Generation of tailored subsurface zones in steels containing metastable austenite by adaptive machining and validation by eddy current testing

In order to withstand high mechanical and tribological loads, it is important that the components not only have a high core ductility but also a hard surface. Typically, a suitable microstructure is created by heat treatment processes before the workpiece is machined. However, these processes are time and energy consuming and can lead to component distortion. It would therefore be of great advantage if no additional heat treatment process would be required to produce a hardened subsurface zone. Since turning is often already integrated as a machining process in production lines, it would be advantageous to create a hardened subsurface within this process. As there is no possibility to measure the hardness directly during the turning process, a soft sensor was developed to determine the properties of the subsurface directly during the machining process. Steels with metastable austenite are of particular interest in this context, as metastable austenite can be converted into martensite by deformation. The amount of martensite produced in the subsurface can be adjusted provided that suitable turning parameters can be found. For this purpose, a process parallel material removal simulation was used to determine the actual conditions governing the process. It was found that there is a correlation between the martensite content and the amplitude of the 3rd harmonic of eddy current testing. Therefore, an eddy current sensor accompanying the process can be used as a basis for controlling the turning process for tailored martensite volume content adjustment.