Microstructure-driven post-seam annealing design for enhanced toughness in X70 electric-resistance welded pipes

In this study, the influence of post-seam annealing (PSA) conditions on the microstructure evolution and low-temperature impact toughness of API X70 electric-resistance welded (ERW) pipes has been investigated. For each PSA condition, key microstructural factors in the ERW bondline—including α grain size, pearlite and martensite-austenite (M-A) fractions, dislocation density, and MX fraction—were qualitatively and quantitatively analyzed. The optimized ERW condition minimized MnO–SiO 2 penetrators to just 0.005% fraction, significantly enhancing the bondlines resistance to hydrogen-induced cracking (HIC). Fracture surface and micro-cracking analyses revealed that elongated M-A distribution and size were the most critical factors affecting low-temperature impact toughness. Crystallographic TEM analysis revealed that within M-A, martensitic lath variant and austenite share their interface with Kurdjumov-Sachs (K–S) relationship, ( 10 1 ) ‾ M // ( 1 1 ‾ 1 ) γ and [111] M //[011] γ . A Gleeble system was used to simulate the PSA process, achieving finer grain sizes and reducing the M-A fraction. Our findings indicate that a combination of low PSA temperature and slow cooling promotes extensive homogenization of austenite-stabilizing elements (C and Mn), facilitating γ decomposition into fine α + Fe 3 C and thereby improving impact toughness. This study provides insight into optimizing process conditions through microstructure control.