TD Coating of Die Steels with Vanadium Carbide
The Toyota Diffusion process, usually shortened to TD, grows a carbide layer on a die steel out of the carbon that is already in the steel. Vanadium, niobium or chromium is carried in a molten borax salt bath, the carbon diffuses out of the substrate, and a carbide such as VC, NbC or Cr7C3 forms on the surface. Because the layer bonds to the substrate through a reaction rather than by adhesion, TD coatings are used where a die has to resist galling, seizure and wear at the same time. The bath temperature is chosen to match the hardening temperature of the die steel, which is what makes the process fit into a hardening route instead of following it.
How the coating is formed
The bath is a borax salt that carries compounds, usually ferroalloys, of carbide forming elements such as vanadium, niobium and chromium. The carbide layer forms through a chemical reaction between those elements dissolved in the fused borax and carbon in the substrate, and it thickens as carbon diffuses from the interior of the die into the outer surface layer. The result the source describes is good surface covering and a strongly bonding carbide coating on the die face.
The cycle on an H13 die
Dies to be treated are degreased, immersed in the carbide salt bath for a set time, quenched for core hardening, tempered, and washed in hot water to remove any residual salt. The bath temperature is selected to conform to the hardening temperature of the die steel. For H13 the bath runs between 1000 and 1050 °C (1830 and 1920 °F), which is the austenitizing temperature of that grade, so the die is hardened from the same heat that grows the coating. Coating thickness is controlled by the bath temperature and the immersion time. An immersion of 4 to 8 h is what the source quotes for H13 to reach the 5 to 10 µm layer used in die casting. The dies are then taken out of the bath and cooled in oil and salt or in air for core hardening, followed by tempering.
Coated grades and coating hardness
The coated tool steels the source names are H12 and H13. Both are reported to show high hardness and excellent resistance to wear, seizure, corrosion and oxidation, and resistance to cracking, flaking and heat checking is also claimed for the coated surface. Coating hardness depends on which carbide grows on the surface. Vanadium carbide is the hardest of the three at 3500 HV, niobium carbide sits at 2800 HV, and chromium carbide is the softest at 1700 HV. For comparison, a conventionally hardened H13 die runs in the region of 45 to 50 HRC, so the layer on the die face is several times harder than the die body that supports it.
What these numbers assume
The thickness, the bath temperature and the immersion time come from one published source, and a real cycle is set by the salt supplier and the die shape. Deep cavities, blind holes and long thin cores are harder to cover evenly than a flat die face, and the layer is thin enough that a later grind on the die face removes it. Because the die is quenched and tempered after the coating, distortion and the final hardness of the core have to be planned together with the coating, not after it. Confirm the cycle and the allowance for the layer before TD goes into a route card.
Source: ASM Handbook, Volume 5, Surface Engineering, ASM International, 1994.