Hydrogen Embrittlement Induced by Hydrogen Charging during Deformation of Ultra-high Strength Steel Sheet Consisting of Ferrite and Nanometer-sized Precipitates

Abstract

The hydrogen embrittlement behavior of an ultra-high strength steel sheet consisting of ferrite and nanometer-sized precipitates has been investigated by a tensile test and sustained tensile-loading test. The amount of absorbed hydrogen of the present ferritic steel is significantly larger than that of the conventional martensitic steels. Hydrogen thermal desorption analysis indicates that a large amount of diffusible hydrogen exists and the nanometer-sized precipitates act as trap sites of hydrogen. In a tensile test in air after the saturation amount of hydrogen charging, the fracture strain decreases slightly. However, in a tensile test during hydrogen charging, the fracture strain decreases markedly despite a small amount of absorbed hydrogen, and the morphology of the fracture surface exhibits a unique brittle mode. Upon prestraining, the saturation amount of hydrogen is more than doubled and the tensile properties deteriorate further. In the sustained tensile-loading test during hydrogen charging, no delayed fracture occurs even under high applied stress. No effects of hydrogen charging on stress relaxation are observed. The results of the present study imply that the increase in the hydrogen content enhances the degradation of tensile properties, but the hydrogen content is not necessarily an index of the hydrogen embrittlement of the ferritic steel. The dynamic interactions between hydrogen and deformation, and particularly, the continuous interactions during hydrogen charging, play important roles in hydrogen embrittlement.

Delayed Fracture Enhanced by Martensite Transformed from Retained Austenite in Ultra-high Strength Steel Sheet

“…As this suggests, an appropriate test method for evaluating the hydrogen embrittlement behavior of automotive steels should be selected. In the tensile test, the continuous interaction between hydrogen and plastic deformation affects hydrogen embrittlement behavior 8,21,22) . In particular, in tensile tests during hydrogen charging, dynamic interactions occur continuously from the early stage of deformation until fracture, and the significant amount of damage induced by these interactions substantially increases the degradation of tensile properties 21,22) .…”