Coupled effect of microstructure heterogeneity and hydrogen on local embrittlement of CGHAZ and IC-CGHAZ in X65 pipeline steel

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2024-12, Vol.917, p.147391, Article 147391
• Main Authors: Li, Qiang, Deng, Caiyan, Wu, Shipin, Zhao, Haiwei, Xu, Xiaohan, Liu, Yong, Gong, Baoming
• Format: Article
• Language: English
• Subjects: Intercritically reheated coarse-grained heat-affected zone; Local embrittlement; Microstructure heterogeneity; Plastic strain localization; Stress triaxiality
• ISSN: 0921-5093
• DOI: 10.1016/j.msea.2024.147391

The local embrittlement of the coarse-grained heat-affected zone (CGHAZ) and intercritically reheated CGHAZ (IC-CGHAZ) in X65 pipeline steel was investigated using an in situ crack-tip opening displacement test in air and H2S, combined with microstructure-based simulation. The IC-CGHAZ exhibited significantly lower cracking resistance, with the fracture toughness reduced by 50 % compared to CGHAZ in both environments (0.160 mm in air and 0.017 mm in H₂S for IC-CGHAZ, compared to 0.307 mm in air and 0.034 mm in H₂S for CGHAZ). The martensite/austenite (M/A) constituents showed exceptionally higher intrinsic hardness than the ferrite matrix. The continuous distribution of M/As along the prior austenite grain boundaries resulted in local hardening and the formation of heterogeneous hard zones in IC-CGHAZ. Such microstructure heterogeneity resulted in the unique partitioning of plastic strain localization and stress triaxiality that promotes premature fracture, particularly with the coupled effect of hydrogen. This study provides a novel perspective on the fracture mechanisms and the typical fracture morphology of IC-CGHAZs in pipeline steel under the coupled effect of microstructure heterogeneity and hydrogen. •Fracture toughness of IC-CGHAZ was reduced by 50 % compared to CGHAZ in air and H2S.•Increased microstructure heterogeneity led to significant toughness degradation.•Formation of heterogeneous hard zones caused increased microstructure heterogeneity.•Partitioning of plastic strain and stress triaxiality influences fracture behavior.•Hydrogen dominated the cleavage fracture related to microstructure heterogeneity.