Historically, the pulsed-DC generation system was developed in Europe and derived from the original concept of continuous-DC generation. The motivation behind the development of the pulse system was the consideration of separating the use of plasma for the heating of the workload up to its process temperature. The continuous-DC plasma furnace was essentially a furnace without heating elements.
It was recognized that with continuous DC, there was a risk (due to the usage of high process voltages for plasma generation) of burning sensitive areas of the charge being processed. Burning due to arc discharge typically occurred at sharp corners. The developmental work of the separation of heating from plasma generation began in the mid-1980s.
It was recognized that an external heating source could be used around the external surfaces of a process vessel. The external heating source needed to be insulated to reduce heat losses. This meant that a bell furnace could encapsulate (with external heating elements) the nitriding process vessel. It further meant that after evacuation of the process vessel to a atmospheric pressure to remove oxygen (air) and substitute with hydrogen at partial pressure, one could use the hydrogen as a conductive gas. The hydrogen molecules transported the heat energy from the internal wall of the process vessel to the process charge. This meant that the charge is now heated (as would be in an air-circulating furnace) by conduction. Hydrogen is the most effective process gas for the transportation of thermal energy.
At a selected temperature – approximately 500°F – plasma energy would be initiated. This means that the plasma voltage choice need not be high voltage, because the work load (charge) was now thermally preheated up to the selected temperature of approximately 500°F.
When the plasma was initiated, the required voltage necessary to generate plasma (which was not now the primary heating source) was considerably lower than would be necessary with continuous DC. Therefore the risk of arc discharge was almost eliminated. Then, the now-generated plasma was used to sputter the workload clean (like atomic shot blasting). As the workload approached the selected process temperature for nitriding, the appropriate process gas of nitrogen begins to flow into the process chamber. The nitrogen flow rate is then determined by the surface metallurgy required for the workpiece.
Further to the reduction of the voltage necessary to generate the plasma, the plasma was now pulsed during the complete process cycle. This meant that the user of the new technology could now adjust the process voltage as well as the pulse time on and the pulse time off of that voltage. The system was now controlling the following:
- Process temperature
- Process cycle time at temperature
- Process plasma-generation voltage
- Pulse time on
- Pulse time off
- Process amperage
- Process current density
- Process current
- Process gas volume (nitrogen and hydrogen)
Pulsed-plasma-generation technology nullifies the risk of arc discharging with the workpiece because of the pulse generation method. The introduction of the PC PLC control for both data acquisition and process control offers a very accurate nitriding process system.
The process metallurgy can be accurately controlled in relation to the surface metallurgy that is necessary to suit the workpiece application as well as producing uniform case depth control and an effective, efficient method of nitriding.