Michiyoshi Yamamoto

SCC Growth Rates and Reference Curves for Low Carbon Stainless Steels in BWR Environment

Nuclear Plant Operation and Maintenance Code has been developed and is going to be applied for nuclear power system components in Japan. If a crack is detected in a component, the evaluation of crack growth due to stress corrosion cracking (SCC) is required. In recent years, the components in BWR primary systems made of low carbon stainless steel, such as core shroud and PLR piping, have suffered from SCC and it is necessary to prepare the crack growth rate reference curves for the materials. In this paper, the development of the SCC growth rate database for low carbon stainless steel in BWR water and the proposed reference curves in Japan are described.Copyright

Effect of Grain Boundary Character Distribution on Stress Corrosion Cracking Behavior in Austenitic Stainless Steels

The effect of grain boundary character distribution (GBCD) on intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels in high temperature water was verified experimentally. GBCD control using the strain annealing method increased the fraction of low- S coincidence site lattice (CSL) boundaries and the segmentalized network of random grain boundaries in austenitic stainless steels. The fractions of low- S CSL boundaries of GBCD controlled steels were 75–85%, while those of uncontrolled steels were 60–70%. Creviced bent beam tests were conducted at 561 K in pure water containing 8 ppm dissolved oxygen for stress corrosion cracking (SCC) evaluation. The tests revealed that GBCD control suppressed IGSCC initiation or propagation and that cracks were predominantly propagated along random grain boundaries. It is considered that induced lower- S CSL boundaries result in high resistance to IGSCC.

Effects of Water Flow Rate on Fatigue Life of Carbon Steel in Simulated LWR Environment Under Low Strain Rate Conditions

The flow rate of water flowing on a steel surface is considered to be one of the important factors strongly influencing the fatigue life of the steel, because the water flow produces difference in the local environmental conditions. The effect of the water flow rate on the fatigue life of a carbon steel was thus investigated experimentally. Fatigue testing of the carbon steel was performed at 289°C for various dissolved oxygen contents (DO) of less than 0.01 and 0.05, 0.2, and 1 ppm, and at various water flow rates. Three different strain rates of 0.4, 0.01. and 0.001 %/s were used in the fatigue tests. At the strain rate of 0.4 %/s, no significant difference in fatigue life was observed under the various flow rate conditions. On the other hand, at 0.01 %/s, the fatigue life increased with increasing water flow rate under all DO conditions, such that the fatigue life at a 7 m/s flow rate was about three times longer than that at a 0.3 m/s flow rate. This increase in fatigue life is attributed to increases in the crack initiation life and small-crack propagation life. The major mechanism producing these increases is considered to be the flushing effect on locally corrosive environments at the surface of the metal and in the cracks. At the strain rate of 0.001 %/s, the environmental effect seems to be diminished at flow rates higher than 0.1 m/s. This behavior does not seem to be explained by the flushing effect alone. Based on this experimental evidence, it was concluded that the existing fatigue data obtained for carbon steel under stagnant or relatively low flow rate conditions may provide a conservative basis for fatigue life evaluation. This approach seems useful for characterizing fatigue life evaluation by expressing increasing fatigue life in terms of increasing water flow rate.

Study on Growth Direction and Characteristics of SCC in Nickel Base Weld Metal in High Temperature Oxygenated Water

SCC growth direction and SCC characteristics in Ni base weld metal (DNiCrFe-1J) in high temperature oxygenated water have been studied. The following results were obtained. (1) Strong direction dependence was observed in which SCC propagated preferentially along the growth direction of columnar crystals, (2) Conventional average SCC length which can be calculated from the equation in which SCC area is divided by the entire CT specimen thickness was about 1/2–3/4 times the average individual SCC length on individual columnar crystals. This was because SCC does not go through the entire thickness of the CT specimen. On the other hand, maximum SCC length was almost equal to the average individual SCC length produced on individual columnar crystals. Thus, the average individual SCC length and/or maximum SCC length were judged to be appropriate lengths for obtaining the SCC growth rate of Ni base weld metal (DNiCrFe-1J).Copyright

Effects of Water Flow Rate on Fatigue Life of Carbon and Stainless Steels in Simulated LWR Environment

Fatigue tests in simulated LWR environment of carbon and stainless steels were performed under high water flow rates between 7 to 10 m/s. 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 high flow rate condition was measured during the fatigue test. Environmental correction factor, Fen , 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 be considered in an evaluation of environmental fatigue.Copyright

Effects of Water Flow Rate on Fatigue Life of Ferritic and Austenitic Steels in Simulated LWR Environment

The flow rate of water flowing over a steel surface is considered to be one of the most important factors influencing the fatigue life of the steel, because the water flow produces differences in the local environment. The effect of the water flow rate on the fatigue life of carbon, low alloy, and austenitic stainless steels was therefore investigated experimentally. Fatigue testing of low (S = 0.008 wt%) and high (S = 0.016 wt%) sulfur content carbon steels and a low alloy steel was performed at 289°C for various dissolved oxygen concentrations (DO) of less than 0.01 and 0.05, 0.2, and 1 ppm, and at various water flow rates. Three different strain rates of 0.4, 0.01, and 0.001%/s were used in the fatigue tests. For high sulfur carbon steel (S = 0.016 wt%), the effect of a high water flow rate on mitigating fatigue life reduction was more clearly observed at a lower strain rate, irrespective of the DO. This effect of high water flow rate was most notable at a DO of 0.2 ppm, which was the DO level that produced a significant sulfur effect. This indicates that the mechanism responsible for the mitigation of fatigue life reduction is the flushing effect of the water, which eliminates the locally corrosive environment. For high sulfur carbon steel (S = 0.016 wt%), no benefit of a high water flow rate was found at a DO of 0.01 ppm. This was because the environmental effect is insignificant at this low DO level. For low sulfur carbon steel (S = 0.008 wt%) and low alloy steel (S = 0.008 wt%), a high water flow rate had little effect on mitigating fatigue life reduction even at a DO of 0.2 ppm. This indicates that the sulfur is much less influential in low sulfur steel than in high sulfur steel. Fatigue testing of Type 316 nuclear grade stainless steel (316NG) and Type 316 stainless steel (SUS316) was performed at 289°C and 320°C for DO levels of less than 0.01 and 0.05, and 0.2. For austenitic stainless steel, no mitigating effect at a high water flow rate was found. It should be noted rather that there is a possibility that a high water flow rate decreases the fatigue life because a tendency to a slight decrease in fatigue life with an increasing flow rate was observed.Copyright