begins at a large under-cooling. Also, non-uniform distribution of flakes increases the brittleness of the cast iron.

Type C with super-imposed flake size and random orientation – In this type of gray iron, the flakes occur in hyper-eutectic iron and form with coarse primary ‘kish’ graphite (Kish graphite is primary graphite which crystallizes directly from the melt in predominantly hyper-eutectic cast iron). This can influence the size of the eutectic cell and distribution of eutectic graphite. It can also reduce the tensile properties and cause pitting on machined surfaces but it can be beneficial when thermal conductivity is important.

Type D with inter-dendritic graphite and random orientation – Type D is fine under-cooled graphite which forms when solidification occurs at a large under-cooling and this structure forms in the presence of titanium. Although finer flakes increase the strength of the eutectic, this morphology is not desirable since it interferes with the formation of a fully pearlitic matrix by providing short diffusion paths for carbon, hence aiding ferrite formation.

Type E with inter-dendritic graphite and preferred orientation – Type E graphite forms in strongly hypo-eutectic irons of low carbon equivalent value which form a strong primary austenite dendrite structure before undergoing eutectic solidification.

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 graphite flakes have no appreciable strength, so they can be treated as voids. The tips of the flakes act as pre-existing notches making the gray iron brittle.  Microscopically, all gray irons contain flake graphite dispersed in a silicon iron matrix. Fig 2 shows types of graphite flakes.

Fig 1 Types of graphite flakes

Properties of gray iron – The properties of gray iron are mainly dependent on its composition. The lower strength grades of gray iron can be consistently produced by simply selecting the proper melting stock. Gray iron castings in the higher strength grades need 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. The properties of gray iron are largely determined by the form and pattern of the graphite, and by the quantity of this constituent. This, in turn, is determined by the changes which can take place during the solidification process.

The important properties of gray iron are (i) low cost of production, (ii) low melting point (around 1,150 deg C to 1,250 deg C), (iii) good castability, (iv) good machinability, (v) good wear resistance, (vi) high damping capacity, (vii) high compressive strength, (viii) high thermal conductivity and capability to withstand thermal shock, (ix) good resistance to atmospheric corrosion, and (x) notch Insensitive.

The typical tensile strength of a gray cast iron of 4 % carbon equivalent value ranges from 230 MPa to 300 MPa. The average strength for gray iron of this composition is 260 MPa with an average hardness of 215 HB (Brinell hardness). The quantity of primary austenite of gray cast iron of this composition ranges from 10 % to 25 %, the rest being eutectic.

Gray irons are normally classified by their minimum tensile strength. A class 220 gray iron indicates that it has a nominal tensile strength of 220 MPa. A class designation can be used to indicate a grade of iron even when tensile strength is not an important consideration and cannot be specified or tested. However, when the class designation is used in conjunction with a standard specification which needs 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 non-destructive test which can be applied in routine inspection. These methods of designation are satisfactory for most applications since the common engineering properties of gray iron can be related to its strength and hardness.

Hardness is the most commonly determined property of metal since it is a simple test and several 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 since differences in graphite structure have more of an effect on tensile properties than on the hardness. Compression strength does correlate very well with hardness for all types of gray iron since hardness is essentially a compression test. Hardness gives normally a good indication of tool life in machining, however, the presence of free carbides in the micro-structure 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 is 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 since it is under an excessive steady stress.

The number of stress applications which induces 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 centred cubic (bcc) ferrous metals. The maximum stress which 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 suppressing 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 worry 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 higher than that of steel or other kinds of iron. This behaviour 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 quantity of graphite present in the lower strength irons increases the energy absorbed. Larger cast section thicknesses increase damping capacity and inoculation normally 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 needed to fracture it. This needed 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) which precedes final fracture. The fracture toughness normally varies with temperature, state of stress and strain-rate, all of which influence the quantity of deformation which precedes fracture.

Gray iron also experiences less solidification shrinkage than other cast irons which do not form a graphite micro-structure. The silicon promotes good corrosion resistance and increases fluidity when casting.  Gray iron is normally considered easy to weld. Compared to the more modern iron alloys, gray iron has a low tensile strength and ductility. Hence, its impact and shock resistance are almost non-existent.

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 solidifies and cools 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 produces a lower hardness in the metallic matrix. Alternately, gray iron which is cast into a section that is too thin solidifies very rapidly and can be file hard. A casting which has 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 has been poured with the same iron. These differences in structure produce differences in mechanical properties.

Heat treatment – Although the majority of gray iron castings are used in the as-cast condition, gray iron can be heat treated for a variety of reasons, such as relieving of residual stresses, improving the machinability, increasing the hardness of the surface either through induction hardening or flame hardening, or hardening of the entire section through an oil quench and draw treatment.

The graphite structure cannot be changed by heat treatment, although the graphite can increase in volume if a pearlitic iron is completely converted into ferrite, in which case, the graphite is normally 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 normally given 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 can be experienced in some softer gray irons. Stress relief heat treatments can be given to improve the dimensional stability of machined castings and are needed for pressure containing parts operating at over 230 deg C and up to 340 deg C made from gray. Heating and cooling rates for such a heat treatment are normally limited to 200 deg C per hour per 25 mm of thickness. This is particularly important for heating since the residual stresses in the casting can increase as a result of thermal expansion of different 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 normally used depending on how much reduction in hardness is needed. If the castings have cementite or carbides in their micro-structure, it is necessary to heat the castings to a 900 deg C to 940 deg C range to break down such carbides.

Gray iron can be successfully hardened by either flame heating or induction heating. The matrix of the iron is to be pearlitic. It is also desirable to keep silicon at the lowest feasible level, normally below 1.75 %. As the silicon content of gray iron is increased, not only the Ac3 temperature is increased, but also a two-phase field of ferrite and austenite is encountered. If silicon is higher, satisfactory hardness is not achieved when the iron is heated in this temperature range. The higher austenitizing temperatures needed for the higher silicon irons also increase the possibility of cracking during the quenching cycle.

Machinability of the gray iron – The gray iron has a good machinability in relation to several other materials and is comparable to steel in this aspect. The chips which are produced during machining are discontinuous and the chip control is rarely a problem. What limits the machinability is frequently the presence of so-called white iron. White iron is a variant of gray iron which is very hard and results in a high level of abrasive wear and is formed when the cooling is very rapid, which is frequently the case in thin sections of a part or at its corners. For reducing the formation of white iron this is to be taken into account when designing the castings, hence overly complex geometries are to be avoided when using gray iron. A wide variety of cutting tool materials can be used to machine gray iron, e.g., cemented carbides, ceramics, and polycrystalline cubic boron nitride (pcBN).

The alloying elements used in gray iron does not automatically determine its mechanical characteristics. Other factors such as the micro-structure, the level of impurities it contains, the cooling rate, and the dimensions of different sections in the part can have a strong impact on the material properties and the machinability.

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 damaging the finished surface. The free cutting behaviour 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 can 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 get 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 – The gray iron is the material used for countless familiar items such as kitchen sink and bath-room sink, bath tubs, type-writer and cash register frames, refrigerators and air conditioner compressors, agricultural and construction equipment, electric motors, machine tools, machinery, equipment for street and highway utilities, and automobile parts such as cylinder blocks, heads, pistons and rings, manifolds, brake drums, and crank-shafts.

Alloyed gray iron is very popular for internal combustion engine cylinder blocks, pump housings, valve bodies, machine tools, heavy earth movers and stationary machinery. In addition, it is suitable for electrical boxes and decorative castings because of its properties. High thermal conductivity and specific heat capacity of gray cast iron are frequently exploited to make cast iron cook-ware and disc brake rotors.

Gray iron 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 get perfect fits, and, ordinarily, high spots in mating metal surfaces can 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.

Leave a Comment