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The properties of metallic materials can be changed by a specific heat treatment and adapted to the increasing demands of stress. With this aim in mind, the department of heat treatment is engaged in application-oriented, technical-scientific and scientific research projects with questions arising in connection with thermal and thermochemical heat treatments. Examples are process developments for saving energy and operating resources or for the improvement of component and material properties.

The competences of the department of heat treatment include both surface layer and penetrating processes and cover both purely thermal heat treatment and thermochemical heat treatment. In the department's own hardening shop, a wide range of furnaces on a technical scale are available for research and development. Many of the scientific questions are dealt with in a transfer-oriented manner in order to ensure a rapid transfer of new findings from research to industrial practice.

Range of topics of the department in the field of stress oriented heat treatment processes.

Further research focuses besides process development are the development and testing of process-accompanying monitoring and control systems, quenching technology as well as dimensional and shape changes during heat treatment. The experimental work is supported by heat treatment simulation and calculations.

In addition to project-related research, the department offers support for industry in the development of adapted heat treatment processes for specific property profiles of the components as well as support for problems with dimensional and shape changes and damage analyses.

Heat treatment activities are organized in four work areas

The activities in the main fields are organised in the four working areas"Case Hardening", "Induction Hardening", "Sensor Technology and Nitriding" and "Simulation and Ashing Technology". In the following, the main topics are presented.

Case Hardening

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

Induction hardening

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

Sensor technology and nitriding

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

Simulation and Quenching Technology

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.

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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.

Projects of heat treatment

ETA in the field – Technology and method building set for increasing energy efficiency in the metal processing industry

The joint project is under the leadership of the PTW of the TU Darmstadt and aims to develop solutions that enable the rapid,
comprehensive and economic dissemination of energy efficiency technologies in the German industrial landscape, which are 
urgently needed to achieve the climate targets.

In regular operations, only the “low hanging fruits” are usually exploited to increase energy efficiency. However, comprehensive exploitation of the complex, systemic energy efficiency potential in existing industrial buildings is absolutely essential in order to achieve the German government's ambitious climate targets. In the course of this, the Leibniz Institute for Heat In sub-project 3 “Energy-efficient heat treatment”, the Leibniz-IWT's Heat Treatment competence area is researching the possibilities of to make the heat treatment process more energy-efficient.
The gas nitriding process was chosen as the use case. Nitriding is a thermochemical surface layer treatment process in which a component is annealed in a nitrogen-containing atmosphere. The basis for further research and efficiency improvement measures was created from the energy analysis of existing plants. In addition to discontinuously operating (chamber) furnace systems,  continuous furnaces were also examined using surface thermography and energy consumption measurements. A thermographic image of a heat treatment furnace is shown in the figure. As the procurement of new heat treatment furnaces is usually associated with an enormous financial outlay, modernization techniques for existing systems are to be considered. In addition to the latest insulation technologies, modernized system technology also includes adapting the system peripherals, in particular the thermal process waste gas treatment.

A thermographic image of a heat treatment furnace is shown in the figure.As the procurement of new heat treatment furnaces is usually associated with an enormous financial outlay, modernization techniques for existing systems are to be considered. In addition to the latest insulation technologies, modernized system technology also includes adapting the system peripherals, in particular the thermal process waste gas treatment. Contamination of the production environment with the gas species present must be ruled out.
The transfer of scientific findings is to be achieved with the help of the development of an expert system. In addition to implementation in standard design software, this should also include heat flow simulation of the furnace system.The software tool will be made available to the furnace construction industry and can identify existing thermal bridges.

 

 

Cooperation: PTW TU Darmstadt
Funding: BMWK-PtJ

 

Contact: 
M.Sc. Tim Oelker
Tel.: +49 421 218 51303
E-Mail: oelker@iwt-bremen.de

Impact of machining-related surface layer changes on the nitriding response of forging tools

The thermochemical heat treatment nitriding is used to improve the surface properties of a workpiece by diffusing nitrogen 
into the surface layer to form precipitates. In tool manufacturing, these precipitates are used to increase the hardness 
and wear resistance of forging dies at elevated temperatures and thus their service life.
 

As no further finishing steps take place after nitriding, the tools are finished by grinding in their hardened or tempered state. During this hard finishing process, the workpiece surface layer is affected by the interaction of the material with the grinding tool and the metal working fluid as well as the thermomechanical load, which in turn affects the surface structure of the workpiece and thus the nitriding response. In this research project, the relationships between the design of the grinding process, the changes in the surface layer and the final nitriding treatment are investigated.


Cooperation: Leibniz-IWT WT/FT
Funding: BMWK-AiF/IGF (AWT)

 

Contact:
M.Sc. Tim Oelker
Tel.: +49 421 218 51303
E-Mail: oelker@iwt-bremen.de

WINDUCTION – Eco-design of an alternative production route for planet gears of wind turbine gearboxes

The main aim of the project WINDUCTION is the design of an eco-friendly and low-energy consuming production route for the planet gears of the wind turbine gearboxes.

Based on the replacement of carburizing by induction hardening, the following advantages are expected: reduction of CO2 emissions, avoiding the use of fossil fuel, and utilization of novel eco-friendly steels designed to improve their performance during the machining operations.

 

 

Funding: EU Projekt/RFCS-2020

 

Contact:
M.Sc. Grace Babb
Tel.: +49 421 218 51331 
E-Mail: babb@iwt-bremen.de

Dr.-Ing. Holger Surm
Tel.: +49 421 218 51342 
E-Mail: surm@iwt-bremen.de 

Use of carburizing processes free of internal oxidation and multi-stage phase transformation for high-strength surface layers

Case hardening is currently typical for highly stressed components such as gears. In contrast, isothermally transformed bainitic surface layers have hardly been investigated so far.

The combination of properties resulting from case hardening of a ductile core and a hard surface layer subjected to residual compressive stresses with a bainitic transformation offers the potential for a further increase in fatigue strength in the field of drive technology.
 


Cooperation: WZL Aachen
Funding: BMWK-AiF-IGF/AWT

Resource-efficient gears for modern high-speed drive train concepts (Light4Speed)

The increasing electrification of the powertrain is significantly reducing the number of transmission components, although electric motors have to be designed larger due to increased torque.

Various approaches are being investigated to reconcile lightweight
design with increased gear stress due to higher torques.

 

Cooperation: FZG München, Volkswagen AG, ALD Vacuum Technologies GmbH, HEESS
                     GmbH & Co KG, OSK-Kiefer GmbH Oberflächen- & Strahltechnik
 

Funding: BMWK

 

Contact: 
Dr.-Ing. Martin Hunkel
Tel.: +49 421 218 51341
E-Mail: hunkel@iwt-bremen.de
 

FVA945I “Induction hardening of 3D printed gears” – Induction hardening of additively manufactured lightweight spur gears

The sustainable treatment of the environment as well as the efficient use of resources enable the preservation of our planet. Efficient and sustainable powertrains can be realized by gears with increased power density.

The combination of additive manufacturing and inductive heat treatment leads to the desired higher power density through increased strength and  reduced weight. Additive manufacturing using PBF-LB/M enables innovative and power-flow-compatible lightweight designs. Furthermore, the printed material has a high dislocation density, which can be utilized for higher surface hardness and increased component strength through efficient induction hardening. In the project, samples and gears of different materials, lightweight designs and heat treatments will be experimentally tested for their load-bearing capacity in order to determine mechanical parameters and thus identify the optimum combination with the greatest possible potential.
 

 

Cooperation: FZG TU-München
Funding: BMWK-AiF/IGF 20150 N/FVA 945 I

 

Contact:
M.Sc. Nicolai Haupt
Tel.: +49 421 218 51439 
E-Mail: haupt@iwt-bremen.de

Identification of distortion critical flows during oil quenching of gear shafts in industrial quench tanks

The quenching process after heat treatment of gear shafts contributes greatly to the resulting amount of distortion.

By combining the results of distortion measurements from heat-treated gear shafts 
with the results of flow measurements in the quench tank used for the experiments, 
a better understanding about the correlations is to be obtained.

 

Funding: AiF / AWT (22411N)

 

Contact: 
M.Sc. Gabriel Ebner
Tel.: +49 421 218 51407
E-Mail: ebner@iwt-bremen.de

Transregional CRC 136 “Process Signatures” – Transfer project T06: Data-based lifetime prediction for a function-oriented induction hardening process

The project aims to predict the process parameters of induction hardening in such a way that required 
component properties, such as the fatigue strength of shafts, are fulfilled in the context of function-orientated 
production. This is achieved by inverting the process signature and calculating local fatigue strengths.

 

 

Cooperation: eldec Induction GmbH
Funding: DFG (Transferbereich SFB TRR136)

 

Contact:
M.Sc. Tobias Heinrich
Tel.: +49 421 218 51338
E-Mail: heinrich@iwt-bremen.de

Influence of optimized low-temperature treatments on gear load capacity

Based on the results of previous research projects, further investigations are carried out to evaluate the 
effects of low-temperature treatments on the load-bearing capacity of case-hardened gears.

 

 

Cooperation: FZG
Funding: BMWK-AiF/IGF + FVA

Case-hardened shafts – Specific consideration of case hardening in the design of shaft-type components

Case hardening is a standard heat treatment in drive technology for the increase in fatigue strength of shaft-type 
components.

However, as was shown in a preliminary study, the existing calculation rules for shaft-type components 
(DIN 743, FKM guideline) are not capable of integrating the resulting surface layer conditions into a 
fatigue strength concept in a differentiated manner.

 

Cooperation: IMM Dresden, IKAT Chemnitz
Funding: BMWK-DLR-IGF/FVA

 

Contact:
Dr.-Ing. Holger Surm
Tel.: +49 421 218 51342
E-Mail: surm@iwt-bremen.de 

ICME-based alloy and process design towards fabrication of high-performance components with nano-bainitic structure

This project aims to set up an integrated simulation framework for the development of a case hardening steel and a 
case hardening process with optimized alloying elements and heat treatment parameters.

It is expected to generate an ultrafine nano-bainitic microstructure at the carburized surface which in turn 
significantly improves the fatigue performance of power transmission components. This is achieved by employing 
the ICME methodology and state-of-the-art experimental verifications as well as benchmarking the results with 
currently available commercial materials. The developed steel grade  and process are used for fabrication of power 
transmission components, initially in the automotive industry and later in the aerospace industry.
 

 

Cooperation: IEHK Aachen
Funding: FOSTA P1545

 

Contact:
Dr. Chengsong Cui
Tel.: +49 421 218 51404
E-Mail: cscui@iwt-bremen.de 

Effect of cryogenic treatment in the heat treatment process of tool steels on corrosion resistance, dimensional and form stability, and fracture toughness (Nanocarbides II)

The influence of a cryogenic treatment, incorporated into the conventional heat treatment process, 
on the distortion, corrosion resistance and fracture toughness of the tool steels X153Cr-MoV12, 
X190CrVMo20-4 and X90CrMoV18 has been investigated.

The dimensional stability of the tool steels can be increased due to transformation of retained 
austenite induced by the cryogenic treatment. Lower tempering temperatures and a lower number 
of tempering cycles improve the corrosive properties of the tool steels, which is attributed to the 
higher chromium content in solid solution. Moreover, the cryogenic treatment positively affects fracture 
toughness and reduces hydrogen susceptibility due to the finely distributed carbide precipitates.
 

 

Cooperation: Leibniz Universität Hannover (IW) und BFI Betriebstechnik GmbH
Funding: AiF-FOSTA P1632

 

Contact:
Dr. Chengsong Cui
Tel.: +49 421 218 51404
E-Mail: cscui@iwt-bremen.de