THERMAL FRICTION CHARACTERISTICS OF STAINLESS STEEL BEARINGS JAESEON LEE, JIHO KIM, JONGIN KIM, MOONHEE CHANG Korea Atomic Energy Research Institute, P.O.Box 105 Yusong-ku Taejon KOREA; e-mail: leejs@kaeri.re.kr SUMMARY The fatigue life of stainless steel bearings is one of the most critical factors to determine the performance of the driving system. Because the bearings which are installed on the control rod driving mechanism in the nuclear reactor are operated at high temperature and high pressure and especially lubricated with dramatically low viscosity water, friction and wear characteristics of the bearing material should be investigated thoroughly. Some control rod driving mechanisms in the nuclear reactor the support bearings are made of the stainless steel and the sliding bearings ceramic material mainly. This study is focused on the characteristics of support bearings which may be used in the nuclear reactor. The ball bearing can be made of standardized 440C stainless steel, and it supports thrust load including the weight of the driving system and external force. Friction and wear characteristics of this material operating under severe lubrication condition is not well known yet, however it will be changed with respect to temperature and boundary pressure. So the friction characteristics are investigated in sliding conditions using the reciprocating tribometer which can simulate the operating conditions. Highly purified water is used as lubricant, and the water is warmed up and pressurized. Friction force on the reciprocating specimens is monitored by the load cells. The results of the experiments are presented in this paper.

Keywords: Stainless steel, Reciprocating tribometer, Friction coefficient, High temperature, Water lubrication

1 INTRODUCTION Generally stainless steel bearings are resistant to water, water vapour, alkaline solutions, photographic developers, and acids. Especially 440C stainless steel can be used at radiation or vacuum environment because of low emission gas, and at high temperature even up to 400°C with light loading. So this steel can be considered to be used in the new type nuclear reactor as a support bearing material. In the nuclear reactor bearings may be lubricated with high temperature and highly pressurized water, and then viscosity of lubricant goes down to very low level just about 10 times of an air [1]. Lubrication condition becomes severe and the load capacity of the bearing is drastically reduced, therefore friction force would be increased because of very low viscosity. Therefore the frictional characteristics of the stainless steel may be affected by operating temperature, however information of frictional characteristics according to temperature rising is not sufficient to refer for estimating system friction force or torque. Many researches focused on the surface coating effect or the effect of low viscosity of oil or solid lubricant with composite or ceramic materials [2-5]. Some studies considered water-lubricated effects with other materials [6].

In this paper, frictional characteristics of 440C stainless steel at high temperature is studied with a special tribometer. The photograph of the tribometer is shown in Figure 1. The tribometer consists of the water chemistry panel and electrical control panel (A), the autoclave (B) and reciprocating tribometer(C). Temperature can be elevated up to 350°C and pressure applied up to 17MPa and adjustably. It has two types of tribometer, one is for rotational bearings and the other is for evaluation of friction force of materials themselves with reciprocating motion (Figure 2). The test results

with the reciprocating tribometer are introduced and analysed.

Figure 1:Photograph of the tribometer system

2 TEST METHOD For the reciprocating test, pin-on-plate specimens are used (Figure 3) and installation status is shown in Figure 4. The plate size is 32mm length and 16mm width and it reciprocates according to the stationary pin which has 5mm diameter and 51mm spherical radius at the contact point. It traverses 11.75mm at 15 cycles per minute. Hydrodynamic lubrication effect is not considered because of low speed and low viscosity of lubricant. Pure water which is chemically controlled is filled in the autoclave and used as a lubricant.

Figure 2:Photograph of the reciprocating tribometer

Figure 3: The pins and the plates

Figure 4: Specimen installation on the tribometer

Water chemistry is controlled as in Table 1. Applied force on the pin is 980N and this is the same value of ball contact force as 4900N thrust load on the 6010 deep groove ball bearing which has 13 mµ diametral clearances [7.8]. Loading is equivalent to about a quarter of the dynamic load ratings of the 6010 bearing and this level of thrust loading is suitable for the deep groove ball bearing. Of course this test does not represent frictional characteristics of stainless steel ball bearings because test result is for sliding contact and not for rolling contact. However this result may use to estimate friction force on the ball-raceway elliptical contact area[7] and overall friction characteristic of this material.

Three levels of operating temperature on the specimens are considered, 30°C, 100°C and 150°C respectively. Pressure in the autoclave is kept at 14.7MPa. Friction force acting on the surfaces is measured by strain gauge

which can be installed under high temperature and high pressure. Output friction values are compensated with temperature and evaluated.

Because 440C stainless steel is magnetic material, the specimens are demagnetised and cleaned with an ultrasonic cleaner before the test.

Chemical Composition Value Ph Ammonia Dissolved Hydrogen Dissolved Oxygen Conductivity

9.5 ~ 10.6 (25 C0 ) 10 ppm ≤0.5 ppb ≤5 ppb 35microsiemens/cm

Table 1: Water chemical composition

The specimens are tempered at 180°C and surface hardness is about 60.5 HRC before the test. 3 TEST RESULTS

Figure 5: Raw data type

Figure 5 shows the raw data from the strain gauge obtained by the oscilloscope. One large division of the graph means 650N that is measured by calibrating mass before the test. So friction force on the graph represents 520N and the friction coefficient as 0.53. All the friction data are compensated with reference calibration force. The upper curve which is similar to sine wave shows LVDT output therefore this means the position of the plate.

Wear shape after the test is shown in Figure 3. Wear length on the pins and plates is 11.75mm and width is within a range from 2.4 to 3.7mm

Wear loss is measured after test and surface roughness and shape are investigated. Wear shape which is measured by Form Talysurf on the plate is shown in Figure 6. This measurement is for the specimen that is tested at 30°C and shows about 6 mµ wear depth. Because amount of wear loss is too small to compare with each specimens, it is not accounted in this paper.

Figure 6: Wear shape after test

Friction force variations are shown in Figure 7 and 8. Test conditions are 30°C (Figure 7) and 100°C (Figure 8) respectively.

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Figure 7: Friction force (at 14.7MPa, 30°C)

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Figure 8: Friction force (at 14.7MPa, 100 C0 )

The test results at 150°C are not presented here because there is too much wear on the specimens and it is not possible to compare the friction coefficients with other test results. This result may come from decrease of surface hardness. Friction coefficient is stabilized from 0.42 to 0.47 at 30°C and from 0.46 to 0.53 at 100°C. At

150°C operating temperature, friction coefficient decreases to 0.4. 4 CONCLUSIONS Water lubricated friction characteristics of 440C stainless steel is examined. This material may be used as bearing material in the nuclear reactor with careful consideration because it owns temperature limitation under medium loading simulated as axial force on the ball bearing. This steel could not be applied as bearing material over 150°C because of severe wear. Further experimental studies will be performed with another stainless steel and be reported soon. 5 ACKNOWLEDGEMENT This work has been performed under the nuclear research and development program sponsored by the Ministry of Science and Technology, Korea.

6 REFERENCES [1] Daugherty R. L., Franzini J. B. and Finnemore E. J.: Fluid Mechanics with Engineering Applications. 8 ed. McGraw-Hill. 1985 [2] Zhao Xingzhong, Liu Jiajun, Zhu Baoliang and Miao Hezhou;Sliding wear of ceramic/metal pairs under boundary lubrication of water and oil. Journal of Materials Science and Technology (1997). 13 vol. 5 n. 409-415 [3] Vairis A.:Investigation of Friction Behaviour of Various materials under sliding conditions. European Journal of Mechanics A/ Solid (1997). 16 v. 6 n. 929-945 [4] Ovaert T. C., Cheng H. S. and Shen M. C.: Temperature effects on friction and elevated temperature behaviour of base oil-additive combinations under boundary lubricated conditions. SAE Transanctions (1991). 100 v. 1131-1160 [5] Dumont B., Blau P. J. and Crosbie G. M.: Reciprocating friction and wear of two silicon nitride-based ceramics against type 316 stainless steel. Wear (2000). 238 v. 2 n. 93-109 [6] Ko Pak L. and Robertson M. F.: Friction and wear studies of nuclear power plant components in pressurized high temperature water-2. ASME Pressure Vessels and Piping Conference, Aug 1-5. 1999. Boston MA [7] Tedric A. Harris: Rolling Bearing Analysis. 3 ed.John Wiley & Sons. 1991 [8] J. S. Lee and D.C. Han: The Static Equivalent Radial Load under the Moment and Radial Force for the Deep Groove Ball Bearings, Journal of KSTLE, 14 (1998). 3. 94-99