Nickel hard castings are mainly made of martensite in the as-cast state, so it is better to produce nickel hard cast iron by metal mold casting. This is mainly due to the rapid cooling rate of the casting in the metal mold, which can effectively restrain the pearlite transformation of austenite, avoid graphitization and ensure the white cast iron. Due to the high cooling rate, the primary crystalline structure is fine, The mechanical properties are much higher than those of sand castings.
Nickel hard castings with simple shapes, especially thick-walled hard cast iron, such as rollers, roll heads, wet ball mills, linings, etc., should be cast by metal as far as possible. Nickel cast iron is heated to 450 ℃ for 4 h, then air-cooled or furnace cooled to room temperature. Double heat treatment can also be used, that is, nickel cast iron can be heated to 450, heat preservation for 4 hours, then cold to 275 ℃ and heat preservation for 4 ~ 16 hours.
To a certain extent, nickel hard castings can reduce the carbon content of retained austenite in the microstructure of nickel hard cast iron by adopting double heat treatment process, so that it can be transformed into martensite during the subsequent cooling process. The purpose of heat preservation at 275 ℃ is to temper the new martensite and transform the austenite into martensite. The impact fatigue life of nickel cast iron with double heat treatment is higher than that of simple heat treatment.