Development of Stress Reduction Method in the Pipe Welded Zone by Heating

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Some cases of Stress Corrosion Cracking (SCC) failures in the proximity of welded zone of core internals and pipes in Primary Loop Recirculation, which are made of austenitic stainless steel, were reported in existing boiling water reactors, from the late 1970s to 1980s. As a countermeasure against SCC, low-carbon stainless steel was developed in order to reduce susceptibility to SCC, and used as standard material at that time. However, SCC failures were still observed in the core internals and pipes made of low-carbon stainless steel in recent years. It is well understood that residual tensile stress due to welding largely affects occurrence and growth of SCC in low-carbon stainless steel. Based on these observations, stress reduction methods for the pipe welded zone have already been developed such as Induction Heating Stress Improvement (IHSI) and Heat Sink Welding (HSW). However, these stress reduction methods are applied to only large-bore, thick-wall pipes, because it is difficult to apply these established countermeasures to the small-bore, thin-wall pipes which diameter is 50A and below. Thus stress reduction method for small-bore pipe has not been established. In this study, residual tensile stress reduction method that is applicable to the welded zone of small-bore pipe has been developed. The stress reduction method uses rapid quenching of inner surface of the pipe by cooling water after heating outer surface of the pipe by a heating device. Just after starting to cool the inside of the pipe, the temperature of the inner surface is low and tensile stress is generated. On the other hand, since temperature of outer surface is high, large temperature difference between inside and outside surface of the pipe develop and the high through-wall thermal stress is generated. When the temperature difference between the inner and outer surfaces of the pipe is large, thermal stress exceeds the yield stress on the pipe inner surface, and plastic deformation occurs on the inner surface of the pipe. The residual stress of the inner surface becomes compressive after the heat treatment due to the residual layer of plastic strain. In this study, the effectiveness of this method is shown by comparing the residual stress on the inner surface of the pipe before and after the application of this method by mock-up tests.