Materials at High Temperature Vol 16, Issue 3, 1999
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Validation of a Code of Practice for notched bar creep rupture testing: procedures and interpretation of data for design
B. Al-Abed1, R.Timmins1, G.A.Webster2 and M.S. Loveday3
1Plant Engineering Division, ERA Technology Ltd, 2Department of Mechanical Engineering, Imperial College of Science, Technology and Medicine, London SW7 2BX, and 3Centre for Materials Measurement and Technology, National Physical Laboratory, UK
The results of a European collaborative project aimed at validating a Code of Practice for notched bar creep rupture testing and interpretation of results are presented. Several laboratories from different European Union member states were selected to undertake a comprehensive test programme using circumferentially double notched bars possessing a range of notch acuity ratios. The effects of notch spacing and specimen size were investigated. Inelastic finite element analysis was conducted for establishing the important stress components and stress ratios that are considered instrumental in determining the multiaxial stress rupture criterion (MSRC) for a given material and loading condition. The results of the work have validated the concepts and procedures recommended in the Code of Practice in terms of carrying out the tests and the simplified procedures for the interpretation of results. New finite element analysis has resulted in revised estimates of the multiaxial skeletal point stress components that enter the MSRC. The finite element work is supported by the experimental results in establishing the minimum notch spacing recommended for testing double notched bars. In addition, selected tests revealed the absence of any significant effect of specimen size on notched bar rupture life, thus supporting the use of miniature testpieces even when only a few grains span the minimum section across the notch throat region of the testpiece. The work is supplemented by metallographic examination of failed notched bars which is used to relate damage evolution to the stress distributions predicted by finite element analysis. The results demonstrate the usefulness of the approach recommended in the Code of Practice for establishing the MSRC. This has been borne out by a series of tests undertaken on three engineering materials possessing a wide range of uniaxial creep ductilities, and each displaying a different MSRC. These differences in multiaxial creep rupture behaviour have been attributed to the differences in uniaxial tertiary creep ductility and its variation with stress state.
Keywords: notched bar creep rupture testing
Creep behaviour of parent, weld and HAZ materials of new, service-aged and repaired 1/2Cr1/2Mo1/4V: 2 1/4Cr1Mo pipe welds at 640°C
T.H. Hyde1,W. Sun1 and J.A.Williams2
1School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham, Nottingham NG7 2RD, UK
2Independent Materials Engineering Consultant, East Leake, Leicester, UK
The results of a series of creep tests on the materials of CrMoV weldments, in main steam pipes, are presented in this paper. The tests were performed at 640°C, using uniaxial, notched, impression and cross-weld creep test specimens. The materials involved include service-aged and ‘as-new’ 1/2Cr1/2Mo1/4V parent materials, service-aged and new 2 1/4Cr1Mo weld metals and the HAZ materials in the new, service-aged and fully repaired welds. Based on the test data obtained, material constants in creep continuum damage constitutive equations for the parent, weld and HAZ materials were determined. Direct comparison of the test data allows the creep deformation and rupture behaviour of the parent, weld and HAZ materials to be identified. The material properties of these CrMoV weldment materials can be used in numerical FE modelling to contribute to the understanding of the performance of the new, service-aged and repaired welds in similar materials.
Keywords: creep properties, CrMoV weldments, weld repair
Corrosion behaviour of four high temperature engineering materials exposed to landfill gas engine flue gas
K.A. Lichti1,T.P. Levi1, S.J. Swann1, S.L. Ferguson1 and P.G.H. McIlhone2
1 Materials Performance Technologies Limited, PO Box 31-310, Lower Hutt, New Zealand
2Aotea College, PO Box 50-046, Porirua, New Zealand
Four engineering materials commonly used in high-temperature applications were exposed to landfill gas engine flue gas for 25 days at 411°C. The flue gas was composed mostly of nitrogen and oxygen, although some carbon monoxide and hydrocarbons were detected together with low concentrations of hydrogen chloride gas and sulphur oxides. Hydrogen fluoride was not analysed, but, due to the nature of the fuel, may have been present at low levels. The materials tested were a carbon steel, a low alloy steel, a 12%Cr stainless steel and an austenitic stainless steel. Thin, protective films were formed on the 12%Cr and austenitic stainless steels, however, the corrosion products formed on carbon and low-alloy steels consisted of several layers, the outermost of which were extremely friable and non-adherent, especially on cooling from the flue gas working temperature. The maximum corrosion rate obtained was 70 _m yr–1 for carbon steel, which may be acceptable for a number of flue gas applications. The exposed coupons were examined using conventional techniques such as X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM). The predominant scale formation mechanism controlling the corrosion appeared to be simple oxidation type reactions, however, the influence of HCl, as evidenced by the presence of akaganeite (_-FeO(OH)), was hypothesised. A model describing the corrosion mechanism is proposed in order to provide an improved life prediction capability for such flue gas environments.
Keywords: high-temperature corrosion, landfill flue gas, engineering materials
Oxidation-limited component lifetime in turbines: adhesion of coatings and oxide scales
S. Osgerby, S.R.J. Saunders and J.P. Banks
Centre for Materials Measurement and Technology, National Physical Laboratory, Teddington, Middlesex. TW11 0LW, UK
Three methods have been used to measure the adhesion of oxide scales and coatings on materials used in turbines. All three methods were suitable for oxide scales grown on 9- and 12-Cr steels, whereas for coated materials the methods were not universally applicable. The data generated by each method were not directly applicable to current predictive models for oxide scale failure. However, a route has been proposed that would allow future workers to correlate data obtained by testing with the input required by the modelling approaches. This could be achieved by an extensive assessment of finite element modelling coupled with critical experiments.
Keywords: oxide scale, coatings, turbines
Proposal of a new concept on creep fracture remnant life for a precracked specimen
A.Toshimitsu Yokobori Jr1 and Martin Prager2
1Fracture Research Institute, Graduate School of Engineering, Tohoku University, Aoba, Aramaki, Aobaku, Sendai City #980-8579, Japan
2MPC, United Engineering Center, 345 East Street, 14th Floor New York 10017, USA
The creep life time of a smooth specimen can be predicted using existing laws for creep deformation and steady state creep rate. When crack growth behaviour is involved, it is necessary to construct a law of creep crack growth rate to predict creep fracture life. Creep fracture life can be measured by integrating the law of creep crack growth rate. One example is the creep crack growth rate, represented by the parameter Q*. In this study, we investigated the applicability of this prediction method to creep fracture remnant life for a cracked specimen. The _ criterion is proposed to predict creep fracture remnant life for a smooth specimen for creep ductile materials. In this study, the correlation between Q*L derived from the paremeters Q* and _ is investigated. The correlation between QL* and _ provided a unified theoretical prediction law of creep fracture remnant life for high-temperature creep-ductile materials in the range from smooth to precracked specimens.
Keywords: creep crack growth rate, creep fracture remnant life, creep fracture life, creep ductile materials