Molten metal for hypoeutectic gray cast irons was poured into a top-open-type sand mold with furan resin. Unsolidified molten metal was discharged from the mold before the molten metal completely solidified. This resulted in the formation of artificial defects on the solid-liquid phase boundary. Dendrites and other microstructures were observed to be formed near the artificial defects. Sharp dendrite stalks with periodic protrusions around the periphery were observed in macroscopic images. This observation confirmed that the dendrites were growing toward the center of the hole formed by the discharge of the unsolidified molten metal. These results corresponded to the observations made using a scanning electron microscope. Graphite flakes formed parallel to the dendrite branches were observed on the cross-sectional microstructures near the solid-liquid phase boundary. Scanning electron microscopy showed that perlite colonies segmented the dendrite stalk. The area around the dendrite stalk, namely the region formed by eutectic solidification, was composed of perlite, ferrite, and graphite flakes.
The wear loss of some steel materials is known to increase significantly under high temperatures than under room temperature. We, therefore, focus on the cast-in insertion casting method which gives materials great functionality. In previous researches, we reported that the combination of cast iron and hard carbide by this method improves heat and wear resistant characteristics. However, some of cemented carbide cast-in insertion specimens were found to crack or deform after solidification. Thus, in this study, we conducted thermal stress analysis using the finite element method, and established a prediction method for the cracks and deformations of cemented carbide cast-in insertion multi-component white cast irons after solidification. Analysis was carried out on the cooing process, from solidification temperature (1423 K) to room temperature (298 K). Multi-component white cast iron was selected for the base metal of the cast-in insertion specimens and cemented carbide (WC and TiC) for the cores. The dimensions of the analyzed model were 50 × 235 × 20 mm, and cuboidal cores (□10 × 235 mm) were placed at bottom and central position. TT
The results of the thermal stress analysis showed that the specimens in which the WC core is placed at the base curved downward due to the difference in the base metal volume between the upper side and the lower side. This enabled reproducibility of the deformation tendency of cast-in specimens after solidification. In addition, thermal stress during cooling significantly reduced when the core was changed from WC to TiC, which is thought to reduce the damage of cast-in specimens by thermal stress. These results suggest that the TiC specimen placed at the center is suitable to reduce thermal stress and deformation.
In hot rolling, rolls generally wear out under normal operations. However mechanical or thermal cracks generate and propagate, when accidents occur during the rolling operation. Therefore, fracture toughness and crack propagation characteristics were investigated using COD and CT test specimens for the typical materials of hot rolling rolls, such as alloy cast steel (CS), adamite (AD), Ni-hard cast iron (IC), high chromium cast iron (HCR) and multi-component white cast iron (HSS). Fracture toughness decreases as eutectic carbides crystallized. The Paris-Erdogan relation, da/dN=C (ΔKI)m holds between the rate of crack propagation (da/dN) and the range of stress intensity factor (ΔKI), and the relation between the constants C and m is expressed by C ＝ 1.88 × 10-5/10.6m. The constant m has a large value over 2 mass% C where the carbides exist, and the higher the hardness, the larger is constant m. Crack growth is faster in the order of CS, AD, HCR, IC and HSS.
Investigations were conducted on the microstructure and elevated temperature properties of SCH13A austenitic heat resistant cast steel which was prepared by the sand mold casting process with self-hardening alkaline phenol binder under the pouring temperatures of 1763K and 1823K. Thermal shock and creep rupture tests were carried out for the evaluation of elevated temperature properties. Results revealed that thermal shock resistance as well as creep rupture time significantly improved with increasing pouring temperature. This can be explained by the crystallization behavior of primary carbides after solidification in terms of their morphology, amount as well as thermal stabilization. The effects of secondary carbides on the suppression of crack propagation emitted under thermal shock and / or creep environment were smaller than primary carbides. Additionally, the precipitated volume of sigma phase which causes high temperature brittleness decreased with increasing pouring temperature.