Abstract: Heat affected zone (HAZ) of the G115 steel pipe was simulated by Gleeble-1500D thermal simulation machine, and the microstructure and hardness of the subregions of heat affected zone were analyzed by means of OM, SEM, EBSD, TEM and microhardness tester. The results indicate that the welded heat affected zone of G115 steel pipe is mainly divided into the coarse grain heat affected zone (CGHAZ) with coarse equiaxed grains, the fine grain heat affected zone (FGHAZ) with a fine mixed grain structure of large grains surrounded by crushed fine grains, and the intercritical heat affected zone (ICHAZ) with tempered martensite, which shows a little difference from the base metal. The geometric dislocation density of the fully phase-transformed microstructure (CGHAZ,FGHAZ) (24.1×10¹⁴, 24.5×10¹⁴ m^-2) is approximately twice that of the incompletely phase-transformed microstructure (ICHAZ) (13.6×10¹⁴ , 11.8×10¹⁴ m^-2), meaning that there is a significant stress-strain gradient at the interface between ICHAZ and FGHAZ. Only a small number of M₂₃C₆ and Laves phases are present within the FGHAZ, which can completely dissolve at higher temperatures. The complete dissolution temperature of the MX phase is higher, and it is still distributed in a small number in the CGHAZ. The Cu-rich phase only precipitates spherically within the grains at the ICHAZ. After the thermal simulation, the hardness and microstructure are positively correlated with the thermal cycling peak temperature. The microhardness of the CGHAZ, FGHAZ and ICHAZ near the base metal is 437.55, 421.85 and about 375 HV0.2, respectively.

Key words: G115 steel pipe, welding thermal cycling, microstructure, hardness