Contact fracture of rolling bodies of open bearings of three-cone rock bits in aqueous environment
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CONTACT FRACTURE OF ROLLING BODIES OF OPEN BEARINGS OF THREE-CONE ROCK BITS IN AQUEOUS ENVIRONMENT E. . Kryzhanivs’kyi,1,2 R. S. Yakym,1 L. E. Shmandrovs’kyi,1 and Yu. D. Petryna1
UDC 621.822.6:620.178.16
We have analyzed the formation of corrosion damages during the standby time of a rock bit in operation and the low crack resistance of high-strength steel under the action of aqueous environment, which lead to the loss of serviceability of the rolling friction pair of the rock bit. We have investigated the relation between the structure of 55SM5FA high-strength steel, used for rollers of the rock-bit bearings, and its short-term crack resistance with regard for the influence of water on it. With decrease in the fracture toughness of steel, the negative influence of environment on its crack growth resistance increases. The optimal structure of steel for rollers is fine-grained martensite with ~ 1% of troostite and hardness HRC 56 to 58. To retard the stress corrosion fracture of rock bits, we recommend conserving them during standby times in the boring process by lubrication not only of the nipple part, but also of the open bearings of bits. Keywords: crack resistance, fracture, corrosion, water, rollers, bearing, rock bit, crack.
The main cause of breakdown of the bearings of three-cone rock bits with rolling bearings is their stall due to the wear of axial plain bearings, the fracture of collars, and the loss of regular service conditions of the ball and roller series of bearings [1]. One of the most characteristic causes of stall of cutter cones is the irregularity of great roller bearing of the rock-bit bearing. Since the driving peripheral rings of cutter cones are situated under the great roller series, the main load-carrying capacity of a rock bit depends on the strength of these series. On the other hand, the location of these series near the bases of cutter cones favors more intense circulation of the flushing fluid inside them and the wear of rolling articles by abrasive particles of the flushing or cleaning agent. Dependences of the wear of rolling bodies on the operating time of a cutter cone in different environments were obtained in [2] on the basis of benchmark tests. It has been shown that oxide films that are formed in the course of wear are permanently removed from the friction zone, and local damages of the surfaces appear, which afterwards develop into cracks. Therefore, it is important to separate the stage of initiation of corrosion damages on a smooth surface and the stage of their propagation. In [3], the following important conclusion is drawn: water lowers the contact strength of steel according to the adsorption mechanism, i.e., decreases the surface energy of deformed metal. On the other hand, in view of the important role of the stage of crack propagation in the fracture of high-strength steel under contact loading, one should take into account the possible manifestation of adsorption effect at the crack tip. Panasyuk and Kovchyk [4] were the first to determine quantitatively the decr
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