Effects of Thermal Processing History on Microstructure and Low Cycle Fatigue Behavior in a Bainitic TRIP Steel

Recent advances in steel alloying have allowed for the formation of carbide free bainitic microstructures via controlled cooling after austenitization. The low cycle fatigue properties of these materials are being studied, with particular interest in the performance of controlled cooled microstructures to that of microstructures created via austempering. Carbide free bainitic microstructures developed via different thermal processing routes exhibit microconstituents of different scale and morphology. In this study, bainitic microstructures are formed by either a controlled cooling process or an austempering process to evaluate the relationship between microstructure and mechanical properties in a 0.2C - 2Mn - 1.5Si - 0.8Cr steel containing small amounts of Nb, Ti, B, and N. When compared to microstructures produced via austempering, microstructures produced with a controlled cool exhibit an increased variety of transformation products, specifically regarding the size and distribution of martensite-austenite and granular bainitic constituents within a lath-like bainitic ferrite matrix. These carbide free bainitic conditions are tested under strain-controlled uniaxial low cycle fatigue with a total strain amplitude of 0.4 pct and R = -1. The low cycle fatigue results are compared to those of a quenched and tempered 4140 steel.