Issue34

K. Nambu et alii, Frattura ed Integrità Strutturale, 34 (2015) 271-279: DOI: 10.3221/IGF-ESIS.34.29 272 an internal oxidation etc. do not occur. Moreover, because this is pollution-free and there is little consumption of carburizing gas, it is saving resources and energy-saving technologies. [7-9]. There is gears as typical parts which perform carburizing processing. In recent years, also in the toothed wheels, a raise in a hardness and a miniaturization are strongly desired with low-fuel-consumption-izing and the weight saving of a motor- car, and also the engine higher power. The steel for DSG2 quantity hardness gears in which there were few internal- oxidation layers which have on the fatigue strength of carburizing material, and they aimed at improvement in the toughness of a carburizing layer from this is developed. In old research, there are few measures of the research on the fatigue strength of vacuum carburizing material, and the research for DSG steel which is an exotic material is hardly found. It is the purpose that this research clarifies influence of vacuum carburizing processing on the fatigue strength of steel for DSG2 quantity hardness gears. T EST MATERIALS AND TEST PIECES est specimens were used for high strength gear steel DSG2. The chemical composition shown in Tab. 1. First, thermal refining (primary quenching 1163K × 30min after holding oil cooling, secondary quenching 1113K × 30min after holding oil cold, tempering 463K × 1h after holding air cooling) is performed. The specimen was processed into the shape and size which are shown Fig. 1, and were mirror-finished by polishing and water buffing. After creating these specimens, in order to remove the residual stress due to processing, vacuum annealing (873K 60 min) was performed. C Si Mn Cu Ni C r Mo 0.21 0.19 0.72 0.12 0.06 0.99 0.16 Table 1 : Chemical composition (mass. ％ ). Figure 1: Shape and dimensions of specimen (in mm) V ACUUM CARBURIZING TREATMENT CONDITIONS pecimens were included in this study were two types. Tempered material (QT material) and carburizing material (VC material) were prepared. VC material vacuum carburization process is performed at a heat pattern shown in Fig. 2. In the vacuum carburization process of the present study were subjected to carburizing hardening rate is 38% in all specimens. F ATIGUE CRACK PROPAGATION TEST METHOD he fatigue-crack progress experiment was carried out with a four-point-bending fatigue-test system. The fatigue- crack progress experiment was carried out with a four-point-bending fatigue-test system. Usually, when doing a crack progress experiment, after introducing a crack beforehand, it is common to perform the stress relieving T S T