Publikační činnost RMTVC (606)

Permanent URI for this collectionhttp://hdl.handle.net/10084/84391

Kolekce obsahuje bibliografické záznamy publikační činnosti (článků) akademických pracovníků Regionálního materiálově technologického výzkumného centra (606) v časopisech registrovaných ve Web of Science od roku 2003 do roku 2023.
Do kolekce jsou zařazeny:
a) publikace, u nichž je v originálních dokumentech jako působiště autora (adresa) uvedena Vysoká škola báňská-Technická univerzita Ostrava (VŠB-TUO),
b) publikace, u nichž v originálních dokumentech není v adrese VŠB-TUO uvedena, ale autoři prokazatelně v době jejich zpracování a uveřejnění působili na VŠB-TUO.

Bibliografické záznamy byly původně vytvořeny v kolekci Publikační činnost akademických pracovníků VŠB-TUO, která sleduje publikování akademických pracovníků od roku 1990.

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Now showing 1 - 20 out of 202 results

Microstructural and magnetic properties of Mn2FeSi and Mn2FeAl alloys prepared in bulk form
(Elsevier, 2023) Životský, Ondřej; Gembalová, Lucie; Jirásková, Yvonna; Szurman, Ivo; Čegan, Tomáš; Malina, Ondřej; Kuriplach, Jan; Čížek, Jakub
Microstructural and magnetic properties of the Mn2FeSi and Mn2FeAl alloys prepared in the bulk form have been investigated. Cylinder-shaped ingots produced by induction melting technique were analyzed in as-quenched state and additionally annealed at 773 K for 5 days in the protective argon atmosphere. The results show that Si and Al have different effects on the microstructural and magnetic properties of the alloys. The Mn2FeAl ingots are single-phase both before and after annealing, and their diffractograms surprisingly correspond not to the Heusler (L21 or XA) but to the primitive cubic β-Mn structure. Conversely, Mn2FeSi alloys show a two-phase behavior in the as-quenched state. From results of X-ray diffraction it was not possible to judge whether Mn2FeSi alloy has the inverse-Heusler (XA) or full Heusler (L21) structure. Annealing of Mn2FeSi leads to the formation of multiple phases. The EDX chemical area analyses resulted in only slight deviations of compositions compared to the nominal ones. The lattice parameters of 0.5672 nm and 0.6339 nm were estimated for the Mn2FeSi and Mn2FeAl samples from X-ray diffraction measurements. From the magnetic viewpoint, all samples are paramagnetic at room temperature and transform into antiferromagnetic state at the N é el temperatures about 50 K and 36 K for Mn2FeSi and Mn2FeAl, respectively. The negative Curie temperatures determined at all samples by Curie-Weiss law indicate an antiferromagnetic ordering of spins. Positron annihilation investigations revealed that Mn2FeSi contains a high concentration of vacancies. The local chemical environment of vacancies characterized by coincidence Doppler broadening is compatible with L21 rather than with XA structure. In contrast vacancy concentration in Mn2FeAl is very low and almost all positrons are annihilated in the free state.
Shallow and deep learning of an artificial neural network model describing a hot flow stress Evolution: A comparative study
(Elsevier, 2022) Opěla, Petr; Schindler, Ivo; Kawulok, Petr; Kawulok, Rostislav; Rusz, Stanislav; Sauer, Michal
In recent years, the utilization of artificial neural networks (ANNs) as regression models to solve the issue of hot flow stress forecasting has become a standard approach. In a connection with this kind of regression issue, employed ANNs are usually learned via a shallow learning technique while only limited attention has been paid to a deep learning method. In the frame of the submitted research, the shallow learning approach is thoroughly compared to the deep learning techniques which are based on the use of a Restricted Boltzmann Machine (RBM) and an Auto-Encoder (AE). To do so, these learning techniques are applied on a feed-forward multi-layer ANN describing the experimental hot flow curve dataset of micro-alloyed medium carbon steel. In comparison with the shallow learning method, both deep learning approaches provided higher accuracy in the network response - especially in the case of a higher number of hidden layers. The results have also shown that neither the RBM-based deep learning method nor the AE-based method had a significant effect on the duration of the necessary calculations. However, it turned out that the RBM-based method can, under certain conditions, lead to a more reliable network performance.
Affecting microstructure and properties of additively manufactured AISI 316L steel by rotary swaging
(MDPI, 2022) Kunčická, Lenka; Kocich, Radim; Benč, Marek; Dvořák, Jiří
The presented work focused on the development of the microstructural and mechanical properties of a AISI 316L stainless steel workpiece prepared through additive manufacturing and subsequently processed by hot rotary swaging. In order to characterize the effects of swaging on the structural development, samples were taken for electron microscopy scanning and microhardness measurements were taken after each swaging reduction. The as-built and final swaged pieces were also subjected to tensile testing at room temperature and at 900 degrees C. The structural analyses showed that the hot swaging introduced a substructural formation; low angle grain boundaries prevailed over high angle ones after each pass. The swaging also imparted an almost complete elimination of the porosity and significant grain size; the average grain area decreased from the original value of 365.5 mu m(2) to 4.4 mu m(2) after the final swaging pass. The changes in the texture between the passes were negligible, however, the grain refinement went hand in hand with the microhardness increase (up to almost 300 HV1). The results of the tensile testing confirmed that the mechanical properties of the swaged pieces which improved dramatically and remained favorable up to high temperatures.
Structural and magnetic properties of inverse-Heusler Mn2FeSi alloy powder prepared by ball milling
(MDPI, 2022) Životský, Ondřej; Skotnicová, Kateřina; Čegan, Tomáš; Juřica, Jan; Gembalová, Lucie; Zažímal, František; Szurman, Ivo
Ternary Mn2FeSi alloy was synthesized from pure elemental powders by mechanical alloying, using a high-energy planetary ball mill. The formation of an inverse-Heusler phase after 168 h of milling and subsequent annealing at 1173 K for 1.5 h was confirmed by X-ray diffraction. The diffractogram analysis yielded XA structure and the lattice parameter 0.5677 nm in a good agreement with the theoretically obtained value of 0.560 nm. The final powder was formed by particles of irregular shape and median diameter D50 of 3.8 mu m and their agglomerates. The chemical analysis resulted in the mean composition of 49.0 at.% Mn, 25.6 at.% Fe and 25.4 at.% Si. At room temperature, the prepared samples featured a heterogeneous magnetic structure consisting of dominant paramagnetic phase confirmed by Mossbauer spectrometry and a weak ferro-/ferrimagnetic contribution detected by magnetization curves. From the field-cooled and zero-field-cooled curves the Neel temperature of 67 K was determined.
Hot deformation behavior of non-alloyed carbon steels
(MDPI, 2022) Kawulok, Petr; Opěla, Petr; Schindler, Ivo; Kawulok, Rostislav; Rusz, Stanislav; Sauer, Michal; Konečná, Kateřina
The hot deformation behavior of selected non-alloyed carbon steels was investigated by isothermal continuous uniaxial compression tests. Based on the analysis of experimentally determined flow stress curves, material constants suitable for predicting peak flow stress sigma(p), peak strain epsilon(p) and critical strain epsilon(crDRX) necessary to induce dynamic recrystallization and the corresponding critical flow stresses sigma(crDRX) were determined. The validity of the predicted critical strains epsilon(crDRX) was then experimentally verified. Fine dynamically recrystallized grains, which formed at the boundaries of the original austenitic grains, were detected in the microstructure of additionally deformed specimens from low-carbon investigated steels. Furthermore, equations describing with perfect accuracy a simple linear dependence of the critical strain epsilon(crDRX) on peak strain epsilon(p) were derived for all investigated steels. The determined hot deformation activation energy Q decreased with increasing carbon content (also with increasing carbon equivalent value) in all investigated steels. A logarithmic equation described this dependency with reasonable accuracy. Individual flow stress curves of the investigated steels were mathematically described using the Cingara and McQueen model, while the predicted flow stresses showed excellent accuracy, especially in the strains ranging from 0 to epsilon(p).
Technological and quality aspects of the use of innovative inorganic binders in the production of castings
(MDPI, 2021) Obzina, Tomáš; Merta, Václav; Folta, Martin; Bradáč, Josef; Beňo, Jaroslav; Novohradská, Nikol; Gawronová, Martina; Kroupová, Ivana; Lichý, Petr; Radkovský, Filip; Janovská, Kamila; Vasková, Iveta; Drobíková, Klára; Nguyenová, Isabel
The production of cores for the pre-casting of holes in castings places high demands on the quality of the molding mixtures used. For this reason, organic binders are still used to a large extent, which, although they meet the technological requirements, are a source of pollutant emissions during the production of castings. The current trend towards greening production is therefore looking for a suitable alternative in 'green' inorganic binders. Although for many decades standard inorganic binders could not be compared with organic resins in terms of technological properties, new inorganic binder systems are currently being developed that can largely eliminate these disadvantages, which include, in particular, significantly lower collapsibility and reclaimability, and lower mechanical strength values. Last but not least, the use of these binder systems may be limited by the technological parameter of shelf-life, which is the main focus of this study. The aim of this paper is to evaluate the influence of technological parameters of core production using a new generation of inorganic binder systems on their shelf-life. Shelf-life, defined as the change in mechanical strength and wear resistance as a function of exposure time in a given environment, is evaluated under different climatic conditions.
Effects of temperature (in)homogeneity during hot stamping on deformation behavior, structure, and properties of brass valves
(Wiley, 2021) Kunčická, Lenka; Kocich, Radim
Herein, assessing the effects of initial temperature (in)homogeneity on a hot-stamped CuZn40Pb2 medical gas valve (fitting) via numerically and experimentally defining mutual relations of selected deformation parameters and optimizing temperature distribution within the original semi-product heated via induction are focused. For these purposes, three preheating regimes are simulated, and deformation behaviors are evaluated. The predicted results are validated by experimental stamping and subsequent evaluation of the structure and properties of stamped fittings. The results show that preheating the semi-product with a certain temperature gradient occurring between the axial and (sub)surface regions is favorable, as homogeneous initial temperature distribution results in rapid surface cooling by the effect of heat transfer from semi-product to die, which imparts inhomogeneous stress distribution, local changes in structural phases, and possible occurrence of oxides. Too low (sub)surface temperature of the semi-product can result in a significant (local) increase in flow stress, which consequently results in the danger of occurrence of forming defects. The brass fitting stamped with the optimized preheating procedure features an average grain size of 5.8 mu m, uniform grains' orientations with maximum texture intensity of two times random, and ultimate tensile strength of almost 400 MPa, while maintaining elongation to failure of more than 35%.
Microstructural aspects of new grade ODS alloy consolidated by rotary swaging
(Elsevier, 2021) Chlupová, Alice; Šulák, Ivo; Kunčická, Lenka; Kocich, Radim; Svoboda, Jiří
In the present paper, the authors have investigated the microstructural aspects of recrystallization of new ODS alloy (Fe-14Cr-10Al-4Y2O3) consolidated by hot rotary swaging. Microstructural characterization of samples was performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). Insight of mechanical properties was performed by Vickers hardness testing and measurement of Young's modulus. A thorough analysis of microstructure concerning grains, oxide particles and dislocations was performed for two states of the material, i.e. in the as-received state after rotary swaging exhibiting the ultra-fine-grained microstructure and in the state after annealing provoking the desirable coarsegrained structure. Different conditions of rotary swaging followed by the same annealing conditions induced different structures, either coarse-grained or bimodal when secondary recrystallization did not happen in the entire sample's volume. The hardness measurements revealed no significant difference between the coarsegrained and the bimodal microstructures, however, the coarse-grained material exhibited systematically higher values of the temperature-dependent Young's modulus in comparison with the bimodal counterpart. Detailed microscopic analysis of the material helped to clarify and discuss the measured properties of the newly developed ODS alloy.
On various multi-layer perceptron and radial basis function based artificial neural networks in the process of a hot flow curve description
(Elsevier, 2021) Opěla, Petr; Schindler, Ivo; Kawulok, Petr; Kawulok, Rostislav; Rusz, Stanislav; Navrátil, Horymír
In recent years, the study of the hot deformation behavior of various materials is significantly marked by an increasing utilization of artificial neural networks, which are frequently employed for a hot flow curve description. This specific kind of description is commonly solved via a Feed-Forward Multi-Layer Perceptron architecture and rarely via a Radial Basis architecture. Both network architectures are compared to assess their suitability in the process of a hot flow curve description under a wide range of thermomechanical conditions. The performed survey is also aimed on the eventual utilization of corresponding modifications of both studied networks, namely on a Cascade-Forward Multi-Layer Perceptron and Generalized Regression network. The main results have shown that the Feed-Forward Multi-Layer Perceptron architecture represents a good choice if very high accuracy is a crucial goal. However, in the case of this architecture, finding the proper parameters can be time-consuming and the hardware burdensome. On the contrary, for the flow curve description the almost unused Radial Basis network offers a very easy training procedure and significantly shorter computing time under acceptable accuracy. The results of the submitted research should then serve as a background for the selection and following application of a suitable network architecture in the process of solving future flow curve description tasks.
Neutron diffraction study of residual stresses in a W-Ni-Co heavy alloy processed by rotary swaging at room and high temperatures
(Springer Nature, 2021) Canelo-Yubero, David; Kocich, Radim; Hervoches, Charles; Strunz, Pavel; Kunčická, Lenka; Krátká, Ludmila
Residual stresses were studied in tungsten heavy alloy bars produced by powder metallurgy and deformed by rotary swaging at room temperature (RT) and at 900 degrees C. Neutron diffraction technique was used to scan the residual stresses across the bars. Both tungsten particles and NiCo2W solid solution matrix were analysed. Maximum axial stresses of similar to 300 MPa and similar to 200 MPa were observed for the tungsten phase at the centre in the RT and in the high-temperature deformed samples, respectively. Compressive residual axial stresses were found close to the sample surface, showing that rotary swaging is a suitable deformation method for tungsten heavy alloys to provide an appropriate surface modification for its use in metallic parts undergoing, e.g., fatigue. Residual stresses developed in the NiCo2W-phase are larger than those found in the tungsten particles although with a secondary role in the overall equilibrium conditions due to its lower strength and smaller volume fraction. Total stresses for each phase were separated into macro- and microstresses. Macrostresses can be mainly influenced by the incompatibility of the elliptical cross-section of the sintered sample with the head of the rotary machine while microstresses are mainly developed by the elastic mismatch between the constituent phases.
Hot deformation activation energy of metallic materials influenced by strain value
(Polska Akademia Nauk, Instytut Metalurgii i Inżynierii Materiałowej, 2021) Schindler, Ivo; Opěla, Petr; Kawulok, Petr; Sauer, Michal; Rusz, Stanislav; Kuc, Dariusz; Rodak, Kinga
Suitable and complete sets of stress-strain curves significantly affected by dynamic recrystallization were analyzed for 11 different iron, copper, magnesium, titanium or nickel based alloys. Using the same methodology, apparent hot deformation activation energy Q(p) and Q(ss) values were calculated for each alloy based on peak stress and steady-state stress values. Linear dependence between quantities Q(p) and Q(ss) was found, while Q(p) values are on average only about 6% higher. This should not be essential in predicting true stress of a specific material depending on the temperature-compensated strain rate and strain.
Mechanical characterization of graphene nanoplatelets-reinforced Mg-3Sn alloy synthesized by powder metallurgy
(MDPI, 2021) Kumar, Pravir; Skotnicová, Kateřina; Mallick, Ashis; Gupta, Manoj; Čegan, Tomáš; Juřica, Jan
The present study investigated the effects of alloying and nano-reinforcement on the mechanical properties (microhardness, tensile strength, and compressive strength) of Mg-based alloys and composites. Pure Mg, Mg-3Sn alloy, and Mg-3Sn + 0.2 GNP alloy-nanocomposite were synthesized by powder metallurgy followed by hot extrusion. The microstructural characteristics of the bulk extruded samples were explored using X-ray diffraction, field-emission scanning electron microscopy, and optical microscopy and their mechanical properties were compared. The microhardness, tensile strength, and compressive strength of the Mg-3Sn alloy improved when compared to those of monolithic Mg sample and further improvements were displayed by Mg-3Sn + 0.2 GNP alloy-nanocomposite. No significant change in the compressive strain to failure was observed in both the alloy and the alloy-nanocomposite with respect to that of the pure Mg sample. However, an enhanced tensile strain to failure was displayed by both the alloy and the alloy-nanocomposite.
Source apportionment of magnetite particles in roadside airborne particulate matter
(Elsevier, 2021) Gonet, Tomasz; Maher, Barbara A.; Kukutschová, Jana
Exposure to airborne particulate matter (PM) is associated with pulmonary, cardiovascular and neurological problems. Magnetite, a mixed Fe2+/Fe3+ oxide, is ubiquitous and abundant in PM in urban environments, and might play a specific role in both neurodegeneration and cardiovascular disease. We collected samples of vehicle exhaust emissions, and of heavily-trafficked roadside and urban background dusts from Lancaster and Birmingham, U.K. Then, we measured their saturation magnetic remanence and used magnetic component analysis to separate the magnetite signal from other contributing magnetic components. Lastly, we estimated the contributions made by specific traffic-related sources of magnetite to the total airborne magnetite in the roadside environment. The concentration of magnetite in exhaust emissions is much lower (3-14 x lower) than that in heavily-trafficked roadside PM. The magnetite concentration in petrol-engine exhaust emissions is between similar to 0.06 and 0.12 wt%; in diesel-engine exhaust emissions similar to 0.08-0.18 wt%; in background dust similar to 0.05-0.20 wt% and in roadside dust similar to 0.18-0.95 wt%. Here, we show that vehicle brake wear is responsible for between similar to 68 and 85% of the total airborne magnetite at the two U.K. roadside sites. In comparison, diesel-engine exhaust emissions account for similar to 7% - 12%, petrol-engine exhaust emissions for similar to 2% - 4%, and background dust for 6% - 10%. Thus, vehicle brake wear is by far the most dominant source of airborne magnetite in the roadside environment at the two sites examined. Given the potential risk posed, post-inhalation, by ultrafine magnetite and co-associated transition metal-rich particles to human cardiovascular and neurological health, the high magnetite content of vehicle brake wear might need to be reduced in order to mitigate such risk, especially for vulnerable population groups.
In situ neutron diffraction investigation of texture-dependent Shape Memory Effect in a near equiatomic NiTi alloy
(Elsevier, 2021) Wang, Zifan; Chen, Jingwei; Besnard, Cyril; Kunčická, Lenka; Kocich, Radim; Korsunsky, Alexander M.
To explore the possibility of customising the functional behaviour of NiTi shape memory alloy via controlling texture, binary Ni55Ti45 (wt.%) alloys were manufactured in as cast and hot swaged conditions, presenting contrasting initial texture and macroscopic performance. In situ time-of-flight neutron diffraction technique was employed to study the texture effect on the microstructural evolution during Shape Memory Effect (SME), and a range of properties were evaluated. It was found that (i) hot swaging process leads to change in grain morphology and increase in microstrain; (ii) thermal expansion coefficients of martensite and austenite variants were weakly affected by the texture and phase transformation constraint; (iii) significant texture effect on the elastic properties at both macroand micro-scale was quantified by Elasto-Plastic Self-Consistent (EPSC) modelling approach, while the anisotropic elastic moduli lie within the range of single crystal state and twinned structure; (iv) texture evolution during SME is weakly related to the initial microstructure; (v) martensite reoriented so that the <010> axis became aligned parallel to the loading direction, and retained this orientation upon unloading, revealing the underlying correlation between texture evolution and detwinning. Based on the experimental results, a multi-variant model was proposed to quantify the lattice strain evolution during SME. Validity of the conceptually simple and parametrically parsimonious model was confirmed by validation against experimental data.
Fundamental improvement of creep resistance of new-generation nano-oxide strengthened alloys via hot rotary swaging consolidation
(MDPI, 2020) Svoboda, Jiří; Kunčická, Lenka; Luptáková, Natália; Weiser, Adam; Dymáček, Petr
New-generation oxide dispersion-strengthened (ODS) alloys with a high volume fraction of nano-oxides of 5% are intended to become the leading creep- and oxidation-resistant alloys for applications at 1100-1300 degrees C. Hot consolidation of mechanically alloyed powders by intensive plastic deformation followed by heat treatment of the alloys are the key aspects for achieving top creep properties, typically ensured by a coarse-grained microstructure strengthened with homogeneously dispersed, very stable yttrium nano-oxides. The rotary swaging method proves to be favourable for hot consolidation of the new-generation ODS alloy presented. Compared to specimens consolidated by hot rolling, consolidation by hot rotary swaging predetermines the formation of coarse grains with a very high aspect ratio during subsequent secondary recrystallization. Such a grain morphology increases the creep strength of the new-generation ODS alloy considerably.
Effects of austenitization temperature and pre-deformation on CCT diagrams of 23MnNiCrMo5-3 steel
(MDPI, 2020) Schindler, Ivo; Kawulok, Rostislav; Opěla, Petr; Kawulok, Petr; Rusz, Stanislav; Sojka, Jaroslav; Sauer, Michal; Navrátil, Horymír; Pindor, Lukáš
The combined effect of deformation temperature and strain value on the continuous cooling transformation (CCT) diagram of low-alloy steel with 0.23% C, 1.17% Mn, 0.79% Ni, 0.44% Cr, and 0.22% Mo was studied. The deformation temperature (identical to the austenitization temperature) was in the range suitable for the wire rolling mill. The applied compressive deformation corresponded to the true strain values in an unusually wide range. Based on the dilatometric tests and metallographic analyses, a total of five different CCT diagrams were constructed. Pre-deformation corresponding to the true strain of 0.35 or even 1.0 had no clear effect on the austenite decomposition kinetics at the austenitization temperature of 880 degrees C. During the long-lasting cooling, recrystallization and probably coarsening of the new austenitic grains occurred, which almost eliminated the influence of pre-deformation on the temperatures of the diffusion-controlled phase transformations. Decreasing the deformation temperature to 830 degrees C led to the significant acceleration of the austenite -> ferrite and austenite -> pearlite transformations due to the applied strain of 1.0 only in the region of the cooling rate between 3 and 35 degrees C center dot s(-1). The kinetics of the bainitic or martensitic transformation remained practically unaffected by the pre-deformation. The acceleration of the diffusion-controlled phase transformations resulted from the formation of an austenitic microstructure with a mean grain size of about 4 mu m. As the analysis of the stress-strain curves showed, the grain refinement was carried out by dynamic and metadynamic recrystallization. At low cooling rates, the effect of plastic deformation on the kinetics of phase transformations was indistinct.
On the Zener-Hollomon parameter, multi-layer perceptron and multivariate polynomials in the struggle for the peak and steady-state description
(MDPI, 2020) Opěla, Petr; Kawulok, Petr; Schindler, Ivo; Kawulok, Rostislav; Rusz, Stanislav; Navrátil, Horymír
Description of flow stress evolution, specifically an approximation of a set of flow curves acquired under a wide range of thermomechanical conditions, of various materials is often solved via so-called flow stress models. Some of these models are associated with a description of significant flow-curve coordinates. It is clear, the more accurate the coordinates description, the more accurate the assembled model. In the presented research, Zener-Hollomon-based relations, multi-layer perceptron networks and multivariate polynomials are employed to describe the peak and steady-state coordinates of an Invar 36 flow curve dataset. Comparison of the utilized methods in the case of the studied alloy has showed that the suitable description is given by the multivariate polynomials although the Zener-Hollomon and perceptron networks also offer valuable results.
Hot deformation behaviour and microstructure evolution of TiAl-based alloy reinforced with carbide particles
(Elsevier, 2020) Lapin, Juraj; Štamborská, Michaela; Pelachová, Tatiana; Čegan, Tomáš; Volodarskaja, Anastasia
The hot deformation behaviour and microstructure evolution of TiAl-based alloy with nominal composition TT 43Al-8Nb-3.6C-0.7Mo (at.%) reinforced with carbide particles were studied. The compression tests were carried out in the temperature range from 900 to 1000 degrees C and strain rates from 0.0001 to 0.01 s(-1) up to a true strain of 0.5. The microstructure of the compression specimens was characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). The work hardening stage of the flow curves results from an increment of the dislocation density and deformation twins. The work softening of the composite results from the dynamic recovery (DRV), dynamic recrystallization (DRX) and fragmentation of some coarse Ti2AlC particles. The size of recrystallised grains increases with decreasing Zener-Hollomon parameter. The non-uniform distribution of local strains in the deformed specimens and deformation temperature affect the orientation and fragmentation of coarse Ti2AlC particles. A constitutive model predicting hot deformation behaviour of the studied alloy is proposed and its validity is evaluated and discussed.
Effect of hot isostatic pressing on porosity and mechanical properties of 316 L stainless steel prepared by the selective laser melting method
(MDPI, 2020) Čegan, Tomáš; Pagáš, Marek; Juřica, Jan; Skotnicová, Kateřina; Hajnyš, Jiří; Horsák, Lukáš; Souček, Kamil; Krpec, Pavel
The manufacturing route primarily determines the properties of materials prepared by additive manufacturing methods. In this work, the microstructural features and mechanical properties of 316 L stainless steel prepared by the selective laser method have been determined. Three types of samples, (i) selective laser melted (SLM), (ii) selective laser melted and hot isostatic pressed (HIP) and (iii) selective laser melted and heat treated (HT), were characterized. Microstructural analysis revealed that SLM samples were formed by melt pool boundaries with fine cellular-dendritic-type microstructure. This type of microstructure disappeared after HT or HIP and material were formed by larger grains and sharply defined grain boundaries. The SLM-prepared samples contained different levels of porosity depending on the preparation conditions. The open interconnected LOF (lack of fusion) pores were observed in the samples, which were prepared with using of scanning speed 1200 mm/s. The blowhole and keyhole type of porosity were observed in the samples prepared by lower scanning speeds. The HIP caused a significant decrease in internal closed porosity to 0.1%, and a higher pressure of 190 MPa was more effective than the usually used pressure of 140 MPa, but for samples with open porosity, HIP was not effective. The relatively high yield strength of 570 MPa, tensile strength of 650 MPa and low ductility of 30-34% were determined for SLM samples with the lower porosity content than 1.3%. The samples after HIP showed lower yield strengths than after SLM (from 290 to 325 MPa) and relatively high ductility of 47.8-48.5%, regardless of the used SLM conditions.
Hot deformation behaviour of Mn-Cr-Mo low-alloy steel in various phase regions
(MDPI, 2020) Schindler, Ivo; Opěla, Petr; Kawulok, Petr; Sojka, Jaroslav; Konečná, Kateřina; Rusz, Stanislav; Kawulok, Rostislav; Sauer, Michal; Turoňová, Petra
The deformation behaviour of a coarse-grained as-cast medium-carbon steel, alloyed with 1.2% Mn, 0.8% Cr and 0.2% Mo, was studied by uniaxial compression tests for the strain rates of 0.02 s(-1)-20 s(-1)in the unusually wide range of temperatures (650-1280 degrees C), i.e., in various phase regions including the region with predominant bainite content (up to the temperature of 757 degrees C). At temperatures above 820 degrees C, the structure was fully austenitic. The hot deformation activation energies of 648 kJ center dot mol(-1)and 364 kJ center dot mol(-1)have been calculated for the temperatures <= 770 degrees C and >= 770 degrees C, respectively. This corresponds to the significant increase of flow stress in the low-temperature bainitic region. Unique information on the hot deformation behaviour of bainite was obtained. The shape of the stress-strain curves was influenced by the dynamic recrystallization of ferrite or austenite. Dynamically recrystallized austenitic grains were strongly coarsened with decreasing strain rate and growing temperature. For the austenitic region, the relationship between the peak strain and the Zener-Hollomon parameter has been derived, and the phenomenological constitutive model describing the flow stress depending on temperature, true strain rate and true strain was developed. The model can be used to predict the forming forces in the seamless tubes production of the given steel.

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