The applied variants of active, reactive and functionalized filters for metal melt filtration of the steel 42CrMo4 (AISI 4140) showed similar effects on the fatigue behavior in the very high cycle fatigue. For all variants large scatter of the fatigue life was observed.

The functionalized Al₂O₃–C filter coated with mullite showed lower fatigue lives compared to the other variants.

The dominant failure type in the investigated cast steel AISI 4140 after hot-isostatic pressing are clusters of endogenous (primary, secondary) non-metallic inclusions mainly of the alumina-type.

Four different internal crack initiation types were clearly identified in the very high cycle fatigue by plotting (i) the radii of the fisheyes and (ii) the inclusion depth versus the fatigue life.

Both, the radius of the fisheye and the inclusion depth decreases with increasing number of cycles to failure.