Papers by Author: Petr Jonšta

Abstract: Cobalt-based superalloys are among the most important materials for application in the demanding conditions of the glass industry. These superalloys with extended life are due to their excellent heat-resistant properties an important part of glass tools, especially of spinning discs. In the submitted paper, a material analysis of the commercially produced cast cobalt superalloy for spinning discs, including metallographic analysis supplemented by an electron X-ray analysis of minority phases, is carried out. An assessment of harmful gases, in particular oxygen and nitrogen, has also been carried out. They appear to be the cause of defects that often lead to the subsequent degradation of the material.

Abstract: The paper deals with the influence of microstructural changes of the nickel superalloy (NiS) on heat transport changes in MAR-M247. The dissolving annealing within the temperature interval of 900 – 1240°C and cooling in water has been applied to enhance structural changes. It has been shown that the narrow connection between the γ’ phase morphology changes, as well as values of density, thermal conductivity and specific heat capacity. In the dissolving annealing process, the γ’ particles change in certain cycles from fine to rough and back to fine one. Obtained results of changes of thermal conductivity, density and specific heat capacity correlate with image analysis and electron microscopy observation.

Abstract: This paper presents the results of structural phase analysis of MAR-M247 superalloy. Analysis was performed at initial and in as casted state after various type of solution annealing in the range 900 °C to 1240 °C with cooling in water. Presented polycrystalline alloy is heat resisting nickel superalloy, especially usable for highly strained components in the industry producing stationary gas turbines and aircraft engines. Analysis was performed using of a Quanta FEG 450 scanning electron microscope with micro-analytic system Trident APEX-4 which have identified presented minor phase. Attention was also paid to its eventual changes after different type of solution annealing.

Abstract: Contamination of steel by excessive amount of non metallic inclusions leads tosignificant deterioration of its quality and consequent degradation. Cause of degradation of forgedrotors for steam turbines made of NiCrMoV steel grade is examined in this paper. Metallographicand electron-microscopic analyses were performed, including X-ray microanalysis for detecting thepurity of the forging being examinated. Concurrently, the consequence of failed technologicalproduction process, namely the influence of broken brick for argon blowing at the casting ladle, wasalso examined.

Abstract: The presented paper deals with comparison of spur gears classical and modern hardening process of the bending fatigue properties. It concerned particularly the hardened surface of the tooth flank using optical source of electromagnetic radiation. The results of fatigue tests were obtained on a servo-hydraulic testing machine LFV 100 kN made by walter+bai ag. These tests were defined by the standard ISO 6336 as method B, when testing is carried out under reference conditions close to the real operation. Comparison of test samples is also carried out on the basic of the achieved local hardness and hardened layer thickness in the tooth root, which is an initiation of fatigue crack.

Abstract: This article deals with testing of fatigue properties of hardened gears at bending. These tests are defined by the standard ISO 6336 - method B, when testing is carried out under reference conditions close to the real operation. This type of testing may be helpful to evaluate the influence of new materials, design parameters, production technology, or the effects of surface finishing. The first part of this article deals with the theory of fatigue testing methodology using modified stair-case method and characteristics of the testing machine. The second part of the paper is devoted to the characteristics of the experimental material and the results achieved. The case hardened steel gears alloyed by Cr-Ni-Mo were selected for the experiment. Hardened gears were subjected to local hardness measurement and subsequently the bending fatigue tests were performed. Ruptured samples were analyzed using a scanning electron microscope. The local bending stress was determined using the modified computational model based on the ISO 6336 standard methodology. The focus of this work consisted in evaluation of the influence of the filler root surface treatment on fatigue limit at bending on the basis of the realized experiments and their confrontation with the equivalents specified in the standards ISO 6336 and ČSN 01 4686.

Abstract: The paper is aimed at abrasive water jetting of non-corroding steels treated by cryogenic temperatures in liquid nitrogen. The investigation of cut walls shows that cryogenic temperatures influence material structure and respective properties. This research is focused on improvement of material reliability in various states of production systems and operation conditions.

Abstract: The paper is devoted to the study of thermal, as well as mechanical properties of polyurethane samples (PU) filled by carbon black (CB), graphite (G) and MWCNT(CNT). Thermal conductivity k(W.m^-1.K^-1), specific heat capacity c_p(J.kg^-1.K^-1) and thermal diffusivity a(mm².s^-1) were measured by the impulse method. An increase of all the above mentioned thermal parameters after addition of fillers under investigation was observed. The dc electrical conductivity drastically increases from PUPU+CBPU+GPU+CNT.

Abstract: Three samples of 10GN2MFA steel were thermally treated at quenching temperatures equal to 900°C, 1000°C and 1100°C, and temperature of the tempering was 670°C. Thermo-physical properties of the steel were modelled by lumped capacity model. The sample was mechanically stressed under yield strength and then the cooling curve of stressed material on air was measured. The heat transfer through grips was taken into account. Thermo-physical properties were obtained by parametric fitting of time-temperature data obtained from cooling curve. Both thermal diffusivity and thermal conductivity increase with the quenching temperature. Specific heat capacity of steel samples after thermal treatment does not change significantly. All the measured thermo-physical parameters were in good agreement with the table values for low carbon Ni steels.

Abstract: This paper is devoted to the study of thermal transport phenomena changes caused by natural aging of rubber blends. Thermal conductivity, diffusivity and heat capacity of rubber blends were measured and compared for the same samples before and after half of a year. Samples were stored at room temperature and daily light. In the frame of our investigations there has been observed the decrease of the thermal diffusivity as well as the thermal conductivity in the interval approximately 40-50 %. The changes of specific heat capacity after sample aging were negligible. The explanation of such behaviour we can see in the sample structure degradation caused by the environmental influence.