Practical application of impression creep data to power plant

The measured "minimum" creep rate in the impression creep (IC) test is currently obtained from a "stabilized" deflection rate, and the time to "stabilization" is somewhat arbitrarily chosen at durations up to 500 h, regardless of the applied load. Nevertheless, IC tests have been successfully utilized for aged ½CrMoV and P91 steam line components by conducting tests at the same load so that the deflection rates can directly be compared and used for ranking. By this experience, the IC test is an excellent tool for material ranking and lifing of service exposed material. However, further development in the IC data assessment procedures and the translation between IC and uniaxial test is still needed for determination of the lower stress range material creep properties from the IC test. It is suggested that the deflection rate should be correlated to a Monkman-Grant type relation with the calculated deflection rate at specified "strain/deflection" against the corresponding uniaxial strain rate at the same specified strain. The modified MG relationship should correct the increasing error expected at IC test conducted at decreasing loads. To be able to do this the strain rate as a function of time (or strain) should be available for primary creep of the uniaxial test. The corrective impact of the modified MG has to be verified by further analysis and testing.