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ABSTRACT Due to the increasing importance of cyclic oxidation as a long term failure mechanism for high temperature components in the energy conversion and transport fields, it is vita1 that reliable predictive models are developed. Unfortunately current test methods are, in general, neither good simulations of what occurs in industry, nor do they provide a fundamental basis for the development of predictive models. A programme is suggested which addresses these issues, through better experimental procedures, and the use of sophisticated finite element analysis and complimentary stress analysis techniques. INTRODUCTION AND BACKGROUND Until quite recently, cyclic oxidation testing has been used as a rough guide for initial screening purposes in the development and assessment of alloys for specific environments. Furthermore, in general, it has been considered that the complexity of the spallation process was too great to predict the life of components subject to frequent thermal cycling. Neither was it felt there was much need for sophisticated modelling ; there were more significant life limiting factors than that of cyclic oxidation. This is not to say that a deeper knowledge of spallation has not been of required in some areas of high temperature technology. In the UK, the work of Evans et al, has been orientated towards spallation of oxides after long term exposure in the Advanced Gas Cooled Reactor. Much of the thinking behind the present paper has been stimulated by these efforts [ Of late, Evans has been part of a UK group, led by the National Physical Laboratory (NPL) and supported by the UK Department of Trade and Industry, and EPRI, which is carrying out fundamental and theoretical work into the adherence of oxide scales. The origin of this programme goes back many years and spans many nations. Hancock and Nicholls at Cranfield, in the UK, were among the first to investigate what was then a somewhat esoteric concern. More recently, workers in Italy, Germany and Holland, have developed specific pieces of equipment to monitor, in-situ, the processes which lead to spallation. In the USA, much of the work, by NASA, Pratt and Whitney, and the South West Research Institute has been directly orientated to developing models of cyclic oxidation. As a result of the recent growth of interest in both long term and cyclic spallation, the NPL organized a Workshop on the Mechanical Properties of Oxide Scales, in 1994 at Teddington, England, to update progress in Europe and North America.[21 Besides nuclear power, long term spallation is of importance in more conventional power generation. In coal and oil fired plant, spallation of oxide from the steam side surfaces of superheater tubing causes significant outages. The debris can collect in the bottom loops of pendant superheaters, reducing steam flows in affected pipework, subsequently inducing stress rupture by local overheating. Oxide debris from spalled areas of steam systems can also lead to erosion- corrosion of the first few rows of steam turbinesL3?. The repairs which are needed, can cost up to OSMS a time. The scale spallation experienced in both fossil and nuclear power plants is, essentially, a result of long term oxidation. In terms of genuine cyclic oxidation, it has been the aeroengine sector which has had most to gain from progress in this field, whether from improved materials, or a better understanding of the mechanisms. As long ago as the First World War, it was recognized that good oxide retention was needed for piston engine exhaust valve materials?. However,
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ABSTRACT ODS alloys are candidate materials for the critical section in the high temperature heater for indirect fired coal conversion systems, although little has been published on their corrosion characteristics in this situation. Salient features of indirect fired systems are described. Under a European Union COST 501 programme, a natural gas fned ODS alloy heater has been built and operated for a short period as part of a Closed Cycle Gas Turbine Demonstrator. Although the planned environment was clean, a number of critical oxidation related problems emerged during development work associated with the programme. These included scale spallation, breakaway corrosion due to aluminum exhaustion, and nitridation. With respect to the material, oxidation led to porosity induced growth, due to the Kirkendall effect, and low temperature embrittlement. The effect of these can be controlled by establishing criteria for the levels of oxide growth and aluminium exhaustion during service, Tentative limits are suggested. Providing that direct attack from coal ashes and slags is eliminated by good furnace design, as seems possible, current alloys should permit the construction of an indirect coal fired system with an efficiency of around 50%. BACKGROUND Among a variety of materials being considered for the main heat exchanger in indirect fired coal conversion systems, ODS (Oxide Dispersion Strengthened) ferritic alloys are among the prime candidates. The interest in this class of materials springs from the high stability of an yttria dispersion at temperature, which implies that unlike more conventional materials, which rely on precipitation hardening, the long term mechanical properties at temperature should be very good. The alloys are alumina farmers, giving excellent oxidation resistance, at least under clean conditions, and the presence of yttrium, coming from the yttria, ensures reasonable spallation resistance. The composition of the alloys is of the type Fe 15-20 Cr 3-5 Al 0.3-0.8Yt203,Ti, MO, and the materials are all produced by mechanical alloying, The yttria dispersion, actually a set of complex alumina-yttria phases, is about 15-20 nm in size, which is quite invisible by normal techniques. Although essentially stable, the dispersoid will coarsen and change in character during high temperature exposure. Hence the dispersoids are able to release yttrium into the ODS alloy matrix. This affects oxide scale properties, both beneficially and adversely. A very practical reason for the interest in these alloys is that an ODS ferritic pressurized heater, using natural gas firing, has been built and operated in the UK, under the aegis of the European Union COST 501 programme. The design thermal input to the heater was about I MW and it was constructed in the UK, using ODM 751 tubing supplied by Documental of Belgium. This material, in the early nineties, was the strongest of the ODS alloys, in the tube form, and was developed with the support of COST funding. The Liege 1994 Conference mentioned in Reference I contains a set of papers which cover much of the development work associated with the COST programme. Reference 2 is a more recent report to the COST 501, Work Package 4 committee, by the author?s former colleague, Dr Q.Mabb
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ABSTRACT An experimental apparatus has been constructed to simulate rapid thermal-cycling conditions in coal- gasitication environments, as experienced by metal components of a ceramic gas-filtration system. A comparative study of the corrosion resistances of several commercial iron-nickel-chromium-base alloys in a equilibrated argon- 14. lYoHz-11. 1VOH20-O. 1%?OHZS-O.08%HCl gas at 600C has been undertaken. Te.sts were carried out under isothermal, conventional thermal-cycling and very rapid thermal-cycling conditions (1 min cooling, giving temperature decreases of 160° to 280C, repeated every 6 rnin). Degradation of the alloys involved the development of oxide scales in the early stages, followed by breakdown and growth of sulphide-rich scales. Thermal cycling involving conventional cool-to-room-temperature cycles generally resulted in increases in the rates of degradation of the alloys, probably by facilitating failure of the transient scales and enhancing the rates of transport across the steady-state scales. However, rapid thermal cycling often resulted in reduced metal loss and thinner scales than isothermal exposure, possibly due to debonding in the scale or at the scale/alloy interface, thereby reducing the rate of cation transport across the scale. INTRODUCTION A prerequisite for efficient operation of pressurized fluidized-bed coal combusters and gasitier combined-cycle systems is the effective removal of particulate and chemical contaminants from the gas stream. There are numerous gas-filtration systems that have been considered with a wide range of construction materials for the filter, such as silicon carbide, fireclay, aluminosilicate fibers and alurnina/mullite. British Coal has opted for a silicon carbide-based porous-ceramic filter elemcntz. This consists of 130 ceramic filter elcmcnta3. During operatinq the porous filters become progressively blocked with elutriated particlea. Thus, a pulse-jet cleaning system has been developed that the dust-cake is, in effect blown off the candle4. The system has been designed so that pulse-cleaning of blocked filters occurs while filtration continum in the unblocked elements. However, as the pulse-cleaning system is largely metallic, it is susceptible to corrosion by the gasifier environment, not only under isothermal conditions, typically 600C, but also under the thermal-cycling conditions imparted by the cleaning process. There have been many studies of the degradation of high-temperature alloys in simulated coal-gasification environments during the past 20 years, leading to extensive literature on the subject and a good understanding of the factors that influence the effectiveness of oxide scales in giving protection in environments of low oxygen and high sulphur potential. Thus, three regimes have been identified to describe the behaviour of chromia-forming alloys5-7. In regime 1, where the activity of oxygen is below the ?threshold? value for development of a CrzOJ scale, defining the so-called ?kinetic? boundary, and the activity of sulphur is below that for sulphidation of the base metal, such alloys may develop chromium sulphide scales and internal precipitates In regime 2, where the activity of oxygen is above the ?threshold? value, a protective oxide scale develops, even if the activity of sulphur is above that for sulphidation of the base metal. However, in regime 3, the activity of oxygen is below the ?threshold? value and the activity of sulphur is above that for sulphidation of the base metal; this results in the development of more rapidly growing scales, rich in sulphides of the base metal. More recently, the emphasis of studies of degradation in simulated coal-gasification environments has shifted to lower g
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