Seznam abstraktů 2023: SEKCE C - VÝROBKY Z OCELI A JEJICH VLASTNOSTI

Steel is by far the world's most important, multi-functional, and most adaptable material. The excellent mechanical properties of steels are determined by their microstructure. The microstructure of advanced steels, such as advanced high strength steels (AHSS), is a combination of different phases or constituents with complex substructures and its classification is extremely challenging. Traditionally used imaging techniques, such as light optical microscopy (LOM) or confocal microscopy (CM), turn out to be insufficient for precise structural characterization of the AHSS, and high-resolution imaging of their surface by state-of-the-art scanning electron microscopes (SEM) is required. However, common metallographic laboratories are equipped only with the LOM or the CM at best. As a result, they are not able to fully and sufficiently characterize the structure of modern steels. Software-based transformation of low-quality images into their high-quality equivalents is a challenging task. We conducted experiments of transforming the LOM images into the SEM images using deep learning techniques. A traditional way of translating one image into another one is using so-called U-Net neural network architecture. This architecture was originally proposed for image segmentation, but a slight modification of the output layer enables us to generate any type of images. We trained the U-Net on a set of 1000 LOM-SEM pairs of 256x256 pixel images. It turned out, however, that U-Net itself is unable to translate the images neither on the testing data set nor on the training data set. In order to improve the result, we used the idea of Generative Adversarial Networks (GANs) and trained the U-Net along with a deep discriminator network in an adversarial way. Even though the GAN transformation is better than standalone U-Net transformation, there is still space for improvement.

Nondestructive testing is an important part of any production process that is required to ensure the quality of materials and parts. The development of reliable NDT methods applicable to novel materials and manufacturing processes such as additive manufacturing is a challenging task because of the complex geometry and anisotropic material properties. The majority of published review works related to the NDT methods in the additive manufacturing process are focused on only one aspect: either the defect classification or NDT methods and quality control systems. In this study, various NDT methods such as X-ray testing, acoustic emission, IR thermography, etc. were analyzed from the point of view of an application for selective laser melting (one of the most common additive manufacturing processes of metal parts). Typical defect types appearing in SLM parts were classified and matched with NDT methods able to detect them. The recommendations on the use of different methods for in-situ process monitoring and printed parts inspection are given.

Additive manufacturing has become one of the most suitable and affordable technologies for manufacturing functionally graded materials (FGMs). These materials, sometimes even classified as advanced composite materials, have become very popular for their unique benefit of distributing desirable properties throughout the volume of the component according to the conditions of the application environment. In this presentation, the mechanical behaviour of an additively manufactured functionally graded system composed of stainless steel 316L and nickel alloy IN 718 is analysed. The focus of this work is on characterizing the local mechanical properties using miniaturized tensile test specimens and evaluating the methodology, including DIC analysis. The results show good rigidity of the transition area between the layers of 316L and IN 718, making this material combination promising for applications where the ductility, non-transition behaviour, and corrosion resistance guaranteed by 316L are required, as well as the strength of the nickel super-alloy IN 718. The mutual compatibility of both materials in the transition region is also confirmed by the defect-free microstructure observed by SEM.

In this work, the dynamic material modelling (DMM) is employed to identify the hot working window that is suitable for hot processing of 2205 DSS. The Gleeble 1500 thermo-mechanical simulator was used to conduct the investigation, where several hot compression tests were conducted at a temperature range of 850 - 1050°C and strain rates of 0.001s-1 - 5s-1. The obtained experimental data were first corrected for friction and adiabatic heating, the correction of adiabatic heating was only done for a strain rate of 1 and 5s-1. After the corrections, the corrected flow stress curves were plotted, and the flow stress data at the strain of 0.1, 0.3, 0.5 and 0,8 were generated. Using the generated flow stress data at a particular strain and combination of temperature and strain rates, the strain rate sensitivity values were calculated using constitutive modelling. The DMM was then applied to calculate the efficiencies and instability values followed by using cubic spline interpolation for more detailed data. The processing maps were then constructed from the results of cubic spline interpolation. The processing maps indicated that, the hot working process of 2205 DSS is more efficient at low strain rates and high temperatures, particularly at true strains below 0.3. With increasing in strain to 0.5 and above, high efficiency seemed to spread to the regions of high strain rates and moderate high temperatures.

The experimental estimation of the fatigue curve is quite expensive and complex. It is very difficult to create an actual reactor's working environment, such as extreme temperatures, etc. Hence, the finite element method can be employed for the investigation, estimation of the fatigue behaviour of the steel, and plotting the fatigue curve, using this curve, the fatigue mechanism can be studied in detail.
In this research, we have created a finite element model for investigating the low cycle fatigue behaviour of steel using LS-Dyna. A strain-controlled low-cycle fatigue test experiment at 300 °C on AISI 304L steel for 0.3% strain was carried out and the results of it are used to estimate the material model parameters and validate the simulation results. The simulation model is created to replicate the experiment. The simulation results are compared with the experimental results. It is observed that the simulation results are close to the experimental results. On the bases of the comparison of the results it can be concluded that the numerical simulation can be employed for the estimation of the fatigue curve and fatigue properties.

During the operation of the axle, the load acting on the axle housing in the vertical direction has a significant effect on the fatigue life of the components. Cracks are caused by a constant stress concentration in the axle housing, resulting in fatigue at the concentration point. If the load exceeds a certain threshold, microscopic cracks will begin to form at the stress concentration point. Later, the cracks grow due to the cyclic load of fatigue. Eventually, the cracks reach a critical limit and then the structure suddenly breaks. Axle housing failures are also affected by factors such as uneven load effect, housing slope, and eccentricity. The paper presents an analysis of reliability and damage to the rear axle of a truck semi-trailer. For this purpose, microscopic observations, FEA and analytical calculations where conducted, preceded by a theoretical introduction was used. Several load cases were considered in order to determine the most likely point of damage initiation to the semitrailer axle.

It is assumed that sever plastic deformation of metallic alloys designed for hydrogen storage can increase the total amount of stored hydrogen while decreasing the temperature of hydrogen absorption and desorption into/from the alloy. We have decided to experimentally verify this assumption on a single-phase high-entropy alloy. This alloy undergoes comprehensive tests of hydrogen absorption in its as-prepared state, after heat treatment, and after severe plastic deformation using high-pressure torsion. At the time of writing this abstract, we are preparing the experiment of hydrogen absorption and desorption, which will provide an answer as to whether significant plastic deformation can improve the parameters of the metal hydride.

Currently, technological development is focused, among other things, on hydrogen and its technologies. Hydrogen storage must be very well secured and material resistance against hydrogen embrittlement is tested using different approaches. Nevertheless, there are three specified methods listed in EN ISO 11114-4 standard, which are disc test, fracture mechanic test and test method to determine the resistance to hydrogen assisted cracking of steel cylinders. Many works showed differences in results evaluated from latter and last method, eventually method for measurement of fracture toughness. Thus, rising crack opening test according to EN ISO 12135 standard, and last method stated in EN ISO 11114-4, i.e., constant displacement test, were done in this paper to find out, how hydrogen damages steel mentioned above. This paper aims on chromium molybdenum steel 34CrMo4, which is often used for production of metal pressure cylinders. Compact tension specimens, respectively modified compact tension specimens with central hole, were prepared and tested for fracture toughness measurement in air and in high-pressure hydrogen gas at 30 MPa, respectively for latter method.

The microstructure of normalized press-hardenable steel 22MnB5 is ferritic-pearlitic. During the hot-stamping process, it transforms into the fully martensitic resulting in desired high strength characteristics. The steel sheets are usually coated with Al-Si to prevent surface oxidation during the austenitization preceding the forming and quenching in a cooled die. The stamped parts are further assembled and joined to other components of a final construction, typically a car body. Laser welding is a suitable method of thin sheets joining thanks to its highly concentrated energy minimizing the heat input and thus the distortions. Together with material characteristics, the heat input determines peak temperature, dwell time, and cooling rate defining the local microstructure of a weld. The effect of sheet surface coating must be also considered because it is usually not removed before welding. Ferrite-stabilizing elements of the coating dissolve in the fused metal and modify its microstructure even though the thickness of the coating is only a few tens of microns. A rapid cooling rate of the fused metal limits its homogenization, which results in the local formation of ferrite decreasing the tensile strength of a weld joint. The ferrite content depends on the beam and processing characteristics. Therefore, it must be kept in mind that every change in the welding process can lead to microstructural changes directly influencing the mechanical properties of a weld joint.

This paper presents of the results of the study of structural-phase state and microhardness changes of structural steels by electrolyte-plasma chemical-thermal treatment (EPCTT). The interest to EPCTT is caused by wide technological capabilities of the given technology which is carried out due to diffusion saturation of a material surface (cathode) with active elements (C, N, C-N ) from electrolyte. Using the EPCTT with durability up to 5-10 min allows to obtain modified materials with improved operational parameters that many times exceed indicators of traditional thermal or chemical-thermal treatment with duration up to 10-30 hours. It is also known that EPCTT improves the wear resistance of steel by increasing the surface hardness with a plastic structure in the core of the material, which provides resistance to shock loading. EPCTT is performed at the electrolyte-plasma treatment unit, where the saturation process is carried out by electric current flowing through the electrolyte, forming a thin layer of vapor-gas shell (VGS) on the surface of the cathode, which is the processed product, where the conversion of electric energy into heat takes place. The surfaces of the part are saturated with the elements of evaporation of compounds from the electrolyte in the VGS, due to thermal decomposition of components and electrochemical reactions on the surface of the part, by diffusion. Studies were carried out by optical microscopy, scanning electron microscopy, X-ray structural analysis and microhardness measurements. The modified samples at different modes of electrolyte-plasma chemical-thermal treatment were obtained.