Improvement of the Hydrogen Microprint Technique on AHSS Steels

Preferential hydrogen diffusion paths in low-carbon ferritic steel and transformation-induced plasticity (TRIP) steel characteristic of new advanced high strength sheet steels (AHSS) were evaluated via the hydrogen microprint technique (HMT). As a consequence of hydrogen diffusion and evolution from the sample after charging, the technique involves reduction of AgBr emulsion applied to the surface of a charged sample to produce discrete Ag grains which identify, via scanning electron microscope (SEM) imaging, locations where hydrogen evolved. Enhancements to the results are revealed through a newly developed image-before-etch HMT method, consisting of SEM imaging before and then again after etching. The modified technique was implemented to facilitate direct comparison of silver grains with the features of an etched multi-phase TRIP steel microstructure. Results from SEM imaging demonstrated the effectiveness of the image-before-etch method to avoid artifacts and to positively identify hydrogen evolution sites in AHSS TRIP steel microstructures.