Vacuum Carburising is used extensively outside the UK, but has only recently become available in this country. The main differences to conventional gas carburising are listed below:
- Oxide free surface. This emlinates initial wear on the surface, enhances resistance to micro cracking which in turn has a beneficial effect on the fatigue resistance
- Excellent Hole Penetration. This is because the gas pressure is pulsed, introducing fresh gas into the holes on a regular basis during the cycle.
- Lower Distortion. As the quench rate is less than oil the distortion is lower. (This is even better if a vacuum carburising steel is specified as the quench rate is even lower)
- Excellent Finish. As the process is carried out in a vacuum chamber the finish is similar to normal vacuum hardening.
- Shorter process times. This is because carburising temperatures can be higher and no post cleaning operations are required.
- Improved Reliability. Once the correct hardness profile has been achieved it can be reproduced with more accuracy than gas carburising.
- No Inter-granular oxidation IGO (Inter-granular attack)IGA. This is important for components that may be prone to failure through fatigue.
- Not Suitable for Low Hardenability Steels. This is because of the limitation of 10 bar nitrogen pressure quenching. Most oil hardending steels will harden in thin sections. Vacuum carburising steels and EN 39 (835M15) will harden in almost any section. The Process is not suitable for mild steels.
- Corrosion Protection Required . Components require protection from corrosion after processing as with other vacuum treated components.
- Masking. Threaded holes can be plugged with bolts to prevent carburising but other areas must be mechanically masked.
- Gears. This is the most common application, particularly if the gears are usually ground after heat treatment. Vacuum carburised gears have no oxide layer and if made from the correct material should distort less, thus eliminating the two main reasons for post griding.
- Fuel Injectors. These and other applications where an even carburised layer in an inacessible place is required.
- Complex Shapes. Components which have a complex shape which would hold oil after oil quenching.