Constitutive analysis of stress–strain curves in dynamic softening of high Nb- and N-containing austenitic stainless-steel biomaterial

Abstract

High N-containing austenitic stainless steels have long been used as materials in orthopedic implants. For almost three decades, research has shown that ASTM F-1586 steel is an alternative for orthopedic applications involving severe loads and long implant survival rates in the human body. However, several studies have detected impaired mechanical strength of prostheses during use as a result of manufacturing processes. In this research work, the dynamic softening of this material is characterized based on a constitutive analysis of the stress–strain curves under conditions resembling those of industrial manufacturing, obtained by continuous isothermal hot torsion tests at different temperatures (900ºC-1200 °C) and strain rates (0.01–10s−1). The results indicate that the hot deformation apparent activation energy (Qdef = 594 kJ/mol) is high compared to other types of 300 series stainless steel, as are the ratios between critical (σc), peak (σp), steady state (σss) and saturation (σsat) stresses: σo/σp = 0.69, σc/σp = 0.94, σss/σp = 0.68 and σsat/σp = 1.01. These high values suggest competition between the mechanisms of work hardening (WH), dynamic recovery (DRV) and dynamic recrystallization (DRX), with delay in the onset and progression of DRX kinetics, significantly affected by the moderate stacking fault energy (γsfe ∼ 68.7 mJ/m2), solute atoms (Nb,N) and by fine Z-phase precipitates (CrNbN) at the grain boundaries, which favor softening with intense DRV. Thus, as can be seen, the parameters of WH (h), DRV (r) and DRX (t0.5, n) determine the shape of the stress–strain curves.