Indian Journal of Engineering & Materials Sciences Vol. 15, October 2008, pp. 377-381 Ferrography A procedure for measuring wear rate N Govindarajan a * & R Gnanamoorthy b a Flat B, Raj Enclave, Balaji Colony, 4 th Cross Street Velachery, Channai 600 042, India b Department of Mechanical Engineering, Indian Institute of Technology, Chennai 600 036, India Received 27 August 2007; accepted 17 July 2008 A contact fatigue phenomenon is most common failure seen in the structural components, which are under high-cyclic fatigue loads. Rail wheels, mating gears, ball bearing and wherever the formal contact between the two structural elements, are affected by contact fatigue failure, is commonly referred as pitting of surface. It can be seen that those structural elements are manufactured by powder metallurgy technology since it has more technical as well as commercial advantages over the conventionally made structural parts. Development in powder metallurgy manufacturing technologies, will give us confident to use of more powder metallurgy structural parts in place of conventional parts. Rolling-sliding contact fatigue (RSCF) experiments on powder metallurgy (PM) steels have been carried out in the laboratories with available experimental set-up. The lubrication oil is collected for regular interval and ferrography test is involved to predict the wear rate of the powder metallurgy steels. Wear morphology of porous steel is predicted. Key words: Contact Fatigue, Powder metallurgy, Wear, Ferrrography Fatigue experiments on porous steels have been performed for defining its fatigue properties under various fatigue parameters. Identification of failure stage of these materials during experiments would be really challenging one as fatigue failures will not give any prior symptoms. If failure stage of structural components, which are under the fatigue loading, is unpredictable, the result would be a catastrophic failure of the components. Hence during the fatigue experiments, it is needed to have condition monitoring of all parameters such as temperature, speed, load, vibration and wear rate. The values of those parameters will be peak at the time of failure stage, than that of normal stage and/or initial stage. One of the life condition monitoring techniques usually followed in fatigue experiments is wear rate measurements on the lubrication oil at regular interval. For this, the lubrication oil used for reducing the sliding/rolling friction in fatigue experiments is collected since wear particles due to fatigue rupture are settled down in the oil. The oil sample is collected at regular interval during the fatigue test so that the failure stage of the structural component can be identified before it fails cataclysmically. A best way to deal with the wear process under contact fatigue, through an experimental approach, is to investigate the resultants of the wear at various stages. Also it attempts to find out what and which *For correspondence (E-mail : nggovind@yahoo.com) mode of failure occurred in a system. The ferrographic particle analysis system is a powerful non-intrusive technique 1. Ferrography provides early detection of abnormal wear of the lubricated critical internal components of mechanical systems. The dualferrograph analyzer prepares the ferrogram slides for analysis through the Ferroscope III microscope 2,3. This is a new method introduced to collect the wear debris using an electromagnet. The ferrography technique was developed in 1970's to overcome the large particle detection deficiencies of spectrometric oil analysis. It is the technique for the separation of particles from fluids for microscopic examination and subsequent analysis. Experimental Procedure Twin disc type rolling contact fatigue test rig available in the laboratory for performing the pure rolling contact fatigue tests is modified for the current project work. The schematic view of test setup is shown in Fig. 1. In the modified test rig, the test roller can be run to the desired speed using a variable speed DC motor while the standard roller is run using the AC induction motor at a constant speed using belt drives and couplings. The speed of the test roller is set to the required level based on the desired slide roll ratio and continuously monitored using the speed sensor connected to the shaft. Both the speeds of the shafts are continuously monitored so that the set slide roll ratio is maintained throughout the test. The
378 INDIAN J. ENG. MATER. SCI., OCTOBER 2008 AC Motor DC Motor Hydraulic Power Pack Test Roller Shaft Test Roller Bearing Block Coupling Standard Roller Shaft Standard Roller Fig. 1 Schematic view of the experimental set up for RSCF test. friction coefficient between the rollers is calculated from the current and voltage readings of the DC motor. The test roller unit is mounted on a linear motion slide and contact load is applied using the hydraulic cylinder connected to a hydraulic power pack. The contact load is maintained constant throughout the test duration and continuously monitored using load cell connected. Lubricant oil, SAE 40, with a viscosity of 0.07 Pa-s that is continuously gravity fed to the contact zone. Temperature of oil coming out of the contact zone is measured using a copper-constantan thermocouple. Lubricant oil flow is kept constant throughout the experiment and the output oil temperature is not allowed to exceed 323 K. The lubricant oil coming out the contact zone is directly collected using a clean oil collector kept below the contact zone for ferrographic analysis. Failure identification during the rolling contact fatigue test is very critical, as the system will continue to run in spite of sufficient damage in the contacting elements. For the research studies even a small surface damage leading to a variation in the sound and/or vibration level of the total system is considered as failure. Severe wear occurring in the roller is also considered as failure and it can be detected only by ferrographic analysis. During the current investigation, sound level of the rolling contact fatigue machine and vibration level of the test roller mount, lubricant oil sample and visual inspection at periodic intervals are carried out to precisely identify the failure point and termination of the experiment. All sensors are connected to the data acquisition system and continuously monitored using a personal computer 4-6. Wear Debris Analysis Procedure The name, ferrography, derives from the initial development of methods to precipitate ferrous wear particles from lubricating oil 1,2. The analysis is generally based on two aspects, the qualitative and quantitative methods. The first method describes the characteristics of the morphology of wear particles whereas other method is based on the size and concentration of the wear debris. The qualitative method is performed to understand the wear mechanism in sintered and wrought steels. Dual- Analytical Ferrograph is used for this analysis.
GOVINDARAJAN & GNANAMOORTHY: FERROGRAPHY 379 Analytical ferrograph instrument is used for preparing the ferrogram slide. Analytical ferrography begins with the magnetic separation of wear debris from the lubricating oil in which it is suspended using a ferrogram slide maker. Figure 2 shows a typical ferrogram slide. The lubricating oil sample is diluted for improved particle precipitation and adhesion if necessary. The prepared sample flows over a specially designed glass slide (ferrogram). The ferrogram rests on a magnetic cylinder, which attracts ferrous particles out of the oil. After all the fluid in a given sample run over the slide, a fixer solution is passed over the slide to remove residual fluid. When the fixer oil has evaporated, the slide is ready for observation using the Ferroscope III. Figure 3 explains the working principle of the ferrogram maker. A nonwetting barrier is painted on one surface of the slide. This coating traps the fluid delivered by a peristaltic pump. The slide is mounted at an angle permitting the fluid to flow by gravity along the glass, but within barrier, where it finally is picked up by a drain tube. This slide is placed above the two permanent magnets, which are separated by an aluminum sheet. The poles of the magnets are counter posed. That is, where one magnet pole is considered north, the pole of the other magnet across the aluminum strip is Fig. 2 Typical ferrogram slide Large Particles deposit at entry point where the magnetic pull is the weaker Oil Flow Slide Magnet Smaller Particles deposit along the slide as the magnetic pull strengthens. Fig. 3 Ferrogram maker Oil flow south. Magnetic particles in the fluid experience a strong downward force. These particles migrate through the fluid down to the glass surface, where they are deposited in strings perpendicular to the direction of fluid flow. Results and Discussion Ferrogram slides were prepared as explained in previous section from the carefully collected oil samples. Ferrous particles are deposited on the slide according to size of the wear particles. The force acting on a particle is proportional to volume, but the viscous resistance of the suspending fluid is proportional to surface area. Therefore, for spheres, force increases with the cube of the diameter, but resistance increases only with the square of the diameter. The largest particles, therefore, are deposited at the entry region of the slide where the lubricating oil first touches down on the glass surface. The examination of the slide in a bichromatic microscope reveals details of size, shape and number of particles. From this information the condition of oil-lubricated parts may be assessed. Figure 4 shows the wear particles of sintered and hardened steels, which were settled down as strings of platelet due to heavy magnetic fields for different sliding friction. Figures 4a and 4b show particles settled during the initial and failure stages of roller tested at 1400 MPa and 0.1% slide/roll ratio. Fine running wear particles were observed in the early stages indicating the significant running-in wear occurring in the sintered roller. Figures 4 c and 4d show the wear particle settled in an oil sample of sintered and hardened steel tested at 1400 MPa contact stress with 1.4% slide/roll ratio. Bigger particles were also observed at the failure stage indicating the presence of pitted material a single particle. The larger particles with striations are probably been generated at higher slide/roll ratio in RSCF test of sintered and hardened roller. These striations of particles are more concentrated at failure stage compared to initial stage of the test roller for same slide/roll ratio. Higher concentration of the wear particles of bigger size also indicates the severe catastrophic form of sliding wear occurring in the system. In the case of ferrogram prepared from the oil sample collected during the RSCF test of wrought EN24 steel tested at 1600 MPa and 0.1% slide/roll ratio, the concentration of the wear particles settled down is lower (Fig. 5a) compared to the sintered and hardened steels. Figures 5b and 5c show the fatigue
380 INDIAN J. ENG. MATER. SCI., OCTOBER 2008 (a) Initial stage of the test (a) Wear Particle Strings (b) Failure stage of the test (i) Strings of Plate Let Type Sliding Wear Particles (1400 MPa and 0.1% Slide/Roll Ratio) (b) Fatigue Wear Particle Settled (c) Initial stage of the test (c) Fatigue Chunks of Metal from Wrought Steel Fig. 5 Wear particles observed in the ferrogram slides prepared from tests conducted for wrought steels (1600 MPa and 0.1% slide/roll ratio -100 ) (d) Failure stage of the test (ii) Strings of Particles Resulting from Sliding Wear (1400MPa and 1.4% Slide/Roll Ratio) Fig. 4 Wear particles observed in ferro gram slides of sintered and hardened steel (100 ). chunks of wear particle settled during the failure stage in wrought steel roller. However, a number of such particles observed are lower in the wrought steels compared to the sintered steels. Conclusions Failure analysis of the rolling sliding contact fatigue specimens tested at the different contact stresses and slide roll ratios are carried out. The
GOVINDARAJAN & GNANAMOORTHY: FERROGRAPHY 381 influence of traction force and porosity on failure pattern is investigated. Surface peeling and pitting type of failures are observed in the sintered and hardened rollers. Severe running in wear particles are seen in the ferrogram prepared in the sintered materials while the concentration of the wear particle is less in the wrought steels. Introduction of coefficient of friction plays important role in wear rate of porous steels compared to wrought steel. References 1 Jones W R & Parker J, ASLE Trans, 22 (1977) 37. 2 Liu Y, Tribol Int, 30 (1997) 279. 3 Harris T A, Rolling Bearing Analysis, 4 th Ed, (John Wiley & Sons, Inc), 1997. 4 Govindarajan N, & Gnanamoorthy R, Mater Sci Eng A, 445-446 (2007) 259. 5 Govindarajan, N, Rolling-sliding Contact Fatigue Behaviour of Sintered and Hardened Steels, M.S. Thesis, Indian Institute of Technology, Madras, 2002. 6 Roylance B J, Tribol Int, 38 (2005) 857. 7 Seifert W W & Westcoot V C, Wear, 21 (1) (1972) 27.