Effect of deformation temperature on the ductile-brittle transition behavior of a modified 9Cr-1Mo steel

A modified 9Cr-1Mo steel received in normalized and tempered condition has been subjected to four different hot-rolling treatments at different rolling temperatures (1050 °C, 1000 °C, 950 °C and 875 °C) applying a constant true strain of ~0.7. The rolled plates were tempered and tested for tensile and Charpy impact properties following standard procedures. Hot-rolling increased the strength by more than 100 MPa, but reduced the ductility of the steel by more than 10%. The ductile-to-brittle transition temperature (DBTT) was found to increase by more than 25 °C after hot-rolling as compared to As-received steel. Among the rolled samples, the plates rolled at 950 °C and 1050 °C showed higher upper shelf energy (comparable to As-received steel), whilst the plate rolled at 1050 °C showed the lowest DBTT. In terms of superior strength-toughness combination rolling at 1050 °C can be considered to be optimum. The results have been analyzed considering the effect of rolling on parameters related to the microstructure and crystallographic texture of the steel, such as, effective grain size, ferrite fraction, fraction of low-angle boundaries, fraction of cleavage planes on the main fracture plane and the fraction of slip planes along the 45° to the main fracture plane.