Atom probe tomographic observation of hydrogen trapping at carbides/ferrite interfaces for a high strength steel

A three-dimensional atom probe (3DAP) technique has been used to characterize the hydrogen distribution on carbides for a high strength AISI 4140 steel. Direct evidence of H atoms trapped at the carbide/ferrite interfaces has been revealed by 3DAP mapping. Hydrogen is mainly trapped on carbide/ferrite interfaces along the grain boundaries. Slow strain rate tensile (SSRT) testing shows that the AISI 4140 steel is highly sensitive to hydrogen embrittlement. The corresponding fractographic morphologies of hydrogen charged specimen exhibit brittle fracture feature. Combined with these results, it is proposed that the hydrogen trapping sites present in the grain boundaries are responsible for the hydrogen-induced intergranular fracture of AISI 4140. The direct observation of hydrogen distribution contributes to a better understanding of the mechanism of hydrogen embrittlement.