Heat Treatment of S.G. Irons | Cast Irons | Metallurgy

S.G. irons have nodules of graphite embedded in the matrix usually of ferrite, or pearlite, or both, depending on the thickness of the casting and/or alloy composition. As the nodule of graphite is not that serious stress-raiser, and that the ends are not that sharp (as flakes) to act as crack, the mechanical properties such as tensile strength, hardness, ductility, toughness, etc. can be enhanced, by changing the matrix (i.e., the microstructure) by various heat treatments.

Stress Relieving:

The as-cast state has residual stresses developed during the process of casting. If no other heat treatment is to be given, then the complex castings of S.G. iron may be heated to as given in Table 15.11 for stress-relieving. The holding time depends on the temperature used, complexity of casting and the extent of stress-relief desired, though, it is normally kept for 1 h plus 1 h per 25 mm of section thickness.

The use of higher temperatures does eliminate virtually all the residual stresses, but with some reduction in hardness (8-10 points on HRB at 650°C) and tensile strength. In order to avoid reintroduction of residual stresses, the castings, after the treatment, are furnace-cooled to 290°C, and then air-cooled. Austenitic S.G. iron castings are air-cooled from the stress-relieving temperature.

Annealing:

The process of annealing is done when maximum ductility and good machinability, and not the high strength are desired to be induced in S.G. iron castings. The resulting microstructure is graphite nodules embedded in complete ferritic matrix.

The amounts of Mn. P and the carbide-Forming elements such as Cr. Mo, V, W should be as small as possible if excellent machin­ability is to be induced as these elements retard the process of annealing. For example, the carbide formed, even when Cr is 0.25% does not dissolve in austenite at 925°C in 20 hours, and remains present as carbide in ferrite, reducing % E to 5%. (0.3% Mo, or 0.05% V, W behave in similar way).

Depending on the nature of S.G. iron, there are three types of annealing cycles:

1. Subcritical Annealing (At 705-720°C for 1h/25 mm of Section):

It directly converts pearlite to ferrite. Carbides may be present in microstructure resulting in lower ductility. After heating, the castings are furnace-cooled at 55°C/h to 345°C, and then air-cooled.

2. Full Annealing for Unalloyed S.G. Irons- (At 870-900°C for 1h/25 mm of Section):

This treatment is given to S.G. irons (unalloyed with 2-3% Si) with no eutectic carbides. After heating, the castings are furnace- cooled at 55°C/h to 345°C, and then air-cooled.

3. Full Annealing of S.G. Irons having Eutectic Carbides (at 900-925°C):

The time of holding normally is a minimum of 2 hrs. but longer times is advised for heavier sections. After soaking, the castings are furnace-cooled at 110°C/h to 700°C, and then held there for 2 hrs; followed by furnace- cooling at 55°C/h to 345°C and later air-cooled to room temperature.

Hardenability of S.G. Irons:

The process of normalising, hardening (and then tempering) or austempering requires the S.G. iron to have good hardenability. Alloying elements are added to increase the hardenability of S.G. iron. Mn and Mo are much more effective (per weight percent added) than Ni or Cu. Moreover, the combinations of elements such as Ni and Mo, or Cu and Mo, or Cu, Ni and Mn are much more effective than the individual elements.

The increase in hardenability is also reflected by the shift towards right of various fields in the TTT or CCT diagrams. Such a shift helps in getting easy transformations to required phases during cooling or holding in the processes of normalising or hardening or austempering. For example, slower cooling rate provided by, oil-quenching results in martensite without distortion and cracks in the S.G. iron castings as compared to if water was used.

Normalising:

It is the process of heating S.G. iron casting to a temperature between 870-940° and holding it for at least 1 h or 1 h per 25 mm of section and then air-cooled. Longer soaking times are required if alloying elements are present which retard carbon diffusion in austenite. Even Sn and Sb prevent the solution of carbon from the nodules as these segregate there.

Normalising results in considerable increase in tensile strength as it generally results in homogeneous structure of Fine pearlite and nodules of graphite, if it does not contain high silicon and has moderate Mn content of 0.3 to 0.5%.

As the cooling rate depends on the mass of the casting, heavier castings usually contain elements such as Ni, Mo, additional Mn (which shift the CCT curve towards right because of increased hardenability) so that even a slower cooling rate ensures the development of a fully fine pearlitic structure and nodules of graphite after air-cooling (normalising).

Thin sections of alloyed S.G. iron may develop martensite or bainite after normalising. Fig. 15.19 illustrates effect of various amounts of Ni and other alloying elements on hardness for various thicknesses after normalising.

Tempering is done after normalising in some cases when martensite forms, to relieve residual stresses; attain desired hardness; high toughness and impact resistance alongwith high tensile strength. Tempering is reheating of castings to 425-650°C and soaking there for 1 h per 25 mm of section.

Hardening and Tempering:

Hardening is the process of (i) heating the casting to the austenitising temperature of 845-925°C, (Fig. 15.20 illustrates that austenitisation temperature between 845 and 870°C result in highest range of hardness, 55-57 HRC, but above 870°C, the decrease in hardness occurs due to larger amount of soft retained austenite obtained because the matrix has higher amount of carbon dissolved in it), (ii) after soaking for 1 h per 25 mm of thickness, water or brine quenching may be done for simple shapes, but oil- quenching is preferred to reduce stresses and quench- cracking; complex castings are hot oil-quenched (80-100°C) to avoid cracks.

Tempering is done immediately after quenching to relieve quenching-stresses, at a temperature 425-600°C for 1 h plus 1 h per 25 mm thickness. Tempered hardness, tensile strength, yield strength, % elongation depend on as-quenched value, alloy content, tempering temperature, time at this temperature.

The tempering of S.G. iron in this range of temperatures is a two-stage process; in the first stage, as happens in steels, there occurs precipitation of carbides; in the second stage, there occurs nucleation and growth of small, secondary graphite nodules at the expense of the carbides.

The resulting decrease in hardness is accompanied with reduction in tensile strength as well as fatigue strength. Alloying elements affect the rate of secondary graphitisation. The tensile strength drops from around 1200 MPa to 450 MPa; yield strength from 1100 MPa to 300 MPa, hardness from 57 HRC to 70 HRB; % elongation increases at 540°C from 3% to 17% at 710°C.

Austempering:

Austempering of ductile iron is similar to that for steels, i.e. austenitising the ductile iron, followed by rapid quenching to an intermediate temperature (above its Ms temperature), held there for some time, and then cooled to room temperature; but the end product bainite, as obtained ill steels, is not allowed to be formed in ductile iron as such a microstructure exhibits lower toughness and ductility.

Austempering in ductile iron results in a microstructure of fine dispersion of acicular ferrite in a unique metastable, carbon enriched (~ 2%) austenite (γ_H) matrix alongwith nodules of graphite embedded in it.

Such an austempered ductile iron (ADI) develops optimum strength and ductility markedly better tensile strength to ductility ratio than by any other heat treatment process such as annealing, normalising, hardening and tempering as illustrated in Fig. 15.21, and a comparison is shown in Table 15.12.

Austempering is a process in which the ductile iron casting is heated to get a matrix of austenite, normally in the range of 845-925°C (in a two phase region of austenite and graphite) and soaking for a time approximately 2h to saturate the austenite with carbon, and also the alloying elements to increase the hardenability.

The carbon content is important because:

(i) It drives the austempering reaction,

(ii) The hardenability depends to a significant degree on the carbon content of the matrix.

The carbon content of austenite increases with the increase of austenitising temperature; but it also depends in a complex way on the type, amount and location of the alloying elements (because these segregate) in the matrix. Silicon content has most dominant effect. For a given austenitising temperature, the carbon content of the matrix decreases as the silicon content increases.

Although, higher austenitising temperatures with, increased austenite-carbon-content increases hardenability, but it slows down the isothermal austenitic transformation even to the required austempering products.

Alloying elements such as Mn, Mo have most powerful effect on pearlitic hardenability, but segregate during casting and promote carbide formation, i.e., have detrimental effects on austempering. Ni, Cu increase hardenability mildly but segregate to graphite nodules, and do not form carbides. Normally, combinations of these elements (which act in opposite directions) are used to get the increase in hardenability.

The austenitised ductile iron casting is cooled to an intermediate temperature above M_s temperature, ≤ 240°C, at a rate sufficiently fast to avoid the formation of pearlite, or other mixed structure (especially in thick-sections) as illustrated in Fig. 15.22; which is accomplished by quenching in a hot-oil bath; nitrate/ nitrite salt bath (whose severity can be increased by adding water); fluidized-bed method (for thin, small parts only) or lead bath (for tool-type applications).

The casting is then held at this austempering temperature for a time, 1-4 h, depending on the alloy content and section size. The time at this temperature is quite critical. At this austempering temperature, with 2-3% Si present in ductile iron, the rapid formation of cementite is prevented. Look at Fig. 15.22, stage I. The nucleation and growth of plate-like ferrite (a) occurs, rejecting the carbon, which enters the matrix austenite, i.e., austenite gets enriched with carbon.

The required austempering reaction is:

The unique, metastable, carbon-rich austenite matrix (γ_H) having fine dispersion of acicular ferrite is the optimum required matrix structure. The austempering reaction is allowed to progress to a point, at which the entire matrix has been transformed to metastable product (stage I, Fig. 15.22), and is terminated before stage II. Fig. 15.22 begins, i.e., true bainitic ferrite and carbide are avoided.

This makes time of holding at the austempering temperature to be very significant. The matrix, γ_H is thermally stable, i.e., as its carbon content increases, i.e., as γ_H forms, both M_s and M_f temperatures decrease rapidly as illustrated in Fig. 15.22, so that when the ductile iron casting is cooled from the austempering temperature to room temperature, γ_H does not transform to martensite, but γ_H and acicular ferrite are retained at room temperature.

The austempering temperature is the primary factor controlling the final structure and thus, the hardness and strength of the ADI. The lower temperature, ≈ 260°C, results in a fine acicular dispersion of ferrite, and is thus a high strength, wear-resistant structure (Tensile strength; 1585 MPa; Yield strength- 1380 MPa; elongation = 3%; Impact unnotched = 54 J; hardness: 475 BHN).

The high austempering tempe­rature, ≈ 370°C results in coarser acicular structure, having high impact strength and good ductility (same ductile iron exhibited- Tensile strength- 1035 MPa; Yield strength- 825 MPa; elongation, = 11%; impact – 130 J; hardness- 321 BHN).

The attainment of maximum ductility at any given austempering temperature depends on the time at the temperature; it increases till γ_H formed has become maximum, and then decreases because stage II starts with the formation of equilibrium bainitic product.

ADI has large applications such as crank-shafts, camshafts, suspension components, steering knuckles of automobiles; bodies, crank shafts, drive shafts of pump and compressors; couplings of railways; impellers of sludge-handling equipments; plough-shears in agriculture; locomotive wheels; conveyer rollers and blades, etc.

Surface-Hardening:

S.G. iron responds well to the common surface-hardening methods; by flame, induction or even laser. As the heating-time is short in these methods, complete pearlitic matrix is preferred as it responds instantly with little holding time at the austenitising temperature, normally between 845 to 900°C in order to harden- fully the thin surface layer. A normalised structure of fine pearlite is quick in response and is preferred.

However, a mostly pearlitic S.G. iron, if stress-relieved and then, surface-hardened by water-quenching results in hardness of 58 to 62 HRC. Care must be observed with pearlitic irons to avoid cracking.

Water-quenching gives high hardnesses, but when the fully pearlitic casting of sufficient mass is allowed self-quenching, it gives hardness figure up to 58 HRC. S.G. iron, predominantly ferritic (partly pearlitic) if stress-relieved earlier then the water-quenching after the surface-hardening results in hardness of 50-55 HRC whereas, self-quenching results in 40-45 HRC. A fully-ferritic S.G. iron responds imperfectly as martensite is formed only around the graphite nodules. Such an iron, when water-quenched behind the flame or induction coil, a hardness of 35 to 45 HRC may be obtained.

The heating temperature, time, amount of dissolved carbon, section-size and the rate of quench controls the microstructure, and thus, the hardness. Instead of water, soluble-oil or polymer quenchants may be used to minimize quench-cracking. Flame or induction-hardened S.G. iron castings have been used for rolls for cold working titanium; ring gears for paper-mill drives; crank-shafts; large sprockets for chain-drives.

The success of induction-hardening depends on the amount of pearlite in the matrix as the time of induction-heating is very short. When the as-cast state contains more than 50% pearlite, a temperature of 900-925°C is used; a minimum of 50% pearlite is satisfactory for induction-heating cycles of 3.5s and longer at temperatures of 955-980°C; with less than 50% pearlite, higher temperatures are used but there is risk of retaining austenite, forming ledeburite, and thus damaging the surface.

For castings in the normalised state, a minimum of 50% pearlite is essential for induction-hardening at 955-980°C for 3.5 s. The rest castings show poor response. S.G. iron, which has already been hardened and tempered, has secondary graphite nodules very close to each other to supply sufficient carbon to the matrix quickly during induction-heating. Such S.G. irons show excellent response over a wide-range of microstructures containing upto 95% ferrite.

Case-Hardening- Nitriding: 

Nitriding here also involves the diffusion of nitrogen in the form of ammonia at a temperature of about 550 to 600°C into a thin layer, normally about 0.1 mm deep and results in a surface hardness of about 1100 VPN. The presence of alloying elements, 0.5-1% Al, Ni, Mo, helps to increase the case-hardness.

As usual, the surface layer is white with nitride needles just below it. The usual advantages of nitriding are, very high hardness, increased wear-resistance, antiscuffing properties, enhanced fatigue life, enhanced corrosion-resistance, and typically used for cylinder liners, bearing pins small-shafts.

Nitriding can also be done in liquid cyanide baths using low temperatures of treatment but results in decreased case-depth. Plasma-nitriding is also done of S.G. irons, but is expensive, process-wise as well as equipment-wise.

Remelt-Hardening:

Plasma torches or even lasers can be used to melt a small area on the surface of the S.G. component, which gets rapidly resolidifies due to sell-quenching effect of the component mass. As the resolidified region cools quite rapidly, it has a structure of white cast iron (free of graphite), which exhibits high hardness (~900 VPN) and resultant wear-resistance.

Although a 2 kW laser melts a very small area, around 1.5 mm in diameter, and 0.5 to 2 mm in depth, but by traversing the component surface, the region hardened by this method can be made to useful size. Remelt-hardening can be used in tappets, cams, or other small parts subjected to sliding wear.