Effect of ageing on properties of pressure vessel steels

Abstract

Manganese-molybdenum-nickel steels are used in the fabrication of nuclear pressure vessels operating at temperatures up to 350°C. In this work the effects of thermal ageing in the temperature range 300–550°C for durations up to 2 × 10 4 h have been studied in conventionally quenched and tempered, and simulated heat-affected-zone (HAZ) microstructural conditions. Quantitative fractography and Auger spectroscopy have been used to relate changes in mechanical properties with associated changes in fracture mode and grain boundary chemistry. The results show that ageing increases the ductile-brittle transition temperature by an amount dependent on material, prior heat treatment, ageing temperature and time. Embrittlement was associated with the segregation of phosphorus to grain boundaries and was successfully modelled using McLean's approach to equilibrium segregation. The embrittling potency of phosphorus was highly dependent on prior heat treatment; largest in the coarse-grained, higher hardness simulated HAZ condition, and least in the conventionally quenched and tempered conditon.