13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 2. Materials and research methods Two types of steel were used in the current research for Zr+ ion beam surface modification: heat resistant ductile 12Cr1MoV steel intended for operation at high temperatures as well as 30CrMnSiNi2 high strength steel used for manufacturing of heavy-loaded machine parts [4]. The choice of 12Cr1MoVsteel for research was caused by the fact that the steel experiences no structural changes at the temperature at which the process of surface layer nanostructuring by ion beam is performed. Besides, the steel is quite ductile, so the study of processes of localized deformation and fracture under cyclic load provides much more evidence at lower rate of deformation processes. High strength 30CrMnSiNi2 steel is used for manufacturing responsible and highly loaded parts that experience action of alternate loadings. Increasing of fatigue durability of this steel is a complex process since its low heat resistance and high level of alloying. Using the experience of 12Cr1MoV steel treatment new regimes for the surface nanostructuring were determined that make possible to slightly decrease mechanical properties while to enhance substantially fatigue durability. Flat specimens of size 70×10×1 mm for 12Cr1MoVsteel and 70×8×1 mm for 30CrMnSiNi2 steel were made from a peace of a pipe by electro-spark cutting. For running fatigue tests the holes of 2 mm were drilled as a stress concentrator in the specimens at a distance of 50 mm from one of its edges. For static tension tests specimens in the shape of dog-bone with gauge length of 20×5×1 mm were also used, as well as ones with the stress concentrator (similar to the specimens for fatigue tests). The specimens were mechanically polished and divided into 2 groups: a) in initial state (without treatment) and b) specimens with a nanostructured surface layer by zirconium Zr+ ion beam (after ion beam treatment). In the initial state steel 12Cr1MoV has the ferrite-pearlite structure with a characteristic grain size of 30 ÷ 50 µm. Ion nanostructuring of surface layer of the steel specimens was carried out with a help of high current vacuum arc source of metal ions UVN-0.2 "Quant". Images of specimen’s surface were obtained by means of optical microscopes Carl Zeiss Axiovert 25 CA and EPIQUANT, as well as scanning electron microscope Carl Zeiss EVO 50.Surface profilometry were performed with the help of Optical Interferometer of white light NewView 6200. X-ray phase analysis was conducted by X-ray difractometer DRON-7. Tests on static tension were performed using electromechanical testing machine Instron 5582 while for the cyclic tension using servo hydraulic testing machine Biss UTM 150 were employed. Surface micrographs of specimen’s were captured by Canon D550 digital photo camera during the process of fatigue tests. Nanohardness of specimens was measured by Nanotest (Micromaterials Ltd., UK). 2. The results of the experiments 2.1. Study of the modified surface layer of 12Cr1MoV steel 2.1.1. Electron-microscopy study The structure of the surface layer of the steel in the initial state is represented by large ferrite grains >1 µm with inclusions of cementite (Fe3C), whose average size makes 120 nm (fig. 1, a). Surface
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