The effect of Si on hydrogen embrittlement of Fe-18Mn-0.6C-xSi twinning-induced plasticity steels

The hydrogen embrittlement (HE) of Fe-18Mn-0.6C-xSi (wt.%) twinning-induced plasticity (TWIP) steels was investigated through slow strain rate tensile tests (SSRTs) and thermal desorption analyses of electrochemically H-charged specimens. Whereas the H-charged Si-free steel showed only the (Fe,Mn)O layer with a fcc crystal structure on the surface, the Si-added steels had double oxide layers; the outer layer was a mixture of (Fe,Mn)O and (Fe,Mn)₂SiO₄ with an orthorhombic crystal structure and the inner layer was only (Fe,Mn)₂SiO₄. When the Si concentration increased, the (Fe,Mn)₂SiO₄ layer became thicker and the charged H concentration decreased. This result indicates that the (Fe,Mn)₂SiO₄ layer is effective in suppressing the permeation of H. Both the elongation loss (E_loss) and the area fraction of the brittle-fractured region increased with increasing Si concentration in the H-charged TWIP steels, particularly in the 3 wt.% Si steel, although the H concentration slightly decreased with increasing Si concentration. The H-charged Si-free and 1.5 wt.% Si steels underwent mechanical twinning and the migration of H atoms from lattices, dislocations and grain boundaries to mechanical twins during the SSRTs. The brittleness of both Si-free and 1.5 wt.% Si steels was caused by H-concentrated mechanical twins. The H-charged 3 wt.% Si steel underwent ε-martensitic transformation as well as mechanical twinning during the SSRT. H atoms migrated to mechanical twins until a strain of 0.24, and then inherited primarily into ε-martensite with further strain. The great E_loss of the H-charged 3 wt.% Si steel was caused mainly by H-concentrated ε-martensite.