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

Abstract In this investigation, the low cycle fatigue properties of carbide free bainitic microstructures are being studied, with particular interest in comparing 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. Bainitic microstructures were formed by either a controlled cooling process or an isothermal 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, which are composed of lath-like bainitic ferrite, microstructures produced with a controlled cool exhibit more variation in the size and distribution of martensiteaustenite constituent in the lath-like or granular bainitic ferrite matrix. These carbide free bainitic conditions were 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. Results indicate that carbide free bainitic microstructures outperform quenched and tempered microstructures of the same hardness in low cycle fatigue, and increased variability in the scale and morphology of CFB microstructure results in less variation in fatigue lives.