EVALUATION OF EMBRITTLEMENT OF WELD METAL IN PRIMARY COOLANT CIRCUIT.

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Two thick panels of SUS304 type stainless steel were welded by SUS308 type stainless steel using submerged arc welding process. A change in the compositional and toughness of the weldmetal during aging at temperatures of 350℃ and 425℃ have been investigated by Mossbauer spectroscopy and Charpy impact testing.The width of the internal magnetic field distribution curve increased in the initial stage of aging followed by an increase in the paramagnetic absorption peak. And also the mean internal magnetic field increased with aging time. These experimental results led to a conclusion that the ferrite undergoes spinodal decomposition. The activation energy for the ferrite decomposition was estimated to be about 160-200kJ/mol. The change in the decomposition fraction of ferrite with aging time was interpreted by the sum of two hyperbolic tangent curves. A calculated curve was presented to predict the change in the composition of ferrite during aging at 320℃ by using an equation combined the activation energy.Charpy impact tests have been undertaken at 0℃ . Reductions in the Charpy impact energy were measured by aging at the above temperatures. In order to take account of the combined effect of aging time(t) and temperature(T), an aging parameter : p(T,t)=logt+0.4343(Q/R)(1/673-1/T), was introduced. The Charpy impact data were plotted as a function of p(T,t) to give a best fit value of Q = 125kJ/mol , for which all experimental points fell on a single master curve. The experimental data were fitted to a hyperbolic tangent function, U = UィイD20ィエD2+D{1+tanh(p(T, t)-E)/F}. The aging time required for the Charpy impact energy to drop to a half of that of the unaged weldmetal was estimated as 500kh at 320℃ on the basis of the above equation.