A: Firstly, the reason why we use plasma or low-pressure carburizing as opposed to gas carburizing is quite simple … to eliminate the risk of intergranular grain-boundary oxidation (IGO).
With plasma or low-pressure carburizing, the basis is the furnace operating pressure. Low pressure (vacuum) has a distinct advantage because there is simply no oxygen to cause IGO. As far as the metallurgy is concerned, the formed-case metallurgy is very similar as it would be with atmosphere carburizing but with no IGO.
The advantages with either process comes with the process time and the ability to make use of high-temperature carburizing for shorter cycle times. One is not locked into the conventional process temperature for gas carburizing of approximately 1700°F (925°C).
Gas-carburizing furnace construction and materials of construction dictate the upper working process temperature of atmosphere carburizing. The “vacuum” furnace is capable of temperatures of up to 2300-2500°F (1260-1370°C).
Carburizing work has been done at operating temperatures of 1900°F (1040°C) with dramatically short cycle times. The consideration now is to distortion. Once the carburizing procedure has been completed, the same principles of heat-treat metallurgy apply. Temperature is reduced from the carburizing temperature to the appropriate austenitizing temperature based on the carbon potential of the surface carbon content of the formed case.
At this point (and depending on the grade of steel), the load can be quenched with high-pressure gas quench. The completed load will not require any further post cleaning work because it will be exceptionally clean.