Microstructural Characterization of Hot Stamped Parts After Partial Hardening 2019-36-0113

Hot forming is a process that has been on the spotlight of body-in-white recent developments, globally it was estimated that around 500 million parts would be produced in 2018. During the process, a blank of a 22MnB5 steel is heated, above 900 °C, in the austenite field, formed and quenched in the pressing machine, with the goal of producing a part with ultimate tensile strength (UTS) around 1500 MPa, achieved by a fully martensitic microstructure. However, from the point of view of crashworthiness, not all body in white components benefit from a fully martensitic microstructure. Alternative process routes have been proposed to achieve properties that are tailored to the part’s final application. In one of these process variants, known as partial hardening, the blank is heated in a furnace that operates with two parallel zones, each at a different temperature (one zone between 650 and 750 °C and one between 900 and 930°C), resulting in the blank having two main zones with different final temperatures, and a transition zone between them. This difference in temperature results in a mixture of microstructures which then results in different properties of the final part. The present works aims to characterize the microstructural and mechanical properties of these zones to better understand the metallurgy behind the partial hardening process. Results show that by applying this difference in temperature, varying microstructural mixtures of martensite and other microconstituents (such as bainite and proeutectoid ferrite) can be found and result in hardness differences of around 75% between the zones.