Can you please shed some light on the metallurgical precautions during casting and heat treatment of high chrome iron, ASTM-A532, Class III, type A (17, 23, 27% Cr)? We are softening for machining (300+ BHN) and hardening (600+ BHN) for better wear resistance during usage in a slurry-pumping (coal-ash) application. Problems we face are cracking in a few impeller castings before and after heat treatment – most typically an S-type crack profile diametrically around the hub.
The white cast irons to which you refer are a specific group of materials classified as a high-alloy type. These materials are primarily used for abrasion-resistant applications such as pumps.
Specification ASTM A532 covers the composition and hardness of the abrasion-resistant white-iron grades. Many castings are ordered according to these specifications, but a large number of castings are produced with composition modifications for specific applications.
The application uses for this class of materials requires a combination of hardness and corrosion resistance. Thus, the presence of carbides in the microstructure provides the hardness needed for applications where crushing and grinding are involved, while the chromium content enhances corrosion-resistant properties. In these applications, a balance is often desirable between the resistance to abrasion and the toughness needed to withstand repeated impact.
While low-alloy white-iron castings, which have an alloy content below 4%, develop hardness in the range of 350-550 HB, the high-alloy irons range in hardness from 450-800 HB.
ASTM A532 Class III white irons have 25-28% Cr with up to 1.5% Mo. These materials can also be alloyed with nickel and copper up to 1%. Class III is used when resistance to corrosion is needed. These 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.
Pump applications also require resistance to corrosion, and in these applications it is not uncommon to see high chromium contents (26-28% Cr) coupled with low carbon contents (1.6-2.0% C). The addition of up to 2% Mo is recommended for improving resistance to chloride-containing environments. Chromium produces a chromium-rich oxide film, providing resistance to scaling at temperatures up to about 1040°C (1900°F).
These high-chromium irons (with carbon contents in the 1–2% range) fall into one of three categories, based on microstructure: martensitic irons (12-28% Cr), ferritic irons (30-34% Cr) or austenitic irons (15-30% Cr plus 10-15% Ni).
Optimum performance is usually achieved by producing a martensitic structure. One of the goals of heat treating (see part 1) is to produce a pearlite-free microstructure. Cooling rates faster than air quenching should not be used due to the tendency of the casting to develop cracks (due to high thermal and/or transformation stresses). The material must have a sufficient alloy content to promote air cooling, but avoid over-alloying (manganese, nickel, and copper) since it will promote retained austenite, reducing resistance to abrasion and spalling.