Effect of boron on creep deformation behavior and microstructure evolution in 9% Cr steel at 650°C

The effect of boron on creep deformation behavior and microstructure evolution during creep has been investigated for a tempered martensitic 9Cr-3W-3Co-VNb steel containing boron up to 140 ppm at 650 degrees C (923 K) with the emphasis on long term behavior. Creep tests were carried out at 650 degrees C for up to about 70000 h. The base steel without boron exhibits degradation in creep rupture strength after times above about 1000 h. The addition of boron retards the onset of tertiary or acceleration creep, where the creep rate increases with time after reaching a minimum creep rate, at low stresses and long times. This effectively decreases the minimum creep rate and increases the time to rupture. The addition of boron reduces the coarsening rate of Ostwald ripening of M23C6 carbides near prior austenite grain boundaries during creep, which retards the onset of acceleration creep. It is considered that boron atoms occupy vacancies in the vicinity of growing carbide interfaces near prior austenite grain boundaries, which makes it difficult to accommodate local volume change around growing carbides and hence causes the reduction in coarsening rate.