Effects of tempering on microstructure, hardness, and fracture toughness of VC/steel surface composite fabricated by high-energy electron beam irradiation

Effects of tempering on microstructure, hardness, and fracture toughness of a VC/carbon steel surface composite fabricated by high-energy electron beam irradiation were investigated in the present study. The mixture of VC powders MgO–CaO flux powders was deposited on a plain carbon steel substrate, and then electron beam was irradiated to fabricate a surface composite layer of 1.8 mm in thickness. Coarse VC carbides were formed along solidification cells, and the matrix inside cells was mostly composed of martensite. When the VC/steel surface composite was tempered, fine VC carbides were precipitated in the tempered martensitic matrix, thereby leading to hardness increase over the untempered composite. Microfracture observation of the surface composite layers revealed that cracks primarily initiated and propagated along intercellular VC carbides, and that they stopped propagating when they met the relatively ductile tempered martensitic matrix. Thus, fracture toughness was determined by intercellular VC carbides working as fracture initiation sites and by matrix property interrupting the crack propagation.