Thermally induced surface roughness in austenitic–ferritic duplex stainless steels Original Research Article

Austenitic–ferritic duplex steels hot forged to rods demonstrate a complex deformational behaviour even in the absence of mechanical loads: purely thermal cycling in the temperature interval from 20 °C to 900 °C can cause either accumulation of inelastic strains (thermal ratchetting) or plastic shakedown, depending on microstructure morphology and thermomechanical properties of the constituting phases. The structural macroscopic response of entire specimens to cyclic thermal loading is investigated by dilatometry experiments. The influence of traction-free external surfaces on the microscopic deformation of ferrite and austenite is examined by measuring the surface roughness evolution by three-dimensional profile scans. Besides an increase of the roughness parameters due to thermal cycling, the measured surface profiles also reveal a strong anisotropy linked to microstructural orientation. Spatial surface roughness parameters based on the autocorrelation function allow a quantitative correlation between roughness and microstructure. Measurements are compared with micromechanical finite-element predictions.