Fatigue With Hold Times Simulating NPP Normal Operation Results for Stainless Steel Grades 304L and 347

Abstract

The cyclic behavior and endurance of austenitic stainless steels tested under NPP-relevant laboratory conditions has been studied. It had been earlier shown that long intervals between fatigue transients can affect the fatigue performance in stainless steels that are generally used in NPP primary piping. If this can be confirmed, then the transferability between laboratory results, design curves and the fatigue behavior of NPP components during plant operation shall be addressed. In addition to coolant water environmental effects, the material response during steady state normal operation should also be accounted for. Advanced Fatigue Methodologies (AdFaM), a joint project of European research laboratories, vendors and plant operators was focused on empirical and mechanistic investigations to confirm the claimed effects of hold times on fatigue life. Strain-controlled fatigue tests incorporating accelerated hold times at temperatures between 290°C and 420°C were performed on stabilized and non-stabilized stainless steel grades, which are used in Germany and the UK. Two material batches of both alloy types (304L and 347) were studied. The mechanisms responsible for the observed variations in stress response and fatigue life have been investigated using a range of microscopy techniques. The results confirmed the extension of fatigue life due to hold times in both stabilized and non-stabilized grades. This life extension appears to be linked to hold hardening observed in the cyclic behavior of both alloys. Tests incorporating hold times may be more representative of material behavior in NPPs, where temperature transients due to start-ups, shutdowns and major power changes may be separated by long periods of steady state operation. This gives reason to consider the transferability of standard laboratory test data to fatigue assessments of NPP components, and to propose a new factor ( Fhold ) as part of an advanced fatigue methodology and realistic transferability factor: Freal = Fen × Fhold.

Characterization of Austenitic Stainless Steels with Regard to Environmentally Assisted Fatigue in Simulated Light Water Reactor Conditions

“…For tests in air, holds were added at 6000 cycle intervals starting from the 6000th cycle for strain range 0.3% and 1000 cycle intervals starting from the 1000th cycle for 0.6%. Preliminary analysis including Phase II tests ( [40,42], confirmed in Section 3.1.3) suggested that there was no observable effect of hold times for the tested conditions, although beneficial effects of hold time on fatigue life have been reported by the AdFaM (Advanced Fatigue Methodologies) project [13].…”

“…It also became apparent that applying hold times during some cycles in this study did not have a major effect on fatigue life. However, significant effects of hold times on fatigue life were reported in a different study [13]. To investigate whether differences in the application of holds led to these differences, a limited sub-programme on hold time tests was started (Section 3.3).…”

“…Therefore, hold times were removed from Phase III testing, and in parallel, a subprogramme on hold time effects was initiated where the test conditions were more closely aligned to those used in the AdFaM study [13].…”

“…This indicates that the guidance provided by CR-6909 includes significant conservatism, which could potentially be reduced without loss of operational plant safety. Accordingly, EAF has received much attention in the last few years [5][6][7][8][9][10][11][12][13][14][15].…”

“…Similarly, a number of studies investigated the influence of the waveform and especially mean strain [8] and hold time periods on environmental fatigue [6,13]. While mean strain did not have a major effect on fatigue life, it turned out that at least under certain conditions, introducing hold times at some cycles during the fatigue life can extend the fatigue life of austenitic steels in the PWR environment.…”