On the creep behaviour of HP40Nb alloy with high-temperature carburisation damage

The effect of carburisation damage on the creep behaviours of HP40Nb alloy was investigated. The carbon concentration profile and microstructural evolution were characterised for the carburised specimens with and without creep, respectively. The variations of creep properties and fracture mode in carburisation treatment were also revealed. The results indicate that massive Cr-rich secondary carbides are precipitated in the matrix after carburising for 5 h, leading to the prolongation of creep lifetime. The further carburisation promotes the coarsening of primary and secondary Cr-carbides and weakens the creep strength of HP40Nb alloy. In creep process, the outward diffusion of Cr atoms contributes to the formation of Cr₂O₃ oxide layer on the surface and the emergence of Cr-depleted zone on the subsurface, while the inward diffusion of C atoms causes the rapid rise and gradual fall in the carbon concentration for all creep-carburisation specimens. Additionally, the creep fracture mode changes from brittle fracture to ductile fracture with the prolonging carburising time and the increasing creep stress, and the continuous carburisation results in the transformation of creep fracture mechanism towards the viscoplastic cavity growth over the applied stress range.