New approaches to determine negligible creep

The working group WG59 "Creep" of the within European Standardization committee CEN/TC54, has drafted an extensive, over 120-page report, regarding new approaches to determine negligible creep. The purpose of this report is an in-depth explanation of the test and assessment methods used for determining the no- and negligible creep temperatures of steels. The work was prepared in the frame of the standard EN 13445-3 for unfired pressure vessels. Presently the standard assumes "rule-of-thumb" values for no-creep temperatures of ferritic and austenitic steels, 375°C and 425°C correspondingly. However, it is well known that the lower ferritic grades could be creeping below the limit temperature and creep strength enhanced ferritic steels may show significant creep at temperatures well beyond it. A similar situation exists for the austenitic steels; nickel and other alloys that are likely to have an even wider range of negligible and no-creep temperatures. This paper also aims to shortly describe the data assessments and test methods used to determine the no- and negligible creep temperatures for specific materials grades in EN material standards. The industrial need for negligible and no creep temperatures has also been reviewed, as well as the negligible creep definitions in current nuclear design codes and related public domain documents and the corresponding reference stresses. The background to this work is also addressed. Then the new approaches to determine the no- and negligible creep temperatures of steels are introduced, promoting test programmes of iso-stress tests in the low temperature range and applying the Wilshire Equations on historical data. The final aim of this work is to minimize the inherent risk and over-conservatism in the current EN13445-3 design procedures for creep-weak grades and creep-resistant grades, respectively.