Plastic Instability in ISO 5832-9 High-nitrogen Austenitic Stainless Steel

Abstract

ISO 5832-9 high-nitrogen austenitic stainless steel has shown promising results in the fabrication of temporary and permanent orthopedic prostheses, exhibiting better mechanical strength and corrosion resistance than the traditional ISO 5832-1 (ASTM F-138) steel. Recent studies have revealed that this alloy possesses unique properties, such as high mechanical strength and corrosion resistance and the presence of second phase particles (Z phase) in the matrix. However, it is not known how the microstructural and mechanical properties and the corrosion rate are correlated in regions of industrial processability of this alloy during hot forming. In this study, continuous and interrupted isothermal hot torsion tests were performed after solubilization heat treatment at 1 473 K for 300 s, with temperature intervals varying from 1 273 to 1 473 K and strain rates ranging from 0.05 to 5 s –1 . The purpose of these tests was to investigate the alloy’s workability and stress corrosion rate by means of electrochemical impedance spectroscopy (EIS) and to characterize its microstructure by optical microscopy. The results indicate that the yield stress is sensitive to the strain parameters. The peak stress decreases with increasing temperature and decreasing strain rate, with a high rate of dynamic recovery, with elongated grains generating areas of stress build-up. These regions of plastic instability present a higher degree of corrosion than as-received samples. The regions of high temperature and low strain rate exhibit good workability with a refined final microstructure of dynamically recrystallized grains.