Austenitic chromium-manganese steels with a comparatively high content of interstitially bound carbon and nitrogen, also known as high-interstitial steels (HIS), are characterised by their versatile properties as high-performance materials: On the one hand, they have high strength in the solution-annealed state, which means they can be used as wear-resistant construction materials for mechanically stressed components. Components made from HIS can be used effectively in acidic environments due to their increased chromium content, which makes them highly resistant to various types of corrosion. They also have paramagnetic properties in the solution-annealed state, which are required for housings in measurement technology, for example, due to their permeability to electromagnetic fields. Compared to other widely used austenitic steels, the CrMn steels remain paramagnetic even with large plastic deformation. Very high austenite stability is achieved without the addition of expensive and limited available alloys such as nickel.

The machining of HIS in a solution-annealed state suitable for application is associated with major challenges due to its extraordinary work hardening: On the one hand, high thermomechanical loads act on the tool, leading to considerable signs of wear and ultimately to process termination. Both the technologically possible metal removal rate and the high tool costs make the roughing process uneconomical. On the other hand, the process efficiency and surface quality required for industrial production cannot be achieved during finishing due to the extremely unfavourable chip formation and burr formation. For quality-compliant production in the solution-annealed condition, time-consuming process interruptions and reworking are required, which can be avoided by adapting the material properties during machining.

In the context of these challenges, this research project in cooperation between ISF and the Chair of Materials Technology at the Ruhr University Bochum is investigating approaches that plausibly lead to a significant improvement in the machinability of HIS.

The aim of the research at a materials engineering level is to improve the machinability of HIS by means of a suitable heat treatment before machining, followed by a restoration of the performance properties of the material by means of a further heat treatment after machining. Carbonitrides are deliberately used to make the material brittle and act as chip breakers. With regard to machining, the aim is to analyse the machinability of the altered microstructure. In addition to the comparison with the solution-annealed initial state, the characterisation of the chip formation process in particular represents the fundamental mechanisms of the altered material behaviour. The results subsequently serve as a basis for simulation-based approaches to optimise tool design with regard to chip formation. In combination with the microscopic observation of the chip formation zone, this will provide the basis for the tool designs and the cooling lubricant strategy. The aim is to develop basic knowledge for machining high-strength CrMn steels by iteratively adapting the cutting edge microstructure and the coating and to create a further basis for increasing wear resistance by designing the cooling lubricant concept in line with the technology.