Ural Federal University (Ekaterinburg, Russia):

+A. A. Bogatov, Dr. Eng., Prof., Chief researcher
D. Sh. Nukhov, Cand. Eng., Researcher, e-mail: d.s.nukhov@urfu.ru
A. O. Tolkushkin, Research engineer

Seversky Pipe Plant (Polevskoy, Russia):
O. A. Panasenko, Head of the SRC tube-rolling laboratory

A large amount of flaws on the tubes' inner surface are forming on some steel grades and alloys due to the adverse strain stress state during piercing. Further, such tubes require repair, which increases their ratio of labour to output. During the piercing process analysis, the term critical reduction δ_c in front of the mandrel is used. Critical reduction can be found by the conical workpieces rolling and determining the cross section in which the internal fracture occurred. Steel ductility, sufficient to obtain the shell without flaws is determined by the results of torsion testing of the cylindrical shape samples. If the number of rotations prior to fracture will be greater, than some critical, corresponding to internal fracture, then piercing will occur without forming of flaws on the shell inner surface. More stable results of the steel ductility evaluation can be received by torsion of the samples with the neck with the radius R and the diameter in the smal lest section d. Rational R/d relation, which allows not only to localize strain but more accurately determine the degree of deformation prior to fracture amounted 1.25. Due to the fact, that during helical rolling occurs alternating deformation, in this work a physical simulation method of the metal fracture process during piercing by alternating torsion of samples with neck R/d = 1.25 is proposed. The change of amplitude-frequency characte ristics of alternating loading allows deforming the sample under conditions adequate to the real technological piercing process on the rolling mill, moreover the amplitude ε_i and the number of alternating torsion stages n determined by the results of computer simulation of the piercing process.

1. The study was made within the base part of state job in the field of scientific activity № 11.9538.2017/8.9.
2. Supported by Act 211 of the Government of the Russian Federation (agreement No. 02.A03.21.0006).
3. This research is supported by the President of the Russian Federation grant number MK-3011.2017.8.

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