不同热处理条件下贝氏体钢的微观组织和疲劳裂纹扩展

以贝氏体钢为研究对象,设计了4种热处理工艺,研究了不同热处理工艺下试验钢的显微组织及疲劳裂纹扩展速率。结果表明,热轧态试验钢的微观组织以粒状贝氏体为主,其上分布有少量的板条贝氏体、马氏体和粗大块状M/A岛,残留奥氏体的体积分数为16.2%,但稳定性较差,裂纹能够直接穿过粗大的块状M/A岛继续扩展,疲劳裂纹扩展速率最快。经900 ℃奥氏体化+空冷后,显微组织以板条贝氏体和马氏体为主,M/A岛仍为粗大的块状,残留奥氏体含量减少至12.3%,疲劳裂纹扩展速率略有降低。经900 ℃奥氏体化+380 ℃盐浴等温30 min +空冷后,显微组织以细密、有序的板条贝氏体为主,残留奥氏体含量减少至10.2%,以薄膜状伴生在板条贝氏体间,板条状贝氏体及板条间的残留奥氏体薄膜会使裂纹端钝化、分叉、偏折,阻碍裂纹扩展的能力增强;经350 ℃回火240 min后,显微组织以马氏体和板条贝氏体为主,残留奥氏体含量比热轧态略微降低,为14.9%;而经450 ℃回火240 min后,显微组织以板条状贝氏体为主,其上分布有少量的马氏体,残留奥氏体体积分数减少至8.6%,也以薄膜状伴生在贝氏体板条间,同时有大量的碳化物析出,裂纹扩展速率最慢。

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

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.