Microstructure and fatigue crack growth of a bainitic steel under different heat treatments

doi:10.13251/j.issn.0254-6051.2022.02.027

Abstract

Abstract: Microstructure and fatigue crack growth rate of a bainitic steel under four different heat treatment processes were studied. The results show that the microstructure of the hot rolled tested steel mainly consists of granular bainite with a small amount of lath bainite, martensite and coarse M/A islands. The volume fraction of retained austenite is 16.2%, but with poor stability. The crack can propagate continuously by directly through the coarse M/A islands, and the fatigue crack propagation rate is the fastest. After austenitizing at 900 ℃ and air cooling, the microstructure turns into lath bainite and martensite mainly, still with coarse massive M/A islands, the content of retained austenite is reduced to 12.3%, and the fatigue crack growth rate is slightly reduced. After austenitizing at 900 ℃ plus holding in salt bath at 380 ℃ for 30 min then air cooling, the microstructure is mainly composed of fine and ordered lath bainite, the retained austenite content reduces to 10.2%, which is associated between bainite laths in the form of thin film. Both such lath bainite and thin film retained austenite can lead to the passivation, bifurcation and deflection of the crack tip, and enhance the ability of hindering crack propagation. After tempering at 350 ℃ for 240 min, the main microstructure evolves into martensite and lath bainite, and the content of retained austenite is 14.9%, which is lower than that of hot rolled tested steel. While, after tempering at 450 ℃ for 240 min, the microstructure mainly consists of lath bainite with a small amount of martensite distributed on it, and the volume fraction of retained austenite reduces to 8.6%, which is also associated between bainite laths in thin film form, with a large number of carbides precipitated, and the crack growth rate is the slowest.

Key words: bainitic steel, heat treatment, microstructure, retained austenite, fatigue crack

CLC Number:

TG111

Dong Rui, Chen Lin, Cen Yaodong, Bao Xirong. Microstructure and fatigue crack growth of a bainitic steel under different heat treatments[J]. Heat Treatment of Metals, 2022, 47(2): 153-158.

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