Rolling Fatigue Crack Propagation on High Speed Cast Steel

Characteristics of rolling contact fatigue of high speed cast steel were examined using a Nishihara type wear testing machine under lubricant. Crack propagation process was discussed referring to direct observation of cracks. The specimens containig 1.8%C, 5%Cr, 5%Mo, 5%W, 3.8 to 6.9%V and 5%Co were melted and poured into CO2 sand mould (34mm 0 X 100mm). These alloys were composed df MC, M2C and matrix. They were heated at 1173 to 1323K for 7. 2ks and quenched into oil bath. Then they were tempered at 803 to 823K for 7. 2ks. A pair of ring-like same species of specimens with diameter of 30mm and thickness of 8mm was set on the machine and rotated at the speed of 12. Om/s to 13. 3m/s. Applied stress was ranged from 1727 to 217OMPa. Large cracks were observed in the end of fatigue life. These cracks were supposed to occur under the specimen surface at first and propagate to the surface. Cracks generate and propagate along Zo position under specimen surface where shear stress becomes maximum. Cracks also go along the interface between matrix and M2C, pass through M2C and propagate along precipitated carbides in the matrix.