Foundation Of Thin Hastelloy C-276Sheet Laser Welding For Nuclear Reactor Coolant Pump Can

The canned motor was used in the nuclear reactor coolant pump of AP1000, and in the canned motor, the can, which was Hastelloy C-276alloy, was important to the work of nuclear main pump. During the manufacture of the can, the welding of can was critical. In this paper, based on the welding requirements of thin large draw ratio Hastelloy C-276alloy can, a laser welding method was proposed. Also, the optimized welding parameters were obtained, and the microstructure, mechanics property and corrosion resistant on the weld joint were investigated. Finally, the effect of welding method on the macro/micro characteristics of Hastelloy C-276was evaluated.(1) Using Nd:YAG laser achieved the well weld joint of Hastelloy C-276, and the effects of single energy, welding velocity, repetition and duration on the weld joint were discussed. Then, the optimization criterion of welding parameters was proposed. Aiming at the smooth, contour line and special profile of weld joint, the buoyancy effect and Marangoni convection was applied to explain the phenomena above. In addition, by the X-Ray nondestructive inspection and layer-by-layer polishing, nearly no any welding defect was found. Hence, it was concluded that the laser welding could achieve the well weld joint nearly without welding defect.(2) Using the SEM, EMPA, XRD and micro-sclerometer, the microstructure, element distribution, phase structure and microharness of weld were tested. In the weld, the obvious sub grain boundary and fined dendrite were found, and the macro segregation was not found, but the micro segregation was significant. A Brody-Flemings model with piecewise revised k was proposed to calculate the element distribution under the rapid cooling condition. The distribution of Cr, Mo, W and Fe during the laser welding was revealed, and it was indicated that the laser welding could restrain the forming of brittle phase. The results of XRD and microharness test indicated the phase structure of weld was the same as that of base metal, and the microharness of weld was not influenced by the laser welding.(3) The yield strength of the weld joint was the same as that of the base metal, but at room temperature, the tensile strength of weld joint was a little smaller than that of the base metal. There were many dimples in the fracture of base metal, and in the weld fracture, the tiny deep dimples were observed. In the fracture of weld, it was not found the enrichment of brittle phases by the XRD test. The engineering stress-strain curve showed all stretched samples presented the same plastic forming, and the true stress-strain curve showed the limiting strain and work hardening trend of weld were the same as that of base metal. (4) Under the corrosion of nitrohydrochloric acid condition, the even corrosion was observed in the base metal, and the intergranular corrosion was found in the weld. The electrochemical results indicated, in the neutral solution, the corrosion resistance of Hastelloy C-276was not affected by the laser welding, but the corrosion mechanisms of base metal and weld were different.