Effects of alloying elements on microstructure and fracture properties of cast high speed steel rolls: Part II. Fracture behavior

This study is concerned with the effects of the microstructural factors such as hard carbides located along solidification cell boundary and the type of the martensite matrix on fracture property of six high speed steel (HSS) rolls manufactured by centrifugal casting method. In situ fracture observation, fracture toughness measurement, and fractographic observation were conducted on these rolls to clarify the microfracture mechanism. The in situ test results indicated that hard carbides located along cell boundary provided easily intercellular fracture sites under low stress intensity factor levels. In the rolls containing a small amount of intercellular carbides and lath-type tempered martensite matrix, fracture occurred along a tortuous transcellular path since carbides were well spaced, thereby leading to the improvement of fracture toughness. In order to obtain better microstructure, hardness, and fracture toughness of the HSS rolls, it is suggested that the optimization of alloying elements is required to achieve the more homogeneous distribution of hard intercellular carbides and the lath-type tempered martensite matrix.