K. Nambu et alii, Frattura ed Integrità Strutturale, 34 (2015) 271-279: DOI: 10.3221/IGF-ESIS.34.29 271 Focussed on Crack Paths Influence of vacuum carburizing treatment on fatigue crack growth characteristic in DSG2 K. Nambu Suzuka national college of technology, Japan N. Egami Meijo University, Japan A BSTRACT . The aim of this research is to clarify the influence of vacuum carburizing on the fatigue-crack progress characteristics of DSG2 steel. The test specimen tempering material (QT material) and vacuum carburizing material (VC material) has been used. The fatigue-crack progress was examined by subjecting the samples to four-point bending. The loading-capacity fixed experiment was done using a maximum load of P max = 4000 – 7000 N. The Δ K fixed experiment was done using a load of Δ K = 18 – 36 MPa√m. 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. 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. K EYWORDS . Vacuum carburizing; DSG2; Fatigue-crack progress characteristics. INTRODUCTION n recent years, longer life-span of the machine construction thing is called for from a viewpoint of earth environment. For longer life-span of a machine construction thing, the fatigue strength of material and wear-resistant improvement are especially required. In order to attain these, coexistence of a hardness and toughness becomes indispensable. The surface modification processing method is mentioned as an effective method for sharing these. It is a disposal method in which surface modification processing hardens the neighborhood of the surface of material, and an inside holds toughness. [1-3]. The surface modification processing methods include many processing methods including carburizing and a nitriding, such as a fine particle peening. [4-6]. The vacuum carburizing processing which is one of the surface modification disposal methods is a disposal method excellent in the improvement in fatigue characteristics, because oxygen is not included during carburizing atmosphere and I