Delay in the static softening kinetics of a Nb-N-bearing austenitic stainless steel

In this work, the retardation in the static softening kinetics of an austenitic stainless steel ASTM F-1586 (ISO 5832-9) used in the manufacture of orthopedic implants is investigated. Double-pass deformations were applied by means of torsion testing under isothermal conditions. The samples were initially heated to 1250 °C using an induction furnace and were held for 300 s to allow for complete solubility. This was followed by cooling down to the deformation temperatures (T_def) in the range 1000 to 1200 °C. Pass strains of 0.30, strain rate of 1.0 s ⁻¹ and interpass times ranging from 0.1–1800 s were employed. These process parameters were selected to simulate the industrial-scale hot forging of this alloy to produce orthopedic implants. The results of the thermomechanical physical simulations indicated that 60% of the softening mechanisms occurred by means of static recovery (SRV). The calculated stacking fault energy (γ _SFE) of the present material is around 69 mJ m⁻², which explains the incubation time for the onset of static recrystallization (SRX) to be greater than 10 s at temperatures below 1100 °C. The static softening curves, i.e. softening fraction (X_s) versus time (t_p) plots, indicate that the delay in the static recrystallization (SRX) kinetics probably resulted from the effect of Z-phase precipitates (NbCrN) located in the grain boundaries which were induced by deformation. (paper)