Cast Iron: Meaning, Heat Treatment and Properties | Industries | Metallurgy

In this article we will discuss about:- 1. Meaning of Cast Iron 2. Melting of Pig Iron to form Cast Iron 3. Production of Different Grades of Cast Iron and their Physical Properties 4. Heat Treatment 5. Important Properties.

Meaning of Cast Iron:

Pig iron on account of its impurities is very weak and cannot be shaped into different articles by processes such as forging or hammering. It is melted in cupola furnace with scrap steel or scrap iron to control the percentage of carbon and impurities and then cast into moulds of the desired shape. It is then called cast iron.

The properties of cast iron are regulated by the control of the amount, type, size, and distribution of the various carbon formations. The important factors are casting design, chemical composition, type of melting scrap, melting process, rate of cooling in the mould, and subsequent heat treatment.

It may be noted that cast iron is generally not specified by the chemical composition but on the basis of the properties. Cast irons have relatively high carbon content (1.5 to 5%) whereas steels contain upto 2% carbon.

The carbon in cast iron can be present in two forms:

(i) Combined carbon as iron carbide and

(ii) Graphite, as a mechanical admixture.

Graphite is in the form of dispersed flakes occupying from 6 to 10% of the volume of the typical grey iron. These flakes impair the continuity of the matrix to such an extent that they exert a very pronounced effect upon the mechanical properties of the metal. However increase in flake size or unfavourable distribution of the graphite may adversely affect the strength of the metal.

In the absence of silicon (or its presence in very little quantity) most of the carbon in cast iron is in the chemically combined form and the iron is called white iron. The presence of silicon causes softening effect and reduces the ability of the iron to retain carbon in chemical combination. Silicon acts as softener in cast iron as it increases the free carbon and decreases the combined carbon.

More silicon is required for light sections as these cool more rapidly, resulting in formation of more combined carbon with consequent increase in hardness. This is counteracted by presence of silicon. Metals like manganese, chromium, molybdenum, titanium and vanadium promote the retention of carbon in the combined form (carbide stabilisers) and counteract silicon, thereby rendering them harder.

Nickel and copper improve the matrix and increase the strength of the iron, but they do not lessen the amount of graphite and keep the iron readily machinable. In foundry, the various elements therefore should be so adjusted as to obtain machinable and strong casting.

Melting of Pig Iron to form Cast Iron:

Melting of pig iron is carried out in cupolas which are miniature blast furnaces. The cupola has a column of about 8 metres and is quite uniform in diameter and is lined with refractory bricks from inside. The hearth portion is provided with tuyeres to blow in air. Molten metal is tapped from the bottom.

Pig iron along with coke and lime as flux are charged from the top. The molten metal is collected from the bottom. As the atmosphere inside cupola is oxidising, some of the impurities are removed by oxidation.

Production of Different Grades of Cast Iron and their Physical Properties: 

Carbon can be present in cast iron as part of the ferrite, part of the cementite or free carbon (graphite).

(i) Grey Cast Iron:

It is produced by melting together low quality foundry pig, scrapped casting and coke in a cupola which is quite similar to a small blast furnace. The salvaged cast scrap is used to control the alloying elements in the finished cast iron.

When this type of cast iron is fractured, it gives a grey appearance. Therefore, it is called Grey iron. Grey cast iron has got most of its carbon in graphite form. It is quite possible to produce such a type of cast iron with all its carbon in the form of free graphite flakes but it is not always desirable. Generally in castings, 0.8% of carbon is in the form of iron carbide Fe₃C, and the rest 2 to 4% is in the form of graphite.

As a matter of fact, a complete series of cast iron is possible ranging from cast iron with all the carbon in graphite form to the cast iron with a good share of the carbon in combined form. As the carbides give hardness and strength to the iron, it is possible to have wide range of properties.

Cast iron having all the carbon in graphite form is soft, easily machinable metal having high damping capacity and high compressive strength and has self-lubricating characteristics. But tensile strength, ductility and impact strength are much lower than steel owing to the weakening effect of the graphite flakes. In such cast irons there is no well-defined yield limit and modulus of elasticity. It is used for basic structures of machine tools and structural members loaded in compression.

Cast irons which have got high content of carbon in the form of carbide are hard, brittle and un-machinable and have got resistance to wear. Close-grained iron containing graphite and pearlite is the strongest, toughest and best finishing type of cast iron. Medium grey irons contain some ferrite with graphite and pearlite and thus have poor strength and poor finish.

A high phosphorus grey cast iron pours very easily and is very cheap also, but is a low quality cast iron. It is used for covers of switch boxes and rain water goods requiring no machining and good physical appearance. High phosphorus cast iron is suited for producing ornamental castings.

The relative amount of free and combined carbon is determined by the variations in composition, melting practice and casting practice. Another factor which causes variation in composition is the rate of cooling of iron in the mould. Slow cooling helps the formation of graphite and when the rate of cooling is rapid, cementite is formed.

Cast iron has lower melting point (1135 to 1250°C) compared to steel (1500°C). The mechanical properties of grey cast iron depend upon its composition, but tensile strength varies between 1500 and 4000 kg/cm², hardness between 155 and 320 HB and compressive strength is 3—4 times the tensile strength.

According to IS – 210—1965, grey iron castings are designated by letters FG followed by ultimate strength in kg/mm², e.g., FG 15, FG 20, FG 25, FG 30, FG 40. It also specifies the important properties like minimum ultimate tensile strength, BHN, results of transverse test such as breaking load, rupture stress and deflection.

Where chemical composition is more important, as the percentage of silicon, then in the designation this percentage is also specified, e.g. FG 30 Si 12 which means grey iron casting having UTS of 30 kg/mm² and containing 12% silicon.

The basic composition of grey cast iron is described in terms of carbon equivalent which is equal to total carbon % + 1/3 (silicon % + Phosphorus %). This factor gives the relationship of % age of carbon and silicon in the iron to its capacity to produce graphite.

(ii) White or Chilled Cast Iron:

It has no graphite and is, therefore, white in colour. The whole of carbon content in this type of cast iron is in the form of either free cementite or cementite in lamellar pearlite.

White or chilled cast iron is prepared by two methods:

(i) The grey iron is cast in such a way that it is cooled rapidly.

(ii) By adjustment of the composition in such a way that carbon and silicon content are low.

For manufacturing such a kind of cast iron, low phosphorus pig iron and steel scrap are melted together in an air furnace which is heated from above, or in a cupola furnace. However, for getting best results generally duplexing or triplexing processes, which are combinations of cupola, air furnace, Bessemer converter and electric furnace, are adopted.

White cast iron is very hard, brittle and wear resistant iron. Hardness of 400 Brinell can be obtained by keeping silicon below one per cent and carbon to about 2% in cast iron.

When chromium is present above 3% in cast iron, it prevents formation of graphite. White cast iron produced in such a way has got better high temperature strength, grain growth resistance and corrosion resistance besides having ordinary properties of white cast irons.

The toughness and strength of white cast iron is doubled by small additions of nickel and chromium (for example, 4.5% Ni and 1.5% Cr). hardness thus obtained is of the order of 700 Brinell.

This being almost un-machinable, is used in parts requiring high abrasion resistance.

(iii) Malleable Cast Iron:

Malleable cast iron is produced by annealing the white cast iron. The annealing process consists of heating it slowly to 870°C and keeping at this temperature for 25 to 60 hours, depending upon size and then cooling slowly. Process of annealing is carried out by two methods.

(i) Annealing in which castings are packed in an oxi­dising material. In this way some carbon is removed and the malleabilized castings are known as white heart.

(ii) Annealing in which castings are packed in inert material such as ferrous silicate scale or slag; such malleabilized castings are called black heart and consist al­most entirely of graphite and ferrite.

It is tougher than grey cast iron and more resistant to bending and twisting. It is used for various automobile, tractor and plough parts, gear housing etc. Malleable cast iron as per BIS is classified as black heart, pearlitic, and white heart and accordingly designated by letters BM, and WM respectively followed by their respective ultimate tensile strength in kg/mm².

(iv) Ductile Cast Iron:

Ductile cast iron is produced by small additions of magnesium (or cerium) in the ladle. By doing so graphite content is rendered nodular or spheroidal in form and is well dispersed throughout the material.

Composition of ductile cast iron is in the following range:

Magnesium controls the graphite form as stated above but there is no effect of magnesium on matrix structure. Since graphite in a spheroidal shape occupies minimum surface for a given volume, there are less discontinuities in the surrounding metal giving it far more strength and ductility.

Nickel and manganese increase the strength but lower the ductility. Manganese content is kept low as the sulphur is generally very low in ductile irons. Silicon is used as an alloying element.

This kind of cast iron has got very high fluidity, cast ability, strength, toughness, wear resistance, pressure tightness, weld ability and machinability. It can be heat- treated in a manner similar to steel. Because of its excellent casting quality, it is suited for both intricate castings as well as big size castings.

Spheroidal graphite iron according to BIS is designated by letters SG followed by UTS in kg/mm² and the percentage elongation. For example, SG 42/12 means spheroidal graphite iron having ultimate tensile strength of 42 kg per sq mm and percentage elongation of 12%.

(v) Special Processed Iron:

Numerous types of cast irons (known as Mechanite) each having a different combination of mechanical and engineering properties suitable for specific purposes, are produced by licensed or patented controlled processes.

Four general classification types are:

(i) General engineering,

(ii) Heat resisting,

(iii) Wear resisting,

(iv) Corrosion resisting.

These have high tensile strength of the order of 1,650 to 3,650 kg/cm²; and when oil quenched and tempered, strength of 5,000 kg/cm² can be obtained.

Nodular iron, or ductile iron, is cast iron with the graphite substantially in spherical or nodular shape and substantially free of flake graphite.

There are two grades of nodular iron:

(i) Cast grade, and

(ii) Graphitizing annealed grade.

It is produced by adding alloys of magnesium or cerium to molten grey iron. The addition of these alloys causes the graphite to take form of small nodules or spheroids instead of the normal angular flakes. Tensile strength varies between 4,400 and 7,000 kg/cm² depending on composition. Nodular iron has the advantages of cast iron (fluidity, low melting point, good machinability) in addition to high tensile strength.

Various types of special processed cast irons are specified as follows by BIS:

AFG Ni 16 Cu 7 Cr 2 —> Austenitic flake graphite iron casting having 16% Ni, 7% Cu and 2%Cr.

ASG Ni 20 Cr 2 —> Austenitic spheroidal graphite iron casting having 20% Ni and 2% Cr.

ABR 33 Ni 4 Cr 2 —> Abrasion resistance iron casting having minimum ulti­mate tensile strength of 33 kg/ mm² and containing 4% nickel and 2% chromium.

(vii) Alloyed Cast Iron:

Alloyed cast irons are produced either in cupola by melting together the alloying compounds or by adding the alloying metal to the pouring ladle after drawing the molten iron from the furnace. Still better methods in which uniform compositions are obtained are by heating the allowing compounds in an air furnace or electric furnace.

The advantages of alloying cast irons are the improvement in strength, hardness, corrosion resistance and response to heat treatment.

Nickel is added upto 5% for improving machinability. Addition of nickel may also increase hardness and strength simultaneously. Presence of nickel also improves the corrosion resistance. It promotes graphitisation, and thus offsets the effect of thin sections by producing uniformity over thick and thin sections. It also lowers the hardening temperature, and so enables cast iron to be quench-hardened without cracking.

Chromium is added upto 3%. It checks the formation of graphite and promotes the formation of carbides. Thus higher percentages of chromium harden the iron by increasing the percentage of combined carbon. It also increases corrosion resistance. It also increases hardness without the extreme brittleness.

Nickel and Chromium when added in the ratio 3 : 1 (total 4%), make their graphite and carbide-forming tendencies neutralize each other. The resultant cast iron obtained has improved grain refinement, hardness and strength without any loss in machinability.

Molybdenum upto 1.5% in cast iron improves strength and wear resistance but decreases machinability. By the addition of molybdenum, graphitization is slowed down, critical transformation is retarded and thus there is improvement in the uniformity of structure. It causes cast iron to become tough.

Vanadium upto the extent of 0.5% improves carbide formation and thus increases the strength and hardness of cast iron very much. Copper in small amounts produces an improvement in the resistance to atmospheric corrosion.

Heat Treatment of Cast Irons:

(i) Stress Relief:

It is carried out by heating the cast iron article to 430—450°C, keeping it at this temperature for 30 minutes to 5 hours and then cooling the article slowly in a furnace. Though the internal stresses are decreased yet there is a slight decrease in hardness or strength at room temperature.

(ii) Annealing:

Cast iron is sometimes softened to facilitate machining. This is carried out by annealing process which consists of heating to 760°C — 825°C (up to 980°C in case of alloyed iron), maintaining at this temperature for some time and then cooling slowly.

Annealing increases the free carbon but decreases the strength though in case of alloyed steels, the strength reduction is less.

(iii) Hardening:

Hardening is generally carried out in case of alloyed steels and accomplished by heating above transformation temperature of 815°C to 870°C, quenching and then tempering to improve hardness and resistance to water. By hardening 0.5 to 0.8% combined carbon is converted to pearlitic or sorbitic structure. Quenching is usually done in oil but sometimes water and air quenching are also used.

Alloyed irons of special compositions are also nitrided to get high surface hardness and wear resistance. Nitriding is carried out at 510°C to 600°C in contact with anhydrous ammonia gas. Time taken is generally 20 to 90 hrs, depending upon the depth and size of hardening contemplated.

(iv) Malleabilising:

This is a lengthy heat treatment process to improve the strength of cast iron by changing the shape and size of the graphite. This treatment suits best on white cast iron in which carbon is more evenly distributed throughout the structure. Three types of cast irons, viz. white heart, black heart and pearlitic are produced by this treatment.

In the white heart process, the castings are packed into boxes with haematite ore, slowly heated to about 900°C, held at that temperature for several days, and finally cooled to room temperature. In this way these sections are completely decarburised. White heart cast iron is used for motorcycle sockets, agricultural machines, etc.

In the black heart process, air is excluded during heating by surrounding the castings in the containers with an inert substance, to prevent decarburisation. The cementite is broken down to form rosettes of graphite in a matrix of ferrite. Casting for this treatment should not contain more than 2.5% carbon. As melting point of cast iron increases with decrease in carbon content, this iron is difficult to cast. Black heart malleable iron is used for axle-boxes, rear axle housings, wheel hubs, etc.

The treatment for pearlitic cast iron is similar to black heart iron, but pearlitic structure is produced by increasing the manganese content is about 1%, or by heating a quenched and tempered black-heart malleable iron. This process produces cast iron structure similar to that of steel and such cast iron is used for axle and differential housings, gears and camshafts.

The tensile strength and elongation of three types respectively are 3500, 3000 and 4500 kg/cm and 5%, 10%, and 5%.

Important Properties of Cast Iron:

(i) Mechanical Properties:

The tensile strength of cast iron varies from 1,350 to 5,350 kg/cm². The elastic limit is close to its ultimate breaking strength. Grey iron can sus­tain indefinitely a static load just short of the tensile strength without distortion or breakage. Grey iron has low ductility and breaks with perceptible distortion. Since grey iron does not distort prior to breaking, it is essential that service stresses be known or else a conservative safety factor be employed.

With static loading the ultimate strength of cast iron in tension is less than that in compression; the impact strength of most cast irons is low. The damping capacity, or the ability to absorb vibrations, is high. The ease of machining grey iron is usually inversely proportional to the strength of the casting.

Chilling, heat treatment and alloy additions reduce the machinability. The white iron or chilled-iron casting is widely used for machinery parts to resist wear. High-alloy cast iron of the chromium, nickel and silicon type is specially resistant to sulphur and acid corrosion.

Cast iron is most widely used in engineering and allied industries because of the ease with which it can be cast and its wide range of useful properties. Cast iron is a very general term. Actually it is available in the forms of soft, weak, hard, brittle, and strong irons.

(ii) Machinability:

Cast iron has wide range of machinability, that is, from very good to the most un-machinability. Annealed permanent mould iron has got the highest machinability as the carbon in such a cast iron is in the state of finely divided and dispersed graphite flakes and not in the combined state as carbide, and moreover it is free from burned-in sand at the surface. Ductile cast iron has also got very high machinability.

The following is the decreasing order of the machinability of the various types of cast irons:

a. Pearlitic ferritic irons.

b. Pearlitic irons.

c. Motteled iron with pearlitic and massive cementite white iron.

d. White iron is particularly very difficult to machine because its structure is largely massive carbide.

(iii) Weldability:

The weldability of all the cast irons is quite low. Forge and submerged-melt welding cannot be used for cast irons. Gas and arc welding can be employed with special rods especially when sections have more than 6 mm thickness provided casting is heated red hot before weld­ing and then cooled slowly to room temperature. Bronze weld­ing is used for grey irons and for white irons before malleabilizing and without pre-heat, provided temperatures obtained are 810°C to 860°C.

(iv) Corrosion Resistance:

Though cast irons are not resistant to rusting yet the formation of rust is very slow and slower in comparison to alloy steels. Cast irons with high silicon and high chromium content are quite resistant to ac­ids.

However, both these and also the unalloyed grades have very little resistance to alkalies. High nickel austenitic irons are resistant to acids, (excepting nitric acid) to stress corro­sion in hot and to alkalies if stresses are low. High silicon (11 to 17% Si) cast irons are remarkably good for withstand­ing all acids except hot hydrochloric acid.

(v) High Temperature Usefulness:

For pressure vessels, grey cast irons are useful upto 340°C and for other applications they can be usefully employed upto 425°C. When grey cast iron is heated repeatedly above 425°C, grain growth, distortion and brittleness are caused. Too much scaling takes place if it is heated above 580°C. Lower carbon, lower silicon and more chromium contents in cast irons increase their permissible temperatures.