Potential implications of step loading in impression creep testing

A Bridges,D Purdy

doi:10.5334/uproc.9

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.

Highlights

IntroductionThe impression creep test has gained interest in the testing community
In recent years, the impression creep test has gained interest in the testing community
Impression creep results in this paper have shown that softening manifests in compressively loaded samples at high temperatures and low strain rates in some alloys, such as martensitic Grade 91

Summary

Introduction

The impression creep test has gained interest in the testing community. The test method can be used as an alternative to standard uniaxial testing to determine steady state creep rates. The test is used when an insufficient amount of material exists for uniaxial creep testing. Traditional uniaxial creep tests use a larger volume of material, about 100-200 times more than impression creep testing, which is considered a ‘destructive’ technique that generally requires weld repair or replacement of the extracted volume of material. Impression creep specimens can be machined from ‘scoop’ samples and is considered a ‘non-invasive’ testing technique. Replications can be performed for microstructure evaluations, but the scoop can be machined into an impression creep specimen.

Methods

Results

Discussion

Conclusion