Metal melt filtration by ceramic foam filters is a promising method of significantly reducing the amount of non-metallic inclusions (NMIs) in metals. As NMIs lead under mechanical load to stress concentrations inducing cracks in the metallic matrix, their removal is beneficial for the resulting mechanical properties.
The present work investigates the influence of plate-like alumina (Al2O3), which are among the most problematic NMIs, in steel that was in contact with several ceramic filters on the resulting mechanical properties.
Various types of carbon-bonded alumina foam filters with different coatings were dipped into steel of type 42CrMo4 (AISI 4140). After treatment, metallographic sections were inspected by optical and scanning electron microscopy (SEM) regarding their NMIs, for estimation of particle distribution, clustering properties and filtration efficiency. Ultrasonic fatigue tests were carried out for the measurement of high cycle fatigue limit on hardened and plasma nitrided specimens. Moreover, strength, deformability and failure behavior at quasi-static loading were determined at temperatures of -40 and 20 °C. Fracture surfaces after mechanical tests were investigated by SEM including energy-dispersive x-ray spectroscopy.
The application of different filters showed a sufficient reduction of relative area of alumina in metallography cross section. However, particle analysis based on maximum Feret diameter is found to be more appropriate for mechanical properties evaluation than equivalent circle diameter and is in a good agreement with fatigue results. Both quasi-static tensile tests and fatigue test showed that plate-like alumina inclusions are damage relevant and crack initiation sites. Hereby fatigue testing is an effective method for detection of plate-like inclusions, “opening” of all plates in the neighborhood of a propagating crack. Surprisingly the reduction of total alumina number could not improve fatigue limit and other mechanical properties. Stress intensity factors and clustering parameters of different NMI types - clusters, plates and dendrites – are compared and analyzed.