Shinobu Okido

Residual Stress Evaluation of Ni Based Weld Metal Using Neutron Diffraction Method

Residual stress generated in Type-316 austenitic stainless steel butt-weld jointed by Inconel-182 was measured using a neutron diffraction method and compared with values calculated using FEM analysis. The measured values of Type-316 austenitic stainless steel as base material agreed well with the calculated ones. The diffraction had high intensity and a sharp profile in the base metal. However, it was difficult to measure the residual stress at the weld metal due to very weak diffraction intensities. This phenomenon was caused by the texture in the weld material generated during the weld procedure. As a result, this texture induced an inaccurate evaluation of the residual stress. Procedures for residual stress evaluation to solve this textured material problem are discussed in this paper. As a method for stress evaluation, the measured strains obtained from a different diffraction plane with strong intensity were modified with the ratio of the individual elastic constant. The values of residual stress obtained using this method were almost the same as those of the standard method using Hooke’s law. Also, these residual stress values agreed roughly with those from the FEM analysis. This evaluation method is effective for measured samples with a strong texture like Ni-based weld metal.

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

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.Copyright

Application of X-Ray and Neutron Diffraction Methods to Reliability Evaluation of Structural Components and Electronic Device

In the X-ray diffraction method, the diffraction intensity, the half-value width, the residual stress and the amount of residual austenitic phase can be measured. By using these parameters, the quality, the mechanical properties and the fatigue strength of materials, the remaining life of fatigue and creep can be evaluated. While the X-ray study has been widely performed for the various kinds of industrial fields in the laboratory, the applications to the actual structure and components have not so many. However, the small size X-ray residual stress analyzer, the position sensitive detector and the micro area diffraction apparatus have been developed for these twenty years. Thus the X-ray diffraction methods have been variously applied to the industrial fields. The X-ray diffraction methods were used to be applied for the large scale structures and machine parts, but recently applied to the semi-conductor fields. On the other hand, the neutron diffraction method has been introduced to measure the residual stresses in the internals of components because of its deep penetration depth. Based on the experiences of X-ray diffraction method, the various kinds of techniques have been proposed. In this paper, the applications of X-ray and neutron diffraction method to the reliability evaluations of structural components and the electronic devices are described.