Atom probe tomographic study of the grain boundary segregation affecting delamination crack in ferritic steels, and the passive film affecting corrosion resistance in austenitic stainless steels

Abstract

The present study conducted two topics on atom probe tomography (APT) study which have not been solved yet in steel research. The first topic is APT study of the grain boundary segregation affecting delamination crack in ferritic high-strength low-alloy (HSLA) steels. The second topic is APT study of the passive film affecting corrosion resistance in austenitic stainless steels. The first topic aims at unveiling the influence of trace amounts of phosphorus on the macro-scale delamination in ferritic HSLA steels. Two different steels with trace amounts of phosphorus were examined to reveal the cause: One was made by long-time high temperature holding to maximize the phosphorus segregation, and the other one was made by short-time high temperature holding to minimize the phosphorus segregation. The atom probe results at the grain boundaries ahead of the delamination-related cracks provide a strong evidence that phosphorus-enrichment and carbon-depletion for the long-time high temperature holding induces a larger degree of delamination as compared to the short-time high temperature holding. Reasons for the different segregation tendency were discussed by correlating the isothermal holding time at high temperature and the competitive segregation between phosphorus and carbon. The second topic focused on the utilization of APT to obtain a direct evidence of Mo-, Ni- and Mn-alloying effects on the corrosion resistance of austenitic stainless steels through characterization of the passive films in atomic scale. Toward this purpose, the passive films of Ni-added commercial SS304L (0.32 wt.% Mo), SS316L (2.05 wt.% Mo), and Ni-free high-Mn stainless steels were analyzed. The results showed that Mo, which exists as Mo atoms and/or Mo-N pairs in the steel matrix, affects the passive film in the form of dense MoO2 molecules. This phenomenon was more pronounced for SS316L in comparison to SS304L, leading to an enhanced corrosion resistance for SS316L. Ni in both SS304L and SS316L affects the passive film by forming a large amount of NiO barrier near the interface between steel and passive film, which may help to prevent further corrosion. As for high-Mn stainless steel, Mn is present in the passive film as the faintly dispersed MnO molecules. Considering the lower corrosion resistance of high-Mn stainless steel, it is inferred that Mn cannot maintain a stable metal-oxide state like Mo and Ni in the passive films, resulting in the negative effects on the corrosion resistance.