The Effects of Thermal Aging on Microstructural and Mechanical Properties of GH3535 Alloy Welded Joint

Ni-Mo-Cr alloys are the main structure materials for the Thorium molten salt reactor. In order to investigate the properties of welded joints of Ni-Mo-Cr alloy under long-term elevated temperature service, mechanical behavior and microstructure of joints aged at 650°C for 250h, 500h, 1000h, 2000h and 4000h were studied. Optical microscopy and scanning electron microscopy were used to study the effects of aging time on the grain size and carbides. Meanwhile, hardness, strength and fracture morphology of welded joints were also analyzed. In this study, the eutectic was observed in the weld metal and heat affected zone. The grain sizes of base metal of welded joints under different aging time were steady at 100μm approximately. Hardness test of welded joints indicated a slight climbing trend by increasing of aging time. The results of tensile test showed that the performance of welded joints after aging was better than that of as-welded joint and increased by growth of aging time, it was stable for the mechanical properties of aged welded joints in long term aging for up to 4000h as well. Moreover, the fracture morphology and the mechanism were observed by mean of SEM, the main fracture mechanism was ductile fracture for all the joints under different aging time. The second phase precipitations with increasing of aging time were the main contributing factor to enhance of mechanical properties.