Fixture hardening

The process developed by EMA Indutec for the inductive heat treatment of carburized ring gears for planetary gears (see image) requires close coordination with soft machining in addition to specialist expertise. Once optimization has been completed, subsequent hard machining can be significantly reduced by inductive fixture hardening. This saves time and costs.

The process is a combination of inductive heat treatment and the use of fixtures. The ring gear on a workpiece carrier is heated homogeneously to austenitizing temperature by a magnetic field using an induction coil. The ring gear expands for physical reasons (approx. 1 % increase in volume). The so-called calibration mandrel, which is intended to give the ring gear the shape and position tolerances, is inserted into the ring gear once the temperature has been reached. By means of the four independently operating showers, the ring gear is first quenched or cooled down to almost room temperature. The warping behavior can be controlled using different pressures and spray times. The controlled cooling process causes the ring gear to shrink. It now sits firmly on the calibrating mandrel.

Conventionally, the ring gear is removed from the calibration mandrel using a hydraulic cylinder under great force. This not only leads to high wear on the calibration mandrel. The desired accuracy values on the workpiece are also often not achieved. The result: lengthy and expensive grinding and cleaning processes. Induction fixture hardening reduces these to a minimum. In contrast to conventional press hardening, inductive fixture hardening involves inductively heating the ring gear, which is seated on the calibration mandrel, to approx. 180 °C. As a result of the renewed thermal expansion, the ring gear “grows” minimally. This creates a small gap between the internal gearing of the ring gear and the calibration mandrel. The ring gear can now be pulled off the calibration mandrel without force. This procedure prevents any abrasion wear on the teeth of the calibrated ring gear and the calibration mandrel.

Another advantage is that the process can be extended to include an additional tempering station. This allows the desired final hardness to be achieved and eliminates the need to invest in a separate tempering furnace. The ring gear now has the desired shape and position tolerances, which it has received as a result of the fixture tools, calibration mandrel, upper and lower hold-down device during hardening.

Due to the accuracy achieved in the 0.05 mm range (axial run-out, parallelism and roundness in several planes), the internal gearing of the ring gear has almost the final contour. In addition to reducing hard machining to a minimum, another special feature of this process is the high surface quality. As the heat treatment takes place in a protective gas atmosphere, scaling on the surface is avoided. This saves the customer subsequent cleaning processes for the ring gear, such as sandblasting. EMA Indutec also uses a water-polymer solution for quenching. When induction hardening their internally toothed workpieces, customers benefit not only from our expertise in induction fixture hardening, but also from decades of experience in the development of customized heat treatment processes.