PVP-Vol. 480, Pressure Vessel and Piping Codes and Standards --2 0 0 4

Abstract

ABSTRACT Fatigue tests in simulated LWR environment of carbon and stainless steels were performed under high water flow rates between 7 to 10 rrds. For carbon steel, high flow rate of water clearly mitigated the environmental effect on a fatigue life at the high sulfur concentration of 0.016 wt% which caused high environmental effect on a fatigue life. On the contrary, high flow rate of water slightly enhanced the environmental effect at the low sulfur concentration at or less than 0.008 wt% which caused very low environmental effect. These results suggested that the environmental fatigue life under various flow rate conditions should be determined by the combination between the mitigating effect caused by flushing of locally severe environment and the enhancing effect caused by increase in corrosion potential. To understand those effects, effects of sulfur concentration on fatigue life for various DO condition were formulated. And corrosion potential under low and h i g h flow rate condition was measured during the fatigue test. Environmental correction factor, F~, which is the ratio of fatigue lives derived from the fatigue life at room temperature in air divided by that in water to be used for the fatigue life prediction at high flow rate condition was assumed based on the MITI guideline equation and considering the hypothetical fatigue life under sulfur free condition and high corrosion potential condition. This assumption was agreed very well with the test data. For stainless steel, flow rate had little effect on a fatigue life of type 316 stainless steel. It suggested that there was no role of water flushing. For type 304 stainless steel, fatigue life has a tendency to decrease with increase in water flow rate. Fatigue lives of type 304 stainless steel under high flow rate of 7 to 10 m/s were shorter than those predicted by MITI guideline equation. This effect should he considered in an evaluation of environmental fatigue. NOMENCLATURE Nleak : The definition N25 : of fatigue life for a hollow cylindrical specimen used in this study: the number of cycles at which the deepest crack penetrates the wall thickness of a hollow cylindrical specimen (cycles) Fatigue life obtained with a solid cylindrical specimen. Number of cycles at which the tensile load decreased by 25% from the value at a cycle of Nz42 (cycles) Environmental correction factor (Ratio of fatigue lives derived from the fatigue life at room temperature in air divided by that in ware0 Ratio of fatigue lives derived from the fatigue life at a water flow rate of 7 m/s divided by that in stagnant water Dissolved oxygen content (ppm) 109 Copyright © 2004 by ASME Reynolds number INTRODUCTION For ferritic and austenitic steels used for primary structures of Light Water Reactor (LWR) plants, the corrosion fatigue behavior must be considered because these steels are exposed to high-temperature oxygenated water, which is known to reduce the fatigue life of these steels [l, 2, 7]. Several equations for evaluating the fatigue life of these steels have been proposed Although there have been many studies on fatigue life in high-temperature-pure-water environments, the effect of water flow rate on fatigue life has not been adequately discussed. Hirano et al. The effects of water flow rate on fatigue life were strongly dependent on DO level in water. Hirano et al. In the present paper, the effects of water flow rate on fatigue life in view of corrosion potential are also discussed. Results are also reported for fatigue testing of austenitic stainless steel performed in a simulated LWR environment under several water flow rat