Seznam abstraktů 2026: POSTEROVÁ SEKCE C

Long-term operation of steam pipelines in power plants, due to high temperatures and pressures, leads to gradual material degradation and changes in mechanical properties. Therefore, it is necessary to apply appropriate diagnostic methods to assess the current condition of the material and estimate the residual service life of the pipeline.
This paper focuses on the application of the Small Punch Test (SPT) method to monitor changes in material properties of steam pipelines during periodic inspections. The SPT method enables the determination of characteristic parameters of mechanical behavior using small samples extracted from components in service.
The study analyzes the evolution of material properties as a function of operating time and discusses their relationship with degradation processes occurring in the microstructure during long-term exposure to elevated temperatures. The results contribute to a more accurate estimation of the residual service life of pipelines and to the optimization of operational monitoring strategies.

Very fine bainitic, including nanobainitic microstructures, are characterised by an increased resistance to crack propagation, which is associated with transformation processes involving retained austenite. The formation of such microstructures is governed by an appropriately selected heat treatment regime influencing both the driving force of bainitic transformation and the transformation kinetics. Higher austenitising temperatures may promote the formation of very fine structures; however, they also prolong transformation times and increase the amount and stability of retained austenite. The resulting microstructure, therefore, represents a compromise between transformation conditions and the required functional properties of the material.
In precision components such as rolling bearings, dimensional stability represents a key parameter in addition to mechanical performance. It was evaluated based on dimensional changes of specimens after defined thermal exposure simulating service conditions. Experimental results obtained on bearing steel 100CrMnSi6-4 indicate that bainitic microstructures containing optically observable retained austenite exhibit significantly greater dimensional changes during stability testing (on the order of 0.2%), most likely due to their additional transformation during thermal loading.
In contrast, lower bainitic and martensitic microstructures without detectable retained austenite showed dimensional changes of approximately 0.02% and 0.01%, respectively, while lower bainite exhibited roughly twice the impact energy compared to martensite. Although microstructural differences may not be pronounced under conventional optical evaluation, they have a decisive effect on the functional reliability of precision components. The results highlight the necessity of finding a technological compromise between transformation kinetics, achieved microstructure, toughness, and dimensional stability when designing heat treatment routes.

Hydrogen is considered one of the primary energy carriers of the future due to the growing demand for clean and renewable energy. Consequently, its global consumption is expected to increase significantly, as indicated by available forecasts. During the production, storage, transportation, and utilization of hydrogen, it interacts with a wide range of structural steels. The presence of elevated hydrogen content in steels can lead to changes in a number of their properties. From a practical standpoint, the most critical effect is that it may result in material degradation due to so-called hydrogen embrittlement, potentially causing complete failure of structures, pressure vessels, and similar components. To ensure the long-term reliability and service life of steels operating in hydrogen-containing environments, it is therefore essential to test their resistance to hydrogen embrittlement under various mechanical, thermal, and pressure conditions that accurately simulate real operating environments. The present paper focuses on the application of conventional and unconventional testing methods for evaluating the resistance of structural steels in hydrogen-containing gaseous environments. The test procedures described in the paper can be used for assessment of the resistance to hydrogen embrittlement of materials used in existing gas infrastructure as well as for optimizing the manufacturing processes for components operating in high-pressure hydrogen environments, or their heat treatment, in order to achieve the desired strength properties, which often also yields significant economic benefits.

Cr-Mo-V grade steel is widely used in steam turbine rotors due to its fatigue resistance, which results from tempered martensitic microstructure. However, conventional post-machining operations, rotor underwent stress reliving heat treatment. Due to higher stress reliving temperature, the microstructure results in ferrite and carbide with low martensite, and inferior mechanical properties, including a yield point phenomenon. To address this, heat treatment optimization was performed between 725-1100°C using Thermo-Calc phase diagram simulations. Peak hardness was achieved at 850°C due to fine austenite grains. Optimized tempering further enhanced mechanical performance, resulting in a 2.5-fold increase in yield strength and suppression of the yield point phenomenon. This improvement is attributed to increased dislocation density from a higher martensite phase fraction, confirmed by X-ray diffraction and EBSD analysis. The study demonstrates that optimized heat treatment can effectively restore and enhance both microstructural and mechanical properties while eliminating undesirable yield point behavior in Cr-Mo-V steel.

Additive manufacturing (3D printing) represents a modern method of material production in many industrial sectors. Materials produced in this way can also be used in the field of hydrogen storage and transportation. This work deals with the influence of hydrogen on 316L stainless steel produced by additive manufacturing using Selective Laser Melting (SLM) technology. Flat tensile test specimens were tested in the as-built condition after printing and after heat treatment. Recrystallization annealing was carried out in a vacuum furnace at a temperature of 1050 °C for 30 minutes, with a heating rate of 20 °C per minute, followed by furnace cooling. The specimens were then electrolytically hydrogen charged in 0.05 M sulfuric acid solution with the addition of KSCN at current densities 1 and 5 mA·cm-2 for 8 hours. The effect of hydrogen on mechanical properties was evaluated by the slow strain rate tensile tests (SSRT). The presence of hydrogen slightly affected the tensile strength and ductility. The microstructure was also examined both in the as-built condition and after annealing. Annealing at 1050 °C resulted in recrystallization. The original layered structure with the presence of melt pools typical for materials produced by this way transformed into the common austenitic structure that can be observed in conventionally produced materials. Microhardness was also measured. In addition, fractographic analysis of the fracture surfaces was performed.

One of the approaches to simulate creep testing conditions as close as possible to the real service conditions of boiler tubes is the use of internal pressure creep tests (burst tests). This paper describes the implementation and results of such creep tests performed on welded joints of superheater tubes ø 44 × 6.3 mm made of HR3C steel, welded by the TIG process using a filler material based on the nickel superalloy Inconel 617. The tests were carried out at two temperatures, 730 °C and 760 °C, under internal pressures up to 29 MPa. The experimentally obtained times to failure exceeded the values predicted by parametric calculations based on available material data. Significant tube bulging was observed, and failure was mainly located in the HR3C base material in the vicinity of the weld. Metallographic analysis confirmed a dominant intergranular creep damage mechanism characterized by the formation of chains of cavities and microcracks initiated from the outer surface. Close to the weldment, the presence of δ-ferrite and precipitates of σ-phase were also detected along grain boundaries in HR3C steel.

This study investigates the influence of welding thermal cycles on the microstructure, local mechanical properties, and corrosion behavior of root and cap regions in multi-pass SMAW joints of API X70 steel. A combination of SEM/EDS analysis, nanoindentation, and potentiodynamic polarization tests was employed. The root region, subjected to rapid cooling, exhibits a fine microstructure dominated by acicular ferrite and bainite, leading to higher hardness. In contrast, the cap region shows a coarser microstructure composed of polygonal and grain boundary ferrite due to slower cooling. Electrochemical tests in 3.5% NaCl solution reveal increased corrosion susceptibility of the welded regions compared to the base metal, with the root being the most affected due to its microstructural heterogeneity and surface roughness. These findings highlight the critical role of welding thermal cycles in governing the local performance of HSLA steel welds in corrosive environments.

Prior information of physical and mechanical properties of materials is very crucial in design and development of a mechanical system. In the normal course, sufficient data is available at around room temperature. However, data of these properties are not easily available at cryogenic temperatures. At the same time, the available data cannot be extrapolated down to cryogenic temperature since the properties suddenly change at low temperatures like ductile to brittle transition for carbon steels, etc. Wide varieties of materials are used for low temperature applications and obtaining information of the low temperature properties becomes a case to case study. Knowledge of fatigue strength is very essential for materials which are subjected to continuous cyclical stresses e.g. springs. Fatigue is a progressive, localised and permanent structural change that occurs in materials subjected to fluctuating stresses and strains that may result in cracks or fracture after sufficient number of stress cycles. Cyclic stresses induce micro cracks within the material which tend to grow with time. These cracks initiate and propagate in regions where strain is very severe. Since most of the engineering materials contain internal defects, fatigue cracks mainly initiate and grow from these structural defects. Inadequate fatigue strengths can lead to premature failure of mechanical components resulting in entire collapse of the system. In view of this critical background, a rotating beam fatigue testing machine for cryogenic temperature zone has been designed and developed. This machine can be used to obtain the data of fatigue properties of any specified material from room temperature down to cryogenic temperature. The cryogenic temperature environment is produced by circulating controlled quantity of liquid nitrogen (LN2) around the test specimen. In this experimental work, fatigue properties of stainless steel (SS304) are determined both at room and cryogenic temperature. The test results are compared and analysed.

This study examines the microstructure, phase composition, and mechanical properties of a lightweight Fe-Mn-Al-C steel alloy containing Cr, Si, and Cu that was processed through electroslag remelting. The alloy was synthesized through induction melting, followed by electroslag remelting. This process resulted in a uniform structure with a lower inclusion content. Detailed investigations using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction, and local chemical analysis revealed that the as-cast steel has a dendritic morphology and a two-phase system consisting of α-Fe and κ-carbides (Fe, Mn)₃AlC. The carbide precipitates were identified as both eutectic lamellae in the interdendritic regions and rounded particles at the dendrite boundaries. These particles are enriched in Al, Mn, and Cr and contribute to grain-boundary strengthening. Mechanical testing revealed that the alloy has an average microhardness of 4.66 GPa, surpassing that of the base Fe-Mn-Al-C composition. These results demonstrate the significant impact of adding Cr, Si, and Cu on improving the phase stability and mechanical performance of electroslag-remelted, lightweight, high-manganese steels, suggesting their potential for advanced structural applications.

Most practical users of optical microscopy think in terms of magnification and resolution. Magnification refers to the ratio of the distances between two points in the sample and to those in its image. Resolution refers to the distance between two points in the sample that appear as two distinguishable points in the image. The image itself is an interference phenomenon formed by an optical system, which we usually assess in a plane called the focal plane. In digital microscopy (but also in observation with the human eye), another factor is the size of the sensor element.
In the last 15 years, digital cameras with sensors of the size of a 35mm film frame and larger have begun to appear on the market. Extreme sensors with 250 and 410 million optical elements (250 and 410 Mpx) exceed the total number of elements in both human eyes. In addition, there have been significant developments in the design and manufacture of optical systems, particularly influenced by applications in the production of mobile phones and silicon chips. In the latter application, the resulting image is composed of fine structures with a resolution below the theoretical limit of light microscopy and thus appears completely different from the original image template, which was a spot. A separate chapter are the design of the sensors themselves, the quality and distribution of filters, and the like.
The aim of developing large-area digital optical microscopy (LADOM) was to extract as much information as possible from the digital image, regardless of the microscope's sensor, lens, or illumination system design. To achieve this, we must first (1) find a focal plane that displays as many distinguished objects as possible. Eventually, we have developed a method for calculating the point divergence gain entropy density [1], a completely new concept for using information entropy to compare large data sets. (2) Another problem is distinguishing between two points. In a grayscale image showing only intensities, the condition for distinguishing between two objects is a combination of the Abbe model (which determines the condition for distinguishing between two adjacent lines on a grid) and the Shannon-Nyquist sampling condition (which determines how many points need to be observed for a given object in order to determine its position). A color image, which is actually a result of the measurement by a color camera (that serves as a spatially resolved colorimeter), contains much more information, especially when combined with knowledge of the incident radiation spectrum, the spectrum of individual filters, and possibly even color errors in the optical system. We showed [2] that it is possible to estimate the absorption/reflection spectrum at each image point. (3) This makes each point a carrier of individual information, with the accuracy limited only by experimental setup.
In LADOM microscopy, we have been able to record images of a steel alloy with a 2 × 3 mm2 area at a real resolution of 120 × 120 nm2. The information is currently limited only by the properties of the illumination system. However, the current technical limit is 75 × 75 nm2, which gives us a 1-Gpx image. In this way, we can observe physical and chemical properties of a crystalline assembly of about 480 iron atoms.

References:
[1] Rychtáriková, R., Korbel, J., Macháček, P., & Štys, D. (2018). Point Divergence Gain and Multidimensional Data Sequences Analysis. Entropy, 20(2), 106.
[2] Lonhus, K., Rychtáriková, R., Platonova, G., & Štys, D. (2020). Quasi-spectral Characterization of Intracellular Regions in Bright-field Light Microscopy Images. Sci Rep 10, 18346.

ASTM A694 F65 is a low-alloy steel valued for its high yield strength, good weldability, and excellent toughness at low temperatures. Owing to its balanced strength and impact resistance, it is extensively employed in the production of flanges and fittings for low-temperature, high-pressure service, where tensile strength and corrosion resistance are not the primary design requirements. This study examines the effect of heat treatment parameters, specifically the type of heat treatment and cooling conditions, on the microstructural and mechanical properties of an industrial hot-forged ASTM A694 F65 steel. Microstructural evolution was characterized using X-Ray Diffraction (XRD) and Scanning Electron Microscopy in Secondary Electron mode (SEM-SE), while mechanical behavior was evaluated through microhardness measurements and tensile testing. Experimental results regarding phase composition, morphology and distribution, lattice strain, yield strength, ultimate tensile strength, elongation at fracture, and microhardness were systematically analysed in relation to the applied heat treatment and cooling regime. Based on the results obtained, double normalizing is recommended for hot-forged A694 F65 steel, with sand cooling for thin-walled components and air cooling for thick sections, to prevent excessively low cooling rates and the formation of undesirable banded microstructures.

Acknowledgments
This work was supported by a grant of the Ministry of Research, Innovation and Digitization, CCCDI-UEFISCDI, project number PN-IV-P7-7.1-PED-2024-2368, contract number 96PED / 29.05.2025, within PNCDI IV.

The long-term global trend in energy consumption indicates that the gradual shutdown of nuclear power units after reaching their planned service life may increase the risk of power outages. One effective way to mitigate this risk is to extend the operation of nuclear units beyond their originally designed lifetime. A crucial prerequisite for reliable long-term operation is a detailed understanding of degradation processes occurring in structural materials, particularly microstructural changes induced by prolonged thermal exposure and the associated changes in the mechanical and physical properties of nuclear power plant materials.
The present study focuses on the analysis of secondary phase precipitation processes in austenitic steel 08CH18N10T, which has been exposed to real operating conditions for 28 years. The results were systematically compared with those obtained for a reference, non-operated material in order to identify possible differences in the extent and character of precipitation. Experimental observations performed using transmission electron microscopy (TEM) were complemented by quantitative statistical analysis of microstructural parameters.
The obtained results indicate a high degree of microstructural stability of the investigated steel even after prolonged thermal exposure. No significant differences in either the nature or the amount of precipitates were observed between the operated and reference materials, confirming the material’s good resistance to long-term thermal ageing and supporting its suitability for extended operation of nuclear facilities.

Niobium is a widely used microalloying element in the production of HSLA steels; however, supply-related and cost-related risks justify the investigation of alternative solutions. The aim of this study was to determine whether the complete or partial replacement of niobium with titanium is feasible in EN 10149-2:2013 grade S355MC steel while maintaining the mechanical properties required by the standard. During the study, three alloying concepts were compared under industrial conditions: conventional Nb microalloying, Ti microalloying instead of Nb, and combined Nb+Ti microalloying. Slabs cast from the different heats were hot rolled using several finishing temperatures, and the resulting coils were evaluated based on their mechanical properties and microstructure.

The results showed that the upper yield strength and tensile strength of the titanium-microalloyed S355MC steels were lower than those of the niobium-microalloyed reference material, while elongation remained nearly unchanged and impact toughness proved to be more favorable. Despite the scatter of the mechanical test results, the titanium-microalloyed steels remained within the standard limits specified for S355MC in all cases. The findings indicate that this microalloying concept can be industrially applicable; however, the inhomogeneity of the properties requires further technological refinement, especially in ladle metallurgy and in controlling the distribution of microalloying elements.

From an economic point of view, the use of titanium may represent a favorable alternative. The thesis indicates that the purchase price of FeTi is significantly lower than that of FeNb, and considerable cost reduction can still be achieved even if approximately twice as much titanium is required to obtain the desired mechanical properties. Based on the results, replacing niobium with titanium appears to be a realistic option in the production of S355MC steel, provided that further optimization of the technological parameters is also carried out while maintaining the standard properties of the steel grade.

In hot rolling mill department of Mobarakeh Steel Company, we are producing hot steel strips in different grades and dimensions. Sometimes based on market demands , we try to produces any new steel grade which is requested by customer. So as a main strategic goal, we are going to develop our products by producing new steel grades. In this case, as we have no previous experience about the requested steel grade, we have to predict related challenges and follow to that, we should find out related tests and also troubleshooting. The main challenges consist of saving stable production, and also obtaining the best mechanical properties and having final product with no defect; regarding customer satisfaction. Recently we have produced the spring steel hot strip CK67 as a new product. It is the most hardened hot steel strips which can be used in some different purposes such as agriculture tools, springs, etc.
In this paper we will describe how to define the hot rolling mill process considering predicted challenges and solutions either to be able to produce this new grade.