Effect of retained austenite stability on cyclic deformation behavior of low‐alloy transformation‐induced plasticity steels

Abstract

The retained austenite (RA) characteristics of Al‐containing TRIP700 steels have been manipulated using varying bainitic isothermal transformation (BIT) processing. The microstructural evolution was investigated using optical microscopy and quantitative image analysis, while the amount of transformed RA was evaluated with the saturation magnetization (SM) technique. Cyclic behavior is found to depend on the applied strain amplitude and stability of RA. At strain amplitudes with comparable elastic and plastic strain components, cyclic softening prevails, facilitated by more stable RA microstructures and Low Cycle Fatigue (LCF) performance benefits from a lower RA stability, which controls the amount of cyclic softening rate. With increasing plastic strain component, a transition to cyclic hardening is observed, and the transition strain increases with increasing RA stability. LCF performance deteriorates because of excessive cyclic strain hardening promoting martensitic transformation. The effect is accompanied by a transition from mixed dimple/cleavage to cleavage‐type fracture characteristics.

Studies of Bainitic Steel for Rail Applications Based on Carbide-Free, Low-Alloy Steel

“…There are also literature reports indicating that the cyclic strengthening or weakening phenomena in steels are not only related to the strain-induced transformation of austenite to martensite. [23,24] Fatigue life is also dependent on the volume fraction of RA. Hu et al [25] described that a high level of retained austenite increases fatigue life due to retarding of crack propagation, resulting from the strain-induced transformation.…”

TRIP Steels: A Multiscale Computational Simulation and Experimental Study of Heat Treatment and Mechanical Behavior

“…A high strain hardening rate at early stages of plastic straining is observed. The stable RA microstructure results in a gradual austenite transformation [76] and progressive hardening until necking at a uniform elongation of 25.4% [77,78]. According to the simulation results of the heat treatment presented in Section 2.3, the initial volume fractions for the individual phases are c (1) = 0.507, c (2) = 0.327, c (a) = 0.166, and c (m) = 0.0.…”