Microstructure evolution and enhanced mechanical properties in SUS316LN steel processed by high pressure torsion at room temperature

Nowadays, it is of interest to enhance the strength of metals for light-weight design of nuclear reactor, so the objective of the present study is to improve the mechanical properties of SUS316LN steel through grain refinement. The nanostructured SUS316LN steels were produced by high pressure torsion (HPT) at room temperature. The variations of microstructure and strain-induced martensite transformation during HPT were investigated. The homogeneous nanostructured SUS316LN steel with an average grain size of approximately 40 nm was achieved by HPT for 5 revolutions under a pressure of 5 GPa, which demonstrated extremely high tensile strength (1828 MPa) and high hardness (530 HV). The interstitial nitrogen atoms can reduce the stacking fault energy of the material and also play an important role in interaction with mobile dislocations during HPT. Therefore, compared to SUS316L steel, the formation of the nanograined structure was much faster and the strain-induced martensite transformation was restrained in the HPT-produced SUS316LN steel.