Heat-Input-Dependent CGHAZ Microstructural Evolution and Impact Toughness of Two X65 Seamless Pipeline Steels with Different Composition–Microstructure Characteristics

This study comparatively investigates the coarse-grained heat-affected zone (CGHAZ) responses of two industrial X65 seamless pipeline steels with distinct composition–microstructure characteristics under simulated girth-welding thermal cycles. One steel exhibits a predominantly bainitic initial microstructure, whereas the other consists of a ferrite–bainite dual-phase microstructure. Low-temperature Charpy impact testing, microhardness measurements, scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), prior-austenite grain reconstruction, and JMatPro 13.0-based continuous cooling transformation (CCT) calculations were employed to evaluate their heat-input sensitivity and microstructural evolution. At heat inputs of 7–10 kJ/cm, both steels maintained high impact toughness at −20 °C, with average absorbed energies of approximately 250 J. A pronounced difference emerged at 15 kJ/cm the bainite-dominated steel retained relatively high impact toughness and higher crack-initiation and -propagation energies, whereas the ferrite–bainite steel exhibited a marked toughness reduction. At higher heat inputs of 20–30 kJ/cm, both steels showed substantial toughness deterioration associated with severe prior-austenite grain growth and coarsening of the bainitic transformation products. Microstructural and crystallographic analyses showed that the bainite-dominated steel generally retained finer prior-austenite grains and more refined crystallographic features under the investigated thermal cycles. Detailed characterization at 15 kJ/cm further revealed finer prior-austenite grain, packet, and block structures, together with more tortuous crack-propagation paths. JMatPro calculations predicted a lower bainitic transformation temperature for this steel, which is consistent with the experimentally observed tendency toward finer bainitic transformation products. The superior CGHAZ toughness retained by the bainite-dominated steel is therefore associated with the combined effects of alloy composition, initial metallurgical state, transformation behavior, and hierarchical crystallographic refinement rather than the initial microstructure alone. The results highlight the importance of coupled composition–transformation–microstructure effects in determining the welding heat-input tolerance of industrial X65 seamless pipeline steels.