| Abstract Scope |
Grade P92 steel is widely used in power generating industries, especially for mainstream pipes, boilers, reheaters and superheaters tubing and rotors. Retention of δ-ferrite and heterogeneous microstructure formation across the weldments make the weldability of Grade P92 steel critical. The detrimental effect of δ-ferrite on the mechanical performance of welds is well known. The precipitation of Laves phase with loss of strengthening elements affects the creep life of welds during service. This research aims to investigate the effect of cooling rates on the retention of δ-ferrite and their subsequent effect on the precipitation of Laves phase during artificial aging. In this research, autogenous tungsten inert gas welding is used to prepare Grade P92 steel bead on plate welds. After welding, the joints are cooled in; (i) air, (ii) water and (iii) furnace from 1200 oC to room temperature. All three weld joints are subjected to post-weld heat treatment of 760 °C for 2h. To see the effect of the cooling rate on δ-ferrite, the heat input is kept constant for all weld joints. The size and fraction of δ-ferrite are found to decrease with a decrease in cooling rate. The weld joints are exposed to artificial aging of 620°C and 650°C for 1000 hrs, 2000 hrs and 3000 hrs. To find out the role of δ-ferrite on the precipitation of Laves phase, the weld fusion zone surfaces will be analyzed using a field emission scanning electron microscope, electron backscatter diffraction and X-ray diffraction. Thermo-Calc will be used to predict driving forces for precipitation of Laves and M23C6 phases. The kinetics of δ-ferrite and Laves phase will also be verified using DICTRA. |