The process of standardisation of small specimen creep testing techniques, specifically the impression creep test requires the repeatability of the test method. In this study it is accomplished through a round robin programme involving four different labs which have slightly different test set-ups adhering to predefined recommendations stated in previous work. The labs all conducted the same stepped stress test on a reference heat of grade 91 power plant steel and the displacement traces of the tests are analysed to outline the effects of different test set-ups and their efficacies. Main differences are in temperature control and loading application and control.

The small sample impression creep test method has recently been of interest, as it can give a good indication of expected creep rates in uniaxial creep testing with minimal use of material. The compressively loaded test has also been shown to provide accurate results under multi-step loading conditions for a low alloy steel (½Cr½Mo¼V) to further extract value from a single test specimen. The Electric Power Research Institute (EPRI) has conducted step tests (step temperature and step loading changes) on another low alloy steel (Grade 22), as well as a tempered martensitic 9 Cr steel (Grade 91). Results have shown that there may be potential problematic areas when conducting step-up and step-down steps in these materials. Additional posttest evaluations have shown that material effects, such as strain hardening and strain softening, may add additional complexities when comparing strain rates of multi-stepped loaded stain rates. Hardness testing on posttest impression creep specimens have confirmed strain softening of tempered martensitic Grade 91 and no observed effect for an ex-service Grade 22 alloy. These findings have shown that careful considerations must be made before using creep rates obtained from multi-stepped loaded tests in situ of single loaded tests.

No abstract

No abstract

The impression creep test method has been used extensively to determine the in-service creep properties of power plant components from a small volume of material. In the standardisation of the impression creep test, the determination of a maximum 'allowable' indentation depth is an important issue. This paper presents the results of finite element analyses obtained using a simplified method based on steady-state creep, for evaluating the variation of conversion factors with indentation depth. The results obtained show that the indentation depth has a significant influence on the conversion parameters and the steady-state creep deformation rate when the indentation depth is sufficiently large. On this basis, recommendation on practical application of these conversion relationships was made to the users, and empirical formulas were provided for the possible corrections of the conversion factors. The effect of plastic zone due to initial loading on the subsequent creep was found to be insignificant.