Materials at High Temperature Vol 21, Issue 3, 2004
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Considerations on the measurement of creep strain in Bridgman notches
Malcolm S. Loveday
Beta Technology Consultant, Division of Engineering and Process Control, National Physical Laboratory, Teddington TW11 0LW, UK
Blunt circumferential (Bridgman) notched tensile creep testpieces are used for determining material property data under a multi-axial stress state at high temperatures. The correlation between axial and diametral strain measurements is considered, together with the practical matters associated with the use of diametral extensometers used for notch creep strain measurement. This paper contains underpinning information relevant to a new Code of Practice for Notch Bar Creep Testing [1].
Keywords: Bridgman notches, diametral strain, creep, multi-axial stress states.
Characterisation of the creep-fatigue behaviour of a 1CrMoV turbine steel
E. Mazzaa, S. R. Holdsworthb and R.P. Skeltonc
aDepartment of Mechanical Engineering, ETH Zurich, 8092, Switzerland,
E-mail: mazza@imes.mavt.ethz.ch
bAlstom Power, Rugby, UK
cDepartment of Mechanical Engineering, Imperial College, London SW7 2AZ, UK
This paper concerns the creep-fatigue lifetime assessment of a 1CrMoV steel. A procedure based on the calculation of fatigue and creep damage fractions and their summation according to a non-linear interaction rule is applied for the endurance evaluation of isothermal fatigue and service-like thermomechanical fatigue tests performed with plain (uniaxial) and notched testpieces. In order to avoid the uncertainties associated with heat-to-heat variability, testpieces were manufactured from a single heat, for which the fatigue and creep deformation/endurance properties were specifically characterised. With the availability of a reliable constitutive model, finite element calculations could be used to determine the local stress-strain response at the position of crack initiation in each test. The predictive capabilities of the proposed approach for lifetime calculation as well as the suitability of uniaxial fatigue and creep data for the damage assessment of notched components are evaluated.
Keywords: creep-fatigue behaviour, 1CrMoV steel
Characterization of the breakaway al content in alumina-forming alloys
B. A. Pint, L. R. Walker and I. G. Wright
Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6156, USA
Residual Al contents were measured in Fe- and Ni-base alloys in order to improve lifetime prediction models based on the consumption of the Al reservoir. Measurements were made using electron probe microanalysis (EPMA) on both foil (50–125 _m thickness) and plate (0.5–2mm) material. For FeCrAl, NiCrAl and Fe3Al plate specimens, significant Al concentration gradients were measured. An Al gradient is not considered in current reservoir type models. Residual Al contents at the onset of breakaway oxidation were somewhat lower than previously reported for several FeCrAl alloys and significantly higher (»10at.%) for Fe3Al and Fe–Al alloys. The implications of these results for performance and lifetime predictions are discussed.
Keywords: residual Al contents, alumina-forming alloys
Calibration and traceability of notch creep strain measurements
Malcolm S. Lovedaya and Gordon Rodgerb
aBeta Technology Consultant, NPL, bDivision of Engineering and Process Control, National Physical Laboratory, Teddington TW11 0LW, UK
Blunt circumferential (Bridgman) notched tensile creep testpieces are used for determining material property data under a multi-axial stress state at high temperatures. Aspects of calibration and traceability for diametral strain measurement are considered, with particular reference to creep strain measurement on Bridgman notched testpieces. Details are given of a new laser interferometer calibrator rig used to provide traceability to the National Measurement system for diametral and axial extensometers used for notch creep strain measurement.
Keywords: calibration, diametral extensometer, creep strain measurement, Bridgman notches, triaxial stress state , standards, laser interferometer
Cyclic crack growth properties of serviceexposed ferritic steels for use in thermal fatigue assessments
R. P. Skelton
Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK
This paper is concerned with the laboratory isothermal (low cycle) fatigue crack growth properties (generally at 550°C) of eleven ferritic steel samples taken from power station components which have seen between 63,000 h and 175,000 h service exposure to high temperatures. Full chemical analyses and abbreviated service histories of the materials are presented. Both the short crack (LCF) and deep crack (LEFM) regimes are examined. In some cases it was possible to continue the growth of actual service cracks under laboratory conditions, by suitable machining of samples into specimens; in other cases it was necessary to start the crack artificially. It is intended that the results can be used in direct application to remanent life assessments. In many tests the creep-fatigue interaction was simulated by the addition of a 0.5 h tensile dwell, it being assumed that major damage was already present in the extracted samples. The short crack growth tests at 550°C with dwell have demonstrated that even where significant prior grain boundary damage is present, the resulting growth rates generally do not violate the upper bound relation at present used in the R5 assessment procedures. The deep crack growth rate tests at 550°C with dwell on the other hand have shown a sensitivity to microstructure such that heavily cavitated large-grained material can give growth rates in excess of the appropriate upper bound relation.
Keywords: unexposed data, cold/hot starts, crack opening range, equivalent stress intensity range, grain boundary cracking, ligament cracking, short/deep cracks, single-edge-notch specimens, total strain range
Potential problems in the identification of the root cause of superheater tube failures in 9Cr martensitic alloys
Fred Starra*, J. Castleb and R. Walkerb
aEuropean Technology Development Ltd, Ashtead Surrey KT21 2HR, UK
bSchool of Engineering University of Surrey, Guildford GU2 7XH, UK
The metallurgy and levels of protective elements in the new P91 and P92 martensitic alloys are quite different to older ferrite pearlite and ferrite bainite steels. This is likely to give problems in the positive identification of failure mechanisms and assessment of the life of superheater tubing. Techniques such as spheroidisation and oxide thickness to assess metal temperatures, or hardness testing and cavitation to estimate the level of creep damage cannot be used with these materials, or are likely to give ambiguous results. These issues are discussed in the paper and were explored in depth in part of a project at the University of Surrey to develop “If-Then” rules for an expert system for power plant failure analysis. The main aim of the proposed expert system is to identify the root causes of such failures in superheater tubing and will encapsulate current knowledge about superheater problems in the form of “If- Then” rules. Root causes of creep failures include furnace design and operation, overestimation of alloy creep properties, inadequate heat treatment and a non-optimum content of strengthening elements. A characteristic of these alloys is that oxidation on the steam side of the tubing can induce premature failures due to the insulating effect of the oxide scales raising tube temperatures. In addition, scale spallation could also increase tube temperatures, as spallation debris may collect in the bottom of tubes, blocking steam flow. Attention is drawn to a potential “runaway affect” in which the tube temperature and rate of oxidation increase with time as the oxide builds up. The root cause of this could either be excessive rates of heat transfer or could be due to inadequate oxidation resistance caused by low levels of protective elements. Methods of identifying which of these is the root cause are discussed.
Keywords: expert systems, superheaters, 9 Cr martensitic steels, creep, oxidation, life prediction