Heat Treating of High-Alloy White Irons

High-alloy cast irons are an important group of materials whose production should be considered separately from that of the ordinary types of cast irons. The producing foundries usually have the equipment needed to handle the heat treatment and other thermal processing unique to the production of these alloys.

The high-alloy white irons are primarily used for abrasion-resistant applications and are readily cast in the shapes needed in machinery used for crushing, grinding, and general handling of abrasive materials. The large volume of eutectic carbides in their microstructures provides the high hardness needed for crushing and grinding other materials. The metallic matrix supporting the carbide phase in these irons can be adjusted by alloy content and heat treatment to develop the proper balance between resistance to abrasion and the toughness needed to withstand repeated impact.

All high-alloy white irons contain chromium to prevent formation of graphite on solidification and to ensure the stability of the carbide phase. Most also contain nickel, molybdenum, copper, or combinations of these alloying elements to prevent the formation of pearlite in the microstructure. While low-alloyed pearlitic white iron castings develop hardness in the range 350 to 550 HB, the high-alloyed white irons range from 450 to 800 HB.

ASTM Specification A 532 covers the composition and hardness of white iron grades used for abrasion-resistant applications. Many castings are ordered according to these specifications: however, a large number of castings are produced with modifications to composition for specific applications. It is most desirable that the designer, metallurgist, and foundry-man work together to specify the composition, heat treatment, and foundry practice to develop the most suitable alloy and casting design for a specific application.

The high-alloy white cast irons fall into three major groups:

Nickel-Chromium White Irons

One of the oldest groups of high-alloy irons of industrial importance, the Ni-Cr white irons, or Ni-Hard irons, have been produced for more than 50 years and are very cost-effective materials for crushing and grinding.

In these martensitic white irons, nickel is the primary alloying element because at levels of 3 to 5% it is effective in suppressing the transformation of the austenite matrix to pearlite, and thus ensuring that a hard, martensitic structure will develop on cooling in the mold. Chromium is included in these alloys, at levels from 1.4 to 4% to ensure that the irons will solidify with carbides to counteract the graphitizing effect of nickel. The optimum composition of the Ni-Cr white iron alloy depends on the properties required for the service conditions and the dimensions and weight of the casting. Abrasion resistance is generally a function of the bulk hardness and the volume of carbide in Cr-Mo iron.

Carbon is varied according to properties needed for the intended service. Carbon contents in the range of 3.2 to 3.6% are prescribed when maximum abrasion resistance is desired. Where impact loading is present, carbon content should be held in the range of 2.7 to 3.2%.

Silicon is needed for two reasons. A minimum amount of silicon is necessary to improve fluidity and produce a fluid slag. But of equal importance is its effect on as-cast hardness. Increased levels of silicon, in the range of 1 to 1.5%, have been found to increase the amount of martensite and the resulting hardness. Late additions of ferrosilicon have been reported to increase toughness. Note that higher silicon contents can promote pearlite and may increase the nickel requirement.

Manganese is usually held to 0.8% max. While it provides increased hardenability to avoid pearlite formation, it is also a potent austenite stabilizer, more so than nickel, and will promote increased amounts of retained austenite and lower as-cast hardness. For this reason higher manganese levels are undesirable. In considering the nickel content required to avoid pearlite in a given casting, the level of manganese present should be a factor.

Copper increases hardenability and the retention of austenite and, therefore, must be controlled for the same reason manganese is limited. Copper should be treated as a nickel substitute and, when properly included in the calculation of the amount of nickel required to inhibit pearlite in a given casting, it reduces the nickel requirement. Molybdenum is a potent hardenability agent in these alloys and is used in heavy section castings to augment hardenability and inhibit pearlite.

Heat Treatment or Nickel-Chromium White Irons. Nickel-chromium white iron castings are given a stress-relief heat treatment because, properly made, they have a martensitic matrix structure, as-cast. Tempering is performed between 205 to 260°C (400 to 450°F) for at least 4 h. This tempers the martensite, relieves some of the transformation stresses, and increases the strength and impact toughness by 50 to 80%. Some additional martensite may form on cooling from the tempering temperature. This heat treatment does not reduce hardness or abrasion resistance.

In the heat treatment of any white cast iron, care must be taken to avoid cracking by thermal shock; never place the castings in a hot furnace or otherwise subject them to rapid heating or cooling. The risk of cracking increases with the complexity of the casting shape and section thickness.

An austenitizing heat treatment usually comprised heating at temperatures between 750 and 790°C (1380 and 1450°F) with a soak time of 8 h. Air or furnace cooling, not over 30°C/h, was conducted followed by a tempering/stress-relief heat treatment. Refrigeration heat treatment is the more commonly practiced remedy for low hardness today.

High-Chromium White Irons

The oldest high-alloy white irons produced commercially were the high-chromium (28% Cr) white irons. The high-chromium white irons have excellent abrasion resistance and are used effectively in slurry pumps, brick molds, coal-grinding mills, rolling mill rolls, shot blasting equipment, and components for quarrying, hard-rock mining and milling. In some applications they must also be able to withstand heavy impact loading.

These alloyed white irons are recognized as providing the best combination of toughness and abrasion resistance attainable among the white cast irons. Through variations in composition and heat treatment these properties can be adjusted to meet the needs of most abrasive applications.

Special High-Chromium Iron Alloys for Corrosion Resistance. Alloys with improved resistance to corrosion, for applications such as pumps handling, are produced with high chromium contents (26 to 28% Cr) and low carbon contents (1.6 to 2.0% C). These high-chromium, low-carbon irons will provide the maximum chromium content in the matrix. Addition of 2% Mo is recommended for improving resistance to chloride-containing environments. Chromium causes the formation of an adherent, complex, chromium-rich oxide film providing resistance to scaling at temperatures up to 1040°C (1900°F).

The high-chromium irons designated for use at elevated temperatures fall into one of three categories, depending upon the matrix structure:

Carbon contents of these alloys range from 1 to 2%.

Optimum performance is usually achieved with heat treated martensitic structures. As described in the previous section, alloying must be sufficient to ensure that a pearlite-free microstructure is obtained in heat treatment. Of necessity, the heat treatment requires an air quench from the austenitizing temperature. Faster cooling rates should not be used, because the casting can develop cracks due to high thermal and/or transformation stresses. Thus the alloy must have sufficient hardenability to allow air hardening. Over-alloying with manganese, nickel, and copper will promote retained austenite, which detracts from resistance to abrasion and spalling.

Austenitization. There is an optimum austenitizing temperature to achieve maximum hardness, which varies for each composition. The austenitizing temperature determines the amount of carbon that remains in solution in the austenite matrix. Too high a temperature increases the stability of the austenite, and the higher retained austenite content reduces hardness. Low temperatures result in low-carbon martensite reducing both hardness and abrasion resistance. Class II irons containing 12 to 20% Cr are austenitized in the temperature range 950 to 1010°C (1750 to 1850°F). Class III irons containing 23 to 28% Cr are austenitized in the temperature range 1010 to 1090°C (1850 to 2000°F).

Quenching. Air quenching (vigorous fan cooling) the castings from the austenitizing temperature to below the pearlite temperature range (that is, between 550 and 600°C, or 1020 and 1110°F) is highly recommended. The subsequent cooling rate should be substantially reduced to minimize stresses; still-air or even furnace cooling to ambient is common. Complex and heavy section castings are often placed back into the furnace, which is at 550 to 600°C, and allowed sufficient time to reach uniform temperature within the casting. After temperature is equalized, the castings are either furnace or still-air cooled to ambient temperature.

Tempering. Castings can be put into service in the hardened (as cooled) condition without further tempering or subcritical heat treatments; however, tempering in the range of 200 to 230°C (400 to 450°F) for 2 to 4 h is recommended to restore some toughness in the martensitic matrix and to further relieve residual stresses.

Sub critical Heat Treatment. Sub critical heat treatment (tempering) is sometimes performed, particularly in large heat-treated martensitic castings, to reduce retained austenite contents and increase resistance to spalling. The tempering parameters necessary to eliminate retained austenite are very sensitive to time and temperature and vary depending on the castings composition and prior thermal history. Typical tempering temperatures range from 480 to 540°C (900 to 1000°F) and times range from 8 to 12 h. Excess time or temperature results in softening and a drastic reduction in abrasion resistance.

Annealing. Castings can be annealed to make them more machinable, either by sub-critical annealing or a full anneal. Subcritical annealing is accomplished by pearlitizing, via soaking in the narrow range between 690 and 705°C for from 4 to 12 h, which will produce hardness in the range 400 to 450 HB. Lower hardness can often be achieved with full annealing, whereby castings are heated in the range 955 to 1010°C followed by slow cooling to 760°C and holding at this temperature for 10 to 50 h depending on composition.

Stress-Relieving. Very little information is available on the amount of stress relief that occurs with tempering. The predominant stresses present in heat-treated castings develop as a result of the volume change accompanying austenite to martensite transformation. Low-temperature tempering, in the range of 200 to 230°C, is particularly desirable because a substantial improvement (20%) in fracture toughness occurs when tempering the martens lie phase. Tempering at temperatures sufficient to significantly relieve stresses, that is, above 540°C, will substantially reduce abrasion resistance.