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Three-body abrasive wear of carbide-free bainite, martensite and bainite-martensite structure of similar hardness
Wear 402–403 (2018) 207–215
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Three-body abrasive wear of carbide-free bainite, martensite and bainitemartensite structure of similar hardness
T
⁎
Minal Shaha, , Subhankar Das Bakshib a b
Materials Engineering Division, CSIR-National Metallurgical Laboratory, Jamshedpur, Jharkhand, India Product Technology, Tata Steel Limited, Jamshedpur, Jharkhand, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbide free bainite Martensite Strain induced martensite
The three-body abrasive wear behavior of three ferrous alloys with different microstructures but similar hardness has been investigated using a standard dry-sand rubber wheel test (ASTM G65-16). Although the hardness of the alloys was similar, the abrasion rates are radically different owing to differences in abrasion mechanisms in their microstructures. A carbide-free bainitic steel having fine bainitic laths exhibited better abrasion resistance owing to the strain- induced transformation of austenite into martensite. Nano-scaled martensitic laths that formed on the surface resisted plastic deformation during abrasion and thereby increased the abrasion resistance. The microstructure containing bainitic ferrite undergoes extensive plastic deformation associated with a large quantity of dislocations, which in turn accommodates the strain of abrasion. The steel with a blockymartensitic microstructure had the least wear resistance. In that case, fragmentation and chipping comprised the prominent abrasion mechanism. The degeneration of a martensitic structure and its subsequent tempering radically reduces the hardness at the surface and makes it more susceptible to abrasion. Steel containing a mixture of bainite and martensite had intermediate abrasion resistance relative to the other two alloys.
1. Introduction The equipment used in various industries such as mining, transport and agriculture undergoes severe abrasive wear during the service life. The cost of abrasive wear to the national economy is estimated to be about 1% of the gross national product, and it can compromise with the safety and reliability of engineering components [1]. The mechanism of wear during three-body abrasion is highly complex in nature and depends on various factors like the properties of material involved in the process, contacting environment, the shape and size of the abrading particles. Mostly abrasion-resistant steels are made of quenched and tempered martensitic microstructure, because the high hardness of the steel intuitively matters in determining the wear resistance. However, the exact relationship between hardness and wear resistance is often quite dubious [2–4]. Hardness resist the penetration of abrasive particles at the onset of wear, whereas, the toughness inhibits the formation of cracks and subsequent material removal at the later stages of wear. In this context, it has been observed that the deformation of the surface region and extent of work hardening of the material beneath plays a significant role in material detachment [5]. Vingsbo et al. reported that once steady-state abrasive wear is achieved, the structure and properties of the abrading surface and underlying material vary considerably
⁎
relative to the bulk of material. It is the properties of this active layer which controls the abrasion resistance of the steel rather than those of the unaffected bulk material [6]. Richardson et al. first reported that bainitic microstructure has better resistance for abrasion than martensitic steel of similar hardness and composition [7] due to its combination of high ductility and hardness [8]. However, the contradictory views exists [9–11], where martensite has been found to exhibit better abrasion resistance compared to pearlite and bainite owing to its high hardness, which were again contradicted under different experimental conditions [8,12–14]. Martensitic steel with higher carbon (C ∼ 0.65 − 1.2 wt%) has been found to possess higher wear rates compared to low carbon martensite (C ∼ 0.16 − 0.37 wt%) owing to its relatively low toughness [15]. Under rolling-sliding wear condition, carbide free bainite microstructures were found to exhibit excellent wear resistance property as it has high capability of accomodating strain of deformation [16–20]. Wang et al. [21] studied the dry sliding–friction wear resistance of nanostructured bainite steel and reported that the dry sliding friction induces the transformation of retained austenite into carbonsupersaturated fine α-phase (~ 3 nm) in the top friction surface. Yang et al. and Leiro et al. [3,4] investigated the rolling/sliding wear resistance of several carbide-free bainitic steels for the application of rail. The nanostructured bainitic steels with chemical composition
Corresponding author. E-mail address: [email protected] (M. Shah).
https://doi.org/10.1016/j.wear.2018.02.020 Received 9 February 2018; Received in revised form 23 February 2018; Accepted 23 February 2018 Available online 24 February 2018 0043-1648/ © 2018 Elsevier B.V. All rights reserved.
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Table 1, were prepared using a vacuum induction furnace to produce 40 kg ingots of dimension 100 × 100 × 500 mm3. The section of ingot was then homogenized at a temperature of 1250 °C for an hour and forged to 70 * 70 * 100 mm3 bars. The forged bars were then homogenized at a temperature 1250 °C for three hours and air cooled to room temperature (27 °C) to attain the desired microstructure. Chemical compositions were measured on specimen using Induced Couple Plasma (ICP) and LECO analysis for carbon and sulfur content (typical measurement uncertainty: 1–3% of stated value). Martensite steel (Table 1) is standard materials being used in the application of earthmovers and mining industries [27].
0.83C–0.81Cr–1.56Si–1.37Mn–0.87W–1.44Al–0.012P–0.0053S (wt%) developed by the isothermal treatment at 220 °C, 240 °C and 260 °C for different hours exhibit significantly better specific wear resistance to that of bainitic steels transformed at higher temperatures with similar hardness values. Bakshi et al. [22,23] conducted experiments in a novel steel of composition 0.83 C − 2.28 Mn − 0.011 P − 0.008 S − 1.9Si0.044Al- 0.12Cu- 1.44Cr- 0.24Mo- 0.11V- 1.55Co- 0.019Sn- 0.023 Nb. The nanostructure bainite is obtained by isothermal transformation at 200 °C of hardness 622 ± 13 HV30. The same steel was heat treated to produce fine pearlite of hardness 378 ± 9 HV30 and untempered martensite of 739 ± 7 HV30. The three steels were investigated for wear resistance under three body abrasion [22] and rolling-sliding wear [5,23] conditions. Untempered martensite steel suffers fragmentation of the wear track, whereas, bainitic steel has significant plastic deformation at the wear surface with good adhesion of the damaged steel material akin to pearlite. Nanostructured bainite of this composition exhibited one of the lowest specific wear rates of any carbide-free bainite investigated previously under similar circumstances [24,25]. The lowest specific wear rates of nanostructured carbide-free bainite is due to the presence of fine scale of (35 nm) of the bainitic lath structure together with considerable amount (27%) of meta-stable retained austenite in form of thin films. This provides a mechanism of work hardening that leads to an increase in hardness in the active surface area [23]. The present work was motivated from observations of the severe abrasion of steel in the buckets being used in the mining industries mainly in excavators and shovels [26]. Such components are made of medium carbon (0.20–0.25 wt% C) steels alloyed with Ni, Cr and/or Mo and are heat treated to either martensite or tempered martensitic condition to achieve bulk hardness of 390 ± 10 BHN [26,27]. It has been reported that the wear of martensite is not only subjected to its bulk hardness, but also depends largely on the size and distribution of alloy carbide precipitates that interacts with the abrading particles [26]. The purpose of present work is to study the exact mechanism involved in abrasion of martensitic steel under three-body abrasion and compare the specific wear rate against carbide free bainite and microstructure comprised of a combination of martensite and carbide free bainite. The study aims towards design and development of a bainitic steel of superior abrasive wear resistance with a probable scope of suitable replacement of the existing martensitic steel for high hardness used in certain components of the earthmoving and other mining components.
2.3. Abrasion testing Dry sand rubber wheel tests were conducted as per ASTM G65-16 follwing type-“A” procedure using the specifications listed in Table 2 [28]. A deviation was taken in terms of duration of test in order or achieve appreciable wear, later, the weight loss was normalized against the load and distance traversed. Rectangular samples of dimension 25.4 × 76.2 × 12.7 mm3 were machined from the initial block using electro-discharge machine (EDM). The broad faces of the samples were ground using 400 µm grit SiC papers to remove any damage caused by EDM prior to the testing and to achieve the desired surface roughness. The schematic of the equipment and the secondary electron image of the abrasive sand used for the testing are shown schematically in Fig. 1a) and b) respectively [22]. The rotating speed of the wheel and sand flow rate were kept uniform throughout the experiment. The experiment was conducted for ten minutes at the specified parameters, and the specific wear rate was calculated using weight loss after abrasion. The abrasion test was conducted on five samples to confirm and validate the repeatability of the results under the same experimental conditions. The specific wear rate (SWR) was averaged out and reported with statistical error. 2.4. Microstructural analysis The heat treated samples were characterized using Field Emission Gun (FEG) assisted Scanning Electron Microscope (make: FEI Nova Nano SEM) and Transmission Electron Microscope (Jeol JEM 2200FS) for finer structural details. For secondary electron imaging the samples were ground and polished followed by etching with 2 vol% nital at room temperature. For TEM, thin slices were cut from the bulk specimen using electro-discharge machining (EDM). The thickness of sample was reduced to ~0.1 mm by manual grinding and coupons of ϕ3.0 mm diameter were produced by shear punching. For the sample preparation of abrasion track special care has been taken by reducing the thickness from opposite side of abrasion track. The coupons so prepared were electro-polished by single jet machine, with impingement of the electrolyte jet from the opposite side of the wear track to create electron transparent region. Electrolyte used was a mixture of 10% perchloric acid and 90% ethanol by volume at an electro-polishing bath temperature of − 30 °C operated at ~ 40 V.
2. Experimental procedures 2.1. Alloy design Alloy 1 and Alloy 2 has been designed to achieve bainite, martensite and retained austenite under the continuous cooling rate 0.1–0.5 °C/s after austenization. Following aspects were considered during designing of the alloys, i. Complete austenization achieved by soaking at1250 °C and continuous cooling up to room temperature (27 °C) with the cooling rate of 0.5 °C/s to produce bainite and retained austenite [28]. ii. Si and Al play a crucial role in alloy composition to encourage the retention of high carbon in austenite over precipitation of cementite, which is desirable to achieve high toughness in steel [29]. iii. Avoidance of allotriomorphic ferrite, widmanstatten ferrite and pearlite formation by the addition of Mo and B [28]. iv. With the indicated fine scale microstructure, hardness in the range of 380–400 BHN should be achieved.
2.5. XRD analysis X-ray diffraction analysis (XRD) was done in Bruker's D8 advance at 0.020° Ɵ step sizes in the range of 2Ɵ = 40 – 100°, 45 kV applied potential at 45 mA current with filtered Cu Kα radiation. The samples were ground and polished using emery papers with increasing fineness followed by polishing with 0.6 and 0.1 µm diamond pastes. Subsequently, the samples were electropolished to remove the strain produced during process of grinding and polishing. Electrolyte used was a mixture of 4% perchloric acid, 21% glycerol and 75% ethanol by volume at an electropolishing bath temperature of − 30 °C operated at ~ 30 V. For sample preparation for abrasive track, sample of 1 cm3 was cut using electrodischarge machine from the abrasive regime. The sample is leveled
2.2. Melting, forging and heat treatment Alloys (Alloy 1 and Alloy 2) with the composition as shown in 208
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Table 1 Chemical composition for the investigated alloys (all elements are in weight %, nitrogen in ppm).
Martensite Alloy 1 Alloy 2
C
Cr
Mn
Si
Mo
B
Ni
P
S
N(ppm)
0.20 0.15 0.08
1.40 3.0 3.0
1.60 0.54 1.6
0.7 1.5 1.7
0.60 0.48 0.40
0.004 0.004 0.004
1.50 – –
0.025 0.020 0.020
0.010 0.025 0.005
20 60 50
3. Results and discussions
Table 2 ASTM G65-16 Abrasion Test Parameters [22,30]. Wheel Rubber Hardness of rubber Abrasive used Mean particle size of silica sand Hardness of silica sand Rotational speed Load Sand flow rate Wheel diameter Time Total sliding distance
3.1. Prior to abrasion
Rubber-clad Steel wheel Chlorobutyl Durometer A−60 Silica sand, grade HR30 300 µm 956 ± 22 kgf-mm−2 (200 g load) 210 revolutions min−1 130 N 300 gmin−1 22.86 cm 10 min 1464.567 m
3.1.1. Metallography The optical and secondary electron images of the microstructure of three steels are shown in Figs. 2 and 3, respectively. Figs. 2(a) and 3(a) shows the martensitic microstructure, Figs. 2(b) and 3(b) shows the carbide free bainite with film retained austenite in Alloy 1 and Figs. 2(c) and 3(c) shows the mixture of bainite and martensite phases in Alloy 2. The lighter etched phases are identified as martensite and darker etched regions are bainite in Fig. 3(c). Due to presence of smaller sub-units or laths [35] forming bundle of sheaves the bainite are etched darker in optical microscopy. The martensite and bainite phases are further confirmed by measuring the micro hardness of the dark and brighter phase at 10 g load in HV scale. The bright phases are having 592 ± 15HV and dark phases having 481 ± 10HV. Based on the contrast difference the volume fraction of martensite and bainite are evaluated through image analysis (ImageJ) and are found to be 40% and 60% respectively [36]. Fig. 10(a) and (b) shows the TEM micrographs indictating the presence of bainite along with retained austenite in the steel with corresponding SADP patterns. Carbide precipitation within bainite matrix is also found to be absent at ten different random locations selected, which may be due addition of high amount of silicon in the Alloy 1, which are sufficient to suppress the carbide precipitation. The mean lineal intercept (L ) of the bainitic ferrite laths were measured from TEM images using the equation (iv) for estimating the true thickness of lath [37]. The bainitic lath thickness (t) was measured to be 100 ± 20 nm for Alloy 1.
from opposite side of the abrasion track to preserve the abrasion track. Extra care is taken to preserve the abrasion track from water or other contaminations. The phase fractions were determined using the method proposed by Dickson [31,32]. This method calculates the austenite Xγ fraction X by considering the contribution of each peak measured by α the inverse of the expected intensity of peaks Iγ(hkl) and Iα(hkl),
Iγ (hkl) Rγ (hkl)[Xγ] = Iα (hkl) Rα (hkl)[Xα]
(1)
where R for each peak were calculated from the volume of the structure factor F, multiplicity factor p, volume of unit cell v, L p the Lorentzpolarization factor and temperature factor e−2m.
R=
[ F 2 (p)(Lp)] e−2m v2
(2)
Carbon concentration in austenite was determined using Dyson and Holmes’ equation which relates the austenite lattice parameter to alloy composition [33,34] which is expressed as,
a °γ
t=
(4)
Where, t is the true thickness of the bainitic lath and L is mean lineal intercept. The presence of bainite and martensite in Alloy 2 are further confirmed by TEM imaging. Fig. 11(a) shows the lighter region being
= 3.578 + 0.033Wc + 0.00095WMn − 0.0002WNi + 0.0006WCr + 0.0056WAl + 0.0031WMo + 0.0018WV
2L π
(3)
Fig. 1. (a) Schematic illustration of the abrasion-testing equipment, (b) SEM micrograph of the abrasive sand [22].
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Fig. 2. Optical micrographs of (a) martensite, (b) bainite in Alloy 1 and (c) bainite-martensite in Alloy 2.
Fig. 3. Scanning electron micrograph of a) martensite, b) Alloy 1 and c) Alloy 2 before abrasion.
3.1.2. XRD analysis XRD analysis was performed on the samples of martensite before abrasion to find the phases present in the steel. No austenite peak is found in XRD profile for Martensite (Fig. 4). XRD analysis for Alloy 1 shows the presence of 17 vol% austenite, 9 vol% martensite and 76 vol % bainite (Fig. 5).
bainite and the darker region as martensite. The dislocation density of the bainite are less as compared to the martensite phase and thus martensite will appear with a dark contrast in TEM [38]. SADP from corresponding bright field (Fig. 11(a)) also confirmed the presence of bainite and martensite. The absence of carbide was observed at ten different random locations. The thickness of martensite laths were found to be 310 ± 20 nm and the same for bainite was estimated to be 120 ± 20 nm as shown in Fig. 11(a) (Table 3). 210
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Table 3 XRD analysis of all the investigated steels. Steel
Microstructure
Conditions
Volume Fraction of Austenite
Martensite
Martensite
Alloy 1
Carbide Free Bainite
Alloy 2
Martensite+ Bainite
Before Abrasion After Abrasion Before Abrasion After Abrasion Before Abrasion After Abrasion
<3 <3 17 <3 <3 <3
Fig. 6. X-Ray Diffraction before and after abrasion for Alloy 2.
loads were kept constant throughout the tests. The specific wear loss was observed to be highest in case of martensitic steel followed by a mixture of martensitic and bainite structure. Carbide free bainite with retained austenite shows best wear resistance under dry-sand abrasion test (Fig. 6). The secondary electron images of the abraded surface of martensite is shown in Fig. 7(a) Micro-cutting is found to be the predominant mechanism involved in material removal with almost no signature of ledge formation. The SEM images that are taken from the cross-section of the abraded surface are shown in Fig. 8(a). Few micro-cracks emanating from the abraded surface were also visible which are more likely to get propagate and coalesced at the sub-surface region causing material removal (Fig. 8(a)). The compressive and tractional components cause severe damage to the surface layer and associated material removal, whereas, the shear stress causes damage to the sub-surface [23]. The difference in carbon concentration was measured through the XRD analysis [39] and is found to be 0.04 wt% in martensite before exposure to abrasion and after the exposure of abrasion. There is slight reduction in Carbon which gives an indication of the tempering of the martensite during abrasion. The nano-indentation test was also performed on the cross-section of the abraded surface which shows the relative softening of martensite below the wear surface. Hardness was found be lowered up to a depth of 200 µm as shown in Fig. 9. This further confirms the slight tempering of martensite on the surface and sub-surface martensite caused due to strain associated with the abrasion. Also fragmentation and chipping of material from the surface confirms the brittle fracture of martensite, which accounts for high material loss. The SEM image of the abraded surface for Alloy 1 is shown in Fig. 7(b) and the corresponding cross sectional image is shown in Fig. 8(b). Micro-pitting and micro-grooving are found to be the prominent wear mechanisms of material removal with signature of adhesive wear. The abraded surface exhibits both groove marks and a large number of pits. The pits are indicative of the obstacles caused by abrasive particles to the motion, causing the termination of few grooves. Few micro-cracks emanating from the abraded surface were also visible which are more likely to get propagated and coalesced under the abraded surface causing material removal. Fig. 10(c-f) shows the TEM images of the carbide free bainite after abrasion. When the stress-field associated with an advancing crack tip or growing void triggers the local transformation of film retained austenite to martensite, volumetric expansion associated with the martensitic transformation act to mitigate the advancing crack propagation or growth of a void, while the hard surrounding martensite acts as a further barrier and thereby improves the wear resistance of the steel. XRD analysis of bainitic structure after the abrasion only shows α(110), α(200), α(220), α(311) peaks (Fig. 5), with complete absence of austenite peaks. This
Fig. 4. X-Ray Diffraction of Martensite before and after abrasion.
Fig. 5. X-Ray Diffraction before and after abrasion for Alloy 1.
3.2. Post abrasion test The abrasion resistance of three steels with different microstructure but comparable hardness was assessed by three body abrasion tests. The specific wear rates calculated from the weight loss of each sample under constant load is listed in Table 4. The test cycles, area of contact and Table 4 Comparison of Specific Wear Rate for different Alloys. Steel
Hardness, BHN
Specific Wear Rate,mm3/N-m (x10−5)
Martensite Alloy 1 Alloy 2
390 ± 15 390 ± 10 390 ± 7
92 ± 2 38 ± 2 45 ± 2
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Fig. 7. Scanning electron microscopy of the abraded surface of (a) martensite, (b) Alloy 1 and (c) Alloy 2.
hardening below the wear surface. Hardness was found be increased up to a depth of 150 µm as shown in Fig. 9 because of the strain induced transformation. These nano martensitic laths formation have improved the wear resistance of the Alloy 1. The bright parallel lines in Fig. 10(e-
confirms that plastic strain caused during abrasion triggers into strain induced transformation causing 17 vol% austenite to transform into martensite. The nano-indentation test was also performed on the crosssection of the abraded surface which gives an interesting observation of
Fig. 8. Sub-surface observations of the abraded surface of (a) Martensite, (b) Alloy 1 and (c) Alloy 2.
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f), associated with deformed substructure in bainite indicates possible formation of microbands. The traces of bainite are lost due to extensive plastic deformation caused in the structure. Micro-voids within the laths of bainite, surrounded by a dense network of crystallographic defects, are frequently observed at many places of the abraded layer. These voids are of a dimension of the order of 20–30 nm located within the dense dislocations network in a heavily plastically deformed bainitic structure, could possibly have formed due to shear-instability of the structure. The Alloy 2 shows intermediate wear resistance between carbide free bainitic and martensitic microstructure. SEM of the abraded surfaces qualitatively confirms the observations (Fig. 7c). Micro-pitting and micro-grooving are found to be the prominent wear mechanism for material removal with the signature of adhesive wear along with micro-cutting mechanism at few regimes. The abraded surface exhibits both groove marks and a large number density of pits. The pits are indicative of the obstacles caused by abrasive particles to the motion, causing the termination of a few grooves at the pits. TEM images and corresponding SADP (Fig. 11(c-f)) shows that the strain resulted in the degeneracy of martensite forming cementite
Fig. 9. Nano indentation of the abraded samples.
Fig. 10. Transmission Electron Microscopy of Alloy 1 with (a) carbide free bainite and retained austenite before abrasion and (b) corresponding SAD patterns (c) and (d) are the microstructure and SAD pattern of carbide free bainite and retained austenite being transformed to martensite after abrasion, (e) Ferrite micro bands formed by abrasion, (f) bainite lath with highly-density dislocation networks formed by abrasion.
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Fig. 11. Transmission Electron Microscopy of Alloy 2 with (a) composite microstructure of bainite and martensite with (b) SAD pattern of martensite and bainite before abrasion (c) coarse martensite plates with void nucleation site caused after abrasion, (d) plates with highly-density dislocation networks, (e) Cementite formation and (f) SAD pattern of the cementite along zone axis of [001].
higher than carbide free or mixture of martensite- bainite steels. 2. The sensitivity of the wear resistance to microstructure is because of the prevalence of different wear mechanisms involved in surface damage in each case. The carbide free bainite wears by a combination of grooving and relatively minor pitting and shows significant plastic deformation at the active surface with good adhesion of the damaged material. The retained austenite undergoes stress induced transformation resulting in formation of nano-scaled martensitic. Such nano martensitic lath formation results in a harder abraded layer on active surface, thus improving the abrasion resistance of steel. 3. In case of martensite, the heat generated and strain during abrasion causes the tempering of martensite and reduces the hardness of surface significantly. The wear mechanism was predominately micro-cutting with extensive removal of wear debris causing higher wear rate. 4. The steel with a mixture of bainite and martensite shows better wear resistance compared to the martensite but lesser compared to carbide-free bainite. The adiabatic heating and strain helped tempering of martensite leading to lowering of hardness of the active surface,
precipitates. Tempering of martensite reduces the brittleness of the structure and believed to improve the abrasion resistance. Micro-voids are initiated at the cementite particle between martensite and bainite lath and are surrounded by a dense network of crystallographic defects, which are frequently observed at many places of the abraded-layer. These voids are of a dimension of the order of 20–30 nm located within dense dislocations network in a heavily plastically deformed martensitic and bainitic structure, could possibly have formed due to shearinstability of the structure. To conclude it can be stated that structure shows ductile fracture with cementite particles serving as nucleation sites for micro-void generation. 4. Summary Three-body abrasive wear behavior has been studied for carbide free bainite with retained austenite, martensite and a mixture microstructure of martensite and bainite of similar hardness. Based on the experimental evidences, the following conclusions can be drawn: 1. The specific wear rate of martensite is approximately two times 214
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which aided more penetration by hard abrasives. [19]
Acknowledgements
[20]
The authors would like to convey thanks to the Director, CSIR-NML for providing his kind permission to publish the work. Fruitful technical discussion with Dr. Mainak Ghosh during TEM study is gratefully acknowledged. One of the authors is indebted for financial support to CSIR-National Metallurgical Lab in form of iPSG Project.
[21]
[22] [23]
References [24] [1] P.J. Blau, ASM Handbook, 18 Friction, Lubrication, and WearTechnology, ASM International, 1992. [2] Y.C. Lin, S.W. Wang, T.M. Chen, A study on the wear behavior of hardened medium carbon steel, J. Mater. Process. Technol. 120 (2002) 126–132. [3] H.-S. Yang, H.K.D.H. Bhadeshia, Designing low-carbon low-temperature bainite, Mater. Sci. Technol. 24 (2008) 335–342. [4] V.A. Leiro, E. Vuorinen, K.G. Sundin, B. Prakash, T. Sourmail, R.E. Smanio, G. Caballero, C. Garcia-Mateo, Wear of nano-structured carbide-free bainitic steels under dry rolling–sliding conditions, Wear 298–299 (2013) 42–47. [5] S. Das Bakshi, A. Leiro, B. Prakash, H.K.D.H. Bhadeshia, Dry rolling/sliding wear of nanostructured pearlite, Mater. Sci. Technol. 31 (2015) 1735–1744. [6] O. Vingsbo, S. Hogmark, Fundamentals of Friction and Wear of Materials, American Society for Metals, Metals Park, OH, 1981, pp. 373–408. [7] R.C.D. Richardson, The wear of metals by hard abrasives, Wear 10 (1967) 291–309. [8] P.L. Hurricks, Overcoming industrial wear, Ind. Lubr. Tribol. (1971) 345–356. [9] K.C. Barker, The Development of Abrasive-corrosive Wear Resistance of Steels by Microstructural Control (Ph.D. Thesis), University of Cape Town, 1988. [10] D.E. Diesburg, F. Borik, Optimizing abrasion resistance and toughness in steel and irons for the mining industry, Mater. Min. Ind., Ann. Arbor., MI (1974) 14–41. [11] J. Suchanek, V. Kuklik, Influence of heat and thermochemical treatment on abrasion resistance of structural and tool steels, Wear 267 (2009) 2100–2108. [12] O.P. Modi, D.P. Mondal, B.K. Prasad, M. Singh, H.K. Khaira, Abrasive wear behaviour of a high carbon steel: effects of microstructure and experimental parameters and correlation with mechanical properties, Mater. Sci. Eng. A 343 (2003) 235–242. [13] G.W. Stachowiak, A.W. Batchelor, Engineering Tribology, 3rd edition, Elsevier, 2005. [14] S. Das Bakshi, D. Sinha, S. Ghosh Chowdhury, V.V. Mahashabde, Surface and subsurface damage of 0.20 wt%-C martensite during three-body abrasion, Wear (2017), http://dx.doi.org/10.1016/j.wear.2017.07.004. [15] N. Prasad, S.D. Kulkarni, Relations between microstructure and abrasive wear of plain carbon steel, Wear 63 (1980) 29–338. [16] P. Clayton, N. Jin, Effect of microstructure on rolling/sliding wear of low carbon bainitic steels, Wear 200 (1996) 74–82. [17] P. Clayton, R. Devanathan, Rolling/sliding wear behavior of a chromium-molybdenum rail steel in pearlitic and bainitic conditions, Wear 156 (1992) 121–131. [18] J. Kalousek, D.M. Fegrdo, E.E. Laufer, The wear resistance and worn Metallography
[25]
[26]
[27] [28]
[29] [30]
[31] [32] [33] [34] [35] [36] [37] [38] [39]
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of Pearlite, bainite and tempered martensite rail steel microstructures of high hardness, Wear 105 (1985) 199–222. P.H. Shipway, S.J. Wood, A.H. Dent, The hardness and sliding wear behaviour of a bainitic steel, Wear 203–204 (1997) 196–205. E. Vuorinen, A. Linstro¨m, P. Rubin, E. Navara, M. Oden Pellets, in: Proceedings of the 2nd World Conference on ‘Pellets', Jonkoping, Sweden, Stockholm: Swedish Bioenergy Association (SVEBIO), June 2006, 2006, pp. 151–155. T.S. Wang, J. Yang, C.J. Shang, X.Y. Li, B. Lv, M. Zhang, F.C. Zhang, Sliding friction surface microstructure and wear resistance of 9SiCr steel with low–temperature austempering treatment, Wear 282–283 (2012) 81–84. S. Das Bakshi, P.H. Shipway, H.K.D.H. Bhadeshia, Three-body abrasive wear of fine pearlite, nanostructured bainite and martensite, Wear 308 (2013) 46–53. S. Das Bakshi, A. Leiro, B. Prakash, H.K.D.H. Bhadeshia, Dry rolling/sliding wear of nanostructured bainite, Wear 316 (2014) 70–78. T. Sourmail, F.G. Caballero, C. Garcia-Mateo, V. Smanio, C. Ziegler, M. Kuntz, R. Elvira, A. Leiro, E. Vuorinen, T. Teeri, Evaluation of the potential of high Si high C steels nanostructured bainite for wear and fatigue applications, Mater. Sci. Technol. 29 (2013) 1166–1173. P. Zhang, F.C. Zhang, Z.G. Yan, T.S. Wang, L.H. Qian, Wear property of low-temperature bainite in the surface layer of a carburized low carbon steel, Wear 271 (2011) 697–704. S.Das. Bakshi, Wear of fine pearlite, nanostructured bainite and martensite, in: S. Das. Bakshi, Wear of Fine Pearlite, Nanostructured Bainite and Martensite, Ph.D. Thesis,University of Cambridge, UK.,University of Cambridge, UK. SSAB, Information materials SSAB, 〈http://www.ssab.com〉. G. Gomez, T. Perez, H.K.D.H. Bhadeshia, Air cooled bainitic steels for strong, seamless pipes Part 1 -Alloy design, kinetics and microstructure, Mater. Sci. Technol. 25 (2009) 1501–1507. H.K.D.H. Bhadeshia, D.V. Edmonds, Bainite in silicon steels: a new compositionproperty approach, Part Ii., Met. Sci. 17 (1983) 420–425. ASTM G65-16, standard test method for measuring abrasion using the dry rubberwheel apparatus, ASTM international, No 03.02, West Conshohocken, Pennsylvania,USA. M.J. Dickson, The significance of texture parameters in phase analysis by X–ray diffraction, J. Appl. Crystallogr. 2 (1969) 176–180. B.D. Cullity, Elements of X–ray Diffraction, Second edition, Addison-Wesley Publishing Company Inc, California, 1978, p. 412. D.J. Dyson, B. Holmes, Effect of alloying additions on the lattice parameter austenite, J. Iron Steel Inst. 208 (1970) 469–474. M.J. Peet., 〈http://mathewpeet.org/thesis/programs/〉. H.K.D.H. Bhadeshia: Bainite in Steels: Theory and Practice, 3rd Edition, 2015, pp. 19. C.A. Schneider, W.S. Rasband, K.W. Eliceiri, NIH image to image J: 25 years of image analysis, Nat. mathods 9 (2012) 671–675. H.K.D.H. Bhadeshia, High performance bainitic steels, Mater. Sci. Forum 500–501 (2005) 63–74. H.K.D.H. Bhadeshia: Bainite in Steels: Theory and Practice, 3rd Edition, 2015, pp. 3–5. M.U. Cohen, Precision lattice constants from X‐ray powder photographs, Rev. Sci. Instrum. 6 (1935) 68–74.
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Three-body abrasive wear of carbide-free bainite, martensite and bainitemartensite structure of similar hardness
T
⁎
Minal Shaha, , Subhankar Das Bakshib a b
Materials Engineering Division, CSIR-National Metallurgical Laboratory, Jamshedpur, Jharkhand, India Product Technology, Tata Steel Limited, Jamshedpur, Jharkhand, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbide free bainite Martensite Strain induced martensite
The three-body abrasive wear behavior of three ferrous alloys with different microstructures but similar hardness has been investigated using a standard dry-sand rubber wheel test (ASTM G65-16). Although the hardness of the alloys was similar, the abrasion rates are radically different owing to differences in abrasion mechanisms in their microstructures. A carbide-free bainitic steel having fine bainitic laths exhibited better abrasion resistance owing to the strain- induced transformation of austenite into martensite. Nano-scaled martensitic laths that formed on the surface resisted plastic deformation during abrasion and thereby increased the abrasion resistance. The microstructure containing bainitic ferrite undergoes extensive plastic deformation associated with a large quantity of dislocations, which in turn accommodates the strain of abrasion. The steel with a blockymartensitic microstructure had the least wear resistance. In that case, fragmentation and chipping comprised the prominent abrasion mechanism. The degeneration of a martensitic structure and its subsequent tempering radically reduces the hardness at the surface and makes it more susceptible to abrasion. Steel containing a mixture of bainite and martensite had intermediate abrasion resistance relative to the other two alloys.
1. Introduction The equipment used in various industries such as mining, transport and agriculture undergoes severe abrasive wear during the service life. The cost of abrasive wear to the national economy is estimated to be about 1% of the gross national product, and it can compromise with the safety and reliability of engineering components [1]. The mechanism of wear during three-body abrasion is highly complex in nature and depends on various factors like the properties of material involved in the process, contacting environment, the shape and size of the abrading particles. Mostly abrasion-resistant steels are made of quenched and tempered martensitic microstructure, because the high hardness of the steel intuitively matters in determining the wear resistance. However, the exact relationship between hardness and wear resistance is often quite dubious [2–4]. Hardness resist the penetration of abrasive particles at the onset of wear, whereas, the toughness inhibits the formation of cracks and subsequent material removal at the later stages of wear. In this context, it has been observed that the deformation of the surface region and extent of work hardening of the material beneath plays a significant role in material detachment [5]. Vingsbo et al. reported that once steady-state abrasive wear is achieved, the structure and properties of the abrading surface and underlying material vary considerably
⁎
relative to the bulk of material. It is the properties of this active layer which controls the abrasion resistance of the steel rather than those of the unaffected bulk material [6]. Richardson et al. first reported that bainitic microstructure has better resistance for abrasion than martensitic steel of similar hardness and composition [7] due to its combination of high ductility and hardness [8]. However, the contradictory views exists [9–11], where martensite has been found to exhibit better abrasion resistance compared to pearlite and bainite owing to its high hardness, which were again contradicted under different experimental conditions [8,12–14]. Martensitic steel with higher carbon (C ∼ 0.65 − 1.2 wt%) has been found to possess higher wear rates compared to low carbon martensite (C ∼ 0.16 − 0.37 wt%) owing to its relatively low toughness [15]. Under rolling-sliding wear condition, carbide free bainite microstructures were found to exhibit excellent wear resistance property as it has high capability of accomodating strain of deformation [16–20]. Wang et al. [21] studied the dry sliding–friction wear resistance of nanostructured bainite steel and reported that the dry sliding friction induces the transformation of retained austenite into carbonsupersaturated fine α-phase (~ 3 nm) in the top friction surface. Yang et al. and Leiro et al. [3,4] investigated the rolling/sliding wear resistance of several carbide-free bainitic steels for the application of rail. The nanostructured bainitic steels with chemical composition
Corresponding author. E-mail address: [email protected] (M. Shah).
https://doi.org/10.1016/j.wear.2018.02.020 Received 9 February 2018; Received in revised form 23 February 2018; Accepted 23 February 2018 Available online 24 February 2018 0043-1648/ © 2018 Elsevier B.V. All rights reserved.
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Table 1, were prepared using a vacuum induction furnace to produce 40 kg ingots of dimension 100 × 100 × 500 mm3. The section of ingot was then homogenized at a temperature of 1250 °C for an hour and forged to 70 * 70 * 100 mm3 bars. The forged bars were then homogenized at a temperature 1250 °C for three hours and air cooled to room temperature (27 °C) to attain the desired microstructure. Chemical compositions were measured on specimen using Induced Couple Plasma (ICP) and LECO analysis for carbon and sulfur content (typical measurement uncertainty: 1–3% of stated value). Martensite steel (Table 1) is standard materials being used in the application of earthmovers and mining industries [27].
0.83C–0.81Cr–1.56Si–1.37Mn–0.87W–1.44Al–0.012P–0.0053S (wt%) developed by the isothermal treatment at 220 °C, 240 °C and 260 °C for different hours exhibit significantly better specific wear resistance to that of bainitic steels transformed at higher temperatures with similar hardness values. Bakshi et al. [22,23] conducted experiments in a novel steel of composition 0.83 C − 2.28 Mn − 0.011 P − 0.008 S − 1.9Si0.044Al- 0.12Cu- 1.44Cr- 0.24Mo- 0.11V- 1.55Co- 0.019Sn- 0.023 Nb. The nanostructure bainite is obtained by isothermal transformation at 200 °C of hardness 622 ± 13 HV30. The same steel was heat treated to produce fine pearlite of hardness 378 ± 9 HV30 and untempered martensite of 739 ± 7 HV30. The three steels were investigated for wear resistance under three body abrasion [22] and rolling-sliding wear [5,23] conditions. Untempered martensite steel suffers fragmentation of the wear track, whereas, bainitic steel has significant plastic deformation at the wear surface with good adhesion of the damaged steel material akin to pearlite. Nanostructured bainite of this composition exhibited one of the lowest specific wear rates of any carbide-free bainite investigated previously under similar circumstances [24,25]. The lowest specific wear rates of nanostructured carbide-free bainite is due to the presence of fine scale of (35 nm) of the bainitic lath structure together with considerable amount (27%) of meta-stable retained austenite in form of thin films. This provides a mechanism of work hardening that leads to an increase in hardness in the active surface area [23]. The present work was motivated from observations of the severe abrasion of steel in the buckets being used in the mining industries mainly in excavators and shovels [26]. Such components are made of medium carbon (0.20–0.25 wt% C) steels alloyed with Ni, Cr and/or Mo and are heat treated to either martensite or tempered martensitic condition to achieve bulk hardness of 390 ± 10 BHN [26,27]. It has been reported that the wear of martensite is not only subjected to its bulk hardness, but also depends largely on the size and distribution of alloy carbide precipitates that interacts with the abrading particles [26]. The purpose of present work is to study the exact mechanism involved in abrasion of martensitic steel under three-body abrasion and compare the specific wear rate against carbide free bainite and microstructure comprised of a combination of martensite and carbide free bainite. The study aims towards design and development of a bainitic steel of superior abrasive wear resistance with a probable scope of suitable replacement of the existing martensitic steel for high hardness used in certain components of the earthmoving and other mining components.
2.3. Abrasion testing Dry sand rubber wheel tests were conducted as per ASTM G65-16 follwing type-“A” procedure using the specifications listed in Table 2 [28]. A deviation was taken in terms of duration of test in order or achieve appreciable wear, later, the weight loss was normalized against the load and distance traversed. Rectangular samples of dimension 25.4 × 76.2 × 12.7 mm3 were machined from the initial block using electro-discharge machine (EDM). The broad faces of the samples were ground using 400 µm grit SiC papers to remove any damage caused by EDM prior to the testing and to achieve the desired surface roughness. The schematic of the equipment and the secondary electron image of the abrasive sand used for the testing are shown schematically in Fig. 1a) and b) respectively [22]. The rotating speed of the wheel and sand flow rate were kept uniform throughout the experiment. The experiment was conducted for ten minutes at the specified parameters, and the specific wear rate was calculated using weight loss after abrasion. The abrasion test was conducted on five samples to confirm and validate the repeatability of the results under the same experimental conditions. The specific wear rate (SWR) was averaged out and reported with statistical error. 2.4. Microstructural analysis The heat treated samples were characterized using Field Emission Gun (FEG) assisted Scanning Electron Microscope (make: FEI Nova Nano SEM) and Transmission Electron Microscope (Jeol JEM 2200FS) for finer structural details. For secondary electron imaging the samples were ground and polished followed by etching with 2 vol% nital at room temperature. For TEM, thin slices were cut from the bulk specimen using electro-discharge machining (EDM). The thickness of sample was reduced to ~0.1 mm by manual grinding and coupons of ϕ3.0 mm diameter were produced by shear punching. For the sample preparation of abrasion track special care has been taken by reducing the thickness from opposite side of abrasion track. The coupons so prepared were electro-polished by single jet machine, with impingement of the electrolyte jet from the opposite side of the wear track to create electron transparent region. Electrolyte used was a mixture of 10% perchloric acid and 90% ethanol by volume at an electro-polishing bath temperature of − 30 °C operated at ~ 40 V.
2. Experimental procedures 2.1. Alloy design Alloy 1 and Alloy 2 has been designed to achieve bainite, martensite and retained austenite under the continuous cooling rate 0.1–0.5 °C/s after austenization. Following aspects were considered during designing of the alloys, i. Complete austenization achieved by soaking at1250 °C and continuous cooling up to room temperature (27 °C) with the cooling rate of 0.5 °C/s to produce bainite and retained austenite [28]. ii. Si and Al play a crucial role in alloy composition to encourage the retention of high carbon in austenite over precipitation of cementite, which is desirable to achieve high toughness in steel [29]. iii. Avoidance of allotriomorphic ferrite, widmanstatten ferrite and pearlite formation by the addition of Mo and B [28]. iv. With the indicated fine scale microstructure, hardness in the range of 380–400 BHN should be achieved.
2.5. XRD analysis X-ray diffraction analysis (XRD) was done in Bruker's D8 advance at 0.020° Ɵ step sizes in the range of 2Ɵ = 40 – 100°, 45 kV applied potential at 45 mA current with filtered Cu Kα radiation. The samples were ground and polished using emery papers with increasing fineness followed by polishing with 0.6 and 0.1 µm diamond pastes. Subsequently, the samples were electropolished to remove the strain produced during process of grinding and polishing. Electrolyte used was a mixture of 4% perchloric acid, 21% glycerol and 75% ethanol by volume at an electropolishing bath temperature of − 30 °C operated at ~ 30 V. For sample preparation for abrasive track, sample of 1 cm3 was cut using electrodischarge machine from the abrasive regime. The sample is leveled
2.2. Melting, forging and heat treatment Alloys (Alloy 1 and Alloy 2) with the composition as shown in 208
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Table 1 Chemical composition for the investigated alloys (all elements are in weight %, nitrogen in ppm).
Martensite Alloy 1 Alloy 2
C
Cr
Mn
Si
Mo
B
Ni
P
S
N(ppm)
0.20 0.15 0.08
1.40 3.0 3.0
1.60 0.54 1.6
0.7 1.5 1.7
0.60 0.48 0.40
0.004 0.004 0.004
1.50 – –
0.025 0.020 0.020
0.010 0.025 0.005
20 60 50
3. Results and discussions
Table 2 ASTM G65-16 Abrasion Test Parameters [22,30]. Wheel Rubber Hardness of rubber Abrasive used Mean particle size of silica sand Hardness of silica sand Rotational speed Load Sand flow rate Wheel diameter Time Total sliding distance
3.1. Prior to abrasion
Rubber-clad Steel wheel Chlorobutyl Durometer A−60 Silica sand, grade HR30 300 µm 956 ± 22 kgf-mm−2 (200 g load) 210 revolutions min−1 130 N 300 gmin−1 22.86 cm 10 min 1464.567 m
3.1.1. Metallography The optical and secondary electron images of the microstructure of three steels are shown in Figs. 2 and 3, respectively. Figs. 2(a) and 3(a) shows the martensitic microstructure, Figs. 2(b) and 3(b) shows the carbide free bainite with film retained austenite in Alloy 1 and Figs. 2(c) and 3(c) shows the mixture of bainite and martensite phases in Alloy 2. The lighter etched phases are identified as martensite and darker etched regions are bainite in Fig. 3(c). Due to presence of smaller sub-units or laths [35] forming bundle of sheaves the bainite are etched darker in optical microscopy. The martensite and bainite phases are further confirmed by measuring the micro hardness of the dark and brighter phase at 10 g load in HV scale. The bright phases are having 592 ± 15HV and dark phases having 481 ± 10HV. Based on the contrast difference the volume fraction of martensite and bainite are evaluated through image analysis (ImageJ) and are found to be 40% and 60% respectively [36]. Fig. 10(a) and (b) shows the TEM micrographs indictating the presence of bainite along with retained austenite in the steel with corresponding SADP patterns. Carbide precipitation within bainite matrix is also found to be absent at ten different random locations selected, which may be due addition of high amount of silicon in the Alloy 1, which are sufficient to suppress the carbide precipitation. The mean lineal intercept (L ) of the bainitic ferrite laths were measured from TEM images using the equation (iv) for estimating the true thickness of lath [37]. The bainitic lath thickness (t) was measured to be 100 ± 20 nm for Alloy 1.
from opposite side of the abrasion track to preserve the abrasion track. Extra care is taken to preserve the abrasion track from water or other contaminations. The phase fractions were determined using the method proposed by Dickson [31,32]. This method calculates the austenite Xγ fraction X by considering the contribution of each peak measured by α the inverse of the expected intensity of peaks Iγ(hkl) and Iα(hkl),
Iγ (hkl) Rγ (hkl)[Xγ] = Iα (hkl) Rα (hkl)[Xα]
(1)
where R for each peak were calculated from the volume of the structure factor F, multiplicity factor p, volume of unit cell v, L p the Lorentzpolarization factor and temperature factor e−2m.
R=
[ F 2 (p)(Lp)] e−2m v2
(2)
Carbon concentration in austenite was determined using Dyson and Holmes’ equation which relates the austenite lattice parameter to alloy composition [33,34] which is expressed as,
a °γ
t=
(4)
Where, t is the true thickness of the bainitic lath and L is mean lineal intercept. The presence of bainite and martensite in Alloy 2 are further confirmed by TEM imaging. Fig. 11(a) shows the lighter region being
= 3.578 + 0.033Wc + 0.00095WMn − 0.0002WNi + 0.0006WCr + 0.0056WAl + 0.0031WMo + 0.0018WV
2L π
(3)
Fig. 1. (a) Schematic illustration of the abrasion-testing equipment, (b) SEM micrograph of the abrasive sand [22].
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Fig. 2. Optical micrographs of (a) martensite, (b) bainite in Alloy 1 and (c) bainite-martensite in Alloy 2.
Fig. 3. Scanning electron micrograph of a) martensite, b) Alloy 1 and c) Alloy 2 before abrasion.
3.1.2. XRD analysis XRD analysis was performed on the samples of martensite before abrasion to find the phases present in the steel. No austenite peak is found in XRD profile for Martensite (Fig. 4). XRD analysis for Alloy 1 shows the presence of 17 vol% austenite, 9 vol% martensite and 76 vol % bainite (Fig. 5).
bainite and the darker region as martensite. The dislocation density of the bainite are less as compared to the martensite phase and thus martensite will appear with a dark contrast in TEM [38]. SADP from corresponding bright field (Fig. 11(a)) also confirmed the presence of bainite and martensite. The absence of carbide was observed at ten different random locations. The thickness of martensite laths were found to be 310 ± 20 nm and the same for bainite was estimated to be 120 ± 20 nm as shown in Fig. 11(a) (Table 3). 210
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Table 3 XRD analysis of all the investigated steels. Steel
Microstructure
Conditions
Volume Fraction of Austenite
Martensite
Martensite
Alloy 1
Carbide Free Bainite
Alloy 2
Martensite+ Bainite
Before Abrasion After Abrasion Before Abrasion After Abrasion Before Abrasion After Abrasion
<3 <3 17 <3 <3 <3
Fig. 6. X-Ray Diffraction before and after abrasion for Alloy 2.
loads were kept constant throughout the tests. The specific wear loss was observed to be highest in case of martensitic steel followed by a mixture of martensitic and bainite structure. Carbide free bainite with retained austenite shows best wear resistance under dry-sand abrasion test (Fig. 6). The secondary electron images of the abraded surface of martensite is shown in Fig. 7(a) Micro-cutting is found to be the predominant mechanism involved in material removal with almost no signature of ledge formation. The SEM images that are taken from the cross-section of the abraded surface are shown in Fig. 8(a). Few micro-cracks emanating from the abraded surface were also visible which are more likely to get propagate and coalesced at the sub-surface region causing material removal (Fig. 8(a)). The compressive and tractional components cause severe damage to the surface layer and associated material removal, whereas, the shear stress causes damage to the sub-surface [23]. The difference in carbon concentration was measured through the XRD analysis [39] and is found to be 0.04 wt% in martensite before exposure to abrasion and after the exposure of abrasion. There is slight reduction in Carbon which gives an indication of the tempering of the martensite during abrasion. The nano-indentation test was also performed on the cross-section of the abraded surface which shows the relative softening of martensite below the wear surface. Hardness was found be lowered up to a depth of 200 µm as shown in Fig. 9. This further confirms the slight tempering of martensite on the surface and sub-surface martensite caused due to strain associated with the abrasion. Also fragmentation and chipping of material from the surface confirms the brittle fracture of martensite, which accounts for high material loss. The SEM image of the abraded surface for Alloy 1 is shown in Fig. 7(b) and the corresponding cross sectional image is shown in Fig. 8(b). Micro-pitting and micro-grooving are found to be the prominent wear mechanisms of material removal with signature of adhesive wear. The abraded surface exhibits both groove marks and a large number of pits. The pits are indicative of the obstacles caused by abrasive particles to the motion, causing the termination of few grooves. Few micro-cracks emanating from the abraded surface were also visible which are more likely to get propagated and coalesced under the abraded surface causing material removal. Fig. 10(c-f) shows the TEM images of the carbide free bainite after abrasion. When the stress-field associated with an advancing crack tip or growing void triggers the local transformation of film retained austenite to martensite, volumetric expansion associated with the martensitic transformation act to mitigate the advancing crack propagation or growth of a void, while the hard surrounding martensite acts as a further barrier and thereby improves the wear resistance of the steel. XRD analysis of bainitic structure after the abrasion only shows α(110), α(200), α(220), α(311) peaks (Fig. 5), with complete absence of austenite peaks. This
Fig. 4. X-Ray Diffraction of Martensite before and after abrasion.
Fig. 5. X-Ray Diffraction before and after abrasion for Alloy 1.
3.2. Post abrasion test The abrasion resistance of three steels with different microstructure but comparable hardness was assessed by three body abrasion tests. The specific wear rates calculated from the weight loss of each sample under constant load is listed in Table 4. The test cycles, area of contact and Table 4 Comparison of Specific Wear Rate for different Alloys. Steel
Hardness, BHN
Specific Wear Rate,mm3/N-m (x10−5)
Martensite Alloy 1 Alloy 2
390 ± 15 390 ± 10 390 ± 7
92 ± 2 38 ± 2 45 ± 2
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Fig. 7. Scanning electron microscopy of the abraded surface of (a) martensite, (b) Alloy 1 and (c) Alloy 2.
hardening below the wear surface. Hardness was found be increased up to a depth of 150 µm as shown in Fig. 9 because of the strain induced transformation. These nano martensitic laths formation have improved the wear resistance of the Alloy 1. The bright parallel lines in Fig. 10(e-
confirms that plastic strain caused during abrasion triggers into strain induced transformation causing 17 vol% austenite to transform into martensite. The nano-indentation test was also performed on the crosssection of the abraded surface which gives an interesting observation of
Fig. 8. Sub-surface observations of the abraded surface of (a) Martensite, (b) Alloy 1 and (c) Alloy 2.
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f), associated with deformed substructure in bainite indicates possible formation of microbands. The traces of bainite are lost due to extensive plastic deformation caused in the structure. Micro-voids within the laths of bainite, surrounded by a dense network of crystallographic defects, are frequently observed at many places of the abraded layer. These voids are of a dimension of the order of 20–30 nm located within the dense dislocations network in a heavily plastically deformed bainitic structure, could possibly have formed due to shear-instability of the structure. The Alloy 2 shows intermediate wear resistance between carbide free bainitic and martensitic microstructure. SEM of the abraded surfaces qualitatively confirms the observations (Fig. 7c). Micro-pitting and micro-grooving are found to be the prominent wear mechanism for material removal with the signature of adhesive wear along with micro-cutting mechanism at few regimes. The abraded surface exhibits both groove marks and a large number density of pits. The pits are indicative of the obstacles caused by abrasive particles to the motion, causing the termination of a few grooves at the pits. TEM images and corresponding SADP (Fig. 11(c-f)) shows that the strain resulted in the degeneracy of martensite forming cementite
Fig. 9. Nano indentation of the abraded samples.
Fig. 10. Transmission Electron Microscopy of Alloy 1 with (a) carbide free bainite and retained austenite before abrasion and (b) corresponding SAD patterns (c) and (d) are the microstructure and SAD pattern of carbide free bainite and retained austenite being transformed to martensite after abrasion, (e) Ferrite micro bands formed by abrasion, (f) bainite lath with highly-density dislocation networks formed by abrasion.
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Fig. 11. Transmission Electron Microscopy of Alloy 2 with (a) composite microstructure of bainite and martensite with (b) SAD pattern of martensite and bainite before abrasion (c) coarse martensite plates with void nucleation site caused after abrasion, (d) plates with highly-density dislocation networks, (e) Cementite formation and (f) SAD pattern of the cementite along zone axis of [001].
higher than carbide free or mixture of martensite- bainite steels. 2. The sensitivity of the wear resistance to microstructure is because of the prevalence of different wear mechanisms involved in surface damage in each case. The carbide free bainite wears by a combination of grooving and relatively minor pitting and shows significant plastic deformation at the active surface with good adhesion of the damaged material. The retained austenite undergoes stress induced transformation resulting in formation of nano-scaled martensitic. Such nano martensitic lath formation results in a harder abraded layer on active surface, thus improving the abrasion resistance of steel. 3. In case of martensite, the heat generated and strain during abrasion causes the tempering of martensite and reduces the hardness of surface significantly. The wear mechanism was predominately micro-cutting with extensive removal of wear debris causing higher wear rate. 4. The steel with a mixture of bainite and martensite shows better wear resistance compared to the martensite but lesser compared to carbide-free bainite. The adiabatic heating and strain helped tempering of martensite leading to lowering of hardness of the active surface,
precipitates. Tempering of martensite reduces the brittleness of the structure and believed to improve the abrasion resistance. Micro-voids are initiated at the cementite particle between martensite and bainite lath and are surrounded by a dense network of crystallographic defects, which are frequently observed at many places of the abraded-layer. These voids are of a dimension of the order of 20–30 nm located within dense dislocations network in a heavily plastically deformed martensitic and bainitic structure, could possibly have formed due to shearinstability of the structure. To conclude it can be stated that structure shows ductile fracture with cementite particles serving as nucleation sites for micro-void generation. 4. Summary Three-body abrasive wear behavior has been studied for carbide free bainite with retained austenite, martensite and a mixture microstructure of martensite and bainite of similar hardness. Based on the experimental evidences, the following conclusions can be drawn: 1. The specific wear rate of martensite is approximately two times 214
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which aided more penetration by hard abrasives. [19]
Acknowledgements
[20]
The authors would like to convey thanks to the Director, CSIR-NML for providing his kind permission to publish the work. Fruitful technical discussion with Dr. Mainak Ghosh during TEM study is gratefully acknowledged. One of the authors is indebted for financial support to CSIR-National Metallurgical Lab in form of iPSG Project.
[21]
[22] [23]
References [24] [1] P.J. Blau, ASM Handbook, 18 Friction, Lubrication, and WearTechnology, ASM International, 1992. [2] Y.C. Lin, S.W. Wang, T.M. Chen, A study on the wear behavior of hardened medium carbon steel, J. Mater. Process. Technol. 120 (2002) 126–132. [3] H.-S. Yang, H.K.D.H. Bhadeshia, Designing low-carbon low-temperature bainite, Mater. Sci. Technol. 24 (2008) 335–342. [4] V.A. Leiro, E. Vuorinen, K.G. Sundin, B. Prakash, T. Sourmail, R.E. Smanio, G. Caballero, C. Garcia-Mateo, Wear of nano-structured carbide-free bainitic steels under dry rolling–sliding conditions, Wear 298–299 (2013) 42–47. [5] S. Das Bakshi, A. Leiro, B. Prakash, H.K.D.H. Bhadeshia, Dry rolling/sliding wear of nanostructured pearlite, Mater. Sci. Technol. 31 (2015) 1735–1744. [6] O. Vingsbo, S. Hogmark, Fundamentals of Friction and Wear of Materials, American Society for Metals, Metals Park, OH, 1981, pp. 373–408. [7] R.C.D. Richardson, The wear of metals by hard abrasives, Wear 10 (1967) 291–309. [8] P.L. Hurricks, Overcoming industrial wear, Ind. Lubr. Tribol. (1971) 345–356. [9] K.C. Barker, The Development of Abrasive-corrosive Wear Resistance of Steels by Microstructural Control (Ph.D. Thesis), University of Cape Town, 1988. [10] D.E. Diesburg, F. Borik, Optimizing abrasion resistance and toughness in steel and irons for the mining industry, Mater. Min. Ind., Ann. Arbor., MI (1974) 14–41. [11] J. Suchanek, V. Kuklik, Influence of heat and thermochemical treatment on abrasion resistance of structural and tool steels, Wear 267 (2009) 2100–2108. [12] O.P. Modi, D.P. Mondal, B.K. Prasad, M. Singh, H.K. Khaira, Abrasive wear behaviour of a high carbon steel: effects of microstructure and experimental parameters and correlation with mechanical properties, Mater. Sci. Eng. A 343 (2003) 235–242. [13] G.W. Stachowiak, A.W. Batchelor, Engineering Tribology, 3rd edition, Elsevier, 2005. [14] S. Das Bakshi, D. Sinha, S. Ghosh Chowdhury, V.V. Mahashabde, Surface and subsurface damage of 0.20 wt%-C martensite during three-body abrasion, Wear (2017), http://dx.doi.org/10.1016/j.wear.2017.07.004. [15] N. Prasad, S.D. Kulkarni, Relations between microstructure and abrasive wear of plain carbon steel, Wear 63 (1980) 29–338. [16] P. Clayton, N. Jin, Effect of microstructure on rolling/sliding wear of low carbon bainitic steels, Wear 200 (1996) 74–82. [17] P. Clayton, R. Devanathan, Rolling/sliding wear behavior of a chromium-molybdenum rail steel in pearlitic and bainitic conditions, Wear 156 (1992) 121–131. [18] J. Kalousek, D.M. Fegrdo, E.E. Laufer, The wear resistance and worn Metallography
[25]
[26]
[27] [28]
[29] [30]
[31] [32] [33] [34] [35] [36] [37] [38] [39]
215
of Pearlite, bainite and tempered martensite rail steel microstructures of high hardness, Wear 105 (1985) 199–222. P.H. Shipway, S.J. Wood, A.H. Dent, The hardness and sliding wear behaviour of a bainitic steel, Wear 203–204 (1997) 196–205. E. Vuorinen, A. Linstro¨m, P. Rubin, E. Navara, M. Oden Pellets, in: Proceedings of the 2nd World Conference on ‘Pellets', Jonkoping, Sweden, Stockholm: Swedish Bioenergy Association (SVEBIO), June 2006, 2006, pp. 151–155. T.S. Wang, J. Yang, C.J. Shang, X.Y. Li, B. Lv, M. Zhang, F.C. Zhang, Sliding friction surface microstructure and wear resistance of 9SiCr steel with low–temperature austempering treatment, Wear 282–283 (2012) 81–84. S. Das Bakshi, P.H. Shipway, H.K.D.H. Bhadeshia, Three-body abrasive wear of fine pearlite, nanostructured bainite and martensite, Wear 308 (2013) 46–53. S. Das Bakshi, A. Leiro, B. Prakash, H.K.D.H. Bhadeshia, Dry rolling/sliding wear of nanostructured bainite, Wear 316 (2014) 70–78. T. Sourmail, F.G. Caballero, C. Garcia-Mateo, V. Smanio, C. Ziegler, M. Kuntz, R. Elvira, A. Leiro, E. Vuorinen, T. Teeri, Evaluation of the potential of high Si high C steels nanostructured bainite for wear and fatigue applications, Mater. Sci. Technol. 29 (2013) 1166–1173. P. Zhang, F.C. Zhang, Z.G. Yan, T.S. Wang, L.H. Qian, Wear property of low-temperature bainite in the surface layer of a carburized low carbon steel, Wear 271 (2011) 697–704. S.Das. Bakshi, Wear of fine pearlite, nanostructured bainite and martensite, in: S. Das. Bakshi, Wear of Fine Pearlite, Nanostructured Bainite and Martensite, Ph.D. Thesis,University of Cambridge, UK.,University of Cambridge, UK. SSAB, Information materials SSAB, 〈http://www.ssab.com〉. G. Gomez, T. Perez, H.K.D.H. Bhadeshia, Air cooled bainitic steels for strong, seamless pipes Part 1 -Alloy design, kinetics and microstructure, Mater. Sci. Technol. 25 (2009) 1501–1507. H.K.D.H. Bhadeshia, D.V. Edmonds, Bainite in silicon steels: a new compositionproperty approach, Part Ii., Met. Sci. 17 (1983) 420–425. ASTM G65-16, standard test method for measuring abrasion using the dry rubberwheel apparatus, ASTM international, No 03.02, West Conshohocken, Pennsylvania,USA. M.J. Dickson, The significance of texture parameters in phase analysis by X–ray diffraction, J. Appl. Crystallogr. 2 (1969) 176–180. B.D. Cullity, Elements of X–ray Diffraction, Second edition, Addison-Wesley Publishing Company Inc, California, 1978, p. 412. D.J. Dyson, B. Holmes, Effect of alloying additions on the lattice parameter austenite, J. Iron Steel Inst. 208 (1970) 469–474. M.J. Peet., 〈http://mathewpeet.org/thesis/programs/〉. H.K.D.H. Bhadeshia: Bainite in Steels: Theory and Practice, 3rd Edition, 2015, pp. 19. C.A. Schneider, W.S. Rasband, K.W. Eliceiri, NIH image to image J: 25 years of image analysis, Nat. mathods 9 (2012) 671–675. H.K.D.H. Bhadeshia, High performance bainitic steels, Mater. Sci. Forum 500–501 (2005) 63–74. H.K.D.H. Bhadeshia: Bainite in Steels: Theory and Practice, 3rd Edition, 2015, pp. 3–5. M.U. Cohen, Precision lattice constants from X‐ray powder photographs, Rev. Sci. Instrum. 6 (1935) 68–74.