- Email: [email protected]
Cyclic loading and a modified ASTM G65 abrasion test
Author’s Accepted Manuscript Cyclic loading and a modified ASTM G65 abrasion test M. Curley, T.G. Joseph
www.elsevier.com/locate/wear
PII: DOI: Reference:
S0043-1648(17)30513-6 http://dx.doi.org/10.1016/j.wear.2017.09.005 WEA102239
To appear in: Wear Received date: 21 March 2017 Revised date: 6 September 2017 Accepted date: 6 September 2017 Cite this article as: M. Curley and T.G. Joseph, Cyclic loading and a modified ASTM G65 abrasion test, Wear, http://dx.doi.org/10.1016/j.wear.2017.09.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Cyclic loading and a modified ASTM G65 abrasion test
M. Curleya, 1, T.G. Josephb a
Department of Civil & Environmental Engineering, School of Mining and Petroleum Engineering Markin/CNRL Natural Resources Engineering Facility 9105 116 St. Edmonton, Alberta, Canada T6G 2W2 b Faculty of Engineering Donadeo ICE Building 9203 116 St. Edmonton, Alberta, Canada T6G 1H9 1 Corresponding Author: [email protected]
Abstract Wear is a multi-faceted phenomenon that occurs through a number of mechanisms. Surface mining operations are principally impacted by abrasive wear during the excavation process. Equipment downtime due to ground engaging tool (GET) abrasive wear significantly affects production of a mining operation. In comparison to the ASTM G65 constant load abrasion test, an equally simple cyclic load test has been designed and trialed to better mimic and predict wear conditions in an oil sand mining scenario. In the modified ASTM G65 test, bitumen stripped oil sand quartz media replaced Ottawa Sand. Coupon specimens, representing GET surfaces, were hardfaced with chromium carbide weld overlay. An actuator controlling applied loads facilitated a more accurate mimic of a dig cycle for an ultra-class electric rope shovel. Shovel kinematics were analyzed to determine resistive forces experienced at the shovel bucket teeth. Recorded field data was scaled to appropriate laboratory testing magnitudes. Wear scar inspection suggested that cyclic loading scenarios potentially generate more severe wear damage versus constant loading. Field tooth life was predicted using a specific energy technique. Test results indicated that a cyclic loading approach provided closer correlation to measured shovel tooth life for an oil sands operation than a constant load approach.
Keywords: three-body abrasion; hardfacing; wear testing; chromium carbide; surface analysis; oil sand
1
Introduction
Wear is a critical factor affecting production rates and maintenance costs in any mining operation. Rabinowicz [1] described wear as “the removal of material from solid surfaces as a result of mechanical action”. Surface mining operations are particularly affected by such an abrasive wear process throughout excavation duties. In Canadian oil sands operations located in Northern Alberta, ultra-class electric rope shovels experience severe GET wear caused by bucket teeth versus abrasive oil sand media interactions. Abrasive wear is a key concern to the mining industry as economic impacts and lost production due to downtime are a significant driver to continually improve wear control practices. The objective of this paper was to develop a laboratory scale test improvement to more accurately represent field abrasive wear conditions (in this case for an oil sand operating environment) experienced by shovel teeth and to apply the test results in a field tooth-life prediction. To accomplish this, a standardized abrasive wear test (ASTM G65) was modified to incorporate a cyclic load-varying component that would facilitate normal forces imparted to a test coupon more reminiscent of actual field loading conditions. This then reflected the discontinuous cyclic forces experienced by shovel teeth as they engage and disengage a mining face. A set of field data from an oil sands operation was analyzed and scaled to establish a laboratory test force range in order to conduct controlled, repeatable tests. Results from the modified abrasion tests were compared to the original ASTM G65 constant load test to discern the effects of cyclic loads on wear rates and wear scars.
1.1
Background
Oil sand is predominately a quartz sand surrounded by a water film with a heavy oil (bitumen) between particles. The quartz is generally sub-angular with high proportion of quartz contributing significantly to an extreme abrasivity. Quartz has a Vickers Hardness (H v) rating of 850–900, indicating that even the hardest steels only provide moderate protection [2]. Ground engaging tools mounted on cable shovel dippers are subjected to high forces during the excavation cycle, resulting in significant wear damage and material loss to teeth tips and adapters. Several wear modes contribute to the loss of material from dipper teeth, yet abrasive wear is the most dominant. The severity of abrasive wear attack depends on a number of factors; most broadly stated as the characteristics of the abrasive media, the properties of the wear material and the interaction between these two elements [3]. Different classification schemes have been developed by researchers over many years to characterize abrasive wear, where, generally it is agreed abrasive wear modes can be classified as low stress, high stress or gouging abrasion. The action of dipper teeth digging through un-blasted ore such as oil sand can be classified as low stress abrasion, as the abrasive particles do not fracture during the dig cycle. A variety of components and protection systems are available to the mining industry to attempt wear alleviation. Llewellyn [4] classified these into four primary groups: ferrous-based materials, elastomers/polymers, ceramics/cermets and surface engineering techniques. Surface engineering techniques, in particular weld overlays, are a common practice to combat the significant abrasivity of oil sands. Hardfacing provides a wear performance and economic advantage, permitting the use of a more common base material that has attractive structural properties but would be too costly to frequently replace weld overlay. Compounds such as tungsten or chromium carbides exhibit a high wear resistance. The process by which the overlay was deposited on the coupons tested in this research was submerged arc welding.
1.2
Abrasive wear test
An ideal laboratory wear test is inexpensive, efficient but is a relative reference test and not one that creates the exact conditions of the real-world scenario. Naturally, cost, scale and practicality usually prevents this. The ASTM G65 test (Figure 1) is an industry recognized abrasion test used for relative ranking the abrasion resistance of various materials. The test is suited for finer abrasives such as oil sand that do not break or fracture during
excavation. The abrasive media is not fixed; the particles are free to slide and rotate between a spinning wheel and a test coupon, simulating three-body abrasive wear [5]. Five procedures characterized by different parameters including normal force, the number of wheel revolutions and the total corresponding lineal abrasion distance outline how the test can be performed. The standard stipulates the test is suited for the testing of any material form, including weld overlays, and uses an abrasive typified by AFS 50/70 Test Sand (Ottawa Sand) [6].
Fig. 1. Basic dry sand/rubber wheel test apparatus (Adapted from [6])
The ASTM G65 test was modified in this work to reflect more accurately the actual mining conditions in an oil sands operation and create the associated abrasive wear environment.
2 2.1
Experimental procedure Abrasive media
A standard G65 test uses “Ottawa” silica sand as the abrasive medium. Previous work experimented with the substitution of different abrasive media, including the use of oil sand on softer metals such as aluminium and mild steel [7]. The purpose of substituting the standard abrasive media is to better represent field conditions. The material used during the modified experiment performed here was bitumen stripped oil sand. The material had significant abrasive qualities and was dried and sieved characterized before testing. Figure 2 details the approximate size and angularity of the abrasive particles used.
Fig. 2. Close-up image of tailings sand abrasive (Adapted from [8])
The figure shows the particles are sub-angular to angular and vary in size and shape. The majority of the particles are quartz. The black background is not bitumen; but rather carbon tape the samples were placed on for imaging. Figure 3 outlines the particle size distribution of the media from sieve analysis. 100%
Cumulative % passing
80%
60% Distribution before test 40%
20%
0% 0
100
200
300
Particle diameter (µm)
Fig. 3. Grain size distribution of abrasive media used in modified G65 test (Adapted from [8])
2.2
Coupon characteristics
The test coupons were chromium carbide (Cr7C3) overlaid on a 44W steel base. The overlay was applied as a submerged arc weld (powder addition) cut to shape and size for the test using a water jet. The coupons measured 76 × 64 × 8 mm and of 50% base substrate and 50% overlay. The samples were ground to a smooth finish using a surface grinder before testing commenced. Chromium carbide is a material frequently used to coat shovel teeth, ripper shanks and crusher feed/discharge areas to prevent wear [4]. The tested coupons were hypoeutectic overlays containing an average of 28% chromium, 4% carbon, 2% manganese and 1% molybdenum & boron, with the balance being iron. Figure 4 shows an (a) unprepared and a (b) test prepared coupon.
(a) Fig. 4. Sample coupon (a) before and (b) after surface grinding
(b)
The coupons were ground until there were no noticeable asperities with all slag material removed. The coupons had a shiny finish with the occasional presence of small, dull grey pits formed as a result of the weld application process. The cracking observed on the surface of the coupon results from the contraction of the weld as it cooled; often called relief or fatigue cracking [9]. Relief cracking was not observed on all samples. Any sand particles caught in the cracks were removed with a vacuum nozzle or blown out with air prior to weighing.
2.3
Modified test apparatus
The most significant addition to the standard G65 test setup was the incorporation of a linear load control actuator. This actuator facilitated a repeatable, cyclical load. The actuator was installed vertically at the end of the G65 system horizontal lever arm, similar to the hanging weight plates in the standard apparatus. A simple moment diagram was constructed in order to determine the downward vertical force necessary to generate the normal resistive force imparted on the coupon. The actuator position, proportional to load, was computer controlled. A load cell was installed between the end of the lever arm and the tip of the actuator cylinder, whose response was recorded by a secondary PC for variations in force as the actuator was extended and retracted. Test runs were performed in order to evaluate and calibrate the system. The actuator also provided the advantage of acting as a rigid beam reducing vibrations that occur in the standard setup where a set of hanging weights deliver a leveraged normal force. To ensure wheel rotational velocity did not decrease when the force was applied, a 10:1 worm gear was installed that supplied the motor with sufficient torque. The mounted rubber wheel had a diameter of 148 mm and a width of 51 mm. The rpm of the wheel was determined from averaging the dig velocity of the dipper from the moment of face entry to exit over all recorded dig cycles. The average dig velocity, 0.775 m/s, corresponded to 100 rpm and was maintained constant throughout the tests. The hopper and nozzle enabled media material flow, controlled via a
valve. The nozzle width was identical to that of the rotating wheel and the opening was 1 mm, a value greater than three times the largest particle diameter to help prevent clogging. In order to calculate abrasion energy and, by association, specific energy, a voltmeter and ammeter were connected to the wheel motor to evaluate the power draw. Figure 5 shows the modified test set-up.
Fig. 5. Modified test apparatus set-up; hopper (1), lever arm (2), load cell (3), actuator (4), specimen and holder (5), rubber wheel (6), voltmeter and ammeter (7) (Adapted from [8])
2.4
Methodology
Shovel data from a Northern Alberta oil sands operation was used to determine the scale laboratory force-range needed for the test. The data set was collected from a P&H 4100 BOSS shovel, including time stamp, hoist rope position, crowd extension, dig velocity and hoist & crowd armature voltage and current values.
2.4.1
Geometric analysis of a shovel
An established geometric analysis technique [8, 10] was used to determine hoist rope and crowd positions, geometrically resolved in a 2D x-y coordinate plane to profile the shovel dig trajectory through the face, relative to the dipper teeth. Figure 6 illustrates the digging profiled created for three passes of the shovel bucket. The y-axis represents the centreline of the shovel’s swing rotation and the x-axis is the ground surface beneath the crawler tracks.
14 Handle-boom saddle rotation point
12
Vertical hoist (m)
10
Dipper exits face
8 Dig Cycle 1 6
Dig Cycle 2
4 2
Dig Cycle 3
Dipper enters face
Tooth tip at tuck motion
0 0
5
10
15
20
25
-2
Horizontal extension (m) Fig. 6. Approximate dig profile of three passes relative to shovel function (Adapted from [8])
Taking the intersection of the shovel boom and dipper handle (saddle point) as the point of orientation, a set of moment equations were generated and the total digging resistance, FR, was obtained. This dig resistance was further resolved to a normal resistance felt by the shovel teeth, permitting scaled laboratory test values to be determined. Figure 7 details a general diagram of an ultra-class shovel’s boom, handle and dipper arrangement. The x and y-axes are defined as above.
Fig. 7. General schematic of ultra-class shovel, including an approximate dig profile (Adapted from [8])
2.4.2
Scaling procedure
Analysis of the field data showed that hoist force was the dominant contributor to the digging resistance. Figure 8 illustrates that hoist force and the digging resistance experienced by the dipper teeth are very similar, as the teeth, dipper and handle are all part of the same fixed arrangement, such that the teeth experience the same direct trajectory the hoist imparts on the entire dig system. It is clear in the example that there is an initial “ramp-up” period of approximately three seconds from 15 to 18 s as the dipper engages the face and beings to progress upward. The force then remains relatively constant, but varying with diggability of the face material, for around 10 s during the “face dig”. The sudden decrease in force at roughly 26 s indicates that the dipper has exited the face. Each dig cycle identified in the data set demonstrated an approximately three-second ramp-up period, a 10 s face dig and a 30 s swing-dump-reposition reprieve.
3500 3000
Force (kN)
2500 ~10 s face dig
2000
Hoist force
1500
Dig resistance
1000
Face exit
500 Ramp-up
0 14
16
18
20
22
24
26
28
Time (s)
Fig. 8. Example of hoist and normal resistive forces (Adapted from [8])
The field force values were much too high to easily test in a laboratory setting. A scale factor was used to determine laboratory appropriate force values used to perform the proposed modified test. It has been documented that area is related through a square-power law; the idea being that if all dimensions of a geometric shape are increased by some factor W the area scales by W2, provided the geometric shape remains unchanged [11]. Volume is scaled via a cubepower law; a cube of side x with all dimensions increased by a factor of two will yield a volume that has increased by eight, or two cubed. The scale factor was calculated by comparing the contact area of an unused ultra-class dipper tooth with a wear scar generated by the modified apparatus on a spare coupon sample. The total surface area of a dipper tooth that interacts with a mining face during excavation was measured to be 1126 cm2. The area of the wear scar generated by a calibration test on a spare hardfaced coupon was 4.4 cm2 [8], such that a the scale factor, SF, could be calculated by taking the square root of the ratio of the measured field tooth wear scar area, Atooth, to the sample wear scar, Acoupon, Equation 1. Inputting the measured surface area and wear scar values yield a scale factor of 16. √
(1)
For all dig cycles in the field data set, a mean dig resistance force, Ftooth, during the 10 s face dig period (Figure 8) was calculated. In order to determine the face dig force applied to the coupons, Fcoupon, in the modified test, the same scale as defined in Equation 1 factor was applied as a cube-root determination proportional to volume or force [11] to the field evaluated mean normal resistive force, Equation 2. An average standard deviation of the face dig force between the dig cycles was also calculated and was similarly scaled to produce a range of normal force values that were tested using the modified set-up. √
(2)
Given that the contact area of the rubber wheel in the modified G65 test on the coupon was observed as constant during calibration, mimicking the field contact where shovel teeth are fully engaged in the dig cycle, it was reasonably assumed that the same contact pressure between media and coupon was evident as found in the field conditions.
2.4.3
Proposed modified test
Figure 9 illustrates one cycle of the proposed modified test, showing the three-second ramp-up period followed by a 10 s face dig and a 30 s reprieve; mimicking a typical dig cycle of an operating ultra-class oil sand shovel. The range of tests (+σ and –σ) depict plus and minus one standard deviation from the scaled mean dig resistance value. Each coupon was subjected to 50 cycles, totalling 525 m of lineal abrasion, at the load category in which they were assessed. The total cyclic load total test time was roughly 36 minutes per coupon. 450 400
Normal applied force (N)
350 300 250
~10 s face dig
Mean test
200
+σ
150
-σ
100 Ramp-up
50
Applied force = 0 N for 30 s
0 0
2
4
6
8
10
12
14
16
Time (s)
Fig. 9. Proposed modified test (Adapted from [8])
Three coupons were tested in each of the respective load categories outlined in Figure 9. For comparison, an additional two coupons were tested using hanging weights and a constant load, 130 N, similar to procedures A, B, C and E of the ASTM G65 test specifications. The wheel rpm, total lineal abrasion distance and abrasive media were kept constant for all tests. The total time of the constant load test was approximately 11 minutes per coupon.
2.4.4
Power draw and specific energy
Graphing the power draw facilitated the determination of the abrasion energy and the specific energy, essentially the energy that caused the volume loss. Figure 10 shows a power–time graph for three of the field data dig cycles. The stippled region indicates the area under the plots that was taken to calculate the field abrasion energy. Since abrasion energy is exerted during the dig cycle, it follows that the shape of the abrasion energy area is similar to the proposed modified test pattern observed in Figure 9.
3500 3000
Power (kW)
2500 2000 Dig cycles
1500
Abrasion energy
1000 500 0 0
20
40
60
80
100
120
Time (s)
Fig. 10. Example of power draw and calculated abrasion energy from field dig cycles (Adapted from [8])
The voltmeter and ammeter connected to the wheel motor of the modified set-up enabled the recording of the power draw during the scaled dig cycle. The use of specific energy to predict field tooth wear has been attempted before [7], and is again reviewed here using a modified approach.
3 3.1
Results Cyclic load capability
The implementation of the actuator was very successful in creating a cyclic load representative of a shovel completing dig cycles in the field. Figure 11 illustrates a snapshot of two complete cycles for the mean test force category. The actual and target mean refer to the average normal force imparted on the coupon during the 10 s face dig portion and the target-exerted force. The variation of the test achieved and target forces during this period is 2%. The dashed lines reflect the range of the three-second ramp up period observed from the field data (Figure 8) and detailed in the proposed modified test (Figure 9).
400 350
Normal force (N)
300 250 Normal force
200
Actual mean
150
Target mean Ramp Δt
100 50 0 180
190
200
210
220
230
240
250
Time (s)
Fig. 11. Example snapshot detailing cyclic load and ramp-up capability of the proposed modified test (Adapted from [8])
3.2
Coupon volume loss
Each coupon was subjected to 525 m of lineal abrasion, equal to 50 complete dig cycles, at a common modified G65 and field tooth speed of 0.775 m/s. Table 1 reports the average volume loss for each target force group for cyclic and constant load tests. The percent deviation refers to the difference between the target face dig force and the mean applied force during the face dig in the cyclical load tests. The constant load tests used a set of hanging weights to apply the resistance. The mass of the hanging plates was calculated based on applying 130 N of normal force on the coupon. Table 1 Volume loss comparison between cyclic load tests and constant load tests
Test type
Cyclic load Constant load
# of test specimens 3 3 3 2
Target face dig force (N) 202 301 399 130
Mean % deviation -3.1 2.6 1.5 -
Mean volume loss (mm3) 3.19 ± 0.11 5.23 ± 1.79 5.62 ± 0.67 3.29 ± 0.21
Figure 12 details an example of a coupon wear scar. The direction of abrasive media flow was perpendicular to the surface grinding preparation direction.
Fig. 12. Example of a coupon wear scar; wear direction top to bottom (Adapted from [8])
3.3
Power draw and abrasion energy
Determining abrasion energy expended during the laboratory test facilitated the determination of specific energy; the energy required to cause a unit volume loss of material. The area under the power-time graph yielded the energy expended during the abrasion process. Figure 13 represents the power draw graph for the mean test (301 N of normal force). Since the coupon was loaded for 50 cycles, the power consumed during the abrasion process was taken as the sum of the differential power (area represented by stippled region) and not simply the power required to spin the rubber wheel. The constant load test’s power draw was calculated similarly, though over one long cycle. 2.5
Power (W)
2
1.5 Power draw at 301 N
1
Abrasion energy 0.5
0 0
5
10
15
20
25
30
Time (s)
Fig. 13. Power draw of modified apparatus at mean normal force (Adapted from [8])
Table 2 summarizes the mean normal force during the face dig and the average abrasion energy expended during the total dig cycle (ramp + dig) for the field data, cyclic and constant load tests. Table 2 Comparison of field and lab force and energy values
Data source Field data Cyclic load Constant load
4
Mean normal force (N) 1 231 000 202 301 399 130
Mean abrasion energy (J) 20 434 482 527 815 1107 182
Symbol Eavg El El El El
Discussion
4.1
Cyclic load capability
The modified abrasion test was successful in mimicking the dig profile generated by an ultra-class excavator in a typical oil sands operation. The application of a linear actuator effectively provided the ability to load a set of test coupons cyclically with repeatable precision. Figure 14 illustrates how reliable the actuator proved to be, demonstrating a very repeatable sequence for 50 consecutive load cycles. For all cyclic load test categories, the actuator produced consistent accuracy with the greatest variation in target-applied force and actual applied force being 3.1%. It is rare for operating mining equipment to experience a constant force for an extended period. The field data analyzed indicates how variable the forces experienced by dipper teeth during excavation can be. 400 350
Normal force (N)
300 250 Normal force 200
Actual mean Target mean
150 100 50 0 0
500
1000
Time (s)
1500
2000
2500
Fig. 14. Example of complete cyclic load test on coupon (Adapted from [8])
4.2
Coupon volume loss and wear scar
It is not surprising that the coupons subjected to a higher mean normal force reported greater volume losses. Chromium carbide is a hard weld overlay that provides significant abrasion resistance. It was expected that the
volumes losses reported in Table 1 would be minimal. Since the distribution of carbide particles was not a controlled part of the experiment, any substantial differences in material loss between similarly tested coupons could be the result of excess matrix material being washed out during abrasion [8]. There is little literature reporting the volume loss results of abrasive wear testing on chromium carbide samples; however, the ASTM G65 test does report some minor volume loss values for a ‘No. 14 hard-chrome plating’ but does not specify the details of the material [6, 10]. An additional purpose of the cyclical loading was to investigate the severity of the difference and examine any potential differences in the wear mechanism. Figure 15, taken at the same location with a scanning electron microscope (SEM), shows the centre of a sample coupon’s wear scar: (a) using a secondary electron mode and (b) using a backscatter technique, which provided the means to observe the carbide particle (darker grey patches) distribution. Examination of the wear scar suggests areas with lower carbide presence appear ‘lower’ or more recessed, indicating the removal of material.
(a)
(b)
Fig. 15. Image taken at centre of wear scar; wear direction is left to right, (a) secondary electron mode and (b) backscatter mode (Adapted from [8])
Cross-sectional images of sample wear scars were taken using an optical and a SEM. The samples were sonic cleaned using acetone and rinsed with methanol. In order to produce quality images the samples were set in a powder-moulding compound and polished with sandpaper, up a maximum of 1200 grit. Final polishing was performed with a 3 µm polycrystalline diamond suspension. Finally, the cut and polished samples were etched with a solution to assist in carbide viewing. Figure 16 shows the optical images taken for the (a) constant load and (b) cyclic load cross sections. The abrasive wear direction is out of the page. White arrows indicate the more frequent and profound grooves formed on the cyclically loaded coupon.
25 µm
(a)
25 µm
(b)
Fig. 16. Optical micrographs of (a) constant and (b) cyclic load tests (Adapted from [8])
The SEM cross-sections in Figure 17 further illustrate the difference between the (a) constant and (b) cyclic load tests.
(a)
(b)
Fig. 17. SEM micrographs of the (a) constant and (b) cyclic load test (Adapted from [8])
The cyclic load tests produced wear scars with deeper and more frequent grooves. The nature of cyclic loading also introduces the possibility of fatigue wear. Figure 18 shows marked contrast in the behaviour of carbides between the two loading scenarios, (a) constant and (b) cyclic. In the constant loading case, the carbides protrude from the surface in typical rod-like forms. They appear intact and, unsurprisingly, the softer matrix material surrounding them appears to have been abraded away. The micrographs of the cyclic loading condition, however, do not indicate any protruding carbides. Instead, the surface appears to be worn down evenly across the wear scar. A possible explanation is that the cyclic load tests successively break down the carbide particles before completely removing them from the matrix surface.
(a)
(b)
Fig. 18. Example SEM wear scar micrographs detailing carbide presence in (a) constant test and lack of carbides in (b) cyclic test (Adapted from [8])
4.3
Specific energy and field tooth-life prediction
The application of specific energy to predict field tooth life has been attempted before [7]. The process described below builds upon previous work and aims to correlate the wear rates experienced in the lab tests performed here to field measured volume loss. New and used field tooth mass was measured in the laboratory; the two used teeth were in operation for 8 hours and 18 hours representing moderate and substantial use, respectively. Table 3 summarizes the volume losses. Table 3 Mass and volume loss data for a single oil sand shovel tooth
Mass (kg) Volume loss, (m3)
Vf
New tooth 95.7 -
Moderate use (8 hours) 81.1 0.00186
Substantial use (18 hours) 59.2 0.00465
The number of cycles the shovel completed in a given period, P, was calculated as a function of the period, P, considered for 8 or 18 hours. An availability, A’, and utilization, U’, and cycle time, Tc, of an ultra-class shovel was determined from a representative data set. Here, the shovel underwent regular maintenance for 3 days per month (90% availability) and the unit was subjected to operational downtime of 1 hour for scheduled personnel breaks, 40 minutes of shift change delays and 45 minutes of data set observed additional downtime (80% utilization). The cycle time was seen in the data set to be 43 s, as the average of the acquired field data. Equation 3 determines the number of cycles. (3) The field average abrasion energy reported in Table 2 was taken to be the energy that caused the field tooth volume losses, Vf, reported in Table 3. The total abrasion energy was multiplied by a tooth-area proportion, Atp, that represents the ratio of a single tooth area to the entire area of the dipper teeth arrangement that engages the face. Considering the field average energy expended per cycle due to abrasion, Eavg, reported in Table 2 and the tooth-area ratio, Atp, the field energy experienced per cycle, per tooth, Efc, converted to GJs is found with Equation 4.
(4) The total energy in a given period, P, to wear one tooth in the field is found by multiplying Equation 3 by Equation 4. The abrasion energy expended per lab cycle, El, is found in Table 2, the number of test cycles is 50 and the measured volume loss of the lab specimens, Vl, reported in Table 1 were used to determine the lab specific energy, Esl, in GPa, described by Equation 5. (5) Given that a specific energy is a constant regardless of whether it is measured in the lab or the field, this permits lab specific energy to predict wear time in the field. A field condition-reflective method to predict dipper tooth replacement would improve the ability to plan maintenance and thus reduce a loss in production due to downtime. Equation 6 yields the predicted time, in hours, a shovel tooth would last under the described operating conditions in an oil sand mining environment, where Tc is the average cycle time of an ultra-class shovel determined from the field data. (6) Table 4 reports the field tooth-life prediction times based on a 90% availability and 80% utilization. Table 4 Tooth life prediction
Mean normal force (N) Cyclic load Constant load
202 301 399 130
Field tooth-life prediction (hours) 8 hour period 18 hour period 5.76 14.41 5.44 13.61 6.89 17.22 1.93 4.83
The cyclic load tests produced significantly better tooth-life prediction results compared to the constant load tests. The total abrasion energy was greater over 50 periodic cycles than one long cycle for the same lineal abrasion distance. Increasing the total abraded distance to match a more severe ASTM G65 procedure would not necessarily improve the prediction results, as the volume loss would correspondingly increase. An additional advantage of the cyclic load pattern was the lack of heat generation on the coupons; something that could contribute to the seemingly aggressive volume loss observed at the lower force of the constant load test. Once the test cycles were completed, the constant load coupons were noticeably warm in comparison to the cyclically tested specimens, which remained quite cool. The drop in field life prediction seen from 202 N to 301 N is most probably explained by a lack of or poor distribution of carbides in the wear scar location on one of the coupons tested in the 301 N mean force category. A lack of deposited carbides would result in a greater concentration of matrix material, offering less protection, which in turn could cause more significant volume loss. Accordingly, the average volume loss in that test group would be influenced and thus the lab specific energy value calculated in Equation 5.
5
Conclusions
A standardized abrasive wear test was successfully modified to represent more accurately wear conditions in a Canadian oil sand mining operation. The test system allows the flexibility to modify abrasive media, specimen materials tested and most importantly the application of field conditions, reflective of imparted forces. Scaled field forces were used as data input and wear volume results were used with a field tooth-life prediction technique to estimate operational time of ultra-class dipper teeth. The key to the test is the application of the same media – material contact pressure during wear abrasion. The substitution of oil sand media and use of chromium carbide overlays on the test coupons created a scenario typical of an oil sands environment. The linear actuator facilitated testing with a more representative load pattern. SEM imaging indicated constant and cyclic loading scenarios generate wear with different mechanisms; cyclic loading potentially being more damaging. A cyclical loading sequence provided a better prediction of tooth life under standard operating conditions than a constant load test. Future adaptations to the modified test could include incorporation of programs such as LabVIEW™ to automate the system completely. Additional testing would also shed light on the possibility of wear rate variance over time; perhaps significantly increasing with the removal of more and more carbides, reinforcing the importance of appropriately timed planned maintenance.
Acknowledgements The authors would like to state their appreciation to Wilkinson Steel and Metals for supplying the sample material at no cost. They also extend gratitude to the members of the Canadian Centre for Welding and Joining (CCWJ) at the University of Alberta for sharing their knowledge on welding, overlays and carbides; as well as providing access to their facility to prepare samples for testing.
References [1] E. Rabinowicz, Friction and Wear of Materials, John Wiley & Sons, Inc., New York, 1965. [2] R. Llewellyn, Resisting wear attack in oil sands mining and processing, CIM Bull. 90 (1012) (1997) pp. 75–82. [3] 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. [4] R. Llewellyn, Materials for controlling wear in surface mining, CIM Bull. 89 (1996) pp. 76–82. [5] 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. [6] ASTM. G65-04. Standard Test Method for Measuring Abrasion Resistance Using the Dry Sand/Rubber Wheel Apparatus, in: Book of Standards, ASTM International. [7] Z. Lin, T.G. Joseph, M. Curley, Specific energy and the modified rubber wheel abrasion test, Wear 370–371 (2017) pp. 9–16. [8] M. Curley. Effects of cyclical load conditions on wear rate and wear scar in a modified ASTM G65 abrasion test, MSc Diss., University of Alberta, Edmonton, 2016. [9] G. Fisher, T. Wolfe, M. Yarmuch, A. Gerlic, P. Mendez, The use of protective weld overlays in oil sands mining, Aust. Weld. J. 57 (2012) pp. 12–14.
[10] T.G. Joseph, N. Shi, A revised dipper-ground equilibrium derivation for shovels operating in oil, sand, and soft ground, CIM J. 3 (1) (2012) pp. 47–53. [11] A. Ghosh, Scaling Laws, in: S. Chakraborty (Ed.), Mechanics over Micro and Nano Scales, Springer, 2011, pp. 61–94.
Highlights i. ii. iii. iv.
A standardized wear test is modified to incorporate cyclic load conditions Proposed modified test is shown to more accurately reflect real-world conditions Wear scars between constant and cyclical loading tests are compared and analyzed Specific energy is used to predict wear performance of shovel tips and their operating life
www.elsevier.com/locate/wear
PII: DOI: Reference:
S0043-1648(17)30513-6 http://dx.doi.org/10.1016/j.wear.2017.09.005 WEA102239
To appear in: Wear Received date: 21 March 2017 Revised date: 6 September 2017 Accepted date: 6 September 2017 Cite this article as: M. Curley and T.G. Joseph, Cyclic loading and a modified ASTM G65 abrasion test, Wear, http://dx.doi.org/10.1016/j.wear.2017.09.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Cyclic loading and a modified ASTM G65 abrasion test
M. Curleya, 1, T.G. Josephb a
Department of Civil & Environmental Engineering, School of Mining and Petroleum Engineering Markin/CNRL Natural Resources Engineering Facility 9105 116 St. Edmonton, Alberta, Canada T6G 2W2 b Faculty of Engineering Donadeo ICE Building 9203 116 St. Edmonton, Alberta, Canada T6G 1H9 1 Corresponding Author: [email protected]
Abstract Wear is a multi-faceted phenomenon that occurs through a number of mechanisms. Surface mining operations are principally impacted by abrasive wear during the excavation process. Equipment downtime due to ground engaging tool (GET) abrasive wear significantly affects production of a mining operation. In comparison to the ASTM G65 constant load abrasion test, an equally simple cyclic load test has been designed and trialed to better mimic and predict wear conditions in an oil sand mining scenario. In the modified ASTM G65 test, bitumen stripped oil sand quartz media replaced Ottawa Sand. Coupon specimens, representing GET surfaces, were hardfaced with chromium carbide weld overlay. An actuator controlling applied loads facilitated a more accurate mimic of a dig cycle for an ultra-class electric rope shovel. Shovel kinematics were analyzed to determine resistive forces experienced at the shovel bucket teeth. Recorded field data was scaled to appropriate laboratory testing magnitudes. Wear scar inspection suggested that cyclic loading scenarios potentially generate more severe wear damage versus constant loading. Field tooth life was predicted using a specific energy technique. Test results indicated that a cyclic loading approach provided closer correlation to measured shovel tooth life for an oil sands operation than a constant load approach.
Keywords: three-body abrasion; hardfacing; wear testing; chromium carbide; surface analysis; oil sand
1
Introduction
Wear is a critical factor affecting production rates and maintenance costs in any mining operation. Rabinowicz [1] described wear as “the removal of material from solid surfaces as a result of mechanical action”. Surface mining operations are particularly affected by such an abrasive wear process throughout excavation duties. In Canadian oil sands operations located in Northern Alberta, ultra-class electric rope shovels experience severe GET wear caused by bucket teeth versus abrasive oil sand media interactions. Abrasive wear is a key concern to the mining industry as economic impacts and lost production due to downtime are a significant driver to continually improve wear control practices. The objective of this paper was to develop a laboratory scale test improvement to more accurately represent field abrasive wear conditions (in this case for an oil sand operating environment) experienced by shovel teeth and to apply the test results in a field tooth-life prediction. To accomplish this, a standardized abrasive wear test (ASTM G65) was modified to incorporate a cyclic load-varying component that would facilitate normal forces imparted to a test coupon more reminiscent of actual field loading conditions. This then reflected the discontinuous cyclic forces experienced by shovel teeth as they engage and disengage a mining face. A set of field data from an oil sands operation was analyzed and scaled to establish a laboratory test force range in order to conduct controlled, repeatable tests. Results from the modified abrasion tests were compared to the original ASTM G65 constant load test to discern the effects of cyclic loads on wear rates and wear scars.
1.1
Background
Oil sand is predominately a quartz sand surrounded by a water film with a heavy oil (bitumen) between particles. The quartz is generally sub-angular with high proportion of quartz contributing significantly to an extreme abrasivity. Quartz has a Vickers Hardness (H v) rating of 850–900, indicating that even the hardest steels only provide moderate protection [2]. Ground engaging tools mounted on cable shovel dippers are subjected to high forces during the excavation cycle, resulting in significant wear damage and material loss to teeth tips and adapters. Several wear modes contribute to the loss of material from dipper teeth, yet abrasive wear is the most dominant. The severity of abrasive wear attack depends on a number of factors; most broadly stated as the characteristics of the abrasive media, the properties of the wear material and the interaction between these two elements [3]. Different classification schemes have been developed by researchers over many years to characterize abrasive wear, where, generally it is agreed abrasive wear modes can be classified as low stress, high stress or gouging abrasion. The action of dipper teeth digging through un-blasted ore such as oil sand can be classified as low stress abrasion, as the abrasive particles do not fracture during the dig cycle. A variety of components and protection systems are available to the mining industry to attempt wear alleviation. Llewellyn [4] classified these into four primary groups: ferrous-based materials, elastomers/polymers, ceramics/cermets and surface engineering techniques. Surface engineering techniques, in particular weld overlays, are a common practice to combat the significant abrasivity of oil sands. Hardfacing provides a wear performance and economic advantage, permitting the use of a more common base material that has attractive structural properties but would be too costly to frequently replace weld overlay. Compounds such as tungsten or chromium carbides exhibit a high wear resistance. The process by which the overlay was deposited on the coupons tested in this research was submerged arc welding.
1.2
Abrasive wear test
An ideal laboratory wear test is inexpensive, efficient but is a relative reference test and not one that creates the exact conditions of the real-world scenario. Naturally, cost, scale and practicality usually prevents this. The ASTM G65 test (Figure 1) is an industry recognized abrasion test used for relative ranking the abrasion resistance of various materials. The test is suited for finer abrasives such as oil sand that do not break or fracture during
excavation. The abrasive media is not fixed; the particles are free to slide and rotate between a spinning wheel and a test coupon, simulating three-body abrasive wear [5]. Five procedures characterized by different parameters including normal force, the number of wheel revolutions and the total corresponding lineal abrasion distance outline how the test can be performed. The standard stipulates the test is suited for the testing of any material form, including weld overlays, and uses an abrasive typified by AFS 50/70 Test Sand (Ottawa Sand) [6].
Fig. 1. Basic dry sand/rubber wheel test apparatus (Adapted from [6])
The ASTM G65 test was modified in this work to reflect more accurately the actual mining conditions in an oil sands operation and create the associated abrasive wear environment.
2 2.1
Experimental procedure Abrasive media
A standard G65 test uses “Ottawa” silica sand as the abrasive medium. Previous work experimented with the substitution of different abrasive media, including the use of oil sand on softer metals such as aluminium and mild steel [7]. The purpose of substituting the standard abrasive media is to better represent field conditions. The material used during the modified experiment performed here was bitumen stripped oil sand. The material had significant abrasive qualities and was dried and sieved characterized before testing. Figure 2 details the approximate size and angularity of the abrasive particles used.
Fig. 2. Close-up image of tailings sand abrasive (Adapted from [8])
The figure shows the particles are sub-angular to angular and vary in size and shape. The majority of the particles are quartz. The black background is not bitumen; but rather carbon tape the samples were placed on for imaging. Figure 3 outlines the particle size distribution of the media from sieve analysis. 100%
Cumulative % passing
80%
60% Distribution before test 40%
20%
0% 0
100
200
300
Particle diameter (µm)
Fig. 3. Grain size distribution of abrasive media used in modified G65 test (Adapted from [8])
2.2
Coupon characteristics
The test coupons were chromium carbide (Cr7C3) overlaid on a 44W steel base. The overlay was applied as a submerged arc weld (powder addition) cut to shape and size for the test using a water jet. The coupons measured 76 × 64 × 8 mm and of 50% base substrate and 50% overlay. The samples were ground to a smooth finish using a surface grinder before testing commenced. Chromium carbide is a material frequently used to coat shovel teeth, ripper shanks and crusher feed/discharge areas to prevent wear [4]. The tested coupons were hypoeutectic overlays containing an average of 28% chromium, 4% carbon, 2% manganese and 1% molybdenum & boron, with the balance being iron. Figure 4 shows an (a) unprepared and a (b) test prepared coupon.
(a) Fig. 4. Sample coupon (a) before and (b) after surface grinding
(b)
The coupons were ground until there were no noticeable asperities with all slag material removed. The coupons had a shiny finish with the occasional presence of small, dull grey pits formed as a result of the weld application process. The cracking observed on the surface of the coupon results from the contraction of the weld as it cooled; often called relief or fatigue cracking [9]. Relief cracking was not observed on all samples. Any sand particles caught in the cracks were removed with a vacuum nozzle or blown out with air prior to weighing.
2.3
Modified test apparatus
The most significant addition to the standard G65 test setup was the incorporation of a linear load control actuator. This actuator facilitated a repeatable, cyclical load. The actuator was installed vertically at the end of the G65 system horizontal lever arm, similar to the hanging weight plates in the standard apparatus. A simple moment diagram was constructed in order to determine the downward vertical force necessary to generate the normal resistive force imparted on the coupon. The actuator position, proportional to load, was computer controlled. A load cell was installed between the end of the lever arm and the tip of the actuator cylinder, whose response was recorded by a secondary PC for variations in force as the actuator was extended and retracted. Test runs were performed in order to evaluate and calibrate the system. The actuator also provided the advantage of acting as a rigid beam reducing vibrations that occur in the standard setup where a set of hanging weights deliver a leveraged normal force. To ensure wheel rotational velocity did not decrease when the force was applied, a 10:1 worm gear was installed that supplied the motor with sufficient torque. The mounted rubber wheel had a diameter of 148 mm and a width of 51 mm. The rpm of the wheel was determined from averaging the dig velocity of the dipper from the moment of face entry to exit over all recorded dig cycles. The average dig velocity, 0.775 m/s, corresponded to 100 rpm and was maintained constant throughout the tests. The hopper and nozzle enabled media material flow, controlled via a
valve. The nozzle width was identical to that of the rotating wheel and the opening was 1 mm, a value greater than three times the largest particle diameter to help prevent clogging. In order to calculate abrasion energy and, by association, specific energy, a voltmeter and ammeter were connected to the wheel motor to evaluate the power draw. Figure 5 shows the modified test set-up.
Fig. 5. Modified test apparatus set-up; hopper (1), lever arm (2), load cell (3), actuator (4), specimen and holder (5), rubber wheel (6), voltmeter and ammeter (7) (Adapted from [8])
2.4
Methodology
Shovel data from a Northern Alberta oil sands operation was used to determine the scale laboratory force-range needed for the test. The data set was collected from a P&H 4100 BOSS shovel, including time stamp, hoist rope position, crowd extension, dig velocity and hoist & crowd armature voltage and current values.
2.4.1
Geometric analysis of a shovel
An established geometric analysis technique [8, 10] was used to determine hoist rope and crowd positions, geometrically resolved in a 2D x-y coordinate plane to profile the shovel dig trajectory through the face, relative to the dipper teeth. Figure 6 illustrates the digging profiled created for three passes of the shovel bucket. The y-axis represents the centreline of the shovel’s swing rotation and the x-axis is the ground surface beneath the crawler tracks.
14 Handle-boom saddle rotation point
12
Vertical hoist (m)
10
Dipper exits face
8 Dig Cycle 1 6
Dig Cycle 2
4 2
Dig Cycle 3
Dipper enters face
Tooth tip at tuck motion
0 0
5
10
15
20
25
-2
Horizontal extension (m) Fig. 6. Approximate dig profile of three passes relative to shovel function (Adapted from [8])
Taking the intersection of the shovel boom and dipper handle (saddle point) as the point of orientation, a set of moment equations were generated and the total digging resistance, FR, was obtained. This dig resistance was further resolved to a normal resistance felt by the shovel teeth, permitting scaled laboratory test values to be determined. Figure 7 details a general diagram of an ultra-class shovel’s boom, handle and dipper arrangement. The x and y-axes are defined as above.
Fig. 7. General schematic of ultra-class shovel, including an approximate dig profile (Adapted from [8])
2.4.2
Scaling procedure
Analysis of the field data showed that hoist force was the dominant contributor to the digging resistance. Figure 8 illustrates that hoist force and the digging resistance experienced by the dipper teeth are very similar, as the teeth, dipper and handle are all part of the same fixed arrangement, such that the teeth experience the same direct trajectory the hoist imparts on the entire dig system. It is clear in the example that there is an initial “ramp-up” period of approximately three seconds from 15 to 18 s as the dipper engages the face and beings to progress upward. The force then remains relatively constant, but varying with diggability of the face material, for around 10 s during the “face dig”. The sudden decrease in force at roughly 26 s indicates that the dipper has exited the face. Each dig cycle identified in the data set demonstrated an approximately three-second ramp-up period, a 10 s face dig and a 30 s swing-dump-reposition reprieve.
3500 3000
Force (kN)
2500 ~10 s face dig
2000
Hoist force
1500
Dig resistance
1000
Face exit
500 Ramp-up
0 14
16
18
20
22
24
26
28
Time (s)
Fig. 8. Example of hoist and normal resistive forces (Adapted from [8])
The field force values were much too high to easily test in a laboratory setting. A scale factor was used to determine laboratory appropriate force values used to perform the proposed modified test. It has been documented that area is related through a square-power law; the idea being that if all dimensions of a geometric shape are increased by some factor W the area scales by W2, provided the geometric shape remains unchanged [11]. Volume is scaled via a cubepower law; a cube of side x with all dimensions increased by a factor of two will yield a volume that has increased by eight, or two cubed. The scale factor was calculated by comparing the contact area of an unused ultra-class dipper tooth with a wear scar generated by the modified apparatus on a spare coupon sample. The total surface area of a dipper tooth that interacts with a mining face during excavation was measured to be 1126 cm2. The area of the wear scar generated by a calibration test on a spare hardfaced coupon was 4.4 cm2 [8], such that a the scale factor, SF, could be calculated by taking the square root of the ratio of the measured field tooth wear scar area, Atooth, to the sample wear scar, Acoupon, Equation 1. Inputting the measured surface area and wear scar values yield a scale factor of 16. √
(1)
For all dig cycles in the field data set, a mean dig resistance force, Ftooth, during the 10 s face dig period (Figure 8) was calculated. In order to determine the face dig force applied to the coupons, Fcoupon, in the modified test, the same scale as defined in Equation 1 factor was applied as a cube-root determination proportional to volume or force [11] to the field evaluated mean normal resistive force, Equation 2. An average standard deviation of the face dig force between the dig cycles was also calculated and was similarly scaled to produce a range of normal force values that were tested using the modified set-up. √
(2)
Given that the contact area of the rubber wheel in the modified G65 test on the coupon was observed as constant during calibration, mimicking the field contact where shovel teeth are fully engaged in the dig cycle, it was reasonably assumed that the same contact pressure between media and coupon was evident as found in the field conditions.
2.4.3
Proposed modified test
Figure 9 illustrates one cycle of the proposed modified test, showing the three-second ramp-up period followed by a 10 s face dig and a 30 s reprieve; mimicking a typical dig cycle of an operating ultra-class oil sand shovel. The range of tests (+σ and –σ) depict plus and minus one standard deviation from the scaled mean dig resistance value. Each coupon was subjected to 50 cycles, totalling 525 m of lineal abrasion, at the load category in which they were assessed. The total cyclic load total test time was roughly 36 minutes per coupon. 450 400
Normal applied force (N)
350 300 250
~10 s face dig
Mean test
200
+σ
150
-σ
100 Ramp-up
50
Applied force = 0 N for 30 s
0 0
2
4
6
8
10
12
14
16
Time (s)
Fig. 9. Proposed modified test (Adapted from [8])
Three coupons were tested in each of the respective load categories outlined in Figure 9. For comparison, an additional two coupons were tested using hanging weights and a constant load, 130 N, similar to procedures A, B, C and E of the ASTM G65 test specifications. The wheel rpm, total lineal abrasion distance and abrasive media were kept constant for all tests. The total time of the constant load test was approximately 11 minutes per coupon.
2.4.4
Power draw and specific energy
Graphing the power draw facilitated the determination of the abrasion energy and the specific energy, essentially the energy that caused the volume loss. Figure 10 shows a power–time graph for three of the field data dig cycles. The stippled region indicates the area under the plots that was taken to calculate the field abrasion energy. Since abrasion energy is exerted during the dig cycle, it follows that the shape of the abrasion energy area is similar to the proposed modified test pattern observed in Figure 9.
3500 3000
Power (kW)
2500 2000 Dig cycles
1500
Abrasion energy
1000 500 0 0
20
40
60
80
100
120
Time (s)
Fig. 10. Example of power draw and calculated abrasion energy from field dig cycles (Adapted from [8])
The voltmeter and ammeter connected to the wheel motor of the modified set-up enabled the recording of the power draw during the scaled dig cycle. The use of specific energy to predict field tooth wear has been attempted before [7], and is again reviewed here using a modified approach.
3 3.1
Results Cyclic load capability
The implementation of the actuator was very successful in creating a cyclic load representative of a shovel completing dig cycles in the field. Figure 11 illustrates a snapshot of two complete cycles for the mean test force category. The actual and target mean refer to the average normal force imparted on the coupon during the 10 s face dig portion and the target-exerted force. The variation of the test achieved and target forces during this period is 2%. The dashed lines reflect the range of the three-second ramp up period observed from the field data (Figure 8) and detailed in the proposed modified test (Figure 9).
400 350
Normal force (N)
300 250 Normal force
200
Actual mean
150
Target mean Ramp Δt
100 50 0 180
190
200
210
220
230
240
250
Time (s)
Fig. 11. Example snapshot detailing cyclic load and ramp-up capability of the proposed modified test (Adapted from [8])
3.2
Coupon volume loss
Each coupon was subjected to 525 m of lineal abrasion, equal to 50 complete dig cycles, at a common modified G65 and field tooth speed of 0.775 m/s. Table 1 reports the average volume loss for each target force group for cyclic and constant load tests. The percent deviation refers to the difference between the target face dig force and the mean applied force during the face dig in the cyclical load tests. The constant load tests used a set of hanging weights to apply the resistance. The mass of the hanging plates was calculated based on applying 130 N of normal force on the coupon. Table 1 Volume loss comparison between cyclic load tests and constant load tests
Test type
Cyclic load Constant load
# of test specimens 3 3 3 2
Target face dig force (N) 202 301 399 130
Mean % deviation -3.1 2.6 1.5 -
Mean volume loss (mm3) 3.19 ± 0.11 5.23 ± 1.79 5.62 ± 0.67 3.29 ± 0.21
Figure 12 details an example of a coupon wear scar. The direction of abrasive media flow was perpendicular to the surface grinding preparation direction.
Fig. 12. Example of a coupon wear scar; wear direction top to bottom (Adapted from [8])
3.3
Power draw and abrasion energy
Determining abrasion energy expended during the laboratory test facilitated the determination of specific energy; the energy required to cause a unit volume loss of material. The area under the power-time graph yielded the energy expended during the abrasion process. Figure 13 represents the power draw graph for the mean test (301 N of normal force). Since the coupon was loaded for 50 cycles, the power consumed during the abrasion process was taken as the sum of the differential power (area represented by stippled region) and not simply the power required to spin the rubber wheel. The constant load test’s power draw was calculated similarly, though over one long cycle. 2.5
Power (W)
2
1.5 Power draw at 301 N
1
Abrasion energy 0.5
0 0
5
10
15
20
25
30
Time (s)
Fig. 13. Power draw of modified apparatus at mean normal force (Adapted from [8])
Table 2 summarizes the mean normal force during the face dig and the average abrasion energy expended during the total dig cycle (ramp + dig) for the field data, cyclic and constant load tests. Table 2 Comparison of field and lab force and energy values
Data source Field data Cyclic load Constant load
4
Mean normal force (N) 1 231 000 202 301 399 130
Mean abrasion energy (J) 20 434 482 527 815 1107 182
Symbol Eavg El El El El
Discussion
4.1
Cyclic load capability
The modified abrasion test was successful in mimicking the dig profile generated by an ultra-class excavator in a typical oil sands operation. The application of a linear actuator effectively provided the ability to load a set of test coupons cyclically with repeatable precision. Figure 14 illustrates how reliable the actuator proved to be, demonstrating a very repeatable sequence for 50 consecutive load cycles. For all cyclic load test categories, the actuator produced consistent accuracy with the greatest variation in target-applied force and actual applied force being 3.1%. It is rare for operating mining equipment to experience a constant force for an extended period. The field data analyzed indicates how variable the forces experienced by dipper teeth during excavation can be. 400 350
Normal force (N)
300 250 Normal force 200
Actual mean Target mean
150 100 50 0 0
500
1000
Time (s)
1500
2000
2500
Fig. 14. Example of complete cyclic load test on coupon (Adapted from [8])
4.2
Coupon volume loss and wear scar
It is not surprising that the coupons subjected to a higher mean normal force reported greater volume losses. Chromium carbide is a hard weld overlay that provides significant abrasion resistance. It was expected that the
volumes losses reported in Table 1 would be minimal. Since the distribution of carbide particles was not a controlled part of the experiment, any substantial differences in material loss between similarly tested coupons could be the result of excess matrix material being washed out during abrasion [8]. There is little literature reporting the volume loss results of abrasive wear testing on chromium carbide samples; however, the ASTM G65 test does report some minor volume loss values for a ‘No. 14 hard-chrome plating’ but does not specify the details of the material [6, 10]. An additional purpose of the cyclical loading was to investigate the severity of the difference and examine any potential differences in the wear mechanism. Figure 15, taken at the same location with a scanning electron microscope (SEM), shows the centre of a sample coupon’s wear scar: (a) using a secondary electron mode and (b) using a backscatter technique, which provided the means to observe the carbide particle (darker grey patches) distribution. Examination of the wear scar suggests areas with lower carbide presence appear ‘lower’ or more recessed, indicating the removal of material.
(a)
(b)
Fig. 15. Image taken at centre of wear scar; wear direction is left to right, (a) secondary electron mode and (b) backscatter mode (Adapted from [8])
Cross-sectional images of sample wear scars were taken using an optical and a SEM. The samples were sonic cleaned using acetone and rinsed with methanol. In order to produce quality images the samples were set in a powder-moulding compound and polished with sandpaper, up a maximum of 1200 grit. Final polishing was performed with a 3 µm polycrystalline diamond suspension. Finally, the cut and polished samples were etched with a solution to assist in carbide viewing. Figure 16 shows the optical images taken for the (a) constant load and (b) cyclic load cross sections. The abrasive wear direction is out of the page. White arrows indicate the more frequent and profound grooves formed on the cyclically loaded coupon.
25 µm
(a)
25 µm
(b)
Fig. 16. Optical micrographs of (a) constant and (b) cyclic load tests (Adapted from [8])
The SEM cross-sections in Figure 17 further illustrate the difference between the (a) constant and (b) cyclic load tests.
(a)
(b)
Fig. 17. SEM micrographs of the (a) constant and (b) cyclic load test (Adapted from [8])
The cyclic load tests produced wear scars with deeper and more frequent grooves. The nature of cyclic loading also introduces the possibility of fatigue wear. Figure 18 shows marked contrast in the behaviour of carbides between the two loading scenarios, (a) constant and (b) cyclic. In the constant loading case, the carbides protrude from the surface in typical rod-like forms. They appear intact and, unsurprisingly, the softer matrix material surrounding them appears to have been abraded away. The micrographs of the cyclic loading condition, however, do not indicate any protruding carbides. Instead, the surface appears to be worn down evenly across the wear scar. A possible explanation is that the cyclic load tests successively break down the carbide particles before completely removing them from the matrix surface.
(a)
(b)
Fig. 18. Example SEM wear scar micrographs detailing carbide presence in (a) constant test and lack of carbides in (b) cyclic test (Adapted from [8])
4.3
Specific energy and field tooth-life prediction
The application of specific energy to predict field tooth life has been attempted before [7]. The process described below builds upon previous work and aims to correlate the wear rates experienced in the lab tests performed here to field measured volume loss. New and used field tooth mass was measured in the laboratory; the two used teeth were in operation for 8 hours and 18 hours representing moderate and substantial use, respectively. Table 3 summarizes the volume losses. Table 3 Mass and volume loss data for a single oil sand shovel tooth
Mass (kg) Volume loss, (m3)
Vf
New tooth 95.7 -
Moderate use (8 hours) 81.1 0.00186
Substantial use (18 hours) 59.2 0.00465
The number of cycles the shovel completed in a given period, P, was calculated as a function of the period, P, considered for 8 or 18 hours. An availability, A’, and utilization, U’, and cycle time, Tc, of an ultra-class shovel was determined from a representative data set. Here, the shovel underwent regular maintenance for 3 days per month (90% availability) and the unit was subjected to operational downtime of 1 hour for scheduled personnel breaks, 40 minutes of shift change delays and 45 minutes of data set observed additional downtime (80% utilization). The cycle time was seen in the data set to be 43 s, as the average of the acquired field data. Equation 3 determines the number of cycles. (3) The field average abrasion energy reported in Table 2 was taken to be the energy that caused the field tooth volume losses, Vf, reported in Table 3. The total abrasion energy was multiplied by a tooth-area proportion, Atp, that represents the ratio of a single tooth area to the entire area of the dipper teeth arrangement that engages the face. Considering the field average energy expended per cycle due to abrasion, Eavg, reported in Table 2 and the tooth-area ratio, Atp, the field energy experienced per cycle, per tooth, Efc, converted to GJs is found with Equation 4.
(4) The total energy in a given period, P, to wear one tooth in the field is found by multiplying Equation 3 by Equation 4. The abrasion energy expended per lab cycle, El, is found in Table 2, the number of test cycles is 50 and the measured volume loss of the lab specimens, Vl, reported in Table 1 were used to determine the lab specific energy, Esl, in GPa, described by Equation 5. (5) Given that a specific energy is a constant regardless of whether it is measured in the lab or the field, this permits lab specific energy to predict wear time in the field. A field condition-reflective method to predict dipper tooth replacement would improve the ability to plan maintenance and thus reduce a loss in production due to downtime. Equation 6 yields the predicted time, in hours, a shovel tooth would last under the described operating conditions in an oil sand mining environment, where Tc is the average cycle time of an ultra-class shovel determined from the field data. (6) Table 4 reports the field tooth-life prediction times based on a 90% availability and 80% utilization. Table 4 Tooth life prediction
Mean normal force (N) Cyclic load Constant load
202 301 399 130
Field tooth-life prediction (hours) 8 hour period 18 hour period 5.76 14.41 5.44 13.61 6.89 17.22 1.93 4.83
The cyclic load tests produced significantly better tooth-life prediction results compared to the constant load tests. The total abrasion energy was greater over 50 periodic cycles than one long cycle for the same lineal abrasion distance. Increasing the total abraded distance to match a more severe ASTM G65 procedure would not necessarily improve the prediction results, as the volume loss would correspondingly increase. An additional advantage of the cyclic load pattern was the lack of heat generation on the coupons; something that could contribute to the seemingly aggressive volume loss observed at the lower force of the constant load test. Once the test cycles were completed, the constant load coupons were noticeably warm in comparison to the cyclically tested specimens, which remained quite cool. The drop in field life prediction seen from 202 N to 301 N is most probably explained by a lack of or poor distribution of carbides in the wear scar location on one of the coupons tested in the 301 N mean force category. A lack of deposited carbides would result in a greater concentration of matrix material, offering less protection, which in turn could cause more significant volume loss. Accordingly, the average volume loss in that test group would be influenced and thus the lab specific energy value calculated in Equation 5.
5
Conclusions
A standardized abrasive wear test was successfully modified to represent more accurately wear conditions in a Canadian oil sand mining operation. The test system allows the flexibility to modify abrasive media, specimen materials tested and most importantly the application of field conditions, reflective of imparted forces. Scaled field forces were used as data input and wear volume results were used with a field tooth-life prediction technique to estimate operational time of ultra-class dipper teeth. The key to the test is the application of the same media – material contact pressure during wear abrasion. The substitution of oil sand media and use of chromium carbide overlays on the test coupons created a scenario typical of an oil sands environment. The linear actuator facilitated testing with a more representative load pattern. SEM imaging indicated constant and cyclic loading scenarios generate wear with different mechanisms; cyclic loading potentially being more damaging. A cyclical loading sequence provided a better prediction of tooth life under standard operating conditions than a constant load test. Future adaptations to the modified test could include incorporation of programs such as LabVIEW™ to automate the system completely. Additional testing would also shed light on the possibility of wear rate variance over time; perhaps significantly increasing with the removal of more and more carbides, reinforcing the importance of appropriately timed planned maintenance.
Acknowledgements The authors would like to state their appreciation to Wilkinson Steel and Metals for supplying the sample material at no cost. They also extend gratitude to the members of the Canadian Centre for Welding and Joining (CCWJ) at the University of Alberta for sharing their knowledge on welding, overlays and carbides; as well as providing access to their facility to prepare samples for testing.
References [1] E. Rabinowicz, Friction and Wear of Materials, John Wiley & Sons, Inc., New York, 1965. [2] R. Llewellyn, Resisting wear attack in oil sands mining and processing, CIM Bull. 90 (1012) (1997) pp. 75–82. [3] 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. [4] R. Llewellyn, Materials for controlling wear in surface mining, CIM Bull. 89 (1996) pp. 76–82. [5] 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. [6] ASTM. G65-04. Standard Test Method for Measuring Abrasion Resistance Using the Dry Sand/Rubber Wheel Apparatus, in: Book of Standards, ASTM International. [7] Z. Lin, T.G. Joseph, M. Curley, Specific energy and the modified rubber wheel abrasion test, Wear 370–371 (2017) pp. 9–16. [8] M. Curley. Effects of cyclical load conditions on wear rate and wear scar in a modified ASTM G65 abrasion test, MSc Diss., University of Alberta, Edmonton, 2016. [9] G. Fisher, T. Wolfe, M. Yarmuch, A. Gerlic, P. Mendez, The use of protective weld overlays in oil sands mining, Aust. Weld. J. 57 (2012) pp. 12–14.
[10] T.G. Joseph, N. Shi, A revised dipper-ground equilibrium derivation for shovels operating in oil, sand, and soft ground, CIM J. 3 (1) (2012) pp. 47–53. [11] A. Ghosh, Scaling Laws, in: S. Chakraborty (Ed.), Mechanics over Micro and Nano Scales, Springer, 2011, pp. 61–94.
Highlights i. ii. iii. iv.
A standardized wear test is modified to incorporate cyclic load conditions Proposed modified test is shown to more accurately reflect real-world conditions Wear scars between constant and cyclical loading tests are compared and analyzed Specific energy is used to predict wear performance of shovel tips and their operating life