Microstructure and mechanical properties of 9Cr-0.5Mo-1.8W–VNb (P92) steel weld joints processed by fusion welding

Grade P92 steel is used for making steam generator components of thermal and nuclear power plants at temperatures up to 923 K. Microstructures, mechanical properties and the service life of the welded components are influenced by the welding process employed. In the present investigation, the effect of Tungsten Inert Gas (TIG) and Activated TIG (A-TIG) welding processes on the microstructure and mechanical properties of P92 steel weld joints are studied. Optical microscope, SEM and TEM were used for characterizing the microstructures. Application of the activated flux on the joint area during A-TIG welding significantly increases the depth of penetration. Due to high current density peak temperature is higher in the weld pool and steep temperature gradient is observed across the joint interface. Therefore, the microstructure is relatively coarse in the A-TIG weld metal. TEM investigations have confirmed that the precipitates present at the grain and lath boundaries are carbides of the form M₂₃C₆ and the precipitates seen within the matrix of the lath are niobium and vanadium rich carbides and nitrides of type MX. The tensile strength of the A-TIG weld joint both at room temperature and at 923 K is higher than that of the TIG weld joint. In comparison to the base metal and TIG weld joint, a decrease in the Charpy impact toughness is observed in the A-TIG weld joint due to the formation of δ-ferrite along the fusion interface. However, in contrast to the TIG weld metal, tensile residual stresses at the A-TIG weld are very low.