Microstructure And Mechanical Properties Of TIG Welded Joint Of CLAM Steel

As is well-known, reduced activation ferritic/martensitic steels (RAFM) are commonly considered as the primary structural materials for the DEMO fusion plant and the first fusion power reactors because of their excellent thermophysical properties and mechanical properties, for example the low irradiation swelling and thermal expansion coefficient, high thermal conductivity etc. , and their relative mature technical foundation. China Low Activation Martensitic steel (CLAM) is the RAFM which have independent intellectual property rights with Chinese and optimized chemical composition and properties. CLAM is considered as the primary structural materials for the China designed ITER test blanket module.As others RAFMs, W, Ta and V etc in CLAM replaced Mo, Nb and Ni etc. in common ferritic/martensitic steels, because of the low activation requirement. Welding technology and process is one of the key technologies for CLAM using in project.In this paper, 5mm CLAM plates were welded by Flat butt welding and T-welding using TIG welding method, and then they were post-weld heat treated at different temperatures. Microstructure and carbide distribution of weld joint were observed by metalloscope and SEM. Hardness and mechanical properties were tested respectively in hardness test, tensile test and impact test experiments. According to the results of observed and test, metallurgical principles and related theories were used to research and analysis welding thermal cycle, microstructure, carbide precipitation, the mechanical properties and hardness of welded joints.The results show that in the experiments of Flat butt welded on 5mm CLAM plates, both Single-layer and double-layer welded joints appeared different sizes of grain. However, all of the microstructure were tempered martensite. With the increase of post-weld heat treatment temperature, the lath beams of martensite gradually uniformed distribution in welding seams, while they also had the trend of increasing. Two kinds of welded joints had micro-M₂₃C₆ carbide precipitate. After post-weld heat treatment the hardness of double-welded joints decreased in every region, and decreased with the increases of heat treatment temperature. The experiments showed that at 810℃the hardness of weld metal had the largest decline to the hardness of the base material. Tensile strength of welded joints also decreased with post-weld heat treatment temperature increased. After heat-treated at 760℃in 30min the tensile strength of welded joints were740MPa,and the tensile strength decreased below 700MPa at 810℃. Single-layer welded joints fractureed in the weld on the 810℃, the rest of the specimen were broken at the base material away from the weld and heat affected zone. Heat treatment made the impact properties of joints significantly enhanced, turned the brittle fracture into a ductile fracture. Impact energy of unheat-treated Weld joints were 9J up to 108J after heat-treated at 760℃in 30min. The fracture was mainly dimple fracture.Weld of CLAM plates T-welded joints was good shape, but there existed an obvious angle distortion in joint flange. In half V-groove and T-joints the grain in flange heat-affected zone was finer than web zone due to the different thermal environments of flange and web zone. Compared to three-panel T-welded joints of different joint forms, W-shaped three-panel T-welded joints had advantage with less filling metal, small grain size and narrow-web heat-affected zone.