Strain rate dependence of deformation behavior of high-nitrogen austenitic steels

Tensile deformation of high-nitrogen austenitic steel under various strain rate conditions was characterized to investigate the deformation mechanism of the steel at different strain rates. Room-temperature tensile tests of the steel with nitrogen contents of 0.51 wt.% and 0.88 wt.% were performed at different strain rates ranging from 5 × 10−5 s−1 to 5 × 10−1 s−1. Tensile curves of the steel were analyzed and compared, to obtain strain hardening characteristics, yield and tensile strengths as well as the elongation. Microstructures of the tested specimens were examined to understand the deformation behavior of the steel. The high-nitrogen steel with a complete austenite microstructure did not exhibit noticeable flow–stress dependence on strain rates. However, the steel with secondary phases such as nitrides in an austenite matrix showed strong strain rate dependency. The flow stress level increased significantly and the elongation decreased at strain rates higher than 5 × 10−2 s−1. The change of the mechanical property became more significant as the nitrogen content in the steel increased. The deformation behavior of the nitrogen steel was elucidated by planar slip and twinning, which operated differently at different strain rates.