Effect of microstructure on tensile fracture of quenched and lightly tempered high carbon and low chromium steel containing undissolved spheroidal carbides

High carbon and low alloy chromium steels have been studied to determine the effect of the microstructure on tensile fracture of quenched and lightly tempered low alloy steel containing undissolved spheroidal carbides. The steels with a volume fraction of 8 and 13 vol % and containing particle sizes from 0.32 to 1.14m were investigated. In the case of steel containing 8 vol % undissolved carbides, many twinned plates were observed in the matrix martensite and microtwinning was observed in the carbide/matrix interfaces. The steel failed in a macroscopically brittle manner and the true fracture stress of the steel was independent of the carbide particle size, while the data exhibited a large scatter. In the case of steel containing 13 vol % of undissolved carbides, the matrix martensite consisted predominantly of lath martensite and a well-defined forest of dislocations was observed around the carbides. Failure of the steel occurred in the relatively early stage of plastic deformation and the true fracture stress of the steel increased with decreasing carbide particle size.