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

doi:10.3901/JME.2024.06.271

Abstract

Abstract: 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.

Key words: low-alloy high-strength bainite steel, coarse-grained heat-affected zone, martensite-austenite constituents, low-temperature fatigue crack growth, fatigue ductile-to-brittle transition

CLC Number:

TG142

WANG Boshi, CHEN Nannan, CAI Yan, WANG Min. 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[J]. Journal of Mechanical Engineering, 2024, 60(6): 271-278.

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