Some Problems of Overlaid Cladding in a Nuclear Reactor Pressure Vessel (Report 2) : Metallurgical Studies on Overlaid Cladding of a Reactor Pressure Vessel

Steel pressure vessels for light-water cooled power reactors are constructed with stainless-steel weld overlay on their inner wall. The overlay weld is usually heat-treated after welding to relieve the residual stresses in the welded joints of the pressure vessel steel. The post weld heat treatment, however, is considered to cause sensitization in the stainless steel overlaid cladding, it is then important to examine the effect of this heat treatment on the overlay weld.The effect of post-weld heat treatment on the mechanical properties of overlay weld was studied previously; a remarkable decrease in the impact strength occurs in the overlaid cladding, and also considerable deterioration in the tensile and bend properties in the weld bond.The purpose of the present sutdy is to obtain information on the relation between the change in mechanical properties and micro structure in the overlaid cladding and weld bond. Test materials and the conditions of heat treatment were the same as previously reported. Experiments in the study include microfractography by electron microscope and scanning electron microscope, optical-micrography, micro-hardness measurement and electron-probe micro analysis.From the results obtained, the following are concluded;1) In the stainless-steel overlaid cladding, the fracture after half-size Charpy test exhibits ductile fracture appearance with a dimple pattern in the as-welded specimens, but the mode changes to intergrarmlar fracture appearance after heat treatment. In the latter case, the fracture occurs at the interface between precipitated carbide particles (M23C6) and the delta ferrite. It is thus considered that ductility loss in the overlaid cladding by post-weld heat treatment is associated with the phase change including those in delta ferrite and carbide formation.2) In the weld bond, the width of carbon migration from the base metal to the weld metal increases with time of the heat treatment and its temperature. The associated loss in ductility also increases similarly.