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Specific energy and the modified rubber wheel abrasion test
Wear 370-371 (2017) ∎∎∎–∎∎∎
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Specific energy and the modified rubber wheel abrasion test Zhihan Lin, T.G. Joseph, M. Curley n University of Alberta, Edmonton, Alberta, Canada T6G 2R3
art ic l e i nf o
a b s t r a c t
Article history: Received 11 October 2016 Received in revised form 31 October 2016 Accepted 5 November 2016
In the oil sands industry a single ultra-class shovel tip can lose more than 35 kg of steel mass in one operating day. Equipment downtime is significantly increased with frequent stoppages to replace worn shovel teeth. This leads to a substantial loss in shovel availability and utilization, as well as a considerable increase in consumable cost. This paper develops a means to predict the wear performance of shovel tips based on field data through the use of specific energy (Es), which is defined as the friction energy required to cause a unit volume loss of material (Nm/m3). A modified rubber wheel abrasion test (similar to ASTM-G65) is presented for the determination of Es. Results show that it is possible to predict the performance of shovel tips. It is also found that Es provides an index to quantify the resistance of wear materials to abrasion under specific abrasive conditions. & 2016 Elsevier B.V. All rights reserved.
Keywords: Dry-sand rubber-wheel Modified rubber wheel test Abrasion Specific energy Hardfacing
1. Introduction
1.1. Background
Electric cable shovels are the most commonly used ultra-class scale excavation equipment in the oil sand mining industry. In the Athabasca oil sand region of Northern Alberta, Canada, the application of cable shovels has proven very effective. However, severe wear caused by interactions between shovel tips and abrasive media leads to significant expenses related to equipment maintenance and production loss. The study of abrasion is of major interest to the mining industry; however, most research has previously concentrated on the theoretical analysis and establishment of micro-scale models, which are difficult to validate for engineering purposes. Some research has aimed to improve wear resistance of materials by means of chemical technologies, which is time consuming and cost intensive. A simple but practical method to facilitate selection of materials to match actual abrasive conditions encountered in the field has been targeted in this paper to realize greater performance from ground engaging tools with little investment. The goal of this study was to investigate a scaled abrasion test to measure specific of energy wear resistant materials interacting with abrasive media and to apply the concept of specific energy to wear life predictions for the ground engaging tools (GET) operating in the Alberta oil sands.
Oil sands are complicated mixtures of quartz, bitumen, and water with quartz accounting for greater than 80% of the total solids and acting as the predominant abrasive and erosive media [1]. 99% of quartz grains in oil sand are waterwet; with the bitumen occupying the interstitial space and a water phase forming a film around the grains. Ground engaging tools mounted on cable shovels operating in oil sand are subjected to severe abrasive wear damage caused by these hard quartz particles. Oil sand displays high shear strength but minor cohesion, with no adhesion damage to ground engaging tools. The abrasive particles are hard but with varying size and shape which makes the application of the accepted G65 rubber wheel abrasion test less than ideal.
n
Corresponding author. E-mail address: [email protected] (M. Curley).
2. Abrasive wear Abrasion, defined as the removal of materials from an abraded surface, is the most common form of wear attack in earthmoving, mining, and mineral processing equipment. For cable shovels and other ground engaging tools, severe abrasion is caused by the interaction between the surface of the shovel teeth and ground. Penetration by hard quartz particles creates plastic deformation of the softer tooth material, which when coupled with sliding motion, results in material removal. Abrasion can be classified as lowstress abrasion, high-stress abrasion, or gouging abrasion according to the degree of severity [2–4]. These three forms of abrasion are encountered by ground engaging tools. Generally, shovel teeth
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are subjected to low-stress abrasion in soft and free excavated materials like oil sands, whereas high-stress or gouging abrasion can occur during excavation of hard, blasted minerals. The actual mechanism with which abrasion works is highly dependent on both the abraded and abrasive material properties but can be characterized as: micro-ploughing, micro-fatigue, micro-cutting and micro-cracking [5]. 2.1. Abrasive failure of shovel teeth In the mining industry, ground engaging tools are subjected to heavy abrasive damage due to severe interactions between teeth and ground. The occurrence of abrasive wear on shovel teeth not only reduces a shovel's operating efficiency, but also leads to significant production loss due to unplanned maintenance. Knights [6] showed that a set of nine teeth was only worth US $2700, but the average production lost caused by an unplanned change-out of a tooth set was US $38,000 [6]. Even though properties of materials such as toughness, ease of fabrication, and weldability have an influence on the performance of ground engaging tools, hardness is the most significant factor considered [1]. In an effort to increase abrasive resistance of shovel teeth, martensitic steel castings have been suggested as the substrate materials, with hardfacing materials employed as protective coatings. Martensitic steel castings have a unique combination of relatively high hardness, suitable toughness, and ease of fabrication; thereby providing an appropriate substrate material for a shovel tooth. Martensitic steels belong to the medium carbon material class of steels with up to 4% alloy [7]. Depending on the digging condition, various combinations of toughness, hardness, weldability, and strength of martensitic steels can be achieved through metallurgical techniques such as alloying and heat treatment. Hardness’ fall in the range of 243–560 HV; much lower than quartz at 850–900 HV. Since quartz sands are the dominant abrasive constituents in oil sands, hardfacing is typically applied to enhance wear resistance and to approach the hardness requirement in practice. The most widely used hardfacing technique is welding deposition. Hardfacing has many advantages including a large range of achievable hardness, corrosion resistance and the ability to permit repairs. The hardness of welding deposits ranges between 513 and 800 HV. Chromium carbide or chrome white irons are the most common hardfacing welding consumables [2,7]. More recently developed consumables include tungsten carbide-based materials, which contain up 75% tungsten carbide particles that have a hardness up to 1900 HV; providing extreme abrasion resistance for shovel teeth. Hardfacing drawbacks include the possible occurrence of cracking, especially on thick deposits, and the influence of high welding temperatures on the microstructure of substrates [8].
Fig. 1. Dry sand/rubber wheel abrasion apparatus (Adapted from ASTM G65).
holder and loading a set force between the specimen and the rubber wheel; setting the revolution counter; adjusting and starting the sand flow; starting the wheel rotation; stopping the drive motor after running the desired number of wheel revolutions; and removing, cleaning and reweighing the specimen. The dry sand/rubber wheel test should be only used for wear ranking, not for specifying absolute wear values. Therefore, to mimic actual circumstances, variants of the standard procedure must be made to obtain the type of wear information required for engineering purposes. Aside from changes in the loading weight and sliding distance, the rate of sand flow, abrasive characteristics, and test duration can be reconfigured. Research has shown that approximately 200 wheel revolutions is adequate to create a steady wear rate and that multiple shorter tests could be run instead of a single long test to protect the rubber wheel [11]. 2.3. Relationship between abrasion and energy Abrasive wear can be described as a hard conical particle penetrating and sliding within a softer material as shown in Fig. 2. In a typical abrasion function, (1), the abrasive wear is quantified as a volume loss generated by a single conical particle sliding over a distance Li [12,13].
Vi =
2 WL ⋅ i i π⋅tan θ H
(1)
Eq. (1) is the typical abrasion function, where 2/(π⋅tan θ ) represents a wear coefficient and is dependent on the ductility of the
2.2. Existing abrasion tests The two most commonly used abrasion tests are the jaw crusher gouging abrasion test (ASTM-G61) and the dry sand rubber wheel test (ASTM-G65) [9,10]. The jaw crusher gouging abrasion test is primarily used to study the wear of ground engaging tools interacting with hard and large abrasives representing conditions commonly associated with quarry and metallic mineral mining operations. For the case of fine abrasives such as oil sands, the dry sand rubber wheel (DSRW) test is more suitable as there is little occurrence of breakage during excavation in a soft abrasive medium like oil sands. The dry sand rubber wheel setup is shown in Fig. 1. The general procedure for the ASTM-G65 test consists of the following steps: cleaning and weighing the specimen; fixing the specimen in the
Fig. 2. A typical model of abrasive wear by a conical particle (after [12]).
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material, interfacial shear strength, and particle shape [14]. In order to use friction energy to study abrasive wear, the friction expression μWi is introduced via Eq. (2) which can be written as:
2 μWi L i ⋅ π⋅tan θ⋅μ H
Vi =
(2)
The possible volume loss caused by friction energy is then given by:
2 AE ⋅ i π⋅tan θ⋅μ H
Vi =
n
n
i=n
i=1
2A 2A Ei = E πμ tan θ πμ tan θH
(4)
In specific abrasion conditions (where 2A /μπ tan θ may be determined), the volume loss (V), of material with hardness H will be a function of the friction energy E. In other words, abrasion results from friction energy caused by the interaction between the material and particles. Therefore it is possible to employ friction energy as a simple but effective method to quantify wear. 2.4. Specific energy Specific energy ( ES =E /V ) is defined as the friction energy required to cause a unit volume loss of material (J/m3 or Pa) and it has been widely used as a measure of energy efficiency in the machining industry [15]. In terms of abrasive wear, the specific energy ES is given by Eq. (5):
ES =
E πμ tan θH = V 2A
V 1 = μFv t Es
(6)
Where, V/t, F, v, and μ are volume loss rate, normal force, velocity, and friction coefficient, respectively. When all parameters are known, specific energy may be obtained. In order to apply specific energy to shovel teeth, the determination of a normal force F and velocity v based on real field data is required. A modified abrasion test has been designed to measure specific energy.
(3)
In Eq. (3) the parameter A is related to the order of magnitude of sliding distance. For example, when the sliding distance Li is 1 cm the value of A should be 10 2 so that the units of friction energy μWiLi will be N m. To some extent the sliding distance depends on particle size. For example, when a particle size is around 1 mm, its sliding distance should be in the range of 0.1 cm to 1 cm, while a particle with size of 1 cm is more likely to slide from 1 cm to 10 cm. Therefore parameter A can be considered as a particle size coefficient. Eq. (3) gives the relationship between abrasion (V), friction energy (E), material hardness (H), and the abrasive conditions characterized by the friction coefficient (μ), particle angularity (θ), and particle size (A). This relationship may be used to explain how material hardness, particle characteristics, contact condition, and normal force influence abrasive wear. When numerous particles abrade material surfaces the total volume loss V may be expressed as:
V = ∑ Vi = ∑
3
3. Modified abrasion test In order to measure specific energy, a modified rubber-wheel abrasion test (MRWAT) has been designed based on the standard dry sand/rubber wheel abrasion test (ASTM-G65) [9]. The main advantage of the modified test is its ability to better replicate actual abrasive field conditions experienced in an oil sands mining environment. A total of 85 test runs were performed; 37 samples for Al 61 and 16 each for Al 63, mild steel A36, and stainless steel 17-4SS. A schematic diagram of the experimental setup of the MRWAT test is shown in Fig. 3. This apparatus is similar to the standard ASTM G65 setup but with two major differences: the abrasive medium used and the wheel. Oil sand has zero cohesion but high friction angle [16]. The quartz particles in oil sand vary in shape from angular to rounded, generally described as being sub-angular. The sand used in the MRWAT is in-situ oil sand (bitumen stripped) rather than the standard AFS 50/70 test sand which consists of rounded quartz grains and is generally used in ASTM G65. Secondly; the ASTM G65 procedure specifies a rubber wheel of with a 22.9 cm diameter and 1.3 cm width whereas the MRWAT use a 15.2 cm diameter wheel that is 5.1 cm wide. This wheel is coated with a thin layer of oil sand which provides multiple advantages: the rubber wheel is protected from severe abrasive damage; full contact is ensured between abrasive sand and the test specimen; and decrease in the change of contact area at different levels of applied load. The hardness of the rubber wheel is Durometer A-60; the hardness was tested at the beginning of testing and before each coupon material substitution. Combined with the layer of oil sand material coating the wheel, the hardness is sufficient enough such that it did not change during the course of the tests. The MRWAT test has been designed as a low-stress abrasion test. The low level of contact stress ensures that most of the friction energy is transferred into abrasion of the targeted materials rather than breakage of the sand particles. Field data normal force values are much too large for laboratory application, and had to be
(5)
Specific energy is defined as the ratio of friction energy to a corresponding volume loss of material. An increase in volume loss occurs from a rise in friction energy by greater applied force or higher velocity. Regardless of the change in force and velocity, the specific energy for a single homogeneous material can be considered as a constant and is independent of scaling. Analysis of Eq. (5) can be used to demonstrate that Es is not influenced by scale, but rather it is determined by material hardness (H), particle size (A), particle angularity (θ), and contact condition (μ). The particle size, particle angularity, and contact condition are collectively termed the abrasive conditions. The specific energy may be regarded as an index, reflecting the performance of a material with hardness H working under specific abrasive conditions characterized by the parameters A, θ, and μ. According to the definition of specific energy ( ES =E /V =μFvt /V ), a measure for specific energy may be:
Fig. 3. Schematic of the MRWAT apparatus.
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Fig. 5. Grain size distribution before and after MRWAT.
Fig. 4. Normal resistance applied to teeth surfaces.
scaled down to appropriate values that facilitated safe, repeatable lab experiments. The scaling process is detailed below. 3.1. Parameters and specimen characteristics of MRWAT 3.1.1. Test parameters 3.1.1.1. Normal force. In order to create a relationship between lab results and field data, a scale factor for contact force was applied. The scale factor was determined from a ratio of the shovel teeth area to the contact area of the coupon produced by exerting a normal force against the rotating rubber wheel. The actual calculation of the scale factor was done using a square power law, since the ratio was dealing in areas rather than a singular length dimension. Using field data from a cable shovel operating in an oil sand environment the normal digging resistance applied to the teeth surface was found to range from 0 to 1600 kN (Fig. 4). A rubber wheel 5.1 cm wide and 15.2 cm in diameter resulted in a scaling factor equal to 12
SF =
ATeeth =12 AContact
(7)
Table 1 shows the conversion from actual normal resistance obtained from the field data (in kN), to lab normal loads and applied weights. After scaling, the lab normal loads ranged from 32 N to 130 N, which fall within an acceptable range for operation of the designed experimental tool. The lab normal loads take in to account the total mass of plates applied, the mass of the lever arm as well as the lever arm leverage ratio which was equal to 2.75. The normal resistance loads are scaled similarly to the contact area shown in Eq. (7); however, a cube root scaling approach is applied in comparison to the square root previously used in the calculation of the contact area.
was not deemed necessary to apply a scaling factor to the rubber wheel's rotational speed. The conversion from the actual digging velocities to rotation speed was solely made proportional to wheel diameter.
3.1.1.3. Abrasive media. The sand used in the abrasion tests were derived from oil sands stripped of bitumen primarily dominated by quartz. This was assumed the predominant abrasive media as hard quartz particles occupy the largest proportion of the total solids [1]. The size distribution for the sand is shown in Fig. 5, where 100% of particles passed 2000 μm (No.10 U.S Standard Sieve) and nearly 80% pass 850 μm (No.20 U.S Standard Sieve). The sub-angular particles were loose and totally dry (Fig. 6). It should be noted that the consistency of the grain size distribution before and after use highlighted that the MRWAT was a low stress abrasion test which allowed the abrasive media to penetrate and slide over a surface without crushing the particles. It was therefore assumed that the majority of friction energy translated into abrasive wear of the material tested. The application of a rubber wheel instead of a steel wheel also precluded occurrence of particle breakage [17]. 3.1.1.3.1. Time and flow rate. As discussed previously, specific energy was defined as the friction energy expended to cause a unit volume loss of material, or the friction power expended for a given volume loss rate; such that specific energy was normalized with respect to time. However; measurements over short periods led to an inaccurate estimation of volume loss; while longer duration measurements were time consuming. Test durations of 4 min were selected although this was an arbitrary decision. The flow rate of the abrasive media was matched to the rotation speed of the wheel so as to mimic the actual digging conditions.
3.1.1.2. Rotation speed. Another important parameter was the rotation speed. Analysis of representative field data shows that the actual digging velocities of the teeth vary from 0 m/s to approximately 1.1 m/s, with a large proportion falling in the range of 0.2 m/s to 0.8 m/s. Since the actual digging velocity was not high, it Table 1 Relationship between the applied mass and lab and field forces. Applied weight (kg)
Lab normal loads (N)
Normal resistance (kN)
0 0.45 0.91 1.36 1.81
32 44 57 69 82
454 625 797 969 1140 Fig. 6. Shape of abrasive media used in MRWAT.
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Table 2 Properties of metal specimens. Avg. Shore #
Vickers Hardness
ASTM B-221 2.7 g/cm3
–
83
3
–
107
Metal
Abbreviation Standard #
Density
Aluminum 6063-T6 Aluminum 6061-T6 Mild Steel A36 Stainless Steel 174SS
Al 63 Al 61
ASTM B-209 2.7 g/cm
MS
ASTM A36
7.85 g/cm3 32
213
SS
ASTM A693
7.85 g/cm3 40
279
3.1.2. Test specimen Material hardness has a significant influence on degree of abrasive wear and it is the major selection criteria for materials. Four metals with different hardness were selected for the tests (Table 2): Aluminum 6063-T6, Aluminum 6061-T6, Mild Steel, and Stainless Steel. These materials were chosen because of the hardness range they represent. Samples measuring 5.1 cm wide by 7.6 cm long by 1.3 cm thick were used as test coupons. 3.2. Interfacial shear test The interfacial shear test (IST) is an auxiliary test designed to determine the friction coefficient between abrasive media and the surface of materials used in the MRWAT. Friction can be considered as equivalence to interfacial shear strength between two different materials. Measurement of the friction coefficient in this research was carried out through an interfacial shear test (IST) developed from a conventional direct shear test (DST) system. The purpose of including the sand/sand and sand/metal interfacial shear box tests are to verify that slip will occur at the sand/metal interface before internal shear within the sand. This is required for successful application of the MRWAT. Table 3 shows that this will be the case for materials harder than those tested here, eliminating the need to redo the tests until it is probable that the sand/ metal friction angle will reach the internal friction angle of the abrasive media. A schematic diagram of the interfacial shear box is shown in Fig. 7, which follows the standard DST setup per ASTM D5321A. A metal sample was placed between the upper and lower boxes, fixed to the lower box with clamping screws. The same abrasive media used in the MRWAT was placed into the upper box, with a layer of media evident between the metal sample and the upper box. When a shear force was applied, the upper box moved while the lower box and the fixed sample remained stationary. In contrast to the direct shear test, the shear area between the abrasive media and material remained constant. The friction coefficient (the interfacial friction angle) was defined as the slope of the resulting shear stress to normal stress plot. The metal specimens tested in the IST were the same as those used in MRWAT (Table 2). However, sand/sand slip may accompany sand/ metal slip during a shear test, giving rise to false interfacial friction
Fig. 7. Schematic of the interfacial shear box.
angles. Brumund and Leonards [18] demonstrated that the sand/ sand slip will occur when the internal friction angle of sand was equal or less than the interfacial friction angle between the sand and material. A direct shear test was also conducted for the abrasive media without the coupon present so as to account for the influence of sand/sand slip on the sand/material slip data. The results of the interfacial and direct shear tests conducted at normal stresses of 150 kPa, 250 kPa, 350 kPa, and 450 kPa are presented in Fig. 8 and Table 3 below. From these results it was concluded that the sand-sand friction angle is over double the sand-metal results suggesting that sand-sand slip will not occur during the MRWAT. 3.3. Modified rubber wheel abrasion test 3.3.1. Abrasion test results The modified rubber wheel abrasion test was designed to measure specific energy for materials via mimicking the abrasive conditions experienced by shovel teeth working in oil sands. From Eq. (6) the specific energy is seen equal to the reciprocal value of the slope of a volume loss rate verses friction power relationship. The MRWAT results for mild steel are presented as an example in Fig. 9. For all materials tested there was a strong linear relationship between volume loss rate and friction power, with the lowest correlation coefficient experienced being 0.97. The hardness, friction coefficient, and specific energy of each material tested are summarized in Table 4. Hardness is defined as the mean pressure to cause materials to undergo a plastic deformation. Harder
Table 3 Comparison of internal friction angle and interfacial friction angles. Test
Correlation coefficient
Friction angle (deg)
Sand/Sand Sand/Al 63 Sand/Al 61 Sand/MS Sand/SS
0.98 1 1 1 1
60.7 25.6 24.6 23.4 22.6 Fig. 8. Relationship between residual shear stress vs. normal stress.
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Fig. 9. Relationship between volume loss rate and friction power for Mild Steel (MS).
ES = BH
Table 4 Characteristics of materials tested. Material Hardness Hv Number Pressure (MPa)
Friction coefficient
Es ( 102 GPa)
Al 63 Al 61 MS SS
0.48 0.46 0.43 0.42
1.69 1.70 7.09 8.07
Very Soft Soft Medium Hard
83 107 213 279
Fig. 10. Relationship between velocity and volume loss rate.
814 1050 2090 2737
materials deform less than softer ones under the same stress. The standard Vickers hardness number can be converted to units of pressure (MPa) by multiplying by 9.81. Volume loss is measured by weighing the sample coupons before and after testing and then using the materials’ density to calculate the volume of material lost. From Table 4 it can be seen that specific energy increases, as would be expected, with an increasing hardness of materials. For example, the specific energy of medium hard stainless steel is roughly 5 times than that of very soft Aluminum 63. In accordance with the definition of specific energy; energy to cause a unit volume loss; soft materials with low specific energy require a lower friction energy to cause the same volume loss as might be seen for hard materials. Therefore specific energy can be used to rank resistance of materials to abrasive wear under similar abrasive conditions.
(8)
Here, B is termed as the specific energy coefficient and is equal to μπ tan θ /2A. The hardness as previously defined is the mean pressure acting on a material to cause fully plastic deformation, commensurate with a Vickers hardness number (HV) which can be described in pressure units by multiplying by gravity. In this paper, all abrasion tests were performed using the same abrasive media; such that a linear relationship between specific energy (Es) and hardness pressure (H) exists. Fig. 11 shows the relationship between specific energy and hardness. Even though the relationship between wear rates, energy and hardness becomes more complicated for the microstructures like alloys, the formula Es¼ BH may estimate the specific energy of any material subjected to the same abrasive conditions. The specific energy coefficient B, determined for the abrasive conditions via (μ tanθ)/2 A, of 2900 – shown in Fig. 11 – was obtained for the abrasive conditions examined in the laboratory to mimic the abrasive wear that occurs on shovel teeth, such that it is only viable for materials which are subjected to this identical abrasive condition. When either the particle characteristics or the contact condition between the particles and material are changed, the specific energy coefficient B would also need to be calibrated. Again, B should be evaluated from a group of samples tested under the same abrasive conditions.
3.3.2. Relationship between friction and specific energy The relationship between friction and specific energy is of interest in verifying specific energy independent of an abrasion index. The abrasion function, Eq. (6), illustrates that the volume loss rate (V/ t) is a function of friction (μF) and velocity (v) while specific energy (Es) is the function coefficient. When the variable μF is fixed, there exists a linear relationship between V/ t and v with a slope of μF/Es, Fig. 10. Fig. 10 provides a sample relationship between volume loss rate (V/t) and velocity (v) for the material. Fig. 10 also shows that as the overall friction energy increases, the volume loss rate increases; a direct result of an increase in friction energy. 3.3.3. Relationship between specific energy and hardness When the particle characteristics (size coefficient A and shape parameter θ) and contact condition (friction coefficient μ) are held constant, the specific energy (Es) should be proportional to the hardness (H); Eq. (8):
Fig. 11. Specific energy vs. Hardness.
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Z. Lin et al. / Wear 370-371 (2017) ∎∎∎–∎∎∎ Table 5 Example data. Parameter Value EC TC A’ U ES
240 kJ/cycle/tooth 40 s/cycle 0.8 0.8 709 GJ/m3
4. Application of specific energy to GET performance One of the most important applications of specific energy is to predict GET performance through the estimation of life expectancy. Eq. (6) may then be adapted to predict the operating time for a GET when a specific energy expenditure is known; Eq. (9):
t =
ES V μFv
ES V ⋅TC ⋅A′⋅U EC
40 s cycle, an availability and utilization both equalling 0.8 and a specific energy value Es¼ 709 GJ/m3 from the lab, tests revealed that the medium wear tooth had a predicted operating time of 44.3 h compared to the 45 h actually recorded from field data. The severe wear tooth was predicted to have experienced 97.3 h of operating time while field data showed the tooth was in use for 96 operating hours. The difference between predicted and actual field life for the medium and severe wear tooth is approximately 1.4%. This level of deviation shows the potential of specific energy to closely predict the performance of ground engaging tools as well as potentially aid in the selection of materials for specific abrasive conditions (Fig. 12). In the case of shovel teeth, even though there is no optimal combination of force and velocity, abrasive wear can still be effectively reduced when both digging velocity and hoist force are at low levels. Practically, cable shovels are suggested to operate at high hoist forces but low digging velocities to keep production up, while decreasing abrasive wear on ground engaging tools.
(9)
Where V is the maximum volume loss before failure, F, v, and μ are the normal force, velocity, and friction coefficients respectively, and their product (μFv) is the friction power. Time (t) is the total time for the materials under continuous abrasion to failure. The value of specific energy (Es) can be obtained through abrasion tests or evaluated from the formula Es¼ BH. Based on Eq. (9), for shovel teeth operating in the Canadian oil sands, the prediction for their actual operating hours is expressed as Eq. (10):
T =
7
(10)
Where Ec is the friction energy per digging cycle per shovel tooth, Tc is the duty cycle time, V is the volume loss of a shovel tooth before failure, A′ is shovel availability and U is the shovel's utilization. According to the field data represented by the failed shovel teeth evaluated in this investigation, one experienced medium wear with a volume of loss of 0.0021 m3 (16.56 kg loss), and another of severe wear with a volume loss of 0.0046 m3 (36.34 kg loss) (Table 5). As an example, using an estimated friction coefficient of 0.45 between the shovel teeth and oil sand, a test material was evaluated. At an energy expenditure of 240 kJ/cycle per shovel tooth, a
5. Conclusion The goal of this paper was to study the abrasive wear of shovel teeth and to facilitate the prediction of performance for ground engaging tools. In order to achieve this goal the principle of specific energy was defined as the friction energy required to cause a unit volume loss of material. A modified rubber wheel abrasion test and an interfacial shear test were designed to measure specific energy of materials. Operating hours for shovel teeth were then estimated through specific energy matched to field data, suggesting that specific energy was useful to predict GET performance. Summarily, some conclusions through this study on abrasive wear of shovel teeth in oil sands are specified: 1. Eq. (6) ( V /t =1/Es μFv ), derived from a typical abrasive model, illustrated that abrasion by nature results from the friction energy, making it possible to predict abrasion from the point view of specific energy defined as the energy required to abrade a unit volume of material. 2. Specific energy can be evaluated through a modified rubber wheel abrasion test (MRWAT) which closely mimics actual abrasive working conditions in a Canadian oil sands mining operation. The specific energy is independent of scaling, normal
Fig. 12. Shovel tooth with severe wear after 18 h use (Used with permission from M. Curley).
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force, and velocity. It is merely determined by particle characteristics and contact conditions via of Eq. (5). Specific energy may be considered as an index quantifying the resistance of a material with hardness H to abrasion by a specific abrasive condition characterized by the particle size coefficient (A), particle angularity (2θ), and contact condition (μ). However, specific energy is not an inherent material property, since it is influenced by the abrasive conditions. 3. A linear relationship between specific energy and hardness exists is evident which is consistent with the equivalent conversion of specific energy units from J/m3 to Pa. Eq. (8) may be used to estimate the specific energy of other materials. The specific energy coefficient B should be evaluated from a group of like-materials tested under the same abrasive conditions, and should only be used for materials subjected to similar abrasive conditions. 4. The operating time predicted via specific energy matches the field data represented by known failed shovel teeth; such that it is possible to facilitate the prediction of the performance of ground engaging tools wear resistant materials.
References [1] R. Llewellyn, Resisting wear attack in oil sands mining and processing, CIM Bull. 90 (1012) (1997) 75–82. [2] C. Cross, D.L. Olson, Friction and wear in the mining and mineral industries, in: P.J. Blau (Ed.), ASM Handbook: Friction, Lubrication and Wear Technology, vol. 18, ASM International, 1992. [3] T. Norman, Wear in ore processing machinery, in: W. Peterson, W. Winer
(Eds.), Wear control handbook, ASME, New York, New York, 1980. [4] J.A. Hawk, R.D. Wilson, Tribology of earthmoving, mining, and minerals processing, in: B. Bhushan (Ed.), Modern Tribology Handbook, vol. II, CRC Press LLC, Boca Raton, Florida, 2001, pp. 1331–1368. [5] K.H. Zum Gahr, Modelling of two-body abrasive wear, Wear 124 (1) (1988) 87–103. [6] P.K. Knights, Optimal replacement intervals for shovel dipper teeth, Int. J. Min. Reclam. Environ. 23 (2009) (157-17). [7] R. Llewellyn, Materials for controlling wear in surface mining, CIM Bull. 89 (1002) (1996) 76–82. [8] S.F. Murray, Wear resistant coatings and surface treatments, in: E.R. Booser (Ed.), Handbook of Lubricants: Theory and Practice of Tribology, CRC Press LLC, Boca Raton, London, New York, New York, 1988. [9] ASTM. G65-04. Standard Test Method for Measuring Abrasion Resistance Using the Dry Sand/Rubber Wheel Apparaturs. In: Book of Standards, ASTM International. [10] ASTM. G81 Standard Test Method for Jaw Crusher Gouging Abrasion Test. In: Book of Standards, ASTM International. [11] J.A. Hawk, Abrasive wear testing, in: H. Kuhn, D. Medlin (Eds.), Mechanical Testing and Evaluation, vol. VIII, ASM International, Materials Park, Ohio, 2000, pp. 325–337. [12] K. Kato, K. Adachi, Wear mechanisms, in: B. Bhushan (Ed.), Modern Tribology Handbook, vol. I, CRC Press LLC, Boca Raton, Florida, 2001, pp. 273–300. [13] R. Gohar, H. Rahnejat, Fundamentals of tribology, Imperial College Press, London, 2012. [14] E. Rabinowicz, Wear coefficients – metals, in: M.B. Peterson, W.O. Winer (Eds.), Wear Control Handbook, ASME, New York, New York, 1980. [15] I.D. Marinescu, W.B. Rowe, D. Dimitrov, H. Ohmori, Tribology of abrasive machining processes, William Andrew, Oxford, 2013. [16] R.N. Morgenstern, D.J. Scott, Shear strength of Athabasca oil sands, Geotech. News 15 (1997) 102–109. [17] J.P. Lafleur, A study of abrasion in steel during comminution, (MSc. dissertation/thesis), McGill University, Canada, 2011. [18] W.F. Brumund, G.A. Leonards, Experimental study of static and dynamic friction between sand and typical construction materials, J. Test. Eval. 1 (1973) 163–165.
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Specific energy and the modified rubber wheel abrasion test Zhihan Lin, T.G. Joseph, M. Curley n University of Alberta, Edmonton, Alberta, Canada T6G 2R3
art ic l e i nf o
a b s t r a c t
Article history: Received 11 October 2016 Received in revised form 31 October 2016 Accepted 5 November 2016
In the oil sands industry a single ultra-class shovel tip can lose more than 35 kg of steel mass in one operating day. Equipment downtime is significantly increased with frequent stoppages to replace worn shovel teeth. This leads to a substantial loss in shovel availability and utilization, as well as a considerable increase in consumable cost. This paper develops a means to predict the wear performance of shovel tips based on field data through the use of specific energy (Es), which is defined as the friction energy required to cause a unit volume loss of material (Nm/m3). A modified rubber wheel abrasion test (similar to ASTM-G65) is presented for the determination of Es. Results show that it is possible to predict the performance of shovel tips. It is also found that Es provides an index to quantify the resistance of wear materials to abrasion under specific abrasive conditions. & 2016 Elsevier B.V. All rights reserved.
Keywords: Dry-sand rubber-wheel Modified rubber wheel test Abrasion Specific energy Hardfacing
1. Introduction
1.1. Background
Electric cable shovels are the most commonly used ultra-class scale excavation equipment in the oil sand mining industry. In the Athabasca oil sand region of Northern Alberta, Canada, the application of cable shovels has proven very effective. However, severe wear caused by interactions between shovel tips and abrasive media leads to significant expenses related to equipment maintenance and production loss. The study of abrasion is of major interest to the mining industry; however, most research has previously concentrated on the theoretical analysis and establishment of micro-scale models, which are difficult to validate for engineering purposes. Some research has aimed to improve wear resistance of materials by means of chemical technologies, which is time consuming and cost intensive. A simple but practical method to facilitate selection of materials to match actual abrasive conditions encountered in the field has been targeted in this paper to realize greater performance from ground engaging tools with little investment. The goal of this study was to investigate a scaled abrasion test to measure specific of energy wear resistant materials interacting with abrasive media and to apply the concept of specific energy to wear life predictions for the ground engaging tools (GET) operating in the Alberta oil sands.
Oil sands are complicated mixtures of quartz, bitumen, and water with quartz accounting for greater than 80% of the total solids and acting as the predominant abrasive and erosive media [1]. 99% of quartz grains in oil sand are waterwet; with the bitumen occupying the interstitial space and a water phase forming a film around the grains. Ground engaging tools mounted on cable shovels operating in oil sand are subjected to severe abrasive wear damage caused by these hard quartz particles. Oil sand displays high shear strength but minor cohesion, with no adhesion damage to ground engaging tools. The abrasive particles are hard but with varying size and shape which makes the application of the accepted G65 rubber wheel abrasion test less than ideal.
n
Corresponding author. E-mail address: [email protected] (M. Curley).
2. Abrasive wear Abrasion, defined as the removal of materials from an abraded surface, is the most common form of wear attack in earthmoving, mining, and mineral processing equipment. For cable shovels and other ground engaging tools, severe abrasion is caused by the interaction between the surface of the shovel teeth and ground. Penetration by hard quartz particles creates plastic deformation of the softer tooth material, which when coupled with sliding motion, results in material removal. Abrasion can be classified as lowstress abrasion, high-stress abrasion, or gouging abrasion according to the degree of severity [2–4]. These three forms of abrasion are encountered by ground engaging tools. Generally, shovel teeth
http://dx.doi.org/10.1016/j.wear.2016.11.002 0043-1648/& 2016 Elsevier B.V. All rights reserved.
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are subjected to low-stress abrasion in soft and free excavated materials like oil sands, whereas high-stress or gouging abrasion can occur during excavation of hard, blasted minerals. The actual mechanism with which abrasion works is highly dependent on both the abraded and abrasive material properties but can be characterized as: micro-ploughing, micro-fatigue, micro-cutting and micro-cracking [5]. 2.1. Abrasive failure of shovel teeth In the mining industry, ground engaging tools are subjected to heavy abrasive damage due to severe interactions between teeth and ground. The occurrence of abrasive wear on shovel teeth not only reduces a shovel's operating efficiency, but also leads to significant production loss due to unplanned maintenance. Knights [6] showed that a set of nine teeth was only worth US $2700, but the average production lost caused by an unplanned change-out of a tooth set was US $38,000 [6]. Even though properties of materials such as toughness, ease of fabrication, and weldability have an influence on the performance of ground engaging tools, hardness is the most significant factor considered [1]. In an effort to increase abrasive resistance of shovel teeth, martensitic steel castings have been suggested as the substrate materials, with hardfacing materials employed as protective coatings. Martensitic steel castings have a unique combination of relatively high hardness, suitable toughness, and ease of fabrication; thereby providing an appropriate substrate material for a shovel tooth. Martensitic steels belong to the medium carbon material class of steels with up to 4% alloy [7]. Depending on the digging condition, various combinations of toughness, hardness, weldability, and strength of martensitic steels can be achieved through metallurgical techniques such as alloying and heat treatment. Hardness’ fall in the range of 243–560 HV; much lower than quartz at 850–900 HV. Since quartz sands are the dominant abrasive constituents in oil sands, hardfacing is typically applied to enhance wear resistance and to approach the hardness requirement in practice. The most widely used hardfacing technique is welding deposition. Hardfacing has many advantages including a large range of achievable hardness, corrosion resistance and the ability to permit repairs. The hardness of welding deposits ranges between 513 and 800 HV. Chromium carbide or chrome white irons are the most common hardfacing welding consumables [2,7]. More recently developed consumables include tungsten carbide-based materials, which contain up 75% tungsten carbide particles that have a hardness up to 1900 HV; providing extreme abrasion resistance for shovel teeth. Hardfacing drawbacks include the possible occurrence of cracking, especially on thick deposits, and the influence of high welding temperatures on the microstructure of substrates [8].
Fig. 1. Dry sand/rubber wheel abrasion apparatus (Adapted from ASTM G65).
holder and loading a set force between the specimen and the rubber wheel; setting the revolution counter; adjusting and starting the sand flow; starting the wheel rotation; stopping the drive motor after running the desired number of wheel revolutions; and removing, cleaning and reweighing the specimen. The dry sand/rubber wheel test should be only used for wear ranking, not for specifying absolute wear values. Therefore, to mimic actual circumstances, variants of the standard procedure must be made to obtain the type of wear information required for engineering purposes. Aside from changes in the loading weight and sliding distance, the rate of sand flow, abrasive characteristics, and test duration can be reconfigured. Research has shown that approximately 200 wheel revolutions is adequate to create a steady wear rate and that multiple shorter tests could be run instead of a single long test to protect the rubber wheel [11]. 2.3. Relationship between abrasion and energy Abrasive wear can be described as a hard conical particle penetrating and sliding within a softer material as shown in Fig. 2. In a typical abrasion function, (1), the abrasive wear is quantified as a volume loss generated by a single conical particle sliding over a distance Li [12,13].
Vi =
2 WL ⋅ i i π⋅tan θ H
(1)
Eq. (1) is the typical abrasion function, where 2/(π⋅tan θ ) represents a wear coefficient and is dependent on the ductility of the
2.2. Existing abrasion tests The two most commonly used abrasion tests are the jaw crusher gouging abrasion test (ASTM-G61) and the dry sand rubber wheel test (ASTM-G65) [9,10]. The jaw crusher gouging abrasion test is primarily used to study the wear of ground engaging tools interacting with hard and large abrasives representing conditions commonly associated with quarry and metallic mineral mining operations. For the case of fine abrasives such as oil sands, the dry sand rubber wheel (DSRW) test is more suitable as there is little occurrence of breakage during excavation in a soft abrasive medium like oil sands. The dry sand rubber wheel setup is shown in Fig. 1. The general procedure for the ASTM-G65 test consists of the following steps: cleaning and weighing the specimen; fixing the specimen in the
Fig. 2. A typical model of abrasive wear by a conical particle (after [12]).
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material, interfacial shear strength, and particle shape [14]. In order to use friction energy to study abrasive wear, the friction expression μWi is introduced via Eq. (2) which can be written as:
2 μWi L i ⋅ π⋅tan θ⋅μ H
Vi =
(2)
The possible volume loss caused by friction energy is then given by:
2 AE ⋅ i π⋅tan θ⋅μ H
Vi =
n
n
i=n
i=1
2A 2A Ei = E πμ tan θ πμ tan θH
(4)
In specific abrasion conditions (where 2A /μπ tan θ may be determined), the volume loss (V), of material with hardness H will be a function of the friction energy E. In other words, abrasion results from friction energy caused by the interaction between the material and particles. Therefore it is possible to employ friction energy as a simple but effective method to quantify wear. 2.4. Specific energy Specific energy ( ES =E /V ) is defined as the friction energy required to cause a unit volume loss of material (J/m3 or Pa) and it has been widely used as a measure of energy efficiency in the machining industry [15]. In terms of abrasive wear, the specific energy ES is given by Eq. (5):
ES =
E πμ tan θH = V 2A
V 1 = μFv t Es
(6)
Where, V/t, F, v, and μ are volume loss rate, normal force, velocity, and friction coefficient, respectively. When all parameters are known, specific energy may be obtained. In order to apply specific energy to shovel teeth, the determination of a normal force F and velocity v based on real field data is required. A modified abrasion test has been designed to measure specific energy.
(3)
In Eq. (3) the parameter A is related to the order of magnitude of sliding distance. For example, when the sliding distance Li is 1 cm the value of A should be 10 2 so that the units of friction energy μWiLi will be N m. To some extent the sliding distance depends on particle size. For example, when a particle size is around 1 mm, its sliding distance should be in the range of 0.1 cm to 1 cm, while a particle with size of 1 cm is more likely to slide from 1 cm to 10 cm. Therefore parameter A can be considered as a particle size coefficient. Eq. (3) gives the relationship between abrasion (V), friction energy (E), material hardness (H), and the abrasive conditions characterized by the friction coefficient (μ), particle angularity (θ), and particle size (A). This relationship may be used to explain how material hardness, particle characteristics, contact condition, and normal force influence abrasive wear. When numerous particles abrade material surfaces the total volume loss V may be expressed as:
V = ∑ Vi = ∑
3
3. Modified abrasion test In order to measure specific energy, a modified rubber-wheel abrasion test (MRWAT) has been designed based on the standard dry sand/rubber wheel abrasion test (ASTM-G65) [9]. The main advantage of the modified test is its ability to better replicate actual abrasive field conditions experienced in an oil sands mining environment. A total of 85 test runs were performed; 37 samples for Al 61 and 16 each for Al 63, mild steel A36, and stainless steel 17-4SS. A schematic diagram of the experimental setup of the MRWAT test is shown in Fig. 3. This apparatus is similar to the standard ASTM G65 setup but with two major differences: the abrasive medium used and the wheel. Oil sand has zero cohesion but high friction angle [16]. The quartz particles in oil sand vary in shape from angular to rounded, generally described as being sub-angular. The sand used in the MRWAT is in-situ oil sand (bitumen stripped) rather than the standard AFS 50/70 test sand which consists of rounded quartz grains and is generally used in ASTM G65. Secondly; the ASTM G65 procedure specifies a rubber wheel of with a 22.9 cm diameter and 1.3 cm width whereas the MRWAT use a 15.2 cm diameter wheel that is 5.1 cm wide. This wheel is coated with a thin layer of oil sand which provides multiple advantages: the rubber wheel is protected from severe abrasive damage; full contact is ensured between abrasive sand and the test specimen; and decrease in the change of contact area at different levels of applied load. The hardness of the rubber wheel is Durometer A-60; the hardness was tested at the beginning of testing and before each coupon material substitution. Combined with the layer of oil sand material coating the wheel, the hardness is sufficient enough such that it did not change during the course of the tests. The MRWAT test has been designed as a low-stress abrasion test. The low level of contact stress ensures that most of the friction energy is transferred into abrasion of the targeted materials rather than breakage of the sand particles. Field data normal force values are much too large for laboratory application, and had to be
(5)
Specific energy is defined as the ratio of friction energy to a corresponding volume loss of material. An increase in volume loss occurs from a rise in friction energy by greater applied force or higher velocity. Regardless of the change in force and velocity, the specific energy for a single homogeneous material can be considered as a constant and is independent of scaling. Analysis of Eq. (5) can be used to demonstrate that Es is not influenced by scale, but rather it is determined by material hardness (H), particle size (A), particle angularity (θ), and contact condition (μ). The particle size, particle angularity, and contact condition are collectively termed the abrasive conditions. The specific energy may be regarded as an index, reflecting the performance of a material with hardness H working under specific abrasive conditions characterized by the parameters A, θ, and μ. According to the definition of specific energy ( ES =E /V =μFvt /V ), a measure for specific energy may be:
Fig. 3. Schematic of the MRWAT apparatus.
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Fig. 5. Grain size distribution before and after MRWAT.
Fig. 4. Normal resistance applied to teeth surfaces.
scaled down to appropriate values that facilitated safe, repeatable lab experiments. The scaling process is detailed below. 3.1. Parameters and specimen characteristics of MRWAT 3.1.1. Test parameters 3.1.1.1. Normal force. In order to create a relationship between lab results and field data, a scale factor for contact force was applied. The scale factor was determined from a ratio of the shovel teeth area to the contact area of the coupon produced by exerting a normal force against the rotating rubber wheel. The actual calculation of the scale factor was done using a square power law, since the ratio was dealing in areas rather than a singular length dimension. Using field data from a cable shovel operating in an oil sand environment the normal digging resistance applied to the teeth surface was found to range from 0 to 1600 kN (Fig. 4). A rubber wheel 5.1 cm wide and 15.2 cm in diameter resulted in a scaling factor equal to 12
SF =
ATeeth =12 AContact
(7)
Table 1 shows the conversion from actual normal resistance obtained from the field data (in kN), to lab normal loads and applied weights. After scaling, the lab normal loads ranged from 32 N to 130 N, which fall within an acceptable range for operation of the designed experimental tool. The lab normal loads take in to account the total mass of plates applied, the mass of the lever arm as well as the lever arm leverage ratio which was equal to 2.75. The normal resistance loads are scaled similarly to the contact area shown in Eq. (7); however, a cube root scaling approach is applied in comparison to the square root previously used in the calculation of the contact area.
was not deemed necessary to apply a scaling factor to the rubber wheel's rotational speed. The conversion from the actual digging velocities to rotation speed was solely made proportional to wheel diameter.
3.1.1.3. Abrasive media. The sand used in the abrasion tests were derived from oil sands stripped of bitumen primarily dominated by quartz. This was assumed the predominant abrasive media as hard quartz particles occupy the largest proportion of the total solids [1]. The size distribution for the sand is shown in Fig. 5, where 100% of particles passed 2000 μm (No.10 U.S Standard Sieve) and nearly 80% pass 850 μm (No.20 U.S Standard Sieve). The sub-angular particles were loose and totally dry (Fig. 6). It should be noted that the consistency of the grain size distribution before and after use highlighted that the MRWAT was a low stress abrasion test which allowed the abrasive media to penetrate and slide over a surface without crushing the particles. It was therefore assumed that the majority of friction energy translated into abrasive wear of the material tested. The application of a rubber wheel instead of a steel wheel also precluded occurrence of particle breakage [17]. 3.1.1.3.1. Time and flow rate. As discussed previously, specific energy was defined as the friction energy expended to cause a unit volume loss of material, or the friction power expended for a given volume loss rate; such that specific energy was normalized with respect to time. However; measurements over short periods led to an inaccurate estimation of volume loss; while longer duration measurements were time consuming. Test durations of 4 min were selected although this was an arbitrary decision. The flow rate of the abrasive media was matched to the rotation speed of the wheel so as to mimic the actual digging conditions.
3.1.1.2. Rotation speed. Another important parameter was the rotation speed. Analysis of representative field data shows that the actual digging velocities of the teeth vary from 0 m/s to approximately 1.1 m/s, with a large proportion falling in the range of 0.2 m/s to 0.8 m/s. Since the actual digging velocity was not high, it Table 1 Relationship between the applied mass and lab and field forces. Applied weight (kg)
Lab normal loads (N)
Normal resistance (kN)
0 0.45 0.91 1.36 1.81
32 44 57 69 82
454 625 797 969 1140 Fig. 6. Shape of abrasive media used in MRWAT.
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Table 2 Properties of metal specimens. Avg. Shore #
Vickers Hardness
ASTM B-221 2.7 g/cm3
–
83
3
–
107
Metal
Abbreviation Standard #
Density
Aluminum 6063-T6 Aluminum 6061-T6 Mild Steel A36 Stainless Steel 174SS
Al 63 Al 61
ASTM B-209 2.7 g/cm
MS
ASTM A36
7.85 g/cm3 32
213
SS
ASTM A693
7.85 g/cm3 40
279
3.1.2. Test specimen Material hardness has a significant influence on degree of abrasive wear and it is the major selection criteria for materials. Four metals with different hardness were selected for the tests (Table 2): Aluminum 6063-T6, Aluminum 6061-T6, Mild Steel, and Stainless Steel. These materials were chosen because of the hardness range they represent. Samples measuring 5.1 cm wide by 7.6 cm long by 1.3 cm thick were used as test coupons. 3.2. Interfacial shear test The interfacial shear test (IST) is an auxiliary test designed to determine the friction coefficient between abrasive media and the surface of materials used in the MRWAT. Friction can be considered as equivalence to interfacial shear strength between two different materials. Measurement of the friction coefficient in this research was carried out through an interfacial shear test (IST) developed from a conventional direct shear test (DST) system. The purpose of including the sand/sand and sand/metal interfacial shear box tests are to verify that slip will occur at the sand/metal interface before internal shear within the sand. This is required for successful application of the MRWAT. Table 3 shows that this will be the case for materials harder than those tested here, eliminating the need to redo the tests until it is probable that the sand/ metal friction angle will reach the internal friction angle of the abrasive media. A schematic diagram of the interfacial shear box is shown in Fig. 7, which follows the standard DST setup per ASTM D5321A. A metal sample was placed between the upper and lower boxes, fixed to the lower box with clamping screws. The same abrasive media used in the MRWAT was placed into the upper box, with a layer of media evident between the metal sample and the upper box. When a shear force was applied, the upper box moved while the lower box and the fixed sample remained stationary. In contrast to the direct shear test, the shear area between the abrasive media and material remained constant. The friction coefficient (the interfacial friction angle) was defined as the slope of the resulting shear stress to normal stress plot. The metal specimens tested in the IST were the same as those used in MRWAT (Table 2). However, sand/sand slip may accompany sand/ metal slip during a shear test, giving rise to false interfacial friction
Fig. 7. Schematic of the interfacial shear box.
angles. Brumund and Leonards [18] demonstrated that the sand/ sand slip will occur when the internal friction angle of sand was equal or less than the interfacial friction angle between the sand and material. A direct shear test was also conducted for the abrasive media without the coupon present so as to account for the influence of sand/sand slip on the sand/material slip data. The results of the interfacial and direct shear tests conducted at normal stresses of 150 kPa, 250 kPa, 350 kPa, and 450 kPa are presented in Fig. 8 and Table 3 below. From these results it was concluded that the sand-sand friction angle is over double the sand-metal results suggesting that sand-sand slip will not occur during the MRWAT. 3.3. Modified rubber wheel abrasion test 3.3.1. Abrasion test results The modified rubber wheel abrasion test was designed to measure specific energy for materials via mimicking the abrasive conditions experienced by shovel teeth working in oil sands. From Eq. (6) the specific energy is seen equal to the reciprocal value of the slope of a volume loss rate verses friction power relationship. The MRWAT results for mild steel are presented as an example in Fig. 9. For all materials tested there was a strong linear relationship between volume loss rate and friction power, with the lowest correlation coefficient experienced being 0.97. The hardness, friction coefficient, and specific energy of each material tested are summarized in Table 4. Hardness is defined as the mean pressure to cause materials to undergo a plastic deformation. Harder
Table 3 Comparison of internal friction angle and interfacial friction angles. Test
Correlation coefficient
Friction angle (deg)
Sand/Sand Sand/Al 63 Sand/Al 61 Sand/MS Sand/SS
0.98 1 1 1 1
60.7 25.6 24.6 23.4 22.6 Fig. 8. Relationship between residual shear stress vs. normal stress.
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Fig. 9. Relationship between volume loss rate and friction power for Mild Steel (MS).
ES = BH
Table 4 Characteristics of materials tested. Material Hardness Hv Number Pressure (MPa)
Friction coefficient
Es ( 102 GPa)
Al 63 Al 61 MS SS
0.48 0.46 0.43 0.42
1.69 1.70 7.09 8.07
Very Soft Soft Medium Hard
83 107 213 279
Fig. 10. Relationship between velocity and volume loss rate.
814 1050 2090 2737
materials deform less than softer ones under the same stress. The standard Vickers hardness number can be converted to units of pressure (MPa) by multiplying by 9.81. Volume loss is measured by weighing the sample coupons before and after testing and then using the materials’ density to calculate the volume of material lost. From Table 4 it can be seen that specific energy increases, as would be expected, with an increasing hardness of materials. For example, the specific energy of medium hard stainless steel is roughly 5 times than that of very soft Aluminum 63. In accordance with the definition of specific energy; energy to cause a unit volume loss; soft materials with low specific energy require a lower friction energy to cause the same volume loss as might be seen for hard materials. Therefore specific energy can be used to rank resistance of materials to abrasive wear under similar abrasive conditions.
(8)
Here, B is termed as the specific energy coefficient and is equal to μπ tan θ /2A. The hardness as previously defined is the mean pressure acting on a material to cause fully plastic deformation, commensurate with a Vickers hardness number (HV) which can be described in pressure units by multiplying by gravity. In this paper, all abrasion tests were performed using the same abrasive media; such that a linear relationship between specific energy (Es) and hardness pressure (H) exists. Fig. 11 shows the relationship between specific energy and hardness. Even though the relationship between wear rates, energy and hardness becomes more complicated for the microstructures like alloys, the formula Es¼ BH may estimate the specific energy of any material subjected to the same abrasive conditions. The specific energy coefficient B, determined for the abrasive conditions via (μ tanθ)/2 A, of 2900 – shown in Fig. 11 – was obtained for the abrasive conditions examined in the laboratory to mimic the abrasive wear that occurs on shovel teeth, such that it is only viable for materials which are subjected to this identical abrasive condition. When either the particle characteristics or the contact condition between the particles and material are changed, the specific energy coefficient B would also need to be calibrated. Again, B should be evaluated from a group of samples tested under the same abrasive conditions.
3.3.2. Relationship between friction and specific energy The relationship between friction and specific energy is of interest in verifying specific energy independent of an abrasion index. The abrasion function, Eq. (6), illustrates that the volume loss rate (V/ t) is a function of friction (μF) and velocity (v) while specific energy (Es) is the function coefficient. When the variable μF is fixed, there exists a linear relationship between V/ t and v with a slope of μF/Es, Fig. 10. Fig. 10 provides a sample relationship between volume loss rate (V/t) and velocity (v) for the material. Fig. 10 also shows that as the overall friction energy increases, the volume loss rate increases; a direct result of an increase in friction energy. 3.3.3. Relationship between specific energy and hardness When the particle characteristics (size coefficient A and shape parameter θ) and contact condition (friction coefficient μ) are held constant, the specific energy (Es) should be proportional to the hardness (H); Eq. (8):
Fig. 11. Specific energy vs. Hardness.
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Z. Lin et al. / Wear 370-371 (2017) ∎∎∎–∎∎∎ Table 5 Example data. Parameter Value EC TC A’ U ES
240 kJ/cycle/tooth 40 s/cycle 0.8 0.8 709 GJ/m3
4. Application of specific energy to GET performance One of the most important applications of specific energy is to predict GET performance through the estimation of life expectancy. Eq. (6) may then be adapted to predict the operating time for a GET when a specific energy expenditure is known; Eq. (9):
t =
ES V μFv
ES V ⋅TC ⋅A′⋅U EC
40 s cycle, an availability and utilization both equalling 0.8 and a specific energy value Es¼ 709 GJ/m3 from the lab, tests revealed that the medium wear tooth had a predicted operating time of 44.3 h compared to the 45 h actually recorded from field data. The severe wear tooth was predicted to have experienced 97.3 h of operating time while field data showed the tooth was in use for 96 operating hours. The difference between predicted and actual field life for the medium and severe wear tooth is approximately 1.4%. This level of deviation shows the potential of specific energy to closely predict the performance of ground engaging tools as well as potentially aid in the selection of materials for specific abrasive conditions (Fig. 12). In the case of shovel teeth, even though there is no optimal combination of force and velocity, abrasive wear can still be effectively reduced when both digging velocity and hoist force are at low levels. Practically, cable shovels are suggested to operate at high hoist forces but low digging velocities to keep production up, while decreasing abrasive wear on ground engaging tools.
(9)
Where V is the maximum volume loss before failure, F, v, and μ are the normal force, velocity, and friction coefficients respectively, and their product (μFv) is the friction power. Time (t) is the total time for the materials under continuous abrasion to failure. The value of specific energy (Es) can be obtained through abrasion tests or evaluated from the formula Es¼ BH. Based on Eq. (9), for shovel teeth operating in the Canadian oil sands, the prediction for their actual operating hours is expressed as Eq. (10):
T =
7
(10)
Where Ec is the friction energy per digging cycle per shovel tooth, Tc is the duty cycle time, V is the volume loss of a shovel tooth before failure, A′ is shovel availability and U is the shovel's utilization. According to the field data represented by the failed shovel teeth evaluated in this investigation, one experienced medium wear with a volume of loss of 0.0021 m3 (16.56 kg loss), and another of severe wear with a volume loss of 0.0046 m3 (36.34 kg loss) (Table 5). As an example, using an estimated friction coefficient of 0.45 between the shovel teeth and oil sand, a test material was evaluated. At an energy expenditure of 240 kJ/cycle per shovel tooth, a
5. Conclusion The goal of this paper was to study the abrasive wear of shovel teeth and to facilitate the prediction of performance for ground engaging tools. In order to achieve this goal the principle of specific energy was defined as the friction energy required to cause a unit volume loss of material. A modified rubber wheel abrasion test and an interfacial shear test were designed to measure specific energy of materials. Operating hours for shovel teeth were then estimated through specific energy matched to field data, suggesting that specific energy was useful to predict GET performance. Summarily, some conclusions through this study on abrasive wear of shovel teeth in oil sands are specified: 1. Eq. (6) ( V /t =1/Es μFv ), derived from a typical abrasive model, illustrated that abrasion by nature results from the friction energy, making it possible to predict abrasion from the point view of specific energy defined as the energy required to abrade a unit volume of material. 2. Specific energy can be evaluated through a modified rubber wheel abrasion test (MRWAT) which closely mimics actual abrasive working conditions in a Canadian oil sands mining operation. The specific energy is independent of scaling, normal
Fig. 12. Shovel tooth with severe wear after 18 h use (Used with permission from M. Curley).
Please cite this article as: Z. Lin, et al., Specific energy and the modified rubber wheel abrasion test, Wear (2016), http://dx.doi.org/ 10.1016/j.wear.2016.11.002i
Z. Lin et al. / Wear 370-371 (2017) ∎∎∎–∎∎∎
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force, and velocity. It is merely determined by particle characteristics and contact conditions via of Eq. (5). Specific energy may be considered as an index quantifying the resistance of a material with hardness H to abrasion by a specific abrasive condition characterized by the particle size coefficient (A), particle angularity (2θ), and contact condition (μ). However, specific energy is not an inherent material property, since it is influenced by the abrasive conditions. 3. A linear relationship between specific energy and hardness exists is evident which is consistent with the equivalent conversion of specific energy units from J/m3 to Pa. Eq. (8) may be used to estimate the specific energy of other materials. The specific energy coefficient B should be evaluated from a group of like-materials tested under the same abrasive conditions, and should only be used for materials subjected to similar abrasive conditions. 4. The operating time predicted via specific energy matches the field data represented by known failed shovel teeth; such that it is possible to facilitate the prediction of the performance of ground engaging tools wear resistant materials.
References [1] R. Llewellyn, Resisting wear attack in oil sands mining and processing, CIM Bull. 90 (1012) (1997) 75–82. [2] C. Cross, D.L. Olson, Friction and wear in the mining and mineral industries, in: P.J. Blau (Ed.), ASM Handbook: Friction, Lubrication and Wear Technology, vol. 18, ASM International, 1992. [3] T. Norman, Wear in ore processing machinery, in: W. Peterson, W. Winer
(Eds.), Wear control handbook, ASME, New York, New York, 1980. [4] J.A. Hawk, R.D. Wilson, Tribology of earthmoving, mining, and minerals processing, in: B. Bhushan (Ed.), Modern Tribology Handbook, vol. II, CRC Press LLC, Boca Raton, Florida, 2001, pp. 1331–1368. [5] K.H. Zum Gahr, Modelling of two-body abrasive wear, Wear 124 (1) (1988) 87–103. [6] P.K. Knights, Optimal replacement intervals for shovel dipper teeth, Int. J. Min. Reclam. Environ. 23 (2009) (157-17). [7] R. Llewellyn, Materials for controlling wear in surface mining, CIM Bull. 89 (1002) (1996) 76–82. [8] S.F. Murray, Wear resistant coatings and surface treatments, in: E.R. Booser (Ed.), Handbook of Lubricants: Theory and Practice of Tribology, CRC Press LLC, Boca Raton, London, New York, New York, 1988. [9] ASTM. G65-04. Standard Test Method for Measuring Abrasion Resistance Using the Dry Sand/Rubber Wheel Apparaturs. In: Book of Standards, ASTM International. [10] ASTM. G81 Standard Test Method for Jaw Crusher Gouging Abrasion Test. In: Book of Standards, ASTM International. [11] J.A. Hawk, Abrasive wear testing, in: H. Kuhn, D. Medlin (Eds.), Mechanical Testing and Evaluation, vol. VIII, ASM International, Materials Park, Ohio, 2000, pp. 325–337. [12] K. Kato, K. Adachi, Wear mechanisms, in: B. Bhushan (Ed.), Modern Tribology Handbook, vol. I, CRC Press LLC, Boca Raton, Florida, 2001, pp. 273–300. [13] R. Gohar, H. Rahnejat, Fundamentals of tribology, Imperial College Press, London, 2012. [14] E. Rabinowicz, Wear coefficients – metals, in: M.B. Peterson, W.O. Winer (Eds.), Wear Control Handbook, ASME, New York, New York, 1980. [15] I.D. Marinescu, W.B. Rowe, D. Dimitrov, H. Ohmori, Tribology of abrasive machining processes, William Andrew, Oxford, 2013. [16] R.N. Morgenstern, D.J. Scott, Shear strength of Athabasca oil sands, Geotech. News 15 (1997) 102–109. [17] J.P. Lafleur, A study of abrasion in steel during comminution, (MSc. dissertation/thesis), McGill University, Canada, 2011. [18] W.F. Brumund, G.A. Leonards, Experimental study of static and dynamic friction between sand and typical construction materials, J. Test. Eval. 1 (1973) 163–165.
Please cite this article as: Z. Lin, et al., Specific energy and the modified rubber wheel abrasion test, Wear (2016), http://dx.doi.org/ 10.1016/j.wear.2016.11.002i