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What is Vanadizing or Thermal Diffusion (TD)?

The method of producing carbide coatings from salt solutions, also called the Toyota Diffusion ( TD ) process, was invented by the Toyota Central Research Laboratory in 1971. This patented technology, which was strictly protected for Japan's own use for many years, is used in many different facilities around the world today.

Toyota 16 valve engine
The vanadizing process was used in the production of the "Twincam" engine, which started mass production by Toyota in 1978.

In this process, a smooth, thin (4-10µm), non-porous, extremely hard (3200-3800 HV) Vanadium Carbide (VC) layer is produced on the surface of steel or sintered tungsten carbide. In the thermal diffusion process, active vanadium atoms on the surface and free carbon in the substrate material are used.

For this reason, the substrate material to be vanadiumized must have a high free carbon content. It is advantageous for the process if the free carbon ratio is higher than 0.2%.

I would also like to point out that the steel carbon ratio and free carbon are not the same.

To give an example, the total carbon content of AISI D2 (1.2379) is 1.55% C, but the free carbon content is ±0.6%.

The TD process is typically carried out in a salt bath of borax and mixed alloy carbides at 1050°C, the hardening/austenitizing temperature of the steel. This is followed by quenching and annealing processes, as in conventional heat treatment. Since the TD process is a diffusion process, the higher the temperature, the faster the VC layer forms. For this reason, although some steels contain high free carbon, they are coated very slowly due to their low hardening temperature.

In order for the process to be effective, steels with high free carbon content and relatively high hardening temperature should be selected.

Some steels suitable for thermal diffusion process

  1. AISI D2, 1.2379

  2. AISI A2, 1.2363

  3. H13, 1.2344

  4. M2, 1.3343

We can see the application of vanadium in many branches of industry. The Japanese automobile manufacturer Toyota, which discovered the process, uses this process in metalworking tools, sheet metal forming, forging and metal injection molds. Spray nozzles that spray abrasive material are also used in pipe bending and ironing molds.

Yamaha engine for cornering
Yamaha R6 chain pins use vanadiuming.

This heat treatment is especially seen in heavy-duty chain pins of Japanese vehicle and machinery manufacturers.

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