Carbon‐bonded ceramic foam filters with different functional coatings are immersed in a 42CrMo4 steel melt within a steel casting simulator. The solidified steel is analyzed with respect to the size distribution and the chemical composition of the remaining nonmetallic inclusions (NMI). Cyclic loading and quasi‐static tests are performed to determine the fatigue limit, the strength, deformability, and toughness of the steel after filter immersion. The immersion of filter with calcium hexaluminate (CA6) coating significantly reduces the population of small (4–20 μm) NMIs. This leads to an increased deformability and, thus, ability for energy dissipation during deformation. However, the maximum size of NMIs is increased from 100 to 150 μm, which results in fatigue limit reduction, despite the decrease in NMIs total density. The majority of inclusions are found to be pure alumina. Large (up to 150 μm) plate‐like alumina inclusions introduce most of the detrimental effects on cyclic strength, whereas significant effect on quasi‐static strength is not found.
In this paper, the effect of a mullite coating on a carbon-bonded alumina filter used in steel melt filtration on the mechanical properties of the cast steel G42CrMo4 (1.7231) at both quasi-static and cyclic loading is presented. The investigations cover quasi-static tensile tests at different temperatures. Additionally, cyclic tests in high-cycle fatigue (HCF) and very high-cycle fatigue (VHCF) regimes were performed. Fracture surfaces as well as cross-sections are examined by scanning electron microscopy and light microscopy to determine the non-metallic inclusions responsible for failure. In comparison to the uncoated filter, a mullite coating on the filter leads to a deterioration of deformation characteristics and fatigue lifetime of the cast steel. This effect is attributed to a higher amount of large non-metallic inclusions, which were not retained by the coated metal melt filter.
The steel 18CrNiMo7-6 (AISI 4317) is treated in three different crucibles based on carbon-bonded alumina. Non-metallic inclusions in the steel are characterized by means of optical microscopy and scanning electron microscopy. Strength, ductility, and dynamic fracture toughness of the steel are evaluated at different temperatures. Furthermore, fatigue lifetimes in the very high cycle regime are determined. The treatment in a carbon-bonded alumina (A-C) crucible resulted in a relatively high inclusion content involving a high number of duplex inclusions consisting of MnS and Si-Al-O. In contrast, less but large pure MnS inclusions are observed when the steel is treated in carbon-bonded alumina-zirconia-titania crucibleswith or without a coating of carbon nanotubes (AZT-C-n and AZT-C). The steel treated in the A-C crucible exhibits the highest strength and fatigue lifetime, but the lowest energy dissipation. The relatively low inclusion content in the AZT-C treated steel results in high energy dissipation during tensile deformation. However, large MnS inclusions in the AZT-C and AZT-C-n treated steels act as crack initiation sites and reduce the fatigue lifetime. The dynamic fracture toughness is not affected by the different melt treatments. This result is explained by cell-like structures within the material that are characterized by a lower strength and an increased concentration of MnS inclusions.
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