Materials Science Forum Vol. 782

Abstract: The mechanisms of metal damage in the pipe of the main pipeline are investigated, the main reasons for their appearance are found and analysed. The main results are obtained by joint use of metallography, indentation, method for processing and analysis of digital images.

Abstract: Alloy elements undergo complex diffusion and segregation processes during steel production. Oxygen diffuses into the steel matrix and forms oxides at the surface (called scale), at the grain boundaries and within the grains. In our work we present assemblies and composition of various oxides found at the grain boundaries and within the grains of a low alloy Mn-Cr hot-rolled steel sheet. We utilise metallographic means (surface etching) and transmission electron microscopy (TEM). After etching we observe four zones of different composition: (i) the scale, followed by a decarburised metallic layer with (ii) oxidised grain boundaries, (iii) non-oxidised grain boundaries and last, (iv) the bulk. Via TEM, we analyse alloy element oxides at grain boundaries and within the grains with high spatial resolution. At the grain boundaries and within the grains we find oxides, such as Mn- or Cr oxides, rather than binary oxides (e.g. Fe-Mn- and Fe-Cr oxides) or even more complex oxides, such as Fe-Mn-Cr oxides. The oxide species found lie next to- or embedded amongst one another.

Abstract: The oxidation behaviour of niobium and titanium enriched, heat resistant cast steel 30Ni-18Cr has been evaluated under cyclic conditions. Cyclic testing was carried out in the air, at temperature of 800, 900 and 1000°C. The alloys underwent oxidation in 5 cycle tests. Characterization of the corrosion products was performed using SEM and X-ray examinations. The cross section and the surface of tested alloys were examined. It has been found that the oxidation resistance of cast steel 30Ni-18Cr in thermal cycling is higher when niobium or especially titanium are added to chemical composition of the alloy. The formation of scale rich in titanium oxides TiO₂ was contributed to better oxidation behaviour.

Abstract: The contribution deals with comparison of microstructure and fatigue properties of synthetic nodular cast irons with a different ratio of steel scrap in a charge. Chemical composition of individual meltages was regulated alternatively by ferrosilicon (FeSi) and carburizer or metallurgical silicon carbide (SiC). The fatigue tests were run at high-frequency sinusoidal cyclic push-pull loading (frequency f 20 kHz, load ratio R = 1, temperature T = 20 ± 5 °C) using the ultrasonic testing equipment KAUP-ZU. The paper shows the influence of charge composition on microstructure, fatigue properties and micromechanisms of failure.

Abstract: In the paper fatigue specimens are extracted from different regions of cast aluminum cylinder heads produced by two foundries. A high strength region and a low strength region were identified within the cylinder head and the A356-T6 material locally characterized in terms of microstructure and defect population. High cycle fatigue testing according to a reduced staircase method was performed to determine the local fatigue strength at 10⁷ cycles in the cylinder heads of the two foundries. The implications of the experimental observations are discussed.

Abstract: Laser surface hardening is an advanced method of surface treatment of structural steels with a great potential for wide industrial applications. The technology is quite new and so, investigations have to be performed in order to gain a comprehensive knowledge about effects on microstructure, hardness, surface properties of treated materials, but also mechanical an particularly fatigue properties. Concerning fatigue resistance of material treated with this technology, results and knowledge recently published in the literature indicate that fatigue resistance can be either reduced or increased, even considerably, depending on numerous parameters of basic material, laser hardening parameters etc. This contribution contains results of a partial study of effect of laser hardening of relatively small specimens on fatigue resistance of 42CrMo4 steel. Two different parameters of the treatment were used, namely two speeds of laser beam on the material surface at constant beam energy. Unlike the lower speed, when fatigue resistance was slightly reduced, higher speed of laser beam resulted in a slight increase of fatigue resistance and fatigue limit. The results are discussed considering an occurrence of residuals stresses. Key words: Laser hardening, residual stress, lifetime, fatigue, fracture, microstructure, surface

Abstract: A pilot 4t valve made of CB2 steel was produced in the frame of COST Action 522. Specimens for long-term creep tests were taken from positions with a wall thickness of 130 mm and 85 mm. Creep testing was carried out at 650°C and a stress range from 50 MPa to 120 MPa. The longest times to rupture of specimens in individual positions were 67,857 h and 82,649 h respectively. Ruptured samples underwent fractographic and microstructural analyses. Quantitative evaluation of substructure was performed using SEM and TEM. Microstructure consisted of tempered martensite with primary boron and niobium nitrides and secondary particles of M₂₃C₆ carbide, Laves phase and vanadium nitride. During creep tests coarsening of Laves phase occurred and new particles nucleated on the site of M₂₃C₆ carbides, NbN or BN nitrides. Significant increase in volume fraction of Laves phase was detected in specimens ruptured up to 20,000 hours in comparison to as received conditions; any further increase was found out after longer creep exposures. Concerning to M₂₃C₆ carbides any important changes were found out in their size and distribution. Density of fine MX nitride was higher after creep tests than in as received conditions with the exception of two longest creep exposures. Several Z-phase particles were also identified after these two exposures.

Abstract: This paper deals with microstructural stability of P23/P91 dissimilar welds during long-term creep exposure at 500°C. Detailed TEM investigations on minor phase evolution were performed on specimens after long-term creep exposure. Microstructural changes in the decarburized layer of the P23 steel were studied using EBSD and TEM. Experimental data on minor phase evolution were compared to the results of thermodynamic (Thermocalc) and kinetic (Dictra) simulations. Evolution of minor phases in the P23/P91 welds during creep at 500°C was in a good agreement with results of microstructural modelling. Partial decarburization of the P23 occurred at the expense of dissolving M₇C₃ phase. Microstructure of the partly decarburized zone in the P23 steel remained bainitic in all specimens investigated. In spite of the partial decarburization in the P23 steel during longterm creep exposure at 500°C, creep failures occurred in the ICHAZ of the P23 steel.

Abstract: Load processes and their interaction over long-term service exposition influence on degradation of construction materials in the equipment of conventional power plants. A wide range of destructive testing methods and number of test for each of them were used to clarify of the extent and mechanism of the damage steel components of the power plants pressure circuit. Changes of material properties were by those methods evaluated in state after operational exposition - till 190 000 hours and after heat treatment reprocessing, for all observed components in the entire profile of the wall thickness. Great attention was paid to metallographic evaluation of the creep damage also. The results of test were compared with the supposed properties of the undegraded material in origin state.

Abstract: High-temperature martensitic steel P91, internationally marked GX12CrMoVNbN91, is the material used in the energy industry. Creep and high-temperature corrosion resistances are important properties that affect the application of this material at higher temperatures. Weldment reduces creep properties. This work deals with the quantification of this decrease in the case of material P91. The main focus is except the evaluation of creep test results given to the mathematical description of the weld creep strength reduction. Further metallographic analyses of weld joint after creep exposures were performed.