Materials Science Forum Vol. 782

Abstract: Steel CB2 developed in frame of the COST actions belongs to the most promising steel for production of cast turbine components working at ultra-supercritical steam conditions. VZÚ Plzeň has participated on assessment of original trial melt CB2 as well as on testing of samples coming from one of pilot valves produced of steel CB2 in full scale. Recently VZÚ Plzeň has cooperated with Doosan Škoda Power in development of weld processes of advanced power energy steels including steel CB2. This contribution deals with a comparison of properties of the steel CB2 produced by four various steelmakers from point of view of their microstructure in relation with their mechanical and creep resistant properties. Evaluation is focused on chemical heterogeneity, porosity, content of non-metallic inclusions, in particular on an occurrence of coarse NB, microstructure and substructure using methods of light and electron microscopy.

Abstract: The austenitic cladding of liquid radioactive waste underground storage tanks was recently required to be under continuous corrosion surveillance with aim to monitor its corrosion stability until all radioactive waste will be processed and safely stored. Tanks are intended primarily to be used for long-term storage of radioactive waste water solutions and nascent sediments. Tanks are components of technology designed for post-processing of radioactive sediments by means of cement matrix fixation. This paper is dealing with an example of metallografical methods focused on corrosion processes monitoring using the special surveillance program. The brief information is presented for the preparation of specific long-run corrosion programme and experimental material samples evaluation. General view of the monitoring system is shown in Fig. 1.

Abstract: The field of high-pressure steel cylinders (HPSC) and vessels used for the variety of applications especially for the CNG (Compressed Natural Gas) in automotive industry is still expanding. With increasing safety requirements, a need of new techniques applied in field of HPSC structural innovations comes into place. Conventional treatment provides generally sufficient parameters of HPSC, but the long-term goal is to achieve as high as possible strength properties vs. as high as possible notch impact strength, which is currently difficult fully accessible, because of the banded structures (segregation banding) created during the process of reversed extrusion and reversed hot rolling.

Abstract: Vespa scooters have been produced since the end of the 50^th, and mechanical parts are produced in license and several reproductions are available. Service centres should know the material properties of the various spare parts for repairing gears. 6 cogwheels and 7 changing gear crosses from various productions were selected (VBA - end of 50^th, GL150 - 1961, PE - end of 70^th, PX200E - 1998, LML 125 - 2008, MEC-EUR - 2010). The analytical investigations showed that 16NiCr4 steel was used mainly for the cogwheels and 18NiCrMo14-6 steel for the changing gear crosses. The samples show negligible differences for elemental composition, impurities and defect distribution. Depending on the various parts the carbon and the hardness gradients from the surface to the core are different. The hardened steel samples showed typical martensitic-bainitic microstructures. The material properties of the different gear parts were compared to find optimal solutions for servicing Vespa gears.

Abstract: The present work investigates texture evolution stages in vacuum-degassed non-oriented electrical steels. The main idea behind the improvement of soft magnetic properties relies on deformation induced grain growth phenomena and heat transport phenomena promoting the preferable formation of columnar grains with so called cube crystallographic orientation {100}<0vw>. In order to achieve the desired orientation with appropriate microstructure state from magnetic properties point of view, we have used an adjusted temper rolling process at elevated temperature and subsequent dynamical annealing in laboratory conditions.

Abstract: The present work deals about nanoindentation measurements between particular grains with various orientation at room and at elevated temperatures ( 100°C, 200°C, 250°C). Nanohardness measurements were carried out in non-oriented silicon steel with columnar microstructure, in order to evaluate local variation of work hardening as function of crystallographic orientation. The dependence of texture on the applied condition was studied by EBSD (Electron Backscatter Diffraction) analysis. Hardness was shown to decrease with increasing temperature in each of individual grains. The differences of hardness values were observed also between particular grain orientations.

Abstract: The stainless steel group AISI 310 is very often used for application in high-temperature conditions. This study investigated the effect of long-term annealing on microstructure and hardness of AISI 310S (24.3% Cr-19.3% Ni 1.8% Mn 0.21% Mo 0.67% Si, wt.%) heat-resistant steel. Microstructural changes and hardness distribution were analyzed after isothermal annealing at 800 °C in long-term exposure (from 1 to 1740 hours). Microstructure and fracture surface mode were analyzed using scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). It was found that after annealing the sigma phase particles were precipitated at grain boundaries and within austenite grains. The chromium content in the sigma phase increased with the prolongation of annealing time. The morphology of sigma phase was changed from fine precipitates at lower annealing times to coarser particles at higher annealing times. Also, presence of sigma phase was confirmed by transmission electron microscopy (TEM). At annealing temperature of 800 °C the hardness began to increase after 16 hours. With longer times of annealing the fracture surfaces were changed. This change is associated with a change of content and the morphology of sigma phase.

Abstract: Dual phase (DP), interstitial free (IF) and advanced high strength low alloy steel (HSLA)sheets have been successfully used for different components of car body. DP and HSLA are used ascrash resistant and IF as cover or “skin” of car body. The development of new vehicles nowadays isbeing driven by the need to simultaneously reduce mass and increase of passenger and pedestriansafety as well as costs saving through cold forming instead of hot forming. Limited publishedinformation is available on the changes in microstructure of these steel grades at different highstrain rates [1-3].This paper deals with changes of mechanical properties, microstructure and fractography of threesteel grades, which were tested at quasi static (10-3 s-1) and high strain rate (3000 s-1). Themicrostructures were characterized in terms of ferrite grain size, aspect ratio of ferrite andelongation of constituent phases. Deformed and undeformed specimens were compared to assess thechanges in the microstructure. The fracture appearance analysis indicates that the fracture patternunder high strain rates is mainly ductile, regardless of steel grades. The microstructure changessignificantly during the deformation process under both quasi-static and dynamic tension in allinvestigated steels. The plastic deformation in ferrite dominates in this process.

Abstract: Susceptibility to cracking of the as-cast C-Mn-Cr-Nb steel was studied by laboratory rolling. The variable parameters were the heating temperature (1150 - 1340 °C) as well as the rolling temperature (950 - 1150 °C). Final microstructure of the free-cooled samples was constituted by bainite, pearlite and ferrite with different morphology and various contribution. Deformation temperature below 1000 °C yielded in the incomplete recrystallization of austenite. Surface cracks originated preferentially on the austenite grains boundaries. Size of the present particles (inclusions and precipitates) varied from 10¹ nm to 10¹ μm. SEM and EDS analysis revealed that the inclusions ware mostly of the MnS type. TEM analysis confirmed that the grain boundaries were not enriched by any particles. In addition to the Fe₃C particles, the discoid niobium carbide particles with approximately 40 nm diameter and 10 nm thickness were detected. These small particles were not connected by any notable pinning of dislocations.

Abstract: Contamination of steel by excessive amount of non metallic inclusions leads tosignificant deterioration of its quality and consequent degradation. Cause of degradation of forgedrotors for steam turbines made of NiCrMoV steel grade is examined in this paper. Metallographicand electron-microscopic analyses were performed, including X-ray microanalysis for detecting thepurity of the forging being examinated. Concurrently, the consequence of failed technologicalproduction process, namely the influence of broken brick for argon blowing at the casting ladle, wasalso examined.