Thermal stability of ultrafine-grained austenitic stainless steels

Ultrafine-grained 316 and 304 austenitic stainless steel samples have been produced by high pressure torsion. Their microstructure, after deformation and annealing at a temperature in the 350–900 °C range, has been characterized using several techniques (transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy). The average grain size in the ultrafine-grained 316 is about 40 nm while it is larger in the ultrafine-grained 304 due to a smaller deformation. Results show the formation of α′-martensite during deformation in both steels while ɛ-martensite is formed only in the 304 steel.Annealing at 350 °C induces the decrease of α′-martensite content in the 316 steel. The trend is different in the 304 steel, in which the α′-martensite content increases.Recrystallization of grains is observed from 700 °C. Mössbauer spectroscopy shows a reduction of the level of solute atoms in α′-martensite during annealing.