Effect of post-weld heat treatment on a friction stir welded joint between 9Cr-ODS and CLF-1 steels

Dissimilar-metals welding is essential to construction of a fusion reactor by using advanced blanket structural materials of oxide dispersion strengthened (ODS) steels. In the present study, Dissimilar-metals bonding between 9Cr-ODS and CLF-1 steels was carried out by friction stir welding (FSW). Effect of post-weld heat treatment (PWHT) on microstructure and bonding properties of the joint was investigated. The results show that, quenched martensite was found in the weld metal (WM) after FSW. PWHT at 740-800 °C can recover the microstructure of the WM to tempered martensite. Mid-size particles and nano-particles were observed and analyzed in the WM. Besides, in the WM on the CLF-1 side, recrystallization was found after PWHT. As the PWHT temperature increased, the recrystallization width and grain size grew up. However, the recrystallization in the WM did not degrade but may be beneficial to the bonding properties of the joint due to support ductile deformation during mechanical tests. The recommend PWHT temperature is 740-760 °C for keeping the mechanical properties of the CLF-1 base metal (BM) as that before FSW.