Contact Durability of the Races of Pins for Tricone Drill Bits
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CONTACT DURABILITY OF THE RACES OF PINS FOR TRICONE DRILL BITS Yu. D. Petryna,1 I. S. Yakym,1, 2 N. V. Kret,3 and Yu. M. Nykyforchyn1
UDC 621.822.6:620.178.16
We study the relationship between the character of distribution of hardness and the concentration of carbon in hardened cemented layers of the races of supports made of 14KhN3MA steel and their contact durability. The optimal hardening of the races depends on the surface hardness HRC (60–63) and the content of carbon (0.95–1.1%) that are equal to 51.5 and 0.45–0.55%, respectively, at a depth of 1.6–2.2 mm. Keywords: drill bit, contact durability, fracture, corrosive media, races, support, cementation, hardness.
At present, the tricone drill bits with open supports based on radial rolling bearings are extensively used for the purposes of drilling. These supports operate under the conditions of high contact loads under the action of corrosive media and abrasives, which significantly accelerates the contact fracture of their elements [1, 2] leading to the premature failures of drill bits in the mine. Note that the highest degree of damage is observed for the arm pins of the bit and especially pronounced defects are formed on the races of rolling bearings in the most loaded lower part of the pin (see Fig. 1).
To harden the working surfaces of the pins, they are cemented and quenched according to a special technology [3] guaranteeing the levels of HRC hardness in the cemented layer of about 60–64 and, in the core of the pin, of about 25–40 [4]. Hence, the leading foreign producers apply Cr-Ni-Mo steels with strictly regulated chemical compositions and physicomechanical properties and with an ability to be hardened according to the requirements of the ASTM.
In the domestic bit building, steels with similar compositions are used in manufacturing the arms [3] (these are, in particular, 14KhN3MA, 19KhGNMA, and 21KhGNMA steels). However, this does not always guarantee the operating parameters of the bits comparable with the world level. Since there exists a correlation between the distributions of hardness of hardened layers and carbon and the quality of hardening, it is possible to establish the optimal parameters of cemented layers under specific technological conditions of manufacturing of the bits.
In the present work, we consider the effective depth of cemented layer specified as the total thickness of the hypereutectoid, eutectoid, and a half of the transient carbonized zones. In the foreign practice, it is defined as the distance from the surface to the metal interlayer with an HRC hardness of 51.5. In what follows, we study the influence of the distributions of hardness and carbon in the hardened surface layer of arm pins on their contact durability as a characteristic of the serviceability of bits. 1 2 3
National Technical University of Oil and Gas, Ivano-Frankivs’k, Ukraine. Corresponding author; e-mail: [email protected].
Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine.
Translated from Fizyko-Khimichna Mekha
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