Mechanical Characterization at Nanoscale of Austenite, Ferrite, and Sigma Phases via Hardness Measurement and Fretting Wear Behavior of a Duplex Stainless Steel

Abstract

This study aimed at the mechanical characterization, on a nanometric scale, of the constituents obtained for different fractions in duplex stainless-steel plates subjected to 850, 950, 1000, and 1150 °C heating treatments via hardness measurements and determining their influences on the fretting wear behavior of the studied steel. The obtained ferrite (α)-, austenite (γ)-, and sigma (σ)-phase fractions were determined using optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) techniques. The mechanical characterization was carried out using hardness measurement and fretting wear techniques via nanoindentation. For comparison purposes, the Vickers microhardness was also characterized to determine the effect of the σ phase, which eventually formed, on the obtained microstructure properties as a whole. Two distinct behaviors were observed, depending on the eventual formation of σ phase as a function of the treatment temperature: (i) specimens treated at 850 and 950 °C showed a hardening effect (HV0.5 values of 333 ± 15 and 264 ± 13, respectively) due to σ-phase precipitation (hereafter termed ‘as-aged’), and (ii) specimens treated at 1000 and 1150 °C (with HV0.5 values of 240 ± 13 and 249 ± 4, respectively) showed no σ-phase precipitation (hereafter termed ‘as-solubilized’). The increases in the microhardness values for the as-aged specimens were attributed to the hardness of the σ-phase precipitates (which showed nanohardness values varying in the 8.0–8.5 GPa range), which was approximately twice that of the austenite and ferrite grains (both phases showed nanohardness values in the 3.6–4.1 GPa range, on average). When formed (for fractions on the order of 8% and 3% at 850 and 950 °C, respectively), σ phase was mainly observed at the α/γ grain interfaces or boundaries. Fretting wear tests, using a diamond sphere with a radius of 10 μm as the counter body and a load of 20 mN, revealed the same wear mechanisms in the α/γ matrix for all studied conditions. However, as-solubilized specimens (heat-treated at 1000 and 1150 °C) displayed higher resistance to fretting micro-wear in the austenitic grains compared to the ferritic grains, indicating lower plastic deformation in the respective wear scars on the obtained tracks. In particular, as-aged specimens (heat-treated at 850 and 950 °C) exhibited lower coefficients of friction due to their higher surface resistances. The localized wear at σ-phase grains was much less pronounced than at ferrite and austenite grains. Overall, this study provides valuable insights into the mechanical behavior of microstructural changes in duplex steel at the nanometric scale.