Softening-precipitation interaction in a Nb-and N-bearing austenitic stainless steel under stress relaxation

In this work, stress relaxation technique was employed to analyze the softening-precipitation interaction in an ASTM F-1586 austenitic stainless steel, an alloy used in orthopedic implants. Hot compression tests were performed at temperature range of 800–1100 °C. Pre-strain of 5.0%, strain rate of 0.1 s−1, and loading time exposure of 40 min were employed. The experiments were carried out in a dilatometer equipment. The stress- time (σ vs. log (t)) curves were plotted, and the microstructure of the specimens was characterized. The acquired results permitted to estimate the start (ts) and the finish (tf) times of the effective interaction between recovery and precipitation. The data revealed three distinct stages in the stress versus log time curves: (i) reduction in the stress levels due to static recovery; (ii) delay in the stress relaxation rate leading to a stress plateau, and (iii) sharp decrease of the stress levels associated with the finish of the precipitation. These results enabled the creation of Precipitation-Temperature-Time (PTT) curves for the present material. It was found that the onset time for the strain-induced Z-phase (CrNbN) is around 120 s and the precipitates delay the kinetics of the softening.