Microstructure transformation of coarse-grained region in heat-affected zone during welding of ultra-low carbon ultrahigh strength pipeline steel

The microstructure transformation of coarsegrained region in heat-affected zone(CG-HAZ) during welding of an ultra-high strength X120 pipeline steel was investigated with thermal simulation.Simulated HAZ continuous cooling transformation(SH-CCT) curves showed that the bainitic microstructure was obtained in CG-HAZ in a wide cooling rate range(0.8-25℃/s).Besides bainite,a small proportion of quasi polygongal ferrite formed in CG-HAZ when the cooling rate was lower than 0.8℃/s whereas a small proportion of martensite formed when the cooling rate was higher than 25℃/s.Bainite was obtained in the CG-HAZ when the welding heat input was in the range of 12-25 kJ/cm.The Vickers hardness of CG-HAZ(276-297 HV0.2) and the impact toughness at room temperature(208-225 J) was stable.Simulated results indicated that the X120 pipeline steel was suitable for welding with wide heat inputs,which was attributed to the ultra-low carbon design.