Mechanisms of hydrogen-induced cracking in high-strength screws

摘要

Low alloyed high strength steels with yield strength above 1200 MPa are increasingly implemented for the reduction of the component weight in the automotive industry. This material type is known for its susceptibility to hydrogen assisted cracking. Both, manufacturing process induced and hydrogen induced embrittlement during component operation have to be considered. The aim of this study was to evaluate the impact of different zinc-containing topcoats on hydrogen re-embrittlement. The susceptibility of high strength fasteners (strength class 12.9) with various zinc-based topcoats (electroplated zinc, electroplated zinc-nickel, galvanized zinc and zinc-flake) were tested under various corrosive environmental conditions. It is shown that the zinc-containing topcoats impose a different susceptibility on hydrogen re-embrittlement. Under the tested environmental conditions, fasteners with electroplated zinc revealed a decrease in the fracture load compared to the heat treated black specimen. In contrast to that, an electroplated zinc-nickel topcoat demonstrated a failure at the loads slightly exceeding the fracture load of the uncoated specimen.