Relation of nonmetallic inclusions to the cyclic properties of steel 42CrMo4 after steel melt cleaning by filtration and related processes

Nonmetallic inclusions (NMIs) detrimentally influence the steel properties. The application of metal melt filter systems promises to reduce NMIs in steel. In the current work, NMIs are analyzed from metallographic sections after metal melt filtration or cleaning by a related process of the steel 42CrMo4 using immersion filtration, flowthrough filtration as well as filters floating on top of the melt. Analyses by automated scanning electron microscopy (ASPEX) as well as light optical microcopy (LOM) revealed the chemistry, size distribution, and morphology of NMIs. The filter systems had a significant effect on these characteristics of NMIs. The threshold value Delta Kth against fatigue crack propagation was found to depend on the amount of NMIs within the steel. A higher amount of NMIs caused less crack deflection leading to less roughness-induced crack closure and in turn resulted in lower threshold values at a stress ratio of R = 0.1. The comparison of the present analysis of NMIs with fatigue data from previous work indicated that the analysis of NMIs from metallographic sections can be used to approximate the fatigue strength of a batch in the same way as the analysis of NMIs from fatigue fracture surfaces for single specimens, as suggested by Murakami.