Influence of thermo-mechanical embrittlement processing on microstructure and mechanical behavior of a pressure vessel steel

Abstract

A thermo-mechanical embrittlement processing (TMEP) consisting of thermal aging and cold strain could cause the deterioration of reactor pressure vessel (RPV) steels in the form of an increase in the ductile to brittle transition temperature (DBTT) and a decrease in the upper-shelf energy (USE). In this study, the TMEP was employed to investigate the microstructure and the evolution of mechanical behavior of the SA508-IV pressure vessel steel. In the microstructure of the as-received state, Cr and Mn atoms replace Fe atoms and form alloying cementites, (Fe,Cr)3C and (Fe,Mn)3C, through in-situ nucleation. Due to the slower diffusion coefficient, Cr precipitates in the outer layer of the Mn clusters. In the subsequent embrittlement process, needle-shaped Mo2C, fine copper-rich precipitates (CRPs) and P-rich precipitates are formed, which play a great role in the mechanical behavior evolution. Mechanical test results show that a series of changes in mechanical behavior occurred. It has been found that DBTT fits a linear function of the square root of embrittling time at 520°C (t1/2) from 10h to 90h degradation and the degree of embrittlement reaches saturation after 90h.