Mechanical Stability of Retained Austenite in the Nanostructured, Carbide Free Bainitic Steels During Tensile Testing and Cold Rolling Process

Terms of service of the steels with retained austenite in the microstructure, is severely depended on the stability of austenite during the course of deformation. The present work aims to evaluate the mechanical stability of austenite in nanostructured, carbide free bainitic steels during tensile testing and cold rolling process. To achieve the microstructure with retained austenite, the steel with chemical composition of (in wt.%) 0.76C-1.95Si-1.4Mn-0.12Ni-0.24Mo-0.69Al isothermally transformed at temperatures of 200, 250 and 300°C for 6, 16 and 72 hours, respectively, after austenitizing at 950°C for 30 min. Austenite stability was evaluated by means of microhardness testing, x-ray diffraction pattern analysis and color metallography. Further study was carried out by comparing the incremental work hardening exponent obtained from the stress-strain curve. According to the results, austenite transforms into martensite in a progressive manner; blocky austenite at the early, but filmy austenite at the later stage of deformation. Samples transformed at the 300 °C have a good capability of retained austenite to martensite transformation. This is as a result of the higher mechanical stability of the austenite in this microstructure.