Transition Zone Tensile Properties within a Tailored Hot Stamping

Recent work at the University of Waterloo addressed the hot stamping of a lab-scale B-pillar using a heated and cooled die to produce a tailored part with a soft and hard region for which the microstructure was predominantly bainitic and martensitic, respectively. This paper addresses the tensile properties of the transition zone (hard to soft region) within this tailored hot stamping using experimental and numerical methods. Vickers hardness measurement showed that the fully softened and hardened material conditions were achieved across a 25 mm transition zone. Sub-size ASTM uniaxial tensile specimens were cut from the transition zone and pulled to failure. Due to the large variation in material properties within the gauge length of the specimens, apparent uniform elongations measured across the gauge length ranged from 0.02 to 0.04 engineering strain, while the calculated engineering ultimate tensile strength (UTS) varied from 798 to 913 MPa. The transition zone tension tests were simulated using the finite element (FE) code LS-DYNA and the tensile dog-bone specimen mesh used in the models was subdivided into “bins” of average hardness values as measured from transition zone hardness distributions. The constitutive response assigned to the elements of each “bin” was calculated using a strain rate sensitive constitutive model that was developed for tailored properties within hot stampings. Agreement between the predicted and measured stress-strain curves was excellent and the predicted strain distributions within the transition zone specimens agreed very well with measured strain distributions that were obtained using digital image correlation measurements.