摘要:

AbstractTraditional component design procedures consider service lifetimes to be limited by either stress or chemical attack from reactive components in the fluid working environment. However, these processes are often coupled; stress can affect the rate of environmental attack and vice versa. The synergy resulting from this coupling is frequently detrimental with regard to service performance, and creep lifetimes, for example, may be reduced dramatically; the paper reviews the evidence and identifies the underlying mechanisms. Physically based equation sets for coupled creep and environmental attack are also presented and some of their predictions are compared favourably with data published previously.MST/1126

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.1.2

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThe mechanical properties of structural materials can be strongly affected by environmental conditions. In particular, during long term exposure at elevated temperatures, i.e. under typical creep conditions, technical atmospheres must be expected, in general, to cause corrosive attack. Thus, to obtain reliable design data it is the interaction of corrosion and mechanical behaviour which must be considered as the lifetime determining factor. The present work is an overview of the influence of sulphidation and carburisation on the creep behaviour of high temperature alloys with emphasis on heat resistant steels. Sulphur is known to be a particularly aggressive species and severe attack on material due to sulphidation is encountered in gas turbines (nickel base alloys) as well as in coal gasification or fluidised bed combustion plants (heat resistant steels). Sulphidation is found to reduce the creep strength of high temperature alloys and the creep ductility in particular can be drastically reduced as a result of accelerated creep crack propagation. Carburisation, which is observed, for example, in ethylene cracker tubes, seems to be less dangerous than sulphidation. Creep experiments on heat resistant steels have shown that carburisation can degrade, but can also improve the high temperature creep properties, depending on the alloy. However, at low temperatures, complete embrittlement of the material always occurs when severe carburisation has taken place. For multicomponent gas mixtures containing, for example, sulphidising, carburising, and oxidising potentials, the corrosion–creep interaction can be rather complex. A high carbon level may eventually overcompensate any deleterious effect caused by sulphidation.MST/1147

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.1.9

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractOxides and oxidation play a very important role in fatigue crack propagation for many materials, not only at elevated temperatures, but also at ambient temperature. The simplest approach to modelling environmental effects on crack growth is to attempt to add a purely mechanical crack growth increment to an environmental component: the oxidation rate multiplied by the cycle time. Limited success has been achieved with models of this type, both for high temperature fatigue in air and in high pressure water environments. One significant result has been to emphasise that at temperatures up to about half the melting point of the material, it is environmental effects (and thus, frequently, oxidation) rather than creep which give rise to accelerated fatigue crack growth rates. However, the problem with these simple models is that there is no account taken of the interaction between the environmental processes taking place. There are many types of interaction between the mechanical and the chemical processes. Examples of synergies which preclude the separate consideration of mechanical and chemical mechanisms are: (i) adsorbed gas on freshly formed crack faces reducing slip reversibility and rewelding; (ii) high modulus oxide films giving rise to image forces on dislocations; (iii) a build-up of oxide as a result of fretting reducing the crack tip loading through the process of oxide induced closure; and (iv) localised oxidation causing embrittlement and a change in the crack path. The paper emphasises the importance of oxidation in fatigue crack propagation studies in gaseous environments. The effects of oxidation are reviewed briefly, with emphasis on the interaction between the fatigue and oxidation processes, highlighting areas where an improved understanding of the role of the oxide is required.MST/1162

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.1.19

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThere is still some debate on the extent of plasticity of oxide scales at high temperatures. Examination of scales after creep of the substrate often shows that at higher strain rates scale cracks are visible after only very low strains (brittle behaviour), whereas at low strain rates no scale cracks are observed after large strains, at least when simple optical methods are applied. In this paper, the possible mechanisms of scale deformation are discussed with the help of deformation mechanism maps constructed for grain sizes of the order of magnitude encountered in surface oxide scales. As well as‘genuine’plasticity by creep of the scale, mechanisms of‘pseudoplasticity’are discussed. These consist of simultaneous cracking and healing processes in the scale and simulate macroscopic plasticity of the oxide. Examples from experiments are shown for through scale cracking plus rapid healing and microcracking plus healing. In model calculations critical strain rates are assessed below which the scales are macroscopically intact.MST/1160

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.1.32

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractOxide growing on mild and low alloy steel components in the high pressure CO2based coolant of UK gas cooled nuclear power stations can become porous and continue to grow at the interfaces of structural components even when any interfacial gap has filled with oxide. Because such growth gives rise to a net volume expansion, the adjacent components can be forced apart. Any fastening, such as a weld or bolt, holding them together can then become stressed as a result of growth of oxide alone, and may sustain mechanical damage. The phenomenon is now well understood and methods of predicting behaviour have been developed and refined over the years to ensure safe and continued plant operation. The basis of such methods is presented for bolted joints, and the way in which joint geometry, environment, and mechanical properties of component material can affect bolt strain is outlined with examples.MST/1125

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.1.39

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThe effects of a compressive stress on the oxidation rate of mild steel have been investigated through the use of novel equipment designed to measure the extension of specimens during scaling. Preliminary results are described for mild steel oxidised in 2% CO–CO2. The rate of breakaway scaling is found to be lower at constrained interfaces by comparison with the free surfaces of the specimens and the rate of oxidation is shown also to decrease as the interfacial stress increases. The microstructures of the various breakaway scales formed with and without an external load, as examined in cross-section in the transmission electron microscope, are also described and possible reasons to account for the observed reduction in the oxidation rate are discussed.MST/1146

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.1.45

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractAcoustic emission (AE) has been investigated as a technique for identifying the onset and type of scale fracture events which occur under applied stress in bend tests at room temperature. Oxide scales have been thermally grown in air at 823 K on mild steel specimens approximately 100×10×1·5 mm in size. Scale was removed from one face of the specimens by carefully abrading with SiC papers. The oxide scale on the opposite face was then subjected to tension or compression stresses by mounting in a three point bend jig with the oxide on either the underside or topside, respectively. Applied load and AE events were monitored during the tests and strain for failure calculated in each mode. The AE waveforms characteristic of scale fracture in tension (through scale) and compression (decohesion) have been identified enabling the mode of failure to be determined non-destructively without the requirement to section material for metallographic examination.MST /1161

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.1.53

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThe influence of oxidation in carbon dioxide and of heat treatment in an inert gas (argon) for up to 2000 h at 1000°C on the metallurgical structure of a 20Cr–25Ni–Nb (wt-%) stabilised stainless steel, annealed before these exposures at 930°C, has been investigated. The effect of oxidation for varying periods and of the corresponding argon heat treatments on the subsequent creep rupture properties of the steel at 750°C has also been studied. The influence of concurrent strain, from a constant applied load, on the oxidation behaviour of the same type of steel has also been examined during 100 h oxidation, in carbon dioxide at 1000°C.MST/1176

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.1.56

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractThe wastage rates of two austenitic steels and one mild steel have been examined as a function of temperature. At low temperatures, oxide growth is slow and there is little wastage. At high temperatures, the increased plasticity of the oxide contributes to protective behaviour and low wastage rates. At intermediate temperatures, the combined effects of oxidation rate and a brittle oxide, which can easily fracture, lead to a maximum in the wastage versus temperature curve.MST/1128

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.1.69

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor

摘要:

AbstractUnder operational conditions in advanced gas cooled reactors (AGRs), fuel cladding alloys are exposed to an oxidising environment for prolonged periods approaching 850°C. It is essential, from safety considerations, that the mechanical integrity of this thin section material is not threatened by any potential oxidation effects. This aspect is addressed in the present work. Sheet specimens of 20Cr–25Ni niobium stabilised steel and nitrided 20Cr–25Ni–Ti stainless steel, both used as fuel cladding materials in AGRs, have been oxidised in simulated reactor gas [CO2–(1–2% CO)] for periods up to 7·0×106s (1900 h) at 850°C, including intermediate thermal cycles to room temperature. These exposures have been carried out in thermobalances which provided a full gravimetric analysis for each specimen. The oxidised specimens have been creep tested subsequently at 750°C, under conditions of constant stress, together with non-oxidised and thermally aged specimens, which formed the‘baseline’mechanical property data. After testing, the specimens have been examined using optical metallography and transmission electron microscopy to determine the microstructural changes caused by thermal aging and by oxidation. The paper compares and contrasts the response in mechanical behaviour to oxidation effects for both of the fuel cladding alloys. The creep rupture properties are affected differently for the two materials, but it is shown that this can be interpreted in terms of the observed changes in microstructure and the nature of the oxidation attack.MST/1127

ISSN:0267-0836    

DOI:10.1179/mst.1990.6.1.73

出版商:Taylor&Francis    

年代:1990

数据来源: Taylor