Ş. Hakan Atapek

Modeling and thermal analysis of solidification in a low alloy steel

In this study, solidification of a low alloyed steel used for armor material was simulated at various transformation temperatures by the help of thermodynamical calculations using Thermo-Calc software. Differential thermal analysis (DTA) was carried out to determine the critical transformation temperatures for the phases and results were compared with those calculated by Thermo-Calc. It is concluded that the temperatures calculated by Thermo-Calc software are in good agreement with the results obtained from DTA. These results will be very useful to determine the proper austenization temperature having an important effect on the properties of steel serving as an armor material.

Wear Behaviour of Heat Treated Hot Work Tool Steels under Dry Sliding Conditions

Abstract In this study, commercial hot work tool steels were selected and their wear behaviour under dry sliding condition was investigated after heat treatment applications. In the first stage of the study, solution annealing, quenching, and finally tempering treatments were applied to the experimental steels. All samples were examined by light microscopy after metallographic preparations. In the second stage, hardness values (HRC) of the steels were determined to evaluate the surface resistance to wear. Finally, wear tests were carried out using ‘ball-on-disc’ type tribometer under dry sliding condition and all worn surfaces were characterized using both light and scanning electron microscopy. It is concluded that (i) the steels had typical tempered martensite matrices, (ii) alloy content affected the final microstructure and a finer matrix was obtained due to vanadium addition, (iii) hardness levels directly affected the coefficient of wear of steels, and (iv) worn surfaces exhibited abrasive and adhesive wear tracks.

Wear Behaviour of Heat Treated 100Cr6 Steels

Abstract In this study, several heat treatments were applied to DIN 100Cr6 steel to obtain different matrices. In the first stage of the study, solution annealing treatment was applied to the steel and cooling was carried out in various media (furnace, oil, and salt bath). In order to eliminate the stresses after transformation from austenization, a low temperature tempering treatment was applied to the quenched samples. All heat treated samples were examined using light microscopy after metallographic preparations. In the second step, ‘ball-on-disc’ type tribometer was used to determine the friction coefficient of the steels depending on the matrix phase. Weight loss was recorded and the friction coefficient versus distance was plotted for each steel. Worn surfaces of the steels were examined using scanning electron microscopy to characterize the wear mechanisms. It turned out that (i) pearlitic, bainitic and martensitic matrices could be obtained depending on the cooling medium, (ii) martensitic matrix had higher wear resistance based on its weight loss, (iii) abrasive and adhesive wear tracks were present on the worn surfaces of the steels.

Formation of Adiabatic Shear Bands in a Tempered Steel under Dynamic Loading and their Damage Effects

Abstract In this study a low alloyed steel was selected and heat treatments including austenization, quenching and tempering were applied to it to obtain a hardened matrix. After heat treatments two steel plates ha-ving hardnesses of 315 and 320 HV were subjected to ballistic shots as target materials. The shots were carried out at 0° from a distance of 30 m to the target materials using API 55-7.62 mm armor piercing projectiles with a velocity of 835–840 m/s. Metallographical and fractographical examinations were carried out after the shots to investigate the behaviour of matrix under dynamic loading and to observe the formation as well as type of adiabatic shear bands and their effects on the nucleation and propagation of cracks within the matrix, in addition to the inclusions. In the etched samples a degenerated matrix compared to the original tempered martensitic-bainitic matrix was observed in the regions close to the penetration. On the other hand deformed and transformed types of adiabatic shear bands were observed as white bands. It is concluded that both types of adiabatic shear bands directly play an important role in the nucleation and propagation of cracks in addition to the inclusions within the matrix.

Effect of cobalt on the aging kinetics and the properties of a CuCoNiBe alloy

Abstract In this study, the effect of cobalt on the aging kinetics of a precipitation hardening CuCoNiBe alloy was studied by variations in hardness and electrical conductivity. Based on the linear relationship between electrical conductivity and volume fraction of the precipitates, phase transformation kinetics of the alloys having different cobalt contents were formulated using the Avrami equation. It was concluded that (i) aging kinetics slowed down in high cobalt alloyed copper alloy, (ii) the calculated electrical conductivity values using Avrami equation were in good agreement with the experimental data, and (iii) aged alloys had higher hardness and electrical conductivity than the as-cast ones.

Effect of tempering temperature and microstructure on the corrosion behavior of a tempered steel

In this study, a tempered martensitic matrix was obtained in a low carbon steel, by applying austenization, quenching and tempering heat treatments. After austenization at 1000°C for 30 minutes, steel samples were quenched in water and then tempered at 200, 540 and 600°C for 30 minutes. Hardness measurements were done and then immersion tests were carried out in a 3.5 wt % NaCl solution for periods ranging between 1–7 days. Weight losses of the samples were determined after each immersion period and microstructural studies were performed on the corroded surfaces. Corrosion rates were calculated using weight loss data and verified by potentiodynamic tests. Results revealed that corrosion behavior of the experimental steels was directly affected by tempering temperature, hardness and microstructure.

Effect of Single and Duplex Thin Hard Film Coatings on the Wear Resistance of 1.2343 Tool Steel

In aluminum extrusion process, tool steels used as die materials suffer from mechanical, thermal and tribological stresses causing plastic deformation, wear and heat checking during hot metal flow. Thin hard film coatings like TiN, (Ti,Al)N and CrN are preferred in order to improve the surface properties of the tools. These coatings can reduce the friction force, minimize the adhesive interaction between the die and billet pairs and decrease the plastic deformation of the tool. In this study, effect of single (CrN and AlTiN) and duplex (CrN + AlTiN) thin hard films on the hot wear behavior of DIN 1.2343 tool steel was investigated. Wear tests were performed both at room temperature and elevated temperature to simulate the conditions of aluminum extrusion process. Based on the evaluation of coefficient of friction values, specific wear rates and worn surface examinations, the duplex coating, which had the best performance in the RT wear test showed good resistance to high temperature wear under the simulated aluminum extrusion conditions.

Microstructural and mechanical characterization of the parabolic spring steel 51CrV4

Abstract Findings about the microstructural features of, spring steels are necessary for the producers to enhance their mechanical properties. There are several reports revealing the basic relation between microstructure and fatigue performance. However, the results are commonly obtained from universal test procedures and have limited use due to the lack of real service conditions. In this study, the microstructural features of 51CrV4 alloy, used as spring steel component, were investigated by metallographic examinations starting from raw material to the final product. Its fatigue behavior was investigated using a self-designed test machine and a test procedure approved by the automotive industry to simulate the service conditions. Fractographic examination of fatigue failed surface was carried out to specify the effect of microstructural features on the fracture. It was concluded that (i) both oxide and decarburization layers were minimized by shot peening and (ii) although tested samples had superior fatigue resistance and failed above 105 cycles limit, oxide layer played a major role for crack initiation.

Tribological behavior of CrN-coated Cr–Mo–V steels used as die materials

DIN 1.2343 and 1.2367 steels are commonly used as die materials in aluminum extrusion, and single/duplex/multi-coatings enhance their surface properties. The design of an appropriate substrate/coating system is important for improving the tribological performance of these steels under service conditions because the load-carrying capacity of the system can be increased by decreasing the plastic deformation of the substrate. In this study, the tribological behavior of CrN-coated Cr–Mo–V steels (DIN 1.2343, 1.2367, and 1.2999 grades) was investigated using different setups and tribological pairs at room and elevated temperatures. The aim of this study was to reveal the wear resistance of a suggested system (1.2999/CrN) not yet studied and to understand both the wear and the failure characteristics of coated systems. The results showed that (i) among the steels studied, the DIN 1.2999 grade steel exhibited the lowest friction coefficient because it had the highest load-carrying capacity as a result of secondary hardening at elevated temperatures; (ii) at room temperature, both abrasive tracks and adhesive layers were observed on the worn surfaces; and (iii) a combination of chemical reactions and progressive oxidation caused aluminum adhesion on the worn surface, and the detachment of droplets and microcracking were the characteristic damage mechanisms at high temperatures.

A case study of a stress corrosion cracking failure in an AA5083 mold material used for curing rubber compounds

In this study the failure analysis of an AA5083 mold material, used for curing rubber compounds, was carried out. The problem revealed itself as the formation of bubbles on the mold surface during curing process and caused quality problems in the final product. Preliminary microanalysis studies showed that there was an accumulation of both Mg and Cl in the bubble. Several samples were taken out from the inner/outer part of the mold material and examined using scanning electron microscope and energy dispersive X-ray spectrometer. It was concluded that (i) the contamination by Cl ions was due to the cutting oil used to open vent holes during mold cleaning, (ii) the outer part of mold material had finer α-Al grains whereas the inner part in contact with the compound had coarse grains due to the thermal effect of curing process, (iii) β-phase (Al3Mg2) formed at the grain boundaries after a sufficient exposure to temperatures above 50°C, (iv) the grain boundaries had Mg-rich phase showing anodic behavior, while the grains had Al-rich phase showing cathodic behavior, (v) the cracks initiated and propagated through the grain boundaries as stress corrosion cracking which resulted in the separation of grains and the accumulation of gases appeared as bubbles due to thermo-mechanical aging and effects of corrosive medium.