Characterization of martensitic-bainitic mixed microstructures created by inductive short time austempering of AISI 4140 - Tom Bell Young Author Award Candidate

The induction hardening process is a well-established surface hardening process to improve the lifetime of steel components. As a fully electric process, it is also independent of fossil energy resources, which emphasizes the future interest in the application. Surface hardening processes are characterized by high temperature gradients from the edge to the center of the workpiece. On the one hand, this can be advantageous, as high compressive residual stresses can be induced at the boundary of the workpiece. On the other hand, there is always a region where the basic microstructure of the workpiece is tempered, which can reduce its lifetime. To minimize this effect, a bainitic-martensitic mixed microstructure was set up and investigated. To achieve this, a short time austempering process was established. The process consists of a heating step, an air-cooled holding step above the martensitic start temperature and subsequent quenching of the workpiece. Therefore, a specialized inductor was used to integrate an airflow for a controlled temperature ramp during the holding step. This study investigates the hardness profiles and the bainitic fraction for different temperature ramps. For the definition of the bainitic fraction within the mixed microstructure three different methods were applied. The hardness profile of the workpiece was used to define the fraction analytically. Further, microscopic images were used to have a similar comparison using the grayscale of the image. Finally, the content was calculated numerically via an FEM-Simulation. The methods were compared and discussed. In the end, the simulation was used to set up an optimal mixed structure for future test series.