13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- high cycle regime. Therefore, if some high cleanliness steel without such inclusions is fabricated, it is supposed that the fatigue fracture caused by the inclusion can be depressed and the corresponding S-N curve is diminished. From such a viewpoint, a conventional spring steel(SUP7) and a high cleanliness spring steel (SWOSC-V) were prepared mainly supposing the practical application as valve spring for automobile gasoline engine. For both kinds of steels, the rotating bending fatigue tests were performed in the very high cycle regime toward gigacycles. Thus, the fundamental S-N properties for the respective spring steels were experimentally clarified and the effect of the cleanliness on the fatigue property was discussed together with the fatigue fracture mechanism of this steel was also discussed from a view point of fractography. 2. Experimental Procedures 2.1. Material and specimen Materials used in this study were two different kinds of spring steels; JIS Material Codes of SUP7 and SWOSC-V, respectively. The former is a spring steel for general use for wide variety of usual springs, and the latter is developed for special use as valve spring for automobile gasoline engine. Chemical compositions of these steels were shown in Teble1. In the case of SUP7 steel, the material was firstly oil-quenched at 1173K(900deg.C) /20min, after which it was tempered at each of 723K(450deg.C)/60min and 659K(386deg.C)/60min, respectively. The former is hereafter so-called as “T-450 series”, whereas the latter is as “T-386 series”. Some of T-386 specimens were electrochemically polished (named T-386EP series), and some of T-386EP specimens were further shot-peened(named T-386SP series). All the preparation process of the respective series for SUP7 steel is summarized as a flowchart in Fig.1. Table 1. Chemical compositions of spring steels Material C Si Mn P S Cu Cr Fe SUP7 0.6 2.0 0.9 0.014 0.019 ― ― Bal. SWOSC-V 0.58 1.45 0.69 0.01 0.01 0.02 0.68 Bal. Figure 1. Flowchart of specimen preparation for SUP7 steel
RkJQdWJsaXNoZXIy MjM0NDE=