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

Modified 9Cr-1 Mo (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 degrees C -2 h was carried out in order to improve the toughness of welds. It has been found that after PWHT at 760 degrees C -2 h, GTA weld (132 J) 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 delta-ferrite. The A-TIG weld requires prolonged PWHT (i.e. more than 2 h at 760 degrees 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 delta-ferrite (0.5%), and more alloy content. After PWHT at 760 C -3 h, the toughness of A-TIG weld was improved and higher than the required toughness of 47J. (C) 2014 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.