Effect of ferrite-austenite morphology and orientation relationship on bainite transformation in low-alloy TRIP steels

Abstract

To reveal the effects of large amounts of ferrite on bainite transformation behavior, the morphological and orientation relationship between ferrite (α) and austenite (γ) reversed from different initial microstructures during intercritical annealing, as well as their effects on subsequent bainite transformation, were investigated by scanning electron microscopy, electron-backscattered diffraction, and dilatometry analysis. The austenite and ferrite reversed from quenched martensite exhibited an alternately arranged lamellar structure. The austenite maintained a Kurdjumov–Sachs (K-S) orientation relationship with ferrite. In the subsequent austempering, bainitic transformation exhibited a strong variant selection phenomenon. Austenite reversed from cold-deformed martensite was dispersed around ferrite, and the orientation relationship between austenite and ferrite was irrational. During austempering, the bainite nucleation rate at K–S-oriented α/γ interfaces was much higher than that at irrational-oriented α/γ interfaces. First, the alternately arranged ferrite and austenite lamellae increased the total length of α/γ interfaces per unit area, i.e., α/γ interfacial density, for bainite nucleation. Second, the activation energy for bainite nucleation at the K–S-oriented α/γ interface was lower. The K–S-oriented α/γ interfaces facilitated bainite nucleation accompanied by strong variant selection, thereby accelerating the kinetics of bainite transformation.