Propriedades da austenita expandida por hidrogênio e sua interação com a fase γN em uma liga Fe-Cr-Ni

Abstract

The permeation of hydrogen produces varied effects in metals. One of these effects is the interstitial retention during cathodic charging, which is of brief duration and is, generally, neglected in studies on hydrogen embrittlement. This phenomenon was evaluated through in situ structural and mechanical analysis on a superaustenitic stainless steel (SASS), in the light of recent advances in the understanding of diffusion and retention of interstitial elements in Fe-Cr-Ni alloys. The hydrogen flux from the electrolyte through the SASS surface saturates interstices of the face-centered cubic (FCC) structure, leading to the formation of a hydrogen-expanded phase. This phase exhibits similarities to the nitrogen-expanded austenite phase, γN. The expansion of the lattice shows two distinct domains, high (γH) and low expansion (γe). The metastable nature of the structure becomes evident through its rapid decay, γH → γe → γ, which is completed in approximately one day. The layer with the expanded phase exhibits hardness twice as high and elastic modulus 17% higher than those of the substrate. These properties return to the levels of the unmodified substrate after the γH phase decay. The hydrogenation affects the elastoplastic response of the surface. At high strain rates, the increase of localized plasticity due to hydrogen, described by the HELP mechanism, is overcome by solid solution strengthening. On the other hand, hydrogen embrittlement dominates the surface plastic interaction at low strain rates. The inhibition of hydrogen flux through a nitrided layer composed of the γN phase was also evaluated under the same cathodic charging conditions. The modified surface partially inhibits the permeation of ions due to the presence of an opposing chemical potential gradient, resulting in a smaller lattice expansion than that observed in the base material. The hardness of the nitrided layer, higher than that of the substrate (6.5 GPa vs. 2.5 GPa), increases further after cathodic charging, reaching up to 10 GPa. Upon hydrogen outgassing, the values return to pre-hydrogenation levels. Hydrogenation alters the elastoplastic response of the nitrided layer, which shifts to a brittle character. The mechanism leading to the partial detachment of regions of the layer is due to the competitive effect between compressive residual stresses imposed by γN and tensile stresses at the γN/γ interface. In the latter, molecular hydrogen H2 is formed, which erupts selectively through the γN nitrided layer along crystallographic directions favorable to the diffusion and accumulation of hydrogen. In conclusion, the expanded phase γH can be properly understood through the interstitial diffusion and trapping mechanisms in the FCC lattice, considering its highly metastable nature.