Experimental research on fatigue performance of niobium stabilized stainless steel (1.4550, X6CrNiNb1810mod) relevant for German NPP primary piping demonstrated good long life performance.

Fatigue tests periodically interrupted for holds indicated time and temperature dependent hardening during holds at 25°C to 325°C. Notable extension of fatigue life was measured when loading patterns consist of cyclic deformation in lower temperatures than hold annealing. Many NPP piping thermal transients separated by normal operation belong to this category and fatigue assessment based on standard fatigue data seems to underestimate fatigue endurance.

Further results for stabilized stainless steel in air at various temperatures will be provided. A parallel paper will deliver unpublished data in PWR water. The influences of temperature and loading pattern will be discussed aiming to improve fatigue assessment of plant components and to reduce confusion concerning applicability of international design codes.

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