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Öğe Comprehensive analysis of pulsed plasma nitriding preconditions on the fatigue behavior of AISI 304 austenitic stainless steel(Springer, 2021) Unal, Okan; Maleki, Erfan; Varol, RemziThis study aims to draw an exact boundary for microstructural and mechanical behaviors in terms of pulsed plasma nitriding conditions. The pulsed plasma nitriding treatment was applied to AISI 304 austenitic stainless steel at different temperatures and durations. Results reveal that nitriding depth increased as process temperature and duration increase. The nitriding depth remarkably increased at 475 degrees C for 8 h and at 550 degrees C for 4 h. An austenite structure was transformed into a metastable nitrogen-oversaturated body-centered tetragonal expanded austenite (S-phase) during low-temperature plasma nitriding. The S-phase was converted to CrN precipitation at 475 degrees C for 8 h and at 550 degrees C for 4 h. Surface hardness and fatigue limit increased through plasma nitriding regardless of process conditions. The best surface hardness and fatigue limit were obtained at 550 degrees C for 4 h because of the occurrence of CrN precipitation.Öğe Effect of Boronizing and Shot Peening on the Fatigue Properties of an Iron Based FeCu-Graphite P/M Material(Carl Hanser Verlag, 2014) Yilmaz, Selim Sarper; Unlu, Bekir Sadik; Kurgan, Naci; Varol, RemziIron based materials manufactured by powder metallurgy (P/M) method are widely used in industry. These materials are very important in applications where no machining is required, they can also be used as journal bearing material due to their self-lubricated property, and can be used for applications in the medical industry. In this study, powder metal parts were manufactured from composites based on FeCu-graphite by P/M method. Fatigue test was carried out on manufactured samples, which were only treated by boronizing or boronizing and shot peeing. The fatigue properties of these parts were investigated by a rotating bending fatigue test rig.Öğe Effect of severe shot peening and ultra-low temperature plasma nitriding on Ti-6Al-4V alloy(Pergamon-Elsevier Science Ltd, 2018) Unal, Okan; Maleki, Erfan; Varol, RemziIn this study, the fatigue behavior of Ti-6Al-4V alloy is tend to be improved by severe plastic deformation via shot peening and plasma nitriding. Shot peening with sub-branches: Conventional shot peening (A12-14), severe shot peening (A28-30 and A34-36) and repeening (N6-8) are exposed. Besides plasma nitriding is implemented at 500 degrees C, 550 degrees C and 600 degrees C with the durations of 4, 8 and 16 h. Due to utilization of shot peening as prior severe plastic deformation during diffusion of nitrogen, two methods are applied in sequence. Severe shot peening forms ultra-fine crystals and oriented grains by disintegrating of a and 13 phases just below the surface and increases surface roughness. Plasma nitriding temperature increase the compound layer thickness and the treatment leads to vanish of 13 particles through a certain line for diffusion layer. Prior severe plastic deformation by severe shot peening generally triggers the formation of thicker compound layer and emerges distinct diffusion depth with finer and oriented precipitation. The phase's intensity obtained from XRD peaks has been amplified by the application of prior shot peening. Each shot peening provides fatigue improvement on the other hand plasma nitriding application regardless of being pre or post treatments diminish the fatigue resistance. (C) 2018 Elsevier Ltd. All rights reserved.Öğe Effects of severe plastic deformation on pre-osteoblast cell behavior and proliferation on AISI 304 and Ti-6Al-4V metallic substrates(Elsevier Science Sa, 2019) Tevlek, Atakan; Aydin, Halil Murat; Maleki, Erfan; Varol, Remzi; Unal, OkanIn this study, titanium alloy (Ti-6Al-4V) and austenitic stainless steel (AISI 304) biomedical alloys were subjected to surface severe plastic deformation (SSPD) via severe shot peening (SSP) with the conditions of A28-30 Almen intensity. SSP is widely accepted as much more effective than the conventional surface modification techniques since it forms a nano-grain layer with large number of dislocations and grain boundaries. The SSP treatment in this study was led to a very thin rough layer in Ti-6Al-4V titanium alloy compared to that of AISI 304. The thicker layer of AISI 304 was created by twin-twin intersections and a martensite structure transformations. SSP treatment was resulted in a severe plastically deformed material surface and led to an increase in full width half maximum (FWHM) that expresses grain size reduction below nanometer regime. Also, SSP treatment increased the surface roughness of materials by forming pits and bumps and led to deterioration on surface topography. Besides physical and microstructural innovations, SSP treatment was improved the surface mechanical properties of the metals. On the other hand, it was noted that the effects of the alteration of surface topography and structural innovations (nano-grain crystal-deformation induced layer) were led to improved cellular behavior and increased cell proliferation regardless of material type.Öğe Investigation of fatigue properties of shot peened and plasma nitrocarburized P/M FC0205 steel(Korean Soc Mechanical Engineers, 2013) Basaran, Ali; Kurgan, Naci; Varol, RemziOne of the widely used powder metal materials in automotive industry, which is copper steel FC 0205 (Fe + 2% Cu + 0.5% C), was used in this study. The pressed samples has been prepared at the compression pressure of 680 MPa and sintered in conventional furnace at 1120 degrees C for 30 minutes. The samples with 7 Mg/m(3) densities were used as fatigue and shot peening processes. Shot peening were applied with 20A and 32A and the others were not peened. Plasma nitrocarburizing process was performed at 555 degrees C for 2 hours under 600-900 Pa pressure. Fatigue tests were conducted at room temperature on a rotating bending type fatigue test device. It was determined that shot peening should be used where increased fatigue strength is needed, and plasma nitrocarburizing should be preferred where surface hardness is required, and plasma nitrocarburizing after shot peening process is much more useful.Öğe Plasma nitriding of gradient structured AISI 304 at low temperature: Shot peening as a catalyst treatment(Pergamon-Elsevier Science Ltd, 2019) Unal, Okan; Maleki, Erfan; Varol, RemziThis study involves the capability of severe shot peening (SSP) as a catalyst intake for plasma nitriding process and the probability of reducing the requirement of thermal energy individually on the diffusion of interstitial atoms. To this end, combination of mechanical-thermal energy is run with pure thermal plasma assisted energy. Therefore, SSP is exposed to AISI 304 austenitic stainless steel as a former treatment and gradient structured surface (nanograined zone, ultrafine grain martensite-twin intersections zone and twin densed zone) is created. Then, plasma nitriding at 400 degrees C-4h and 475 degrees C-2h temperature-duration conditions. The condition of 475 degrees C-2h provides the requirements of diffusion individually however former treatment SSP increases the diffusion layer effectively. In contrast, 400 degrees C-4h condition is not adequate for diffusing atoms and white layer formation on the surface. The support of SSP as a mechanical energy triggers the process and white layer and the interstitial atoms diffusion take place influentially.Öğe Tribological properties of boronized ferrous based PM journal bearings(Carl Hanser Verlag, 2015) Unlu, Bekir Sadik; Yilmaz, Selim Sarper; Kurgan, Naci; Varol, RemziFerrous based PM (powder metallurgical) materials are widely used in journal bearings since they provide a good tribological performance. These bearings are self-lubricating and can be used in places where lubricating is not possible. This also applies to the shaft material which shows adhesive wear. Tribological properties can be improved and adhesive wear can be decreased by boronizing. In this study, tribological properties of ferrous based boronized and non-boronized Fe-graphite, FeCu-graphite and CuSnFe-graphite bearings manufactured by PM method have been determined and compared. Boronizing treatment was carried out at 950 degrees C for 4 hours. The SAE 1050 steel shaft was used as counter abrader. Experiments were carried out at 20 N loads and 1500 rpm every 30 minutes for 2.5 hours.
