Influence of thermal stress on the formation of plastic-flow zones in the hot forging of cylindrical blanks

Abstract

The plastic-flow zone is simulated in the initial stage of hot extension of a cylindrical blank in hammer forging. The stress-intensity curve is obtained for the total elastic-stress tensor (corresponding to mechanical and thermal loads). The stresses are determined for the plane deformation of a blank of round cross section to which three point forces are applied. The plastic-flow zone is determined on the basis of the yield point of the material, which is the upper limit on the elastic stress. Tensile azimuthal thermal stress appears in the plastic-flow zone of the blank’s surface layer, thereby facilitating the forging process. As an example, the formation of mechanical and thermal stress in the extension of a 7-t 40XH steel ingot after billeting (with a radius R = 0.35 m of the middle cross section) is analyzed. The minimum permissible forging temperature (880°C) is reached 20 min after removal from a furnace at 1180°C. The distribution of the components of the mechanical and thermal stress tensors is plotted, along with the corresponding stresses over the blank’s cross section.