Abstract
Effects of Mn and Cr addition on tensile properties and low temperature toughness have been investigated with extra hevavy gauge pressure vessel steel plates that have the chemical composition of 0.12% C-1.4%Mn-0.3%Mo-0.75% Ni, then the results have been discussed with metallurgical view points.
The Cr addition, not less effectively than the increase of Mn content, increases the tensile strength at both room temperature and 350°C, with less deterioration of Charpy absored energy. With the cooling rates that are equivalent to those at quenching of such extra heavy gauge plates as 150mm thick, the Cr addition works to suppress the elevation of starting temperatures of bainitic transformation (Bs). This suppression of elevation of Bs could be the reason for the improvement of tensile strength with Cr addition. The Cr addition also changes the morphology of cementite in the region surrounded by acicular ferrite (carbide free bainite) form lamellar one to small sized granular one. This morphological change of cementite could lighten the deterioration of the Charpy absorbed energy through lowering the possibilities of crack initiation.
