Modelling of Grain Boundary Segregation in 1/2Cr1/2Mo1/4V Boiler Shell Steel Welds

摘要

Ductile-brittle transition temeprature (DBTT) shifts of up to 60 deg C have been observed in 1/2Cr1/2Mo1/4V steel welds after service for a period of up to 30 years at about 350 deg C. These changes are absent at lower temperatures, for example, at 180 deg C. There is a suggestion that vanadium and molybdenum carbide precipitation is responsible for the effect. It is anticipated that there may be some non-hardening embrittlement caused by grain boundary phosphorus segregation. To examine the possibility of grain boundary phosphorus segregation being a cause of the effect, grain boundary segregation in the steel welds is modelled by simplifying the steel into a Fe-C-Mo-P alloy and considering the weld cycle and post-weld heat treatment. In the modelling, the segregation of C, Mo and p is treated at the same time and all sites at the grian boundary are assumed to be available for solute segregation. The model predictions indicate that there is a large amount of C segregation after 6-hour stress relieving at 650 deg C (approx 48 at.