Issue 7

S. Bagheri Fard et alii, Frattura ed Integrità Strutturale, 7 (2009) 3-16 ; DOI: 10.3221/IGF-ESIS.07.01 7 E XPERIMENTAL INVESTIGATIONS ON NC SURFACES OBTAINED BY SHOT PEENING P processes to obtain NC layers have been successfully used on a variety of materials including pure metals, alloys and intermetallics. Majority of these experiments have included characterization of structure and properties of the surface layers by scanning electron microscopy (SEM), transmission electron microscopy (TEM), microhardness, scratch and so many other tests in order to assess the contribution of the process to improvement of material behavior. Here some notable effects of surface nanocrystallization on special properties which have been studied in literature are discussed. Fatigue Since most fatigue cracks initiate from the surface and propagate to the interior, a component with a nanostructured surface layer and coarse-grained interior is expected to have highly improved fatigue properties because both fatigue-crack initiation and propagation are inhibited by fine grains near the surface and coarse grains in the interior, respectively. Moreover, the residual compressive stresses introduced during the severe plastic-deformation process can also effectively stop or retard the initiation and propagation of fatigue cracks [71-74]. There are so many results confirming improvement of fatigue life of different materials using SP nanocrystallization methods [46, 62, 80, 81, 84, 85, 87] . In an experiment conducted by SNH process, a C-2000 alloy was treated by Five tungsten carbide and cobalt balls with a diameter of 7.9 mm for duration times of 30, 60, 90, and 180 min. load-controlled four-point-bend fatigue tests revealed that the surface nanocrystallization process affected the fatigue behavior of the material in two ways: the nanostructured surface layer, work-hardened region, and residual compressive stresses could enhance the fatigue strength especially in the high-cycle fatigue range (> 10 6 cycles), while the surface contamination and micro-damages caused by the SNH process could somehow deteriorate the fatigue strength. As shown in Fig. 5, the 30 min treatment resulted in the best improvement in the fatigue resistance, while prolonged treatments (60, 90, and 180 min) either leaded to no improvements or even decreases in the fatigue resistance. In the shorter cycle fatigue range (<10 6 cycles), the fatigue lifetimes of all the treated samples except the 30-min treated sample were lower than those of the as-received one. The longer the processing time, the lower the fatigue lifetimes in shorter cycle fatigue range. Thus, to fully utilize the SNH process to improve the fatigue behavior of the material with a nanostructured surface layer, processing conditions need to be optimized [80]. Figure 5 : Fatigue behavior of SNH treated Ni-based C-2000 super alloy samples [80]. Specimens of the austenitic stainless steel AISI 304 were also shot peened using S170R with coverage of 98% and Almen intensities of 0.175, or 0.120 mmA, respectively. Tension/compression fatigue tests were performed under stress control without mean stresses ( R = -1) with a cycling frequency of 5 Hz. The investigations revealed that the microstructural changes severely influence the cyclic deformation behavior of the near surface regions as well as of the soft specimen 300,00 350,00 400,00 450,00 500,00 550,00 600,00 650,00 700,00 750,00 100000,00 1000000,00 10000000,00 Number of cycles to failure Maximum stress (MPa) 180 min-SNH treated 90 min-SNH treated 60 min-SNH treated 30 min-SNH treated as recieved S