Seznam abstraktů 2023: POSTEROVÁ SEKCE C

Hot-work tool steels are extensively used in industrial applications that require high resistance to mechanical and chemical degradation at elevated temperatures. To meet these requirements, hot-work tool steels must exhibit good mechanical properties, including high tensile strength, hardness, wear resistance, and tempering resistance, as well as high thermal conductivity and ductility. This study investigates the ductility of the hot-work tool steel HTCS-130, which suffers from low ductility due to the presence of stable molybdenum-tungsten carbides (M6C) on the prior austenite crystal grain boundaries. Increasing austenitisation temperatures or prolonging the dwelling time at temperature can promote intensive migration of grain boundaries, leading to negative effects on the mechanical properties of the steel. To address this issue, isothermal transformation in the bainitic area between 350 and 500 °C was performed. Isothermal transformation at around 350 °C leads to the formation of lower bainite, which has similar hardness to tempered martensite. As the temperature of isothermal transformation increases, the hardness of the material decreases, due to the formation of upper bainite. The hardness analysis was measured using the Vickers method, the impact toughness of the steel samples was measured using a Charpy test with V-notched samples. The microstructure characterization was performed using optical and scanning electron microscopy. The improvement of ductility can be achieved by controlling the isothermal transformation of bainite and adjusting the heat treatment conditions. These findings provide useful insights into the design and optimization of heat treatment processes for hot-work tool steels.

Paper presents analysis of laser lap joint welding of similar and dissimilar materials. Proces analysis is based on numerical simulations using Simufact Welding software and experimental trial joint. Numerical investigation of some joints properties are based on thermo-mechanical simulation. Based on parameters estimated during numerical simulations trial joint was performed. Specimens obtained from experimental laser lap welding were analysed using mechanical and metallograpnic tests, including hardness test, tensile strength, micro and macrostructure and SEM analysis. Problem of laser lap joint welding was investigated and results were shown in this article.

Low carbon high-strength steel sheets are in scope of the material research mainly as a structural materials for automotive industry. Cold forming is the most prominent technology of achieving the final shape of the parts made of thin sheet. This requires sufficient plasticity of the material so that the forming procedure can be performed without sheet fracture. High strength steels are attractive due to possibility of weight saving; however, their higher strength comes with a reduction of plasticity. Especially widely used dual and complex phase steels, providing an exceptional combination of plasticity and strength, are not performing well during forming due to their susceptibility to an edge fracture. It emerged that this feature is caused by alternation of hard martensite and soft ferrite in their structure causing crack propagation by separation of these phases. This experiment is aimed at development of a high-strength low-carbon steel sheet of a homogeneous low-tempered martensite microstructure with a tensile strength in an interval of 1000-1200 MPa. This strength is comparable with the highest grades of dual and complex-phase steels. However, the homogeneous microstructure gives an advantage in the formability, which can be crucial factor for the cold forming operation. Influence of different concentration of Si an Mo was examined by tensile test and hole expansion test for 0.8 mm thick sheets.

Nowadays, the use of high strength steels (HS) and ultra-high strength steels (UHS) is increasing, especially when welded constructions are made. These types of steels are mainly used as a material for heavily loaded constructions such as bridges, cranes, and excavators, as well as for pressure vessels, different kinds of vehicles, ships, drilling constructions etc., working at room temperature or below. As the requirements for the abovementioned constructions are specific, the use of high strength and ultra-high strength steels necessitates the need for research of the factors that influence their weldability. Possible negative consequences when welding these types of steels are cold cracks formation, softening and brittleness. Following the common welding rules HS steels and UHS steels can be welded by all conventional welding methods. The studied in the present work steel S960QL falls into the group of high strength quenched and tempered fine-grained steels with carbon content up to 0.2% and warranted impact toughness at -40°C. The types of joint preparation of HS and UHS steels is specified in EN ISO 9692-1:2013 and EN ISO 9692-2:2001 standards. However, the welding gap is the closing section and doesn’t comply with the standard recommendations frequently. This fact necessitates the research on the influence the welding gap has on the properties of the weldments. This work represents results, showing the influence of the welding gap on some mechanical properties of submerged arc weldments of high strength steel S960QL.

One of the methods of evaluating the mechanical properties of a material in the case of a limited amount of material is the use of methods that focus on the use of miniaturized test samples. The basic properties used mostly for life evaluation are tensile strength, impact notch toughness or impact notch toughness transition curve, fracture toughness, creep and high cycle fatigue. This research investigates the creep properties using sub-size specimens with a cross section 2x1mm. This article describes the methodology of studying the creep properties of laser-directed energy deposition (L-DED) steel made from Inconel 718 (IN718) creep steels and 10Cr-3W-3Co-1Mo creep-resistant steels without nitrogen at a temperature of 650°C. Emphasis was placed on the practical use of test results.

New mathematical expression for prediction of hardness of quenched and tempered steel were established. Mathematical expression is similar to the relationship established by E. Just. The hardness of quenched and tempered steel can be predicted based on as-quenched hardness, by taken into account the tempering temperature and the degree of hardening. Other than these parameters the steel’s hardness after tempering can depend on other steel properties which depend of chemical composition of steel. The prediction of quenched and tempered steel’s hardness could be more precise if, except the temperature of tempering and the degree of hardening, chemical composition of steels will be taken into account. In accordance with it, the chemical composition of steel are included in the expression to achieve a more precise prediction of quenched and tempered steel hardness. The chemical composition of steel are taken into account by carbon equivalent (CE). By implementation of the chemical composition of steel in relation for prediction of tempered steel’s hardness, the influence of diffusivity of carbon and other elements on the kinetics of the tempering processes of steel will be taken into account. Established mathematical expression were applied in computer simulation of tempering of as-quenched steel workpiece. Prediction of the as-quenched hardness of steel workpiece is based on the relationship between as-quenched hardness and the characteristic cooling time from 800 °C to 500 °C.

DUPLEX steels are new generation austenitic-ferritic type of stainless steels. Duplex steels have better mechanical strength, corrosion resistance and fatigue strength in aggressive environment comparing to traditional stainless steels such as AISI 316 or 304. However, DUPLEX steels have poorer weldability because of inability to achieve higher welding speeds due to the poor weld penetration and fluidity in weld pool. That is why the responsible selection of welding process is important for manufacturing of components and structures made of DUPLEX steels. This paper presents the investigation of free different welding process (BW.141, FW.136 and FW.141) for x-type specimen. The initial static tension experiment has not showed significant difference in strength of specimens with different welding processes. Therefore, it was decided to conduct low cycle fatigue experiment. Stress controlled fatigue test with pulse type cycle loading was chosen for this analysis. Three specimens for each welding process were tested. Low cycle fatigue results have revealed that the highest number of cycles x-type specimen has held out when FW.136 welding process was used. Comparing to BW.141 and FW.141 welding processes the durability of x-type specimens was accordingly 1.2 and 2 times lower.

Weathering steel structures are protected by patina layer. The development of protective patina depends on exposure conditions. The specific parameters of this protective patina were characterized during long-term exposure of standard samples exposed at atmospheric test sites and bridge structures together with evaluation of corrosion mass loss. The new method for evaluation of protective ability of patina is verified.

Steel 16Mo3 (P1 according to ASTM) represents still a popular material used in waterwalls, pipelines and other components of steam power plant boilers operating at temperature up to 530 °C. However, the absence of other alloying elements (especially chromium) reduces the long-term structural stability of this steel especially at temperatures higher than 450 °C. The paper describes the results of an analysis of a pipeline elbow that burst after nearly 200 000 hours of operation. The complex analysis of material properties and microstructure revealed the extensive graphitization, although mechanical strength, plasticity as well as notch toughness were affected only negligibly. Particles of graphite were randomly distributed in the microstructure and heavy cavitation damage was detected namely close to the fracture surface. Analysis of operation data then showed that the temperature in this part of the steam pipeline was higher due to the changes in the fuel mix and exceeded the maximum 470 °C recommended for the steel 16Mo3 just in order to avoid the danger of massive graphitization.

The paper deals with the issue of temperature dependence of emissivity in reference paints for surface homogenization during non-contact temperature measurement using infrared camera. Two different paints for high-temperature applications are under investigation. Emissivity is measured with a FLIR E95 infrared camera using procedures according to ČSN EN ISO 18434-1 standard. The obtained results are important especially for practical applications in which it is necessary to quantitatively measure the variable surface temperature on objects for which the homogeneity of the surface cannot be unequivocally guaranteed.

Ferritic-martensitic Eurofer-97 and ODS-Eurofer steels are potential candidates for structural applications in advanced nuclear reactors. Influence of scandium, yttrium, and aluminum addition was studied at 600 °C using 10-6 wt.% oxygen concentration. Microstructure and corrosion behaviour was evaluated after 500 h and 1000 h of exposure to liquid lead. Microscopical investigation identified oxidation, solution based attack, and Pb penetration into the material. Addition of scandium and aluminum changed the mechanism of corrosion attack. Eurofer-97 was partially protected by outer oxide and inner oxidation and Pb penetration was found. In contrast ODS-Eurofer steels after 1000 h of exposure showed mainly inner oxidation and SBA including Pb penetration into the steel. The inner oxidation consisted of Pb+Cr-O and Cr-O+Al-O layers mixture. Degradation of ODS steel was significantly lower than Eurofer-97. It seems that the inner oxidation layer beneath the surface did not prevent the Pb penetration but protected the surface from dissolution.