Seznam abstraktů 2022: SEKCE C - VÝROBKY Z OCELI A JEJICH VLASTNOSTI

Hydrogen has a strong potential for use as an alternative fuel provided that it can be stored in a safe and efficient way. One possibility is to store hydrogen as a solid hydride using suitable metals or alloys. Metal hydrides have been widely studied as storage materials but most alloys are unable to fulfil the requirements of a competitive hydrogen storage unit that can be exploited in practical applications. A “Superior“ step forward in this respect represents publication of M. Sahlberg, et al. entitled “Superior hydrogen storage in high entropy alloys” where the authors studied the hydrogenation of the high-entropy solid solution alloy TiVZrNbHf of BCC structure and observed that extremely large amounts of hydrogen (2.7 wt.% of H) can be absorbed in it. The amount of hydrogen corresponds to H/M ratio of 2.5 and becomes a world record in terms of volumetric energy density 219 kg H/m3 exceeding by far the requested 40 kg H/m3. The authors explain these phenomena by lattice strain favouring to absorb hydrogen in both tetrahedral and octahedral interstitial sites. In my talk, I will summarize the current state-of-the-art of metal hydrides development and show direction in material science for development of new-generation alloys capable of storing hitherto unprecedented amounts of hydrogen.

Thanks to their good corrosion resistance and excellent mechanical properties, duplex stainless steels (DSS) are widely used in many industries (petrochemical, nuclear, gas transportation…). Therefore, it is important to investigate the plasticity of the material and its sensitivity to hydrogen embrittlement. The different metallurgical aspects of ferrite and austenite represent a real challenge for the understanding of hydrogen effect on their mechanical behavior. Thus, the present work concentrates on the effect of hydrogen and thermal aging on the plastic deformation of each phase of a DSS2304. Different specimens were considered: annealed, cathodically hydrogen charged and thermally aged at 475 °C. The materials were cyclically loaded to accumulate the deformation in each phase without leading to the rupture of the specimen. Fatigue tests were performed under controlled total strain in order to obtain a plastic strain variation equal to 0.07 % at each cycle for each specimen. Atomic Force Microscopy was used to characterize the slip marks pattern. The main changes were observed in austenite of the hydrogenated DSS which were covered by slip steps, including secondary slip system, instead of extrusions observed in the uncharged specimen. The height and the distance between the slip marks decreased from [30-5] nm to [8-2] nm and from [3-0.7] µm to [1-0.2] µm respectively with hydrogen addition. The slip marks morphologies suggested a more planar slip mode after hydrogen charging. After thermal aging, ferrite became harder which reduced the slip activity inside the phase. These results highlight the importance of the metallurgical state of DSS.

The aim of study is to compare mechanical response, phase stability and dislocation structures of 316L steel cycled in two multiaxial (axial/torsional) loading modes: in-phase and 90° out-of-phase and two uniaxial modes: push/pull and pure torsion. All tests were done at room temperature with constant strain rate. Tests were performed on hollow cylindrical specimens manufactured from austenitic stainless steel 316L. Fatigue lives measured in pure torsional mode are significantly longer compared to push/pull mode, SN curves of both multiaxial modes lie between uniaxial ones. Similar situation can be observed in the case of hardening/softening curves - the biggest secondary hardening was measured in pure torsion and the lowest in push/pull mode. Such behavior is probably caused by phase transformation from austenite to deformation induced α´ martensite. The evidence of the presence of α´ martensite was obtained by transmission electron microscope (TEM). A distribution of α´ martensite along the specimen thickness wall was evaluated by diffraction of back scattered electrons. A comparison of TEM foils made from specimens cycled in particular loading modes was carried out as well and the following conclusions are mutual to all loading modes: vein structure prevails in case of low amplitudes meanwhile dislocation cells are very common for high amplitudes. Dislocation structures are similar in in-phase and uniaxial modes (for the same equivalent strain amplitude) but differ in out-of-phase mode where multiple slip operates at all loading levels.

The presence of primary niobium particles and their morphology affected by changes in nickel concentration in the as-cast microstructure of stainless steels with 18 wt% Cr were analysed. The formation of primary particles is due to the decreasing solubility during cooling and solidification of the steel melt, and additionally, segregation of alloying elements occurs during solidification, which contributes to the formation of particles. The coarse, sharp-edged nitrides formed during solidification reduce the mechanical properties of the material, highlighting a significant reduction in impact toughness and resistance to high temperature creep. According to the thermodynamic models, the solubility of niobium and nitrogen was estimated. Solubility product of niobium nitrides decreases by lowering temperature and increasing the nickel content. With the Scheil-Gulliver model, it was predicted that, in a solution with 18 wt% Cr, 0.5 wt% Nb and 0.03 wt% N precipitation of primary niobium particles start at solid fractions 0.91. The results were also confirmed using the Scheil solidification calculator with Thermo-Calc program. Three batches containing 9, 4.7 and 0.16 wt% nickel were prepared in an open induction furnace and cast into sand moulds. The presence of primary niobium particles, eutectic phases and heterogeneous nucleation particles on MnS non-metallic inclusions were observed in the microstructure using an optical microscope and scanning electron microscope. By lowering the nickel content in the solution, the solubility product of niobium nitride increases and the proportion of primary particles lowers. During cooling, the solubility of niobium particles decreases, small niobium carbonitrides precipitate on grain boundaries and on MnS non-metallic inclusions.

Because of the complexity of microstructure of welding joints, their fatigue behaviour assessment is more complicated than for monolithic materials. Indeed, a welded joint consists of a fusion zone (FZ), a heat affected zones (HAZ) and the base metal (BM). Each part has its own metallurgical feature (chemical composition, grain size…) and therefore its own mechanical properties (hardness, work-hardening…). The present communication deals with the fatigue response of a welded joint made of 316L stainless steel plates processed by gas tungsten arc welding, by using the digital image correlation (DIC) technique. Low cycle fatigue tests were performed both on monolithic specimen and welded joint, either under strain control or load control. The DIC technique was first applied to estimate the strain partition around a surface defect or internal inclusion to explain the poor fatigue life of some welded joint specimen. In healthy specimen, fatigue crack always initiated in the fusion zone. Scanning electron microscopy revealed that both the monolithic and welded materials were covered by slip markings. Thanks to the possibility of DIC to record locally the strain during cycling in each part of the welded joint, it is shown that the deformation concentrated either early in the fatigue life at high stress or progressively. Because of the fusion zone and the base metal did not accommodate the cyclic deformation with the same manner, a deformation transfer from the base metal to the fusion zone at low stress is suggested.

To ensure proper maintenance and remaining lifespan of railway axles it is important to precisely estimate the distribution of mechanical properties within the railway axle. In past many methods were introduced and standardized. However conventional methods often require samples to be extracted from examined parts which inevitably leads to destruction of the examined component. Recently featured Advanced Indentation system enables quasi-non-destructive testing and evaluation of hardness and tensile properties either in-situ or in laboratory environment.
This paper deals with characterization of hardness and tensile properties of railway axle made of EA4T steel using Advances Indentation System. While standard methods of hardness evaluation use optical microscopy and size of indent, indentation system uses depth of indent as main parameter for calculations. Load-depth curves were obtained from loading-unloading cycles and later used to analyse tensile properties. The results were compared with standard Vickers hardness tests and data given by standards for railway axles and EA4T steel and other literature. Experimental data revealed that indentation method is suitable substitute for conventional methods of hardness and tensile properties.

The topic of low density steels, also called light weight steels, has emerged in the last decade. These steels have remarkably low densities of around 6.7 kg/l. They are based on the Fe-Mn-Al-C alloy system and contain approx. 15 % Mn and approx. 10 % Al, with high contents of C, approx. 0.8 %. Due to high amounts of alloying elements, their microstructures can be ferritic, austenitic to duplex (also sometimes triplex). Besides the low density, the tensile tests can show elongations up to 90% and strengths above 1000 MPa. The promising steel properties are, however, accompanied by a complicated microstructural evolution, with kappa carbide precipitation and ordering of ferrite. The solidification intervals are about 100 °C, and the slow cooling leads to B2 ordered phase precipitation, and the formation of different morphologies of kappa carbides, that cause embrittlement. Fast cooling prevents secondary precipitation, and avoids phase transformations but the internal and thermal stresses cause cracking. The melting itself is not trivial due to the complex chemistries. The solidification, microstructural evolution and practical cases of heat treatments and hot working, with examples of material failure are presented.

One of the important tasks of this work is the experimental and numerical simulation of the fracture mechanical behaviour of the sub sized specimens. The numerical simulation of the stress and strain field in the vicinity of the crack front, of the J integral, of the loss of constraint assessment and finally the evaluation of the size effect are being performed. These numerical simulations will help to rationalize the experimental data.
This paper presents the influence of simulated loading conditions and finite element mesh on the stress field in the vicinity of the crack front and J integral. Two options were investigated of the load transfer to the specimen: by a pin with a zero friction and by added material chunk with the triangular tip. The evaluation of the size effect to the fracture behaviour was presented also. For this purpose, the Finite Element (FE) code ANSYS was used.
The paper is for the dissemination of the project results.

During the rotary friction welding process is a joint made at the interface of two materials using the parameters such as temperature, pressure, relative motion speed and the secondary parameter is time. This method offers a wide range of applications, especially when are welded rotating parts. The research is focused on homogenous and heterogenous rotary friction welds between subsequent materials: steel DIN 975, stainless steel 304L, aluminum alloy AlCu4PbMgMn, aluminum alloy AlMgSi, copper Cu - ETP, brass CuZn39Pb3. Heterogenous welds were formed by various combinations of the mentioned materials, where the microstructural analysis was performed and the physical nature of the joint formation during rotary friction welding was created. The influence of the geometry on the contact surfaces was monitored on homogenous welds - the results were used in the selection of the effective geometry also in heterogenous welds. Furthermore, sets of homogenous welds were subjected to a static tensile test, a hardness test and also microstructural analysis. Welded workpieces were cylindrical with 10 mm in diameter.

Surface hardening is a process of forming high hardness layer at the surface of metal object, thus improving wear resistance without the need for hardening whole section. It allows to reduce stress, deformations and cost of the finished product compared to through hardening. Electron beam due to high heating rate makes it possible to obtain layers with wide range of thickness with required properties. Also, because of the ease of dynamic deflection and focusing, hardening of components with complex geometries is possible. This paper presents the results of metallographic and hardness test of electron beam hardened C45 grade steel shaft.

42CrMo4 steels are widely used in automotive, aerospace applications, and machine tools where wear resistance needs to be maintained throughout its lifetime. It is possible to change the microstructure of these steels with various heat treatments and improve their mechanical properties. For this purpose, the effect of different heat treatment processes on wear characteristics of 42CrMo4 quality steel was investigated in this study. After various heat treatments methods, spheroidized ferrite-perlite, tempered martensite, tempered martensite with spherical carbides microstructures were obtained in 42CrMo4 steel. Hardness measurements and the wear tests were performed with microhardness tester and reciprocating type tribometer, respectively. The microstructural features developed by the applied heat treatments were examined with an optical microscope, and the wear characteristics were studied in terms of friction coefficient, wear volume loss, and worn surfaces of samples.

Heat treatment is usually preferred to achieve the pre-determined material properties of steel components thereby suited for several engineering applications. Gas quenching after austenitisation in a vacuum is an established process for this purpose, as it is clean and environment friendly. The selection of quenching parameters depends on many factors such as sample geometry as well as material and the batches. The process parameters are adopted by many years of expert knowledge, complex calculations or from trial and error methods. This problem is addressed in this scientific study by developing a prognosis tool, which can predict the heat treatment results based on an artificial neural network (ANN). This is attained by training the ANN on the basis of experimental and numerical investigations. Therefore, the heat treatment experiments were carried out on specific components made from 42CrMo4 and 100Cr6 in a two chambered quenching setup, where N2 gas act as quenching fluid. The cooling behavior will be investigated under the variation of process parameters such as gas pressure, geometry and batches. The development of the microstructure and hardness as a function of the process parameters are analysed metallographically. For the detailed investigation as well as to improve the training quality of ANN, FEM simulations are developed and validated, which serves afterwards to research the influence of parameter variation numerically. Thereby, sufficient data are generated numerically and experimentally for the successful training of the ANN of the prognosis tool, which can finally predict the heat treatment results.

The effect of repeatedly impinging the surface of austenitic stainless steel AISI 316L with a high number of water droplets was studied. A pulsating water jet ultrasonically excited with a sonotrode of a frequency of 21 kHz was used for creating separate clusters of water. The impact of distinct droplets of water causes the water hammer phenomenon, which for transient time, significantly increases pressure compared to stagnation Bernoulli pressure. The water hammer effect leads to increased cutting effectivity of pulsating water jet, where water hammer pressure is dominant, compared to continuous water jet, where stagnation pressure is dominant. The water droplets are impinging the material with high-frequency, the surface is therefore subjected to repetitive loading. In this study, the surface of treated austenitic steel was observed by scanning electron microscopy as well as transmission electron microscopy. The main variable was the number of impingements affecting the surface. It was observed that the main roughening mechanism consists of twinning and grain tilting. No signs of fatigue dislocation structure were observed in the subsurface. The cross-section of the treated surface was subjected to microhardness measurement to understand the effect of water clusters on the strengthening of the surface. The final step of the study was applying a low-intensity pulsating waterjet to the surface of fatigue samples. Then, the preliminary fatigue tests were conducted. The preliminary results show a promising increase in the number of cycles to failure. The results and observations can be also adopted from controlled surface modification to erosion prediction.