Heat Treating of Gray Irons: Part Two

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The heat treatment of gray irons can considerably alter the matrix microstructure with little or no effect on the size and shape of the graphite achieved during casting.
The matrix microstructures resulting from heat treatment can vary from ferrite-pearlite to tempered martensite. However, even though gray iron can be hardened by quenching from elevated temperatures, heat treatment is not ordinarily used commercially to increase the overall strength of gray iron castings because the strength of the as-cast metal can be increased at less cost by reducing the silicon and total carbon contents or by adding alloying elements.

Gray irons are a group of cast irons that form flake graphite during solidification, in contrast to the spheroidal graphite morphology of ductile irons. The flake graphite in gray irons is dispersed in a matrix with a microstructure that is determined by composition and heat treatment.

The heat treatment of gray irons can considerably alter the matrix microstructure with little or no effect on the size and shape of the graphite achieved during casting. The matrix microstructures resulting from heat treatment can vary from ferrite-pearlite to tempered martensite. However, even though gray iron can be hardened by quenching from elevated temperatures, heat treatment is not ordinarily used commercially to increase the overall strength of gray iron castings because the strength of the as-cast metal can be increased at less cost by reducing the silicon and total carbon contents or by adding alloying elements.

Hardening and Tempering

Gray irons are hardened and tempered to improve their mechanical properties, particularly strength and wear resistance. After being hardened and tempered, these irons usually exhibit wear resistance approximately five times greater than that of pearlitic gray irons.

Furnace or salt bath hardening can be applied to a wider variety of gray irons than can either flame or induction hardening. In flame and induction hardening, a relatively large content of combined carbon is required because of the extremely short period available for the solution of carbon in austenite. In furnace or salt bath hardening, however, a casting can be held at a temperature above the transformation range for as long as is necessary; even an iron initially containing no combined carbon can be hardened.

Unalloyed gray iron of low combined carbon content must be austenitized for a longer time to saturate austenite with carbon. With increased time, more carbon is dissolved in austenite and hardness after quenching is increased.

Because of its higher silicon content, an unalloyed gray iron with a combined carbon content of 0.60% exhibits higher hardenability than a carbon steel with the same carbon content. However, because of the effect of silicon in reducing the solubility of carbon in austenite, unalloyed irons with higher silicon contents necessarily require higher austenitizing temperatures to attain the same hardness.

Manganese increases hardenability; approximately 1.50% Mn was found to be sufficient for through hardening a 38 mm section in oil or for through hardening a 64 mm section in water.

Manganese, nickel, copper, and molybdenum are the recognized elements for increasing the hardenability of gray iron. Although chromium, by itself, does not influence the hardenability of gray iron, its contribution to carbide stabilization is important, particularly in flame hardening.

Austenitizing. In hardening gray iron, the casting is heated to a temperature high enough to promote the formation of austenite, held at that temperature until the desired amount of carbon has been dissolved, and then quenched at a suitable rate.

The temperature to which a casting must be heated is determined by the transformation range of the particular gray iron of which it is made. The transformation range can extend more than 55°C above the At (transformation-start) temperature. A formula for determining the approximate A, transformation temperature of unalloyed gray iron is:

A (°C) = 730 + 28.0 (% Si) - 25.0 (% Mn) Chromium raises the transformation range of gray iron. In high-nickel, high-silicon irons, for example, each percent of chromium raises the transformation range by about 10 to 15°C. Nickel, on the other hand, lowers the critical range. In a gray iron containing from 4 to 5% Ni, the upper limit of the transformation range is about 710°C.

Castings should be treated through the lower temperature range slowly, in order to avoid cracking. Above a range of 595 to 650°C, which is above the stress-relieving range, heating may be as rapid as desired. In fact, time may be saved by heating the casting slowly to about 650°C in one furnace and then transferring it to a second furnace and bringing it rapidly up to the austenitizing temperature.

Quenching. Molten salt and oil are the quenching media used most frequently for gray iron. Water is not generally a satisfactory quenching medium for furnace-heated gray iron; it extracts heat so rapidly that distortion and cracking are likely in all parts except small ones of simple design. Recently developed water-soluble polymer quenches can provide the convenience of water quenching, along with lower cooling rates, which can minimize thermal shock.

The least severe quenching medium is air. Unalloyed or low-alloy gray iron castings usually cannot be air quenched because the cooling rate is not sufficiently high to form martensite. However, for irons of high alloy content, forced-air quenching is frequently the most desirable cooling method.

Tempering. After quenching, castings are usually tempered at temperatures well below the transformation range for about 1h per inch of thickest section. As the quenched iron is tempered, its hardness decreases, whereas it usually gains in strength and toughness.

Austempering

In austempering, the microstructural end product of the gray iron matrix formed below the pearlite range but above the martensite range is an acicular or bainitic fer-rite, plus varying amounts of austenite depending on the transformation temperature. The iron is quenched from a temperature above the transformation range in a hot quenching bath and is maintained in the bath at constant temperature until the austempering transformation is complete.

In all hot quenching processes, the temperatures to which castings must be heated for austenitizing and the required holding times at temperature prior to quenching in the hot bath correspond to the temperatures and times used in conventional hardening, that is, temperatures between 840 and 900°C (1550 and 1650°F). The holding time depends on the size and chemical composition of the casting.

Gray iron is usually quenched in salt, oil, or lead baths at 230 to 425°C for austempering. When high hardness and wear resistance are the ultimate aim of this treatment, the temperature of the quench bath is usually held between 230 and 290°C. The effect of iron composition on the holding time may be considerable. Alloy additions, such as nickel, chromium, and molybdenum, increase the time required for transformation.

Martempering

Martempering is used to produce martensite without developing the high stresses that usually accompany its formation. It is similar to conventional hardening except that distortion is minimized. Nevertheless, the characteristic brittleness of the martensite remains in a gray iron casting after martempering, and martempered castings are almost always tempered. The casting is quenched from above the transformation range in a salt, oil, or lead bath: held in the bath at a temperature slightly above the range at which martensite forms (200 to 260°C or 400 to 500°F. for unalloyed irons) only until the casting has reached the bath temperature; and then cooled to room temperature.

If a wholly martensitic structure is desired, the casting must be held in the hot quench bath only long enough to permit ii to reach the temperature of the bath. Thus, the size and shape of the casting dictate the duration of martempering.

Flame Hardening

Flame hardening is the method of surface hardening most commonly to gray iron. After flame hardening, a gray iron casting consists of a hard, wear-resistant outer layer of martensite and a core of softer gray iron, which during treatment does not reach the At transformation temperature.

Both unalloyed and alloyed gray irons can be successfully flame hardened. However, some compositions yield much better results than do others. One of the most important aspects of composition is the combined carbon content, which should be in the range of 0.50 to 0.70%, although irons with as little as 0.40% combined carbon can be flame hardened. In general, flame hardening is not recommended for irons that contain more than 0.80% combined carbon because such irons (mottled or white irons) may crack in surface hardening.

Effects of Alloying Elements. In general, alloyed gray irons can be flame hardened with greater ease than can unalloyed irons, partly because alloyed gray irons have increased hardenability. Final hardness also may be increased by alloying additions. The maximum hardness obtainable by flame hardening an unalloyed gray iron containing approximately 3% total carbon, 1.7% Si, and 0.60 to 0.80% Mn ranges from 400 to 500 HB. This is because the Brinell hardness value for gray iron is an average of the hardness of the matrix and that of the relatively soft graphite flakes. Actually, the matrix hardness on which wear resistance depends approximates 600 HB. With the addition of 2.5% Ni and 0.5% Cr, an average surface hardness of 550 HB can be obtained. The same result has been achieved using 1.0 to 1.5% Ni and 0.25% Mo.

Stress Relieving. Whenever practicable or economically feasible, flame-hardened castings should be stress relieved at 150 to 200°C.

Induction Hardening

Gray iron castings can be surface hardened by the induction method when the number of castings to be processed is large enough to warrant the relatively high equipment cost and the need for special induction coils.

Considerable variation in the hardness of the cast irons may be expected because of a variation in the combined carbon content. A minimum combined carbon content of 0.40 to 0.50% C is recommended for cast iron to be hardened by induction, with the short heating cycles that are characteristic of this process. Heating castings with lower combined carbon content to high hardening temperatures for relatively long periods of time may dissolve some free graphite, but such a procedure is likely to coarsen the grain.

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