Materials Science Forum Vol. 752

Abstract: Dual phase (DP) steels belong to the group of high-strength low-alloy (HSLA) steels. Our study focuses on the causes of crack formation of sheets, which are designed for deep drawing, of DP780-grade dual phase steel. Also, a series of heat treatment experiments were performed regarding the effect of intercritical annealing temperature on the structure and mechanical properties of the finished product (in this case a rolled sheet).

Abstract: In designing of plastic metalworking technologies, one determining amount used is the flow stress (kf ) of the material to be formed. A few experimental methods are used to measure it, but in all case it is intended to establish a specific stress and strain state in the whole volume of the specimen, or at least in its majority. In our work the limitations of upsetting in axial symmetrical and plane strain states were investigated by FE modeling and experimental methods within cold working conditions. Both methods were executed on traditional testing machines and Gleeble 3800 thermo-mechanical simulator. It was intended to determine the flow stress up to as high strain values as possible. The processing of the experimental results showed, reliable flow stress values were obtainable in the equivalent strain range of 0 to 3 using the step-by-step upsetting in plane strain, and in the 0 to 1.2 range by cylindrical upsetting.

Abstract: Directly-generated compressive residual stress has become a widely used surface modification technique. Intentional residual stress in solid materials can be produced by different surface compression methods. It is well known how residual stress field is influenced by the parameters of inducing technology, but during the operation of a surface compressed machine element the problem of compressive residual stress relaxation can occur. Only few articles address this phenomenon. The literature on residual stress decrease caused by fatigue load is scarce and the kinetics of this mechanism is not yet investigated. The present work reports on the examination of the effect of the fatigue load on compressive residual stress strengthened specimens. Investigations have been conducted at three stress levels. Residual stress data have been measured before fatigue load and measurements have been repeated after defined fatigue cycles.

Abstract: This paper presents an investigation about the occurrence of the dynamic recrystallization (DRX) during hot forming. Two 7075 aluminum alloy samples in different initial states were examined by compression tests at temperatures between 573 K and 723 K and constant strain rates ranging from 0.002 to 2 (s-1) with the maximum strain of 0.5 mm/mm. The activation energies of the examined aluminium were calculated, being 137.7 kJ/mol and 142.4 kJ/mol. The critical stresses and strains for the initiation of the dynamic recrystallization were determined using a numerical method based on the changes of the work hardening rate (θ) as a function of the flow stress (σ) or a strain (ε), respectively. Based on the micrographs taken after the deformations it was revealed that in some cases only dinamyc recovery (DRV) whereas in others DRX occurred under the applied examination conditions. The critical stress which belongs to the onset of the DRX or the DRV depend on the temperature, the strain rate, and the initial grain size. The critical and maximal stresses and the corresponding strain values at different conditions were determined. In the examined range of deformation, temperature and strain rate, the critical rations are σ_c/σ_p= (0.93-0.99) and εc/ εp = (0.3-0.74), respectively.

Abstract: The quality demands of shape characteristics of strips have especially been increased recently by the modern industrial branches processing strips. The final shape of sheet is influenced by the construction and stiffness of mill stand and the shaping parameters through the deformation patterns of roll gap. The purpose of our research work was the modelling of shape-defects (crown) developing during cold rolling in order to increase the technological precision of production of sheets.

Abstract: The drawing of wires with round cross section is a widely used technological method for making a great number of wire products of different range of sizes having excellent material- and surface quality. The planning of any kinds of wire drawing technology is greatly facilitated by knowing the models describing the technological parameters in the best possible way. Mainly the finite element method (FEM) and the analytical method are used for modelling the above parameters. Contrary to FEM, the advantage of the continuous analytical models is that they require less calculation processes and this fact seems to be really a great advantage when planning the technology in industrial circumstances. In the course of planning, the choosing of the individual technological parameters is determined by the cost effectiveness (i.e. the min. specific energy consumption), the quality of products and the productivity. The aim of our research work was to determine and develop a complex optimizing target function for the cold drawing technology which takes into consideration each of the above conditions and hasn’t been published in the references yet. By utilizing the fast computability of analytical model, a design software meeting all the expectations and giving IN-TIME result in industrial circumstances was developed by means of the complex optimization target function

Abstract: Frequently, the cause of the failure of different structures or structural elements is the cyclic loading. Both fatigue design curves and methods for determination of these curves can be found in the literature. Even so, there are structural details whereabouts executing of examinations is necessary. The aims of the study are as follows: to give a short summary of important design curves can be found in different standards or specifications; and to demonstrate of own high cycle fatigue tests on a soldered structural element and the comparing of our results and the results of an empirical method.

Abstract: Simultaneous measurement of electric resistance and thermopower the concentration of manganese and magnesium in the solid solution were determined by using the Matthiessen and Nordheim-Gorter relationships, in case of homogenized continuously cast AlMn1Mg3 alloy. It can be stated that the quantity of magnesium changes only in a small extent in the course of heat treatment while the manganese precipitates from the oversaturated structure between 400°C and 600°C then it dissolves again over 600°C.

Abstract: A measurement of the angular velocity/revolution number of magnetically stirred liquid gallium-indium alloy was realized with newly developed angular velocity measuring equipment. To get additional information about the flow of the melt, a numerical simulation model was performed with ANSYS FLUENT 13.0 with a single phase 2D k-ε turbulence solver. The aim was to reproduce the flow as accurate as possible, so the measured and computed angular velocity data was compared, to see if the system can be modeled fairly well.

Abstract: A lot of phases can be present in the structure of cast iron depending on the quality and the heat treatment processes. By using the color etching, the different phases – e.g. the carbides – can exactly be identified and their quantity can be measured. The carbide of iron (cementite) or other types of carbides existing in the structure have a significant influence on the properties of cast product therefore strict prescriptions regulate their quantity mainly in case of making cast products for the motor-car industry. In the course of our experiments performed by using the color etching, we proved that carbides can be detected and their quantity can be determined by using the digital image processing method by means of the different reagents. Other different characteristics of the structure can become visible by means of the reagents used during the color etching. During the investigations, it was experienced that some of the ferrite grains are etched in different color. This is probably connected to the orientation of grains as the thickness of interference-film is different on the anode phases having different orientation and therefore they color in different tones.