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Effects of thermal and thermomechanical treatments on sliding wear of graphite crystallised white cast iron
Wear 301 (2013) 656–662
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Effects of thermal and thermomechanical treatments on sliding wear of graphite crystallised white cast iron X.J. Gao a, Q. Zhang a, D.B. Wei a, S.H. Jiao b, Z.Y. Jiang a,n a b
School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong NSW 2522, Australia Baosteel Research Institute, Baoshan Iron & Steel Co. Ltd., Shanghai, China
a r t i c l e i n f o
a b s t r a c t
Available online 23 November 2012
The effects of thermal and thermomechanical treatments on sliding wear of graphite crystallised white cast iron (GWCI) were studied. Due to the inherent embrittlement of GWCI, a laminated metal in which the GWCI is cladded by low carbon steel was prepared for this study. Three cylindrical samples (GWCI-A, GWCI-B and GWCI-C) were machined from the same laminate. GWCI-A was kept in as-cast state while GWCI-B and GWCI-C underwent the thermal and thermomechanical treatments, respectively. The pin-ondisc type sliding wear tests were performed on the GWCI layers at room temperature. The microstructures and wear mechanisms were analysed by optical microscope, scanning electron microscope and the Vickers hardness test. Experimental results demonstrated that the GWCI, after laminating with ductile steel, can be deformed at high temperature with crack-free. The thermomechanical treatment produced a finer microstructure and crushed primary carbides in GWCI-C. Both GWCI-B and GWCI-C displayed plenty of secondary carbides in supercooled austenitic matrix, which was more favourable to squeeze the graphite and form the oxide layers than the matrix of martensite plus retained austenite in GWCI-A. The wear resistance of GWCI-C was superior to that of GWCI-A and GWCI-B because the oxidational wear rather than delamination dominated the sliding wear process. & 2012 Elsevier B.V. All rights reserved.
Keywords: Thermomechanical treatment Graphite crystallised white cast iron Sliding wear Laminated metal
1. Introduction The so-called graphite crystallised white cast iron (GWCI) [1], which is different from the general graphite-free white cast iron (WCI), can be regarded as a composite consisting of carbides, ferrous matrix and graphite phase. On one hand, GWCI still possesses good hardness (450–650 HV in as-cast state) and enhanced wear resistance due to the high volume fraction of hard carbides embedded in a relatively ductile ferrous matrix [2–4], although the toughness of the matrix also contributes to the wear resistance [5]. On the other hand, the graphite phase is expected to work as a solid lubricant in the cast iron attributing to its low hardness and easily slipped [6,7]. Therefore, the development of GWCI may make an excellent combination of wear resistance and lubricity for sliding wear [1]. However, as the same as the general WCI, GWCI also exhibits severe brittleness because of the carbides, which are very brittle and distributed in a netlike form [8] or a rather massive form [9]. Both of these forms are detrimental to the continuity of the matrix. This brittleness not only limits the range of applications of the cast irons but also
n
Corresponding author. Tel.: þ61 2 4221 4545; fax: þ 61 2 4221 5474. E-mail address: [email protected] (Z.Y. Jiang).
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.11.019
impedes the development of techniques that committed to improving their mechanical properties. Extensive research has been carried out to improve the properties of WCI while maintaining a relatively high volume fraction of hard carbides. Most of these studies attempted to modify the characteristic of the microconstituents (i.e. ferrous matrix, carbides and graphite phase) including the type, size, morphology and distribution as well as orientation through the methods of heat treatment [10], alloying [11,12] or spray casting [13]. With varying and limited degrees of success, these previous studies have found that both a refined microstructure and less interconnected carbides can improve the toughness and wear resistance of WCI [10–13]. However, few investigations were implemented to produce the fine microstructure by hot working WCI because of its embrittlement. This work conducted a study on the sliding wear resistance of GWCI to determine the influence of the modification on its microconstituents by applying thermal and thermomechanical treatments. In order to achieve the deformation of GWCI without any crack occurred, a sandwich-structure laminated metal, in which the GWCI is surrounded by low carbon steel (LCS), was fabricated by composite casting. Three specimens were machined from the same laminated metal, and after undergoing different treatments, the cast iron layer with different microstructures was subjected to pin-on-disc type sliding wear test.
X.J. Gao et al. / Wear 301 (2013) 656–662
2. Experimental procedure In this study, the chemical composition of GWCI is listed in Table 1 and LCS contains about 0.2 wt% C. To begin with, the GWCI and the LCS were composite cast into a sandwich structural laminated metal with the cast iron as the core. During solidification, the laminated metal billet was cooled by air down to room temperature. Three cylindrical samples of 12 mm diameter and 14 mm length were wire cut from the same billet. One sample was dubbed GWCI-A and it was kept in as-cast state. The second sample named GWCI-B was heated up to 1323 K at a heating rate of 20 K/s. Then it was held for 480 s and quenched by water jet. Significantly, the third sample called GWCI-C underwent a similar thermal treatment as GWCI-B, except that hot compression was conducted at a strain rate of 1/s with 40% of overall reduction prior to water quench. Both the thermal treatment of GWCI-B and the thermomechanical treatment of GWCI-C were performed on a Gleeble-3500 thermomechanical simulator, which is a fully integrated digital closed loop control thermal and cooling as well as mechanical testing system [14]. Vickers macrohardness was measured on polished samples by using 10 kgf load for 15 s. The microstructure of the samples was examined by optical microscope (OM) and scanning electron microscope (SEM) after etching with 2.5% Nital solution. The phase constituent of the cast iron layer was determined by X-ray diffractometer (XRD) using monochromated Cu Ka radiation. The pin-on-disc configuration, which has the advantage of having a simple set-up and a low testing cost [15], was used to measure the friction and sliding wear properties of the GWCI. Although this technique does not attempt to describe all the conditions that may be experienced in service [16], it has proved useful in providing valuable information on the tribological behaviour of the sliding wear system [17–19]. Therefore, the aim of this work is the comparative study of the same GWCI with Table 1 Chemical compositions of GWCI (wt%). C
Si
Mn
Cr
Ni
Mo
Cu
P
2.0
0.990
0.778
1.50
4.80
0.270
0.074
0.023
657
different microstructures rather than the replication of a real applied condition. The specimens for pin-on-disc wear test were produced by machining the samples of GWCI-A, GWCI-B and GWCI-C to reveal the fresh surface of the cast iron layer. According to the requirement of CETR UMT pin-on-disc apparatus, the specimens were mounted in polyester resin in a stainless steel stud with a diameter of 50 mm and then played the role of disc part, as shown in Fig. 1. Spherical pin of 6.35 mm diameter which is made of AISI E52100 steel with hardness of 890 HV was used in the test. Dry sliding wear test was carried out at room temperature and room humidity. The test methodology followed the guidelines presented in ASTM G99-05 standard [16]. The disc surface was polished to a roughness level of 1 mm and cleaned up with ethanol before testing. A normal load of 40 N was applied as deadweight to press the pin wear against the rotating disc plate, while the rotating velocity was 191 rpm and the duration time was 7.5 min. The average diameter and sliding distance of wear track were 7.799 mm and 35 m respectively in each wear test. During each test, the variation of the friction coefficient was recorded as a function of sliding distance. Wear resistance and wear mechanism were obtained by evaluating the wear tracks with OM and SEM. Ferrous matrix and wear surface of the test specimens were also analysed by means of microhardness using a Vickers hardness tester, model M-400-H1, with a load of 500 g for 10 s. Microhardness values are the average of 15 measurements.
3. Results and discussion 3.1. Microstructures Fig. 2(a), (b) and (c) shows the cross-sectional microstructure of GWCI-A, GWCI-B and GWCI-C, respectively. The differences in contrast and microstructure between the GWCI and the LCS clearly reveal the interface. In this study, laminating the steel to the cast iron is aiming to achieve the deformation of the brittle GWCI. Previous research of metal matrix composites has demonstrated that laminating a soft material to a brittle material can significantly improve the formability or ductility for the latter one
Load Stainless steel stud
Friction/Load force sensor Specimen Pin holder
Sample table
with pin Disc
Polyester resin
Fig. 1. Schematic diagram of pin-on-disc wear tester (a) and photograph of test sample (b).
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Interface GWCI
LCS LCS
Interface
GWCI
GWCI
Interface
LCS
Fig. 2. SEM micrographs of the bonding interface of the test samples: (a) GWCI-A, (b) GWCI-B, (c) GWCI-C.
[20,21]. While on the other hand, the interfacial bonding quality is an important factor which affects the formability and overall quality of laminated composite [22]. As can be seen in Fig. 2, it is considered that, for all the three cases, the interface has a good bonding quality between the GWCI and the LCS since no defect or discontinuity such as void, hole and lack of bonding was detected along the interface, although the interface morphology was varied after different treatments. Moreover, GWCI-C was successfully deformed by compressing the laminated metal billet with 40% of reduction. There was no crack occurred event it is very brittle, as shown in Fig. 2(c). The detailed microstructures of the cast iron layer in the three test samples are shown in Fig. 3. First of all, Fig. 3(e) and (f) points out once again that no defects like micro cracks can be found in GWCI-C. The mechanism for this improvement requires further investigation. The evolution of the microstructure is noticeable. In GWCI-A, as shown in Fig. 3(a) and (b), the matrix is multiphase and characterised by the typical microstructure of WCI, i.e., by the presence of needle shaped martensite and retained austenite. The netlike carbides divide the matrix greatly and weaken its continuity, while the graphite distributes randomly with anomalous shape. Significant changes took place in GWCI-B and GWCIC. Fig. 3(c) and (d) represents microstructural features of GWCI-B. It is observed that the matrix turned to single phase and XRD results indicated that it is supercooled austenite. The continuity of the matrix increased clearly because the island structure of the primary carbides substituted the netlike structure. Moreover, the secondary carbides precipitated from carbon-enriched austenite, as shown in Fig. 3(d). There is little difference in the distribution of the graphite whereas, more or less, its morphology became a regular shape like cotton pellet. The effect of thermomechanical treatment on microstructure of GWCI-C is shown in Fig. 3(e) and (f). Although the matrix of GWCI-C is also a single phase as well as GWCI-B, the smaller austenite grains and many sub-grains were produced because of hot working. The primary carbides were crushed and meanwhile lots of secondary carbides
were detected. After compression, the ‘‘cotton pellet’’ was flattened somewhat. It is interesting to note that martensite was totally substituted by supercooled austenite when the samples underwent the thermal and thermomechanical treatments. This significant change can be explained by the metallography principle. As shown in Table 1, the alloying elements such as Si (0.99%), Mn (0.778%), Ni (4.8%) and Mo (0.27%) have an effect on the stabilisation of the supercooled austenite by right shifting the CCT curve and lowering the martensite transition temperature [23]. There perhaps has another reason presented here even it needs a further confirmation. As is well known, the specific volume usually expands dramatically when the matrix changes from austenite to martensite, because austenite has the smallest specific volume in all kinds of ferrous matrixes [23]. Therefore, very fast cooling rate ( 150 K/s) resulting from water quench probably generates a certain of isotropic compressive stress in the samples, and this stress may restrain the martensite transition by hindering the expansion of the specific volume. Hot compression on GWCI-C may encourage this resistance significantly. However, there is no doubt that the supercooled austenite in GWCI-B and GWCI-C took the place of the martensite in GWCI-A will dramatically decrease the macrohardness of the material. Because the hardness of supercooled austenite is well lower than that of martensite [23]. Test results show that the bulk hardness of GWCI-A, GWCI-B and GWCI-C are 455 710 HV10, 255 710 HV10 and 275710 HV10, respectively. According to the research of Gahr and Eldis [24] and Moore [25], for each structure the wear tends to decrease as the hardness increase, while for different structures the matrix may have a greater influence on wear resistance than the bulk hardness. Therefore, making an estimation of the wear resistance for the current samples in sliding wear condition is premature. Generally, WCI is graphite-free because the relatively high nominal concentration of chromium helps to prevent the formation of graphite [26]. However, this phase emerges when the ratio of chromium-to-silicon (Cr/Si) is less than 2.7 [27]. For the current GWCI, the value of Cr/Si is about 1.515.
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Primary carbides
659
Martensite
Retained austenite Graphite 50μm
Primary carbides Secondary carbides
Graphite 50μm
Secondary carbides
Refined carbides 50μm Fig. 3. OM (a, c and e) and SEM (b, d and f) micrographs showing microstructures in cast iron layers: (a, b) GWCI-A, (c, d) GWCI-B, (e, f) GWCI-C.
3.2. Wear response Observation of the wear tracks reveals significant differences among the three specimens as shown in Fig. 4. The OM micrographs of Fig. 4(a), (c) and (e) clearly show the squeezing of graphite on the sliding tracks in all the three cases. Such squeezing was caused by wearing of the graphite which emerged on the sliding surface [28]. Fig. 5 shows the average area ratio of squeezing graphite-to-wear track (AR %) by measuring five images for each specimen using the image analysis technique. The results show that the quantity of squeezing graphite increased when the samples underwent the thermal and thermomechanical treatments. This increase indicates a softer matrix is more favourable to squeeze graphite. As a solid lubricant in the cast iron, the increase of squeezing graphite has an important influence on the friction coefficient between the sliding pair, and finally affects the wear rate of the samples [28]. In addition, deep scratches were found in GWCI-A and GWCI-B but not in GWCI-C. The reason for this difference may be related to the evolution of the microhardness of the wear surface. The details will be discussed later in Fig. 7. In Fig. 4(b), (d) and (f), to varying degree, the surfaces were grooved, delaminated and covered with oxide layers. The extent of delamination was steadily lessen on the sliding surface of GWCI-B compared with that of GWCI-A. After hot working, the
delamination became less and finally lost its dominance in GWCI-C. Inversely, the oxide region grew fast in breadth and thickness when the microconstituents were modified by employing the treatments, especially by the thermomechanical treatment. This evolvement indicates the wear mechanism may be transformed from mechanical peeling to oxidational wear. Based on the research of Quinn [29], the oxide layer prevents intermetallic contact and adhesion between the sliding pair, which results in an oxide-metal contact or an oxide–oxide contact dominating the sliding process. Such protection makes the wear turning to mild and lowers the metal loss. Surface smearing was also observed in Fig. 4(b) but not in Fig. 4(d) and (f). Normally, a softer material is prone to smearing rather than a harder one [30,31]. The situation seems discrepant in this study. Taking the oxidational wear and hardness evolution into consider, two hypotheses may respond to this issue. One is the smeared surface was covered by oxide layers, while the other one may be the wear surface of the treated specimens has a higher microhardness than that of the untreated specimen. In order to verify these hypotheses, the wear tracks were analysed by SEM in a higher magnification (see Fig. 6) and the microhardness of the wear surface was measured using a Vickers hardness tester (see Fig. 7). Fig. 6(a), (b) and (c) shows the wear track of GWCI-A, GWCI-B and GWCI-C, respectively. The results show that
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Peeling Deep scratch Surface smearing
Oxide layer
Squeezing of graphite
200μm
Peeling
Squeezing of graphite Oxide layer
Deep scratch
200μm
Oxide layer Peeling Squeezing of graphite
200μm Fig. 4. OM (a, c and e) and SEM (b, d and f) micrographs showing wear tracks of the test specimens: (a, b) GWCI-A, (c, d) GWCI-B, (e, f) GWCI-C.
30 25
AR %
20 15 10 5 0
GWCI-A
GWCI-B
GWCI-C
Test sample Fig. 5. Average area ratio of squeezing graphite-to-wear track.
the surface smearing not only occurred in GWCI-A but also in GWCI-B. However, as shown in Fig. 6(b), the oxide layer on the wear surface of GWCI-B covered most of the area of the metal surface so that the surface smearing was more difficult to detect.
Fig. 7 shows the microhardness of the matrix in free-sliding regions and the metallic areas on sliding surface where was not covered by the oxide scale. The microhardness of the matrix is in agreement with the bulk hardness of the samples. While on the wear surface, it can be seen that a significant increase in microhardness took place because of sliding. Such an increase is due to strain hardening, and different strain hardening capacities in different samples result in different increments. As previously mentioned, the matrix structures in GWCI-A, GWCI-B and GWCI-C are martensite plus retained austenite, supercooled austenite and deformed supercooled austenite, respectively. Therefore, GWCI-C possesses the highest strain hardening capacity, which results in a higher microhardness on its wear surface than that of the other two samples. Thus the excellent scratch resistance and surface smearing resistance can be obtained in GWCI-C, while the deep scratches and surface smearing were found in GWCI-A and GWCI-B, as shown in Figs. 4 and 6. Fig. 8 shows the friction coefficient as a function of the sliding distance for three of the GWCI with different microstructures. At the early stage of sliding, the value of friction coefficient increased quickly to a peak in all the three cases. This increase attributes to direct metal on metal contact which probably caused adhesion. The sequence of peak values decreasing from GWCI-B to GWCI-A through GWCI-C is in line with the bulk hardness
X.J. Gao et al. / Wear 301 (2013) 656–662
661
Surface smearing
Peeling Oxide layer Surface smearing Oxide layer
Metal surface Oxide layer
Fig. 6. SEM micrographs showing wear tracks of GWCI-A (a), GWCI-B (b) and GWCI-C (c).
1.0
1000 Matrix Wear surface
900
0.9
Friction Coefficient
Microhardness (HV)
700 600 500 400 300
0.7 0.6
GWCI-B
GWCI-A
0.5 0.4 0.3 0.2
200
0.1
100 0
GWCI-C
0.8
800
0.0
GWCI-A
GWCI-B
GWCI-C
Test sample
0
3
6
9
12
15
18
21
24
27
30
33
36
Sliding Distance (m) Fig. 8. Friction coefficient of the GWCI specimens with sliding distance.
Fig. 7. Microhardness of the matrix and the wear surface of the test samples.
increasing sequence. After a running-in stage, the sliding wear process became stable when the sliding distance is over 12 m. From this point, the slopes of friction coefficient curves of GWCI-B and GWCI-C were negative while that of GWCI-A was positive. Reversed tendency suggested wear conditions including contact state and wear mechanism were starting to change. With the help of the squeezing graphite and the formation of oxide layers, friction coefficients of GWCI-B and GWCI-C became lower than that of GWCI-A at the sliding distance of 24 m and 28 m respectively, as indicated by the arrows in Fig. 8.
4. Conclusions In this paper, the sliding wear behaviour of GWCIs with and without thermal or thermomechanical treatment was studied by pin-on-disc type unidirectional sliding wear tests. A soft LCS was
cladded to the brittle GWCI so that the hot compression on the cast iron can be carried out with crack-free. With a sound bonding interface, the GWCI layers in as-cast state (GWCI-A), thermal treatment state (GWCI-B) and thermomechanical treatment state (GWCI-C) were sliding worn by a chrome steel spherical pin. The development of microstructure and wear behaviour can be summarised as follows: 1. Both GWCI-B and GWCI-C displayed a supercooled austenitic matrix and plenty of secondary carbides because of the thermal and thermomechanical treatments, respectively. A finer microstructure and crushed primary carbides were obtained in GWCI-C due to hot working. 2. The matrix of supercooled austenite in GWCI-B and GWCI-C was more favourable to squeeze the graphite phase and form the oxide layers than the matrix of martensite plus retained austenite in GWCI-A.
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3. Wear of GWCI-A with higher bulk hardness was incurred by delamination and surface smearing. While for GWCI-B and GWCI-C with lower bulk hardness, the oxidational wear dominated the sliding process. This evolution leads to the conclusion that the matrix microstructure plays a more important role in controlling wear loss than the bulk hardness. 4. Strain hardening occurred on the wear surface in all the three cases, which to a more extend in GWCI-C. Such wear induced hardening may also accompany with the occurrence of martensite transition in the subsurface that requires further investigations.
Acknowledgements The first author wishes to thank the China Scholarship Council (CSC) for supporting the current research work. References [1] S. Aso, S. Goto, Y. Komatsu, W. Hartono, Sliding wear of graphite crystallised chromium white cast iron, Wear 250 (2001) 511–517. [2] H. Berns, Comparison of wear resistant MMC and white cast iron, Wear 254 (2003) 47–54. [3] D.Z. Guo, L.J. Wang, J.Z. Li, Erosive wear of low chromium white cast iron, Wear 161 (1993) 173–178. [4] H. Li, C.F. Burdett, Y.M. Wang, Solidification characteristics of atomized white cast iron powders, Scripta Materialia 29 (1993) 249–254. [5] O.N. Dogan, J.A. Hawk, G. Laird, Solidification structure and abrasion resistance of high chromium white irons, Metallurgical and Materials Transactions A 28 (1997) 1315–1328. [6] Y. Wang, Z.Y. Pan, W. Zheng, X.G. Sun, W. Liang, Sliding wear behaviour of Cr–Mo–Cu alloy cast irons with and without nano-additives, Wear 271 (2011) 2953–2962. [7] A. Rac, Influence of load and speed on wear characteristics of grey cast iron in dry sliding-selection for minimum wear, Tribology International 18 (1985) 29–33. [8] L.M. Chang, L. Liu, J.H. Liu, Properties of cross-rolled low alloy white cast iron grinding ball, Journal of Iron and Steel Research International 14 (2007) 47–51. [9] H. Li, C.F. Burdett, Y.M. Wang, Superplastic behaviour of fine-grained white cast irons, Journal of Materials Science 21 (1993) 5901–5908. [10] X. Chen, Y.X. Li, Effect of heat treatment on microstructure and mechanical properties of high boron white cast iron, Materials Science and Engineering: A 528 (2010) 770–775. [11] A.B. Jacuinde, R. Correa, I. Mejia, J.G. Quezada, W.M. Rainforth, The effect of titanium on the wear behaviour of a 16%Cr white cast iron under pure sliding, Wear 263 (2007) 808–820.
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Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Effects of thermal and thermomechanical treatments on sliding wear of graphite crystallised white cast iron X.J. Gao a, Q. Zhang a, D.B. Wei a, S.H. Jiao b, Z.Y. Jiang a,n a b
School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong NSW 2522, Australia Baosteel Research Institute, Baoshan Iron & Steel Co. Ltd., Shanghai, China
a r t i c l e i n f o
a b s t r a c t
Available online 23 November 2012
The effects of thermal and thermomechanical treatments on sliding wear of graphite crystallised white cast iron (GWCI) were studied. Due to the inherent embrittlement of GWCI, a laminated metal in which the GWCI is cladded by low carbon steel was prepared for this study. Three cylindrical samples (GWCI-A, GWCI-B and GWCI-C) were machined from the same laminate. GWCI-A was kept in as-cast state while GWCI-B and GWCI-C underwent the thermal and thermomechanical treatments, respectively. The pin-ondisc type sliding wear tests were performed on the GWCI layers at room temperature. The microstructures and wear mechanisms were analysed by optical microscope, scanning electron microscope and the Vickers hardness test. Experimental results demonstrated that the GWCI, after laminating with ductile steel, can be deformed at high temperature with crack-free. The thermomechanical treatment produced a finer microstructure and crushed primary carbides in GWCI-C. Both GWCI-B and GWCI-C displayed plenty of secondary carbides in supercooled austenitic matrix, which was more favourable to squeeze the graphite and form the oxide layers than the matrix of martensite plus retained austenite in GWCI-A. The wear resistance of GWCI-C was superior to that of GWCI-A and GWCI-B because the oxidational wear rather than delamination dominated the sliding wear process. & 2012 Elsevier B.V. All rights reserved.
Keywords: Thermomechanical treatment Graphite crystallised white cast iron Sliding wear Laminated metal
1. Introduction The so-called graphite crystallised white cast iron (GWCI) [1], which is different from the general graphite-free white cast iron (WCI), can be regarded as a composite consisting of carbides, ferrous matrix and graphite phase. On one hand, GWCI still possesses good hardness (450–650 HV in as-cast state) and enhanced wear resistance due to the high volume fraction of hard carbides embedded in a relatively ductile ferrous matrix [2–4], although the toughness of the matrix also contributes to the wear resistance [5]. On the other hand, the graphite phase is expected to work as a solid lubricant in the cast iron attributing to its low hardness and easily slipped [6,7]. Therefore, the development of GWCI may make an excellent combination of wear resistance and lubricity for sliding wear [1]. However, as the same as the general WCI, GWCI also exhibits severe brittleness because of the carbides, which are very brittle and distributed in a netlike form [8] or a rather massive form [9]. Both of these forms are detrimental to the continuity of the matrix. This brittleness not only limits the range of applications of the cast irons but also
n
Corresponding author. Tel.: þ61 2 4221 4545; fax: þ 61 2 4221 5474. E-mail address: [email protected] (Z.Y. Jiang).
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.11.019
impedes the development of techniques that committed to improving their mechanical properties. Extensive research has been carried out to improve the properties of WCI while maintaining a relatively high volume fraction of hard carbides. Most of these studies attempted to modify the characteristic of the microconstituents (i.e. ferrous matrix, carbides and graphite phase) including the type, size, morphology and distribution as well as orientation through the methods of heat treatment [10], alloying [11,12] or spray casting [13]. With varying and limited degrees of success, these previous studies have found that both a refined microstructure and less interconnected carbides can improve the toughness and wear resistance of WCI [10–13]. However, few investigations were implemented to produce the fine microstructure by hot working WCI because of its embrittlement. This work conducted a study on the sliding wear resistance of GWCI to determine the influence of the modification on its microconstituents by applying thermal and thermomechanical treatments. In order to achieve the deformation of GWCI without any crack occurred, a sandwich-structure laminated metal, in which the GWCI is surrounded by low carbon steel (LCS), was fabricated by composite casting. Three specimens were machined from the same laminated metal, and after undergoing different treatments, the cast iron layer with different microstructures was subjected to pin-on-disc type sliding wear test.
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2. Experimental procedure In this study, the chemical composition of GWCI is listed in Table 1 and LCS contains about 0.2 wt% C. To begin with, the GWCI and the LCS were composite cast into a sandwich structural laminated metal with the cast iron as the core. During solidification, the laminated metal billet was cooled by air down to room temperature. Three cylindrical samples of 12 mm diameter and 14 mm length were wire cut from the same billet. One sample was dubbed GWCI-A and it was kept in as-cast state. The second sample named GWCI-B was heated up to 1323 K at a heating rate of 20 K/s. Then it was held for 480 s and quenched by water jet. Significantly, the third sample called GWCI-C underwent a similar thermal treatment as GWCI-B, except that hot compression was conducted at a strain rate of 1/s with 40% of overall reduction prior to water quench. Both the thermal treatment of GWCI-B and the thermomechanical treatment of GWCI-C were performed on a Gleeble-3500 thermomechanical simulator, which is a fully integrated digital closed loop control thermal and cooling as well as mechanical testing system [14]. Vickers macrohardness was measured on polished samples by using 10 kgf load for 15 s. The microstructure of the samples was examined by optical microscope (OM) and scanning electron microscope (SEM) after etching with 2.5% Nital solution. The phase constituent of the cast iron layer was determined by X-ray diffractometer (XRD) using monochromated Cu Ka radiation. The pin-on-disc configuration, which has the advantage of having a simple set-up and a low testing cost [15], was used to measure the friction and sliding wear properties of the GWCI. Although this technique does not attempt to describe all the conditions that may be experienced in service [16], it has proved useful in providing valuable information on the tribological behaviour of the sliding wear system [17–19]. Therefore, the aim of this work is the comparative study of the same GWCI with Table 1 Chemical compositions of GWCI (wt%). C
Si
Mn
Cr
Ni
Mo
Cu
P
2.0
0.990
0.778
1.50
4.80
0.270
0.074
0.023
657
different microstructures rather than the replication of a real applied condition. The specimens for pin-on-disc wear test were produced by machining the samples of GWCI-A, GWCI-B and GWCI-C to reveal the fresh surface of the cast iron layer. According to the requirement of CETR UMT pin-on-disc apparatus, the specimens were mounted in polyester resin in a stainless steel stud with a diameter of 50 mm and then played the role of disc part, as shown in Fig. 1. Spherical pin of 6.35 mm diameter which is made of AISI E52100 steel with hardness of 890 HV was used in the test. Dry sliding wear test was carried out at room temperature and room humidity. The test methodology followed the guidelines presented in ASTM G99-05 standard [16]. The disc surface was polished to a roughness level of 1 mm and cleaned up with ethanol before testing. A normal load of 40 N was applied as deadweight to press the pin wear against the rotating disc plate, while the rotating velocity was 191 rpm and the duration time was 7.5 min. The average diameter and sliding distance of wear track were 7.799 mm and 35 m respectively in each wear test. During each test, the variation of the friction coefficient was recorded as a function of sliding distance. Wear resistance and wear mechanism were obtained by evaluating the wear tracks with OM and SEM. Ferrous matrix and wear surface of the test specimens were also analysed by means of microhardness using a Vickers hardness tester, model M-400-H1, with a load of 500 g for 10 s. Microhardness values are the average of 15 measurements.
3. Results and discussion 3.1. Microstructures Fig. 2(a), (b) and (c) shows the cross-sectional microstructure of GWCI-A, GWCI-B and GWCI-C, respectively. The differences in contrast and microstructure between the GWCI and the LCS clearly reveal the interface. In this study, laminating the steel to the cast iron is aiming to achieve the deformation of the brittle GWCI. Previous research of metal matrix composites has demonstrated that laminating a soft material to a brittle material can significantly improve the formability or ductility for the latter one
Load Stainless steel stud
Friction/Load force sensor Specimen Pin holder
Sample table
with pin Disc
Polyester resin
Fig. 1. Schematic diagram of pin-on-disc wear tester (a) and photograph of test sample (b).
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Interface GWCI
LCS LCS
Interface
GWCI
GWCI
Interface
LCS
Fig. 2. SEM micrographs of the bonding interface of the test samples: (a) GWCI-A, (b) GWCI-B, (c) GWCI-C.
[20,21]. While on the other hand, the interfacial bonding quality is an important factor which affects the formability and overall quality of laminated composite [22]. As can be seen in Fig. 2, it is considered that, for all the three cases, the interface has a good bonding quality between the GWCI and the LCS since no defect or discontinuity such as void, hole and lack of bonding was detected along the interface, although the interface morphology was varied after different treatments. Moreover, GWCI-C was successfully deformed by compressing the laminated metal billet with 40% of reduction. There was no crack occurred event it is very brittle, as shown in Fig. 2(c). The detailed microstructures of the cast iron layer in the three test samples are shown in Fig. 3. First of all, Fig. 3(e) and (f) points out once again that no defects like micro cracks can be found in GWCI-C. The mechanism for this improvement requires further investigation. The evolution of the microstructure is noticeable. In GWCI-A, as shown in Fig. 3(a) and (b), the matrix is multiphase and characterised by the typical microstructure of WCI, i.e., by the presence of needle shaped martensite and retained austenite. The netlike carbides divide the matrix greatly and weaken its continuity, while the graphite distributes randomly with anomalous shape. Significant changes took place in GWCI-B and GWCIC. Fig. 3(c) and (d) represents microstructural features of GWCI-B. It is observed that the matrix turned to single phase and XRD results indicated that it is supercooled austenite. The continuity of the matrix increased clearly because the island structure of the primary carbides substituted the netlike structure. Moreover, the secondary carbides precipitated from carbon-enriched austenite, as shown in Fig. 3(d). There is little difference in the distribution of the graphite whereas, more or less, its morphology became a regular shape like cotton pellet. The effect of thermomechanical treatment on microstructure of GWCI-C is shown in Fig. 3(e) and (f). Although the matrix of GWCI-C is also a single phase as well as GWCI-B, the smaller austenite grains and many sub-grains were produced because of hot working. The primary carbides were crushed and meanwhile lots of secondary carbides
were detected. After compression, the ‘‘cotton pellet’’ was flattened somewhat. It is interesting to note that martensite was totally substituted by supercooled austenite when the samples underwent the thermal and thermomechanical treatments. This significant change can be explained by the metallography principle. As shown in Table 1, the alloying elements such as Si (0.99%), Mn (0.778%), Ni (4.8%) and Mo (0.27%) have an effect on the stabilisation of the supercooled austenite by right shifting the CCT curve and lowering the martensite transition temperature [23]. There perhaps has another reason presented here even it needs a further confirmation. As is well known, the specific volume usually expands dramatically when the matrix changes from austenite to martensite, because austenite has the smallest specific volume in all kinds of ferrous matrixes [23]. Therefore, very fast cooling rate ( 150 K/s) resulting from water quench probably generates a certain of isotropic compressive stress in the samples, and this stress may restrain the martensite transition by hindering the expansion of the specific volume. Hot compression on GWCI-C may encourage this resistance significantly. However, there is no doubt that the supercooled austenite in GWCI-B and GWCI-C took the place of the martensite in GWCI-A will dramatically decrease the macrohardness of the material. Because the hardness of supercooled austenite is well lower than that of martensite [23]. Test results show that the bulk hardness of GWCI-A, GWCI-B and GWCI-C are 455 710 HV10, 255 710 HV10 and 275710 HV10, respectively. According to the research of Gahr and Eldis [24] and Moore [25], for each structure the wear tends to decrease as the hardness increase, while for different structures the matrix may have a greater influence on wear resistance than the bulk hardness. Therefore, making an estimation of the wear resistance for the current samples in sliding wear condition is premature. Generally, WCI is graphite-free because the relatively high nominal concentration of chromium helps to prevent the formation of graphite [26]. However, this phase emerges when the ratio of chromium-to-silicon (Cr/Si) is less than 2.7 [27]. For the current GWCI, the value of Cr/Si is about 1.515.
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Primary carbides
659
Martensite
Retained austenite Graphite 50μm
Primary carbides Secondary carbides
Graphite 50μm
Secondary carbides
Refined carbides 50μm Fig. 3. OM (a, c and e) and SEM (b, d and f) micrographs showing microstructures in cast iron layers: (a, b) GWCI-A, (c, d) GWCI-B, (e, f) GWCI-C.
3.2. Wear response Observation of the wear tracks reveals significant differences among the three specimens as shown in Fig. 4. The OM micrographs of Fig. 4(a), (c) and (e) clearly show the squeezing of graphite on the sliding tracks in all the three cases. Such squeezing was caused by wearing of the graphite which emerged on the sliding surface [28]. Fig. 5 shows the average area ratio of squeezing graphite-to-wear track (AR %) by measuring five images for each specimen using the image analysis technique. The results show that the quantity of squeezing graphite increased when the samples underwent the thermal and thermomechanical treatments. This increase indicates a softer matrix is more favourable to squeeze graphite. As a solid lubricant in the cast iron, the increase of squeezing graphite has an important influence on the friction coefficient between the sliding pair, and finally affects the wear rate of the samples [28]. In addition, deep scratches were found in GWCI-A and GWCI-B but not in GWCI-C. The reason for this difference may be related to the evolution of the microhardness of the wear surface. The details will be discussed later in Fig. 7. In Fig. 4(b), (d) and (f), to varying degree, the surfaces were grooved, delaminated and covered with oxide layers. The extent of delamination was steadily lessen on the sliding surface of GWCI-B compared with that of GWCI-A. After hot working, the
delamination became less and finally lost its dominance in GWCI-C. Inversely, the oxide region grew fast in breadth and thickness when the microconstituents were modified by employing the treatments, especially by the thermomechanical treatment. This evolvement indicates the wear mechanism may be transformed from mechanical peeling to oxidational wear. Based on the research of Quinn [29], the oxide layer prevents intermetallic contact and adhesion between the sliding pair, which results in an oxide-metal contact or an oxide–oxide contact dominating the sliding process. Such protection makes the wear turning to mild and lowers the metal loss. Surface smearing was also observed in Fig. 4(b) but not in Fig. 4(d) and (f). Normally, a softer material is prone to smearing rather than a harder one [30,31]. The situation seems discrepant in this study. Taking the oxidational wear and hardness evolution into consider, two hypotheses may respond to this issue. One is the smeared surface was covered by oxide layers, while the other one may be the wear surface of the treated specimens has a higher microhardness than that of the untreated specimen. In order to verify these hypotheses, the wear tracks were analysed by SEM in a higher magnification (see Fig. 6) and the microhardness of the wear surface was measured using a Vickers hardness tester (see Fig. 7). Fig. 6(a), (b) and (c) shows the wear track of GWCI-A, GWCI-B and GWCI-C, respectively. The results show that
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Peeling Deep scratch Surface smearing
Oxide layer
Squeezing of graphite
200μm
Peeling
Squeezing of graphite Oxide layer
Deep scratch
200μm
Oxide layer Peeling Squeezing of graphite
200μm Fig. 4. OM (a, c and e) and SEM (b, d and f) micrographs showing wear tracks of the test specimens: (a, b) GWCI-A, (c, d) GWCI-B, (e, f) GWCI-C.
30 25
AR %
20 15 10 5 0
GWCI-A
GWCI-B
GWCI-C
Test sample Fig. 5. Average area ratio of squeezing graphite-to-wear track.
the surface smearing not only occurred in GWCI-A but also in GWCI-B. However, as shown in Fig. 6(b), the oxide layer on the wear surface of GWCI-B covered most of the area of the metal surface so that the surface smearing was more difficult to detect.
Fig. 7 shows the microhardness of the matrix in free-sliding regions and the metallic areas on sliding surface where was not covered by the oxide scale. The microhardness of the matrix is in agreement with the bulk hardness of the samples. While on the wear surface, it can be seen that a significant increase in microhardness took place because of sliding. Such an increase is due to strain hardening, and different strain hardening capacities in different samples result in different increments. As previously mentioned, the matrix structures in GWCI-A, GWCI-B and GWCI-C are martensite plus retained austenite, supercooled austenite and deformed supercooled austenite, respectively. Therefore, GWCI-C possesses the highest strain hardening capacity, which results in a higher microhardness on its wear surface than that of the other two samples. Thus the excellent scratch resistance and surface smearing resistance can be obtained in GWCI-C, while the deep scratches and surface smearing were found in GWCI-A and GWCI-B, as shown in Figs. 4 and 6. Fig. 8 shows the friction coefficient as a function of the sliding distance for three of the GWCI with different microstructures. At the early stage of sliding, the value of friction coefficient increased quickly to a peak in all the three cases. This increase attributes to direct metal on metal contact which probably caused adhesion. The sequence of peak values decreasing from GWCI-B to GWCI-A through GWCI-C is in line with the bulk hardness
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661
Surface smearing
Peeling Oxide layer Surface smearing Oxide layer
Metal surface Oxide layer
Fig. 6. SEM micrographs showing wear tracks of GWCI-A (a), GWCI-B (b) and GWCI-C (c).
1.0
1000 Matrix Wear surface
900
0.9
Friction Coefficient
Microhardness (HV)
700 600 500 400 300
0.7 0.6
GWCI-B
GWCI-A
0.5 0.4 0.3 0.2
200
0.1
100 0
GWCI-C
0.8
800
0.0
GWCI-A
GWCI-B
GWCI-C
Test sample
0
3
6
9
12
15
18
21
24
27
30
33
36
Sliding Distance (m) Fig. 8. Friction coefficient of the GWCI specimens with sliding distance.
Fig. 7. Microhardness of the matrix and the wear surface of the test samples.
increasing sequence. After a running-in stage, the sliding wear process became stable when the sliding distance is over 12 m. From this point, the slopes of friction coefficient curves of GWCI-B and GWCI-C were negative while that of GWCI-A was positive. Reversed tendency suggested wear conditions including contact state and wear mechanism were starting to change. With the help of the squeezing graphite and the formation of oxide layers, friction coefficients of GWCI-B and GWCI-C became lower than that of GWCI-A at the sliding distance of 24 m and 28 m respectively, as indicated by the arrows in Fig. 8.
4. Conclusions In this paper, the sliding wear behaviour of GWCIs with and without thermal or thermomechanical treatment was studied by pin-on-disc type unidirectional sliding wear tests. A soft LCS was
cladded to the brittle GWCI so that the hot compression on the cast iron can be carried out with crack-free. With a sound bonding interface, the GWCI layers in as-cast state (GWCI-A), thermal treatment state (GWCI-B) and thermomechanical treatment state (GWCI-C) were sliding worn by a chrome steel spherical pin. The development of microstructure and wear behaviour can be summarised as follows: 1. Both GWCI-B and GWCI-C displayed a supercooled austenitic matrix and plenty of secondary carbides because of the thermal and thermomechanical treatments, respectively. A finer microstructure and crushed primary carbides were obtained in GWCI-C due to hot working. 2. The matrix of supercooled austenite in GWCI-B and GWCI-C was more favourable to squeeze the graphite phase and form the oxide layers than the matrix of martensite plus retained austenite in GWCI-A.
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3. Wear of GWCI-A with higher bulk hardness was incurred by delamination and surface smearing. While for GWCI-B and GWCI-C with lower bulk hardness, the oxidational wear dominated the sliding process. This evolution leads to the conclusion that the matrix microstructure plays a more important role in controlling wear loss than the bulk hardness. 4. Strain hardening occurred on the wear surface in all the three cases, which to a more extend in GWCI-C. Such wear induced hardening may also accompany with the occurrence of martensite transition in the subsurface that requires further investigations.
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