(A) study on the intercritical annealing treatment on the mechanical properties of SA106 Gr.C piping steel

Abstract

From the point of view on material properties in Leak-Before-Break (LBB) analysis, the premise of LBB is that the materials used are sufficiently tough (ductile) that small through-wall crack would remain stable. It is reported, however, that the toughness and LBB safety margins of SA106 Gr.C piping steel is reduced due to dynamic strain aging (DSA) at the reactor operating temperature. In this study, intercritical annealing in two-phase ($\alpha+\gamma$) region was performed to investigate the possibility of improving the toughness and reducing DSA susceptibility for giving allowable LBB safety margins. Tensile tests were carried out under various temperatures and strain rates for assessing the material properties with the heat treatments. Also, Charpy impact tests were conducted to measure impact toughness at room temperature. After intercritical annealing, the manifestations of DSA were observed in the tensile properties. However, the ductility loss caused by DSA was smaller than the as-received material. Furthermore, the intercritical annealing was able to increase the impact toughness by 1.5 times compared to as-received material. With the heat treatment, we could obtain some kinds of microstructural changes such as the cleaner retained ferrite, increased ferrite content and somewhat finer grain size. It is considered that the reduced DSA sensitivity was induced by cleaner retained ferrite and it resulted in higher impact toughness in addition to the general toughening due to finer grain sizes and increased ferrite content.