Cast Iron

Basic Classification of Cast Iron

    The goal of the metallurgist is to design a process that will achive required mechanical properties in a material. For that we need to know what effects on mechanical structure. When discussing the metallurgy of cast iron, the main factors of influence on the structure are;

-Chemical composition

-Cooling rate

-Liquid treatment

-Heat treatment

    In addition; we could say a matrix can be internally decarburized or carburized by depositing graphite on existing sites or by dissolving carbon from them. Depending on the silicon content and the cooling rate, the pearlite in iron can vary in carbon content. This is a ternary system, and the carbon content of pearlite can be as low as 0.50%  if there is 2.5% Si. The critical temperature of iron is influenced (raised) by silicon content, not carbon content.

Gray Iron (Flake Graphite Iron)

   When we select gray iron there is three things that we need to pay attention

1.       Required graphite shape (in this case flakes)

2.       Stress free structure which has no carbon content in it.

3.       Required matrix

    For common cast iron, the main elements of the chemical composition are carbon and silicon. A 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 = % C + 0.3(% Si) + 0.33(% P) - 0.027(% Mn) + 0.4(% 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 (irileştirme/büyütme) 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.

For other minor elements such as phosphorus and sulfur are the most common and are always present in the composition. Manganese balances sulfur content. İf it does not, undesired FeS (ironsulfur) placed on grain boundries. Here is a ferrite structure can contain maximum how much Manganese-Sulfure demonstrated by a their balance equation;

% Mn = 1.7(% S) + 0.15

Other minor elements can affect both carbon or graphite morphology and matrix.

Here is typical chemical composition range of Gray iron;

In conculusion; Both major and minor elements have a direct influence on the morphology of flake graphite.

       There is five types of graphite flakes disturbiton types can be found in gray cast iron micro structure. Uniform distribution of flakes with random orientation is found in inoculated irons cooled with moderate rates and shows best mechanical properties.  And others are rosette groupings, kish graphite (superimposed flake sizes with random orientation), interdendritic segregation with random orientation and interdendritic segregation with preferred orientation

     

 Effect of maximum graphite flake length on the tensile strength of gray iron;

Alloying elements

     Alloying elements can be added in common cast iron to enhance some mechanical properties. They influence both the graphitization potential and the structure and properties of the matrix. The main elements are Carbon, Tin, Phosphorus, Silicon, Aluminum, Copper, Nickel, Neutral and Iron. Also the elements effects negatively are Manganese, Chromium, Molybdenum, Vanadium. This classification is based on the thermodynamic analysis of the influence of a third element on carbon solubility in the Fe-C-X system, where X is a third element. Keep in mind as it said Sulphur and Manganese can neturalize each others.

In general, alloying elements can be classified into three categories

1.       Silicon and aluminum: 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. Because they increase the ferrite/pearlite ratio, they lower strength and hardness.

2.       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. Because they increase the amount of pearlite, they raise strength and hardness.

3.       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 α 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 will determine the solidification of a structure with both Gr and Fe3C (mottled structure), which will have lower strength but higher hardness.

        Equation for the influence of composition and cooling rate on tensile strength is ;

TS = 162.37 + 16.61/D - 21.78(% C) -61.29(% Si) - 10.59 (% Mn - 1.7% S) + 13.80(% Cr) + 2.05(% Ni) + 30.66(% Cu) + 39.75(% Mo) + 14.16 (% Si)2 -26.25(% Cu)2 - 23.83 (% Mo)2 (where D is diameter in inch and equation is valid for bar diameters of 20 to 50 mm (0.78 to 2 in.) and certain chemical composition range.

Cooling Rate

    Cooling rate affects mechanical properties as much as chemical composition and The cooling rate of a casting is primarily a function of its section size. Increaseing cooling rate can do;

a.                  Refine graphite and matrix size to increse strenght and hardness

b.                  Increase chilling tendency that increase hardnes as a result however decrases strenght.

The liquid treatment

    The liquid treatmen relatively effects mechanical properties and it is name of process that adding some minute amount of minor elements into melted cast iron. Material can change properties during solidification and show unexpected mechanical properties because of this graphite structure or matrix can be significantly change thats why it has relatively effect  on mechanical props . The main effects of liquid treatment are;

    An increased graphitization potential because of decreased undercooling during solidification and affect of the chilling tendency is diminished, and graphite shape changes from type interdendritic segregation with random orientation or interdendritic segregation with preferred orientation to uniform distrubuted with random orientation.

    And a finer structure, that is, higher number of eutectic cells, with a subsequent increase in strength.

Heat Treatment

    Heat teatment process  affects the matrix structure, although graphite shape and size remain basically unaffected. Common heat treatment may consist of stress relieving or of annealing to decrease hardness.

Ductile Iron (Spheroidal Graphite Iron)

Composition

Composition effects mostly same as gray cast iron but there is quantitative effects on graphite morphology. Carbon Equivalent (CE) has mild-minor- infulence on properties and structure becouse of graphite shape of ductile cast iron.

However there is a optimum Carbon-Silica equavelent to prevent high impact transition, high chilling tendency, graphite floatation or excessive shrinkage.

Also as it told on gray cast iron, minor elements have influence on graphite shape (which is most important) as well as chilling tendency and matrix structure. Graphite shape is the most important thing affects mechanical properties of cast iron.

Figure: Influence of graphite morphology on the stress-strain curve of several cast irons

    The generic infulence of various elements on graphite shape are;

Spheroidizers; Magnesium, calcium, rare earths (cerium, lanthanum, etc.), yttrium

Neutrals; Iron, carbon, alloying, elements

Antispheroidizers (degenerate shape); Aluminum, arsenic, bismuth, tellurium, titanium, lead, sulfur, antimony

Mostly used spheroid element is magnesium. The amount of residual magnesium, Mgresid, required to produce spheroidal graphite is generally 0.03 to 0.05%. The precise level depends on the cooling rate. A higher cooling rate requires less magnesium. The amount of magnesium to be added in the iron is a function of the initial sulfur level, Sin, and the recovery of magnesium, η, in the particular process used

Figure: İnfluence of residual Magnesium on graphite shape.

A residual magnesium level that 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 iron. The presence of antispheroidizing (deleterious) minor elements may result in graphite shape deterioration, up to complete graphite degeneration. Therefore, upper limits are set on the amount of deleterious elements to be accepted in the composition of cast iron.

Alloying elements have in principle the same influence on structure and properties as for gray iron. Because a 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 discussed for gray iron also occur in ductile iron, but the section sensitivity of ductile iron is lower. This is because spheroidal graphite is less affected by cooling rate than flake graphite.

Liquid Treatment

Liquid Treatment is different  from gray cast iron and much more complex. There is two purpose of this treatment. One is Changing grpahite shape flakes to spherodical by adding magnesium or magnesium allyoys. Other is the increase nodule count by inoculation (aşılama). Higher nodule count means less chilling tendency.

Heat Treatment

Heat treatment is extensively used in the processing of ductile iron because better advantage can be taken of the matrix structure than for gray iron. The heat treatments usually applied are as follows:

· Stress relieving

· Annealing to produce a ferritic matrix

· Normalizing to produce a pearlitic matrix

· Hardening to produce tempering structures

· Austempering to produce a ferritic bainite

Compacted Graphite Irons

Graphites of this iron has a shape between spheoridical and flakes. Chemical composition and Cooling rate properties between Gray and Ductile Cast Irons. When it comes to liquid tratment Magnesium alloying is preffered but inoculation must be kept as low as posible. Heat treatment is not common for this type.

Malleable Cast Iron (Annealed White Cast Iron)

Produces by heating of White Cast Iron structure from 800 °C to 970°C which results of decomposition of  and formation of temper graphite

The structure of matrix is a function of cooling rate after annealing. This process called Blackheart Malleable Cast Iron. In europe they produce it by decarburuzation of White Cast Iron, and it is called Whiteheart Malleable Cast Iron.

Composition

The composition of malleable irons must be selected in such a way as to produce a white as-cast structure and to allow for fast annealing times. Although higher carbon and silicon reduce the heat treatment time, they must be limited to ensure a graphite-free structure. Both tensile strength and elongation decrease with higher carbon equivalent. Nevertheless, it is not enough to control the carbon equivalent. The annealing time depends on the number of graphite nuclei available for graphitization, which in turn depends on C-Si rate. Tempered graphite clusters (kümeler) is decreases when C-Si Rate is increases under stable CE conditions.

Mangenese contents and Mg/S rate must be controlled closely. Mg/S rate has direct affect to shape of Graphites and elongation rate of material. The Mn/S ratio also influences the number of temper graphite particles. If material hold on low tepmerature(350°C) for 12h, it has more tendency to produce tempered carbon cluster than non holded one.

Alloying elements can be used in some grades of pearlitic malleable irons such as manganese, copper, nickel and molybdenium. But must be avoided chromium usage because it has a producing stable carbides affect and which makes difficult to decarburization.

Cooling Rate

To Be Continued...

References;
[1] ASM  Metal Handbooks by the AMERICAN SOCIETY FOR METALS