Microstructural evolution and mechanical properties of heat affected zones for 9Cr2WVTa steels with different carbon contents

•The weldability of 9Cr2WVTa steel with different carbon contents was studied.•The HAZs of the 9Cr2WVTa steel include three characteristic regions.•Residual delta-ferrite significantly deteriorates the impact property of HAZs.•The twin martensite deteriorates the impact toughness as a whole.•The 9Cr2WVTa steel with 0.14–0.17 wt.% carbon content has good weldability.

The microstructures and mechanical properties of heat affected zones (HAZs) by Gas Tungsten Arc Welding (GTAW) were studied for 9Cr2WVTa steels with carbon content varying from 0.07 wt.% to 0.25 wt.%. Enlarged HAZs samples with 8 mm to 10 mm wide uniform temperature zone were prepared by the thermal–mechanical physical simulator Gleeble 1500 based on the Finite Element Method (FEM) numerical simulation and experimental measurement for the welding thermal cycle process and weld profile. The microstructures were observed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). In addition, the mechanical properties tests including micro-hardness test, tensile test and impact test were carried out to investigate the effects of the carbon content and the welding thermal cycle. The results show that the big blocky delta ferrite in 9Cr2WVTa steel with lower carbon content deteriorates the impact property. On the other hand, the quenched martensite, especially for the twin martensite in 9Cr2WVTa steel with higher carbon content, deteriorates the impact toughness as well. The weldability of 9Cr2WVTa steel can be improved by adjusting the carbon content between 0.14 wt.% and 0.17 wt.%.