Fatigue of Niobium stabilized austenitic stainless steel (X6CrNiNb1810 mod) is studied using specimens extracted from a solution annealed and quenched primary piping material sample. This paper reports and discusses new results complementing the data published in PVP2009 and 2011, where good long life performance was demonstrated and relevance of the new design curve in ASME III was questioned.

Effects of temperature and operational loading sequences are in focus here. A typical case with steady state operation between fatigue cycles was roughly simulated in periodically interrupted tests. An extension of fatigue life due to hardening during the holds in elevated temperature was demonstrated and here we show that the hardening effect is generic. It occurs also in isothermal conditions, where straining and holds are both at 325 °C, irrespective of the phase within a cycle, where the hold is introduced, and also for other austenitic steel grades.

Fatigue assessment based on standard fatigue data seems to underestimate fatigue performance of materials subjected to typical thermal transients in nuclear piping during operation.

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