In various industries in recent years, there is an increasing demand for large-diameter wells (from 0.5 to 5 m) with depths of 1000 m or more. Such wells are needed for water systems in flooded deposits of minerals, for ventilation and other purposes in coal and hard-rock mines, for drilling below the surface casing string in super-deep oil/gas and exploratory wells, for the construction of oil mines, and for other purposes.The All-Union Scientific-Research Institute of Drilling Technology (VNIIBT) has developed a method for drilling these wells based on the use of a turbo&ill; this technique has been termed the "jet-turbine drilling (JTD) method." JTD is accomplished by means of a downhole assembly consisting of two, three, or four turbodrills with bits, operating in parallel with a rigid interconnection. In the operation of this assembly, the only elements that come into contact with the rock are the peripheral rows of bit teeth. Thus it becomes possible, with a limited total axial load, to create contact stresses that would require extreme loads when turbodrilling conventional wells [1].VNIIBT has developed drill bits that are specially designed for the JTD method: Sh490TZ-TsVR and Sh490S-TsVR. The distinctive feature of these bits is that the cutting structure is confined to the periphery of the rollers. Bits of these types are in regular production at the joint stock company Sarapul'skii Mashzavod [Sarapul Machinery Plant], which specializes in the manufacture of large-diameter bits [2].The process used in manufacturing the Sh490S-TsVR bit differs from those for intermediate-size bits (215.9-295.3 mm) with milled teeth, in that a different method is used to obtain the roller-cutter blanks. In the manufacture of rollercutters for intermediate-size bits, forged blanks are used; in contrast, roller-cutter blanks are manufactured at the Sarapul Machinery Plant by investment casting.In order to increase the wear resistance of the toothed cutters of the bits in regular production, a process of surface hardening by "relite" is used; this is accomplished by mixing granular relite with the molten surface layer of metal. The teeth are heated by art oxyacetylene torch or by induction heating. The hardened layer that is obtained is brittle and nonuniform; and as a consequence, when the surfaces are subjected to any significant alternating loads, microcracks appear [3], followed by brittle spalling. The surface hardening of the cutting structure of the bit is of particularly low quality in the case of cast teeth, since the layer of relite is melted onto the surface of teeth coated with skin from the casting operation, which contains various nonmetallic inclusions.The surface-hardened cutters have a very low resistance to shock and fatigue [4] owing to the presence of internal stresses caused by nonuniformity of metal bonding and the concentration of hard alloy in the hardened zone; therefore, in the course of service, the hardened layer will peel and scale, resulting in premature wear of the cutter. Still another shortc...

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