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Cast Iron

• satyendra
• March 28, 2013
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• cast iron, CG iron, ductile iron, Flake graphite, graphite, Gray cast iron, Iron carbon matastable system, Malleable cast iron, SG iron, white iron,

Cast Iron

The term ‘cast iron’ identifies a large family of ferrous alloys. Cast irons are multi component ferrous alloys, which solidify with a eutectic. They contain major elements such as iron (Fe), carbon (C), and silicon (Si), minor elements (less than 0.1 %), and often alloying elements (higher than 0.1 %). Cast iron has higher C and Si contents than steel. Because of the higher C content, the structure of cast iron, as opposed to that of steel, shows a rich C phase.

Cast iron in its basic form is a brittle material which has a very little impact strength. It has a little or practically no toughness when compared to low C steels.  It has a fraction of the tensile strength of low C steels.  When a cast iron piece fails, it does not deform in a noticeable way and appears to snap apart or break in a manner consistent with a snap.  There is no early warning of a failure.

Typical percent of C in cast iron rages from 2.5 % to 4 %. Cast iron is very brittle, not amenable to deform. It is easy to cast (due to lower melting point) into complicated shapes and is a low cost material. With alloying, good foundry practice and heat treatment, properties of cast iron can be varied over wide range.

Depending primarily on composition, cooling rate, and melt treatment, cast iron can solidify according to the thermo-dynamically meta-stable Fe-Fe3C (iron-iron carbide) system or the stable iron- graphite system (Fig 1). When the metastable path is followed, the rich C phase in the eutectic is the iron carbide (Fe3C). When the stable solidification path is followed, the rich C phase is graphite.

Fig 1 Iron-carbon diagram showing stable and metastable phases

Referring only to the binary Fe-Fe3C or iron- graphite system, cast iron can be defined as an iron-carbon alloy with more than 2 % C. However, Si and other alloying elements can considerably change the maximum solubility of C in austenite. Hence, in exceptional cases, alloys with less than 2 % C can solidify with a eutectic structure and hence still belong to the family of cast iron.

The formation of stable or metastable eutectic is a function of many factors including the nucleation potential of the liquid, chemical composition, and cooling rate. The first two factors determine the graphitization potential of the iron. A high graphitization potential results in cast irons with graphite as the rich C phase, while a low graphitization potential results in cast irons with iron carbide. A schematic of the structure of the commercial types of different cast irons, as well as the processing needed to obtain them, is shown in Fig 2.

Fig 2 Basic micro-structures and processing for obtaining commercial cast irons

The two basic types of eutectics namely (i) the stable austenite-graphite, or (ii) the metastable austenite-iron carbide (Fe3C), have wide differences in their mechanical properties, such as strength, hardness, toughness, and ductility. Hence, the basic scope of the metallurgical processing of cast iron is to manipulate the type, amount, and morphology of the eutectic in order to achieve the desired mechanical properties.

Historically, the first classification of cast iron was based on its fracture. Two types of iron namely (i) white cast iron, and (ii) gray cast iron, were initially recognized.

White cast iron – It shows a white, crystalline fracture surface since fracture occurs along the iron carbide plates. It is the result of metastable solidification (Fe3C eutectic).

Gray iron – It shows a gray fracture surface since fracture occurs along the graphite plates (flakes). It is the result of stable solidification (graphite eutectic).

With the advent of metallography, and as the knowledge related to cast iron increased, the following other classifications based on micro-structural features became possible.

Graphite shape – As per the graphite shape, cast iron is classified as lamellar (flake) graphite (FG), spheroidal (nodular) graphite (SG), compacted (vermicular) graphite (CG), and temper graphite (TG). Temper graphite results from a solid-state reaction (malleabilization).

Matrix – As per the matrix, cast iron is classified as ferritic, pearlitic, austenitic, martensitic, and bainitic (austempered).

These classifications are seldom used by the foundries. The most widely used terminology is the commercial one. A first division can be made in two categories. The first one is the common cast irons which are for general-purpose applications and these are unalloyed or low alloy cast irons. The second one is special cast irons which are for special applications and are normally high alloy cast irons. The relationship between classification by designation, micro-structure and the fracture of cast iron after the final processing stage is given in Tab 1.

Special cast irons differ from the common cast irons mainly in the higher content of alloying elements (higher than 3 %), which promote micro-structures having special properties for elevated-temperature applications, corrosion resistance, and wear resistance. A classification of the main types of special cast irons is shown in Fig 3.

Fig 3 Classification of special high alloy grade cast irons

Metallurgical principles of cast iron

The main factors which have influence on the structure of cast iron (i) chemical composition, (ii) cooling rate, (iii) liquid treatment, and (iv) heat treatment. In addition, there are some aspects of combined C in cast irons which are required to be considered. These are described below.

• In the original cooling or through subsequent heat treatment, a matrix can be internally decarburized or carburized by depositing graphite on existing sites or by dissolving C from them.
• Depending on the Si content and the cooling rate, the pearlite in cast iron can vary in C content. This is a ternary system, and the C content of pearlite can be as low as 0.50 % with 2.5 % Si.
• The conventionally measured hardness of graphitic cast irons is influenced by the graphite, especially in gray iron. Martensite micro-hardness can be as high as 66 HRC, but measures as low as 54 HRC conventionally in gray cast iron (58 HRC in ductile cast iron).
• The critical temperature of cast iron is influenced (raised) by Si content, not by C content.

Gray cast iron (flake graphite cast iron)

When the composition of the cast iron and the cooling rate at solidification are suitable, a substantial portion of the C content separates out of the liquid to form flakes of graphite. The fracture path of such cast iron follows the graphite flakes. The fracture surface of this cast iron appears gray because of the predominance of exposed graphite.

The composition of gray cast iron is to be selected in such a way as to satisfy three basic structural requirements namely (i) required graphite shape and distribution, (ii) carbide-free (chill-free) structure, and (iii) required matrix. For common cast iron, the main elements of the chemical composition are C and Si. Fig 4 shows the range of C and Si in steels and cast irons. It can be seen that the cast irons have C in excess of the maximum solubility of C in austenite, which is shown by the lower dashed line. A high C content increases the quantity of graphite or Fe3C. High C and Si contents increase the graphitization potential of the cast iron as well as its castability.

Fig 4 C and Si percent ranges in cast irons and steels

The combined influence of C and Si on the structure is usually taken into account by the CE (carbon equivalent) which is given by the equation CE = % C + 0.3 (% Si) + 0.33 (% P) – 0.027 (% Mn) + 0.4 (% S). Although increasing of the C and Si contents improves the graphitization potential and thus decreases the chilling tendency, the strength is adversely affected. This is due to ferrite promotion and the coarsening of pearlite.

The Mn (manganese) content varies as a function of the desired matrix. Typically, it can be as low as 0.1 % for ferritic cast irons and as high as 1.2 % for pearlitic cast irons, since Mn is a strong pearlite promoter. From the minor elements, P (phosphorus) and S (sulphur) are the most common and are always present in the composition. These elements can be as high as 0.15 % for low-quality cast irons and are significantly less for high-quality cast irons, such as ductile iron or compacted graphite iron.

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

Other minor elements, such as Al (aluminum), Sb (antimony), As (arsenic), Bi (bismuth), Pb (lead), Mg (magnesium), Ce (cerium), and Ca (calcium), can considerably change both the graphite morphology and the micro-structure of the matrix. Typical range of composition for the unalloyed cast irons is given in Tab 2.

Both the major and the minor elements have a direct influence on the morphology of flake graphite. Fig 5 shows typical flake graphite shapes. Type A graphite is found in inoculated cast irons cooled with moderate rates. Normally, it is associated with the best mechanical properties and cast irons with this type of graphite show moderate under-cooling during solidification. Type B graphite is found in cast irons of near eutectic composition, solidifying on a limited number of nuclei. Large eutectic cell size and low undercoolings are common in cast irons showing this type of graphite. Type C graphite occurs in hyper-eutectic irons as a result of solidification with minimum undercooling. Type D graphite is found in hypoeutectic or eutectic irons solidified at rather high cooling rates, while type E graphite is characteristic for strongly hypo-eutectic irons. Types D and E are both associated with high undercoolings during solidification. Not only graphite shape but also graphite size is important, because it is directly related to strength.

Fig 5 Types of flake graphite shapes

Alloying elements can be added in common cast iron to improve some mechanical properties. They influence both the graphitization potential and the structure and properties of the matrix. Elements with high positive graphitization potential (in decreasing order of positive potential) are (i) C, (ii) Sn (tin), (iii) P, (iv) Si, (v) Al, (vi) Cu (copper), and (vii) Ni (nickel). Fe (iron) has neutral graphitization potential. Elements with high negative graphitization potential (increasing order of negative potential) are (i) Mn, (ii) Cr (chromium), Mo (molybdenum), and V (vanadium). This classification is based on the thermodynamic analysis of the influence of a third element on C solubility in the Fe-C-X system, where X is a third element.

Although listed as a graphitizer (which is true thermodynamically), P also acts as a matrix hardener. Above its solubility level (perhaps around 0.08 %), P forms a very hard ternary eutectic. The above classification is also to include S as a carbide former, although Mn and S can combine and neutralize each other. The resultant MnS also acts as nuclei for flake graphite. In industrial processes, nucleation phenomena can sometimes override solubility considerations. In general, alloying elements can be classified into three categories.

Silicon and aluminum – These elements increase the graphitization potential for both the eutectic and eutectoid transformations and increase the number of graphite particles. They form solid solutions in the matrix. Since they increase the ferrite/pearlite ratio, they lower strength and hardness.

Nickel, copper, and tin – They increase the graphitization potential during the eutectic transformation, but decrease it during the eutectoid transformation, hence raising the pearlite / ferrite ratio. This second effect is due to the retardation of C diffusion. These elements form solid solution in the matrix. Since they increase the amount of pearlite, they raise strength and hardness.

Chromium, molybdenum, tungsten, and vanadium – They decrease the graphitization potential at both stages. Hence, they increase the amount of carbides and pearlite. They concentrate in principal in the carbides, forming (FeX)nC type carbides, but also alloy the alpha Fe solid solution. As long as carbide formation does not occur, these elements increase strength and hardness. Above a certain level, any of these elements determine the solidification of a structure with both graphite and Fe3C (mottled structure), which have lower strength but higher hardness.

In alloyed gray iron, the typical ranges for the elements are Cr – 0.2 % to 0.6 %, Mo – 0.2 % to 1 %, V – 0.1 % to 0.2 %, Ni – 0.6 % to 1 %, Cu – 0.5 % to 1.5 % and Sn – 0.04 % to 0.08 %. The influence of composition and cooling rate on tensile strength can be estimated using an empirical equation.

The cooling rate, like the chemical composition, can significantly influence the as cast structure and hence the mechanical properties. The cooling rate of a casting is primarily a function of its section size. The dependence of structure and properties on section size is termed section sensitivity. Increasing of the cooling rate results into (i) refine both graphite size and matrix structure which results in increased strength and hardness and (ii) increase the chilling tendency which can result in higher hardness, but can decrease the strength. Thus, composition is to be adjusted in such a way so as to provide the correct graphitization potential for a given cooling rate. For a given chemical composition and as the section thickness increases, the graphite becomes coarser, and the pearlite / ferrite ratio decreases, which results in lower strength and hardness. Higher C equivalent (CE) has similar effects.

The liquid treatment of cast iron is of vital importance in the processing of cast iron since it can dramatically change the nucleation and growth conditions during solidification. As a result, graphite morphology, and hence, properties can be significantly affected. In gray iron practice, the liquid treatment used is termed inoculation and consists of minute additions of minor elements before pouring. Typically, ferro-silicon with additions of Al and Ca or proprietary alloys are used as inoculants.

The main effects of inoculation are (i) an increased graphitization potential because of decreased undercooling during solidification, as a result of this, the chilling tendency is diminished, and graphite shape changes from type D or type E to type A, and (ii) a finer structure, that is, higher number of eutectic cells, with a subsequent increase in strength. Inoculation also improves tensile strength. This influence is more pronounced in the cast irons with low CE.

Heat treatment can significantly change the matrix structure, although graphite shape and size remain basically unaffected. A rather low proportion of the total gray iron produced is heat treated. Common heat treatment can consist of stress relieving or of annealing to decrease hardness.

Ductile iron (spheroidal graphite iron)

The main effects of chemical composition are similar to those for gray iron, with quantitative differences in the extent of these effects and qualitative differences in the influence on graphite morphology. The CE has only a mild influence on the properties and structure of the ductile iron, since it affects graphite shape significantly less than in the case of gray iron. However, to prevent excessive shrinkage, high chilling tendency, graphite flotation, or a high impact transition temperature, optimum amounts of C and Si are to be selected.

Further, minor elements can significantly change the structure in terms of graphite morphology, chilling tendency, and matrix structure. Minor elements can promote the spheroidization of graphite or can have an adverse effect on graphite shape. The minor elements which adversely affect graphite shape are said to degenerate graphite shape. A variety of graphite shapes can occur (Fig 6). Graphite shape is the single most important factor affecting the mechanical properties of cast iron.

Fig 6 Typical graphite shapes

Based on the generic influence of various elements on graphite shape, they can be divided into three groups. The spheroidizer elements (group 1) are Mg, Ca, rare earths such as Ce, and lanthanum (La) etc., and yttrium (Y). The neutral elements (group 2) are Fe, C, and alloying elements. The anti spheroidizer elements (group 3) are Al, As (arsenic), Bi (bismuth), Te (tellurium), (Ti) titanium, Pb (lead), S, and Sb (antimony).

The elements in the first group, i.e. the spheroidizing elements, can change graphite shape from flake through compacted to spheroidal. The most widely used element for the production of spheroidal graphite is Mg. The amount of residual magnesium, Mg (residual) required to produce spheroidal graphite is generally in the range 0.03 % to 0.05 %. The precise level depends on the cooling rate. A higher cooling rate requires less Mg. The amount of Mg to be added in the cast iron is a function of the initial S level, S (initial), and the recovery of Mg, and a constant ‘K’ depending on the particular process used. The relationship equation is Mg (added) = [0.75 S (initial) + Mg (residual)]/ K.

A residual Mg level which is too low results in insufficient nodularity (that is, a low ratio between the spheroidal graphite and the total amount of graphite in the structure). This in turn results in a deterioration of the mechanical properties of the cast iron. If the Mg content is too high, carbides are promoted. The presence of anti spheroidizing (deleterious) minor elements can result in graphite shape deterioration, upto complete graphite degeneration. Hence, upper limits are set on the amount of deleterious elements to be accepted in the composition. These values can be influenced by the combination of various elements and by the presence of rare earths in the composition. In addition, some of these elements can be deliberately added during liquid processing in order to increase the nodule count.

Alloying elements have in principle the same influence on structure and properties as for gray iron. Because better graphite morphology allows more efficient use of the mechanical properties of the matrix, alloying is more common in ductile iron than in gray iron.

Cooling rate – When changing the cooling rate, effects similar to those described in the case of gray iron also occurs in ductile iron, but the section sensitivity of ductile iron is lower. This is since spheroidal graphite is less affected by cooling rate than flake graphite.

The liquid treatment of ductile iron is more complex than that of gray iron. The two stages for the liquid treatment of ductile iron are (i) modification, which consists of Mg or Mg alloy treatment of the melt, with the purpose of changing graphite shape from flake to spheroidal, and (ii) inoculation (normally, post inoculation, that is, after the Mg treatment) to increase the nodule count. Increasing the nodule count is an important goal, since a higher nodule count is associated with less chilling tendency and a higher as-cast ferrite / pearlite ratio.

Heat treatment – It is extensively used in the processing of ductile iron since better advantage can be taken of the matrix structure than for gray iron. The heat treatments usually applied are (i) stress relieving, (ii) annealing to produce a ferritic matrix, (iii) normalizing to produce a pearlitic matrix, (iv) hardening to produce tempering structures, and (v) austempering to produce a ferritic bainite. The advantage of austempering is that it results in ductile irons with twice the tensile strength for the same toughness.

Compacted graphite irons

Compacted graphite (CG) cast irons have a graphite shape intermediate between spheroidal and flake. Typically, compacted graphite looks like type IV graphite (Fig 6). Hence, most of the properties of CG cast irons lie in between those of gray and ductile iron. The chemical composition effects are similar to those described for ductile iron. CE influences strength less obviously than for the case of gray iron, but more than for ductile iron. The graphite shape is controlled, as in the case of ductile iron, through the content of minor elements. When the goal is to produce compacted graphite, it is easier from the stand point of controlling the structure to combine spheroidizing (Mg, Ca, and / or rare earths) and anti spheroidizing (Ti and /or Al) elements.

The cooling rate affects properties less for gray iron but more for ductile iron. In other words, CG iron is less section sensitive than gray iron. However, high cooling rates are to be avoided because of the high propensity of CG cast iron for chilling and high nodule count in thin sections.

Liquid treatment can have two stages, as for ductile iron. Modification can be achieved with Mg, magnesium, Mg + Ti, Ce + Ca, and so on. Inoculation is to be kept at a low level to avoid excessive nodularity. Heat treatment is not common for CG cast irons.

Malleable cast irons

Malleable cast irons differ from the types of other cast irons in that they have an initial as-cast white structure, that is, a structure consisting of iron carbides in a pearlitic matrix. This white structure is then heat treated (annealing at 800 deg C to 970 deg C), which results in the decomposition of Fe3C and the formation of temper graphite. The basic solid state reaction is given by equation Fe3C = gamma iron + graphite. The final structure consists of graphite and pearlite, pearlite and ferrite, or ferrite. The structure of the matrix is a function of the cooling rate after annealing.

Most of the malleable iron is produced by this technique and is called black-heart malleable iron. Some malleable iron is produced by decarburization of the white as-cast iron, and it is called white-heart malleable iron.

The composition of malleable cast irons is to be selected in such a way as to produce a white as-cast structure and to allow for fast annealing times. Although higher C and Si reduce the heat treatment time, they are to be limited to ensure a graphite-free structure upon solidification.

Both tensile strength and elongation decrease with higher CE. Nevertheless, it is not enough to control the CE. The annealing time depends on the number of graphite nuclei available for graphitization, which in turn depends on, among other factors, the C / Si ratio. A lower C / Si ratio (that is, a higher Si content for a constant CE) results in a higher temper graphite count. This in turn translates into shorter annealing times.

Mn content and the Mn / S ratio are to be closely controlled. In general, a lower Mn content is used when ferritic rather than pearlitic structures are desired. The correct Mn / S ratio can be calculated by equation ‘% Mn = 1.7 (% S) + 0.15’. Below this, all S is stoichiometrically tied to Mn as MnS. The excess Mn is dissolved in the ferrite. In the range delimited by the S stoichiometrically tied to Mn and Mn/S = 1, a mixed sulphide, (Mn,Fe)S, is formed. For Mn/S ratios smaller than 1, pure FeS is also formed. It is assumed that the degree of compacting of temper graphite depends on the type of sulphides occurring in the cast iron. When FeS is predominant, very compacted, nodular temper graphite forms, but some undissolved Fe3C can persist in the structure, resulting in lower elongations. When MnS is predominant, although the graphite is less compacted, elongation is higher because of the completely Fe3C-free structure. The Mn/S ratio also influences the number of temper graphite particles. From this standpoint, the optimum Mn/S ratio is around 2 to 4.

Alloying elements can be used in some grades of pearlitic malleable irons. The Mn content can be increased to 1.2 %, or Cu, Ni, and / or Mo can be added. Cr is to be avoided since it produces stable carbides, which are difficult to decompose during annealing.

Cooling rate – Like all other cast irons, malleable cast irons are sensitive to cooling rate. However, since the final structure is the result of a solid-state reaction, they are the least section sensitive cast irons.

The liquid treatment of malleable cast iron increases the number of nuclei available for the solid-state graphitization reaction. This can be achieved in two different ways. The first is by adding elements which increase undercooling during solidification. Typical elements in this category are Mg, Ce, Bi, and Te. Higher undercooling results in finer structure, which in turn means more gamma iron-Fe3C interface. Because graphite nucleates at the gamma iron-Fe3C interface, this means more nucleation sites for graphite. Higher undercooling during solidification also prevents the formation of unwanted eutectic graphite. The second is by adding nitrite-forming elements to the melt. Typical elements in this category are Al, B, Ti, and Zr (zirconium).

Heat treatment – The heat treatment of malleable iron determines the final structure of this cast iron. It has two basic stages. In the first stage, the Fe3C is decomposed in austenite and graphite. In the second stage, the austenite is transformed into pearlite, ferrite, or a mixture of the two. Although there are some compositional differences between ferritic and pearlitic irons, the main difference is in the heat treatment cycle.

When ferritic structures are to be produced, cooling rates in the range of 3 deg C/hour to 10 deg C/hour are needed through the eutectoid transformation in the second stage. This is necessary to allow for a complete austenite to ferrite reaction. When pearlitic irons are to be produced, different schemes can be used. The goal of the treatment is to achieve a eutectoid transformation according to the austenite-to-pearlite reaction. In some limited cases, quenching-tempering treatments are used for malleable irons.

Special cast irons

Special cast irons are alloy cast irons which take advantage of the radical changes in structure produced by rather large amounts of alloying elements. Abrasion resistance can be improved by increasing hardness, which in turn can be achieved by either increasing the amount of carbides and their hardness or by producing a martensitic structure. The least expensive material is white iron with a pearlitic matrix. Additions of 3 % to 5 % Ni and 1.5 % to 2.5 % Cr result in cast irons with (FeCr)3C carbides and an as-cast martensitic matrix. Additions of 11 % to 35 % Cr produce (CrFe)7C3 carbides, which are harder than the iron carbides. Additions of 4 % to 16 % Mn result in a structure consisting of (FeMn)3C, martensite, and work-hardenable austenite.

Heat resistance depends on the stability of the microstructure. Cast irons used for these applications can have a ferritic structure with graphite (5 % Si), a ferritic structure with stable carbides (11 % to 28 % Cr), or a stable austenitic structure with either spheroidal or flake graphite (18 % Ni, 5 % Si). For corrosion resistance, irons with high Cr (upto 28 %), Ni (upto 18 %), and Si (upto 15 %) are used.