Localized attack at inclusions in 42CrMo4 QT steel

Localized attack can negatively affect the mechanical properties and durability of high strength low alloy steels. The present work investigates the effect of inclusions on the susceptibility to pitting attack of 42CrMo4 QT steel (quenched and tempered condition). A detailed characterization of the microstructure of the steel was performed by field emission scanning electron microscopy and energy dispersive X-ray spectroscopy in order to evaluate the morphology and chemical composition of the inclusions. The initiation of pitting corrosion at the inclusion sites was investigated with a localized approach combining topographic and surface potential maps obtained by means of the scanning Kelvin probe force microscopy technique and the morphology of inclusion sites after the immersion in an aggressive electrolyte. The effect of different inclusion types (clusters combining Al2O3 and MnS and containing Ca and Mg or single MnS based inclusions) is discussed in the present work taking into account their galvanic coupling with the surrounding steel matrix and the existence of gaps at the periphery of the inclusions promoting the formation of an occluded cell.