Potential implications of step loading in impression creep testing

Abstract

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.

Post-test impression creep evaluation methods and findings for improved code of practice

“…A reference stress approach has been used to convert stress relationships from traditional uniaxial testing to testing stresses used in impression creep testing [2]. It is also important to note that this approach is based solely upon sample geometry and has shown to be material independent based on published research of CrMoV, stainless steels, 9Cr steels, 2.25Cr1Mo steels and a variety of filler weld metals [5][6][7][8][9].…”

“…It shows that the percent variations fluctuate more in the Grade 91 tests compared to the Grade 22 test. This question further explored in reference [7].…”

“…It is also evident that the time needed to reach a steady state rate can be dependent on the material (Grade 22 > Grade 91), and that some materials have shown more fluctuations than others (Grade 91 > Grade 22), as shown in Figure 5. The potential material effects on strain rate due to loading and temperature changes are not discussed in this paper, but another in these conference proceedings [7]. The following comments are justification for recommended parameters in guidelines above: 1.…”