The Effect of Long Ageing Treatments on Steady-State Creep in 20% Cr–25% Ni, Nb-Stabilized Stainless Steel

Abstract

The variation in the steady-state creep behaviour with ageing time of 20Cr–25Ni–1Nb stainless steel has been studied over a stress range of 60–160 N/mm² and at a temperature of 1023 K. After short ageing times at 1023 K before creep, activation-energy values were found to be greater than that for self-diffusion (∆H _{S D} ) and to decrease with increasing applied stress. With increasing ageing time, the creep activation energy became less stress-dependent and approached ∆H _{S D} . This change was accompanied by a gradual reduction in the magnitude of the stress exponent. As the ageing time increases, NbC precipitates coarsen and the NbC→M₆C transformation occurs. The creep behaviour is attributed to the variations of internal backstress that accompany these changes in structure.