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In this study, the effects of Ti addition on the microstructure, the tensile properties at 550 ℃, and the thermal stabil- ity 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 sig- nificant 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 be- fore and after thermal exposure. ICHAZ exhibited the substantial retention of fine laths and high density of dis- location 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.
