Materials at High Temperature Vol 17, Issue 3, 2000
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An energetic approach in thermomechanical fatigue for silicon molybdenum cast iron
E. Charkaluk1 and A. Constantinescu2
1P.S.A. Peugeot-Citroën, Direction de la Recherche et de l’Innovation, Chemin de la Malmaison, 91570 Bievres, France
2L.M.S, Laboratoire de Mécanique des Solides (CNRS UMR 7649), Ecole Polytechnique, 91128 Palaiseau, France
The purpose of this paper is to define a low cycle fatigue criterion in order to predict the failure of engineering structures. The major problem in defining a predictive fatigue criterion is that it should be applicable for structures submitted to complex multiaxial thermo-mechanical loadings but should be identifiable from simple experiments on specimens. After a short critical review of the principal criteria used in low cycle fatigue it will be shown that the dissipated energy per cycle permits a correlation of isothermal and anisothermal results obtained on silicon molybdenum cast iron in the case of specimens and also on structures.
Keywords: thermomechanical fatigue, silicon molybdenum, cast iron
Structural changes in 12–2.25% Cr weldments – an experimental and theoretical approach
Thomas Helander1,3, Henrik C. M. Andersson2,3, and Magnus Oskarsson1,3
1Materials Science and Engineering, KTH, S-100 44 Stockholm, Sweden
2Swedish Institute for Metals Research, S-114 28 Stockholm, Sweden
3Brinell Centre, S-100 44 Stockholm, Sweden
In welds between low alloy and 12% Cr-steels, a depletion of carbon occurs in the material of lower carbon activity, the low alloy steel, during post weld heat treatment. A carbon depleted zone is formed along the fusion boundary. This zone consists of large ferritic grains and affects the creep strength of the material by promoting type IIIa cracking. In this work the absence of carbides in the carbon depleted zone is verified by transmission electron microscopy, and identified as the cause of the grain growth. A computer simulation of the diffusion processes with the DICTRA software shows good agreement with experimental microprobe carbon content measurements. The simulations could also predict the types of carbides appearing in the weld zone. Methods for reducing the carbon depletion by varying the weld and heat treatment parameters are identified.
Keywords: 12–2.25% Cr weldments, structural changes, computer simulation, TEM, creep strength, post weld heat treatment,
diffusion
Corrosion of thermal barrier coatings by vanadium and sulfur compounds
Basil R. Marple1*, Joël Voyer1, Christian Moreau1 and Douglas R. Nagy2
1Industrial Materials Institute, National Research Council of Canada, Quebec, Canada
2Liburdi Engineering Limited, Ontario, Canada
Hot corrosion studies of two plasma-sprayed coatings, yttria-stabilized zirconia and calcium silicate, were undertaken in order to compare the performance of these materials for use as thermal barrier coatings in high-temperature combustion environments. The coatings were tested in contact with vanadium pentoxide at 1,000°C and, also, under conditions in which they were exposed to sulfur-containing compounds at 900°C or 1,000°C. The samples were subsequently characterized by scanning electron microscopy and X-ray diffraction analysis to identify the effects of these tests on the microstructure and composition. The results indicate that reactions with V2O5 lead to a disruptive phase transformation in zirconia that rapidly degrades the coating. For calcium silicate, the reactions with V2O5 appear to be more limited and less disruptive so that the coating is much more slowly degraded by the vanadium compounds. Exposure to SOx and sulfate salts at high temperature caused rapid degradation of the calcium silicate coatings through a reaction involving the formation of CaSO4. Under similar conditions, the yttria-stabilized zirconia coatings experienced much less attack.
Keywords: hot corrosion, thermal barrier coatings, zirconia, calcium silicate
Cyclic oxidation – guidelines for test standardisation, aimed at the assessment of service behaviour†
J. R. Nicholls1 and M. J. Bennett2
1Cranfield University, Cranfield, Bedfordshire MK43 0AL, UK
2Materials Research Consultant, Three Chimneys, South Moreton, Oxfordshire OX11 9AH, UK
This paper reviews current procedures used for cyclic oxidation testing and their ability to simulate service behaviour for life time prediction modelling. Test complexity varies from a simple laboratory, slow cycle, furnace test to the use of high velocity dynamic combustion rigs. It is shown that the response of an alloy under cyclic oxidation testing varies, depending on the exact test conditions chosen and the length of test exposure, but for many alloy systems this response has common features. It is proposed that all alloys that are protected by a stable, slow growing oxide scale conform to a common, generic behaviour under cyclic oxidation conditions. This life cycle of such alloys involves first the formation of a stable, protective oxide. At a critical thickness this may start to spall, increasing the rate of scale forming element consumption. When the activity of such elements falls below a critical level, in the near surface region, internal oxidation results together with the formation of less protective scales. Ultimately, it is no longer possible for the alloy to self repair following a thermal cycle and then breakaway corrosion ensures, marking the end of the alloys life. Thus the choice of cyclic test procedure must be tailored to the particular phase of the alloy life cycle under investigation.
Keywords: cyclic oxidation testing
Solders as high temperature engineering materials
W. J. Plumbridge
Department of Materials Engineering, The Open University, Milton Keynes, UK
Although they operate at temperatures around ambient, it is argued that tin-based solders may be regarded as conventional high temperature engineering alloys, such as steels, titanium and nickel-base alloys, which normally experience much higher temperatures. This proposition is based upon a comparison at similar homologous temperatures, rather than their absolute values. The demand for structural integrity of soldered joints in modern electronic devices is growing. Design challenges, quite similar to those in power generation and aerospace, require reliable life prediction under complex operating conditions. The paper compares monotonic and cyclic properties of solders with those of the more conventional high temperature alloys. Particular attention is focussed on the emerging lead-free solders which are being introduced for environmental reasons. It is concluded that solders are amongst the most vulnerable of high temperature alloys particularly with respect to their strain rate sensitivity and time dependent behaviour.
Keywords: soldiers, high-temperature alloys