The TD process is very similar to a conventional salt bath heat treatment process. The difference is that the coating stage is performed at the austenitising temperature of the steel. The actual Thermal Diffusion process involves fully immersing parts in a molten salt bath, mostly in the temperature range between 850 and 1050 °C.
TD process salts contain active vanadium dispersed which, together with the free carbon contained in a steel, combines to form the vanadium carbide layer. This high temperature reaction and diffusion of elements also creates the extremely high bond strength. Due to the temperature ranges concerned with the diffusion process, this means some steels are not suitable for the TD process. Click here to see a table of suitable tool steels for TD.
TD Coating through the salt bath method means that the coating reaction occurs wherever the salt comes in contact with the steel. This means that coating of difficult geometries is easily possible due to the full submersion of part. Also the high density salt helps to displace the weight of the tool through Archimedes principle. This reduces distortional creep of tools under their own weight at high temperature.
The thickness of the TD coating can be precisely controlled through adjusting the processing time, bath temperature and accounting for the composition of the substrate. Parts can also readily be re-coated. Re-coating of up to nine times have been reported, however this depends on application, and practically a new tool is usually required after approximately four to five re-coats.
TD coating is a hot process but the TDCC uses specific methods to minimize size changes and distortion.