Heat Treatment of Grey Irons | Cast Irons | Metallurgy

Grey irons have graphite flakes dispersed in the steel-matrix, usually, of pearlite or ferrite (the amount of Mn could be as high as 1.2% for pearlitic, or as low as 0.1% for ferritic matrix). The heat -treatment has little or no effect on the size and shape of the graphite as obtained after casting.

However, the carbon-diffusion-paths being shorter in grey iron with fine-graphite-flakes, the ferti­lization, or the saturated-austenite-formation is achieved in shorter time i.e. the fine-flakes not only make the heat-treatment easier, but also induce superior mechanical properties.

The grey iron is usually hardened and tempered to increase the hardness (graphite embedded in hard martensite) to increase the resistance to wear and abrasion; but not ordinarily used commercially to increase the strength as the flakes act as stress-raisers to cause brittle fracture. It is economical to increase the strength by reducing the silicon content and total carbon (to reduce the volume of the graphite). Grey irons are more often given stress-relieving and annealing heat-treatments.

Stress Relieving:

It is practically impossible to produce a casting from the foundry completely free from internal stresses, though in most cases, these stresses are of little significance; but in some cases, their sufficient magnitude can cause fracture.

Most iron-castings, even the structural castings for precision machinery having internal walls are essentially internal-stress free, if these are allowed to cool to below 425°C in the sand-moulds, often called the mould-stress-relieving. As moulding sand is a bad conductor of heat, it thus, retains most of the heat absorbed from casting when (i) it solidifies at around 1090°C, (ii) the iron undergoes critical transformation (γ→ α at usually 750° to 850°C), liberating the heat.

The heat tends to equalise the temperature throughout the casting, while it is cooling. In complex castings, i.e., which have thick alongwith very thin sections, by the time the thick section (which cools at a much slower rate) finally cools and contracts, the thin-section is already cool and firm, i.e., the thin section restrains the contraction of thick- section; induces tensile stresses in the volume of iron in between these two sections. Such residual stresses introduced during solidification, and during further cooling to room temperature, adversely affect strength; cause distortion, and even result in cracking in some cases.

Some plastic yielding (fast creep) to relieve the internal stresses, occurs even at room temperature, which justifies the long established practice of use of ‘weathering’ of critical castings. Weathering, i.e., allowing the castings to stand in the open to expose them to fluctuating atmospheric temperatures for long periods of time of up to months, does effect the points of maximum stress, (stress gets reduced) but has been seen to improve the dimensional stability.

Weathering is able to remove a very small percentage of the internal stresses, and has little effect on major casting stresses. As the stress-relief can occur only by changing elastic strain to plastic strain, it is unlikely to be significant when the residual stresses are less than one third of the maximum tensile strength of iron. Heat-treatment is much more certain and can be carried out in a few hours.

Stress-relieving is the process of heating the grey iron castings to a temperature (normally much below Ac₁ temperature) in the range of 540° to 565°C (for unalloyed cast irons), or as given in table 15.9 for one hour so that the stress (75 to 85%) is relieved by rapid creep (i.e., elastic strain gets changed to plastic strain).

Unalloyed grey iron castings are almost completely stress-relieved (> 85%) at 595°C. Complete relief of stress may not necessarily be desirable in many cases. Grey irons of higher CEV require lesser temperature, say about 5I0°C; Low-alloy irons need 595°C to 650°C temperature, depending on the composition.

For example, diesel engine cylinder block (3.25% C. 2.20% Si, 0.30% Cr) is stress-relieved at 620°C for 2 hours to be further furnace cooled to 370°C. Higher-strength grey irons require higher-stress- relieving temperatures.

However, there is a limitation to use higher temperatures. As the temperature is higher than 540°C, there is an increasing tendency for the cementite of pearlite to dissociate to ferrite and carbon (which diffuses to and precipitates on the existing graphite).

Such a reaction is increased by the presence of higher silicon content, but is reduced by carbide-formers (Cr, Mn etc.). As such a reaction changes the microstructure it results in the decrease of hardness (Fig. 15.15) and tensile strength of iron. A good stress-relief treatment should allow little or no decrease in mechanical properties.

The rate of heating for stress-relief treatment is not critical for castings of normal shape and size, but it should be slow enough not to develop new stresses. The complex-shapes are charged for example, in a batch-type of furnace kept at a temperature not exceeding 95°C. It is then heated fast to 620°C in about 3 hours, held for I hour, and then cooled to 315°C in about 4 hours before removing the castings from the furnace to be cooled further in air. The heating up speeds for large or very critical castings should not exceed 10-15°C/h. and cooling rates be 20°-40°C/h.

Annealing:

Annealing is performed mainly to soften the castings in order to improve their machinability by breaking down the pearlite (to ferrite and graphite), and by minimising or eliminating the massive eutectic carbides or chill. This heat-treatment reduces substantially the mechanical properties; for example, it reduces the properties of grade 40 grey iron to grade 30.

The extent of reduction depends on the annealing temperature, time at that temperature and the alloying elements present in grey cast iron. Phosphorus (0.3%) has the most pronounced effect of all the elements normally present by virtue of the high hardness of phosphide eutectic, called steadite. Silicon affects the hardness as it strengthens the ferrite. Fine graphite Hakes too increases the hardness.

The annealing cycles are of three types:

1. Ferritizing Annealing:

This heat-treatment is done to improve the machinability of unalloyed or low-alloy cast irons of normal compositions by changing the original microstructure of graphite and pearlite to graphite and ferrite, i.e., by breaking down the pearlite. i.e., by decomposing cementite of pearlite to graphite and ferrite, i.e., the matrix becomes completely ferritic, and that is why the name, ferritizing annealing. As the temperature increases above 595°C, the rate of decomposition of cementite (to ferrite plus graphite) increases markedly to reach the maximum at lower critical temperature of around 760°C.

Most grey irons are ferritizing-annealed by heating between 700° and 760°C (without forming austenite) for 1h/25 mm of section of casting. The holding time, though depends on the composition, but may be as short as 10 minutes at 780°-820°C in some industrial short-term annealing process for unalloyed irons.

The presence of small percentages of Cr (0.2-0.3%) markedly increases the stability of pearlite, and thus, longer annealing times are needed. After the treatment, cooling to room temperature by a rate such as 100°C/h to 300°C/h is good for all except very complex castings. Ferritizing annealing is often done on castings that are subjected in service to temperatures of 450°-550°C, and still have dimensional stability.

2. Full Annealing:

The presence of excess amount of alloying elements increases the stability of iron carbide (of pearlite), such that the ferritizing annealing is ineffective. Elements which stabilise pearlite are Sb, Sn. V, Mo, Cr. Mn. P, Ni, Cu. etc., whereas carbon and silicon accelerate the decomposition of pearlite and free carbide. Iron castings containing Cr, or Mo, above about 0.3%, on ferritizing annealing change pearlite to spheroidised cementite and ferrite.

In such cases, full annealing is done to obtain softness and high machinability. Full annealing is the process of heating alloyed cast iron castings to a suitable temperature between 790° and 900°C for about 1h/25 mm of thickness. After the treatment, the castings must be slowly-cooled in range 790° to 675°C.

3. Graphitising Annealing:

The presence of excess amount of strong carbide formers, or inadequate inoculation or a fast rate of cooling may result in massive free carbide-particles, or chill in the microstructure of grey iron. Graphitising annealing aims to convert the carbides to graphite and austenite at the annealing temperature.

Graphitising annealing is the process of heating such grey iron castings to a temperature of 900°-955°C for a few minutes to several hours (depending on the composition) in a controlled atmosphere (otherwise the moisture of the furnace causes scaling). Pack-annealing using coke-packing is frequently employed. Care must be taken; otherwise steadite present may melt at 925°C if iron has more than 0.1% P.

The rate of cooling from such high annealing temperatures depends on the properties to be induced in the grey iron. If the castings are to be free of free-carbides, but must have high strength and good wear resistance, i.e., the microstructure should be pearlite and graphite, then the castings are air-cooled from the annealing temperature to about 540°C, and then cooled to about 300°C at not more than 110°C/h to avoid induction of residual stresses.

If maximum machinability is the main aim of graphitising annealing, then the ultimate microstructure should be ferrite and graphite; then, the castings are furnace-cooled from the anneal­ing temperature to 540°C, taking care to cool them very slowly through the transformation range. The cooling from 540°C to about 300°C should not be at a rate more than 110°C/h to avoid induction of residual-stresses.

Normalising:

Normalising may be used to increase the mechanical properties such as tensile strength and hardness of grey iron-castings, or to restore (reclaim) the as-cast properties which got modified due to earlier heat- treatment done deliberately or accidentally such as graphitising annealing, or the pre-heating and post-heating done for repair-welding. Normalising is the process of heating the grey iron castings to a temperature of around 885° to 925° C; soaking for a period of about 1h/25mm of the maximum thickness of casting; cooling in still air to room temperature.

The microstructure of normalised iron consists of pearlite and graphite flakes. Austenitisation temperature, (alloying elements if present) strongly effects the microstructure, and thus, the resulting tensile strength and hardness- Higher austenitisation temperature increases the carbon solubility in austenite, which increases the amount of cementite in the resulting pearlite, increasing ultimately both, the tensile strength and hardness.

The presence of some elements increases the carbon solubility in austenite, while some elements decrease it; thus, the carbon content of the matrix is dependent on the combined effects of alloying elements. Moreover, the hardness and the tensile strength increase as pearlite- interlamellar spacing (distance between two adjoining cementite plates) decreases; fast cooling in air results in smaller pearlite-spacing (but too high cooling rates promote martensite formation, which is the process of hardening).

If the CEV of un-alloyed grey iron is low (3.27-3.37), the as-cast strength and hardness is high (case 1 in Table 15.10); Normalising (heat at 900°C for 1½ hr., air-cool) almost restores these properties; but the unalloyed cast iron with high CEV and low Mn%, not only has low as-cast tensile Strength and hardness, but normalising decreases it further (case 2, Table 15.10). As alloying elements such as Mn, Cr, Mo, Ni stabilise pearlite, alloyed cast irons having these elements have high tensile strength and hardness in the normalised state as compared to as-cast state in spite of high CEV.

Hardening and Tempering:

Hardening and tempering of grey iron aims to improve the mechanical properties such as hardness, wear-resistance (increases five times of cast-state), and some tensile strength, hi hardening, the castings are heated to a high temperature of about 850°-885°C (around 95°C above the calculated A₁ transformation temperature from equation 15.2) in a salt bath, or in an electrically heated, gas-fired, or oil-fired furnace; soaked at this temperature for 20 min./25mm of thickness (so that the desired amount of carbon is dissolved), and then quenched more often in an agitated oil-bath, or in a molten-salt bath, or in a polymer-quenchant.

The austenitisation temperature, the soaking time depends on the composition and microstructure of the grey iron. The austenitisation temperature is kept above the transformation range. This range can extends to more than 55°C above A₁, is raised by Cr, but is decreased by Ni, Mn. Cu, etc. If the original matrix is ferritic, i.e., there is no, or low combined carbon content, the casting has to be austenitised for a longer time to satu­rate the austenite with carbon.

A higher silicon content, say 2.5% in grey iron restricts the solubility of carbon to 0.5% in austenite (as silicon decreases the solubility of carbon in austenite), higher austenitising temperatures have to be used to increase the solubility of carbon in austenite, because the hardness after hardening (i.e., of martensite) depends mainly on the carbon dissolved in austenite. However, very high temperatures should be avoided as then, quenching increases the danger of distortion, cracking and even of retained austenite (particularly in alloyed irons), which is avoided.

Heating to the austenitisation temperature may often be done in steps to avoid the development of cracks during heating- first heat the castings slowly to about 650°C in one furnace, then transfer them to a second furnace to heat then up rapidly to the austenitising temperature.

High silicon content, the presence of Mn increases the hardenability of unalloyed grey iron; it is further increased if elements like Ni, Mn, Cu, etc. are present in alloyed cast irons. Cr, Mo, the carbide-formers, increase the risk of cracking in thick-sections and complex-shapes; but in simple shapes, their presence as carbides increase significantly the wear resistance. High hardenability of even unalloyed irons permits the use of oil, molten-salts as quenching mediums.

Water is not recommended as it invariably leads to distortion and cracking in parts due to its too rapid a cooling power; without getting any extra increase in hardness. Highly alloyed cast irons, having high hardenability, may be given forced-air quenching. It is a careful practice to plunge the heavier section first in the bath, keeping the bath agitated for better quenching efficiency.

As the temperature of the castings falls to about 150°C, these are tempered immediately to avoid cracks being developed (due to the core of the casting transforming to martensite). Untempered hardnesses in the range 450-550 VPN are obtained with substantial loss in tensile strength.

Hardness (BHN) of cast iron (3.10% TC, 0.70% CC, 2.05% Si, 0.80% Mn, 0.27% Cr, 0.37% Ni, 0.45% Mo

Tempering is the process of reheating of castings up to a temperature maximum to below A₁, soaking for about 1 h/25 mm of maximum thickness. As tempering decreases the hardness, usually increases the tensile strength and toughness, the quenched castings are tempered at a temperature to meet the requirements. Tempering upto 300°C may reduce the hardness by 10-50 points it increases the tensile strength to its original unhardened value, or more.

The tempering temperature which results in maximum strength depends on the chemical composition, but in general, increases with higher alloy content. The normal range of tem­pering temperature to get high strength is between 370°-510°C and 100-150°C for getting high-hardness. Valve guides (3.40% C, 2.40% Si, 0.21% Cr, 0.50 Cu) are austenitised at 870°C for 1 h, oil-quenched, and then tempered at 495°C for 1h. The as-quenched hardness of 52 HRC gets tempered to 27 HRC.

Pearlitic grey iron responds to hardening much better than annealed or ferritic grey irons because the carbon is already dispersed in the matrix as pearlite, ready for solution when α à γ transformation occurs to result in getting saturated austenite in short time. In ferritic iron, however, carbon must diffuse from graphite-flakes into matrix; if sufficient time is not allowed, carbon content in austenite shall be low; the iron shows poor response to hardening.

Austempering:

Austempering is a hardening-treatment in which austenite transforms isothermally to acicular bainite (plus varying amount of austenite depending on the composition of cast iron and transformation temperature), which being softer and less brittle than martensite, helps objectively to reduce distortion and cracks in the castings.

This takes place in three steps:

1. The casting is heated to the conventional austenitising temperature, i.e. between 840° to 900°C, and soaked for time depending on the size and chemical composition of the casting, normally 1 h per 25 mm of section.

2. The casting is usually quenched in salt, oil, or lead bath maintained at 230°C-425°C (the bath temperature is in range 230° to 290°, if high hardness and wear-resistance are the main aims). The casting is held in the bath at this temperature sufficiently long to permit the austenite to transform completely to bainite.

This holding time could be long and depends on the temperature of the quenching bath and the chemical composition of the iron. Ni, Cr, Mo increase this time required for complete transformation. As bainite forms when the temperature has equalled in the cross-section of the casting, little stresses are developed during austem­pering.

3. Air cooling is done from bath tempe­rature to room temperature. No tempering is done after austempering.

Although grey irons have reasonable harden­ability, but depending on the section thickness and shape of the casting, hardenability agents, Ni, Mo, Cu may be added in the casting so that the cooling is fast enough to prevent any transfor­mation of austenite until the casting reaches the temperature of the quenching bath. Fig. 15.16 illustrates the effect of transfor­mation temperature (quenching bath) on the hardness of austempered unalloyed grey iron and Ni-grey iron.

Although austempering results in lesser maximum hardness than obtainable by martempering but the usually necessary tempering treatment following martempering cancels this difference. In such a state, the impact resistance (charpy) of austempered cast iron is more than martempered casting; but both the treatments result in less distortion and growth than the conventional hardening and tempering. As no tempering is done after austempering it helps to save the cost of tempering.

Martempering:

It is the process of hardening with an objective to minimise distortion and cracking, Martempering produces martensite without developing the high stresses that accompany its formation. However, the charac­teristic brittleness of the martensite necessitates the grey iron castings to he almost always tempered immedi­ately after hardening.

Martempering is heating of the grey iron castings to its conventional austenitising temperature (of hardening) i.e., between 840°-900°C in a controlled neutral atmosphere (such as endothermic atmosphere of 0.2 to 0.4% CCM and soaking at this temperature for around 1 h (to saturate the austenite with around 0.7% C), and then the casting is quenched in a salt, oil or lead bath maintained at a temperature slightly above the range at which martensite forms (between 200-260°C for unalloyed irons), only until the casting has attained the bath temperature; and then air-cooled to room temperature.

The process of tempering is done immediately at around I50-200°C for 1-2 hrs. If through martensitic structure is desired, only then the castings are held in hot quenching-bath long enough to permit the core to reach the temperature of the bath. This duration of hold depends on the size and shape of the casting.

The macrohardness after hardening could be 45-50 HRC, but the microhardness of the matrix is more than 60 HRC. Some allowance for the growth of the component must be made prior to the heat-treatment. A ferritic matrix as compared to pearlitic matrix causes more expansion. Martempering results in minimum distortion and cracking.

Flame-Hardening:

Many engineering applications require production of a hard surface subsequent to the machining, or grinding of the cast iron component. Flame-hardening is the most commonly used surface-hardening method applied to grey iron, by which, a grey-iron casting gets a hard, wear-resistant outer layer of martensite (around 1.8 mm thick on a flat surface), and a core of softer grey iron.

Flame-hardening is done by using oxy-acetylene or oxy-coal gas flame, followed by immediate quenching in water (at 30°C), or soluble-oil mixtures or polymer quenchants (oil in contact with flame causes fire-hazards). Unalloyed as well as alloyed grey irons can be successfully flame-hardened. Good hardness is obtained in low phosphorus irons.

Unalloyed grey iron respond well to flame-hardening provided CEV is below 4.1, i.e., the total carbon should be as small an amount as is consistent with the production of sound castings; although irons with as little as 0.40% combined carbon can be flame-hardened, but good results are obtained when the combined carbon is between 0.50 to 0.70% (i.e., for effective hardening, the matrix should be fully pearlitic as ferritic matrix needs much high temperature and longer times to dissolve carbon from flakes; thus not responding well to flame-hardening); irons containing more than 0.80% combined carbon or presence of free carbide, invariably show cracking (white irons or mottled irons are not (lame-hardened).

Coarse graphite flakes are avoided as these get burnt during flame-application resulting in porous, pitted and unattractive surface. P is kept below 0.2%, otherwise melting of steadite gives pitted surface. As silicon is a graphitiser, its content should not normally exceed 2%.

To increase the carbon solubility in austenite, Mn is kept between 0.8 to 1.0%. Unalloyed grey iron (3% TC, 1.7% Si, 0.6-0.8% Mn) results in a macrohardness of 400-500 BHN, but the matrix hardness is around 600 BHN. The casting surface should be free of porosity and the rough surfaces should be sand or shot blasted before flame-hardening is done.

Alloyed cast iron is flame-hardened more easily because of the high hardenability (due to presence of Cr, Mo, Mn etc.); results in higher macrohardness of around 550 BHN or more. Small amount of Cr (0.5%) helps to retain high combined carbon during austenitisalion for hardening, but Cr and Mo delay the solution of carbides in austenite.

Automobile camshaft (1% Cr. 0.50% Mo, 0.80% Mn) is flame-hardened to 52 HRC. Stress-relieving (at 150-200°C for a few hours in a furnace, or hot oil) is desirable but is often safely omitted as in case of camshafts, it decreases the hardness of the surface.

Fig. 15.17 illustrates the hardness-gradient in surface layers normally observed in a flame-hardened grey iron casting. The retention of soft austenite at the surface is responsible for lower surface hardness than immediately inside, which can be corrected by heating to 195-250°C. or by sub-zero treatment to – 40°C for 1 hour. Flame-hardening induces compressive stresses in the surface layers which increases the fatigue strength of the castings.

Induction-Hardening:

Induction-surface-hardening method can be adopted for grey iron if the number of castings to be induction-hardened is large enough to justify the relatively expensive equipment cost as well as the need for special induction coils.

A minimum combined carbon of 0.40 to 0.50% as pearlite in grey iron can be easily hardened. As the heating cycle is short in induction-hardening, ferritic grey irons do not respond well. The minimum induction-hardening temperature for grey iron is 870° to 925°C.

As the carbon equivalent value of the grey irons increases, the graphite flakes become more in number and larger in size (with the same conditions of casting). This results in lower apparent surface hardness (macro-hardness), yet the hardened matrix consistently shows hardness of around 60 HRC.

After induction heating, the castings may be water-quenched, though thin castings or castings with cross-drilled holes may be hardened by the use of oil (to prevent excessive distortion). The extent of distortion by the process of induction-hardening as compared to through-hardening is very small.