Study on the Effect of the Grain Refinement on Mechanical Properties of the P92 Welded Joint

The welded joint of martensitic P92 steel is associated with heterogeneous microstructure arising due to the thermal history of the welding thermal cycle. The complex microstructure of the welded joint results in degradation of mechanical strength of the joint in-service condition. The present work showed experimental investigation where an improvement in mechanical properties and homogeneous microstructure formation for P92 welded joint has been achieved through grain refinement of virgin P92 steel plate. The grain refinement of virgin P92 steel has been achieved using the double austenitization based normalizing and tempering treatment. The as-received P92 steel having ‘normalized and tempered’ (1040 °C/40 min/air cool+760 °C/2 h/air cool) condition had an average grain size of 9.54±2.50 μm, while after ‘double austenitization based normalizing and tempering’ P92 steel (1040 °C/60 min/water quench+950 °C/40 min/air cool+760 °C/2 h/air cool), average grain size of 15.75 ± 3.75 μm had been obtained. The welded joint has been prepared for both the condition (normalized and tempered; double austenitization based normalized and tempered) of the plates. The paper discusses the effect of grain refinement of virgin base metal on tensile properties, impact toughness and microhardness of the P92 welded joint, and results obtained for the ‘double austenitization based normalized and tempered’ welded plate have been compared with the conventional ‘normalized and tempered’ welded plate. The welded joint was also characterized for heat treatment conditions. The welded joint did not qualify the ultra-supercritical boiler requirements in as-welded condition due to poor impact energy. The fairly good impact energy and tensile properties were obtained after the double austenitization based normalizing and tempering post-weld heat treatment (DHT). After the direct post-weld heat treatment, a superior weld strength was obtained, but impact energy just qualified the minimum required value and also inhomogeneity in microstructure was obtained along welded joint. The DHT showed a significant improvement in tensile strength of welded joint without any loss in ductility.