Hardening optimization of high chromium-manganese austenitic steel

The study is focused on the assessment of the best thermal range for plastic deformation of Cr-Mn austenitic steel, to obtain a correct hardening and mechanical properties at room temperature. This steel grade is featured by a fully austenitic microstructure deriving from the high concentration of Mn and N, and is mainly used for the retaining rings bearing of power generation shafts. These components should not have magnetic permeability and thus, the mechanical strengthening can be performed by strain hardening and activation of twinning systems during rolling and forging at high temperature. Different specimens were tensile tested at different temperatures and different strains without arriving at the fracture point. Once the strained specimens were cooled, they have been tested by complete tensile tests at room temperature to determine the final mechanical properties. The best combination of the final mechanical properties have been obtained for plastic deformation performed between 250°C and 350°C, but the formation of martensite at 250°C narrows the useful thermal range between 300°C and 350°C. The metallographic observations indicated that the best hardening conditions can be obtained through the exploitation of the twinning plasticity effect and when the deformation temperature avoids any recovery that can reduce the dislocation density maintained after the cooling at room temperature. The performed experimental trials have also allowed stating the most favorable thermal range for the strain hardening of Cr-Mn steels through forging process to maximize the strengthening effect without the detrimental chromium carbide precipitation.