Our research on fatigue performance of stainless steel and transferability of laboratory data to nuclear power plant operational conditions continues. The focus is in quantification of time and temperature dependent damage relaxation during holds introduced within strain controlled LCF fatigue tests with niobium stabilized X6CrNiNb1810mod steel. These holds aim to simulate steady state normal operation between fatigue relevant cycles at start-up, shut-down or power changes in PWR primary circuit components, e.g. the pressurizer spray lines and surge line.

Amplified cyclic hardening was observed at strain rates approaching zero at normal operation temperatures (≤325°C). Even more pronounced static hardening is consistently measured during holds in elevated temperatures (≥200°C). Beneficial effects of holds in material endurance were shown five years ago. The latest results suggest another beneficial change in component fatigue performance. In addition to improved material response, de-localization of strain is demonstrated in this paper. Our target is a thermodynamic prediction model for improved assessment of fatigue with normal operation periods. The model should quantify the life extension due to long periods in normal operation at operational temperatures.

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