Research On Microstructures And The Thermal Fatigue Crack Forming Mechanism Of WC/steel Matrix Composites With Centrifugal Casting

The WC/ Steel matrix composite LGJW20 was made by means of centrifugal casting, taking waste bearing steel GCr15 as raw material and WC particles as hard phase. Microstructures and properties of the composites and evolution of carbides were investigated by using optical microscopy, microhardness tester, X-ray diffraction (XRD), scanning electron microscopy(SEM) and energy dispersive (EDS). Influences of heat treatment on structure and properties of them were analyzed, the thermal fatigue crack forming mechanism was studied emphatically.The results show that the microstructure of the composite is composed by low temperature ledeburite(P+Fe₃C_â…¡), primary cementite(Fe₃C_â… ), secondary cementite(Fe₃C_â…¡), alloy cementite((Fe,M)₃C), other carbides and granular pearlite, a large number of fine WC, W₂C, recrystal W-Fe-C and M₆C,M₇C₃,M₂₃C carbide particles are separated out in the steel matrix, which form many different shapes such as fine particles, reticulation, fishbone, arborization, and conglomeration.After annealing, the microstructure of the composite is composed of spheroidal pearlite and different types of carbides; the structures of which quenched at different temperatures are composed of quenching martensite, residual austenite and various kinds of carbides; the uniformity of those fine carbides distributing in the steel matrix can be enhanced in the process of tempering, and some residual austenites are transformed into tempered martensites. The reciprocity of took place among carbides, WC particles and steel matrix, which are dissolution, diffusion and separation.The period of the thermal fatigue crack incubation of the LGJW20 is short. The crack can germinate on the root of the V-shape notch after several cycles, and it spreads rapidly in the length. The main morphologies of cracks are trapezium, Y shape, beenline and round. The interface between carbides and steel matrix, reticulation, conglomeration and large WC particles are mostly spreading ways. As the thermal cycling number increasing, cracks can be formed and propagated in the steel matrix, but hardly in fishbone carbides and arborization carbides.