Papers by Author: Süleyman Tekeli

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Authors: Ahmet Güral, Süleyman Tekeli, Dursun Özyürek, Metin Gürü
Abstract: The effect of repeated quenching heat treatment on microstructure and dry sliding wear behavior of low carbon PM steel was investigated. For this purpose, atomized iron powder was mixed with 0.3 % graphite and 1 % Ni powders. The mixed powders were cold pressed and sintered at 1200°C for 30 min under pure Ar gas atmosphere. Some of the sintered specimens were intercritically annealed at 760°C and quenched in water (single quenching). The other sintered specimens were first fully austenized at 890°C and water quenched. These specimens were then intercritically annealed at 760°C and re-quenched in water. The martensite volume fraction in the double quenched specimens was higher than that of the single quenched specimen. Wear tests were carried out on the single and double quenched specimens under dry sliding wear condition using a pin-on-disk type machine at constant load and speed. The experimental results showed that the wear coefficient effectively decreased in the double quenched specimen.
673
Authors: Süleyman Tekeli, Metin Gürü
Abstract: Yttria-stabilized cubic zirconia is well known the material that possesses high oxygen ionic conductivity and chemical stability over wide ranges of temperature and oxygen partial pressure and thus it is widely used as an oxygen sensor, thermal barrier and solid oxide fuel cell (SOFC) electrolyte. In the present study, 8 mol% yttria-stabilized cubic zirconia with SiO2 addition up to 10 wt% was studied with respect to the microstructure, fracture toughness and hardness. XRD results showed that SiO2 had very limited solubility of 0.3 wt% in cubic zirconia. This suggests that only small part of SiO2 dissolved in cubic zirconia and the rest of SiO2 segregated at grain boundaries and multiple junctions. This glassy phase also wetted the zirconia grains and prevented the grain growth and the formation of facetted grains. Both hardness and fracture toughness were measured using a Vickers indenter. It is observed that the introduction of SiO2 decreased the hardness and increased the fracture toughness of cubic zirconia. The hardness and fracture toughness also showed the same trend with increasing SiO2 content.
2418
Authors: Süleyman Tekeli, Ahmet Güral, Metin Gürü
Abstract: The effect of tempering temperature and microstructure on dry sliding wear behavior of quenched and tempered PM steels was investigated. For this purpose, atomized iron powder was mixed with 0.3 % graphite and 1-2 % Ni powders. The mixed powders were cold pressed and sintered at 1200°C. The sintered specimens were quenched from 890°C and then tempered at 200°C and 600°C for 1 hr. Wear tests were carried out on the quenched+tempered specimens under dry sliding wear conditions using a pin-on-disk type machine at constant load and speed. The experimental results showed that the wear coefficient effectively increased with increasing tempering temperature. With increasing Ni content, the wear coefficient slightly decreased at all tempering temperatures due to the high amount of Ni-rich austenitic areas.
629
Authors: Metin Gürü, Süleyman Tekeli, Emin Akin
Abstract: The amount of marble dust occurred during machining and cutting of marble pieces and fly ash emitted from coal power plant is rather high and these wastes create significant environmental pollution. In fact, these wastes can be utilized in various industrial applications. In this study, various amount of fly ash, marble dust and polyester as base material, methyl ethyl keton peroxid as hardener and cobalt naphtanats as accelerator were used to produce polyester matrix composite material. Mechanical properties of composite materials were investigated and the optimum values were determined. In the first step of the manufacturing of composite material, the amounts of hardener, accelerator and polyester were kept constant and only fly ash/marble dust ratio was changed. The experimental results showed that while fly ash/marble dust ratio up to 1/3 was increased, the strength and hardness of the composite materials increased. Thus, the composite materials with high strength and hardness were produced. The optimum three point bending strength and hardness values were 30.42 N/mm2 and 98 Shore A, respectively. In the second step, the amounts of hardener, accelerator and fly ash/marble dust ratio were kept constant and the effects of the change in the amount of polyester were investigated. It was seen that the highest tree point bending strength and hardness were obtained at polyester/filler (marble dust +fly ash) ratio of 0,38. The optimum three point bending strength and hardness values were 32.78 N/mm2 and 99 Shore A, respectively.
1353
Authors: Metin Gürü, M. Korçak, Süleyman Tekeli, Ahmet Güral
Abstract: The properties of ceramic-metal (Cermet) composites as tensile strength, hardness and resistance to corrosion and high temperature are superior than ceramics and metals. Because of the enhanced characteristics of cermets, they are commonly used in various applications and industries. The main objective of this study is to produce a cheap, easy produced, strong and high corrosion resistant composite material. For these purposes, zinc is used for its natural capacity against corrosion, low density, low melting point and softness. Magnesium aluminates spinel oxide (MgAl2O4) is chosen because of its high melting point and low density. Fly ash is a waste from coal power plant having puzzolanic properties. In this study, the effect of various amounts of zinc and fly ash addition on density and hardness behaviour of zinc-based MgAl2O4 composites was investigated. The experimental results showed that zinc and fly ash addition improved the hardness behavior of zincbased MgAl2O4 composite.
1203
Authors: Metin Gürü, Yüksel Akyüz, Süleyman Tekeli
Abstract: The discovery of superconductivity at 39 K in magnesium diboride (MgB2) has attracted much attention from theoretical and experimental points of view because it has the highest superconducting transition temperature in the conventional intermetallic superconductors reported so far. After the discovery of superconductivity in MgB2, many attempts have been made to synthesize this material with an aim of improving superconducting properties. In this study, MgB2 was synthesized from anhydrous B2O3. To provide homogeneous mixing of magnesium and boron oxide powders and to make easy sintering, powders were mechanically milled in an attritor under argon gas atmosphere. The mixture was treated with ethylenediamine solution to separete MgB2 from the product. The suspension solution was filtrated under vacuum and the solid particulates of MgO were removed. The filtrate was evaporated in a vacuum oven at 80 °C and sintered at 900 °C for 2 h under flowing argon gas atmosphere. XRD results showed that the final product was MgB2 with a small amount of MgO. Mechanical milling decreased the synthesis time and temperature of MgB2. The critical temperature of the specimens was determined as 37.4 K by an A.C susceptometer.
723
Authors: Süleyman Tekeli, Metin Gürü
Abstract: To achieve improvements in the mechanical properties, fine grain size, homogeneous microstructure and high density are desirable. The poor dispersion of the powders produce difficulties in the densification and the presence of agglomerates is responsible for poor mechanical properties. Slurry casting is an important colloidal processing method for the ceramic industry and helps to prevent the agglomeration of fine particles. In the present study, the effect of processing parameters, namely solid content, dispersing agent concentration, slurry viscosity and milling time on slurry casting of 8YSCZ ceramics were investigated and optimum values were determined. The results showed that ceramic powder used was castable under limited conditions. The effect of shaping processon sintering behaviour and microstructure was also investigated for slurry-cast and die-pressed specimens. The results showed that specimen processed by slurry-casting had a faster sintering rate and lower sintering temperatures, compared to die-pressed specimen. The reason for better sintering was due to the homogeneous dispersion of the powder and elimination of agglomerates in slurry-cast specimen.
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