Die load and stresses in press forging

Abstract

Several axi-symmetric EN3B steel components differing in shape and size were forged on a 100 ton joint knuckle press. A load cell fitted under the lower die inserts recorded the total deformation forces. Job parameters were measured off the billets and the forged parts. Slug temperatures were varied and two lubricants - aqueous colloidal graphite and oil - were used. An industrial study was also conducted to check the results of the laboratory experiments. Loads were measured (with calibrated extensometers attached to the press frames) when adequately heated mild steel slugs were being forged in finishing dies. Geometric parameters relating to the jobs and the dies were obtained from works drawings. All the variables considered in the laboratory study could not, however, be investigated without disrupting production. In spite of this obvious limitation, the study confirmed that parting area is the most significant geometric factor influencing the forging load. Multiple regression analyses of the laboratory and industrial results showed that die loads increase significantly with the weights and parting areas of press forged components, and with the width to thickness ratios of the flashes formed, but diminish with increasing slug temperatures and higher billet diameter to height ratios. The analyses also showed that more complicated parts require greater loads to forge them. Die stresses, due to applied axial loads, were investigated by the photoelastic method. The three dimensional frozen stress technique was employed. Model dies were machined from cast araldite cylinders, and the slug material was simulated with plasticene. Test samples were cut from the centres of the dies after the stress freezing. Examination of the samples, and subsequent calculations, showed that the highest stresses were developed in die outer corners. This observation partly explains why corner cracking occurs frequently in industrial forging dies. Investigation of die contact during the forging operation revealed the development of very high stresses.