Mismatch between fracture and impact toughness in local brittle zone (LBZ) of FH420 offshore jacket pipe steel

Résumé fourni par la source

This study challenges the common premise of a linear correlation between impact and fracture toughness in LBZ of High-strength low-alloy steels, revealing a fundamental mismatch between them. In this study, fracture/impact toughness tests were conducted on simulated LBZ specimens with different heat inputs, and their fracture mechanisms under quasi-static loading and impact loading were investigated respectively. The results showed the crack tip opening displacement (CTOD) value of coarse-grained HAZ (CGHAZ) exhibited a three-fold increase with increased t 8/5 (cooling time from 800℃ to 500℃), while the impact absorbed energy remained stable at about 200 J without significant fluctuations. Under low heat input ( t 8/5 = 4 s and 7 s), a lath bainite (LB)/lath martensite (LM) mixed microstructure in the CGHAZ provided high impact toughness via martensite plasticity deformation, but quasi‑static cracks propagated along brittle bainite, lowering fracture toughness. At higher heat inputs ( t 8/5 = 11 s and 17 s), a “softened” acicular ferrite (AF)/granular bainite (GB) mixed microstructure effectively resisted crack propagation under both loading conditions, ensuring excellent comprehensive toughness. In the intercritically-reheated CGHAZ (ICCGHAZ), numerous martensite–austenite (M−A) constituents induced microcracks and embrittled all microstructures, causing a drastic toughness deterioration. This work provides a mechanistic framework for designing welding processes to ensure structural integrity under complex loading scenarios.

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