Micro-hardness measurement and micro-structure characterization of T91 weld metal irradiated in SINQ Target-3

This work is concerned with the micro-structure and mechanical behavior of T91 weld metal before and after an irradiation in SINQ Target-3. Optical and TEM observations and micro-hardness tests were performed to identify the irradiation effects. Before irradiation, the micro-structure of the T91 weld metals consisted of mainly tempered martensite and retained ferrite area. Precipitates in the weld metal are predominately M7C3 carbides, and few M23C6 particles are observed along the martensitic lath and primary austenite grain boundaries. The dislocation density in the weld metal is much higher than that in the base metal. The main feature of the irradiated micro-structure of the weld metal are small defects (black dots) and faulted Frank interstitial loops at lower irradiation temperature and a high density of helium bubbles appear at higher irradiation dose and temperature. The results are comparable with those of the T91 base metal irradiated under the same condition in the previous work. The weld metal and heat affected zone (HAZ) show much higher hardness than the base metal before irradiation, showing that no post-weld heat treatment (PWHT) has been applied to the weld metal. Irradiation hardening increases with irradiation dose below 10 dpa, but decreases at higher dose, which might be related to the transformation of M7C3 precipitates to M23C6 at higher irradiation temperatures.