Comprehensive experimental research on fatigue performance of niobium stabilized (type 347) X6CrNiNb1810mod steel has revealed particular features of cyclic performance. This paper reports and discusses on the endurance limit behavior studied by strain controlled HCF tests at room and elevated temperatures. In contrast to carbon steels, stainless steels can tolerate notable amounts of plastic strain and display broad hysteresis loops at and below endurance limit. Together with pronounced secondary hardening, this results to abrupt endurance limit behavior. The modified Miner rule with S-N–curves extrapolated to low amplitudes is generally applicable for carbon steels, but not for stainless. Effectiveness of the endurance limit even with variable amplitude straining was shown for 347 type steel. Extrapolation of ε-N curves beyond a few million cycles is conservative. An increase of temperature decreases, but does not vanish the endurance limit.
Another peculiar feature of the studied stainless steel is its superior ductility, which is not affected by partial fatigue ‘damage’. Five LCF tests at 325°C and 0.22 ≤ εa ≤ 0.5% were interrupted beyond the half-life condition (N/N25 ≥ 0.5) at 0.5 ≤ CUF ≤ 0.9. Cyclic straining was directly followed by monotonic pulling to fracture. Surprisingly low correlation between tensile properties and fatigue usage was observed. The ultimate tensile strength was preserved within 10 MPa (2.5%) in all cases. Yield strength and elongation remained practically unchanged by pre-fatigue.
This paper provides new experimental results. They are discussed together with earlier observations on the same steel extracted from a pipe, which has been manufactured for use in primary loop of a German NPP.