Jens Fruhstorfer

Thermal Shock and Thermo-Mechanical Behavior of Carbon-Reduced and Carbon-Free Refractories

The thermal shock behaviour of novel carbon-reduced refractories with maximum grain size of 1 mm was investigated. A wedge splitting test for small specimen geometries (max. 40 × 40 × 20 mm 3 ) was successfully implemented with different loading configurations to determine “work of fracture” and thermal shock parameters. Additionally, heating-up thermal shock tests were performed with an electron beam facility. The addition of 2.5 wt% ZrO 2 and TiO 2 to Al 2 O 3 refractories appears to improve their thermal shock resistance due to microstructural changes that reduce brittleness and inhibit critical crack growth. However, a phase transition of ZrO 2 affects the properties at elevated temperature. For another pure alumina refractory, no geometry-independent value for the work of fracture could be obtained for the sample geometry used, which is probably related to the formation of a large interaction zone of the fracture surfaces. Al 2 O 3 -C materials with addition of semi-conductive Si and nanoparticles revealed a strong effect of the pressing direction on the work of fracture. However, the thermal shock parameter R’’’’ was hardly affected by the different additives. Furthermore, thermal shock tests using the electron beam facility JUDITH 1 did not indicate any significant differences in the damage pattern of the different Al 2 O 3 -C materials.

Characterization of Nonmetallic Inclusions in 18CrNiMo7-6

The detrimental effect of nonmetallic inclusions (NMIs) in steels on, e.g., fatigue lifetime is well known. In order to increase the durability and safety of materials and components, inclusion control and a deep understanding of inclusion formation are essential. The present study examines the formation of inclusions as well as their content, type, morphology, and size distribution for different batches of the steel alloy 18CrNiMo7-6 (AISI 4317), which was processed with various refractory crucible materials. To this end, extensive metallographic investigations were carried out including fracture surface analyses, metallographic sections, and the chemical extraction of inclusions. Scanning electron microscopy supported by energy-dispersive X-ray spectroscopy was used alongside electron backscatter diffraction for the qualitative and quantitative analysis of the NMIs. Oxide contents were found to have a significant effect on sulfide precipitation behavior, and had a strong impact on the resulting sizes and numbers of inclusions. The formation and growth of sulfides and oxide-sulfides featuring different morphologies is discussed on the basis of these experimental results.