When considering the use of wear-resistant steels for the manufacture of wear parts that operate in highly demanding environments such as the quarrying and mining industries, are operators viewing this option as a positive return on investment or as just a higher cost against limited internal budgets?
To prove the cost benefits of wear-resistant materials, Abraservice Group, a European supplier of wear-resistant and high yield steel plates, ran a comparative test program based in a chromium mine in Turkey, with results showing that a 20 per cent investment in higher cost wear-resistant bucket edges provided an increase in wear life of the components from +21 per cent up to an outstanding +92 per cent depending on the size and density of the aggregate being mined, as well as reducing equipment downtime and significantly increasing periods between maintenance.
BACKGROUND
Mining excavator buckets on earthmoving machinery are typically manufactured with high yield strength steel for the body, with standard 450HB or 500HB water-quenched steel wear plates welded into position on the front.
The comparative test program run by Abraservice Turkey was designed to compare the use and the component life of bucket wear edges produced in a commonly sourced, industry standard water-quenched 450HB steel from northern Europe against those manufactured in a specialist abrasion resistant steel, Creusabro 8000.
The Creusabro range of steels is produced by Industeel ArcelorMittal Group and distributed across Europe by Abraservice Group.
The test program was designed to compare the cost of investment for replacement wear edge components with the wear life achieved from the parts, to establish if wear-resistant steels can provide a positive return on investment for companies operating daily in these extreme market sectors.
TESTING PROCESS
The comparative test program carried out by Abraservice Turkey was run at the chromium mine in Eti Krom City, in east Turkey.
This very harsh external environment provided extremely abrasive conditions. Identical bucket wear edges were produced in the two grades of steel, each 30mm in thickness and installed on identical wheel loaders within the quarry. The two buckets were then operated in a side-by-side comparison program to evaluate the wear life of the components in abrasion testing, focusing on the areas of impact, dragging and sliding within crushed aggregate.
The wheel loaders fitted with the manufactured wear edges were operated in two different areas of the quarry. The first was used for the mining of chromite, an iron chromium oxide (FeCr2O4), and the second for ferrochrome, a chromium iron alloy (FeCr). The size and density of the aggregate being mined in these two areas is significantly different, and the characteristics of the two aggregates are shown in Table 1.
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COMPARISON OF STEELS
The two grades of steel run on the side-by-side comparison program were analysed for their chemical composition and hardness qualities. The quality control test results for each steel grade are outlined in Tables 2 and 3, and are shown against the manufacturers’ specifications. As can be seen, the chemical composition varies quite significantly between the two samples but the hardness of the steel materials on test are very similar, with the water-quenched 450HB having a tested hardness of 454HB and the oil-quenched Creusabro 8000 having a tested hardness of 461HB.
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COMPARATIVE TEST RESULTS
The first wheel loader operated in the chromite stock area of the quarry, where the test wear parts on the buckets were working with aggregate of which 50 per cent was ~500mm in diameter and 50 per cent was 15mm to 60mm in diameter, with an average density of 4.5.
The second wheel loader operated in the ferrochrome stock area of the quarry, where the test wear parts on the buckets were operating with aggregate of 200mm to 500mm in size, with an average density of 7.2.
The results obtained in the four comparative abrasion tests are summarised in Table 4. The tests’ aim was to determine the number of hours worked by the wheel loaders to reduce the fitted bucket wear edges from the original 30mm thickness down to 5mm thickness, at which point the wear edges would need replacing.
The working hours were carefully monitored for each set of bucket wear edges on each wheel loader, including downtime for changing sets of wear edges. The average results based on side-by-side testing of the wear parts are reported in Table 4.
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INDUCED PLASTICITY
The comparative abrasion test clearly shows that the outstanding wear-resistant properties of the Creusabro 8000 offer dramatic increases in the working life of the wear edge components, ranging from an increased working life of 21 per cent in the chromite mining area to an outstanding +92 per cent when operating in the ferrochrome area of the quarry.
The results show the hardness of the two steel grades are very similar, with the water-quenched 450HB having a tested hardness of 454HB and the oil-quenched Creusabro 8000 having a tested hardness (before work-hardening) of 461HB, so the extended wear life and superior performance achieved by the Creusabro 8000 wear parts must come from another metallurgical property found within this specialist steel.
In addition, the lifetime differences seen between the two test environments of chromite (21 per cent increase in wear life) and ferrochrome (92 per cent) are strongly influenced by the material density, the stone and alloy block size and the energy of impact within the mining operation.
The metallurgical property observed in the results is known as transformation induced plasticity, and such steels have a triple-phase micro-structure containing austenite.
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This phenomenon allows the Creusabro 8000 material to be delivered to customers with an intentionally limited hardness, specification 430HB to 495HB, allowing the steel to be more easily profiled through many processes including rolling, machining, bending, countersinking, cutting or milling to produce the components required.
Then when the material components are in use, the retained austenite within the composition of the metal allows the steel to work-harden in service, triggered through impact or pressure, providing enhanced strength and ductility to the steel. The Creusabro 8000 work-hardening properties produce a 70HB hardness increase in service over the properties achieved by the industry standard TE 450HB, as shown in Figure 1.
The investment in producing bucket wear edges in abrasion-resistant Creusabro 8000 is certainly higher, with these specialist steels being about 20 per cent higher in cost, plate for plate, compared with a standard 450HB steel.
But with tighter limits on their budgets, companies need equipment and replacement components such as critical wear parts to have a longer lifespan.
The return achieved through maximising investment in maintaining equipment not only provides prolonged wear life of products and fabrications, but also significantly reduces the on-site downtime of key equipment used in these extreme environments, increases periods between required maintenance and reduces labour costs associated with fitting replacement parts, proving that the benefits of these wear-resistant solutions should be seen as a positive return on investment for any company trying to drive down costs.
Gilles Gros and Gary Chandler are respectively the product manager and the general director of Abraservice Group.
This story originally appeared in the November 2013 issue of Quarry Management UK and reappears in Quarry with kind permission.