Tempforming in medium-carbon low-alloy steel

Deformation of tempered martensitic structure, i.e., tempforming using a caliber rolling at 500 C was applied to a 0.4%C-2%Si-1%Cr-1%Mo steel to obtain ultrafine elongated grain structure with a strong 〈1 1 0〉//RD fiber deformation texture and then tensile and Charpy impact properties were investigated. An inverse temperature dependence of impact toughness was demonstrated in the tempformed (TF) samples in subzero temperatures, where conventionally quenched and tempered samples showed a ductile-to-brittle transition. The enhancement in the impact toughness of the TF sample was attributed to the occurrence of a delamination, where cracks branched to the longitudinal direction of the impact test bar. The delamination was shown to occur due to the microstructural anisotropy. The dominating factors controlling the delamination were considered to be the transverse grain size, the grain shape and the deformation texture.