Improved creep resistance of 20Cr25NiNb heat resistant steels through grain boundary intermetallic precipitation strengthening

The creep performance of 20Cr25NiNb austenitic steel through the addition of W, Mo, and B elements was investigated. W and Mo enhanced high temperature creep resistance primarily through solid solution strengthening σss and precipitation strengthening σp by introducing Laves phase. Boron enrichment was predominantly observed in Laves phase at grain boundaries (GBs) rather than in σ and G phases. The significant improvement in creep life of B-bearing steel was primarily attributed to a remarkable increase in σp, as trace amounts of B greatly accelerated Laves precipitation at GBs and inhibited σ phase coarsening. The creep resistance decreased significantly after 5000 h thermal aging due to reduced dislocation density, grain coarsening, and decreased solid solution elements in austenite. Creep cracks mainly propagated along GBs, where coarse σ phase provided sites for creep cavity nucleation. The applied stress accelerated σ precipitation and coarsening, resulting in the absence of tertiary creep stage during 4700 h creep of B containing 2025WMoB steel.