低合金贝氏体高强钢模拟焊接粗晶热影响区低温疲劳韧脆转变行为及机制

Low-temperature Fatigue Ductile-to-brittle Transition Behavior and Mechanism for Simulated Coarse-grained Heat-affected Zone of a High-strength Low-alloy Bainite Steel

低合金贝氏体高强钢接头粗晶热影响区是薄弱位置,针对粗晶热影响区中贝氏体组织的低温疲劳性能缺少系统研究。对一种低合金贝氏体高强钢模拟焊接粗晶热影响区进行组织特征分析及一系列温度(20℃到-60℃)的疲劳裂纹扩展试验。结果表明,该粗晶热影响区组织为板条贝氏体,板条块之间为条状的马氏体-奥氏体组元,且马氏体-奥氏体组元的碳含量和纳米硬度值都大于板条贝氏体基体。该粗晶热影响区疲劳韧脆转变温度为-40℃,在-40℃及以上时,疲劳裂纹扩展速率随温度的降低而降低,疲劳断裂模式是以辉纹为主的韧性断裂,裂纹扩展路径较为曲折,贝氏体板条束和马氏体-奥氏体组元会使扩展路径发生“之”字形偏折。而在-40℃以下时,疲劳裂纹扩展速率陡增,疲劳断裂模式是以辉纹、二次裂纹和准解理面为主的循环解理断裂,裂纹扩展路径较为平直,沿着贝氏体板条束和马氏体-奥氏体组元处出现二次裂纹。研究结果揭示了板条贝氏体组织特点及疲劳韧脆转变行为,能为低合金贝氏体高强钢焊接接头在低温疲劳环境中的应用提供数据支撑和参考。

The coarse-grained heat-affected zone(CGHAZ)of welds of bainite high-strength low-alloy(HSLA)steels is a weak position.To date,few systematic studies have been reported on the low-temperature fatigue performance of the bainite microstructure in CGHAZ of bainitic steel welds.Fatigue crack growth rate(FCGR)tests and microstructure features are performed in the simulated a CGHAZ of a bainite HSLA steel at the low-temperature range(from 20℃to−60℃).The results show that the microstructure in the CGHAZ is dominated by lath bainite embedded with film martensite-austenite(M-A)constituents.The carbon content and nano-indentation value in the M-A constituents are higher than those in the matrix of lath bainite.The fatigue ductile-to-brittle transition(FDBT)temperature is−40℃.Above−40℃,the FCGR decreases with decreasing temperature,and the fatigue fracture mode is a striation-dominated ductile mechanism.The crack propagation path is relatively tortuous,and the bainitic lath bundle and M-A constituents cause zigzag deflections of the path.While the FCGR increases rapidly below−40℃,and the fatigue fracture mode is cyclic cleavage mechanism,which features quasi-cleavage facet with mixture of striations secondary cracks and tearing ridges.The crack propagation path is relatively straight with secondary cracks along the bainite lath bundles and at the M-A constituents.The results reveal the microstructure characteristics and FDBT behavior of lath bainite,which could provide data support and reference for the application for broader applications welded joint of bainite steels subjected in low-temperature fatigue environments.