Quantitative dependence of ductile-to-brittle transition on phosphorus grain boundary segregation for a novel Ni–Cr–Mo RPV steel

Abstract

SA508-4 N steel is considered a novel Ni–Cr–Mo low-alloy steel for reactor pressure vessels. The low-temperature embrittlement in the steel is an important issue that needs to be addressed. Herein, the main aim of this work is to investigate the non-hardening embrittlement caused by grain boundary segregation (GBS) of P in a P-doped SA508-4 N steel with the use of impact testing in conjunction with Auger electron spectroscopy (AES) and microstructural characterization. The steel samples are quenched from 1000 °C, followed by tempering at 650 °C and subsequent ageing at different lower temperatures. The ductile-to-brittle transition temperatures (DBTTs) of the aged specimens are determined by mini-Charpy impact testing (specimen: 2.5 mm × 2.5 mm in cross section), and the GBS of P is evaluated using AES. The results show that there is a linear dependence of DBTT (oC) on the boundary concentration of P (Cp/at.%), when the steel hardness is fixed after the 650 °C-tempering (∼260 HV10), being expressed as DBTT=13.31Cp−339.28. Employing this relation, one can forecast the DBTT once the boundary level of P is known. With the DBTT−segregation relation in combination with segregation thermodynamics and kinetics, a time–temperature embrittlement diagram is also developed. This may be employed to predict the tendency of embrittlement in the steel, caused by GBS of P after serving at a temperature. It is believed that this study is helpful in monitoring the safe operation of devices manufactured with this novel low-alloy steel.

Keywords

• RPV steel
• Embrittlement
• Ductile-to-brittle transition
• Grain boundary segregation
• Auger electron spectroscopy