Ductile Fracture of Carbon Steel under Cold Metal Forming Conditions : 2nd Report, Effect of Metallurgical Structure

Abstract

Torsion tests under pressures of up to 4 kbars are carried out to investigate the mechanism of ductile fracture of 0.033 ∼ 0.54 % carbon steels of various microstructures. It is found that the strain at which fracture-nuclei begin to grow is affected by the carbon content, shape of cementite and hydrostatic pressure, whereas the strain from the beginning of nuclei-growth to fracture is not affected by these factors. A criterion for ductile fracture which accommodates the metallurgical factors is proposed. Microscopic observation of fracture process of 0.25 % carbon steel is carried out under a scanning electron microscope. The observed result is related with the initiation strain of nuclei-growth.