276 Int. J. Nuclear Energy Science and Technology, Vol. 10, No. 3, 2016 Design and development of high-temperature tribometer for material testing in liquid sodium environment Hemant Kumar* Department of Metallurgy & Materials Engineering, Indian Institute of Technology, Kharagpur, West Bengal 721302, India Email: [email protected] *Corresponding author S. Vijayaraghavan, Shaju K. Albert and A.K. Bhaduri Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu 603102, India Email: [email protected] Email: [email protected] Email: [email protected] K.K. Ray Department of Metallurgy & Materials Engineering, Indian Institute of Technology, Kharagpur, West Bengal 721302, India Email: [email protected] Abstract: It is well known that high friction coefficients due to stripping of oxide films, wear and galling are common to many mating surfaces of the components of sodium cooled Fast Breeder Reactors (FBRs). Therefore, understanding tribological behaviour of various materials in liquid sodium is an important input for design of reactor components. In order to generate this information, a first-of-the-kind high-temperature Pin-on-Disc Tribometer (PODT) has been designed and fabricated for carrying out in-sodium sliding friction and wear studies in conditions simulating reactor operating conditions. The design of the tribometer incorporated consideration for convenience of operation in liquid sodium environment. The PODT system has been designed for the maximum temperature of 873 K and contact stress of 55 MPa; these parameters cover the temperature and stress levels prevailing in the operation of Indian FBRs. The appropriateness of the designed tribometer has been demonstrated with some typical experimental results. Keywords: fast breeder reactor; pin-on-disc tribometer; friction; wear; galling; high temperature; liquid sodium. Copyright © 2016 Inderscience Enterprises Ltd. Design and development of high-temperature tribometer Reference to this paper should be made as follows: Kumar, H., Vijayaraghavan, S., Albert, S.K., Bhaduri, A.K. and Ray, K.K. (2016) ‘Design and development of high-temperature tribometer for material testing in liquid sodium environment’, Int. J. Nuclear Energy Science and Technology, Vol. 10, No. 3, pp.276–285. Biographical notes: Hemant Kumar holds BTech degree in Metallurgy and Materials Engineering from University of Ranchi and also undergone one year orientation course in Nuclear Sciences and Engineering at BARC Training School, Bhabha Atomic Research Centre, Mumbai. Presently he is with Materials Technology Division, Metallurgy & Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam. Since 2003, he has been working in the area of hard-facing of various nuclear components, wear & friction studies in flowing sodium at high temperature and so on. He was awarded INS Young Engineer Award for the year 2011, IEI Young Engineer Award for the year 2012, DAE Group Achievement Award for the year 2008, 2009, 2011 and 2013 from DAE Govt. of India. S. Vijayaraghavan is a Mechanical Engineer from The Institution of Engineers, Kolkata, India, and works as a Scientific Officer in IGCAR, DAE at Kalpakkam, India. He has undergone specialised training in tool engineering for five years before joining IGCAR in 1995. He is involved in the manufacture, erection, testing and commissioning of experimental facilities for dynamic high-temperature in-sodium testing of materials for study of phenomena such as creep, fatigue, thermal striping, friction and wear. He has worked in the various areas of sodium technology including transfer, operation, modification, surveillance and maintenance of sodium systems. Shaju K. Albert holds a BE from IISc Bangalore and PhD from IIT Bombay. He is a Metallurgical Engineer and specialises in the field of welding and hardfacing. He joined the Indira Gandhi Centre for Atomic Research, Kalpakkam, in 1985 and is presently head in its Materials Technology Division. He was the recipient of the fellowship of the Japan Society for Promotion of Science (JSPS) in 2002 and the Gold Medal of the Indian Nuclear Society for the year 2005. A.K. Bhaduri is a FNAE, FIIM and FIIW and holds a BTech and doctorate from IIT at Kharagpur. He is the Director of Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam. He is a Metallurgical Engineer and specialises in the field of welding and hard-facing. He was awarded the Research Fellowship of the Alexander von Humboldt Foundation of Germany in 1993, National Metallurgists’ Day Award in 2003, Homi Bhabha Science and Technology Award in 2002, Indian Nuclear Society Gold Medal in 2002, and many best paper awards of the Indian Institute of Welding. K.K. Ray is a FIIM, holds a BE (Metallurgical Engineering) from B.E. College Shibpur (currently IIEST) and doctorate (in Metallurgical Engineering) from IIT Bombay. He is associated with teaching and research for more than 45 years, was a Professor for over three decades with IIT Kharagpur till recently and is currently an adjunct Professor with the same IIT. His broad field of research area is Mechanical Metallurgy with emphasis in Deformation, Creep, Fatigue, Fracture, Tribology, NDT and Structure–Property relations in metallic, ceramic and composite materials with current h-index of 24. He is the recipient of Binani Gold Medal in 1985, National Metallurgists’ Day Award in 1995, and Distinguished Educator award of IIM in 2011 in addition to a large number of best paper awards from the Indian Institute of Metals. 277 278 1 H. Kumar et al. Introduction Use of liquid sodium as coolant in Fast Breeder Reactors (FBRs) results in tribological issues that are very specific to FBRs (Wild and Mack, 1980; Yoshida et al., 1980). Being a highly reactive element, liquid sodium removes the oxide film from most metal surfaces, making these amenable to extensive adhesive wear, high friction and selfwelding (Yukota and Shimoyashiki, 1988). Austenitic stainless steel, the major structural material for FBRs, is prone to these issues in liquid sodium environment because surfaces of these components are in contact with each other at elevated temperatures and stresses and are often having relative movements between them. One of the possible solutions to overcome these degradations is to provide a hard coating on the contact surfaces between the mating parts. Several coatings on austenitic stainless steel have been examined in the past to achieve improved wear resistance for structural materials employed in FBRs; these include materials like hard chromium, Stellite, Colmonoy, and Triboloy (Wild and Mack, 1980; Yoshida et al., 1980). Assessment of these coating materials in liquid sodium environment under different operating conditions has remained a difficult task owing to non-availability of a suitable tribometer. However, several friction and wear studies in sodium have been carried out (Johnson et al., 1976; Kanoh at al., 1976; Kumar et al., 2007; Kumar et al., 2010) using tribometers having limited features that simulate actual service conditions such as elevated temperature, sodium exposure, type of contact, rubbing speed and load. Further, in a single system, it is difficult to simulate all types of movements such as sliding, reciprocating, rotational and oscillatory. One of the main objectives of this work is to design a suitable tribometer that can be installed in a sodium test facility, which would enable understanding the friction and wear behaviour of different material combinations and evaluation of the effect of various parameters such as sodium temperature, contact load and nature of sliding contacts. The design and development of high-temperature Pin-on-Disc Tribometer (PODT) that would enable tribological testing in liquid sodium environment following the common pin-ondisc principle is the inherent highlight of this report. 2 Development of tribometer for material testing in liquid sodium The aim during the design stage was to develop a tribometer that can be used up to a temperature of 873 K and contact stress of 55 MPa. During design, various factors such as convenience of operation, ease of removal of the tribometer from liquid sodium, sodium cleaning, quick dismantling, mounting of specimens, insertion of the system into liquid sodium and the necessary accuracy of measurements were considered. While many conventional tribometers are available for testing materials in vacuum or in lubricant medium, the principles of these systems cannot be used for in-sodium testing owing to issues such as sodium fire and contamination of liquid sodium from lubricant. Necessary considerations have been given towards avoiding lubricant contamination, achieving sodium compatibility and leak tightness to avoid sodium fire. The tribometer consists of two major parts, viz. a rotating disc and a static pin holder. The disc holder is connected to a central rotating spindle whose rotation is given through a gear system and Design and development of high-temperature tribometer 279 is housed above the flange in the upper part of the tribometer. The central spindle is designed as two pieces that are integrated with a coupler to minimise vibration and consequent issues. Analysis of the rotating and loading mechanisms was carried out to ensure safety related issues with respect to liquid sodium aerosols, liquid sodium draining and leak tightness of sealings (Vijayaraghavan et al., 2010). This included FEM analysis of the rotating shaft under high load at elevated temperature in order to avoid buckling of the spindle during operation. Based on the above studies, the design of the tribometer was finalised. A schematic cross-sectional view of the set-up along with the assembly of the pins and its holder is shown in Figure 1. The PODT system has four main parts, viz. the spindle-specimen assembly, loading assembly, drive mechanism and control-measuring systems. In the design, it was a challenge to keep all these assemblies away from any contact with liquid sodium and its vapours. This necessitated a cover gas space above the free level of liquid sodium, leading to an increase in the length of the central rotating spindle to 1600 mm. The system is designed to operate in liquid sodium, with the capability to provide relative motion between the pins and disc, and the ability to apply normal load to the specimens fully immersed in liquid sodium flowing at 300 litres/min. Elevated temperature sealing for inert cover gas system over the free liquid sodium level is provided with further facilities to measure the applied load, contact stresses, wear loss and sliding speed. The relative motion as stated earlier is by means of a disc and pin configuration, where the disc rotates while it is being loaded axially against three pins fixed on a stationary housing. The long main rotating spindle is made of two annular parts fitted collinearly and erected vertically. At the bottom of this spindle, the specimen holder is fixed for mounting the wear disc that is exposed to liquid sodium. A short part of about 470 mm length is coupled at the top for supporting the inside bearing of the long spindle, to enable easy rotation and connected to a pulley for rotary belt drive. To achieve leak tight sealing for preventing escape of argon cover gas containing sodium aerosol a few seals were tested. An indigenous VITON-type elastomeric seal, with double-lip configuration, has been developed and fabricated. This has been introduced at two locations above the test vessel on the spindle assembly and between the spindle and the housing. A digital pressure gauge is used to monitor argon cover gas pressure in the chamber between the two seals. Sufficient heat sinks have been provided both on the tribometer and in the test vessel to maintain the sealing integrity. The complete filling and draining of sodium in the main hollow spindle casing to the required levels is ensured by providing slots at different elevations. The selection of the materials for different components of the system was another problem domain. Various materials were considered for fabrication of the different parts depending on their elevated temperature mechanical properties as well as their compatibility with liquid sodium. After a few trials, 316LN stainless steel and Nimonic 80A were chosen for the fabrication of different parts. The design drawings were made in three-dimensional computer-aided diagnosis (3D CAD); the fabrication and the assembly drawings were suitably prepared to ensure smooth integration during assembly and dismantling. During trial testing at high speeds, a marginal run-out beyond acceptable limits was observed. This problem was overcome by suitably providing guide bushes with 200 µm clearance to ensure smooth rotation of spindle without large deflection. All sensors required for measuring the normal load, frictional force and wear were mounted above the base plate to prevent contact with high-temperature liquid sodium. The 280 H. Kumar et al. development of this tribometer has been carried out in several stages: (i) fabrication of all individual mechanical components with required level of tolerances, (ii) assembly of all these components, (iii) calibration of the different parts of the tribometer, (iv) checking leak tightness between seals by measuring pressure drop after fabrication and (v) assembly and installation of the tribometer in a liquid sodium test loop. Figure 1 Schematic (a) cross-sectional view of the pin-on-disc tribometer for testing in hightemperature flowing sodium, and (b) configuration of the pins on their holder (a) Design and development of high-temperature tribometer Figure 1 281 Schematic (a) cross-sectional view of the pin-on-disc tribometer for testing in hightemperature flowing sodium, and (b) configuration of the pins on their holder (cont) (b) 3 Performance testing Before commencing in-sodium testing, performance testing to validate the design and fabrication of the PODT was carried out in a separate leak-tight stainless steel vessel filled with argon. The heating in-sodium environment was simulated by means of a barrel-type furnace, with a 3 kW coil heater housed inside a thermally insulated chamber. This furnace was suspended inside the test vessel enveloping the spindle mounted with specimens. The PODT mechanisms were tested at elevated temperatures, in air and argon environments and qualified for service in liquid sodium. The performance tests were carried out at 823 K in argon using disc and pin specimens of 316L austenitic stainless steel at 300 N load using sliding speed of 16 mm/s for a total sliding distance of 200 m. During the tests, friction force and normal load were monitored using separate load cells and the data were converted online into friction coefficient. The results of friction coefficient from the two tests, carried out in Ar atmosphere, are plotted against sliding distance as shown in Figure 2. The average friction coefficients from these tests are found to be 0.545 ± 0.065 and 0.560 ± 0.120. The mean and standard deviation associated with these average values from the above tests are 0.552 and 0.011 respectively. The obtained results are in accordance with the reported values for 316L and its equivalent austenitic stainless steels (Hsu et al., 1980; Parthasarathi et al., 2013; Radcliffe, 1980; Smith, 1984). Next, two consecutive tests were carried out on the same material using identical load (at 300 N load) in liquid sodium environment at 823 K. The results of friction coefficient versus sliding distance obtained from these tests are depicted in Figure 3. These results indicate average friction coefficients of 0.374 ± 0.081 and 0.385 ± 0.060. The mean and standard deviation associated with the above-mentioned average values from the individual tests are calculated as 0.380 and 0.008, respectively. The results from Ar and sodium tests clearly indicate that the coefficient of variation (100 standard deviation/mean) is less than 2% in Ar and 3% in sodium, which extend support to the reliability of the developed system in both the environments. 282 H. Kumar et al. The guide bush of specimen assembly, disc and pin specimens after performance tests are shown in Figure 4. The worn surfaces of the pin specimens were analysed by Scanning Electron Microscope (SEM). Figure 5 shows the micrograph of the worn surfaces of the pin specimens after testing in Ar and sodium environments. Figure 2 Coefficient of friction plot for 316L stainless steel at 823 K in argon at 300 N load 1.0 Test-1 Test-2 Both at 823 K in Ar-environment 0.9 Friction Coefficient (µ) 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 25 50 75 100 125 150 175 200 Sliding distance (m) Coefficient of friction plot for 316L stainless steel at 823 K in liquid sodium at 300 N load 0.8 Test-1 Test-2 Both at 823 K in liquid sodium 0.7 0.6 Friction Coefficient (µ) Figure 3 0.5 0.4 0.3 0.2 0.1 0.0 0 25 50 75 100 125 Sliding distance (m) 150 175 200 Design and development of high-temperature tribometer 283 Figure 4 Disc and pin specimens along with guide bush after performance test Figure 5 SEM images of worn surfaces of pin specimens after testing in (a) Ar and (b) sodium environments, respectively (a) (b) 284 4 H. Kumar et al. Conclusion A high-temperature PODT with design features suitable for testing in flowing liquid sodium environment has been successfully designed. This tribometer has been fabricated, installed and commissioned in an elevated temperature liquid sodium test loop. The performance of PODT has been established by testing 316L stainless steel at 300 N load at 823 K in argon gas and liquid sodium environment. The mean friction coefficient for above test conditions in Ar environment is 0.552 and that in liquid sodium environment is 0.380. These obtained values are in good agreement with some earlier reported values (Hsu et al., 1980; Parthasarathi et al., 2013; Smith, 1984). Acknowledgement The authors acknowledge the support received from Materials Technology Division and Fast Reactor Technology Group of Indira Gandhi Centre for Atomic Research, Kalpakkam, India, for carrying out the studies. References Hsu, K.L., Ahn, T.M. and Rigney, D.A. (1980) ‘Friction, wear and microstructure of unlubricated austenitic stainless steel’, Wear, Vol. 60, pp.13–37. Johnson, R.N., Aungst, R.C., Hoffman, N.J., Cowgill, M.G., Whitlow, G.A. and Wilson, W.L. (1976) ‘Development of low friction materials for LMFBR components’, Proceedings of International Conference on Liquid Metal Technology in Energy Production, Champion, PA, pp.122–130. Kanoh, S., Mizobutchi, S. and Asumo, H. (1976) ‘Friction and wear behaviour of Colmonoy and Stellite alloys in sodium environment’, Proceedings of International Conference on Liquid Metal Technology in Energy Production, ANS-ASME Meeting, Champion, PA, pp.153–159. Kumar, H., Ramakrishnan, V., Albert, S.K., Meikandamurthy, C., Tata, B.V.R. and Bhaduri, A.K. (2010) ‘High temperature wear and friction behaviour of 15Cr-15Ni-2Mo Titanium-modified austenitic stainless steel in liquid sodium’, Wear, Vol. 270, pp.1–4. Kumar, H., Ramakrishnan, V., Rajanbabu, V., Vijayashree, R., Albert, S.K., Sritharan, R., Meikandamurthy, C., Balasubramaniyan, V. and Bhaduri, A.K. (2007) ‘In-sodium tribological studies pertaining to core and reactor assembly components of PFBR’, DAE-BRNS Theme Meeting on Nuclear Tribology, Mumbai, pp.35–49. Parthasarathi, N.L., Borah, U. and Albert, S.K. (2013) ‘Correlation between coefficient of friction and surface roughness in dry sliding wear of AISI 316L(N) stainless steel at elevated temperature’, Computer Modelling and New Technologies, Vol. 17, No. 1, pp.51–63. Radcliffe, S.J. (1980) ‘Friction coefficients of AISI 316 stainless steel in impure and in zirconium hot-trapped sodium’, Leeds-Lyon Symposium on Friction and Traction, University of Leeds, Leeds, pp.1–5. Smith, A.F. (1984) ‘The friction and sliding wear of unlubricated 316 stainless steel at room temperature in air’, Wear, Vol. 96, pp.301–318. Vijayaraghavan, S., Shanmugasundaram, M., Rajasundaram, P. and Chandran, T. (2010) Introduction of Tribometer in Fatigue Loop of INSOT Facility – Safety Report, IGC/FRTG/ SFD/99136/SR/3037. Design and development of high-temperature tribometer 285 Wild, E. and Mack, K.J. (1980) ‘Tribological problems in sodium-cooled nuclear power plants’, Wear, Vol. 64, pp.151–162. Yoshida, E., Hirakawa, H., Kano, S. and Nihei, I. (1980) ‘In-sodium tribological study on cobalt free hardfacing materials for contact and sliding parts for FBR component’, Liquid Metal Engineering Technology, Vol. 502, pp.1–10. Yukota, N. and Shimoyashiki, S. (1988) ‘Investigation of self-welding rate of structural materials in liquid sodium’, Nuclear Technology, Vol. 81, pp.407–414.