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Prior to any gaseous carburising process the removal of the passive oxide layer is required. This is because the layer acts as a barrier to carbon atoms, resulting in carbon diffusion not being able to take place with the lattice.

 

The removal of the layer is achieved via an ‘ex-situ’ or ‘in-situ’ process.

 

Ex-Situ

 

An ‘ex-situ’ process is where the oxide layer is replaced by a thin, metallic catalytic layer of for example nitrogen. This is done for

 

“preventing the stainless steel surface from re-passivation and catalysing the dissociation of the gas species at the surface, thus promoting the surface kinetics and allowing processing at lower temperatures.”

 

This technique has the benefit of allowing the material to be stored prior to the carburising process, providing the catalytic layer is stable under the storage conditions. After the carburising treatment, the surface is once again re-passivated by the removal of the catalytic layer. A visual representation of the effect of the catalytic layer is shown below.

 

 

 

 

 

 

 

 

 

 

 

In-Situ

 

An ‘in-situ’ process takes place within the atmosphere of the furnace itself and involves the addition of halogen containing compounds or a mixture of acetylene-hydrogen. These gases activate the surface by either removing or transforming the oxide layer prior to the carburising process.