Effect Of Heat Treatment On Microstructure And Properties Of WC/steel Composites By Centrifugal Casting

The WC/steel composites with 20wt% WC particles in GCr15 matrix were produced by centrifugal casting. The microstructure and phases transformation of the material was characterized by optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and Energy dispersive spectroscopy (EDS). The effect of heat treatment on the microstructure of the composite was observed. This paper mainly studied on the thermal fatigue behavior of the composite: propagation mechanism of thermal fatigue cracks, effect of heat treatment on thermal fatigue properties, and the changes in microstructures and properties of the composite after thermal cycles.Because of the low content (20 wt%) of WC particles added into LGJW20, WC particles mostly dissolved into the molten steel. W element mainly precipitated during eutectic reaction, and the precipitated phases were mainly composed of Fe3W3C. The matrix is a kind of nodular troostite, where there were a large number of carbide particles precipitated. Three kinds of multiple carbides: fishbone-like eutectic carbide, network secondary carbide and massive carbide, precipitated in the microstructure of LGJW20 during casting process. Their formation mechanism, composition, and physical properties are very different.Heat treatments improved the microstructure and the properties of LGJW20 effectively. During isothermal spheroidizing process, microstructure defects were eliminated; carbides dissolved and precipitated; network secondary carbide was broken and spheroidized; and the microstructure became more homogenous. During the quenching process, the carbides dissolved seriously; matrix transformed into quenched martensite. The quenched hardness reached maximum at the temperature of 950℃. After tempered at temperature of 200℃, segregation of carbon, decomposition of martensite, transformation of residual austenite and precipitation and spherodization of carbide occurred in the matrix, where there were lots of carbides particles precipitating.During thermal cycle, stress concentration occurred on the interface between the carbide and matrix, which caused the micropore structure in LGJW20. The micropores caused the initiation of thermal fatigue cracks. They propagated along carbides in form of main crack. Heat treatment can improved the microstructure and reduced the hardness difference between carbides and matrix, so the thermal fatigue properties were improved. The results show that the composite owns the best thermal fatigue resistance by quenched at 980℃and tempered at 200℃. There a kind of linear relation existing between the propagation rate of thermal fatigue crack and the thermal cycle index. After times of thermal cycles the hardness of the composite reduced because of the cumulative tempering transformation.