Formation of Nano/Ultrafine Grains in AISI 321 Stainless Steel Using Advanced Thermo-Mechanical Process

Production of nano/ultrafine grains through deformation-induced martensite formation and its reversion to austenite in an AISI 321 stainless steel was studied. The repetitive cold rolling and subsequent annealing were conducted to obtain nanocrystalline structure. Heavy cold rolling (90% reduction) at +20 and −20 °C was carried out to induce the formation of ά-martensite from metastable austenitic material. The process was followed by annealing treatment at 700–900 °C for 0.5–30 min. Effects of process parameters, i.e., “reduction percentage,” “rolling temperature,” “annealing temperature” and “annealing time”, on the microstructural development were considered. Microstructural evolutions were conducted using feritscope, X-ray diffractometer and scanning electron microscope. Hardness of the specimens was measured by Vickers method. Results revealed that the higher thickness reduction and lower rolling temperature provided more martensite volume fraction and further hardness. X-ray diffraction patterns and feritoscopic results indicated that saturated strain (εs) was reduced from 2.3 to 0.9 when temperature declined from +20 to −20 °C. The smallest grain size (about 70 nm) was achieved in the condition of cold rolling at −20 °C followed by annealing at 750 °C for 5 min.