Effect of Stretch-forming on Hydrogen Diffusion Behavior in High-strength Steel Sheet

The hydrogen diffusion behavior in a tempered martensitic steel sheet with 1-GPa grade tensile strength was investigated using a newly developed hydrogen visualization technique with an Ir complex, whose color changes from yellow to orange due to its reaction with hydrogen. Hydrogen permeation through the steel sheet could be monitored via the color change of the Ir complex. Furthermore, the breakthrough time of hydrogen through the specimen could be qualitatively evaluated based on changes in the brightness of the Ir complex. Additionally, this hydrogen visualization technique was applied to a stretch-formed steel sheet using a hemispherical punch to simulate the press-forming of automotive structural components. The hydrogen breakthrough time around the top of the specimen increased and then decreased as the distance from the top increased. Based on the plastic strain distribution of the specimen calculated using the finite element method, the hydrogen breakthrough time increased with the plastic strain. The introduction of plastic strain decreased the hydrogen diffusion coefficient due to the introduction of dislocations acting as hydrogen trap sites, thus increasing the hydrogen breakthrough time.

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