Effect of thermo-mechanical treatment on mechanical properties of high-nitrogen containing Cr-Mn-Ni austenitic stainless steels

To develop a high strength nonmagnetic stainless steel with excellent toughness, thermo-mechanical control process (TMCP) was applied to hot-forged products of Cr-Mn-Ni austenitic stainless steels with various levels of C and N. The yield strength in the as-hot-forged condition increases with increasing nitrogen contents. Carbon addition is detrimental to toughness because it promotes carbide precipitation at the grain boundary during hot forging, causing intergranular fracture at 293 K. In a low carbon and high nitrogen steel, 0.2% yield strength increases and toughness slightly decreases with lowering finish-forging temperature. It was found that a steel with a chemical composition of Fe-0.06C-20Cr-15Mn-4Ni-2Mo-0.64N can be strengthened by TMCP to a level of more than 1 000 MPa in 0.2% yield strength with maintaining its ductility and toughness. This excellent strength-toughness balance can be explained by austenite grain refinement and substructures generated by TMCP as well as strong solution hardening and grain size hardening by nitrogen.