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In the present investigation, a thermal welding simulation technique was used to investigate the mechanical properties and microstructure features of the coarse-grained heat-affected zone （CGHAZ） of ASTM4130 steel. The effect of post welding heat treatment （PWHT） and welding heat inputs on the toughness of CGHAZ was also analyzed. The results show that CGHAZ has the lowest toughness, which is only 5.5%–7.1% of the base metal. CGHAZ is mainly composed of dislocation martensite, up-per and lower bainite, and M-A constituents. But after PWHT, carbides precipitate from non-equilibrium microstructures of CGHAZ accompanying some retained austenite which transforms into low bainite, thereby enhancing toughness over the base metal. Therefore, the key microstructure factors affecting fracture toughness are lathlike non-equilibrium microstructure and lowered supersaturation before and after PWHT respectively. When welding heat input is between 12 kJ/cm and 28 kJ/cm, the mechanical properties in CGHAZ of ASTM4130 with single-pass welding can satisfy the requirements when PWHT is applied.

The microstructures and mechanical properties of coarse grain heat-affected zone (CGHAZ) of domestic X70 pipeline were investigated. The weld CGHAZ thermal cycles having different cooling time Δ t 8/5 were simulated with the Gleeble-1500 thermal/mechanical simulator. The Charpy impact absorbed energy for toughness was measured, and the corresponding fractographs, optical micrographs, and electron micrographs were systematically investigated to study the effect of cooling time on microstructure, impact toughness, and fracture morphology in the CGHAZ of domestic X70 pipeline steel during in-service welding. The results of simulated experiment show that the microstructure of CGHAZ of domestic X70 pipeline steel during in-service welding mainly consists of granular bainite and lath bainite. Martensite–austenite (M–A) constituents are observed at the lath boundaries. With increase in cooling time, the M–A constituents change from elongated shape to massive shape. The reduction of toughness may be affected by not only the M–A constituents but also the coarse bainite sheaves. Accelerating cooling with cooling time Δ t 8/5 of 8 s can be chosen in the field in-service welding X70 pipeline to control microstructures and improve toughness.