A comparative study on the effect of GTAW processes on the microstructure and mechanical properties of P91 steel weld joints

•A-TIG has low delta ferrite content (0.5%).•GTA weld shows complete absence of delta ferrite.•GTA weld has higher toughness than A-TIG weld.•A-TIG weld requires prolonged PWHT to achieve the required toughness. Modified 9Cr-1Mo (P91) steel is widely used in the construction of power plant components. In the present study, a comparative study on influence of activated flux tungsten inert gas (A-TIG), and gas tungsten arc (GTA) welding processes on the microstructure and the impact toughness of P91 steel welds was carried out. P91 steel welds require a minimum of 47J during the hydrotesting of vessels as per the EN1557: 1997 specification. Toughness of P91 steel welds was found to be low in the as-weld condition. Hence post-weld heat treatment (PWHT) was carried out on weld with the objective of improving the toughness of weldments. Initially as per industrial practice, PWHT at 760°C – 2h was carried out in order to improve the toughness of welds. It has been found that after PWHT at 760°C – 2h, GTA weld (132J) has higher toughness than the required toughness (47J) as compared with A-TIG weld (20J). The GTA weld has higher toughness due to enhanced tempering effects due to multipass welding, few microinclusion content and absence of δ-ferrite. The A-TIG weld requires prolonged PWHT (i.e. more than 2h at 760°C) than GTA weld to meet the required toughness of 47J. This is due to harder martensite, few welding passes that introduces less tempering effects, presence of δ-ferrite (0.5%), and more alloy content. After PWHT at 760°C – 3h, the toughness of A-TIG weld was improved and higher than the required toughness of 47J.