Abstract Scope |
Grade P92 steel is widely used in power generating industries, especially for mainstream pipes, boilers, reheaters and superheaters tubing and rotors. Corrosion is one of the significant causes for the failure of weld joints and materials in power plant applications. Retention of δ-ferrite and heterogeneous microstructure formation across the weldments make the weldability of Grade P92 steel critical. This research aims to investigate the effect of cooling rates on the retention of δ-ferrite and their subsequent effect on corrosion performance of the weld fusion zone. The detrimental effect of δ-ferrite on the mechanical performance of welds is well known. 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 oC for 2h. The retention of δ-ferrite depends on peak temperature and cooling rate. The heat input is kept constant for all weld joints to see the effect of the cooling rate on δ-ferrite. δ-ferrite size, as well as the content, are found to decrease with a decrease in cooling rate. The morphology, composition, structure, and hardness of δ-ferrite against the un-tempered and tempered martensite phase are verified for all weld joints. All weld joints are tested for corrosion performance concerning for δ-ferrite content and size in 2% NaCl + 2% Na2So4 solution. An as-received P92 steel sample in the normalizing and tempered condition is used as a reference microstructure against the δ-ferrite containing welds for corrosion performance. To find out the corrosion product or oxide layer thickness, the weld fusion zone surfaces will be analyzed using a field emission scanning electron microscope. The kinetics of δ-ferrite will also be verified using DICTRA. |