Tula State University (Tula, Russia):
O. I. Boriskin, Dr. Eng., Prof., Director of Polytechnic Institute

National University of Science and Technology “MISiS” (Moscow, Russia):
G. A. Nuzhdin, Cand. Eng., Associate Prof., e-mail: nuzhdin.54@mail.ru
V. Yu. Vvedenskiy, Cand. Phys.-Math., Associate Prof.

The paper presents the results of tests for multi-cycle fatigue of laboratory samples of welded joints of exploration drill pipes in a combination of steel s 32-2-Mn of strength group L (pipe body) with 40-Cr-Ni (tool joint part) and solid samples of pipe body material. Welding of the joint part with the pipe body was carried out by rotary friction welding. Multi-cycle fatigue tests were carried out according to GOST 25502 on a test machine with a two-support rotating specimen under constant torque. Mathematical processing of the results of fatigue tests was carried out, linear regression equations were obtained, fatigue curves were constructed, the endurance limits σ-1 of samples with a welded joint and solid samples were determined, taking into account the relative error of the regression equations. On the basis of metallographic analysis, vulnerabilities in welded joints were identified, in which the initiation and development of fatigue cracks occurred. Fractograms are presented that illustrate the mechanism of fatigue fracture of solid and welded samples. It is shown that the development of a fatigue crack in specimens of welded joints occurs with a greater proportion of microplastic deformation. It has been established that rotary friction welding of a pipe body made of 32-2-Mn steel of strength group L and a joint made of 40-Cr-Ni steel contributes to an insignificant decrease in the fatigue limit of welded joints relative to the pipe body, providing its value at the level of 43-47 % of the ultimate strength of 32-2-Mn steel. The data obtained assume trouble-free operation of the welded joint zone in the drill pipe string made of steel 32-2-Mn, strength group L with welded joints made of steel 40-Cr-Ni.

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