What are the different annealing processes for wear-resistant castings?

Recrystallization annealing of wear-resistant castings involves one phase transformation and recrystallization during both heating and cooling, hence the term “recrystallization annealing,” often abbreviated simply as “annealing.” This annealing method is quite commonly used for steel. The recrystallization annealing process for steel is as follows: the material is slowly heated to 30–50°C above Ac1 (for austenitic or hypereutectoid steel), holding at that temperature for an appropriate period, and then cooling slowly. This process involves the transformation of pearlite (or, in some cases, prior-eutectoid ferrite or cementite) formed during heating into austenite (the first phase transformation and recrystallization), as well as the reverse transformation and recrystallization occurring during cooling (the second phase transformation and recrystallization), resulting in pearlite (or, in some cases, eutectoid ferrite or cementite) with finer grains, thicker lamellae, and a uniform microstructure. Annealing at temperatures above Ac3 (for hypoeutectoid steels) that results in complete recrystallization is called complete annealing. Annealing at temperatures between Ac1 and Ac3 (for hypoeutectoid steels) or between Ac1 and Acm (for hypereutectoid steels), which results in partial recrystallization, is called incomplete annealing. The former is primarily used for castings made of hypoeutectoid steel to eliminate microstructural defects (such as Widmanstätten structure and banding) and to refine and homogenize the microstructure, thereby improving the plasticity and toughness of the steel component. The latter is primarily used for castings made of medium-carbon, high-carbon, and low-alloy steels. Additionally, spheroidizing annealing of hypereutectoid steel at temperatures between Ac1 and Acm is also considered incomplete annealing.　　Spheroidizing annealing involves heating the casting to a temperature 20–40°C above the point at which the steel begins to form austenite, holding it at that temperature, and then cooling it slowly. During cooling, the lamellar carbides in the pearlite transform into spherical carbides, thereby reducing hardness.