Borut Kosec

Analysis of casting die failures

Cracks in the surface of a fixed die half resulting in imprints on AlSi9Cu3 alloy castings were analysed. The cracks were revealed and identified by the use of penetrants. Some of them were clearly seen by the use of a magnifying glass or even by the naked eye. Non-destructive metallographic examination by SEM of polymeric replicas was applied.

An Approach to Multi-criteria Environmental Evaluation with Multiple Weight Assignment

In the domain of environmental protection, multi-criteria evaluation is used in cases where multiple alternatives, on the bases of multiple parameters, need to be evaluated. Depending on the particular goal of investigation, various approaches have been developed and applied. This various multi-criteria approaches differ in parameter weighting method, in data normalization method as well as in the method for assessment of alternatives. According to the applied multi-criteria methodology, the result, i.e., the rank of alternatives may differ to some extent. In this context, parameter weighting bears special significance in multi-criteria evaluation, while the choice of method is crucial for final result. The specificity of parameter weighting process in environment protection is directly related to pronounced interdisciplinary character of this area, as well as the large number of influential parameters. With this in mind, this paper presents an approach to multi-criteria evaluation which—through integration of three specific methods for parameter weighting—allows more flexible and multi-purpose application. Based on the established concept, a software application was developed. Besides automated parameter weighting, it also provides graphical interpretation of results. The developed approach and software have been verified on the case study related to evaluation of environmental loadings at six locations in the city of Novi Sad.

Comparison of the corrosion behavior and surface morphology of NiTi alloy and stainless steels in sodium chloride solution

The corrosion behavior of NiTi alloy and stainless steels (AISI 316L and X2CrNiMoN22-5- 3) in 0.9% sodium chloride (0.154 mol/l) solution was investigated using open circuit potential measurements, potentiodynamic polarization and electrochemical impedance spectroscopy measurements. Microstructural analyses before and after electrochemical tests were performed with the scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). The lowest corrosion current density has NiTi alloy and the extent of the passive range increased in the order AISI 316L stainless steel < NiTi alloy < X2CrNiMoN22-5-3 duplex stainless steel. The oxide film formed on all samples has a double-layer structure consisting of a barrier- type inner layer and a porous outer layer. Oxide films formed on the surface of steels mainly contains iron oxides and chromium oxide, while the surface film of the NiTi alloy mainly contains TiO2 oxide.

The characterisation of microstructural changes in rapidly solidified Al-Fe alloys through measurement of their electrical resistance

Abstract Microstructural changes of rapidly solidified Al alloy ribbons of various thicknesses and containing different concentrations of iron were analysed. The kinetics and sequence of microstructural changes occurring at a constant heating rate were analysed through the measurement of electrical resistance. This method allowed detection of the transition between metastable and stable phases of rapidly solidified Al – Fe alloy ribbons with various concentrations of iron and various thicknesses. After determining the temperature regions throughout the heating cycle, the quenched microstructure was analysed at transition points using both optical and transmission electron microscopy. Precipitation is the most common reaction for these types of alloys, and the most important alloy formed is Al13Fe4.

The effect of sol–gel boehmite coatings on the corrosion and decarburization of C45 steel

AbstractSol–gel boehmite coatings were produced on the polished surfaces of hypo-eutectoid C45carbon steel by dip coating. After the deposition the coatings were heat treated by drying at RT, 150 and 200 °C. It was found that for crack-free coatings, the drying temperature needs to be ≥150 °C. Properly heat-treated boehmite coatings then improve the corrosion resistance of steel, while proposed alumina coatings derived from the boehmite coatings during annealing of the steel affect the decarburization. The coated steel samples were annealed at temperatures that are typical for the heat-treatment and thermomechanical process of C45 steel, i.e., AC3 (austenitization, thermomechanical process). The sol–gel alumina coating decreased the decarburization rate of the quenched steel during the tempering (≤600 °C), which meant that the hardness reduction on the steel surface was smaller. Alumina coatings have a smaller effect on the decarburization at higher temperatures; however, the metallographic analyses indicate that the coatings can retard the oxidation rate in comparison to the decarburization rate and thus, somewhat increase the visual level of the decarburization. Under annealing conditions (T = 950 °C, t = 0.5–2 h) at which the oxidation rate is higher than the decarburization rate the decarburization process is not important since the decarburized layer is peeled off the steel surface together with the scale. No visual decarburization is then observed.

Identification of liquid/solid transformations in eutectic Pb-Sn alloy

Abstract In this paper the applicability and the efficiency of in-situ electrical resistance measurements with a four-probe method for monitoring the solidification process are presented. For this purpose a special measurement cell was developed. It enables simultaneous in-situ electrical resistance and temperature measurements during solidification to characterize the L → S phase transformation of metals and alloys. The method was experimentally demonstrated with a eutectic Pb – Sn (i. e. Pb-61.9 wt.% Sn) alloy. Additionally, the L → S phase transformation was monitored and compared with conventional differential thermoanalysis. The results of our investigation show that the simultaneous in-situ electrical resistance and temperature measurement method is able to determine the exact position of the start and end points of the L → S phase transformations from the resistance curves R(t) and the corresponding temperatures.

Microstructural characterization of Cu82.3Al8.3Mn9.4 shape memory alloy after rolling

In this paper, the microstructure of Cu82.3Al8.3Mn9.4 (in wt. %) shape memory alloy after hot and cold rolling was investigated. The Cu82.3Al8.3Mn9.4 alloy was produced by a vertical continuous casting method in the form a cylinder rod of 8 mm in diameter. After the casting, hot and cold rolling was performed. By hot rolling a strip with a thickness of 1.75 mm was obtained, while by cold rolling a strip with a thickness of 1.02 mm was produced. After the rolling process, heat treatment was performed. Heat treatment was carried out by solution annealing at 900 °C held for 30 minutes and water quenched immediately after heating. The microstructure characterization of the investigated alloy was carried out by optical microscopy (OM), scanning electron microscopy (SEM) equipped with a device for energy dispersive spectroscopy (EDS). Phase transformation temperatures and fusion enthalpies were determined by differential scanning calorimetry (DSC) method. The homogenous martensite microstructure was confirmed by OM and SEM micrographs after casting. During rolling the two-phase microstructure occurred. Results of DSC analysis showed martensite start (M s ), martensite finish (M f ), austenite start (A s ) and austenite finish (A f ) temperatures.

The mechanics behind formation of secondary ledeburite during tool steel welding

In this article we describe the microstructural changes in the heat affected zone (HAZ) of the tool steel W. Nr. 1.2379, which was surfaced or welded by the submerged arc welding technique (SAW) with different welding parameters. Microstructure of the welds and of the surfacing welds was analysed by optical and scanning electron microscope. In this research, we particularly studied microstructural changes in the area of primary chromium carbides in the HAZ and their effect on the weld crystalization. We came to the conclusion that the temperature in the HAZ is high enough during the welding process that it caused primary carbides to dissolve and concentration of the carbide-forming elements and carbon increased in the surrounding austenite matrix area to eutectic composition, which remelts and solidified as secondary eutectic (ledeburite).

The Effect of Long-Term Annealing at Elevated Temperature on Microstructure and Hardness of Heat-Resistant Steel

The stainless steel group AISI 310 is very often used for application in high-temperature conditions. This study investigated the effect of long-term annealing on microstructure and hardness of AISI 310S (24.3% Cr-19.3% Ni 1.8% Mn 0.21% Mo 0.67% Si, wt.%) heat-resistant steel. Microstructural changes and hardness distribution were analyzed after isothermal annealing at 800 °C in long-term exposure (from 1 to 1740 hours). Microstructure and fracture surface mode were analyzed using scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). It was found that after annealing the sigma phase particles were precipitated at grain boundaries and within austenite grains. The chromium content in the sigma phase increased with the prolongation of annealing time. The morphology of sigma phase was changed from fine precipitates at lower annealing times to coarser particles at higher annealing times. Also, presence of sigma phase was confirmed by transmission electron microscopy (TEM). At annealing temperature of 800 °C the hardness began to increase after 16 hours. With longer times of annealing the fracture surfaces were changed. This change is associated with a change of content and the morphology of sigma phase.

Damage of a screw in the seal coupling

The damage to a screw on the head of a 1.8 liter personal car engine was nucleated as the result of common disadvantageous environmental influences and reversed loads leading to corrosion fatigue.