Effect of dynamic strain aging on the leak-before-break assessment in SA106 Gr.C piping steel

Abstract

Tensile and J-R tests were carried out under various temperatures at displacement rates of quasi-static to quasi-dynamic to understand the characteristics of dynamic strain aging (DSA) in SA106 Gr.C piping steel and its weld metal. Also, the characteristics of DSA on material properties used in leak-before-break (LBB) analysis were discussed. The manifestations of DSA were clearly observed in the tensile properties of SA106 Gr. C base and weld metal. However, the magnitude of serration and strength increase by DSA in SA106 Gr. C base metal was relatively small compared to similar grade carbon steels. This may be due to high manganese content in the material. The weld metal was more susceptible to serrated flow than base metal. DSA region of weld metal was broader and appeared at higher temperatures than that of base metal. These are caused by higher cooling rate, smaller grain size, and higher manganese content in weld metal. The plateau and small peak in yield stress (sys) exhibited and strain rate dependence disappeared at the nuclear reactor operating temperature which falls in DSA region. Ramberg-Osgood parameter n decreased in the region of DSA, and increased with increasing strain rate near the plant operating temperature. The influence of DSA manifested in Ramberg-Osgood parameter a was similar to that in sys. But, it was less clear than that on sys and parameter n. Crack initiation resistance, Ji, and crack growth resistance, dJ/da, in DSA region were about 40 ~ 50% and 30 ~ 40% lower than those at room temperature, respectively. The temperature corresponding to minimum fracture resistance was shifted to higher temperature with increasing loading rate for both metals. However, the values of minimum fracture toughness were almost same regardless of loading rate. For both metals, the minimum J-R curve at normal operating temperature appeared at displacement rate of 4.0 mm/min rather than quasi-static loading rate or quasi-dynamic loading rate correspondin...