The creep strength of a 20%Cr-25%Ni-Nb steel containing controlled particle dispersions

Abstract

A 20%Cr-25%Ni-Nb steel subjected to mechanical working and precipitation strengthening was creep-tested at a temperature of 1073 K in the stress range 25–180 MPa. Coarse and fine NbCN precipitate distributions were investigated. At stresses less than that to bow dislocations between precipitates each alloy exhibited a linear dependence of creep rate on stress. At high stresses, increases occurred in stress dependence, n, from 1 to 9.5 in the coarse and from 1 to 10.3 in the fine distribution materials. Dislocation densities measured on samples strained in secondary creep also showed a marked transition in behaviour above and below the yield stress for bowing. Below YS the densities were independent of stress; above YS the density could be described by the relation: σ_A=σ_B+αμb(P _A ^½−P _o ^½), where σ_A is the applied stress, σ_p the YS due to bowing between particles, σ_A the dislocation line tension factor, μ the shear modulus, b the Burgers vector, P _A the dislocation density in secondary creep, and P _o the limiting lower dislocation density. The creep mechanisms at low and high stress are discussed. Although the low-stress creep rates agree to within a factor of three with the Coble grain boundary diffusion creep process, insufficient data on the influence of grain size on creep are available at present to confirm this mechanism. At higher stresses the observed creep rates may be rationalized according to the relation ε_s ∞(σ_A−σ_p)⁴.