Contact rolling fatigue of rail steels

Transcription

1 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII 357 Contact rolling fatigue of rail steels S. Nishida 1, N. Hattori 1 & T. Miyake 2 1 Faculty of Science and Engineering, Saga University, Japan 2 Graduate school of Science and Engineering, Saga University, Japan Abstract The authors have focused on the dark spots, which are initiated from white phase due to the contact between wheel and rail, especially in the Shinkansen track. Most of these defects have been initiated on the high-speed rail track at more than 150 km/h and in the accelerated train s section. Though the dominating factors for the dark spot s initiation have not necessarily been clarified yet, it has been observed that the white phase exists and the contact rolling fatigue cracks are propagated through the white phase. As the white phase consists of 90% martensite and 10% residual austenite, it is very brittle and the contact rolling fatigue cracks are easily initiated due to cyclic loading. Therefore, it is considered that this white phase would be one of the causes for generating the contact rolling cracks in Shinkansen rails. The object of this study is to investigate the mechanism of contact rolling fatigue defects in view of the initiation mechanism of dark spot, which is also known as the shelling defect. The authors considered that the initiation of the contact rolling defects relates to Hertizn stress, lubrication, tangential forces etc. The main results obtained in this study are as follows: (1) The white phase would be generated due to the alternate conditions between water lubrication and dryness. (2) The tangential force affects the contact rolling fatigue crack initiation and propagation properties. (3) White phase has been generated at an early stage according to the observation results of the contact rolling fatigue mechanism. Keywords: contact rolling fatigue, rail steels, white phase, shelling, fatigue crack, lubrication, Hertzian stress. 1 Introduction As far as the surface defects in the running rail head, there are mainly two kinds of defects called the head checks and dark spots (or shelling defects) in the

2 358 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII Japanese Shinkansen s rail track [1-7]. The former is considered to be due to sever contact between wheel flange and gauge corner of the rail head, especially in the curved track. On the other hand, the latter has not been clarified about its initiation mechanism yet. One of the authors has tried to investigate the above problem and proposed the relation between the dark spot and white phase, which was observed in the running surface layer of the rail head in service [2-7]. The authors have focused on the dark spots, which would be initiated from the white phase due to the contact between wheel and rail. Most of such kinds of defects have been initiated in the high-speed rail track more than 150 km/h and in the accelerated train s section. Though the dominated factors for the dark spot s initiation have not necessarily clarified yet, it has been observed that the white phase exists and the contact rolling fatigue cracks are propagated through the white phase. As the white phase was consisted of 90% martensite and 10% residual austenite, it was very brittle and the contact rolling fatigue cracks are easily initiated due to cyclic bending [3][5][6]. Therefore, it is considered that this white phase would be one of the causes for generating the contact rolling cracks in Shinkansen rails. The object of this study is to investigate the mechanism of contact rolling fatigue defect from a view point of the initiation mechanism of the dark spot, which is called the shelling defects by another name. Figure 1: System diagram of testing machine. 2 Experimental procedure 2.1 Testing machine The testing machine used in this test is a two-disk contact rolling fatigue testing machine. Fig.1 shows the system diagram of the testing machine. It is possible to

3 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII 359 drive each disk of the machine independently and its maximum frequency is settled at 2500 rpm. The thrust force is applied by the two numbers of coils (see fig.1, L1 and L2) and its maximum load is 9.8 kn. 2.2 Test specimen The material used in this test is the representative rail steel, that is, eutectoid carbon steel (JIS60kg). Tables 1 and 2 list the chemical compositions and mechanical properties of the specimen material, respectively. Fig. 2 shows the shape and dimensions of the specimen. As shown in this figure, the specimen of the low-speed side has curvature whose radius is 600 mm. The reason for this is to be able to apply adequate Hertzian stress equal to the practical rail track in Shinkansen [7,8]. The surface roughness of the specimen at the middle of the width is about 0.4µm not only for the high-speed side but also the low-speed side. Table 1: Chemical composition (mass%). C Si Mn P S Table 2: Mechanical properties. Tensile strength (MPa) 2.3 Experimental procedure 0.2% proof stress (MPa) Elongation (%) Vickers hardness (Hv) Table 3 lists contact load, maximum Hertzian stress, lubrication, and difference in revolution speed under no loading. In addition, the maximum Hertzian stress is about 1GPa in Shinkansen s new representative rail [8], [9]. Therefore, the drop of lubricant rate is set to 0.2 cc/sec by the fatigue limit of cycles in the low-speed side. Dry and water lubricant are applied every cycles or every cycles, respectively [10]. Tangential force is given by rolling slippage setting at 2080rpm for high-speed side cycle and the low-speed side cycle can be changed arbitrarily on contact because it is possible to drive the two-disk rolling-sliding machine independently. The oil lubrication is introduced to decrease the friction of the rail surface. It is considered to be able to control the wear of the rail using the oil and to reproduce the damage easily. In addition, the damage during dry conditions could be neglected in this study. The judgment of fatigue strength is chosen from a variety of ways considering the experimental conditions and object. In this study, the test is stopped at every cycles and the specimen s surface is successively observed at the specified cycles by a scanning electron microscope (SEM). The critical fatigue strength is determined when the crack length becomes greater than 2 mm.

4 360 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII *1 Dry and water lubricant are applied every cycles. *2 Dry and water lubricant are applied every cycles. (1) High-speed side (2) Low-speed side Figure 2: Shape and dimensions of the specimen. Table 3: Test conditions. Test conditions Specimen s No. Load (N) Hertzian stress (GPa) Lubrication condition Difference in rev. speed under no loading (rpm) A Dry+Water*1 50 A Dry+Water*1 50 A Dry+Water*1 50 B Dry+Water*2 50 B Dry+Water*2 50 B Dry+Water*2 50 C Water 10 D Water 50 E Oil 50 This study gives importance to the surface observation. Therefore an optical microscope and SEM were used after specified cycles. Hardness distribution was measured about the fatigue test specimens by Vickers hardness testing machine.

5 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII 361 White phase 500µm 500µm of A1 (b): Surface observation of A1 (Nf=2 106 cycles) 200µm 500µm (d): Surface observation of A1 (Nf= cycles) (c): Surface observation of A1 (Nf=3 106 cycles) 50µm White Phase * 200µm Vertical section (a): Surface appearance (Nf=1 105 cycles) * Contact rolling surface (e): Contact rolling fatigue crack of A1 (Nf= cycles vertical section) (f): White Phase in A1 (SEM). (Nf= cycles) Loading direction Figure 3: 3 Photographs of specimen A1 at the running surface and in horizontal section (σhz =1.0GPa). Results and discussion 3.1 Effect of lubrication The contact rolling fatigue cracks are always initiated in the low-speed side specimen. This phenomenon is a representative tendency in the cases of normal

6 362 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII contact rolling fatigue damage. The low-speed side specimen is corresponding to the section of a rail where trains are accelerated up to a high speed. Hereinafter, testing results are focused on low-speed side specimen. Fig.3 shows the photographs of specimen A1 at the running surface and in the horizontal section. The rolling contact surface becomes brownish one after cycles (Fig.3(a)). The scratch marks and ultra-thin avulsion are observed in this surface (Fig.3(b)). The white phase is generated in the running surface in the area of mm 2. In the following stage, the small pits occur but some of them dissipate according to the successive contact rolling. In addition, the width of contact point expands (Fig.3(c)), because the high-speed side specimen abrades the surface of the low speed side one during no lubrication condition. Fig.3(d) shows the photograph of specimen A1 in horizontal section after contact rolling fatigue test. Fig.3(e) shows the crack in specimen A1.The surface damage will have been accumulated by no lubrication condition and the contact rolling fatigue crack initiates and propagates due to water intrusion. The secondary crack grows due to the successive repetition by the opposite specimen and reaches the original of the specimen s original surface and it is called flaking or spalling (ref. Section 3.2). Cracks propagated to a depth of about 200µm in condition A1 (Fig.3(e)). It is about 400µm in condition B1 (Fig.4). The angle of crack progress for the running surface is about 15 both ways. Flaking and pits were not observed in condition C. Fig.5 shows the plots of Maximum hertzian stress vs. number of cycles. From this figure, it is clear that contact rolling fatigue strength decreases in accordance with the lubrication time and no lubrication time. Therefore, the lubrication condition influences the damage of rails. These phenomena are very similar to those of the Shinkansen. At the same time, after pent-up passing tonnage goes beyond 0.15 billion tons in Shinkansen s rail, the shelling is considered to increase at a great rate. If it is assumed that passing of one wheel is proper for one revolution in this study, a period of shelling increase at a great rate is suitable for cycles in this study. Therefore, the cracks that grow up shelling occur much earlier with this prediction. 3.2 Crack observation of results The subsurface cracks were observed with an optical microscope. The above crack occurs from contact rolling surface. It propagates in the running direction with inclination angle about 15 from the running surface. The secondary crack occurs afterwards due to successive bending by the opposite specimen from the primary crack (Fig.4(a)). The impression is observed in the neighbour hard crack initiation point (Fig.4(b)). It is considered that the primary cracks occur at first and the secondary cracks are generated due to contact rolling fatigue by the highspeed side disk. In addition, the corrosion by water etc. accelerates the growth of the depression.

7 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII µm 200µm (a) Loading (b) Figure 4: Contact rolling fatigue crack of B1. (N f = cycles, vertical section). Max. hertzian stress σ Hz [GPa] E+06 Condition A Condition B 1.00E+07 Number of cycles to failure N f (cycles) Figure 5: Max. Hertzian stress VS. cycles. 3.3 Effect of Hertzian stress It is clear that the fatigue strength decreases when applied load increases as shown in fig.5. Fig.6 shows the results of Vickers hardness test. The hardness number shows Hv=290 to 300 in virgin mother material and Hv=370 to 380 at work-hardened contact rolling surface. There exists the part of extraordinarily high hardness number Hv=600, where the White Phase is observed (ref. fig.3(f)). It is considered that the fatigue strength decreases when maximum Hertzian stress increases due to embrittled surface by work-hardened contact rolling.

8 364 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII Hardness Hv (0.49N) Condition A1 Condition A2 Condition A Distance from surface mm Figure 6: Result of Vickers hardness tests. 3.4 Effect of slip The authors draw a comparison between condition C and condition D. The tangential force of C is smaller than D. The depth of crack propagation is about 50µm in C. On the other hand, it is about 150µm in D (fig.7). Therefore, the tangential force affects crack initiation and propagation properties. Fig.4(a) and (b) show the subsurface crack. 3.5 SEM observation results The White phase is observed in the early stage of contact rolling fatigue in conditions A and B (for example, ref. fig.3(b)). The thickness is about 5µm (ref. fig.3(f)), and hardness is about 600 Hv. In this study, the authors can t find the cracks that penetrate the White Phase in C and D. The reason why there are cracks and not the White Phase in C and D is that the White Phase has been removed by the successive repetition of loading. In fact, the surface at these locations is remarkably uneven. It is considered that the friction force can control the wear of the rail by the oil and it will be possible to reproduce the damage easily. But the White Phase and cracks aren t observed, because the friction force is too small. Thus, the frictional force impinges on the occurrence of the White Phase in some way. Fig.8 illustrates the crack initiation mechanism by contact rolling fatigue. At first, the primary crack is initiated at the surface of the low speed specimen and the secondary crack will be generated as a branching crack due to the successive loading. When this secondary crack propagates and reaches the original surface, the portion surrounded by the above two kinds of cracks will be removed. This phenomenon is called spalling.

9 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII 365 Specimen s surface High speed Low speed Primary crack Secondary crack Figure 7: Crack of subsurface in D. Figure 8: Illustration of crack initiation by contact rolling fatigue. 4 Conclusion The main results obtained in the present study are as follows: (1) The white phase would be generated due to the alternating between water lubrication and dry conditions. (2) The tangential force affects the contact rolling fatigue cracks initiation and propagation properties. (3) The White phase occurs in the early stage in the case of contact rolling fatigue mechanism. (4) The authors have succeeded in reproducing the shelling defects in the laboratory using the two-disk contact rolling fatigue testing machine. References [1] Masumoto, H., Sugino, K., Nishida, S., et al., ASTM STP 644, pp , [2] Nisida, S, Sugino, K, Urasima, C, Masumoto, H, Trans. of JSME (in Japanese), , pp , [3] Nisida, S, Sugino, K, Urasima, C, Masumoto, H, Trans. of JSME (in Japanese), , pp , [4] Nishida, S., Sugino, K., Urashima, C., Masumoto, H., Bulletin of JSME, , pp , [5] Nishida, S., Sugino, K., Urashima, C., Masumoto, H., Bulletin of JSME, , pp , [6] Nishida, S., Sugino, K., Urashima, C, Role of Fracture Mechanics in Modern Technology, Elsevier Science Publishers BV, pp , [7] Nishida, S., Sugino, K., Urashima, C., Masumoto, H., Computational and Experimental Fracture Mechanics, ,pp , [8] Akaoka, J., Report of Railway Tech. Res. Int., 6-183, pp89-101, [9] Ito, A., Wear of Rail, Lubrication, 16-1, pp.3-8, [10] Kaneta, M, Matsuda, K, etc., JSME, l61-588, pp , 1995.