A study of optimum die-design in the heading process with two blows. Part 1: Experimental study

Abstract

The conventional cold-heading process for the production of a bolt-shaped product involves two or three blows. The strength of a bolt-shaped product produced by multi-blow heading depends on the mechanical properties of the material and on the working conditions of the heading process such as the corner radius of the necked portion of the product, the pre-forming die angle and the reduction in height during pre-forming. The effects of the corner radius of the necked portion and the die angle in pre-forming on the internal state of the product, such as the shape of the deformed zone, the hardness distribution and the fracture strength, of a product produced by double-blow cold heading have been investigated experimentally. A rigid-plastic finite-element program has been coded and the program vindicated by comparison of its predictions with the results of experimentation. The results are summarized as follows: (1) The corner radius of the necked portion of the product is a significant factor not only for fracture strength but also for the internal state of the product. (2) The die angle of the pre-forming die also affects the internal state, the shape of the head and the forming load, but its effect on the fracture strength is not remarkable. (3) The effects of the above two factors are not independent of each other. (4) The rigid-plastic finite-element code can provide accurate information on double-blow cold heading.