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sliding wear of ferrite–pearlite steel
Wear 358-359 (2016) 62–71
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Effect of different strengthening methods on rolling/sliding wear of ferrite–pearlite steel Dongfang Zeng, Liantao Lu n, Ning Zhang, Yubin Gong, Jiwang Zhang State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China
art ic l e i nf o
a b s t r a c t
Article history: Received 18 November 2015 Received in revised form 2 April 2016 Accepted 4 April 2016 Available online 8 April 2016
The object of this paper is to study the influence of different strengthening methods on wear resistance of ferrite–pearlite steel. Rolling/sliding wear tests were conducted for five railway wheel steels which were hardened by carbon addition, solid solution strengthening and precipitation strengthening, respectively. Wear rate, subsurface plastic deformation and strain-hardening of tested steels were examined. The test results show that wear resistance of ferrite–pearlite steel is improved by both carbon addition and solid solution strengthening, whereas it is deteriorated by precipitation strengthening. Wear resistance of ferrite–pearlite steel depends on the worn surface hardness that is influenced by bulk hardness and strain-hardening. Strengthening methods increase the bulk hardness to different extents, where the highest and lowest bulk hardness increments are obtained by the solid solution strengthening and precipitation strengthening, respectively. The strain-hardening is promoted by carbon addition, while it is reduced by solid solution strengthening and precipitation strengthening where precipitation strengthening makes a greater reduction in strain-hardening. Strain hardening of ferrite–pearlite steel is reduced by a high content of proeutectoid ferrite with a low ductility, which is caused by solid solution strengthening and precipitation strengthening. & 2016 Elsevier B.V. All rights reserved.
Keywords: Rail-wheel tribology Rolling–sliding Steel Wear testing Hardness
1. Introduction Wear and rolling contact fatigue of rails and railway wheels are costly problems for the railway system. They cost about 1.2 billion US dollars annually in China [1]. ER8 steel defined by EN13262 is a widely used wheel material for Chinese high-speed train. Wear is the principal reason for replacing railway wheels made of ER8 steel. Consequently, it is necessary to develop a wheel steel with a higher wear resistance. Almost all railway wheels are made of ferrite-pearlitic steels. Wear resistance of these steels is most generally characterized by the bulk hardness, which is highly influenced by the microstructure and thereby can be improved by optimizing chemical compositions. Besides, increasing the worn surface hardness through strain-hardening has been proved to improve the wear resistance of the ferrite–pearlite steel [2–7]. Softer ferrite-pearlitic steels have better wear resistance than the initially harder bainite steel since ferrite-pearlitic steels develop greater strain-hardening during the wear process [8–11].
n
Corresponding author. Tel.: þ 86 28 86466025; fax: þ 86 28 87600868. E-mail address: [email protected] (L. Lu).
http://dx.doi.org/10.1016/j.wear.2016.04.003 0043-1648/& 2016 Elsevier B.V. All rights reserved.
In the past years, many strengthening methods, including carbon addition, solid solution strengthening and precipitation strengthening, have been proposed to improve the wear resistance of the ferrite–pearlite steel. 1.1. Carbon addition Considerable effort has gone into understanding the role of carbon addition on the wear behavior of the ferrite–pearlite steel [3,4,12–19]. One clear conclusion is that carbon addition enhances the wear resistance of the ferrite–pearlite steel through increasing the bulk hardness of the steel [3,4,12–17]. Investigations conducted by Ueda and Naka et al. have shown that the ferrite–pearlite steel with higher carbon content has greater strain-hardening rate since carbon addition promotes the grain refinement in the vicinity of the worn surface; this further improves the wear resistance of the steel [3,4]. 1.2. Solution strengthening Silicon and manganese can be used to increase the wear resistance of the ferrite–pearlite steel through solid solution strengthening [12,20–25]. In recent years, a ferrite–pearlite steel containing high contents of silicon and manganese was developed
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and applied for the railway wheel [20–22,25]. Silicon and manganese contents are increased to stabilize the material at elevated temperatures, which reduces the thermal sensitivity of the steel [20–22]. However, the wear resistance of this steel has not been reported.
0.07V steel are hardened by solid solution strengthening and precipitation strengthening, respectively. U71Mn steel was selected as the tested rail materials; it has a carbon content of more than 0.72%.
1.3. Precipitation strengthening
Rolling/sliding wear tests were carried out using a twin-disc machine. Fig. 1 shows the schematic illustration of wear testing. Microstructures and mechanical properties of the wheel steel vary with the depth below the wheel tread. In order to ensure the uniformity of material properties, wheel discs were removed from wheel rims at a depth of approximately 15 mm with their top surfaces parallel to the wheel tread. Rail discs were removed from the U71Mn rail with their top surfaces parallel and close to the top surface of railhead. Then the tested discs were machined into the shape and dimension as shown in Fig. 1. After that, the contact surface of test discs was polished to achieve an average roughness (Ra) of about 0.2 μm. A profilometer (MarSurf PS1) was used to measure the roughness of the contact surface before the testing. Five measurements were taken in the axial direction of the disc and an average value was calculated for each disc. The result shows that the values of Ra vary between 0.189 μm and 0.206 μm. All test discs can be considered to have a similar surface roughness. The line contact between two cylindrical test discs was used to simulate the normal loading and slip present at rail/wheel contact. Wear tests were conducted under a maximum contact pressure of 800 MPa and a slip ratio of 5.4% to simulate the wearing condition in curved tracks. In order to prevent the change of the microstructure caused by friction heating and remove wear debris, the contact area was cooled with dry compressed air during the testing. A torque sensor, with a maximum torque capacity of 15 N m and relative error of 71%, was used to measure the friction force during the testing, from which the friction coefficient was calculated. Previous work has shown that after a certain number of rolling cycles (running in stage), the accumulated plastic deformation within the subsurface of test discs reaches its maximum and thus a steady wear state is obtained for the remainder of the test [31]. Under the condition used in this study, the number of rolling cycles needed to establish a steady wear state is about 15,000 [32,33]. Therefore, all tests in this study were carried out by applying the discs 20,000 cycles to establish a steady wear state. Then, the discs were taken down and cleaned in
The bulk hardness of the ferrite–pearlite steel can also be improved by additions of vanadium and niobium through the precipitation strengthening [26–30]. However, works conducted by Katsuki et al. have shown that although vanadium addition raises the bulk hardness of the steel, it reduces the strainhardening rate of the worn surface [26,27]. And thus, the worn surface hardness, which depends on both the bulk hardness and the strain-hardening, is complicated for precipitation strengthening steels. Therefore, it is difficult to determine the effect of precipitation strengthening on the wear resistance of ferrite–pearlite steels; researches using different materials have led to different conclusions [26–29]. In order to improve the wear resistance of the railway wheel steel by optimizing the chemical compositions, it is necessary to study the effect of different strengthening methods on the wear behavior of the ferrite–pearlite steel. However, such a study has not been conducted so far. In this study, rolling/sliding wear tests were performed for five railway wheel steels which were hardened by carbon addition, solid solution strengthening and precipitation strengthening, respectively. Wear rate, plastic deformation and strain-hardening of the tested steels were examined. The effects different strengthening methods on the wear resistance were analyzed.
2. Materials and experiment 2.1. Materials All tested materials were machined from the railway wheel and the rail that were never used in service. Table 1 gives the chemical compositions of the tested materials. In this study, wear tests were conducted for five railway wheel steels, which were denoted by ER7, ER8, ER9, HiSi and 0.07V, respectively. The main difference in compositions among ER7, ER8 and ER9 steels, which are defined by the EN 13262, is the carbon content. ER7, ER8 and ER9 steels have a carbon content of 0.48%, 0.52 and 0.57%, respectively. The contents of silicon and manganese of HiSi steel are increased compared with those of ER8 steel, where HiSi steel has a more than threefold increase in silicon content. The main difference in compositions between ER8 steel and 0.07V steel is the vanadium content. 0.07V steel has a vanadium content of 0.07%, while the vanadium content of ER8 steel is negligible. Compared with ER8 steel, HiSi steel and
2.2. Experiment
Table 1 Chemical compositions of test materials w/%. Materials
Carbon Silicon Manganese Sulfur Phosphorus Chromium Vanadium a
Wheel steel
Rail steel
ER7
ER8
ER9
HiSi
0.07Va
U71Mn
0.48 0.28 0.75 0.016 0.002 0.22 0.003
0.52 0.26 0.73 0.006 0.002 0.25 0.002
0.57 0.26 0.73 0.007 0.002 0.26 0.003
0.52 0.93 0.93 0.009 0.001 0.21 0.003
0.53 – – – – – 0.07
0.72–0.82 0.65–0.90 0.75–1.05 r 0.04 r 0.035 r 0.035 /
Chemical compositions of 0.07V steel are confidential.
Fig. 1. Shapes of test discs and schematic illustration of wear tests.
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an acetone ultrasonic bath for 10 min, dried in a jet of hot air, and weighed using an electronic balance with a sensitivity of 7 0.1 mg. After that, the discs were remounted in the twin-disc machine to conduct another 100,000 rolling cycles and weight loss during the testing were determined using the electronic balance. In order to obtain reliable results, two pairs of wheel and rail discs shown in Fig. 1 were tested for each kind of wheel steel listed in Table 1. 2.3. Observation of microstructures and hardness Before the testing, the microstructures of tested steels were observed using a KEYENCE VK-9710 confocal laser scanning microscope (CLSM) and a JSM-6610LV scanning electron microscope (SEM). The proeutectoid ferrite content was determined using Image-Pro software from the CLSM images. The interlamellar spacing corresponding to the minimum apparent spacing was measured perpendicular to lamellae within pearlite colony having the shortest spacing [34]. The prior austenite grain size was assessed with linear intercept method according to ASTM E112. Transmission electron microscopy (TEM, JEM-2100F) studies were conducted on thin foils at an accelerating voltage of 200 kV. The composition of the precipitate was investigated by energy dispersive X-ray spectroscopy (EDX). After wear testing, the hardness of worn surface was measured using a microhardness tester with a load of 25 g. In order to obtain reliable results, over 20 indentations were carried out at randomly selected locations on each worn surface of wheel disc. Then, wheel
discs were sectioned along the track center and prepared for metallographic observation by CLSM and SEM. The hardness profile below the worn surface was measured with the microhardness tester.
3. Results 3.1. Microstructure and bulk hardness Fig. 2 shows CLSM micrographs of the tested steels, from which the proeutectoid ferrite content was measured. Fig. 3 sketches the procedure to determine microstructural characterizations, including interlamellar spacing and prior austenite grain size. A summary of microstructural characterizations for all tested steels is presented in Table 2. The microstructural observation of ER7, ER8 and ER9 steels shows that the proeutectoid ferrite content within the steel decreases with an increase in carbon content. When compared to ER8 steel, the proeutectoid ferrite content of HiSi and 0.07V steel is increased to 12.1% and 18.4%, respectively. Interlamellar spacing and grain size seem not to be affected by carbon addition and solid solution strengthening, since these microstructural characterizations for ER7, ER8, HiSi and ER9 steel are similar. However, the microstructure of ferrite–pearlite steel can be refined by increasing vanadium content, since interlamellar spacing and grain size for 0.07V steel are obviously smaller than those of other examined
Fig. 2. CLSM micrographs of (a) ER7, (b) ER8, (c) ER9, (d) HiSi, (e) 0.07V and (f) U71Mn steel.
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Fig. 3. Brief summary of microstructure analysis for (a) interlamellar spacing and (b) prior austenite grain size.
Table 2 Microstructural characterizations and bulk hardness of tested steels.
Table 3 Results of wear testing.
Materials
ER7
ER8
ER9
HiSi
0.07V
U71Mn
Proeutectoid ferrite content (%) Interlamellar spacing, λ (μm) Grain size (ASTM No.) Vickers hardness (HV0.3)
14.4 0.142 8.0 267
7.6 0.143 7.5 273
4.6 0.139 7.5 284
12.1 0.145 7.5 289
18.4 0.126 8.5 281
0 0.261 6.5 305
Materials
ER7 ER8
ferrite–pearlite steels. U71Mn is a full pearlite steel; this can be attributed to its high carbon content. ER7, ER8, ER9, HiSi and 0.07V wheel steels have an average hardness of 267, 273, 284, 289 and 281 HV0.3, respectively. For ER7, ER8 and ER9 steels, the hardness of ferrite–pearlite steel increases with an increase in carbon content. Both HiSi and 0.07V steel exhibit a higher hardness than ER8 steel, while HiSi steel possesses the highest hardness of all examined railway wheel steels. For the materials used in this study, the strengthening methods increasing the hardness of ferrite–pearlite steel in a descending order are solid solution strengthening, carbon addition and precipitation strengthening. The hardness of U71Mn rail steel exhibits a higher hardness than all of wheel steels. 3.2. Wear behavior Results of rolling/sliding wear testing are summarized in Table 3. The difference of wear losses between two tests using the same wheel steel is negligible; this indicates the reliability of test results. The average friction coefficient for all tests varies between 0.58 and 0.61. It seems that the friction coefficient cannot be affected by the wheel steels under rolling/sliding condition. For ER7, ER8 and ER9 steels, the wear loss decreases with an increase in carbon content. Wear loss for HiSi steel is the lowest of all examined materials, while 0.07V steel has the highest wear loss. In this study, wear resistance of ferrite–pearlite steel is improved by both carbon addition and solid solution strengthening, where the solid solution strengthened steel has the highest wear resistance. However, wear resistance of ferrite–pearlite steel is reduced by the precipitation strengthening method used in this study. All wheel discs have a similar worn surface micrograph, as shown in Fig. 4. The appearance of the worn surface indicates that the mechanism by which the wheel disc loses material is delamination, where thin metallic flakes are observed. 3.3. Subsurface microstructure Rolling/sliding wear tests produce severe deformed microstructure within a small volume of material near the contact surface, as shown in Fig. 5. The material is deformed towards the friction direction and the most severe deformation occurs at the worn surface, where surface cracks are formed. Then, cracks
ER9 HiSi 0.07V
Friction coefficient
1 2 1 2 1 2 1 2 1 2
0.60 0.59 0.58 0.60 0.61 0.59 0.58 0.60 0.59 0.60
Wear loss/mg Wheel
Rail
336 340 319 324 292 301 287 293 368 372
60 58 63 61 57 62 57 60 55 56
gradually grow along the highly strain-flattened proeutectoid ferrite. These cracks are thought to be the origins of the metallic flakes observed in Fig. 4. Fig. 6 shows a SEM micrograph of the subsurface of ER8 steel. It can be seen that the microstructure near the surface is almost aligned parallel to the worn surface by massive plastic deformation. The severe plastic deformation causes considerable bending, fracturing and realignment of the hard cementite lamellae within pearlite. The softer pearlite ferrite was severely deformed, allowing a reduction in the interlamellar spacing at the subsurface. 3.4. Strain hardening behavior Fig. 7 shows the microhardness variation of wheel discs as a function of depth from the worn surface. The microhardness at the depth of zero was measured from the worn surface. It can be seen that the maximum strain-hardening occurs at the worn surface and then it gradually decreases until a certain depth, at which the bulk hardness of the steel is reached. In this study, the increment between the worn surface hardness and the bulk hardness is defined as strain hardness, as shown in Fig. 7. And the relationship between the bulk hardness, strain hardness and worn surface hardness for tested wheel steels is presented in Fig. 8. For ER7, ER8 and ER9 steels, the average strain hardness slightly increases from 267 to 275 Hv when the carbon content increases from 0.48% to 0.56%. Compared with ER8 steel that has an average strain hardness of 272 Hv, the average strain hardness slightly reduces to 266 Hv for HiSi steel and remarkably reduces to 240 Hv for 0.07V steel. The results show that the worn surface hardness does not always increase with an increase in the bulk hardness since the strain hardness of the ferrite–pearlite steel is influenced by the strengthening method. Although 0.07V steel is harder than ER7 and ER8 steels at the beginning of the test, it has the lowest worn surface hardness when the test is finished.
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Fig. 4. A typical worn surface micrograph of wheel discs. (a) Overall view and (b) metallic flakes at a high magnification (Region A). This micrograph was observed from the worn surface of ER8 wheel disc.
Fig. 5. Cross-sectional microstructures of (a) ER7, (b) ER8, (c) ER9, (d) HiSi and (e) 0.07V steel.
4. Discussions 4.1. Strengthened mechanisms, microstructure and bulk hardness Microhardness of proeutectoid ferrite and pearlite within the microstructure of examined steels was measured by a Vickers hardness tester with a load of 10 g and the tested results are shown in Fig. 9. It has been reported that the hardness of ferrite– pearlite steel depends on the volume fraction and mechanical properties of the individual phases (proeutectoid ferrite and pearlite) [18,19]. To analyze the influence of compositions on the microstructure, phase diagrams for wheel steels were calculated using Thermo-Calc software. The resulting diagrams are presented in Fig. 10.
For ER7, ER8 and ER9 steels, carbon addition reduces the volume fraction of proeutectoid ferrite through decreasing the A3 temperature, as shown in Fig. 10. Besides, the cementite density within the pearlite can be increased by increasing the carbon content [3]; this improves the pearlite hardness, as shown in Fig. 9. Both of the factors increase the hardness of the ferrite–pearlite steel. Thus, for ER7, ER8 and ER9 steels, the higher carbon content leads to the higher hardness. Silicon is well known as an alloying element favoring ferrite phase by raising the A3 temperature [19]. Silicon content of HiSi steel rises over threefold compared to ER8 steel and thus A3 point is increased from 757 °C for ER8 steel to 769 °C for HiSi steel, as shown in Fig. 10. The proeutectoid ferrite content is therefore significantly increased from 7.6% for ER8 steel to 12.1% for HiSi
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Fig. 6. SEM micrograph of the subsurface of ER8 steel.
Precipitates are not observed within the V free steels, including ER7, ER8, ER9 and HiSi steels. EDX analysis performed on these precipitates identifies them as vanadium carbide, as shown in Fig. 11(c). The microstructural improvement of 0.07V steel can be associated with the presence of precipitates. The precipitates restrict the prior austenite grain size and pearlite colony size. This increases the proeutectoid ferrite content because the grain boundaries present the sites for the nucleation of proeutectoid ferrite. The precipitates within the pearlitic ferrite can refine interlamellar spacing by hindering the growth of cementite. Thus, the higher hardness for 0.07V steel when compared to ER8 steel can be attribute to multiple strengthening methods, including precipitation strengthening, fine grained strengthening and refinement of interlamellar spacing.
600 ER7 ER8 ER9 HiSi 0.07V
Worn surface hardness
500 Strain hardness
Microhardness / HV0.01
550
450 400 350
Bulk hardness 300 250
4.2. Worn surface hardness and wear resistance
0
20
40
60
80 100 Depth / µm
120
140
160
Fig. 7. Microhardness variation of wheel discs as a function of depth from the worn surface.
steel. Although HiSi steel has an obvious higher proeutectoid ferrite content than ER8 and ER9 steels, it exhibits a higher hardness. This is caused by the solid solution strengthening of silicon and manganese. Silicon is a comparatively strong solid solution strengthening element for ferrite. It has been reported that 1% silicon raises yield stress by 81 MPa [35]. Thus HiSi steel exhibits a higher hardness of the proeutectoid ferrite than ER8 and ER9 steels, as shown in Fig. 9. Additionally, ferrite and cementite within the pearlite are hardened by the solid solution strengthening of silicon and manganese, respectively. The solid solution strengthening of manganese is reported to be promoted by a high content of silicon [36]. Thus the pearlite phase within HiSi steel exhibits a higher hardness than that within ER8 steel, as shown in Fig. 9. It can be seen from Fig. 10 that A3 temperature is further increased to 771 °C for 0.07V steel; this also promotes the formation of proeutectoid ferrite. The TEM observation of 0.07V steel shows that a fine dispersion of precipitates is presented in proeutectoid ferrite and pearlitic ferrite, as shown in Fig. 11.
Fig. 12 shows the relationship between the wear loss and the reciprocal of the bulk hardness. For ER7, ER8, ER9 and HiSi steels, the wear loss appears to linearly increase with an increase in the value of (bulk hardness) 1. However, although 0.07V steel exhibits higher bulk hardness than ER7 and ER8 steels, it exhibits the highest wear loss of all examined steels. Previous studies have shown that softer pearlite steel was work hardened more than the initially harder bainite steel during wear process, and thus the worn surface hardness of the pearlite steel is higher than that of bainite steel [8,9]. This leads to a better wear resistance of pearlite steel than that of bainite steel. It seems that the worn surface hardness is the dominating factor in wear resistance of the ferrite–pearlite steel. Therefore, the relationship between the wear loss and the reciprocal of the worn surface hardness is plotted, as shown in Fig. 13. There is almost a linear relationship between the wear loss and the reciprocal of the worn surface hardness. Although 0.07V steel exhibits higher bulk hardness than ER7 and ER8 steels, it has the lowest wear resistance because of the lowest worn surface hardness. Archard wear equation, given by V ¼kWL/H, where V is the wear volume, k the wear coefficient, W the normal load, L the sliding distance and H is the hardness, is a widely used model to estimate the wear loss of metal [37]. It can be seen from Fig. 12 and
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Microhardness / Hv
ER7
ER8
ER9
HiSi
0.07V
300
300
570
290
290
560
280
280
Average value for ER8
550
270
540
260
260
530
250
250
520
240
240
510
230
500
270
Average value for ER8
230 Average bulk hardness
Average strain hardness
Average value for ER8
Average worn surface hardness
Fig. 8. Relationship between (a) bulk hardness, (b) strain hardness and (c) worn surface hardness.
325
Average hardn ess / Hv
300
Proeutectoid ferrite Pearlite
275 250 225 200 175 150 ER7
ER8
ER9
HiSi
0.07V
Fig. 9. Microhardness of proeutectoid ferrite and pearlite within the microstructure.
13 that the wear data do not fit the wear equation since the lines shown on these figures do not go through the origin. Similar observations have also been shown in previous studies that were conducted to assess the wear behavior of the ferrite–pearlite steel [3,12,26,38]. The laboratory and field tests have identified two separate mild and severe wear regimes for railway wheel steel [39–41]. In order to maintain the equation in the different wear regimes, a constant K has been added in the equation, given by V ¼kWL/HþK [42]. It seems that the wear data well fit this modified equation, as shown in Fig. 12 and 13. 4.3. Strengthened mechanisms and worn surface hardness For ER7, ER8 and ER9 steels, the worn surface hardness increases with an increase in the carbon content (Fig. 8c). This can be explained by the following factors: Firstly, the near-surface region of test disc is composed of highly strained proeutectoid ferrite and pearlite. The nanohardness measurement conducted on the near-surface of worn ferrite–pearlite steel has shown that the
Fig. 10. Phase diagrams calculated using Thermo-Calc software. Line I: Fe–(0.4–0.8) C–0.26Si–0.73Mn–0.25Cr (wt%) system. The alloys, with 0.48, 0.52, 0.57 wt% C, correspond to the compositions of ER7, ER8 and ER9 steel, respectively. Line II: Fe– (0.4–0.8)C–0.93Si–0.93Mn–0.04Cr (wt%) system. The alloy, with 0.52 wt% C, corresponds to the composition of HiSi steel. Line III: a Fe–C–Si–Mn–Cr–V system. The alloy, with 0.53 wt% C, corresponds to the composition of 0.07V steel. Phases are indicated as α (ferrite), γ (austenite) and P (pearlite).
strained pearlite exhibits higher hardness value than the strained proeutectoid ferrite [43]. As shown in Fig. 5, carbon addition increases the volume fraction of the strained pearlite; this increases the worn surface hardness of test disc. Besides, an increment in the cementite density within the pearlite has been reported to promote the strain-hardening by increasing the amount of dislocation in pearlite ferrite and promoting the grain refinement of the pearlite ferrite [3]. Carbon addition increases the cementite density within the pearlite and therefore promotes the strain-hardening during wear testing, as shown in Fig. 8b.
D. Zeng et al. / Wear 358-359 (2016) 62–71
Precipitates
69
Precipitates
Intensity / a.u.
Fe
V 4
5
Cr
Fe
6 7 Energy / keV
8
9
Fig. 11. Transmission electron micrograph of 0.07V steel. Precipitates (a) at proeutectoid ferrite and (a) between cementite lamellae. (c) EDX of precipitates in (a).
400
400
380
ER8 ER9
360
HiSi 0.07V
340
380 360 Wear loss / mg
Wear loss / mg
ER7
320 300
340
ER7 ER8 ER9 HiSi 0.07V
320 300 280 40
280
20
20
0 1.20
0 1.5
1.6
1.7
1.8 3.4 3.5 3.6 (Bulk hardness)-1/ (GPa)-1
3.7
3.8
3.9
1.25
1.30 1.35 1.80 1.85 1.90 (Worn surface hardness)-1 / (GPa)-1
1.95
2.00
Fig. 13. Relationship between wear loss and microhardness of worn surface.
Fig. 12. Relationship between wear loss and bulk hardness.
Previous studies have shown that the accumulation of plastic deformation begins earlier and increases at a higher rate in the proeutectoid ferrite than in the pearlite [43,44]. Although the proeutectoid ferrite has a greater initial ductility than pearlite, it reaches the ductility exhaustion earlier than pearlite. When the ductility exhaustion occurs, the surface crack would initiate along the highly strained proeutectoid ferrite, as shown in Fig. 5. Proeutectoid ferrite with better ductility can sustain more plastic deformation before crack initiation, and therefore the refinement of interlamellar spacing (shown in Fig. 6) would be more remarkable when the critical plastic deformation of proeutectoid ferrite is reached. This would lead to a higher worn surface hardness according to a Hall–Petch relationship with interlamellar spacing that hardness of pearlite phase is in inversely proportion to the square root of the spacing [2,45].
Compared with ER8 steel, the proeutectoid ferrite within HiSi steel and 0.07V steel is hardened due to the solid solution strengthening induced by silicon and manganese addition and precipitation strengthening induced by vanadium addition, respectively. However, it has been reported that the ductility of ferrite would be reduced by both of the strengthening methods [26,27,46,47]. The proeutectoid ferrite within HiSi and 0.07V steels would reach the ductility exhaustion earlier than that within ER8 steel. HiSi and 0.07V steels would therefore strain harden less than ER8 steel, as shown in Fig. 8(b). Silicon and manganese have relatively small effect on the ductility of the steels when their individual contents are less than 1% [46]. However, precipitates shown in Fig. 11 could lead to the generation of residual stress on the ferrite; this can remarkably decrease the ductility of ferrite [26,47]. Therefore, the degree of strain-hardening on the worn surface of 0.07V steel is less than that of HiSi steel, as shown in Fig. 8(b).
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Wear resistance of ferrite–pearlite steel depends on the worn surface hardness (Fig. 13) that can be influenced by both bulk hardness and strain-hardening, as shown in Fig. 8. Microstructure features can influence the wear resistance through imposing the effect on the strain-hardening of worn surface. For ER7, ER8, ER9 and HiSi steels, the difference that microstructure features make to the strain-hardening is relatively small, in a comparison to 0.07V steel. Therefore, except for the 0.07V steel, the wear resistance appears to be proportional to the (bulk hardness) 1, as shown in Fig. 12. The analysis shows that the strengthening methods can influence the wear resistance of ferrite–pearlite steel by influencing the strain-hardening. An appropriate wear model must consider the effect of strain-hardening, given that the 0.07V steel, which is harder than ER7 and ER8 steels at the beginning of the tests, exhibits the lowest wear resistance. This would lead to more reasonable predictions of rolling/sliding wear loss.
5. Conclusions Wear resistance of ferrite–pearlite steels which were hardened by different strengthening methods was tested under rolling/ sliding condition. The influence of strengthening methods, including carbon addition, solid solution strengthening and precipitation strengthening, on wear resistance was investigated and the following conclusions were drawn from this investigation. (1) Wear resistance of ferrite–pearlite steel is improved by both carbon addition and solid solution strengthening, whereas it is deteriorated by precipitation strengthening. (2) Wear resistance of ferrite–pearlite steel depends on the worn surface hardness that can be influenced by both bulk hardness and strain-hardening. (3) Strengthening methods can increase the bulk hardness of ferrite–pearlite steel to different extents, where the highest and lowest bulk hardness increment is obtained by the solid solution and precipitation strengthening, respectively. (4) Strain hardening is promoted by carbon addition, while it is reduced by solid solution strengthening and precipitation strengthening where precipitation strengthening steel has a greater reduction in strain-hardening. (5) Strain hardening of ferrite–pearlite steel is reduced by a high content of proeutectoid ferrite with a low ductility, which is caused by solid solution and precipitation strengthening.
Acknowledgments This work was supported by Basic Research Association Foundation of High–speed Rail (U 1134202 and U 1334206), the 2014 Doctoral Innovation Funds of Southwest Jiaotong University and the Fundamental Research Funds for the Central Universities.
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Effect of different strengthening methods on rolling/sliding wear of ferrite–pearlite steel Dongfang Zeng, Liantao Lu n, Ning Zhang, Yubin Gong, Jiwang Zhang State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China
art ic l e i nf o
a b s t r a c t
Article history: Received 18 November 2015 Received in revised form 2 April 2016 Accepted 4 April 2016 Available online 8 April 2016
The object of this paper is to study the influence of different strengthening methods on wear resistance of ferrite–pearlite steel. Rolling/sliding wear tests were conducted for five railway wheel steels which were hardened by carbon addition, solid solution strengthening and precipitation strengthening, respectively. Wear rate, subsurface plastic deformation and strain-hardening of tested steels were examined. The test results show that wear resistance of ferrite–pearlite steel is improved by both carbon addition and solid solution strengthening, whereas it is deteriorated by precipitation strengthening. Wear resistance of ferrite–pearlite steel depends on the worn surface hardness that is influenced by bulk hardness and strain-hardening. Strengthening methods increase the bulk hardness to different extents, where the highest and lowest bulk hardness increments are obtained by the solid solution strengthening and precipitation strengthening, respectively. The strain-hardening is promoted by carbon addition, while it is reduced by solid solution strengthening and precipitation strengthening where precipitation strengthening makes a greater reduction in strain-hardening. Strain hardening of ferrite–pearlite steel is reduced by a high content of proeutectoid ferrite with a low ductility, which is caused by solid solution strengthening and precipitation strengthening. & 2016 Elsevier B.V. All rights reserved.
Keywords: Rail-wheel tribology Rolling–sliding Steel Wear testing Hardness
1. Introduction Wear and rolling contact fatigue of rails and railway wheels are costly problems for the railway system. They cost about 1.2 billion US dollars annually in China [1]. ER8 steel defined by EN13262 is a widely used wheel material for Chinese high-speed train. Wear is the principal reason for replacing railway wheels made of ER8 steel. Consequently, it is necessary to develop a wheel steel with a higher wear resistance. Almost all railway wheels are made of ferrite-pearlitic steels. Wear resistance of these steels is most generally characterized by the bulk hardness, which is highly influenced by the microstructure and thereby can be improved by optimizing chemical compositions. Besides, increasing the worn surface hardness through strain-hardening has been proved to improve the wear resistance of the ferrite–pearlite steel [2–7]. Softer ferrite-pearlitic steels have better wear resistance than the initially harder bainite steel since ferrite-pearlitic steels develop greater strain-hardening during the wear process [8–11].
n
Corresponding author. Tel.: þ 86 28 86466025; fax: þ 86 28 87600868. E-mail address: [email protected] (L. Lu).
http://dx.doi.org/10.1016/j.wear.2016.04.003 0043-1648/& 2016 Elsevier B.V. All rights reserved.
In the past years, many strengthening methods, including carbon addition, solid solution strengthening and precipitation strengthening, have been proposed to improve the wear resistance of the ferrite–pearlite steel. 1.1. Carbon addition Considerable effort has gone into understanding the role of carbon addition on the wear behavior of the ferrite–pearlite steel [3,4,12–19]. One clear conclusion is that carbon addition enhances the wear resistance of the ferrite–pearlite steel through increasing the bulk hardness of the steel [3,4,12–17]. Investigations conducted by Ueda and Naka et al. have shown that the ferrite–pearlite steel with higher carbon content has greater strain-hardening rate since carbon addition promotes the grain refinement in the vicinity of the worn surface; this further improves the wear resistance of the steel [3,4]. 1.2. Solution strengthening Silicon and manganese can be used to increase the wear resistance of the ferrite–pearlite steel through solid solution strengthening [12,20–25]. In recent years, a ferrite–pearlite steel containing high contents of silicon and manganese was developed
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and applied for the railway wheel [20–22,25]. Silicon and manganese contents are increased to stabilize the material at elevated temperatures, which reduces the thermal sensitivity of the steel [20–22]. However, the wear resistance of this steel has not been reported.
0.07V steel are hardened by solid solution strengthening and precipitation strengthening, respectively. U71Mn steel was selected as the tested rail materials; it has a carbon content of more than 0.72%.
1.3. Precipitation strengthening
Rolling/sliding wear tests were carried out using a twin-disc machine. Fig. 1 shows the schematic illustration of wear testing. Microstructures and mechanical properties of the wheel steel vary with the depth below the wheel tread. In order to ensure the uniformity of material properties, wheel discs were removed from wheel rims at a depth of approximately 15 mm with their top surfaces parallel to the wheel tread. Rail discs were removed from the U71Mn rail with their top surfaces parallel and close to the top surface of railhead. Then the tested discs were machined into the shape and dimension as shown in Fig. 1. After that, the contact surface of test discs was polished to achieve an average roughness (Ra) of about 0.2 μm. A profilometer (MarSurf PS1) was used to measure the roughness of the contact surface before the testing. Five measurements were taken in the axial direction of the disc and an average value was calculated for each disc. The result shows that the values of Ra vary between 0.189 μm and 0.206 μm. All test discs can be considered to have a similar surface roughness. The line contact between two cylindrical test discs was used to simulate the normal loading and slip present at rail/wheel contact. Wear tests were conducted under a maximum contact pressure of 800 MPa and a slip ratio of 5.4% to simulate the wearing condition in curved tracks. In order to prevent the change of the microstructure caused by friction heating and remove wear debris, the contact area was cooled with dry compressed air during the testing. A torque sensor, with a maximum torque capacity of 15 N m and relative error of 71%, was used to measure the friction force during the testing, from which the friction coefficient was calculated. Previous work has shown that after a certain number of rolling cycles (running in stage), the accumulated plastic deformation within the subsurface of test discs reaches its maximum and thus a steady wear state is obtained for the remainder of the test [31]. Under the condition used in this study, the number of rolling cycles needed to establish a steady wear state is about 15,000 [32,33]. Therefore, all tests in this study were carried out by applying the discs 20,000 cycles to establish a steady wear state. Then, the discs were taken down and cleaned in
The bulk hardness of the ferrite–pearlite steel can also be improved by additions of vanadium and niobium through the precipitation strengthening [26–30]. However, works conducted by Katsuki et al. have shown that although vanadium addition raises the bulk hardness of the steel, it reduces the strainhardening rate of the worn surface [26,27]. And thus, the worn surface hardness, which depends on both the bulk hardness and the strain-hardening, is complicated for precipitation strengthening steels. Therefore, it is difficult to determine the effect of precipitation strengthening on the wear resistance of ferrite–pearlite steels; researches using different materials have led to different conclusions [26–29]. In order to improve the wear resistance of the railway wheel steel by optimizing the chemical compositions, it is necessary to study the effect of different strengthening methods on the wear behavior of the ferrite–pearlite steel. However, such a study has not been conducted so far. In this study, rolling/sliding wear tests were performed for five railway wheel steels which were hardened by carbon addition, solid solution strengthening and precipitation strengthening, respectively. Wear rate, plastic deformation and strain-hardening of the tested steels were examined. The effects different strengthening methods on the wear resistance were analyzed.
2. Materials and experiment 2.1. Materials All tested materials were machined from the railway wheel and the rail that were never used in service. Table 1 gives the chemical compositions of the tested materials. In this study, wear tests were conducted for five railway wheel steels, which were denoted by ER7, ER8, ER9, HiSi and 0.07V, respectively. The main difference in compositions among ER7, ER8 and ER9 steels, which are defined by the EN 13262, is the carbon content. ER7, ER8 and ER9 steels have a carbon content of 0.48%, 0.52 and 0.57%, respectively. The contents of silicon and manganese of HiSi steel are increased compared with those of ER8 steel, where HiSi steel has a more than threefold increase in silicon content. The main difference in compositions between ER8 steel and 0.07V steel is the vanadium content. 0.07V steel has a vanadium content of 0.07%, while the vanadium content of ER8 steel is negligible. Compared with ER8 steel, HiSi steel and
2.2. Experiment
Table 1 Chemical compositions of test materials w/%. Materials
Carbon Silicon Manganese Sulfur Phosphorus Chromium Vanadium a
Wheel steel
Rail steel
ER7
ER8
ER9
HiSi
0.07Va
U71Mn
0.48 0.28 0.75 0.016 0.002 0.22 0.003
0.52 0.26 0.73 0.006 0.002 0.25 0.002
0.57 0.26 0.73 0.007 0.002 0.26 0.003
0.52 0.93 0.93 0.009 0.001 0.21 0.003
0.53 – – – – – 0.07
0.72–0.82 0.65–0.90 0.75–1.05 r 0.04 r 0.035 r 0.035 /
Chemical compositions of 0.07V steel are confidential.
Fig. 1. Shapes of test discs and schematic illustration of wear tests.
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an acetone ultrasonic bath for 10 min, dried in a jet of hot air, and weighed using an electronic balance with a sensitivity of 7 0.1 mg. After that, the discs were remounted in the twin-disc machine to conduct another 100,000 rolling cycles and weight loss during the testing were determined using the electronic balance. In order to obtain reliable results, two pairs of wheel and rail discs shown in Fig. 1 were tested for each kind of wheel steel listed in Table 1. 2.3. Observation of microstructures and hardness Before the testing, the microstructures of tested steels were observed using a KEYENCE VK-9710 confocal laser scanning microscope (CLSM) and a JSM-6610LV scanning electron microscope (SEM). The proeutectoid ferrite content was determined using Image-Pro software from the CLSM images. The interlamellar spacing corresponding to the minimum apparent spacing was measured perpendicular to lamellae within pearlite colony having the shortest spacing [34]. The prior austenite grain size was assessed with linear intercept method according to ASTM E112. Transmission electron microscopy (TEM, JEM-2100F) studies were conducted on thin foils at an accelerating voltage of 200 kV. The composition of the precipitate was investigated by energy dispersive X-ray spectroscopy (EDX). After wear testing, the hardness of worn surface was measured using a microhardness tester with a load of 25 g. In order to obtain reliable results, over 20 indentations were carried out at randomly selected locations on each worn surface of wheel disc. Then, wheel
discs were sectioned along the track center and prepared for metallographic observation by CLSM and SEM. The hardness profile below the worn surface was measured with the microhardness tester.
3. Results 3.1. Microstructure and bulk hardness Fig. 2 shows CLSM micrographs of the tested steels, from which the proeutectoid ferrite content was measured. Fig. 3 sketches the procedure to determine microstructural characterizations, including interlamellar spacing and prior austenite grain size. A summary of microstructural characterizations for all tested steels is presented in Table 2. The microstructural observation of ER7, ER8 and ER9 steels shows that the proeutectoid ferrite content within the steel decreases with an increase in carbon content. When compared to ER8 steel, the proeutectoid ferrite content of HiSi and 0.07V steel is increased to 12.1% and 18.4%, respectively. Interlamellar spacing and grain size seem not to be affected by carbon addition and solid solution strengthening, since these microstructural characterizations for ER7, ER8, HiSi and ER9 steel are similar. However, the microstructure of ferrite–pearlite steel can be refined by increasing vanadium content, since interlamellar spacing and grain size for 0.07V steel are obviously smaller than those of other examined
Fig. 2. CLSM micrographs of (a) ER7, (b) ER8, (c) ER9, (d) HiSi, (e) 0.07V and (f) U71Mn steel.
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Fig. 3. Brief summary of microstructure analysis for (a) interlamellar spacing and (b) prior austenite grain size.
Table 2 Microstructural characterizations and bulk hardness of tested steels.
Table 3 Results of wear testing.
Materials
ER7
ER8
ER9
HiSi
0.07V
U71Mn
Proeutectoid ferrite content (%) Interlamellar spacing, λ (μm) Grain size (ASTM No.) Vickers hardness (HV0.3)
14.4 0.142 8.0 267
7.6 0.143 7.5 273
4.6 0.139 7.5 284
12.1 0.145 7.5 289
18.4 0.126 8.5 281
0 0.261 6.5 305
Materials
ER7 ER8
ferrite–pearlite steels. U71Mn is a full pearlite steel; this can be attributed to its high carbon content. ER7, ER8, ER9, HiSi and 0.07V wheel steels have an average hardness of 267, 273, 284, 289 and 281 HV0.3, respectively. For ER7, ER8 and ER9 steels, the hardness of ferrite–pearlite steel increases with an increase in carbon content. Both HiSi and 0.07V steel exhibit a higher hardness than ER8 steel, while HiSi steel possesses the highest hardness of all examined railway wheel steels. For the materials used in this study, the strengthening methods increasing the hardness of ferrite–pearlite steel in a descending order are solid solution strengthening, carbon addition and precipitation strengthening. The hardness of U71Mn rail steel exhibits a higher hardness than all of wheel steels. 3.2. Wear behavior Results of rolling/sliding wear testing are summarized in Table 3. The difference of wear losses between two tests using the same wheel steel is negligible; this indicates the reliability of test results. The average friction coefficient for all tests varies between 0.58 and 0.61. It seems that the friction coefficient cannot be affected by the wheel steels under rolling/sliding condition. For ER7, ER8 and ER9 steels, the wear loss decreases with an increase in carbon content. Wear loss for HiSi steel is the lowest of all examined materials, while 0.07V steel has the highest wear loss. In this study, wear resistance of ferrite–pearlite steel is improved by both carbon addition and solid solution strengthening, where the solid solution strengthened steel has the highest wear resistance. However, wear resistance of ferrite–pearlite steel is reduced by the precipitation strengthening method used in this study. All wheel discs have a similar worn surface micrograph, as shown in Fig. 4. The appearance of the worn surface indicates that the mechanism by which the wheel disc loses material is delamination, where thin metallic flakes are observed. 3.3. Subsurface microstructure Rolling/sliding wear tests produce severe deformed microstructure within a small volume of material near the contact surface, as shown in Fig. 5. The material is deformed towards the friction direction and the most severe deformation occurs at the worn surface, where surface cracks are formed. Then, cracks
ER9 HiSi 0.07V
Friction coefficient
1 2 1 2 1 2 1 2 1 2
0.60 0.59 0.58 0.60 0.61 0.59 0.58 0.60 0.59 0.60
Wear loss/mg Wheel
Rail
336 340 319 324 292 301 287 293 368 372
60 58 63 61 57 62 57 60 55 56
gradually grow along the highly strain-flattened proeutectoid ferrite. These cracks are thought to be the origins of the metallic flakes observed in Fig. 4. Fig. 6 shows a SEM micrograph of the subsurface of ER8 steel. It can be seen that the microstructure near the surface is almost aligned parallel to the worn surface by massive plastic deformation. The severe plastic deformation causes considerable bending, fracturing and realignment of the hard cementite lamellae within pearlite. The softer pearlite ferrite was severely deformed, allowing a reduction in the interlamellar spacing at the subsurface. 3.4. Strain hardening behavior Fig. 7 shows the microhardness variation of wheel discs as a function of depth from the worn surface. The microhardness at the depth of zero was measured from the worn surface. It can be seen that the maximum strain-hardening occurs at the worn surface and then it gradually decreases until a certain depth, at which the bulk hardness of the steel is reached. In this study, the increment between the worn surface hardness and the bulk hardness is defined as strain hardness, as shown in Fig. 7. And the relationship between the bulk hardness, strain hardness and worn surface hardness for tested wheel steels is presented in Fig. 8. For ER7, ER8 and ER9 steels, the average strain hardness slightly increases from 267 to 275 Hv when the carbon content increases from 0.48% to 0.56%. Compared with ER8 steel that has an average strain hardness of 272 Hv, the average strain hardness slightly reduces to 266 Hv for HiSi steel and remarkably reduces to 240 Hv for 0.07V steel. The results show that the worn surface hardness does not always increase with an increase in the bulk hardness since the strain hardness of the ferrite–pearlite steel is influenced by the strengthening method. Although 0.07V steel is harder than ER7 and ER8 steels at the beginning of the test, it has the lowest worn surface hardness when the test is finished.
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Fig. 4. A typical worn surface micrograph of wheel discs. (a) Overall view and (b) metallic flakes at a high magnification (Region A). This micrograph was observed from the worn surface of ER8 wheel disc.
Fig. 5. Cross-sectional microstructures of (a) ER7, (b) ER8, (c) ER9, (d) HiSi and (e) 0.07V steel.
4. Discussions 4.1. Strengthened mechanisms, microstructure and bulk hardness Microhardness of proeutectoid ferrite and pearlite within the microstructure of examined steels was measured by a Vickers hardness tester with a load of 10 g and the tested results are shown in Fig. 9. It has been reported that the hardness of ferrite– pearlite steel depends on the volume fraction and mechanical properties of the individual phases (proeutectoid ferrite and pearlite) [18,19]. To analyze the influence of compositions on the microstructure, phase diagrams for wheel steels were calculated using Thermo-Calc software. The resulting diagrams are presented in Fig. 10.
For ER7, ER8 and ER9 steels, carbon addition reduces the volume fraction of proeutectoid ferrite through decreasing the A3 temperature, as shown in Fig. 10. Besides, the cementite density within the pearlite can be increased by increasing the carbon content [3]; this improves the pearlite hardness, as shown in Fig. 9. Both of the factors increase the hardness of the ferrite–pearlite steel. Thus, for ER7, ER8 and ER9 steels, the higher carbon content leads to the higher hardness. Silicon is well known as an alloying element favoring ferrite phase by raising the A3 temperature [19]. Silicon content of HiSi steel rises over threefold compared to ER8 steel and thus A3 point is increased from 757 °C for ER8 steel to 769 °C for HiSi steel, as shown in Fig. 10. The proeutectoid ferrite content is therefore significantly increased from 7.6% for ER8 steel to 12.1% for HiSi
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Fig. 6. SEM micrograph of the subsurface of ER8 steel.
Precipitates are not observed within the V free steels, including ER7, ER8, ER9 and HiSi steels. EDX analysis performed on these precipitates identifies them as vanadium carbide, as shown in Fig. 11(c). The microstructural improvement of 0.07V steel can be associated with the presence of precipitates. The precipitates restrict the prior austenite grain size and pearlite colony size. This increases the proeutectoid ferrite content because the grain boundaries present the sites for the nucleation of proeutectoid ferrite. The precipitates within the pearlitic ferrite can refine interlamellar spacing by hindering the growth of cementite. Thus, the higher hardness for 0.07V steel when compared to ER8 steel can be attribute to multiple strengthening methods, including precipitation strengthening, fine grained strengthening and refinement of interlamellar spacing.
600 ER7 ER8 ER9 HiSi 0.07V
Worn surface hardness
500 Strain hardness
Microhardness / HV0.01
550
450 400 350
Bulk hardness 300 250
4.2. Worn surface hardness and wear resistance
0
20
40
60
80 100 Depth / µm
120
140
160
Fig. 7. Microhardness variation of wheel discs as a function of depth from the worn surface.
steel. Although HiSi steel has an obvious higher proeutectoid ferrite content than ER8 and ER9 steels, it exhibits a higher hardness. This is caused by the solid solution strengthening of silicon and manganese. Silicon is a comparatively strong solid solution strengthening element for ferrite. It has been reported that 1% silicon raises yield stress by 81 MPa [35]. Thus HiSi steel exhibits a higher hardness of the proeutectoid ferrite than ER8 and ER9 steels, as shown in Fig. 9. Additionally, ferrite and cementite within the pearlite are hardened by the solid solution strengthening of silicon and manganese, respectively. The solid solution strengthening of manganese is reported to be promoted by a high content of silicon [36]. Thus the pearlite phase within HiSi steel exhibits a higher hardness than that within ER8 steel, as shown in Fig. 9. It can be seen from Fig. 10 that A3 temperature is further increased to 771 °C for 0.07V steel; this also promotes the formation of proeutectoid ferrite. The TEM observation of 0.07V steel shows that a fine dispersion of precipitates is presented in proeutectoid ferrite and pearlitic ferrite, as shown in Fig. 11.
Fig. 12 shows the relationship between the wear loss and the reciprocal of the bulk hardness. For ER7, ER8, ER9 and HiSi steels, the wear loss appears to linearly increase with an increase in the value of (bulk hardness) 1. However, although 0.07V steel exhibits higher bulk hardness than ER7 and ER8 steels, it exhibits the highest wear loss of all examined steels. Previous studies have shown that softer pearlite steel was work hardened more than the initially harder bainite steel during wear process, and thus the worn surface hardness of the pearlite steel is higher than that of bainite steel [8,9]. This leads to a better wear resistance of pearlite steel than that of bainite steel. It seems that the worn surface hardness is the dominating factor in wear resistance of the ferrite–pearlite steel. Therefore, the relationship between the wear loss and the reciprocal of the worn surface hardness is plotted, as shown in Fig. 13. There is almost a linear relationship between the wear loss and the reciprocal of the worn surface hardness. Although 0.07V steel exhibits higher bulk hardness than ER7 and ER8 steels, it has the lowest wear resistance because of the lowest worn surface hardness. Archard wear equation, given by V ¼kWL/H, where V is the wear volume, k the wear coefficient, W the normal load, L the sliding distance and H is the hardness, is a widely used model to estimate the wear loss of metal [37]. It can be seen from Fig. 12 and
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Microhardness / Hv
ER7
ER8
ER9
HiSi
0.07V
300
300
570
290
290
560
280
280
Average value for ER8
550
270
540
260
260
530
250
250
520
240
240
510
230
500
270
Average value for ER8
230 Average bulk hardness
Average strain hardness
Average value for ER8
Average worn surface hardness
Fig. 8. Relationship between (a) bulk hardness, (b) strain hardness and (c) worn surface hardness.
325
Average hardn ess / Hv
300
Proeutectoid ferrite Pearlite
275 250 225 200 175 150 ER7
ER8
ER9
HiSi
0.07V
Fig. 9. Microhardness of proeutectoid ferrite and pearlite within the microstructure.
13 that the wear data do not fit the wear equation since the lines shown on these figures do not go through the origin. Similar observations have also been shown in previous studies that were conducted to assess the wear behavior of the ferrite–pearlite steel [3,12,26,38]. The laboratory and field tests have identified two separate mild and severe wear regimes for railway wheel steel [39–41]. In order to maintain the equation in the different wear regimes, a constant K has been added in the equation, given by V ¼kWL/HþK [42]. It seems that the wear data well fit this modified equation, as shown in Fig. 12 and 13. 4.3. Strengthened mechanisms and worn surface hardness For ER7, ER8 and ER9 steels, the worn surface hardness increases with an increase in the carbon content (Fig. 8c). This can be explained by the following factors: Firstly, the near-surface region of test disc is composed of highly strained proeutectoid ferrite and pearlite. The nanohardness measurement conducted on the near-surface of worn ferrite–pearlite steel has shown that the
Fig. 10. Phase diagrams calculated using Thermo-Calc software. Line I: Fe–(0.4–0.8) C–0.26Si–0.73Mn–0.25Cr (wt%) system. The alloys, with 0.48, 0.52, 0.57 wt% C, correspond to the compositions of ER7, ER8 and ER9 steel, respectively. Line II: Fe– (0.4–0.8)C–0.93Si–0.93Mn–0.04Cr (wt%) system. The alloy, with 0.52 wt% C, corresponds to the composition of HiSi steel. Line III: a Fe–C–Si–Mn–Cr–V system. The alloy, with 0.53 wt% C, corresponds to the composition of 0.07V steel. Phases are indicated as α (ferrite), γ (austenite) and P (pearlite).
strained pearlite exhibits higher hardness value than the strained proeutectoid ferrite [43]. As shown in Fig. 5, carbon addition increases the volume fraction of the strained pearlite; this increases the worn surface hardness of test disc. Besides, an increment in the cementite density within the pearlite has been reported to promote the strain-hardening by increasing the amount of dislocation in pearlite ferrite and promoting the grain refinement of the pearlite ferrite [3]. Carbon addition increases the cementite density within the pearlite and therefore promotes the strain-hardening during wear testing, as shown in Fig. 8b.
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Precipitates
69
Precipitates
Intensity / a.u.
Fe
V 4
5
Cr
Fe
6 7 Energy / keV
8
9
Fig. 11. Transmission electron micrograph of 0.07V steel. Precipitates (a) at proeutectoid ferrite and (a) between cementite lamellae. (c) EDX of precipitates in (a).
400
400
380
ER8 ER9
360
HiSi 0.07V
340
380 360 Wear loss / mg
Wear loss / mg
ER7
320 300
340
ER7 ER8 ER9 HiSi 0.07V
320 300 280 40
280
20
20
0 1.20
0 1.5
1.6
1.7
1.8 3.4 3.5 3.6 (Bulk hardness)-1/ (GPa)-1
3.7
3.8
3.9
1.25
1.30 1.35 1.80 1.85 1.90 (Worn surface hardness)-1 / (GPa)-1
1.95
2.00
Fig. 13. Relationship between wear loss and microhardness of worn surface.
Fig. 12. Relationship between wear loss and bulk hardness.
Previous studies have shown that the accumulation of plastic deformation begins earlier and increases at a higher rate in the proeutectoid ferrite than in the pearlite [43,44]. Although the proeutectoid ferrite has a greater initial ductility than pearlite, it reaches the ductility exhaustion earlier than pearlite. When the ductility exhaustion occurs, the surface crack would initiate along the highly strained proeutectoid ferrite, as shown in Fig. 5. Proeutectoid ferrite with better ductility can sustain more plastic deformation before crack initiation, and therefore the refinement of interlamellar spacing (shown in Fig. 6) would be more remarkable when the critical plastic deformation of proeutectoid ferrite is reached. This would lead to a higher worn surface hardness according to a Hall–Petch relationship with interlamellar spacing that hardness of pearlite phase is in inversely proportion to the square root of the spacing [2,45].
Compared with ER8 steel, the proeutectoid ferrite within HiSi steel and 0.07V steel is hardened due to the solid solution strengthening induced by silicon and manganese addition and precipitation strengthening induced by vanadium addition, respectively. However, it has been reported that the ductility of ferrite would be reduced by both of the strengthening methods [26,27,46,47]. The proeutectoid ferrite within HiSi and 0.07V steels would reach the ductility exhaustion earlier than that within ER8 steel. HiSi and 0.07V steels would therefore strain harden less than ER8 steel, as shown in Fig. 8(b). Silicon and manganese have relatively small effect on the ductility of the steels when their individual contents are less than 1% [46]. However, precipitates shown in Fig. 11 could lead to the generation of residual stress on the ferrite; this can remarkably decrease the ductility of ferrite [26,47]. Therefore, the degree of strain-hardening on the worn surface of 0.07V steel is less than that of HiSi steel, as shown in Fig. 8(b).
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Wear resistance of ferrite–pearlite steel depends on the worn surface hardness (Fig. 13) that can be influenced by both bulk hardness and strain-hardening, as shown in Fig. 8. Microstructure features can influence the wear resistance through imposing the effect on the strain-hardening of worn surface. For ER7, ER8, ER9 and HiSi steels, the difference that microstructure features make to the strain-hardening is relatively small, in a comparison to 0.07V steel. Therefore, except for the 0.07V steel, the wear resistance appears to be proportional to the (bulk hardness) 1, as shown in Fig. 12. The analysis shows that the strengthening methods can influence the wear resistance of ferrite–pearlite steel by influencing the strain-hardening. An appropriate wear model must consider the effect of strain-hardening, given that the 0.07V steel, which is harder than ER7 and ER8 steels at the beginning of the tests, exhibits the lowest wear resistance. This would lead to more reasonable predictions of rolling/sliding wear loss.
5. Conclusions Wear resistance of ferrite–pearlite steels which were hardened by different strengthening methods was tested under rolling/ sliding condition. The influence of strengthening methods, including carbon addition, solid solution strengthening and precipitation strengthening, on wear resistance was investigated and the following conclusions were drawn from this investigation. (1) Wear resistance of ferrite–pearlite steel is improved by both carbon addition and solid solution strengthening, whereas it is deteriorated by precipitation strengthening. (2) Wear resistance of ferrite–pearlite steel depends on the worn surface hardness that can be influenced by both bulk hardness and strain-hardening. (3) Strengthening methods can increase the bulk hardness of ferrite–pearlite steel to different extents, where the highest and lowest bulk hardness increment is obtained by the solid solution and precipitation strengthening, respectively. (4) Strain hardening is promoted by carbon addition, while it is reduced by solid solution strengthening and precipitation strengthening where precipitation strengthening steel has a greater reduction in strain-hardening. (5) Strain hardening of ferrite–pearlite steel is reduced by a high content of proeutectoid ferrite with a low ductility, which is caused by solid solution and precipitation strengthening.
Acknowledgments This work was supported by Basic Research Association Foundation of High–speed Rail (U 1134202 and U 1334206), the 2014 Doctoral Innovation Funds of Southwest Jiaotong University and the Fundamental Research Funds for the Central Universities.
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