Key Engineering Materials Vol. 923

Abstract: This work deals with the influence of the processing technology of ultrafine-grained titanium by the application of severe plastic deformation (SPD) on its degradation processes in creep at elevated temperatures. Commercial grade titanium heats with different content of carbon were processed by multi-stage rolling. Creep tests under constant tensile stress were performed in a protective atmosphere of argon at a temperature of 673 K with application of different stress levels. The microstructure of the fractures creep specimens were analysed by metallographic and fractographic analyses using scanning and transmission electron microscopy (SEM, TEM). Under the same creep loading conditions the heat with higher content of carbon exhibited better creep resistance due to stronger effect of precipitation hardening by titanium carbides.

Abstract: Carbon and low-alloys steel components are used in various stages of steam power plants which are operating at elevated temperatures (up to 530°C). Long-term exposure of low-alloy steel components at elevated temperatures inevitably result in some kind of structural degradation; for example, creep cavitation, carbide coarsening and/or spheroidization, and, less commonly, also graphitization. Graphitization can be detected mainly in carbon steels, but also in 16Mo3 steel where the ferritic matrix is strengthened by Mo in the solid solution. The absence of strong carbide forming elements (typically Cr, V, Nb) is the principal reason why these steels are susceptible to cementite decomposition and graphitization much more than the other creep resistant steels. The paper describes microstructure and material properties of superheater 16Mo3 tube, in which graphite was found after 70,000 hours of exposure at 445 °C. The actual material properties were compared to another tube of the same material delivered in the as-received state. The comparison was based on evaluation and testing of mechanical strength, fracture properties and microstructure. Creep resistance of the steel was studied using small punch creep tests (SPCT).

Abstract: With high operating parameters of the medium in medium-pressure and high-pressure steam pipelines of fossil power plants, creep damage occurs, especially in welded joints leading to complete rupture of the pipe wall in the last phase. Detection of creep damage at an early stage before major cracks may occur can prevent these accidents. For these purposes, phased array ultrasonic testing was performed using a high frequency probe. This testing was performed on real welded joints cut from the power plant. To verify the possibility of detection, metallographic analysis on the tested locations was performed. When comparing the results, it is possible to refine the interpretation of the ultrasonic data and the metallographic results can also be used in the eventual qualification of non-destructive testing.

Abstract: Magnesium (Mg) and its alloys have been intensively investigated as potential biocompatible materials with appropriate structural, mechanical and corrosion properties for preferable orthopedic applications. Selective Laser Melting (SLM) technique was developed for the fabrication of three-dimensional components with high structural integrity at a low cost and is compatible with various materials, including biocompatible magnesium alloys. In this study, WE43 magnesium alloy samples produced by SLM technique were examined by in vitro static test in Hank's balanced salt solution (HBSS) under physiological temperature conditions for six days. A hydrogen evolution method for the estimation of the corrosion rate was used. The phase composition of samples before and after the immersion test was investigated by X-ray diffraction (XRD). The surface morphology and chemical composition of the initial material and tested samples were characterized by scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDX).

Abstract: The primary coolant of High-Temperature Gas-cooled Reactor (HTGR) is expected to contain impurities that can make corrosion to structural metallic materials at elevated temperatures. According to the chemical thermodynamics and kinetics, the carbon activity of helium can be calculated, and it is indicated that a high “CH₄/H₂O” ratio may lead to severe carburizing of the alloys. On this basis, corrosion tests were conducted on the three heat-resistance alloys Inconel 617, Hastelloy X, and Incoloy 800H at 950°C using helium environment with impurities, and mainly the effect of carburization was investigated. The corrosion samples were observed by Scanning Electron Microscopy (SEM) with Energy Disperse Spectroscopy (EDS), Electron Probe Microanalyzer (EPMA), and Carbon-sulfur Analyzer. These all alloys showed the oxidation and carburizing behavior, in which the carburized depth of Hastelloy X was shallow due to the dense oxide scale.

Abstract: The paper deals with analysing the causes of damage to the injection mould insert block to produce plastic parts. Mould insert block material has been declared as alloy tool steel for hot working 1.2344 (X40CrMoV5-1). Part of the analysis is also evaluating an attempt to repair a damaged block using welding methods temporarily. However, there was an intense cracking of the welded joint already during the welding process. Therefore, a non-destructive capillary test was used to analyse surface defects. Complex material analysis of the mould insert block was performed using light and electron microscopy methods, the chemical composition was determined based on optical emission spectrometry.

Abstract: The paper deals with complex material analysis of a shaft after failure. The shaft was operated as part of an "Abradable rig" device used to test the resistance of coatings at high speeds, which simulates a turboprop engine's operating conditions. The shaft is made of 16MnCr5 material with subsequent cementation. The subject of interest is a complex material analysis (i.e. control of the chemical composition of the material and complete fractographic and metallographic analysis) and verification of the shaft design. The aim of the study is a precise determination of the specific cause of component failure.

Abstract: The aim of the work was to find out the causes of cracking of the A120 crane track rail. Although the track was regularly inspected by defectoscopic techniques, after five years of operation, the rail was broken at the weldment. The elemental composition of the welded joint materials was verified and the mechanical properties were determined. A fractographic description of the fracture surface was performed and the structure of the material was evaluated. Several factors contributed to the formation of crack, the combination of which led to a rail failure. Defects of the welded joint, acting as notches, from which they initiated fatigue cracks, unfavourable microstructure of the weld metal, low toughness of the base material and operating environment and conditions were dominant.

Abstract: The paper summarizes results of failure root cause analysis of the driven part of the screw conveyor shaft for transfer of combustion residues (ash and slag) of a power plant steam boiler. The paper presents also the results of material analysis of selected components of this conveyor to be evaluated their material quality.

Abstract: A temperature stress testing machine (TSTM) was used to investigate effect of cement composition and temperature history on thermal stress of concrete. Results show that the benefit of higher amount of C3S on concrete strength enhancement was compromised by the relatively higher temperature rise at early age, leading to a lower temperature difference. However, by means of a delicate design of cooling history, the deficiency of concrete with higher amount of C3S can be compensated and the cracking sensitivity was closer to that of concrete with lower amount of C3S.