Abstract
Impression creep testing is an established small-scale testing technique in which the indentation rate of a rectangular indenter can be converted into equivalent creep strain rate. It is a versatile technique in that, once a stable indentation rate is established, either the stress, temperature, or both, can be varied to provide data under multiple test conditions on the same specimen. It does not however produce a specimen failure. In order to overcome this limitation, use can be made of an empirical relationship between the creep strain rate obtained in the impression test and the rupture life obtained in a conventional uniaxial creep test at the same stress and temperature. This relationship, termed the Impression Monkman Grant relationship, has been applied successfully to grade 91 steel where it has been shown that rupture life predicted from impression testing is in good agreement with actual rupture life obtained by conventional uniaxial testing. The relationship has proved particularly useful for plant application in situations where mis-heat treated grade 91 pipework with lower than expected creep strength has been encountered, requiring an estimate of creep strength to justify continued operation in service.
Highlights
Impression creep testing was originally developed at Nottingham University in the UK primarily to obtain creep data from different microstructural regions within weldments to be used in finite element modelling
The widespread presence of this form of mis-heat treated material on plant necessitated that an equivalent approach be developed. This led to the concept of an Impression Monkman Grant relationship, in which the measured impression creep strain rate could be correlated directly with uniaxial rupture life obtained at the same test conditions
The use of an Impression Monkman Grant relationship is a practical means of converting impression creep strain data into estimated uniaxial rupture life
Summary
Impression creep testing was originally developed at Nottingham University in the UK primarily to obtain creep data from different microstructural regions within weldments to be used in finite element modelling. The technique is supported by a substantial body of theoretical work and full details about specimen preparation and the testing technique are available [1] In recent years it has become a useful method of evaluating the creep strength of grade 91 materials in high temperature steam pipework and headers, where routine inspection/on-site metallography has indicated the possible presence of materials with inadequate properties [2][3]. For all three materials the rupture life predicted from impression creep data in combination with the Parker Monkman Grant relationship and the uniaxial rupture lives measured were found to be in good agreement [6]
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