圧力容器製造におけるステンレス鋼肉盛り熔接の研究(第1報)

Abstract

Steel pressure vessels of light-water cooled nuclear power reactors are constructed with stainless steel weld overlay liners on their inner wall. Occasional cracking in the stainless steel cladding of pressure vessel is considered one of the problems in the development, and also maintainance, of the reactor in several countries including Japan.Since the stainless steel overlay cladding is subjected to thermal and internal-pressure loads in reactor operation, it is desirable for the cladding to have high strength and ductility from the standpoint of structural safety. In Section III of ASME Boiler and Pressure Vessel Code, post-weld heat treatment of more than one hour per inch at over 1100°F (593°C) is required for the weld joints of low-alloy pressure vessel steels. This heat treatment to relieve residual stresses in the weld joints during construction of the pressure vessel is considered to cause sensitization of the cladding.The purpose of the present study is to obtain information on the effect of post-weld heat treatment on the mechanical properties of overlay weld. The tests used include half size Charpy-V notch impact test, micro tensile test and guided side bend test. The test specimens were machined from the overlaid cladding, weld-bond and base metal heat treated at 600°, 625° and 650°C for 20, 50 and 100 hours. From the results obtained, the following conclusions are drawn:(1) The decrease in ductility occurs in overlaid cladding or weld-bond, depending on the post-weld heat treatment applied. A remarkable decrease occurs in the impact property of overlaid cladding; while a considerable decrease, in tensile and bend properties, was observed in the weld-bond.(2) The post-weld heat treatment should be limited to within 20 hours at 625°C, because of an excessive decrease in ductility caused in the weld-bond by prolonged heating.(3) In order to reduce the embrittlement of overlaid cladding due to post-weld heat treatment, the Gow and Harder's empirical ratio K (%Cr+4×%Mn+3(%Si-1)-16×%C/%Ni+1/3(%Mn-1)), Cr/Ni ratio and ferrite content in Schaeffler's diagram should be limited to below about 1.9, 2.1 and 7.5% respectively.