Plasma carburizing is characterized by two things – plasma generation and low pressure. In order for the gas ionization to take place, it is necessary for the furnace to have the low-pressure capability and the ability to generate the plasma – gas ionization – conditions. This necessitates a plasma generator.
There are two types of plasma generators. The first is a continuous DC electrical generator, and the second is a pulsed DC generator. The continuous electrical DC generator is sized based upon the maximum surface area of work within the furnace to be carburized. As the name implies, the power is continuous.
If the continuous DC generator is undersized to the work surface area, then it will be difficult for the workload to come up to the process temperature set point. On the other hand, if the continuous DC generator is oversized, there is a strong likelihood that overheating would occur and have the potential to burn thin cross sections within the workload area.
The process gas has traditionally been city gas (methane) with a carrier gas (dilutant gas) of nitrogen or even hydrogen. Hydrogen is a reducing gas, however, and has the potential to cause some surface decarburization.
The method of continuous DC power for plasma generation is now being replaced with the pulsed DC plasma-generation system. One can now vary the following:
- Plasma-generation voltage
- The power-generation working amperage
- The time that the power is on (pulse time on)
- The time that the power is off (pulse time off)
- The process gas ratios
- The process gas operating pressure (vacuum level)
Low-pressure carburizing is simply another term used for what we have previously known as vacuum carburizing. Atmosphere generation was always a concern due to the ability of methane to crack successfully without sooting. If the process gas went to soot, it would accumulate at the cold areas of the furnace. When this occurs at the water-cooled power feed throughs, it has the potential to cause electrical shorts.
For many years, acetylene has been known of in terms of its chemistry (C2H2). It can be seen from the equation that the carbon potential is 100% greater than that of methane (CH4). Additionally, the acetylene gas had the potential to “crack” at low pressure and high process temperatures and could be readily diluted with nitrogen and hydrogen. Another product has come to the fore – ethylene (C2H4). This too can be carried into the low-pressure furnace using either nitrogen or hydrogen as a carrier and dilutant gas.
The two gases of acetylene and ethylene are now being significantly used as the hydrocarbon gas to provide the source of carbon for diffusion into the surface of carburizing steel. Low-pressure carburizing is now quickly becoming a mature process that has been commercialized. The equipment does not produce the excess carbon, which has been traditional with vacuum carburizing. The resulting surface metallurgy is almost the same as what one would produce with gas carburizing but without the problems of intergranular grain-boundary oxidation. Further, and unlike plasma carburizing, it does not require a plasma generator. So, the capital investment tends to be lower and requires significantly less maintenance than would be necessary with the plasma-generation system.
The low-pressure carburizing systems have developed to such an extent that one can accomplish continuous carburizing using modular systems in conjunction with high-pressure gas quenching.
If you would like to read Part 1 of this series, click here.