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% and 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.
As you most likely already know, optimum properties are usually achieved in high-chromium white irons with heat-treated martensitic structures. Cooling rates from austenitizing temperature faster than air-cooling are not recommended due to the propensity to develop cracks (due to high thermal and/or transformational stresses). Over-alloying (Mn, Ni, Cu) will promote retained austenite that may also be problematic. Here are a few well-known heat-treating tips.
1. Subcritical or full annealing can be used to improve the machinablity of castings. Subcritical annealing is performed in the range of 690-705°C (1275-1300°F) for 4-12 hours. The resultant hardness is in the range 400-450 HB, and the microstructure is typically pearlitic. Full annealing in the range 955-1010°C (1750-1850°F) followed by slow cooling to 760°C and holding at this temperature for 10-50 hours (depending on composition) lowers hardness.
2. Each composition of white iron has an optimum austenitizing temperature to achieve maximum hardness. The austenitizing temperature will determine the amount of carbon that remains in solution in the matrix. Class III irons (see part 2) containing 23-28% Cr are typically austenitized in the temperature range 1010-1090°C (1850-2000°F). High austenitizing temperatures increase the stability of the austenite, resulting in higher retained-austenite levels and reduced hardness. Low austenitizing temperatures results in low-carbon martensite reducing both hardness and abrasion resistance.
3. Aggressive fan-assisted air-cooling of castings from austenitizing temperature to below 550-600°C (1020-1110°F) is highly recommended so as to avoid the pearlitic range. Below 500°C (1020°F) still-air or furnace cooling is recommended to minimize stress. Complex and/or heavy section castings are often placed back into the furnace, which is at 550-600°C, and allowed sufficient time to reach uniform temperature within the casting. After temperature is equalized, the castings are either furnace cooled, cooled while covered in sand or still-air cooled to ambient temperature.
4. Tempering in the range of 200-230°C (400-450°F) for 2-4 hours is recommended but not mandatory. Tempering will restore properties such as fracture toughness and relieve residual stresses. Tempering in the range of 480-540°C (900-1000°F) for 8-12 hours acts as a partial anneal. This is recommended for large castings to reduce retained-austenite contents in the martensite matrix and to increase resistance to spalling. These tempering parameters are very sensitive to the time/temperature profile and vary depending on composition and prior thermal history. Excess time or temperature can result in softening and subsequent reduction in properties such as abrasion resistance.