Microstructural features and mechanical properties of similar and dissimilar ferritic welded joints for ultra-supercritical power plants

The grade P91 and P92 steel are commonly used materials for the components in ultra-supercritical power plants in the temperature range of 590–650 °C. For welded joints, the notch toughness and microstructure stability played an essential role in meeting the requirement of high temperature creep properties. The microstructural stability and notch toughness were strongly affected by the composition of base and filler materials, welding processes and pre and post-weld heat treatments (PWHTs). In the present research, four different combinations of similar/dissimilar welded joints of P91 and P92 steel are prepared using shielded metal arc welding (SMAW). The microstructure of different welded joints has been characterized in the as-welded condition. For the as-welded state, the room temperature tensile properties, flexural strength, microhardness and notch toughness have been studied. The δ-ferrite patches were observed for P92 filler welded joints. It has been concluded that in the weld fusion zone, the higher amount of ferrite stabilizers promotes the formation of the δ ferrite. The tensile strength and flexural strength were increased in P92 filler welds due to the presence of solid solution hardening elements in the P92 filler. The detrimental effect of δ-ferrite patches has been observed on the Charpy impact toughness and microhardness values of the weld fusion zone. The Charpy toughness values in the weld fusion zone were 12 ± 2, 7 ± 2, 8 ± 2 and 5 ± 3 J for P91-P91-P91, P92-P92-P92, P91-P91-P92 and P91-P92-P92 weld joints, respectively, which were lower than the minimum recommended Charpy toughness value (47 J) for ferritic steel welded joints.