Laboratory Testing of Materials for Tunnel Boring Machine Drag Bits

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III. DRAG BIT MATERIALThe part of the drag bit that is subjected to toughest wearconditions is usually made of ceramic-metal (cermet)composite material. Conventionally it is cermet made oftungsten carbide and cobalt (WC-Co). Cobalt content usuallyranges from 6 to 15 % by weight. WC grain size is rangingfrom 2 to 20 µm. Any excess or deficiency of carbon hassignificant effect on hardness and strength. In general, anyalloying impurities like iron chromite, nickel, sodium orsulphur can result in poor combination of hardness andstrength. Small additions of titanium carbide (3 – 5% byweight), however, could prevent grain coarsening andincrease hardness without affecting the transverse rupturestrength. Equally important is the grain size control; hardnessand compressive strength increasing with decreasing grainsize, whereas the desirable grain size for best rupture strengthis from 1µm to 3 µm. Porosity in the alloys’ structure is anunwanted parameter. High porosity gives rise to poortransverse rupture strength but in hard metals, high densitiesup to 99.5% are achieved and uniformly distributed porosityis usually present, which is not so harmful [5, 6].Improvement of wear resistance of WC-Co cermets isachievable by combining the coarse and fine carbides in onestructure, so that the smaller ones fill the spaces between thelarger ones. Double-structuring may help to improve fracturetoughness. Development of complex structures with improvedwear resistance is possible by additions of different hardphase types (Cr 3 C 2 , TiC, VC, TaC, NbC, etc.). Resistance tocorrosive environments and thermal shocks may be improvedby modification of binder phase by Ni, Cr, Re, etc [5, 7].As far as abrasive wear is concerned polycrystallinediamond compact (PDC) tools are now available with 5-6times higher hardness than tungsten carbide. However, PDCis more susceptible to brittle failure than cemented tungstencarbide, because its fracture toughness is almost twice lowerthan that of tungsten carbide with 6% cobalt [5].Fig. 1. Multi Modular Tribosystem with adjustable inertia and rigidity ofloading system (MMTS) for Block-on-Ring abrasive wear testing (TUT).Abrasive is supplied between disk and sample. It is usuallydry abrasive but spraying of liquid onto abrasive after nozzleis possible. Sample may be heated or cooled to provideconstant temperature from 10 to 450 ºC. If testing with slurryis required then it is possible to make tests similar to ASTMG 105 or B 611 standards (Fig. 2). Usually abrasive of sizesmaller than 1 mm are used. These are one sample tests.Fig. 2. Device for abrasive wear testing with slurry (TUT).Ultra-low stress abrasion used for evaluation of the wearproperties of oxide scales at elevated temperatures is given inFig. 3. The load is generated only due to immersion into theabrasive. Up to 36 samples may be treated simultaneously intwo abrasives.IV. LABORATORY DEVICES FOR DRAG BIT WEAR TESTINGWITH FREE ABRASIVEIt is possible to make testing applying fixed (for exampleagainst sand paper that is called two-body abrasion) or againstfree abrasive. When free abrasive is present between twobodies the test is called three-body abrasion.A. Testing in Conditions of Ultra-Low. Low or High StressAbrasionTesting in Block-on-Ring configuration may be performedusing rubber-coated (according to ASTM G 65 standard) orsolid steel ring. During testing using rubber wheel theabrasive is usually not broken and it is called low-stressabrasion. Multi Modular Tribosystem (Fig. 1) allows testingusing both wheels while enabling to adjust rigidity and inertiaof loading system that is required to make test conditions asclose as possible to those experienced in real applications.Fig. 3. Device for ultra-low stress abrasion-oxidation testing at elevatedtemperatures (TUT).B. Testing in Conditions of Impact by Abrasive of DifferentSizeIn dry erosion test the specimen is attacked by the jet ofparticles that is accelerated by means of centrifugal forcesduring the rotation of the rotor (Fig. 4). It is possible to varythe impact angle (10º-90º), impact speed (0-80 m/s) andtemperature (20-650ºC in elevated erosion test device). Up to20 samples are tested simultaneously. Similar principle isused in centrifugal type slurry erosion tester (Fig. 5) thatallows to attack samples by the jet of slurry that is the mixtureof abrasive and liquid. The slurry may not be changed during301
test. Both methods do not allow using abrasive larger than1mm. Up to 30 samples may be treated simultaneously.Fig. 4. Dry abrasive erosion test device CAK (TUT).Fig. 5. Centrifugal type slurry erosion tester (TUT).Larger stones (up to 25 and up to 10 mm in impeller-typeand in disintegrator devices accordingly) may be usedapplying impeller [8] or in disintegrator type impact testdevices. Disintegrator allows speeds up to 200 m/s while inimpeller-type device it is below 10 m/s. Edges are notprotected in impeller-type device that intensify the edge effect(edges are usually worn first and the wear rate depends onradius of the edge).Fig. 8. NTNU device for dry abrasion testing with abrasive of mixed sizes[9].In [10] and [11] the sample is moved up and down togenerate impact. During high temperature cyclic impactabrasion test the plunger impact the specimen and then slideproviding high stress abrasion. The volume removed formzones of impact and abrasion are measured and wear rates inthese both conditions could be estimated.Fig. 6. Impeller-in-drum impact-abrasive wear tester [8].Fig. 9. Three-body pin-on-disk device [10].Fig. 7 Disintegrator type impact wear tester (TUT).C. Testing in Combined Conditions of Impact and AbrasionIn [9] it is proposed to use abrasive of mixed size to addimpact conditions (Fig. 8). The test is similar to Block-on-Ring abrasive wear testing while it has speed limitation. Anadvantage is that the sample is tested with fresh abrasivehowever abrasive removal by vacuum cleaner may result inhigh wear rate of removal system.Fig. 10. High temperature cyclic impact abrasion [11].The soil abrasion testing system proposed in [12] is theonly system alloying to make tests under different pressure(0-10 bar). The samples are rotated inside the slurry. Thetorque required to provide rotation is measured. The edges of302
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Laboratory Testing of Materials for Tunnel Boring Machine Drag Bits

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