GRAY IRON

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 that exceeds the solubility in austenite precipitates as flake graphite.

Gray irons usually contain 2.5 to 4% C, 1 to 3% Si, and additions of manganese, depending on the desired microstructure (as low as 0.1% Mn in ferritic gray irons and as high as 1.2% in pearlitics). Sulphur and phosphorus are also present in small amounts as residual impurities.

The composition of gray iron must be selected in such a way to satisfy three basic structural requirements:

• The required graphite shape and distribution
• The carbide-free (chill-free) structure
• The required matrix

For common cast iron, the main elements of the chemical composition are carbon and silicon. High carbon content increases the amount of graphite or Fe3C. High carbon and silicon contents increase the graphitization potential of the iron as well as its castability.

The combined influence of carbon and silicon on the structure is usually taken into account by the carbon equivalent (CE):

CE = %C + 0.3x(%Si) + 0.33x(%P) - 0.027x(%Mn) + 0.4x(%S)

Although increasing the carbon and silicon contents improves the graphitization potential and therefore decreases the chilling tendency, the strength is adversely affected. This is due to 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 irons and as high as 1.2% for pearlitic irons, because manganese is a strong pearlite promoter.

The effect of sulfur must be balanced by the effect of manganese. Without manganese in the iron, undesired iron sulfide (FeS) will form at grain boundaries. If the sulfur content is balanced by manganese, manganese sulfide (MnS) will form, which is harmless because it is distributed within the grains. The optimum ratio between manganese and sulfur for a FeS-free structure and maximum amount of ferrite is:

%Mn = 1.7x(%S) + 0.15

Other minor elements, such as aluminum, antimony, arsenic, bismuth, lead, magnesium, cerium, and calcium, can significantly alter both the graphite morphology and the microstructure 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. Because 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, thus raising the pearlite/ferrite ratio. This second effect is due to the retardation of carbon 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 decrease the graphitization potential at both stages. Thus, they increase the amount of carbides and pearlite. They concentrate in principal in the carbides, forming (FeX)_nC-type carbides, but also alloy the aFe solid solution. As long as carbide formation does not occur, these elements increase strength and hardness. Above a certain level, any of these elements will determine the solidification of a structure with Fe₃C (mottled structure), which will have lower strength but higher hardness.

Generally, it can be assumed that the following properties of gray cast irons increase with increasing tensile strength from class 20 to class 60:

• All strengths, including strength at elevated temperature
• Ability to be machined to a fine finish
• Modulus of elasticity
• Wear resistance.

On the other hand, the following properties decrease with increasing tensile strength, so that low-strength irons often perform better than high-strength irons when these properties are important:

• Machinability
• Resistance to thermal shock
• Damping capacity
• Ability to be cast in thin sections.

Successful production of a gray iron casting depends on the fluidity of the molten metal and on the cooling rate, which is influenced by the minimum section thickness and on section thickness variations.

Casting design is often described in terms of section sensitivity. This is an attempt to correlate properties in critical sections of the casting with the combined effects of composition and cooling rate. All these factors are interrelated and may be condensed into a single term, castability, which for gray iron may be defined as the minimum section thickness that can be produced in a mold, cavity with given volume/area ratio and mechanical properties consistent with the type of iron being poured.

Scrap losses resulting from missruns, cold shuts, and round corners are often attributed to the lack of fluidity of the metal being poured.

Mold conditions, pouring rate, and other process variables being equal, the fluidity of commercial gray irons depends primarily on the amount of superheat above the freezing temperature (liquidus). As the total carbon content decreases, the liquidus temperature increases, and the fluidity at a given pouring temperature therefore decreases. Fluidity is commonly measured as the length of flow into a spiral-type fluidity test mold.

The significance of the relationships between fluidity, carbon content, and pouring temperature becomes apparent when it is realized that the gradation in strength in the ASTM classification of gray iron is due in large part to differences in carbon content (~3.60 to 3.80% for class 20; ~2.70 to 2.95% for class 60). The fluidity of these irons thus resolves into a measure of the practical limits of maximum pouring temperature as opposed to the liquidus of the iron being poured.

The usual microstructure of gray iron is a matrix of pearlite with graphite flakes dispersed throughout. Foundry practice can be varied so that nucleation and growth of graphite flakes occur in a pattern that enhances the desired properties. The amount, size, and distribution of graphite are important. Cooling that is too rapid may 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.

Flake graphite is one of seven types (shapes or forms) of graphite established in ASTM A 247. Flake graphite is subdivided into five types (patterns), which are designated by the letters A through E. Graphite size is established by comparison with an ASTM size chart, which shows the typical appearances of flakes of eight different sizes at l00x magnification.

Type A flake graphite (random orientation) is preferred for most applications. In the intermediate flake sizes, type A flake graphite is superior to other types in certain wear applications such as the cylinders of internal combustion engines.

Type B flake graphite (rosette pattern) is typical of fairly rapid cooling, such as is common with moderately thin sections (about 10 mm) and along the surfaces of thicker sections, and sometimes results from poor inoculation.

The large flakes of type C flake graphite are formed in hypereutectic irons. These large flakes enhance resistance to thermal shock by increasing thermal conductivity and decreasing elastic modulus. On the other hand, large flakes are not conducive to good surface finishes on machined parts or to high strength or good impact resistance.

The small, randomly oriented interdendritic flakes in type D flake graphite promote a fine machined finish by minimizing surface pitting, but it is difficult to obtain a pearlitic matrix with this type of graphite. Type D flake graphite may be formed near rapidly cooled surfaces or in thin sections. Frequently, such graphite is surrounded by a ferrite matrix, resulting m soft spots in the casting.

Influence of Alloying Elements on Steel Microstructure

It is a long-standing tradition to discuss the various alloying elements in terms of the properties they confer on steel. For example, the rule was that Chromium (Cr) makes steel hard whereas Nickel (Ni) and Manganese (Mn) make it tough. In saying this, one had certain types of steel in mind and transferred the properties of particular steel to the alloying element that was thought to have the greatest influence on the steel under consideration. This method of reasoning can give false impressions and the following examples will illustrate this point.

When we say that Cr makes steel hard and wear-resisting we probably associate this with the 2% C, 12% Cr tool steel grade, which on hardening does in fact become very hard and hard-wearing. But if, on the other hand, we choose a steel containing 0,10% C and 12% Cr, the hardness obtained on hardening is very modest.

It is quite true that Mn increases steel toughness if we have in mind the 13% manganese steel, so-called Hadfield steel. In concentrations between l% and 5%, however, Mn can produce a variable effect on the properties of the steel it is alloyed with. The toughness may either increase or decrease.

A property of great importance is the ability of alloying elements to promote the formation of a certain phase or to stabilize it. These elements are grouped as austenite-forming, ferrite-forming, carbide-forming and nitride-forming elements.

Austenite-forming elements

The elements C, Ni and Mn are the most important ones in this group. Sufficiently large amounts of Ni or Mn render a steel austenitic even at room temperature. An example of this is the so-called Hadfield steel which contains 13% Mn, 1,2% Cr and l% C. In this steel both the Mn and C take part in stabilizing the austenite. Another example is austenitic stainless steel containing 18% Cr and 8% Ni.

The equilibrium diagram for iron-nickel, Figure 1, shows how the range of stability of austenite increases with increasing Ni-content.

An alloy containing 10% Ni becomes wholly austenitic if heated to 700°C. On cooling, transformation from g to a takes place in the temperature range 700-300°C.

Ferrite-forming elements

The most important elements in this group are Cr, Si, Mo, W and Al. The range of stability of ferrite in iron-chromium alloys is shown in Figure 2. Fe-Cr alloys in the solid state containing more than 13% Cr are ferritic at all temperatures up to incipient melting. Another instance of ferritic steel is one that is used as transformer sheet material. This is a low-carbon steel containing about 3% Si.

Multi-alloyed steels

The great majority of steels contain at least three components. The constitution of such steels can be deduced from ternary phase diagrams (3 components). The interpretation of these diagrams is relatively difficult and they are of limited value to people dealing with practical heat treatment since they represent equilibrium conditions only. Furthermore, since most alloys contain more than three components it is necessary to look for other ways of assessing the effect produced by the alloying elements on the structural transformations occurring during heat treatment.

One approach that is quite good is the use of Schaeffler diagrams (see Figure 3). Here the austenite formers are set out along the ordinate and the ferrite formers along the abscissa. The original diagram contained only Ni and Cr but the modified diagram includes other elements and gives them coefficients that reduce them to the equivalents of Ni or Cr respectively. The diagram holds good for the rates of cooling which result from welding.

A 12% Cr steel containing 0,3% C is martensitic, the 0,3% C gives the steel a nickel equivalent of 9. An 18/8 steel (18% Cr, 8% Ni) is austenitic if it contains 0-0,5% C and 2% Mn. The Ni content of such steels is usually kept between 9% and 10%.

Hadfield steel with 13% Mn (mentioned above) is austenitic due to its high carbon content. Should this be reduced to about 0,20% the steel becomes martensitic.

Carbide-forming elements

Several ferrite formers also function as carbide formers. The majority of carbide formers are also ferrite formers with respect to Fe. The affinity of the elements in the line below for carbon increases from left to right.

Cr, W, Mo, V, Ti, Nb, Ta, Zr.

Some carbides may be referred to as special carbides, i.e. non-iron-containing carbides, such as Cr7C3 W2C, VC, Mo2C. Double or complex carbides contain both Fe and a carbide-forming element, for example Fe4W2C.

High-speed and hot-work tool steels normally contain three types of carbides, which are usually designated M6C, M23C6 and MC. The letter M represents collectively all the metal atoms. Thus M6C represents Fe4W2C or Fe4Mo2C; M23C6 represents Cr23C6 and MC represents VC or V4C3.

Carbide stabilizers

The stability of the carbides is dependent on the presence of other elements in the steel. How stable the carbides are depends on how the element is partitioned between the cementite and the matrix. The ratio of the percentage, by weight, of the element contained in each of the two phases is called the partition coefficient K. The following values are given for K:

Al	Cu	P	Si	Co	Ni	W	Mo	Mn	Cr	Ti	Nb	Ta
0	0	0	0	0,2	0,3	2	8	11,4	28	Increasing

Note that Mn, which by itself is a very weak carbide former, is a relatively potent carbide stabilizer. In practice, Cr is the alloying element most commonly used as a carbide stabilizer.

Malleable cast iron (i.e. white cast iron that is rendered soft by a graphitizing heat treatment called malleablizing) must not contain any Cr. Steel containing only Si or Ni is susceptible to graphitization, but this is most simply prevented by alloying with Cr.

Nitride-forming elements

All carbide formers are also nitride formers. Nitrogen may be introduced into the surface of the steel by nitriding.

By measuring the hardness of various nitrided alloy steels it is possible to investigate the tendency of the different alloying elements to form hard nitrides or to increase the hardness of the steel by a mechanism known as precipitation hardening.

The results obtained by such investigations are shown in Figure 4, from which it can be seen that very high hardnesses result from alloying a steel with Al or Ti in amounts of about 1,5%.

On nitriding the base material in Figure 4, hardness of about 400 HV is obtained and according to the diagram the hardness is unchanged if the steel is alloyed with Ni since this element is not a nitride former and hence does not contribute to any hardness increase.