Effects of Hot Water and Holds on Fatigue of Stainless Steel

Abstract

Direct strain controlled LCF data for solid specimens is still very rare. In PVP2013-97500 and PVP2014-28465 we reported results for niobium stabilized X6CrNiNb1810mod steel (type 347) fatigued in 325°C and 200°C PWR water according to VGB water chemistry specification. New data in this paper further confirms the conclusions: we are unable to repeat as high Fen factors or short lives as predicted according to NUREG/CR-6909. The slowest strain rate used 4·10−6 in 325°C water would predict Fen > 12, i.e. laboratory specimen data below the current ASME design curve, but our results are superior for this steel generally used in German NPP’s. However, the difference is not necessarily grade specific. Use of 100% relevant fabricated material batch and standard LCF methodology are regarded to play an important role. Notable hardening can be measured, when long duration holds in elevated temperatures are introduced between blocks of cyclic strains at lower temperatures. This is the case for thermal gradient loaded primary circuit components, e.g. the PWR pressurizer spray lines or surge line, which connects the pressurizer to primary coolant line. In PVP2011-57942 we reported improved endurances in fatigue tests aiming to roughly simulate steady state operation between fatigue transients in such NPP components. New test types have been introduced to generalize the results. Mechanisms of time and temperature dependent relaxation of fatigue damage and/or improvement of material fatigue performance during holds are not yet fully revealed, but the rate controlling thermal activation energy is below shown to be near that for vacancy and interstitial atom diffusion. This allows us to draft a thermodynamic prediction model. Improved accuracy of fatigue assessment helps in focusing optimally scheduled nondestructive testing to the most relevant locations and maintaining high level of reliability without excessive cost and radiation doses for inspection personnel. This paper provides previously unpublished experimental results and proposes methods to improve transferability of laboratory test data to fatigue assessment of NPP components. The effects of material, water environment, temperature and service loading patterns are discussed.