Effect of temperature on cavitation erosion of 9Cr steel in liquid metal

1. Effect of temperature on cavitation erosion of 9Cr steel in liquid metal University of Fukui, Japan 　　　　　○ Akihiro Nimura ShujiHattori Hiroki Yada

2. Research background • Research on cavitation erosion in liquid metal is very important to confirm the safety of the “Monju”. • But, research on cavitation erosion on liquid metal has been hardly done compared with research in water. Fast breeder reactor “Monju”. • Cavitation erosion rate at a temperature of 260℃ in sodium was 9 times higher than that in water. • We are afraid that cavitation erosion rate increases at 500℃. • Cavitation erosionin a fast breeder reactor environment has hardly been studied.

3. Previous research • We carried out the cavitation erosion tests in three kinds of lead bismuth ( PbBi ) alloy. • Effect of test temperature is larger than that of metal composition on cavitation erosion. • In the previous research, we clarified that erosion rates in PbBi at relative temperature of 14° is 10 - 12 times higher than that in deionized water. ※ Hattori et al. Wear 267 (2009) 2033-2038

4. Problems Objectives Present test region • Test temperature range is very limited for 75 to 150℃ (1.4 - 16° of relative temperature ranges) • Erosion rate at high temperatures remains uncertain. • Cavitation erosion tests were carried out at various temperatures. • Effect of the test temperature on the cavitation erosion rates is clarified. • A method for predicting the erosion rate in sodium is proposed using the test results of deionized water and PbBi alloy obtained in this study.

5. Chemical composition and physical properties of test materials • Test specimen is 9Cr steel which is proposed to be used for pipes in the next-generation fast breeder reactor and stainless steel SUS304 which is used for pipes in the present fast breeder reactor “Monju”. • Liquid metal is a low melting-point PbBi alloy which consists of the elements Bi, Pb, Sn, and Cd. Melting temperature is 68℃. PbBi 100 mm

6. Test apparatus and test conditions Piezo-electric oscillation apparatus (according to ASTM G32) Test temperature available : 50 - 400℃ Vibrational frequency : 20kHz Peak-to-peak displacement amplitude : 40μm Covering gas : Ar ( for the tests at 250 to 400℃) Amplifying horn • Cavitation erosion tests were carried out with the vibratory specimen method specified in ASTM G32. • Cavitation erosion was evaluated in terms of mass loss and instantaneous MDER (Mean Depth of Erosion Rate) of the test specimen. Piezo converter Specimen Mantleheater Test method Fig. Whole test apparatus Thermocouple Cooling coil Fig. Cooling coil

7. Performance of test apparatus Melting 5.6 • Test temperature increased and deceased in a short time. • Test temperature was controlled with cooling air with a tolerance ±3℃. 7.9 3.5 8.2℃/min Variation in temperature for heating Temperature decrease for cooling Variation in temperature during test

8. Mass loss curves in liquid metal SUS304 9Cr steel 488 267 • For 9Cr steel, incubation period were 1 hour at 100℃, 0.3 hour at 250℃ and 300℃,and 0.2 hour at 350℃ and 400℃. • Mass loss rate in the maximum rate stage were about 3 times higher at 100℃, 12 times at 250℃ and 300℃, and23 times at 350℃ and 400℃compare withthat in deionized water. • Mass loss rates increased with the temperature. • Incubation period of SUS304 is similar to that of 9Cr steel. 515 265 553 490 266 250 69 50 22 mg/h Incubation period

9. Eroded specimen surfaces In water In PbBi 9Cr steel 9Cr steel • In deionized water, the test surface has an uneroded ring region. • In PbBi, the test surface is eroded over the whole surface. • The difference in surface profile is due to the difference in the mobility of vapor bubbles. 5mm In deionized water of 25℃ In PbBi of 400℃ μm 15.6mm Specimen surface of before test Uneroded region In deionized water at 400℃ 9Cr steel after 5 hours In PbBi at 400℃ 9Cr steel after 2 hours

10. 6.6％ 6.1％ 4.3％ 3.3％ Mean depth of erosion rate and temperature ( ) • MDERmax of 9Cr steel and SUS304 changes similarly. • In deionized water, each increase of 1℃ increases the erosion rate by 1 to 2 % at near 25℃. • In PbBi, each increases 1°in relative temperature increased the erosion rate in PbBi by 3 - 4 % at 10 - 40°and by 6 - 7 % at 40 - 50°. • The increasing ratio in PbBi was almost 3 times higher. 1 – 2 %

11. Prediction method of erosion rate in sodium : Basic idea • MDERmax can be evaluated in various liquids and liquid metals by using the this parameter. • MDERmax can be expressed with a power law as a function of . The lower equation can be obtained. • We predicted the erosion rate by using this equation. ※ Hattori et al.Wear 265 (2008) 1649-1654 ρ: Density, C: Sound velocity, S: Solid, L: Liquid

12. Prediction method of erosion rate in sodium : Procedure Select a temperature to obtain the MDERmax in deionized water and in PbBi. k and n are obtained. MDERmax in sodium is obtained using the value of sodium. Prediction curve of MDERmax is obtained as a function of relative temperature. Young’s test results in sodium (green points) agree with the prediction curve. ② ④ ③ ① • Prediction erosion rate curve in sodium is locatedhalfway from the rates between PbBi and deionized water.

13. Conclusions • Performance test of newly developed apparatus showed that test temperature increased and deceased in a short time and test temperature was controlled with a tolerance of ± 3℃. • Each increase of 1°relative temperature increased the erosion rate in PbBi by 3 - 7 % and the increasing ratio in PbBi is almost 3 times higher than that in deionized water • Erosion rate in sodium was estimated to be located halfway from the rates between lead bismuth and deionized water. Acknowledgement The present study includes the FY2010 result of the “Core R&D program for commercialization of the fast breeder reactor by utilizing Monju” entrusted to the University of Fukui by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).