Observation of Hydrogen Distribution Around Non-Metallic Inclusions in Steels with Tritium Microautoradiography

Hydrogen distributions around non-metallic inclusions in steels are successfully characterized with high-resolution tritium autoradiography. The autoradiographs show that hydrogen accumulation characteristics around the inclusions depend on types of the inclusions. In the case of MnS, hydrogen was inhomogeneously distributed in the ferrite matrix surrounding the MnS inclusion, probably because hydrogen is trapped in defects formed around MnS. The inhomogeneous distribution of hydrogen may be originated from the asymmetric stress field produced by a contraction of the MnS phase in the heat treatment, i.e. the inhomogeneous volumetric change of MnS owing to its larger thermal expansion than that of the ferrite phase. In the case of Al₂O₃, hydrogen was intensely localized at boundary layers of the ferrite matrix surrounding the Al₂O₃ inclusion. This could be attributed to hydrogen trapping at defects introduced by a residual stress in the boundary layers of the ferrite matrix due to larger contraction of the ferrite phase than that of the Al₂O₃ phase on cooling. Similarly hydrogen was accumulated in the surrounding ferrite matrix but more widely distributed around Cr carbide probably because difference in the thermal expansion between the Cr carbide and ferrite phases is less than that between the Al₂O₃ and ferrite phases.