S. Tekeli

The effect of martensite particle size on tensile fracture of surface-carburised AISI 8620 steel with dual phase core microstructure

In this study, the production of dual phase microstructure in the core of surface-carburised AISI 8620 cementation steel and the effect of martensite particle size at constant martensite volume fraction on tensile fracture have been investigated. The results showed that specimens with dual phase microstructure in the core exhibited slightly lower tensile and yield strength but superior ductility without sacrificing surface hardness than a specimen with nearly fully martensitic microstructure in the core produced by conventional heat treatment involving quenching from ∼900 °C. Tensile properties increased with decreasing martensite particle size. The effects of martensite particle size on tensile fracture has also been observed. In the core of the specimens with dual phase having fine and coarse microstructures, microvoids formed at martensite particles, inclusions and martensite-ferrite interfaces in the necked region. Martensite particle size had an influence in determining martensite cracking. In the specimen with coarse martensite particles, interconnected martensite distributed along ferrite grain boundaries cracked easily and the fracture mode was predominantly cleavage type. Martensite cracking was less frequent and the microvoids were smaller and microvoid density were higher in the specimen with fine martensite particles and the form of fracture was dimple depression type. In this specimen, voids initiated mostly by decohesion at the interface, and by some examples of fracture of martensite. Microvoid coalescence was the dominant form of fracture in both fine and coarse microstructures.. The specimen with nearly fully martensitic microstructure in the core produced by conventional heat treatment showed completely cleavage type fracture.

Enhancement of fatigue strength of SAE 9245 steel by shot peening

Abstract In this study, the effect of shot peening on fatique strength of SAE 9245 steel, which is used in industry for spring production, was investigated. Shot peening process was carried out to create residual compressive stresses caused by deformation hardening at the surface and to remove decarburized layer of the surface resulted in heat treatment. The fatique specimens were heated up to 850 °C and held at that temperature for 20 min to transform microstructure completely to austenite and hardened by water quenching. The specimens were then tempered at 500 °C to eliminate residual stresses caused by the quenching. A group of fatique specimens, only as heat-treated and other specimens shot peened by using an air high-pressure type of equipment with CS 230 shot, was tested on Wohler fatique test machine. The fatique results for unpeened and peened conditions were compared. It was seen that shot peening improved the fatique life by about 30%.

The effect of martensite volume fraction and particle size on the tensile properties of a surface-carburized AISI 8620 steel with a dual-phase core microstructure

This study is focused on the production of a dual-phase steel structure in the core of a surface-carburized AISI 8620 cementation steel and the effect of martensite volume fraction (MVF) and martensite particle size (MPS) on tensile properties. Experimental results showed that, compared with specimens with a fully martensitic microstructure in the core, those with a dual-phase microstructure in the core exhibited slightly lower tensile and yield strength but superior ductility without sacrificing surface hardness. In specimens with a dual-phase microstructure in the core, the tensile strength increased and ductility decreased with increasing MVF. Both the tensile strength and the ductility increased with decreasing MPS at constant MVF. The best combination of tensile strength and ductility was obtained with a fine MPS at a constant MVF of 25%.

A comparative study of superplastic deformation and cavitation behaviour in 3 and 8 mol.% yttria-stabilized zirconia

Superplastic deformation, grain boundary structure and cavitation behaviour during high temperature tensile testing in fine-grained 3 and 8 mol.% yttria-stabilized zirconia have been compared. The 8 mol.% yttria stabilized zirconia (8Y-CSZ) had a single cubic phase, while the 3 mol.% yttria-stabilized zirconia (3Y-TZP) was predominantly tetragonal. Extensive tensile ductility was obtained in 3Y-TZP, reaching ∼283% at 1673 K and 1×10−4 s−1, whereas under the same test conditions, tensile elongation in 8Y-CSZ was limited to ∼20%. These differences in elongation were related to differences in segregation of the yttria cation at grain boundaries. Transmission electron microscopy (TEM) observations with energy dispersive spectrometer (EDS) analyses revealed strong segregation of the yttria on grain boundaries in the 3Y-TZP; this segregation suppressed grain growth and lowered grain boundary mobilities. There was no evidence of preferential segregation of the yttria at grain boundaries in the 8Y-CSZ. Internal cavities developed in both materials during superplastic deformation. The results demonstrated that the extent of cavitation in 8Y-CSZ was much higher than in 3Y-TZP. In 8Y-CSZ, most of the cavities propagated in a direction perpendicular to the tensile axis as in most ceramics (with increasing strain these grew and interlinked to form cracks leading to failure at relatively low strains), whereas, in 3Y-TZP cavities were elongated parallel to the tensile axis as in metals. This difference arose from severe grain growth in 8Y-CSZ.

Influence of a transition metal oxide (CuO) on the superplastic behaviour of 8 mol% yttria-stabilised cubic zirconia polycrystal (8Y-CSZ)

The microstructure and superplastic deformation of fine-grained undoped 8Y-CSZ and 1 wt% CuO doped 8Y-CSZ have been investigated in tension in the temperature range 1503 to 1623 K and strain rate range 5 × 10−5 to 1 × 10−3 s−1. Deformation of the undoped 8Y-CSZ was characterized by large strain-hardening with limited tensile elongations of 20%; this was mainly due to severe grain growth during deformation. The addition of a small amount of a transition metal oxide (CuO) resulted in a decrease in strain-hardening and enhanced tensile elongations up to 78%. The ductility enhancement in the CuO doped 8Y-CSZ was due to copper segregation to grain boundaries, thus facilitating grain boundary sliding. In addition, the enhanced ductility in the doped material was related to a reduction in flow stress which, in turn, suppressed cavitation and delayed fracture.

A quantitative assessment of cavities in 3 mol% yttria-stabilized tetragonal zirconia specimens containing various grain size

It is well established that cavitation occurs in most materials during superplastic deformation, and cavitation has an effect on elongation to failure and mechanical properties of the materials after deformation. The cavities most likely nucleate at grain boundary particles or at triple points in quasi-single phase materials, at triple points and grain boundary ledges in microduplex materials and at particulate or whisker reinforcement in metal matrix composites. To minimize cavitation during superplastic flow it is necessary to exercise sound microstructural control. The starting materials should be processed to develop a fine uniform grain size and if dispersions are present, these should be fine and uniformly dispersed. In the present study, the effect of grain size on superplastic deformation and cavity formation in 3 mol% yttria-stabilized zirconia polycrystal has been examined. Also the distribution of cavity size and circularity have been quantitatively determined as a function of grain size. The results demonstrated that extensive internal cavities formed during high temperature deformation and the amount of cavitation increased with increasing grain size.

Effect of prestraining on cavity morphology in a superplastic 3 mol% yttria-stabilized tetragonal zirconia (3Y-TZP)

Whereas early studies on superplastic ceramics associated large elongations with an absence of cavitation, recent studies have clearly shown that most ceramics cavitage during superplastic deformation although the level of cavitation varies from one material to another; this cavitation may lead to inferior post-forming properties and premature failure. It is therefore desirable to restrict the concurrent cavitation and retard interlinkage of cavities in a direction perpendicular to the tensile axis during superplastic deformation. Cavitation in metallic alloys can be controlled by various procedures including annealing and/or the application of hydrostatic pressure prior to, during or after superplastic deformation. Only limited quantitative studies have been performed on ceramics, in particular cavitation as a function of strain, strain rate and grain size. The present investigation was undertaken to characterize quantitatively the occurrence of cavitation in superplastic 3Y-TZP as a function of prestraining.

Manufacturing of Polymer Matrix Composite Material Using Marble Dust and Fly Ash

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.

Exchange of experience the effect of synthesis time on the wear behavior of Al–8%Ti alloy produced by mechanical alloying

In this study, Al–8%Ti alloy was produced by mechanical alloying. The produced powders were cold pressed at 630 MPa and synthesized at 600°C for 12 and 24 h under argon gas atmosphere. After synthesis processes, the specimens were examined by SEM, XRD, and hardness tests. Wear tests were carried out under dry sliding conditions using a pin-on-disk type machine at a constant load of 30 N and a sliding speed of 1 m/sec. Total sliding distances were selected as 500, 1000, 1500, and 2000 m. The experimental results showed that the hardness and density of Al–%8Ti alloy increased with increasing synthesis time. It was also observed that volume reduction increased with increasing sliding distance and decreased with increasing synthesis time.

Wear Properties of Titanium and Ti6Al4V Titanium Alloy by Mechanical Milling

Abstract In this study, wear characteristics of titanium and titanium alloyed materials produced with mechanical alloying/milling (MA/MM) method were studied. In production of titanium and titanium alloyed materials, sponge titanium, gas atomised titanium powder and pre-alloyed Ti6Al4V alloy powder were used. The produced materials were characterised by scanning electron microscope, optical microscope, X-rays diffractometer (XRD) and EDS. The wear tests were carried out on a pin-on-disc type wear device under three different loads (15–30–45 N), four different sliding distances (500–1000–1500–2000 m) and a sliding speed of 1 ms–1. The results showed that no porosity was found on the gas atomised titanium and the Ti6Al4V alloy while very low amounts of micro porosity were formed on the sponge titanium samples. EDS analysis, on the other hand, revealed some Fe contamination on the materials, arising from the milling environment. It was seen that the hardness values of the samples produced from the gas atomised titanium powder were higher either than the samples produced from the sponge titanium, or the samples produced from the Ti6Al4V alloy. The fact that the wear losses obtained for the samples produced from the TiAl4V and gas atomised titanium powders were lower when compared to the sponge titanium. This was considered as the result of porous structure of sponge titanium.