Decarburization effects on high-cycle fatigue of uncoated press hardened steels

The decarburization layer can be detrimental and lead failures in press hardened steels. This work investigates the effect of decarburization layer on the fatigue crack origination and growth on press hardened parts of uncoated 22MnB5 steel. The decarburization layer was analyzed by light microscope, scanning electron microscope, and microhardness measurements. It was found that decarburization layer has two distinct zones indicating complete and partial decarburization. To understand the effect of decarburization on crack origination, high cycle fatigue tests were performed on samples from press hardened parts in decarburized and polished (nondecarburized) conditions. The results have shown that the fatigue performance of press hardened steels in polished condition outperforms the fatigue samples in decarburized condition. In fractographic investigations, it was seen that decarburization is not only responsible for crack origination but also affects crack propagation by having full and partial decarburization zones. The full decarburization zone was acted as surface crack origination source. In the partial decarburization zone, the crack propagation behavior was changed by a complex phase microstructure of bainite, ferrite, and martensite.