Heat Treating of High Alloy Graphitic 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. In these cast iron alloys, alloy content is well above 4% and, consequently, they cannot be produced by ladle additions to irons of otherwise standard compositions. 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 cast iron alloys discussed in this article are alloyed for increased abrasion resistance, for strength and oxidation resistance at elevated temperatures, and for improved corrosion resistance. They include the high-alloy graphitic irons and the high-alloy white irons.

The heat treatment practices for the following high-alloy graphitic irons are described:

The high-alloy graphitic cast irons have found special use primarily in applications requiring (1) corrosion resistance or (2) strength and oxidation resistance in high-temperature service. Those alloys used in applications requiring corrosion resistance comprise the nickel-alloyed (13 to 36% Ni) gray and ductile irons, and the high-silicon (14.5% Si) gray irons.

The alloyed irons produced for high-temperature service comprise the austenitic, nickel-alloyed gray and nodular irons, the high-silicon (4 to 6% Si) gray and nodular irons and the aluminum-alloyed gray and nodular irons. Two groups of aluminum-alloyed irons are recognized: the 1 to 7% Al irons and the 18 to 25% Al irons.

Austenitic Nickel-Alloyed Graphitic Irons

These nickel-alloyed austenitic irons have found usefulness in applications requiring corrosion resistance, wear resistance, and high-temperature stability and strength. Additional properties of benefit are low thermal expansion coefficients, nonmagnetic properties, and cast iron materials having good toughness at low temperatures. The procedures and temperatures of the heat treatments for these ductile irons with nodular graphite are similar to those for gray (flake-graphite), corrosion-resistant austenitic cast irons.

ASTM Specification A 436 defines eight grades of austenitic gray iron alloys, four of which are designed to be used in elevated-temperature applications and four types are used in applications requiring corrosion resistance.

The ASTM Specification A 439 defines the group of austenitic ductile irons. There are nine alloys listed in the specification. The austenitic ductile iron alloys have similar compositions to the austenitic gray iron alloys but have been treated with magnesium to produce nodular graphite. The ductile iron alloys have high strength and ductility combined with the same desirable properties of the gray iron alloys. They provide resistance to frictional wear, corrosion resistance, strength and oxidation resistance at high temperatures, nonmagnetic characteristics and, in some alloys, low thermal expansivity at ambient temperatures.

Heat Treatment of Austenitic Ductile Irons. Heat treatment of the nickel-alloyed austenitic irons serves to reduce residual stresses and to stabilize the microstructure for increased durability. Heat treatments are similar with the graphite in nodular form (ductile iron) or flake form (gray iron).

Stress Relieving. For most applications, it is recommended that austenitic cast irons be stress relieved at 620 to 675°C (1150 to 1250°F), for 1 h per 25 mm (1 in.) of section, to remove residual stresses resulting from casting or machining, or both. Stress relieving should follow rough machining, particularly for castings that must conform to close dimensional tolerances, that have been extensively welded, or that are to be exposed to high stresses in service. Stress relieving does not affect tensile strength, hardness, or ductility. For large, relatively thin-section castings, mold-cooling to below 315°C (600°F) is recommended rather than stress relief heat treatment.

Spheroidize Annealing. Castings with hardness above 190 HB may be softened by heating to 980 to 1040°C (1800 to 1900°F) for 1/2 to 5 h except those alloys containing 4% or more chromium. Excessive carbides cause this high hardness and may occur in rapidly cooled castings and thin sections. Annealing dissolves or spheroidizes carbides. Although it lowers hardness, spheroidize annealing does not adversely affect strength.

High-Temperature Stabilization. This treatment consists of holding at 760°C (1400°F) for 4 h minimum or at 870°C (1600°F) for 2 h minimum, furnace cooling to 540°C (1000°F), and then cooling in air. This treatment stabilizes the microstructure and minimizes growth and warpage in service. The treatment is designed to reduce carbon levels in the matrix and some growth and distortion often accompanies heat treatment. Thus, it is usually advisable to stabilize castings prior to final machining.

Dimensional Stabilization. This treatment normally is limited to castings that require true dimensional stability, such as those used in precision machinery or scientific instruments. The treatment is not applicable to castings of type I alloys. Other alloys may be dimensionally stabilized by the following treatment:

Solution Treating. Although this treatment is seldom used, quenching from high temperatures is capable of producing higher-than-normal strength levels and slightly higher hardnesses by dissolving some carbon in austenite at elevated temperatures and by preventing precipitation of the carbon by rapid cooling. This treatment consists of heating to 925 to 1010°C (1700 to 1850°F) and quenching in oil or water. Because no metallurgical phase change occurs, the possibility of cracking is lessened.

High-Silicon Irons for High-Temperature Service

Graphitic irons alloyed with from 4 to 6% Si have provided good service, and low cost, in many elevated-temperature applications. These irons, whether gray or nodular, provide good oxidation resistance and stable ferritic matrix structures that will not go through a phase change at temperatures up to 815°C.

The elevated silicon content of these otherwise normal cast iron alloys reduces the rate of oxidation at elevated temperatures, because it promotes the formation of a dense, adherent film at the surface, which consists of iron silicate rather than iron oxide. This layer is much more resistant to oxygen penetration and its effectiveness improves with increasing silicon content.

High-Silicon Nodular Irons. The advent of ductile iron led to the development of high-silicon nodular irons, which currently represent the greatest tonnage of these types of irons being produced. Converting the eutectic flake graphite network to isolated graphite nodules further improved resistance to oxidation and growth. The higher strength and ductility of the nodular iron versions of these alloys qualifies them for more rigorous service.

The high-silicon nodular iron alloys are designed to extend the upper end of the range of service temperatures viable for ferritic nodular irons. These irons are used to temperatures of 900°C. At 5 to 6% Si, oxidation resistance is improved and critical temperature is increased, but the iron can be very brittle at room temperature. For most applications, alloying with 0 to 1% Mo provides adequate strength at elevated temperatures and creep resistance.

The high-silicon gray and nodular irons are predominantly, ferritic as-cast, but the presence of carbide stabilizing elements will result in a certain amount of pearlite and often intercellular carbides. These alloys are inherently more brittle than standard grades of iron and usually have higher levels of internal stress due to lower thermal conductivity and higher elevated-temperature strength. These factors should be taken into account where deciding on heat treatment requirements.

For the high-silicon nodular irons, high-temperature heat treatment is advised in all cases to anneal any pearlite and stabilize the casting against growth in service. A normal graphitizing (full) anneal in the austenitic temperature range is recommended where undesirable amounts of carbide are present.

For the 4 to 5% Si irons this will require heating to at least 900°C (1650°F) for several hours, followed by slow cooling to below 700°C (1300°F). At higher silicon contents (>5%), in which carbides readily break down, and in castings relatively carbide-free, subcritical annealing in the temperature range 720 to 790°C (1325 to 1450°F) for 4 h is effective in ferritizing the matrix. Compared to full annealing, the subcritically annealed material will have somewhat higher strength, but ductility and toughness will be reduced.

High-Silicon Irons for Corrosion Resistance. Irons with high silicon content (14.5% Si) comprise a unique corrosion-resistant ferritic cast iron group. These alloys are widely used in the chemical industry for processing and for transporting highly corrosive liquids. The most common of the high-silicon iron alloys are covered in ASTM Specification A 518M.

Because of the very brittle nature of high-silicon cast iron, castings are usually shaken out only after mold cooling to ambient temperature. However, some casting geometries demand hot shakeout so that the castings can be immediately stress-relieved and furnace cooled to prevent cracking.

Castings are stress relieved by heating in the range of 870 to 900°C (1600 to 1650°F) followed by slow cooling to ambient temperatures to minimize the likelihood of cracking. Heat treatments have no significant effect on corrosion resistance.