Solid-solution hardening by hydrogen in Fe-Cr-Ni-based austenitic steel: Temperature and strain rate effects

Solid-solution hardening caused by dissolved hydrogen (H) atoms in face-centered cubic metals is a favorable phenomenon that counteracts the H-induced degradation of mechanical performance in structural alloys, i.e., hydrogen embrittlement. In the present study, the changes of yield and flow stresses by solute H with the concentrations of 2000-7600 at ppm were systematically investigated in a Fe-24Cr-19Ni-based austenitic stainless steel under the temperature range of 173-423 K and two different strain rates: 5 x 10-5 and 5 x 10-3/ s. Stress relaxation tests were subsidiarily employed in order to elaborate the underlying mechanisms predominating the H-related hardening at low and ambient temperatures. Four essential ingredients of the H-induced hardening were identified: (i) H atoms in the matrix lattice as dispersed obstacles; (ii) pinning of stationary dislocations by H atmosphere; (iii) dynamic pinning of dislocations resting at obstacles; (iv) drag force to moving dislocations by migratable H clouds. The hardening around 173 K was attributed to (i) and (ii), where the primary importance of interstitial-substitutional interaction between Cr and H was explicitly invoked. Meanwhile, the magnitude of hardening was maximized at around 298 K under the slow strain rate condition owing to the increasing contributions from (iii) and (iv).