Issue34

K. Nambu et alii, Frattura ed Integrità Strutturale, 34 (2015) 271-279: DOI: 10.3221/IGF-ESIS.34.29 278 the mixed fracture surface of transgranular fracture and an intergranular fracture is shown in a fracture surface, an intergranular fracture is a dominant fracture surface.  Stage C: Because the influence of a carburizing layer becomes weak, the influence of a plastic region appears and a crack progress speed rises gradually. In a fracture surface, it becomes a mixed fracture surface of a brittle fracture and a ductile rupture, and transgranular fracture is main.  Stage D: Because a crack progresses considerably and the influence of a carburizing layer is lost completely, a crack progress speed turns into a fixed speed like QT material. A fracture surface is only a ductile rupture. Figure 12: Change of fracture surface with applied stress intensity factor range and distance from notch root. As shown in the schematic diagram, in high delta K -value, stage area A continued to some extent, the stages B and C were short, and going to the stage D immediately was checked. Moreover, the crack progress behavior which begins from a stage area B was shown by low delta K-value. Thus, also from the fracture-surface configuration which changes with external load or crack progress parts being shown, it was suggested that the crack progress behavior is governed by external load and the crack progress part. If in other words the maximum-stress value which acts on the application part of material is decided, it will be thought possible to decide the rate of optimal effective carburizing as all the lifetime. Moreover, about the point shown by Fig. 8~12, even if materials differed, it was suggested that these things are applicable to steel material by performing vacuum carburizing processing. C ONCLUSION n this research, the surface modification effect over the fatigue-crack progress characteristic of DSG2 steel that vacuum carburizing processing was performed was examined, and the following results were obtained. 1. The maximum surface hardness of VC material showed one about 2.6 times the value of this as compared with QT material. Moreover, the compressive residual stress was about -100MPa. 2. The crack progress speed of VC material fell, after the high crack progress speed was shown, and after it showed the minimum, it showed the tendency to go up again. This is considered to be what is depended on the compressive residual stress given to the carburizing layer. From this, it is thinkable that there is a crack progress depression effect in a carburizing layer. 3. In VC material, a carburizing layer has a crack progress depression effect from a plunger-helix bottom to about 2.6 mm, and it turned out that it is larger than an effective carburizing layer. Moreover, in each Δ K , it was shown that depression effect revelation differs and the crack progress process accompanying it was able to be shown typically. R EFERENCES [1] Yoshida, A., Fujii, M., H. Nakajima, Influence of Surface Modification on Rolling Contact Fatigue of Steel Rollers : In Case of Pure Rolling Contact Condition, Transactions of the Japan Society of Mechanical Engineers. C, J. Soc. Mat. Sci., Japan, 49(11) (2000), 1235-1241; 66 (2000) 275-282. I