Laboratory Testing of Materials for Tunnel Boring Machine Drag Bits

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