Hydrogen microprint analysis on the effect of dislocations on grain boundary hydrogen distribution in steels

Abstract

Hydrogen related intergranular fracture in high strength steels and steel welds are often explained by the hydrogen segregation at grain boundary that induces decohesion of the boundary. In this study, distribution of hydrogen in the grain boundary was visualized by a hydrogen microprint technique (HMT), and the effect of dislocations on hydrogen distribution in steel was analyzed. 0.02%C steel with ferrite microstructure was subjected to a plastic deformation up to 20% and HMT analysis, and then quantitative analysis of hydrogen concentration was conducted by measuring the area of silver particles formed by the reductive reaction between AgBr and hydrogen. The grain boundary hydrogen density was defined by an area of silver particle at the grain boundary per unit grain boundary length and diffusible hydrogen charged. And, it was found that the grain boundary hydrogen density decreases with increasing prestrain since hydrogen is trapped by dislocations inside the grain. The effect of dislocations introduced by martensitic transformation was also investigated using 0.1%C steels in as-quenched and quenched and tempered conditions. Silver particles were mainly observed on lath boundary in the as-quenched condition, while hydrogen density at the prior austenite boundary increased after tempering. It can be said that hydrogen segregation at the prior austenite grain boundary is affected by dislocations and lath boundary in martensite microstructure. Effect of dislocations on hydrogen distribution was also discussed based on dislocation densities and hydrogen contents.