Unlubricated sliding wear of pearlitic and bainitic steels

Unlubricated sliding wear of pearlitic and bainitic steels

Wear 259 (2005) 405–411 Unlubricated sliding wear of pearlitic and bainitic steels C.C. Vi´afara, M.I. Castro, J.M. V´elez, A. Toro ∗ Tribology and S...

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Wear 259 (2005) 405–411

Unlubricated sliding wear of pearlitic and bainitic steels C.C. Vi´afara, M.I. Castro, J.M. V´elez, A. Toro ∗ Tribology and Surfaces Group, National University of Colombia, Medellin, Colombia Received 2 August 2004; received in revised form 22 January 2005; accepted 1 February 2005 Available online 23 May 2005

Abstract Sliding wear has a great influence on the performance of rail–wheel systems, especially because the wheel flange slides over the rail in a curve track. Since rail–wheel sliding introduces adhesive effects, high slip ratios strongly affect the rolling contact fatigue wear acting on the surfaces. Sliding wear tests were carried out in a pin-on-disk device to study the behavior of AISI 1070 pearlitic and AISI 15B30 bainitic pins sliding against AISI 1085 pearlitic disks. The sliding speed was 1 ms−1 for all the tests and normal loads of 10, 30 and 50 N were used. A bainitic microstructure was obtained after austempering the AISI 15B30 steel at 310 ◦ C for 30 min, while the AISI 1070 samples were normalized in air, being the austenitizing temperature 850 ◦ C for both materials. The wear resistance was related to the mass loss measured after the tests and the worn surfaces, as well as particle debris, were analyzed by optical and scanning electron microscopy. Micro-hardness profiles were also obtained to analyze strain hardening effects beneath the contact surfaces. The pearlitic steel showed higher sliding wear resistance than bainitic steel, due to the excellent strain hardening of pearlite compared to bainite. Oxidative wear regimes were observed in the pearlitic steel, while in the bainitic one adhesive wear was the main removal mechanism, leading to a much more accentuated damage of the surface. In fact, the wear regime for bainitic samples was always severe, even for the lower loads applied. © 2005 Elsevier B.V. All rights reserved. Keywords: Wheel–rail contact; Adhesive and oxidative wear; Pearlite; Bainite; Strain hardening

1. Introduction Continuous improvements in rail steels have been made since the first wheel–rail system started to operate a 100 years ago. In the last years, the study of wheel–rail contact mechanics and the material response under rolling contact fatigue became an important issue due to the increase in loads and speeds of vehicles, which produce high critical stresses in metallic components [1]. When the wheel flange slides over the outer rail in a curve track, it develops an axial force that leads to deformation and cracking processes, changing the dynamics of wheel–rail contact and reducing the expected rail life. On the other hand, sliding wear has a great relevance in rolling contact fatigue mechanism since a slip ratio increase could cause a higher mass loss from surfaces in contact [2,3].



Corresponding author. Tel.: +57 44 255254; fax: +57 42 341002. E-mail address: [email protected] (A. Toro).

0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.02.013

Optimization of chemical composition and thermal treatment is fundamental to improve the wear behavior of rail steels. In addition, other properties related with manufacture and welding processes should be taken in account to maintain an adequate performance of wheel–rail system. Traditionally, pearlitic steels have been used as rail materials. However, some authors [4,5] have suggested that bainitic steels could be comparable to pearlitic steels from the point of view of their wear resistance when submitted to rolling contact fatigue and adhesion. Some bainitic steels show an unusual combination of strength and toughness (especially those alloyed with silicon) and it is well known that these properties are required to obtain an excellent wear resistance [6,7]. The aim of this work was to study the relation between microstructure and wear behavior of pearlitic and bainitic pins sliding against pearlitic disks under different conditions of normal load.

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Table 1 Nominal chemical composition of the studied steels (in wt.% except boron) Steel

C

Mn

P

S

B (ppm)

Fe

AISI 15B30 AISI 1070 AISI 1085

0.35 0.65–0.75 0.8–0.94

1.5 0.6–0.9 0.7–1

0.26 0.04 0.04

0.02 0.05 0.05

70 – –

Balance Balance Balance

2. Experimental procedure 2.1. Testing materials AISI 15B30, 1070 and 1085 steels were used. The AISI 15B30 steel was selected to obtain a bainitic microstructure since boron increases the hardenability of the alloy; the AISI 1070 and AISI 1085 steels were used to achieve a fully pearlitic microstructure, taking advantage of their near-toeutectoid chemical composition. The chemical composition of tested materials is shown in Table 1. 2.2. Heat treatments All the specimens were annealed at 850 ◦ C for 30 min to homogenize the microstructure previously to any final heat treatment. The AISI 15B30 steel pins were austempered at 310 ◦ C for 30 min, which allowed obtaining a bainitic microstructure. The 1070 pins and 1085 disks were normalized in forced convection to obtain a very fine pearlitic microstructure, similar to that commonly found in steel rails and wheels.

loads (10, 30, and 50 N) were applied, the maximum sliding distance was 3200 m for pearlitic and 800 m for bainitic pins and the sliding speed was 1 ms−1 for all the tests. Both disks and pins surfaces were polished with grade 600 silicon carbide emery paper before each test, in order to guarantee the same initial surface finishing conditions. The mass losses were measured by using a scale with 0.1 mg resolution. The wear rates were calculated from the slopes of mass loss versus sliding distance plots and normalized with the apparent contact area of the pins (3.17 × 10−5 m2 ) and the sliding distance. Time-variation curves of friction coefficient were obtained for each test in order to determine the wear regime and the nature of the contact between the surfaces. 2.4. Surface examination and microstructure observation The average roughness Ra of pins was periodically measured during the tests by using a stylus profile roughness tester Mitutoyo SJ-201. The worn surfaces were analyzed using a PGH S15 optical stereomicroscope and a JEOL 5910LV Scanning Electron Microscope. The microstructure of pearlitic and bainitic steels was also analyzed in SEM.

3. Results and discussion 3.1. Heat treating and microstructure

Wear tests were conducted in a pin-on-disk wear testing machine shown in Fig. 1. The tribological pairs tested were AISI 15B30 bainitic and AISI 1070 pearlitic pins sliding against pearlitic AISI 1085 disks. Three different normal

Fig. 2 shows the pearlitic microstructure obtained in pins of AISI 1070 steel after air cooling. A fine structure with an interlamellar spacing of approximately 0.08 ␮m can be observed. The micrograph also shows how the pearlitic nodules delineate the prior austenitic grains. The microstructure of austempered AISI 15B30 steel, composed by groups of parallel plates of bainitic ferrite

Fig. 1. Detail of the home-built pin-on-disk testing machine used in all the experiments.

Fig. 2. Microstructure of normalized pins of AISI 1070 steel. Nital 2%.

2.3. Sliding wear tests

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Fig. 3. Microstructure of austempered pins of AISI 15B30 steel. Lepera’s reagent.

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Fig. 5. Mass loss as a function of sliding distance and normal load for AISI 1070 pins sliding against AISI 1085 disks.

3.2. Mass losses and wear rates

Fig. 4. Microstructure of normalized disk of AISI 1085 steel. Nital 2%.

Figs. 5 and 6 show the variation of mass loss with sliding distance for AISI 1070 pearlitic and AISI 15B30 bainitic pins sliding against AISI 1085 pearlitic disks, respectively. In pearlitic pines the higher mass losses were obtained when a normal load of 30 N was applied, which possibly reveals transition in the initial accommodation of the surfaces from 30 to 50 N. The wear rates for each load condition were calculated by using mass loss data starting from 400 m, value in which the running-in period has already ended. After that point, a linear behavior was observed with virtually no differences among the slopes for each normal load condition. Unlike the pearlitic pins, the bainitic ones showed different wear rates as a function of the normal load. Moreover, the running-in period was shorter than that observed

(lower bainite), is shown in Fig. 3. Also, some martensite formed during cooling from austempering to room temperature due to incomplete bainitic reaction [8,9]. The microstructure of the AISI 1085 disks is shown in Fig. 4. The pearlitic structure has a degenerate shape, probably as a consequence of a faster cooling that promoted a loss of cooperation in lamellae growth. Table 2 shows the hardness of the studied steels. The variations around the mean values are probably due to small differences in cooling rates from one sample to another. Table 2 Measured hardness of the studied steels Steel

Microstructure

Hardness [HV30]

15B30 (pins) 1070 (pins) 1085 (disks)

Lower bainite Fine pearlite Fine pearlite

400–420 320–340 325–345

Fig. 6. Mass loss as a function of sliding distance and normal load for AISI 15B30 bainitic pins sliding against AISI 1085 pearlitic disks.

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Fig. 7. Wear rate as a function of the normal load for pearlitic and bainitic pins.

for pearlite–pearlite contact, since the linear relation between mass loss and sliding distance started in 200 m. This short accommodation period can be related to the large hardness difference between pin and disks, which promotes more accentuated roughness changes and reduces adhesive interactions. The variation of wear rates against normal load for pearlitic AISI 1070 and bainitic AISI 15B30 pins is shown in Fig. 7. The wear rate of bainitic pins increased continuously with normal load and the measured values were at least two orders of magnitude greater than those found for pearlitic pins. 3.3. Roughness and friction coefficient The average roughness Ra of pin surfaces is plotted as a function of the sliding distance in Figs. 8 and 9 for both pearlitic and bainitic pins. Generally speaking, the average roughness of bainitic pins was always higher than that of pearlitic ones, which can be associated to a more severe mass removal condition in bainite. The transition from the surface accommodation period to the

Fig. 8. Variation of average roughness Ra with sliding distance for AISI 1070 pearlitic pins. The Ra values in zero sliding distance correspond to the condition of the surfaces before the tests.

Fig. 9. Variation of average roughness Ra with sliding distance for AISI 15B30 bainitic pins. The Ra values in zero sliding distance correspond to the condition of the surfaces before the tests.

steady state regime was quite different for both pairs studied: while the roughness of AISI 1070 pearlitic pins sharply increased at the beginning of the tests and then decreased to relatively low values after circa 400 m, the AISI 15B30 bainitic pins maintained the high roughness values reached in the running-in period. The surface smoothing observed in pearlitic pins is commonly found in metals during the accommodation period and has been typically associated to the formation of an interfacial oxide layer, which reduces adhesive forces and leads to a stationary oxidative wear regime [12]. Figs. 10 and 11 show the variation of friction coefficient as a function of the sliding distance and normal load for pearlitic and bainitic pins, respectively. No significant changes in friction coefficient were observed with variation of normal load, although the mass losses were quite different in each case as can be seen in Figs. 5 and 6. 3.4. Worn surfaces and particle debris Worn surfaces of AISI 1070 pearlitic pins for different normal loads are shown in Fig. 12. Intense formation of ad-

Fig. 10. Variation of friction coefficient with sliding distance and normal load. AISI 1070 Pearlitic pins sliding against AISI 1085 pearlitic disk.

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Fig. 11. Variation of friction coefficient with sliding distance and normal load. AISI 15B30 bainitic pins sliding against AISI 1085 pearlitic disk.

hesion joints was observed in samples tested with a normal load of 30 N (Fig. 12b) if compared to those tested with 10 and 50 N. Furthermore, only Fe3 O4 (black) iron oxide was observed at the surface of the pins tested with a normal load of 30 N, while both Fe3 O4 and Fe2 O3 (red) iron oxides could be seen in the specimens tested with 50 N. The Fe2 O3 oxide is formed at higher temperatures (or higher normal loads) and helps preventing metallic contact between the surfaces, leading therefore to a moderate oxidative wear regime that has been reported by several authors in pearlitic steels [4,6,10]. The main damage mechanisms observed in AISI 1070 pearlitic pins were formation of adhesion joints and sub-

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superficial crack propagation. Fig. 13a shows the typical aspect of an adhesion joint in a sample tested with a normal load of 30 N, while in Fig. 13b some sub-superficial cracks can be observed beneath the worn surface of a specimen tested with a normal load of 50 N, in which a delamination process similar to that reported by Zum Gahr [11] is in progress. Examination of wear debris gives additional information concerning the mechanisms of mass removal from the surfaces. Fig. 14a shows the characteristic aspect of a broken adhesion joint, while in Fig. 14b several shear marks related to cracks propagation are revealed parallel to the sliding direction. The worn surfaces of several AISI 15B30 bainitic pins tested with different normal loads are shown in Fig. 15. Adhesion arises as the most important damage mechanism together with intense plastic deformation, which becomes more evident at the edges of the pins in the tests with higher normal loads (Fig. 15b and c). SEM images showing adhesion marks in pins tested with normal loads of 10 N (a) and 50 N (b) are presented in Fig. 16. Surface examination of the pins supports the results presented in Fig. 7, according to which the rate of mass removal from the surface increases with the normal load. 3.5. Strain hardening effects Fig. 17 shows the variation of micro-hardness as a function of depth from the worn surface, for pearlitic pins tested with different normal loads. It can be seen that both the micro-

Fig. 12. Typical aspect of worn surfaces of AISI 1070 pearlitic pins for different normal loads.

Fig. 13. Adhesion joints and sub-superficial cracks in AISI 1070 pearlitic pins.

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Fig. 14. Broken adhesion joints (a) and shear marks (b) identified in wear debris of AISI 1070 pearlitic pins. SEM examination in low-vacuum mode.

Fig. 15. Typical aspect of worn surfaces of AISI 15B30 bainitic pins for different normal loads.

hardness at the surface and the size of the hardened volume increase with normal load. By considering the friction coefficient and roughness results, it is proposed that when a low normal load is applied most of the friction work is not converted in plastic strain but in heat dissipation along the surface, which leads to an increase in surface temperature and to the subsequent oxide formation. The high strain hardening of pearlitic microstructures has been attributed to the shape and distribution of carbides, which can be represented by the interlamellar spacing [1]. In the tests carried out in this work, the small interlamellar spacing of pearlite was decisive to obtain a higher sliding

wear resistance in AISI 1070 than in bainitic AISI 15B30 steel. Nevertheless, it is worth noticing that the higher carbide fraction of AISI 1070 steel compared to that of AISI 15B30 steel also played an important role in the wear response of the pins. On the other hand, no strain hardening was observed in bainitic pins after micro-hardness measurements, which explains their low resistance to adhesive sliding wear. The friction work is therefore spent in plastic deformation and material shearing, and the low toughness of the microstructure promotes crack propagation. Therefore, it would be necessary to evaluate other types of bainitic steels,

Fig. 16. Adhesion marks in AISI 15B30 bainitic pins.

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regime in which the wear rates were low and the surfaces remained relatively smooth. In bainitic AISI 15B30 steel pins sliding against AISI 1085 steel disks the adhesive effects were much more accentuated, mainly due to the poor strain hardening of the microstructure. The mass losses were almost three orders of magnitude higher than in pearlitic pins and roughening of the surfaces was significant. References

Fig. 17. Variation of micro-hardness as a function of depth from the worn surface in AISI 1070 pearlitic pins.

like silicon-alloyed ones, where the presence of carbides is avoided [7]. Moreover, controlled amounts of retained austenite could be useful to optimize the strain hardening capacity in bainitic microstructure [7]. The analysis of initial hardness of pins (Table 2) indicates that an appropriate modelling of sliding wear must consider the strain hardening effects, given that the bainitic pins, which are circa 100 HV harder than the pearlitic ones at the beginning of the tests, presented much more accentuated adhesion marks, as well as higher mass losses and wear rates. This would improve the classical models proposed by Archard [12] and Borland & Bian [13] and lead to more reasonable predictions of sliding wear rates.

4. Conclusions The main wear mechanisms identified in pearlitic AISI 1070 pins sliding against AISI 1085 steel disks were adhesion and oxide layer removal. This configured an oxidative wear

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