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High-temperature oxidation of pure Fe and the ferritic steel 2.25Cr1Mo

The global pressure for recycling and ecological energy production increases steadily in combination with the demand of cost-effective application of materials. However, some severe corrosion problems, associated with the high internal/intergranular corrosion rates in boiler components need to be avoid. Some commercial boiler materials contain a Cr content of 0.55 (wt. (%)) - 2.25 (wt. (%)). This Cr concentration in the alloys is not sufficient for the formation of a complete dense Cr2O3 scale. Hence, high oxidation kinetics may result. In this study, pure Fe and the steel 2.25Cr1Mo were oxidized in laboratory air at 550 °C using a thermobalance system. The surface as well as the cross section of oxidized specimens were analysed using scanning electron microscopy in order to quantify several factors (e.g. surface finishing, cold working and grain size) on the overall oxidation kinetics. For alloys with low Cr content, a decreasing in the grain size leads to an acceleration of the oxidation rate by facilitating the oxygen diffusion along alloy grain boundaries leading to an inward oxide layer formation. The study of effects of surface finish and cold working yielded results revealing that the oxidation process is complex and comparison of results from different laboratories is difficult and should be done.

Effect of alloy grain size on the high-temperature oxidation behavior of the austenitic steel TP 347

Generally, oxide scales formed on high Cr steels are multi-layered and the kinetics are strongly influenced by the alloy grain boundaries. In the present study, the oxidation behaviour of an austenite steel TP347 with different grain sizes was studied to identify the role of grain-boundaries in the oxidation process. Heat treatment in an inert gas atmosphere at 1050 °C was applied to modify the grain size of the steel TP347. The mass gain during subsequent oxidation was measured using a microbalance with a resolution of 10-5 g. The scale morphology was examined using SEM in combination with energy-dispersive X-ray spectroscopy (EDS). Oxidation of TP347 with a grain size of 4 µm at 750 °C in air follows a parabolic rate law. For a larger grain size (65 µm), complex kinetics is observed with a fast initial oxidation followed by several different parabolic oxidation stages. SEM examinations indicated that the scale formed on specimens with smaller grain size was predominantly Cr2O3, with some FeCr2O4 at localized sites. For specimens with larger grain size the main oxide is iron oxide. It can be concluded that protective Cr2O3 formation is promoted by a high density of fast grain-boundary diffusion paths which is the case for fine-grained materials.

High temperature oxidation of mechanically alloyed Mo–Si–B alloys

Abstract Nowadays, even fourth generation nickel base superalloys are approaching their fundamental limitation, the melting point. Hence, a further increase in efficiency, i.e. of jet engines, can only be realised by developing new materials for the use at temperatures beyond 1200°C. A new alloy concept using the Mo–Si–B system for ultrahigh temperature applications is discussed. Those alloys have melting points ∼2000°C, while retaining good mechanical properties and oxidation resistance in the desired temperature range. A three phase Mo–9Si–8B alloy (composition in at.-%) consisting of α-Mo, Mo3Si and Mo5SiB2 (T2) was produced by powder metallurgical processing route. At temperatures higher than 1000°C in laboratory air, a protective SiO2/B2O3 glass layer develops on the alloy surface giving excellent oxidation resistance. However, in the temperature range between 700 and 900°C, non-protective and highly volatile molybdenum oxide cause the disintegration of the material (the so called pesting phenomenon). Additions of Zr and La2O3 to the Mo–Si–B alloy systems were investigated to improve the performance of the alloys in the pesting temperature range. The oxidation kinetics was determined by means of thermogravimetric analysis and discontinuous oxidation experiments. Microstructural examinations were performed by means of optical and scanning electron microscopy in combination with energy dispersive X-ray spectroscopy. The microstructural observations were compared with the theoretical prediction of phase stability using computational thermodynamic calculations. A significant improvement of the alloys during oxidation in the pesting temperature range was found. The rate of formation of molybdenum oxides could be drastically reduced at intermediate temperature range. At high temperatures (>1000°C), a homogeneous and protective SiO2 oxide layer was formed on the alloy surface leading to a slow growing oxide scale.

The Effect of Grain-Boundary Diffusion on the Oxidation of Low-Chromium Steels

Even though the oxidation behavior of steels is generally considered as to be widely understood, a closer look reveals some open questions, e.g. regarding the influence of the substrate grain size on the overall oxidation kinetics. At temperatures below 570°C the main constituent of the oxide scale formed on top of low alloy steels is magnetite. As shown by gold marker experiments it grows outward and inward at the same time, the latter exhibiting a gradual transition to the more stable spinel compound FeCr2O4. As indicated by intergranular-oxidation attack below the superficial scale, inward oxide growth seems to be driven by oxygen transport along the grain boundaries serving as fast diffusion paths. This is supported by thermogravimetric oxidation tests in air on low-Cr steels with varying grain size: The smaller the grains the higher the oxidation rate. Recently, a numerical model for the diffusive transport processes based on the finite-difference approach has been developed, which distinguishes between fast grain-boundary diffusion and bulk diffusion. Qualitatively, it is capable to predict the relationship between substrate grain size and inward oxide growth kinetics. Together with the thermodynamic tool ChemApp and in combination with a data set for the Fe-Cr-O system the mechanism-based simulation of the overall oxidation process of low-Cr steels is possible.

Studying the role of the alloy-grain-boundary character during oxidation of Ni-base alloys by means of the electron back-scattered diffraction technique

Abstract Most polycrystalline steels and nickel-base alloys exhibit a tendency to preferred oxidation attack along the alloy grain boundaries during exposure to corrosive atmospheres at high temperatures. This is due to the much faster intergranular transport of the reacting elements in combination with easy nucleation of precipitates. In general, Ni-base superalloys exhibit a superior oxidation resistance compared to low-alloy steels due to the presence of elements with very high oxygen affinity, e.g., Cr, Al and Ti, which are often responsible for pronounced intergranular oxidation. Oxidation tests on the Ni-base superalloy IN718 were carried out at temperatures between 850°C and 1000°C using thermogravimetry supported by analytical scanning electron microscopy in combination with EBSD (electron back-scattered diffraction). Evaluation of oxidation kinetics have revealed that special grain boundaries with a high fraction of coincident lattice sites (low Σ values) seem to exhibit a higher resistance to intergranular attack as compared to random high-angle grain boundaries. Hence, grain-boundary engineering might be a promising way to improve high-temperature oxidation resistance.

Effect of shot peening on high temperature oxidation behaviour of boiler steel: experimental results and simulation

Abstract A prediction of the service life of boiler steels in aggressive atmospheres requires a full understanding of the degradation mechanisms of the material due to high temperature oxidation. The developed model is a useful tool to simulate such degradation processes under complex conditions and hence, to contribute to new mechanism based life prediction methods. In the present study the effect of shot peening on the oxidation behaviour is studied and given importance to model the behaviour and simulate the effect. Thermogravimetric measurements were carried out by using a microbalance with a resolution of 10–5 g at 750°C. The scale morphology was examined by using scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX) and X-ray diffraction (XRD). The oxidation of the alloys was modelled by a mechanism based computer simulation that has been developed in order to predict the corrosion rate of alloys depending on the chemical composition and microstructure. The computer simulation is based on the numerical Crank–Nicholson solution for the diffusion differential equation in combination with the powerful thermodynamic subroutine ChemApp. The program deals with the grain boundary diffusion and bulk diffusion in a different way and calculates the individual localised equilibrium state using ChemApp with the help of parallel processing units. Considering the effect of the grain size of the substrate, the developed model gives the distribution of alloying elements, oxide phases, the oxidation kinetics and the concentration of penetrating oxygen in a two-dimensional manner. The numerical description of oxide formation is in agreement with experimental observations.

Modeling of Intergranular Oxidation by the Cellular Automata Approach

Within this study, the application area of the cellular automata model for the prediction of internal corrosion during high-temperature applications has been extended to intergranular oxidation. Besides a significant mass transport by diffusion, chemical reactions and phase transformations have to be accounted for in a modeling framework for internal corrosion. In addition, grain boundaries play an important role as they are acting as fast diffusing paths where the transport of matter is by orders of magnitudes higher than inside the grain. Here, a numerical model is presented to describe intergranular oxidation attack. The model is applied to the nickel-based superalloy 80a and the low-Cr steel X60. It is shown that experimental and simulated results are in good agreement.

Computer-based simulation of kinetics of internal corrosion of engineering alloys at high-temperatures

A reasonable prediction of the service life of structures or equipment operating at high-temperatures in aggressive atmospheres requires a full understanding of the degradation mechanisms of the material due to mechanical loading and corrosion. The overall objective of this study is to simulate high-temperature corrosion processes under near-service conditions, which require both, a thermodynamic model to predict phase stabilities for given conditions and a mathematical description of the kinetic process, i.e., solid state diffusion. A computer program was developed in which the thermodynamic program library ChemApp is integrated into a numerical finite-difference diffusion calculation to treat internal oxidation, nitridation and sulfidization processes in various commercial alloys. The model is capable of simulating multi-phase internal corrosion processes controlled by solid-state diffusion into the bulk metal as well as intergranular corrosion occurring in low-alloy steels by fast inward oxygen transport along the grain boundaries of the substrate. In this article, dilute and monophase solutions are considered.

Theoretical and experimental study of carburisation and decarburisation of a meta-stable austenitic steel

Metastable austenitic stainless steels are known to undergo a partial transformation of austenite to martensite as a consequence of plastic deformation. In the case of cyclic loading, a certain level of plastic strain must be exceeded, and phase formation takes place after an incubation period, during which the necessary amount of plastic deformation is accumulated. The susceptibility of the austenitic phase to deformation-induced martensite formation is strongly affected by the temperature of loading and the stability of austenite, which itself depends on the chemical composition. A key element in this regard is carbon which stabilizes the austenitic phase. It is shown in this study that the carbon concentration can be analysed systematically and reproducible by means of annealing treatments, if the parameters of these treatments are carefully defined on the basis of advanced theoretical thermodynamic and kinetic considerations. First results on the effect of carbon concentration and temperature of fatigue testing on the austenite/martensite transformation are presented, in order to illustrate the significance of these parameters on the martensite formation rate.

Strain ageing in welded joints of API5L X65Q seamless pipes

Abstract In this work, we investigated the effects of the phenomenon of strain ageing in joints obtained by gas-shielded arc welding (gas metal arc welding, GMAW), of seamless pipes of API5L X65Q steel. Test specimens obtained from the welded joints were submitted to cold plastic strain of 3% and then aged for 1 h at 250 °C, so as to induce static ageing. For evaluating dynamic ageing, the test specimens were strained to 3% and loading was maintained for 1 h at 250 °C. The aged specimens were submitted to tensile testing and representative samples were examined in the light microscope and the scanning electron microscope. It was observed that the phenomenon altered the volume fraction of secondary constituents in the weld metal and in the HAZ, with increase in the proportion of ferrite–carbide aggregates. Regarding the mechanical properties, it was found that the phenomenon reduced the elastic ratio of the welded joints due to an increase in the ultimate strength, besides increasing the total elongation, but without impairing the tensile properties of the welded joints. On comparing static ageing with dynamic ageing, it was observed that the increase in the capacity for plastic strain of the welded joints was greater after static strain ageing.