Effects of cold rolling deformation on microstructure, hardness, and creep behavior of high nitrogen austenitic stainless steel

Effects of cold rolling deformation on the microstructure, hardness, and creep behavior of high nitrogen austenitic stainless steel (HNASS) are investigated. Microstructure characterization shows that 70% cold rolling deformation results in significant refinement of the microstructure of this steel, with its average twin thickness reducing from 6.4 mu m to 14 nm. Nanoindentation tests at different strain rates demonstrate that the hardness of the steel with nano-scale twins (nt-HNASS) is about 2 times as high as that of steel with micro-scale twins (mt-HNASS). The hardness of nt-HNASS exhibits a pronounced strain rate dependence with a strain rate sensitivity (m value) of 0.0319, which is far higher than that of mt-HNASS (m = 0.0029). nt-HNASS shows more significant load plateaus and a higher creep rate than mt-HNASS. Analysis reveals that higher hardness and larger m value of nt-HNASS arise from stronger strain hardening role, which is caused by the higher storage rate of dislocations and the interactions between dislocations and high density twins. The more significant load plateaus and higher creep rates of nt-HNASS are due to the rapid relaxation of the dislocation structures generated during loading.