Effect of Aging Temperature on Plastic Flow Behaviour and Toughness of Nickel Free High Nitrogen Austenitic Stainless Steels

In the present study, tensile properties, plastic flow behaviour and impact toughness of Nickel free High Nitrogen Austenitic Stainless Steels (NFHNSS) were evaluated under solution annealed and aged condition (at 700 ^oC, 800 ^oC and 900 ^oC for 14 hours). Plastic flow behaviour was analyzed using Holloman and Ludwigson flow equations. Samples in solution annealed and aged conditions exhibited a flow transition behaviour. Ludwigson flow equation produces best fit for flow transition behaviour. Transmission Electron Microscope (TEM) investigations of NFHNSS samples after tensile test revealed dislocation network and planar arrangement of dislocations adjancent to Grain Boundaries (GB) in the solution annealing and aged conditions respectively. Plastic deformation of NFHNSS occurs by a combination of planar glide and twinning. Impact energy was significantly higher in solution annealed condition than aged condition. Impact energy values decreased with increasing aging temperature. Precipitation and growth as well as morphology of Cr₂N at the GBs reduce the impact energy values significantly. Presence of larger precipitates readily pull out from GB easily than smaller precipitates at low temperatures. Large precipitate, at the GB’s readily pull out at low temperature, compared to slower ones.

Keywords:
High Nitrogen Austenitic Stainless Steels; plastic flow behaviour; dislocations; TEM; Ludwigson flow equation