Nano-grain evolution in austenitic stainless steel during multi-directional forging

Nano-grain evolution in an austenitic stainless steel (SUS 316) during multidirectional forging (MDF) was investigated at temperatures of 77 K and 300 K. The flow stress during MDF and the room-temperature hardness increased significantly with increasing cumulative strain. The initial grains were subdivided by mechanical twinning and martensitic transformation. The formation of packets, which are composed of lamellar-structured mechanical twins with a spacing of 10–300 nm, enhanced grain fragmentation. The packet size ranged from 40 nm to 100 nm depending on the MDF temperature and the cumulative strain. Tensile tests at ambient temperatures revealed a maximum proof strength of 2.1 GPa. While the proof strength increased with cumulative strain, the plastic strain at fracture was approximately 10% independent of the cumulative strain over ∑Δɛ = 2.4.