Volume 72, Issue 3

Kurzfassung Aus wirtschaftlichen und ökologischen Gesichtspunkten kommt dem Thema Leichtbau in der Fahrzeugindustrie in den letzten Jahren eine besondere Bedeutung zu. Dabei werden oftmals konstruktive Lösungen verfolgt, um dieses Ziel erfolgreich umzusetzen. Allerdings müssen Aspekte einer fertigungsgerechten Auslegung der Bauteile, insbesondere im Antriebsstrang, entlang der gesamten Prozesskette berücksichtigt werden, um infolge der abschließenden Wärmebehandlung gravierende Verzugsprobleme zu vermeiden. Im Moment liegen allerdings noch keine Konstruktionsrichtlinien vor, die den Einfluss von konstruktiven Änderungen auf das Maß- und Formänderungsverhalten von Zahnrädern berücksichtigen. Daher wurde in einem Forschungsprojekt das Verzugsverhalten von Zahnradgrundkörpern auf der Basis von experimentellen und numerischen Studien analysiert. Zentrale Aspekte waren der Einfluss der Bauteilgröße, der Maße von Steg und Zahnkranz, der Zahl und des Durchmessers von Bohrungen im Steg, der Gestaltung von Querschnittsübergängen und der Abschreckprozess. Weitere Fragestellungen vor dem Hintergrund der Reduzierung des Getriebebauraums betrafen den Einfluss eines verschobenen bzw. verkippten Stegs. In diesem Teil der Veröffentlichung werden die experimentellen Untersuchungsergebnisse für das Einsatzhärten mittels Gasaufkohlung und Ölabschreckung vorgestellt. Die Ergebnisse der numerischen Untersuchungen folgen im zweiten Teil dieses Berichts.

Carbonitriding of shafts in drive technology is strongly connected with distortion. Bend straightening is an important process step in order to eliminate distortion without removing the hardened surface layer. As shown in several investigations, even for simple part geometries, bending induced stresses and plastic deformations have a strong impact not only on the residual stress state but also on the microstructure after straightening. As gear shafts become increasingly filigree and complex in their geometry, the components state after straightening is only understood in rare cases. Due to indispensable, function based notches as well as changes in the cross section, these effects will occur especially in areas of small cross sections or notches in conjunction with high bending stresses. It is the objective of this work, to characterize these critical areas with respect to the distribution of residual stresses and retained austenite along the circumference as well as in the in-depth direction. Therefore, the measured distributions of residual stresses, integral width values and retained austenite near the surface will be presented and discussed. For this purpose a carbonitrided gear shaft taken out of a large production volume was investigated before and after a commonly used straightening operation.

The aim of most applications of nitriding treatments at steel components is to obtain a compact compound layer and/or a deep diffusion layer. The possibility of a survey of the nitriding treatment by analyzing directly the component´s surface state during the nitriding process is particularly interesting, since it allows a process monitoring and control based on the actual nitriding result. In the present study, two measurement methods were developed and combined with the aim of direct surface state analysis during a nitriding treatment: the in-situ X-ray diffraction method and the photothermal radiometry. An experimental setup including a miniature nitriding furnace was developed in order to allow the combined application of both methods during a nitriding process under controlled atmosphere. In the present work, results of combined in-situ measurements on the steel AISI 4140 regarding the nitride layer formation during nitriding process as well as the nitride layer change during the following denitriding of the layer in nitrogen gas are presented and discussed. The investigations show that the photothermal radiometry is sensitive to the changing surface properties due to growing compound layers and when porous layers are generated. This method has a high potential for implementation in industrial nitriding furnaces, but for this, further development for quantitative evaluations of the measurements will be required.

Some high manganese austenitic steels show extraordinarily high ductility due to twinning induced plasticity (TWIP). The TWIP-effect is based on the formation of crystal twins in the austenitic lattice structure as a response to mechanical strain resulting in an extraordinary high ductility. Therefore, TWIP steels are very interesting for forming processes like deep drawing. Within the collaborative research center 747 (SFB 747 “Mikrokaltumformung”) of the German Research Foundation (DFG), micro-components are produced by deep drawing from thin metal foils with thicknesses from 20 μm to 50 μm. The production of such foils by conventional methods as multiple rolling processes of cast sheets is very time consuming since the initial defect free crystal lattice has to be recovered repeatedly by heat treatments after each rolling process to reach a final foil thickness below 50 μm. In the present work, physical vapor deposition (PVD) by DC magnetron sputtering was used as an alternative way to produce thin (≈ 15 μm) TWIP steel foils. An iron based alloy with 25 mass % manganese, 3 mass % silicon and 3 mass % aluminum was deposited onto 100 μm copper foils. After the deposition process the copper substrate foils were removed by selective etching without affecting the resulting TWIP steel films. Microstructure, phase contents and mechanical properties of the resulting PVD-X5MnAlSi25-3-3 TWIP-steel foils were characterized as a function of post annealing conditions. Additionally, the influence of the roughness of the copper substrate foils on the mechanical properties of the resulting PVD-TWIP steel foils was investigated.

Traditionally, case hardening is based on carburizing in atmospheres and oil quenching; this is carried out in sealed quench furnaces and in continuous lines (pusher, roller or rotary furnaces). They are technologies and devices developed more than 50 years ago and, over the course of time, they have exhausted their development potential. At present, they hardly meet the incoming requirements of the modern industry regarding quality and replicability, integration and organization of production, and environment protection. A solution for weak points of traditional case hardening is the use of vacuum technologies and equipment. Vacuum carburizing increases the resulting precision and replicability, and the variety of vacuum equipment for heat treatment allows the adaptation to modern industry requirements. Two applications are described in the article. The first one – evolutionary – is based on the triple- chamber vacuum furnace for semi-continuous production as a wide alternative for traditional devices. The second system – innovative – is based on the true single-piece flow method. Both solutions have specific and characteristic properties that predispose them for different applications depending on quality requirements and organization of production.