Influence of Rail Surface Roughness Formed by Rail Grinding on Rolling Contact Fatigue

Transcription

1 PAPER Influence of Rail Surface Roughness Formed by Rail Grinding on Rolling Contact Fatigue H. CHEN, Ph.D Senior Researcher, Track Dynamics Laboratory, Railway Dynamics Division M. ISHIDA General Manager, JR Affairs, Marketing and Business Development Division Based upon stress analysis that indicated that surface roughness causes higher contact pressure on contact surfaces, the question arises about whether the initial surface roughness formed by rail grinding may speed up the onset of rail rolling contact fatigue (RCF). In order to clarify whether initial surface roughness has an adverse effect on rail RCF, the authors carried out experiments by means of a twin-disc rolling contact machine and then investigated the experimental results from several perspectives, such as surface roughness, plastic flow, as well as the hardness and axis density of crystals beneath the rail surface. This paper describes the details of the experiments, the variation of roughness on contact surfaces that accompanies repeated rolling contact and the influence of the initial surface roughness on RCF. Keywords: rail grinding, surface roughness, rolling contact fatigue, RCF, inverse pole figure 1. Introduction Preventative rail grinding is currently becoming very popular in Japan as a way of removing the surface layer of rail rolling contact fatigue (RCF) damage caused by repetitive trainloads. In addition, curative rail grinding is carried out to remove the longitudinal rail surface irregularities such as rail corrugations and rail welds. Such rail grinding is contributing to extended rail service lives and reduced rolling noise and vibration. On the other hand, elastic-plastic stress analysis of asperity contact has revealed the high degree of contact stress on the wheel/rail interface in terms of surface roughness 1). For instance, in cases where von Mises stress surpasses the shear yield stress of rail materials, plastic deformation may take place and cause cracks due to ratcheting and other factors. Focused on rail roughness from the point of view of rail grinding work efficiency, an appropriately high level of roughness has some advantage on grinding speed and ground thickness per cycle. However, an initially high level of roughness formed on the rail surface by rail grinding poses a problem with regard to high-frequency rolling noise and has the potential to cause RCF damage, as mentioned above. On the other hand, such high levels of initial roughness are usually reduced to normal due to repetitive trainloads when in operation. This poses two interesting questions. One is how long it takes or how much accumulated passing tonnage is required to reduce the initial level of roughness. The other concerns the degree of RCF damage accumulated from the initial level of roughness to the time when trainloads have reduced it to normal roughness. It is hence essential to study the optimal initial roughness formed by rail grinding to avoid rolling noise and/or RCF, and to discover the most appropriate and efficient way of carrying out grinding work. In this study, we carried out some experiments using a twin-disc rolling contact machine 2) to investigate the variation in accumulated passing tonnage needed to settle down the initial roughness formed by rail grinding. In addition, we analyzed the rail discs, focusing on the plastic flow of the surface layer using an optical microscope and the crystal axis density using X-ray diffraction. This report describes the experimental results obtained by the twin-disc rolling contact machine and the results of metallurgical analysis carried out on the rail discs. 2. Repeating rolling experiments 2.1 Test machine Figure 1 shows the twin-disc rolling contact machine adopted in the experiments. The wheel disc was set as the driving side and the rail disc as the following side. The wheel disc (diameter: 3 mm, thickness: 5 mm) was made from the same materials as an actual wheel, and the rail disc (diameter: 17 mm, thickness: mm) was cut out from JIS 6kg rail and formed. Upon carrying out the experiments, the contact surface of the wheel disc was polished with #8 abrasive papers, and an Rz, the maximum height of roughness, of 1 µm to 4 µm were applied onto the contact surface of the rail disc (Fig. 2(a)). The direction of the roughness groove was formed parallel to the axis direction (Fig. 2(b)) referencing the actual roughness of the grinding marks. 216 QR of RTRI, Vol. 47, No. 4, Nov. 26

2 Load cell (4 units) Universal joint Rotary meter Radial load Wheel disc Rotary encoder Surface roughness 6mm 5mm D.C. motor Torque sensor Rotary encoder Rail disc E.C.B D.C. motor Ø17mm (a) Roughness made on rail disc mm (b) Dimensions and roughness orientation of rail disc Fig. 1 Twin-disc rolling contact machine Fig. 2 Rail disc dimensions Test discs Roughness Rz (μ m) No No No Contact pressure (MPa) Rolling speed (km/h) Table 1 Test arrangements Slip ratio (%) Repetitive cycles (cycles) Passing tonnage(equivalent) (MGT) Temperature & Humidity ( ), (%) , < , < , < 32 Maximum roughness Rz, m Repetitive cycles 1 3 Initial roughness No m No m No m No m Maximum roughness Rz, m Maximum roughness Rz, m Repetitive cycles Initial roughness No m No m No m No m Repetitive cycles 1 3 Initial roughness No m No m No m No m (a) Rail disc No. 1 (b) Rail disc No. 2 (c) Rail disc No. 3 Fig. 3 Variation of roughness with repetitive cycles on rail disc Nos.1 to 3 QR of RTRI, Vol. 47, No. 4, Nov

3 Maximum roughness Rz, 㱘m 3. Experimental results and discussions V ariation of roughness with repetitive cycles Variation No. 1-㽲 Slip ratio.% No. 2-㽲 Slip ratio.% 3 Figure 3 indicates the variation of roughness with repetitive cycles in terms of rail disc Nos.1 to 3. It can be seen that Rz decreased gradually in response to the increase of repetitive cycles. In addition, even the largest initial roughness disc settled down to almost the same level of roughness as other smaller initial roughness discs after 8, repetitive cycles. No. 㽲 Slip ratio.2% Repetitive cycles 䋨㬍13䋩 Fig. 4 V ariation of roughness comparison between rail Variation disc Nos.1 to Experimental arrangements Table 1 describes the test arrangements. Nos. 1 to 3 denote the rail disc numbers. ① to ④ show the location of roughness (Rz) applied to a rail disc (Fig. 2(a)). The accumulated passing tonnage was calculated taking into consideration of the vertical axle load corresponding to an axle load of 1kN. A constant maximum Hertzian pressure of 75 MPa was adopted, taking into account the contact between a worn wheel and rail. In addition, a noslip case and a slip (slip ratio.2%) were applied. During the experiments, the roughness and hardness of the contact surface were measured at 11,9 repetitive cycles corresponding to the number of wheel passes a day on a revenue line of an annual 4 million gross tonnage (MGT: a parameter to define track damage caused by trainloads, which is calculated as axle load multiplied by the number of wheel passes) 3 11,9, 7 11,9 and 11 11,9, respectively. After finishing the experiments, the plastic flow of the contact surface, its hardness, and crystal axis density of crystals beneath the surface were measured using an optical microscope, hardness tester, and X-ray diffractometer. Figure 4 shows a variation of roughness comparison between rail discs Nos.1 to 3 with repetitive cycles. With a slip ratio of.2%, rail disc No. 3 showed the largest variation of roughness, which was reduced earlier than the rail discs without slip. The initial roughness reduction trends in this study were roughly consistent with the findings obtained so far on tracks in operational service. In order to confirm the variation of roughness, Fig. 5 shows the surface status of rail discs Nos.1 and 3 after 11,9 repetitive cycles and 11 11,9 repetitive cycles. It is clear that the roughness of rail disc No.3 with slip reduced and almost vanished even at 11,9 cycles, which was earlier than that of No. 1 without a slip. 3.2 Effect of initial roughness on plastic flow The plastic flow of rail discs No. 1- ① and No. 1- ④, which had different initial roughness but the same repetitive cycles without slip, are shown in Fig. 6(a), that of rail discs No. 1- ① and No. 2- ①, which had different repetitive cycles, are shown in Fig. 6(b). Furthermore, that of rail disc No. ① and No. ④, which had different initial roughness but the same repetitive cycles with slip, are shown in Fig. 6(c). The scale for all micro photos was set at 4 times. In the case of rail disc Nos.1 and 2 without slip (Figs. 6(a) and 6(b)), almost no plastic flow was identified, irrespective of initial roughness. In the case of Repetitive cycles䋺㩷12.3㬍13 Repetitive cycles䋺㩷11.9㬍13 No. 1-㽲 1- No. 11-㽵 No. 㽲 Repetitive cycles䋺㩷138.8㬍13 No. 11-㽲 1-㽵 No. 1- (a) Rail disc No. 1 㽵 No. Repetitive cycles䋺㩷138.3㬍13 No. 㽲 No. 3㽵 (b) Rail disc No. 3 Fig. 5 Investigation of surface status of rail disc Nos.1 and 3 with repetitive cycles 218 QR of RTRI, Vol. 47, No. 4, Nov. 26

4 Rz 9.61㱘m Rz䋺9.61 No. 1-㽵 1- No.1-㽲 No.1- Rz䋺32.98 Rz 32.98㱘m (a) Optical micrographs of rail disc No. 1 (different initial roughness) 1-㽲 No. 1- Repetitive cycles cycles䋺 㬍 㽲 No. 2- Repetitive cycles䋺 cycles 㬍1 (b) Optical micrographs of rail disc Nos. 1 and 2 (different repetitive cycles) No. 㽲 Rz 8.98㱘m Rz䋺8.98 No. 㽵 Rz 31.63㱘m Rz䋺31.63 㱘 (c) Optical micrographs of rail disc No. 3 (different initial roughness) Fig. 6 Investigation of plastic flow on rail disc Nos. 1 to 3 rail disc No.3 with a slip ratio of.2% (Fig. 6(c)), a slight plastic flow was apparent in their very thin layers. However, the difference between various levels of initial roughness was not clear. From these metallurgical inspections of the rail discs on which repetitive loads of up to 13, cycles were applied, the initial roughness formed by the rail grinding is considered to have less effect on plastic flow. 3.3 Effect of initial roughness on hardness We measured rail disc surface hardness under some repetitive cycles during the experiments and measured the hardness of the rail discs beneath the surface after the experiments had been completed. A Shore-type hard- QR of RTRI, Vol. 47, No. 4, Nov. 26 ness tester was used in measuring the surface hardness of the rail discs and a Knoop-type hardness (HK) tester used for measuring the hardness beneath the surface. In advance of the measurements with the Knoop-type hardness tester, it was calibrated with a standard test specimen of 3 Hv (Hv: Vickers hardness) and.245n selected as its applied load. The measurement depths decided upon were at intervals of 2 µm up to 6 µm and intervals of.1mm up to 1mm. Figure 7 indicates the surface hardness Hv of rail disc No. 3 measured before, during and after the experiments. As shown in the figure, the surface hardness increases from the beginning to 8, repetitive cycles. However, the surface hardness at 13, repetitive cycles is less than that at 8, cycles. The reason is related to the 219

5 Hardness, Hv No. Initial roughness m No. Initial roughness m No. Initial roughness 16.3 m No. Initial roughness 9.61 m Repetitive cycles 1 3 Fig. 7 Surface hardness variation of rail disc No. 3 precision of the hardness tester or the activation of materials during repetitive rolling contact cycles. Figure 8 depicts the variation of hardness beneath the surface of rail disc No. 1-1, No. 1-4, No. 1 and a new disc. Each datum plotted in the figures is the average of three data measured at the same depth location laterally 2 mm apart. The hardness of rail disc No. 1-4 with little initial roughness appears less than that of No. 1-1 as a whole, but the difference between the two levels of roughness is not clearly identified due to some data dispersion. The hardness of rail disc No. 1 with slip is slightly greater than that of rail disc No.1-1 without slip in the case of the same roughness level. This agrees with the findings obtained so far that indicate that the strain hardening of the surface to which slip is applied progresses more than that with no slip because a shear force was applied. On the other hand, the hardness of rail discs to which repetitive loads have been applied is greater than that of a new rail disc, irrespective of slip because of strain hardening. 3.4 Effect of initial roughness on crystal axis density The crystal axis density was measured using the X- ray diffraction of inverse pole figure method 3) to investigate the influence of initial roughness on RCF damage. An X-ray diffraction of inverse pole figure measurement gives information on the orientation of constituent crystals, from which it is considered possible to evaluate the degree of RCF. The ratio of the X-ray diffraction intensity of a test piece to the intensity from the standard material that has a random orientation is defined as the crystallographic axis density. An axis density of 1 means a condition in which there is no preferred orientation with respect to the crystallographic axis. In the measurements, Mo was used as the X-ray origin at 6 kv and 2 ma and the target of crystallographic axis was 222 since it is recognized as one of the potential parameters to evaluate the degree of RCF damage accu- Hardness, HK No. 1- Initial roughness m No. 1- Initial roughness 9.61 m No. Initial roughness m New rail disc Depth from surface, m Fig. 8 Variation of hardness beneath the surface of rail disc Nos. 1 and 3, plus new rail disc Axis density No. 1- Initial roughness m No. 1- Initial roughness 9.61 m Depth from surface, m Fig. 9 Variation of crystal axis density beneath the surface of rail disc No QR of RTRI, Vol. 47, No. 4, Nov. 26

6 mulated in rail surface layers in operational service 4). The inverse pole figure of the contact surface at a depth of 1 µm was measured first and later the measurements were continued up to the depth of 2 mm with electro-solution polishing. Figure 9 shows an example of the measured results of rail disc No.1 for crystallographic axis 222. The axis density value in the very thin layer is greater than 1, so that the order of the crystal plane is slightly recognized. However, the difference between various initial degrees of roughness is apparently unidentified. In the measured results of rails ground on tracks in operational service 5), the axis density of the thin layer of the rail surface, where 26 MGT of accumulated passing tonnage were applied after grinding, was almost equivalent to 1. From the findings obtained by the actual rail samples 5), it may suggest that the minor damaged surface layer indicated in Fig. 9 would be removed while increasing repetitive loadings, and the axis density of the thin layer corresponding to 1.4 MGT of accumulated passing tonnage in this study would be less or closer to 1 by increasing repetitive loading cycles. In-depth studies into this are anticipated. However, the possibility of minor effects of initial surface roughness, grinding marks, formed by current rail grinding work on RCF damage was identified from the results of this study. 4. Conclusions It was apparent that the surface roughness formed by rail grinding work was decreased and reduced to some normal roughness level value after almost identical repetitive loading cycles, irrespective of the initial roughness levels. In addition, based upon the investigated results on the hardness, plastic flow, and rail disc crystal axis density, after the initial roughness had settled down to almost the same level of normal rail roughness, the difference of initial roughness was apparently unrecognized. Hence, the grinding marks formed by the current rail grinding work have less potential to generate RCF cracks and make the cracks progressive. RCF damage cracks may mainly be generated after the initial roughness of the grinding mark has been reduced to a normal rail surface level. 5. Acknowledgements The authors would like to express their sincere appreciation to Mr. Yukio Satoh and Dr. Kengo Iwafuchi for their very valuable discussions and help with the X-ray diffraction of inverse pole figure method. References 1) Kapoor, A., Frank, F. J., Wong, S. K. Ishida, M.: Surface Roughness and Plastic Flow in Rail Wheel Contact, Wear 253, pp , 22. 2) Ishida, M., Ban, T. and Moto, T.: Development of High-Precision Rolling Contact Machine and a Few Initial Test Results, Proc. J-rail 98, Japanese Society of Electrical Engineers, pp (in Japanese). 3) Nagashima, S.: Texture Structure, Maruzen, ) Satoh, Y. and Iwafuchi, K.: Crystal Orientation Analysis of Running Surface of Rail Damaged by Rolling Contact, Wear 258, pp , 25. 5) Aabe, T. et al.: Rolling Contact Fatigue Layer of Rail Used in Narrow Gauge Line, Proc. J-rail 4, Japanese Society of Electrical Engineers, pp. 554 (in Japanese). QR of RTRI, Vol. 47, No. 4, Nov