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Vacuum-Based Carburizing

Processes
Vacuum and plasma carburizing are processes where the carburizing gas remains under an absolute pressure of a few mbar. Carburizing gases are hydro-carbons such as methane, propane or acetylene, whereby primarily acetylene is used for vacuum carburizing.

Methane requires an additional plasma activation in order to obtain adequate carburizing results. Plasma carburizing with methane has its advantage, if the parts need partial carburizing. In this case, a simple and easy removable metallic mask is placed on that part of the workpiece which should not be carburized. This process is easier than using paste which is difficult to remove afterwards.

1. Cross-section of a partially case-hardened injection nozzle,
2.
Typical parts of case hardening (transmission for cars)

Vacuum-Based Carburizing Processes
In vacuum carburizing, propane or acetylene are usually selected for all carburizing processes without any specific geometrical requirements. However, it has been proven that acetylene offers better carbon efficiency compared to propane because of its instability and higher carbon content per mol of gas. Therefore, by using acetylene, densely packed loads, especially parts with complicated shapes can be carburized at high, reproducible quality.

Thermal decomposition during vacuum carburizing

Carburizing Gas

Carboncontent*

Carbonyield**

Mathane CH4

75 %

< 3 %

Propane C3H8

82 %

~ 25 %

Acetylene C2H2

92 %

~ 60 %

* in weight-%,   **  % of carbon transferred from gas into load

Small quantities of carburizing gas are introduced in the hot zone and are drawn-off by the vacuum pumps. Process parameters like temperature and gas flow are selected according to the parts requirements and are used for the process control. To achieve the specified carbon profile, the carburizing is done in alternating steps for carburizing and diffusion, followed by a final diffusionstep. These sequences are also parameters for the process control.

Process Advantages
Compared to the processes using atmospheric conditions, vacuum carburizing has a lot of advantages. Due to a higher carbon mass-flow rate the cycle time is considerably reduced. Vacuum furnaces easily allow higher temperatures, thus process times can be reduced dramatically, especially for bigger case depths.

Because of the absence of oxygen (air), the quality and properties of the parts surfaces are improved.

1. Various parts for vacuum carburizing, 2. Typical load of vacuum-carburized gear parts, 3. Vacuum carburizing and gas quenching process

Advantages of Vacuum Carburizing
• Fast carbon transfer
• No surface oxidation
• Good case depth uniformity
• Integration into manufacturing lines
• Small consumption of carburizing gas
• No formation of furnace atmosphere
• High carburizing temperatures possible