Hardening during deformation of an 18Cr/12Ni/Nb austenitic steel at 650°C

Abstract

The hardening mechanisms operating when niobium carbide precipitation occurs during deformation of an austenitic stainless steel have been studied by tensile testing sheet specimens of an 18Cr/12Ni/Nb steel at 650°C after solution treatment at 1300°C. The structures at fracture and at small strains were examined by extraction replica and thin film electron microscopy. Three types of hardening mechanisms were obtained at slow strain rates. The first occurred directly after the proportional limit and is thought to be produced by an increase in dislocation density as a result of an atmosphere drag force, the second is the hardening produced by decorated dislocations, and the third is attributed to the presence of precipitation on stacking faults. Plots of (dislocation density) 1 2 against flow stress are used to illustrate the occurrence of Orowan hardening due to decorated dislocations. The effect of deformation on the growth of stacking faults was also examined; it was found that deformation produced earlier nucleation of faults and an increase in their rate of growth. These effects are considered to be due to the increased rate of niobium diffusion during deformation.