Effect of Crucible Material for Ingot Casting on Detrimental Non-Metallic Inclusions and the Resulting Mechanical Properties of 18CrNiMo7-6 Steel

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 crucibles-with 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.