Tula State University, Tula, Russia

A. V. Chernyaev, Dr. Eng., Prof., Dept. of Mechanics and Plastic Forming Processes, e-mail: sovet01tsu@rambler.ru

Russian University of Transport (MIIT), Moscow, Russia
V. N. Chudin, Dr. Eng., Prof., Dept. of Strength of Materials and Structural Mechanics

Hot upsetting processes are used in the forming of high-strength iron-carbon alloys using hydraulic press equipment. The upsetting technology includes force calculations and the permissible degree of workpiece deformation. During deformation, the workpiece material exhibits viscous properties (creep) and is in a state of viscoplasticity. Relaxation of internal stresses occurs, which affects the force and deformation modes of the operation. The relaxation is greater, the slower the forming rate. The energy upper-bound method of plastic mechanics was used to calculate the upsetting pressure. A discontinuous velocity field with permissible kinematics is proposed, consisting of rigid blocks and velocity discontinuity surfaces. In a plane-strained state of the material, deformations occur only on the velocity discontinuity surfaces. The upsetting process is non-stationary, since the velocity discontinuity surfaces change their position during deformation, which is accounted for by adjusting the normal velocity component. Friction at the contact surfaces of the tool and workpiece is accounted for in accordance with Coulomb’s law. The strengths of external and internal forces allow the upsetting pressure to be determined in accordance with the extreme upper-bound theorem. It is shown that a decrease in velocity leads to a decrease in the pressure and the process upsetting force. Using damage criteria of fracture mechanics, the finite continuity of the workpiece material is assessed. Continuity depends on the stress pattern stiffness and the degree of deformation, and for some materials, on the speed of the operation. It is shown that the loss of initial continuity (damage) decreases with decreasing upsetting rate. Calculations were performed for upsetting a rectangular workpiece (bar) made of alloy steel.

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