Effect of microstructure on the cold headability of a medium carbon steel

Cold headability is the ability of a cylindrical metallic specimen to be shaped at high strain rate into the head of a bolt, screw or other cold-formed part without cracking. This property is material dependent and can be influenced by many factors such as chemical composition, surface condition, and microstructure. The effect of microstructure upon the cold headability of a medium carbon steel (1036M) was investigated. Six different microstructures were produced by various heat treatments. Drop weight tower (DWT) testing methods, previously developed at McGill University, were used on samples of these materials. Visual inspection, metallographic and SEM analysis were carried out to detect cracks on the surfaces of tested samples and to identify their causes. The axial and circumferential strains of the tested samples were measured and the strains at which cracks were initiated were used to assess the headability. The cold headability was found to be particularly sensitive to the microstructure and was greatest in the completely spheroidized structures. This indicates that DWT testing is a valid method for evaluating the cold headability of metallic materials.