EFFECT OF THERMOMECHANICAL 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 i ncreases with increasing nitrogen contents. Carbon addition is detrime ntal to toughness because it promotes carbide precipitation at the gra in boundary during hot forging, causing intergranular fracture at 293 K. In a low carbon and high nitrogen steel, 0.2% yield strength increa ses and toughness slightly decreases with lowering finish-forging temp erature. 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 ducti lity and toughness. This excellent strength-toughness balance can be e xplained by austenite grain refinement and substructures generated by TMCP as well as strong solution hardening and grain size hardening by nitrogen.