오스테나이트 스테인리스강의 변형모드와 수소취성에 관한 연구

Abstract

Materials in which hydrogen gas can be stored and transported have become increasingly in demand in response to the focus on H2 as an alternative to fossil fuels. General carbon steels cannot be used for these applications due to hydrogen embrittlement (HE), which leads to a drastic failure of components. Austenitic stainless steels (STSs) are appropriate for these tasks owing to low H diffusivity and high H solubility. However, the most commercialized STS 316L suffers from low strength and high cost. Thus, a new type of economical as well as practical STS is needed, but the alloy design strategy has been unclear or absent until now. Firstly, austenite stability and HE was investigated in metastable Cr-Ni-Mn-N austenitic STSs. When Ni was replaced with Mn, the thermodynamic and mechanical austenitic stabilities of these elements were different: Mn was more beneficial to increasing thermodynamic stability against martensitic transformation than the equivalent amount of Ni. The tendency for strain-induced martensite (SIM) transformation was governed not by the thermodynamic stability but by stacking fault energy (SFE), which was increased more effectively by Ni than by the equivalent amount of Mn. Furthermore, SIM transformation and H redistribution became different after pre-charging and deformation, which could be explained with SFE: H2 out-gassing during deformation was predominant in a high-SFE STS, and H trapping at austenite/SIM interfaces mainly occurred in a low-SFE STS. Hence, SFE determined HE susceptibility of metastable austenitic STSs. Secondly, the way to design a low-Ni STS was suggested in terms of Ni+Mn and Ni/Mn. SIMT should be suppressed to increase a resistance against HE, but the fraction of strain-induced a´-martensite was dependent on deformation amount and H charging. Thus. deformation indices were newly introduced to represent this tendency, and a stable austenite region against strain-induced a´-martensite was introduced on the plots of Ni+Mn vs Ni/Mn. Lastly, the effect of grain size on HE was investigated with a high-N austenitic STS and a high-Mn twining-induced plasticity steel. They showed an opposite HE susceptibility to grain refinement, which was related to different deformation modes and resultant dislocation densities: increase in dislocation density led to increase in susceptibility to HE. Hence, deformation mode and resultant dislocation density determined HE susceptibility of stable austenitic STSs. The present work investigated critical factors to determine HE susceptibility in Cr-Ni-Mn-N austenitic STS and how to design a low-Ni austenitic STS, which was mainly correlated with deformation mode. Therefore, alloy design strategy and HE-related failure analysis should focus on this aspect.