The Anomalous Redundant Deformation and Work Hardening of the AISI 420 Stainless Steel During Axisymmetric Drawing

The cold axisymmetric drawing of metals leads to effective strains that increase from the centerline to the surface of the material cross section. This strain heterogeneity depends on the die semi-angle and reduction in area related through a "Delta" parameter. The average strain in the product is evaluated through a redundant deformation coefficient, "phi," which has a minimum value of unity and rises as Delta is increased. Anomalous experimental results for this relationship (phi values below unity and insensitive to variations in Delta) have been reported for the AISI 420 stainless steel. Strain path affects the work hardening of metals during sheet forming, where some materials harden more and others less than under pure tension, for the same strain path. The present paper analyses the possibility that a similar dependence of the work hardening on the strain path, during the axisymmetric drawing of AISI 420 stainless steel causes the anomalous phi versus Delta relationship. The strain path followed along various material streamlines in axisymmetric drawing involves the superposition of a radially varying reversed shear strain on a basic radial compression/longitudinal tension pattern. A new method was developed for the determination of the effective stress versus effective strain curves of the material along three material streamlines, located close to the material surface, along its centerline and following a midcourse between these two flow lines. A relationship between the local microhardness of the material and its flow stress was established and visioplasticity was employed for the determination of local strains in the deformation region. Data were obtained for six situations resulting from the combinations of two reductions of area (8% and 20%) and three die semi-angles (3 deg, 8 deg, and 15 deg). The various strain paths followed in axisymmetric drawing of AISI 420 stainless steel led to effective stress versus effective strain curves tending to be often lower than that obtained in pure tension. The degree of lowering seems to depend on the reduction in area and die semi-angle. The effect of strain path on the work hardening during axisymmetric drawing causes the anomalous experimental results for the phi versus Delta relationship of the AISI 420 stainless steel. The present paper seems to be the first report in literature covering such effects under cold bulk forming conditions.