Heat Treatment

Case hardening is the process of choice for the treatment of highly stressed components, such as gears. A prerequisite for reliable process control is knowledge of the processes and their sequences, i.e. the thermodynamic laws of reactions in the gas atmosphere and in the edge layer of the workpiece. In this context, the use of suitable measuring and control methods is of great importance.

In addition to process development/further development during case hardening (carburizing and carbonitriding), the work of the working area includes the investigation of the process influence on the surface layer structure and the resulting component properties. The focus of the developments is also on the targeted adaptation of the surface layer structure to the respective specific loads by a targeted modification of the phase mixtures.

In industrial practice, martensitic surface layers with small proportions of retained austenite are usually produced during case hardening. The development of new surface layer structures of carburized and carbonitrided components by bainitic transformation or varying proportions of martensite, bainite and retained austenite with carbides and carbonitrides is being pursued as a central development trend for improved component properties.

Gas carburizing of helical gears

Inductive surface hardening is an energy-efficient, environmentally friendly and fast technology for hardening the surface layer of components while maintaining the core strength of the quenched and tempered steels used. Due to tactile hardening and short heat treatment times, inductive heat treatment can also be flexibly integrated into the production chain. This allows optimized material flows to be achieved and throughput times and inventories to be reduced. In this process, heat is generated by means of Joule heat from eddy currents generated directly in the edge layer of the ferromagnetic material by means of electromagnetic induction, with current intensity in the inductor and frequency being the main parameters.

More recent developments allow the simultaneous application of different frequencies in order to adjust the energy input into the component in a targeted manner. The working area focuses on the process development with regard to the adaptation of the component properties to the respective requirement profile. The work contents are the consideration of the material dependence of corresponding heat treatments. Furthermore, the effect of the process parameters on the temperature field in the component is analysed. In addition, material and component properties resulting from a corresponding treatment are of interest. Moreover, possibilities of process modelling and simulation of the corresponding processes are considered. Investigations into contour hardening focus on the gear wheel as a component. The available dual-frequency technology offers the possibility to harden components (e.g. gears) close to the contour, i.e. similar to a case hardening layer.

Hardening of a rotating bending sample with a sharp notch during austenitizing of the notch area

Sensors enable automation in many areas of production with the associated improved quality assurance. In particular, the industrial transformation towards "Industry 4.0" requires further automation also in the various heat treatment processes. In the field of heat treatment, sensors are already successfully used in many areas, especially for temperature and atmosphere control. An important example is the use of oxygen and hydrogen probes in the carburizing and nitrocarburizing processes. With these sensors, reactive treatment atmospheres can be recorded, controlled and regulated. In carbonitriding, a sensor system with integrated simulation of the diffusion and precipitation processes has been successfully developed and brought to market in recent years.

The use of gas sensors is necessary but not sufficient, since they do not provide information about the current material condition, which is the main focus of interest as a target variable in heat treatment processes. Further work is therefore concentrated on the development of sensors to measure the current heat treatment condition. Successful developments such as the nitriding sensor for nitriding and nitrocarburizing processes, the development of sensors for the in situ qualification and quantification of the material microstructure such as bainite, martensite and tempering microstructure during heat treatment including adapted sequence controls could be realized in the past.

In the field of nitriding and nitrocarburizing processes, the focus is on process developments for stress-optimized component applications such as deep nitriding of gears and applications for hot and cold working tools as well as applications with narrow specifications in the steel spectrum from unalloyed to austenitic steels. In this context, facilities for the entire process and combination spectrum of nitriding and nitrocarburizing from plasma (incl. active lattice) and low pressure up to number-regulated normal pressure processes can be used.

In addition, we are also working on aspects of economy, sustainability and ecology such as the energy efficiency of nitriding plants and nitriding processes. Finally, basic topics such as pore formation or nitriding of non-ferrous materials such as aluminium, titanium and nickel alloys are also being pursued in close cooperation with industry. This also includes the further development of post-oxidation.

Plasma nitriding of a gear wheel

The computational modelling of heat treatment processes opens up new possibilities for a heat treatment-compatible design. The focus is on the simulation of hardening processes and in particular of the quenching process, considering the influence of material inhomogeneities on the transformation behavior. According to the current state of the art, such work can only be designed with a basic orientation, since only a fraction of the influencing variables can be recorded and taken into account in the models. Furthermore, existing models are continuously being expanded with the aim of integrating process steps such as tempering into the simulation. Current topics in the modeling of heat treatment processes are bainitic transformation under stress, tempering and phase transformations in the additive production of hardenable steels.

Dimensional changes and distortion are a central problem in the production of components. They are often associated with heat treatment alone as one of the final manufacturing steps. In many cases, however, heat treatment steps only trigger plasticization due to thermally induced residual stress reduction, which is caused by previous manufacturing steps. Due to the extraordinary complexity of such processes, individual aspects have to be investigated and combined to form an overall picture on the basis of a long-term strategy. Currently, the heat treatment simulation deals with the influence of the component geometry on the dimensional and form changes, especially in the context of lightweight construction developments and the consideration of effects from previous processes (e.g. forming) with regard to the dimensional and form changes.

Calculation of deformation: Temporal development of the tilting of the gear rim and phase transformation during a quenching process in oil

The quenching technology focuses on the characterization of the quenching effect of oils and aqueous polymer solutions with regard to the target parameters microstructure and hardness. In addition, work in the field of gas quenching will continue. The further development of vacuum heat treatment systems in the field of high-pressure gas quenching has opened up new possibilities for replacing liquid with gaseous quenching media if the hardenability of the materials used is sufficient. The quenching effect is primarily determined by the parameters gas type, quenching pressure and inflow velocity. Investigations are under way to characterize the quenching effect in connection with vacuum heat treatments as well as following inert gas heat treatments. Besides aspects of distortion minimization, ecological aspects are in the foreground of the investigations.