J. Zurek

Effect of water vapour on growth and adherence of chromia scales formed on Cr in high and low pO2-environments at 1000 and 1050°C

Abstract The oxidation behaviour of pure Cr at 1000 and 1050°C was studied in Ar–O2 and Ar–H2–H2O mixtures. It was found that in the low-pO2 gases the oxide scales exhibited higher growth rates than in the high-pO2 gases. The scales formed in the low-pO2 gases showed substantially better adherence during cooling, than scales formed in the high-pO2 gases. These differences in growth rate and adherence can be correlated with differences in size and location of the in-scale voids formed during the isothermal exposure. Exposures in Ar-O2-H2O mixtures revealed that the differences in scale growth rates as well as in scale void formation and growth are not primarily related to differences in the oxygen partial pressure of the atmosphere but to the presence of water vapour in the test gas. At sufficiently high H2O/O2-ratios, water vapour promotes oxide formation at the scale/metal interface thereby suppressing excessive growth of existing voids, and also as a consequence improved scale adherence. Whether the enhancement of inward scale growth is related to transport of H2O- or H2-molecules or due to OH− ions, cannot be derived with certainty from the present results.

Effect of Alloying Additions in Ferritic 9-12%Cr Steels on the Temperature Dependence of the Steam Oxidation Resistance

The oxidation behaviour of a number of commercially available 9 – 12 % Cr steels as well as model alloys with a systematic variation of selected alloying elements was studied at temperatures between 550 and 650°C. For characterizing the oxidation behaviour, the results of gravimetric studies were combined with data obtained from a number of analysis techniques, such as optical metallography, scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX), Raman spectroscopy (LRS) and sputtered neutrals mass spectrometry (SNMS). The critical Cr-content for obtaining protective oxidation appeared to depend on presence of minor alloying addition and probably alloy microstructure. In the mentioned temperature range some of the studied materials exhibited a decreasing oxidation rate with increasing temperature. Introduction A number of high strength, ferritic 9-12%Cr steels have recently been developed to be used as construction materials in advanced, high efficiency steam power plants [1]. The materials possess substantially higher creep strength than conventional low-alloy steels however, a large variety of long-term tests have illustrated, that the oxidation resistance of the steels in water-vapour rich gases is worse than that in air or oxygen [2, 3]. A further problem in respect to component design using these materials is their anomalous temperature dependence of the oxidation rates. This means, that in the envisaged temperature range of applications, i.e. 550 to 650°C, the maximum oxidation rate is in most cases not found for the highest temperature, however it frequently occurs at 600 or even 550°C. Experimental The compositions of the studied materials are listed in Table 1. Experimental details were given elsewhere [4, 5]. After exposure the oxidation products were characterised by optical metallography, scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX), sputtered neutrals mass spectrometry (SNMS) and Raman spectroscopy (LRS). Materials Science Forum Online: 2004-08-15 ISSN: 1662-9752, Vols. 461-464, pp 791-798 doi:10.4028/www.scientific.net/MSF.461-464.791

Basic creep models for 25Cr20NiNbN austenitic stainless steels

Abstract Basic models for solid solution and precipitation hardening during creep are presented for the austenitic stainless steels 25Cr20NiNbN (TP310HNbN, HR3C, DMV310N). The solid solution hardening is a result of the formation of Cottrell clouds of solutes around the dislocations. In addition to slowing down the creep, the solutes increase the activation energy for creep. The increase in activation energy corresponds to the maximum binding energy between the solutes and the dislocations. The formation of fine niobium nitrides during service enhances the creep strength. It is found that the nitrides have an exponential size distribution. In the modelling the critical event is the time it takes for a dislocation to climb over a particle. The creep models can accurately describe the observed time and temperature dependence of the creep rupture strength.

Why the Growth Rates of Alumina and Chromia Scales on Thin Specimens Differ from those on Thick Specimens

For a number of chromia and alumina forming high temperature alloys and coatings, recent studies revealed, that in some cases the specimen/component or coating thickness may substantially affect the growth rates of the surface oxides. For the alumina formers the thickness dependence is mainly governed by depletion of oxygen active elements such as Y, Zr, Hf, Mg which are either intentionally added alloying elements or manufacturing related alloy impurities. In the case of the chromia forming materials, which tend to exhibit a more substantial dependence of oxidation rate on specimen/component thickness, depletion of minor alloying additions is also an important factor to be considered. However, for these alloys relaxation of oxide growth stresses by plastic deformation of the metallic substrates seems to be the dominant parameter which governs the observed behaviour.

Enhanced Internal Oxidation as Reason for Breakdown of Protective Chromia Scales on FeCr-Alloys in Water Vapour Containing Gases

The oxidation behaviour of binary Fe-Cr alloys containing 10 and 20 mass % Cr, respectively, was studied in Ar-20%O2, Ar-7%H2O and in Ar-4%H2-7%H2O at temperatures between 800 and 1050°C. Thermogravimetric analyses in combination with analytical studies using SEM/EDX and Raman Spectroscopy revealed, that in atmospheres in which water vapor is the source of oxygen, Cr exhibits a higher tendency to become internally oxidized than in the Ar-O2 gas. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the surface oxide scale, the present results reveal that the presence of water vapor also affects the transport processes in the alloy. The enhanced internal oxidation, which is likely the result of water vapor increasing the solubility and/or the diffusivity of oxygen in the alloy, explains the frequently observed effect that Fe(Ni)Cr alloys with intermediate Cr contents (e.g. 10-20%, depending on temperature) exhibit protective oxidation in dry gases but breakaway type oxidation in steam. The temperature dependence of the change from protective to non-protective behaviour in Ar-H2O differs quantitatively, but not qualitatively from that in Ar-O2.

Oxidation and reduction kinetics of iron and iron based alloys used as storage materials in high temperature battery

Abstract The background of the present studies relates to the development of a novel high temperature energy storage system based on a solid oxide cell. The energy is stored in a metal/metal oxide system which is part of the fuel side of the cell. The aim of the present study was to evaluate the suitability of pure iron and iron based model alloys as possible energy storage material for this type of high temperature battery system at a service temperature of 800°C. For this purpose the oxidation and reduction behaviour of iron in Ar–H2–(H2O) environments has been examined. The reduction process in Ar–2%H2 of the wüstite scale formed on pure iron in Ar–H2–H2O was hampered by the formation of a continuous, gas tight metallic iron layer on its surface. Possible approaches to increase oxidation and reduction kinetics by optimised alloy compositions for the Fe storage are discussed.

Effect of Surface Condition on Steam Oxidation of Martensitic Steels and Nickel-Based Alloys

The steam oxidation behavior of inner tube surfaces in the as-manufactured condition for selected martensitic steels and nickel-based alloys was compared with that of standard laboratory test coupons that were ground to a 600 grit surface finish prior to the steam exposure. Exposures up to 10,000 h in the temperature range of 550°C to 700°C revealed that the as-delivered inner tube surfaces may exhibit substantial internal oxidation as a result of the (solution) heat treatment in the final stages of the manufacturing process. The internal oxides contained mainly chromia in the martensitic steels and alumina and/or titania in the nickel-based alloys. For the latter materials, the internal oxidation, especially of Ti, caused the steam-grown external chromia scales to be generally thinner than those observed on the specimens that were ground prior to oxidation testing. For the martensitic steels, the internal oxidation modified the incorporation of Cr (Mn, Si) into the surface scales, thus suppressing the fo...

The Effect of Gas Flow Rate on the Oxide Scale Morphology of a 10%Cr-Ferritic Steels in Ar-H2O and Ar-H2-H2O Mixtures

The oxidation behaviour of a Ferritic 10%Cr steel in Ar-H2O mixtures was investigated at 650°C. The studies aimed at elucidating the effect of water vapour content as well as the gas flow rate on the mechanisms of oxide scale formation. An important observation of the present investigation is, that H2 produced by the reaction of water vapour with the steel, can play a significant role in the oxidation process. It affects the possibility to form an external haematite layer and may alter the oxide scale growth rate. The extend by which the H2 affects the oxidation behaviour depends on the gas flow conditions, the water vapour content and the exposure time. To confirm these observations a number of specimens were oxidized in Ar-H2-H2O mixtures. This atmosphere guarantees a very low equilibrium oxygen partial pressure, in which H2 formed by reaction of the gas with the metal, does not substantially alter the thermodynamic properties of the gas.

Influence of Temperature and Composition of SO2/O2 Atmosphere on the Surface Morphology of Alumina Scales Formed on FeCrAl Steel

The paper deals with the oxidation of an FeCrAl-type steel in SO2 and in SO2/O2 gas mixtures in the temperature range 800 850°C. It has been found that at 850°C, independently of the composition of the oxidizing atmosphere, the α-Al2O3 scale is strongly deformed and poorly adherent to the metallic substrate. At lower temperatures (800-830°C), the scale is adherent and composed of α-Al2O3 and θ-Al2O3, with a characteristic plate-like morphology of the external surface. At 820°C, in the atmospheres containing less than 60% SO2, the plate-like structure tends to disappear and the rate of scale formation increases with the concentration of O2 in the SO2/O2 mixtures. In SO2 and in the SO2/O2 mixtures containing more than 95% SO2, a well-developed plate-like structure is observed and its mass is comparable to the mass of the inner compact layer of α-Al2O3.

Microstructure Evolution and Precipitation Modeling in Ni-Based Alloy C-263

Due to its high creep strength and oxidation resistance, C-263 is a promising Ni-based alloy for applications in superheater tubes in coal fired thermal power plants. The creep strength is mostly based on finely distributed gamma-prime precipitates. In this work, the microstructural evolution of this material during heat treatment and thermal ageing has been investigated. The investigations were carried out by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Selected Area Diffraction Pattern (SADP) and Energy Dispersive Spectroscopy (EDS). Besides, equilibrium and Scheil calculations were carried out using the thermodynamic software MatCalc to analyze the stable phases and the solidification process, respectively. Precipitation calculations during solution annealing and subsequent ageing at 700°C and 750°C up to 10.000h ageing time were performed to predict the phase fraction and precipitates radius. SEM and TEM investigations of aged specimens revealed three different precipitates: M23C6, γ’ and MX. MatCalc also predicted these precipitates. The calculated phase fraction and mean radius show good agreement with experimental data.