Gas Nitride or Plasma Nitride. Well known heat treat consultant David Pye, davidpye@pyemet.com is providing us with some details about what to consider when choosing between Gas or Plasma Nitriding. April 11/11
The choice between gas nitriding and plasma nitriding is usually a very difficult choice to make. The choice will come down to depend on;
Capital investment,
Logistics
Necessary surface metallurgy and control that is required for the processed component.
Capital Investment: The capital investment always seems to be considered as the driving force for the decision. Yes, the plasma systems are more costly than with the gas nitride. That is a correct assumption. But that seems to be where it stops. When making the cost comparison, between the two process techniques will generally be finished at the purchase price line. To have a true cost comparison, one really needs to consider the cost comparison of operating costs as well as the primary investment. It will then come down to considering the corporate amortization period of depreciation. After that, it really comes down to the operating costs. Once the equipment is amortized then it comes down to the direct operating costs of the equipment. It will be seen, that after the amortization period is complete, and then operating costs of the plasma technology are reduced, because of the nature of the process. This is because of process gas usage and power costs are lower. The writer has experienced the following approximate value to obtain a conventional nitrided case, as being;
100ft³ of ammonia gas for 100ft² (approximate) of available surface area. (This is based on 5 to 7 process gas volume changes per hour). It further considers process temperature and time at process temperature. The reason for the process gas volume on gas nitriding being high is simply because the process vessel is being pressurized to a slight positive atmospheric pressure to act as a ‘sweep gas’ to ensure process gas circulation within the process retort and work.
3 ft³ of process gas per hour (approximately)(nitrogen and hydrogen) for 100ft² work surface area. The reason for the low flow volume is because we are only using the gas necessary to generate the activity within plasma glow seam.
Logistics
The logistics consideration will be based on the;
Should the process be conducted ‘in house’ or sub-contracted out to a commercial heat treater, and why?
What is the distance of travel from the manufacturing house to the heat treatment shop?
Do we have the equipment to enable us to observe and control the surface metallurgy achievable. (Laboratory sample evaluation equipment)?
How quickly will the processed part be processed by the commercial heat treater?
Do we have the appropriate floor space to accommodate the equipment?
Can we maintain and service the equipment?
Process evaluation
The question is now, how to test, evaluate, and most importantly, how do we control the process? A small metallurgical laboratory can be developed (or it is a pre-existing laboratory) for the process evaluation that would include the following equipment;
Abrasive sample cut off saw
A pre-grind system for rough grind with silicon carbide papers
A rough polish using silicon carbide papers
Final polish using the appropriate grade of polishing cloth and either an aluminum slurry or diamond paste
Micro-hardness test unit
Inverted metallurgical microscope
Sample storage dessicator
Fume hood for simple acid etching
Spill kit
The above will enable you to process most metallurgical samples and not just nitride evaluation. The process control will come from the furnace control panel and its method of;
Data acquisition
Data storage
Gas delivery through either micrometer gas flow valves or mass flow controllers
Process gas flow ratios
Temperature control
Time cycle control
Heating power control
The above is just some of control parameters that should be observed. Nitriding (gaseous or plasma) process parameters need to be controlled if we are to produce continuous surface metallurgy and reproducible results.
