Relation of embrittlement to phosphorus grain-boundary segregation for an advanced Ni–Cr–Mo RPV steel

Abstract

Embrittlement is an essential issue for reactor pressure vessel (RPV) steels. Grain-boundary segregation (GBS) of impurity phosphorous is one of the major reasons for non-hardening embrittlement of an RPV steel. The main aim of the present work is to establish a relation of embrittlement to phosphorous GBS for an advanced SA508-4N Ni–Cr–Mo RPV steel so that one can forecast its embrittling tendency. To obtain this type of relation, the steel samples doped with phosphorous are quenched from 890 to 1100 °C to obtain different prior austenite grain sizes (PAGSs) and then tempered at 650 °C, followed by ageing at different lower temperatures so as to achieve different phosphorous segregation levels at prior austenite grain boundaries. The characterization work is done with the use of mini-Charpy impact tests (specimen: 2.5 mm × 2.5 mm in cross section) together with Auger electron spectroscopy, scanning electron microscopy, and optical microscopy. The results indicate that there is a linear relationship between ductile-to-brittle transition temperature (DBTT/oC) and phosphorous GBS concentration (Cp/at.%) for both cases (quenching from 890 °C: PAGS = 34 μm; quenching from 1100 °C: PAGS = 112 μm). The relations can be expressed as DBTT=13.13Cp−335.70 (PAGS = 34 μm) and DBTT=6.69Cp−223.87 (PAGS = 112 μm). The rate of material embrittlement, characterized by the slope of the line, is associated with both the phosphorous segregation-induced grain-boundary (GB) embrittlement and the total GB area per unit volume. Interestingly, there is a critical phosphorous GBS concentration, below which the DBTT of the coarse-grained specimen is higher due to its higher intrinsic brittleness and above which that of the fine-grained specimen is higher at the same phosphorous GBS level due to the considerable phosphorous segregation-induced GB embrittlement along with a much larger total GB area per unit volume.