Effect of environment and grain size on cyclic deformation and surface hardening of iron

Abstract

Specimens of commercially pure vacuum-melted iron with different grain sizes (0.02 to 0.9 mm diameters) were subjected to push-pull cyclic loading under plastic strain control at an amplitude of 5 × 10-4 in both ultrahigh vacuum (UHV) and oxygen to investigate the effects of grain size and environment on cyclic surface deformation and microcrack initiation. The cyclic stress response curve is not affected by the environment. Fatigue in UHV produces diffuse, wavy but discrete slip lines in which tiny microcracks are initiated. In the presence of oxygen, fatigue produces prominent slip bands along which intense slip band cracks develop. Slip lines developed in UHV, although weaker in intensity, cover more surface area than those developed in oxygen. With increasing grain size, initiation of fatigue cracks along grain boundaries becomes more important. Surface hardening of the specimens and the depth of the surface hardened layer were examined by microhardness measurements after cyclic deformation. In both environments, the surface layer shows an enhanced microhardness over that in the bulk, and the depth of the hardened surface layer increases with grain size. The depth profile of microhardness is not significantly affected by the environment.