Reliable Vacuum for Plasma Nitriding Ensures Batch Quality and Saves Material
HWL Löttechnik GmbH
Process of plasma nitridingPlasma nitriding is becoming more and more important for heat treatment at HWL. They have more than 30 years of experience with this process. Today, state-of-the-art system technology and control ensure that the structure and composition of the compound and diffusion layers can be continually controlled and monitored.
Pulsed direct current plasma is used to achieve uniform heat treatment results. The advantage of this thermochemical process is that heat treatment can be performed at comparatively low temperatures between 520° and 580° Celsius.
Free charge carriers for electricity transmission must be available to make the plasma electrically conductive. At atmospheric pressure, economically unrealistic temperatures would be required to produce electrical conductivity of the plasma.
HWL works with pressures of 2.5 millibar, which, in practice, enables heat treatment below 600° Celsius. The temperatures are low when compared to other heat treatment methods and this has an extremely positive effect on the warping behaviour of the components. Another advantage to this method is that individual sections of components that should not be nitrided can be mechanically masked and thus selectively excluded from the nitriding process. This does not change the properties of outer layers underneath the masked sections.
Fig. 1: Vacuum system consisting of an R5 rotary vane vacuum pump and a PUMA vacuum booster from Busch. Source: Busch Vacuum Solutions.
After the charging procedure and after the furnace is closed, it is evacuated to the required process pressure and heated using the wall heater.
After this heating process, the components are exposed to glow discharge in a nitrogen atmosphere. A plasma is created in this glow discharge. The nitrogen dissociates in the process, ionises, and is fired at the surface of the components. The exact handling temperature and nitriding duration depend on the material, size and composition of the components, and the nitriding depths to be achieved. After the nitriding procedure, the furnace with the components is cooled down.
The entire process lasts between 17 and 30 hours. The vacuum system is in operation during this period (fig. 1).
Vacuum system solution and its benefits for the customerAfter HWL already had positive experience with other heat treatment systems with vacuum pumps from Dr.-Ing. K. Busch GmbH, a new nitriding furnace was acquired in 2013, and it also had a Busch vacuum system. It consists of R5 rotary vane vacuum pump as a backing pump and a PUMA vacuum booster. This vacuum system achieves an ultimate pressure of <1 x 10-2 mbar while the actual operating pressure during the process is 2.5 millibars. This utilizes the optimal pumping speed of the vacuum system, which is highest in this operating range (fig. 2).
Fig. 2: Vacuum system pumping speed. Source: Busch Vacuum Solutions.
The PUMA vacuum booster is turned on only now. As a booster, it considerably increases the pumping speed of the vacuum system to quickly achieve and reliably maintain the process pressure.
Precise maintenance of the operating pressure and pumping speed guarantees the ability to run and document replicable processes. This makes it possible to precisely achieve the desired product properties. Most of the time, high-alloy stainless steels are plasma nitrided at HWL but construction steels or sintered metals are also heat treated using this process.
Since starting up the nitriding furnace in 2013, there has never been a vacuum system malfunction or failure even though it is in operation around the clock. Continuous operation is only interrupted by set-up or placement times.
For Kai Lembke, absolute reliability of the vacuum technology has the highest priority. This is because failure of the vacuum system during the process can make the entire batch of top-quality and costly precision components unusable.