Properties of superplastic Cr-Ni steel

Abstract

Superplastic flow has been investigated in the steel X5 CrNiTi 26.6 by means of the hot tensile test. Particular attention has been paid to the magnitude of flow stresses, strain-rate sensitivity, and total elongation with its dependence on microstructure (controlled by technological treatment), strain rate, and temperature. Of all the microstructural parameters examined, the matrix grain size seems to have the predominant influence. The steel exhibits superplastic behaviour if the matrix has a grain size Dα, ≤4.5μm,if the strain rate ε is <10⁻¹ min⁻¹, and if the temperature is above 875° C. At temperatures ≥ 1000°C in the α- and γ- regions, grain growth occurs, and the superplastic flow consequently terminates. As temperature and strain rate are connected in an Arrhenius-type relationship, an activation energy for flow processes in the ferrite-austenite phase mixture can be determined for the range of ferrite grain sizes D_α,= 3.2 to 5.4 μm.The activation energy depends linearly on matrix grain size as long as the requirements for superplastic flow are sufficiently fulfilled. For D_α,=3.2 to 5.0 μm the values of the activation energy Q are between 220 and 280 kJ/mol. Comparison of these values with results reported in the literature shows that self-diffusion is the rate-controlling process for the flow considered. This might explain the relatively low deformation velocities during superplastic flow.