Characterization of High-Temperature Tensile Properties and Thermal Stability in Gas Tungsten Arc Welds of Ti-added Reduced Activation Ferritic/Martensitic Steel

In this study, the effects of Ti addition on the microstructure, the tensile properties at 550 ℃, and the thermal stability in gas tungsten arc (GTA) welds of reduced activation ferritic/martensitic (RAFM) steel were studied. Ti was added to promote precipitation of MX in order to enhance high-temperature properties. For reference, Ti-free RAFM (reference RAFM steel with the composition of 9Cr-1W-0.2V-0.1Ta-0.1C) was compared with 0.013 wt% Ti added RAFM steel (Ti-added RAFM). The addition of Ti contributed to the increase in the area fraction of MX precipitates (2.0 → 4.6 %) and a decrease in the average size of M23C6 (149 → 119 nm) in the base metal. After the cross-weld tensile test at 550 ℃, the tensile properties of Ti-added RAFM steel were superior to that of Ti-free RAFM steel. Both steels were fractured at inter-critical heat-affected zone (ICHAZ) showing the lowest hardness due to over-tempering. However, the higher area fraction of MX precipitates in the Ti-added RAFM produced more significant strengthening, compared to the Ti-free RAFM steel. After heat exposure at 550 ℃ for 500 h, Ti-added RAFM steel was highly resistant to degradation; hardness distribution and tensile properties were almost similar before and after thermal exposure. ICHAZ exhibited the substantial retention of fine laths and high density of dislocation with marginal recovery even after thermal exposure. It is conceivable that excellent thermal stability of Ti-added RAFM steel can be attributed to the high fraction of MX particles by suppressing lath boundary migration.