Coating Greatly Expands Tool Life

By Tohru Arai, TD Center, Edited by Diane L. Hallum
(Reprinted with permission from American Machinist November 1995 issue)

Long used in Japan, the thermal diffusion process modifies tool surfaces by depositing a diffusion layer that forms a hard coating. Carbon and nitrogen in the steel substrate diffuse from the steel to combine with carbide or nitride-forming element such as niobium, vanadium, chromium, and molybdenum to form a very hard, wear-resistant surface. TD-processed materials exhibit properties of carbides and nitrides: high hardness and excellent resistance to wear, seizure, and corrosion. The physical properties of TD coatings significantly increase machine up-time and reduce maintenance and lubricating costs.

The TD coating is suitable for applications where hard-coatings applied by chemical vapor deposition or physical vapor deposition are commonly used. The thermal diffusion process dramatically hardens the surface of the material being treated while remaining thin, (8.0×10^-5 to 8.0×l0^-4 in.), very dense, smooth, and thoroughly bonded to the substrate. In many instances, the original surface finish of the part remains unchanged after TD processing.

High hardness and excellent wear resistance substantially increase the life of tooling for all heavy-wear applications such as blanking dies, forming punches and blocks, swaging dies, core pins for aluminum, expanding and draw dies, mandrels, cold forging dies, flange dies, and pierce and notch dies. The TD process effectively coats many tool steels, including cold and hot-working die steels, high-speed steels, specialty steels, cemented carbide and low-alloy steels.

Initially developed in the early 1970s by Toyota Central Research and Development Laboratories, hence called the Toyota diffusion process, TD was quickly recognized for its practical industrial applications.

The TD process requires full immersion of parts in a fused salt bath kept at temperatures of 1600° to 1900°F for 1 to 8 hours. This temperature range is suitable for quench hardening many grades of low alloy steels and tool steels. Carbide constituents dispersed in the salt bath combine with carbon atoms contained in the tooling substrate, which must contain a carbon content of 0.3% or greater.

A carbide layer forms into and onto the surface of the substrate by diffusion of carbon and nitrogen from the substrate. The vanadium and chromium elements in the bath diffuse into the steel substrate to form iron-chromium or iron-vanadium layers beneath the carbide layer. The carbide layer produced has a fine, non-porous composition and bonds metallurgically into the surface through diffusion rather than by coating.

Parts undergo TD processing at the austenizing temperature recommended for the grade of steel being treated. After processing, the parts are quenched in air or salt to produce a hardened substrate. The now coated parts undergo a tempering cycle.

Depending on the carbide-forming elements used in the salt bath, the TD process can produce layers of vanadium carbide, niobium carbide, and chromium carbide. Tantalum, titanium, tungsten, and molybdenum can also be used. Vanadium and niobium-based TD coatings exhibit excellent peel strength and resistance to wear, corrosion, and oxidation than other coating processes. Chromium carbide has low wear resistance, but it has high oxidation resistance.

The sole U.S. licensee to the process, TD Center, Columbus, IN, applies vanadium carbide, since it is the hardest coating possible. Substrates coated with the TD vanadium carbide will have surface hardness in the range of 3,200 to 3,800 VHN. For comparison, most cemented carbide used in tooling applications register near 1,800 VHN. Substrates successfully TD processed with the vanadium carbide coating include air-hardenable tool steels like AISI-A2, AISI-D2, and AISI-H13. Other substrates have included high-speed steels, including powdered-particle high-performance steels, and cemented carbides. Carbon-deficient metals like iron and nickel alloys can be TD processed after carburizing. The substrate can have the same or lower hardness than normal in some applications.

Tool design requires certain considerations for successful TD processing. Because the composition and properties of the TD coatings are almost independent of the substrate materials, inexpensive and easily machinable metals can be used for a number of applications.

Where tool chipping or breaking is a problem, the user can opt for a substrate with lower hardness and increased toughness. The hard carbide coating provides the more-than-sufficient wear resistance. Tools seeing high surface pressure, like extruding and cold-forging dies, require a hard substrate to support the carbide coating.

Parts that undergo pre-heat-treatment before processing see less distortion. Because of the relatively high-temperature processing of the TD treatment, long slender parts may distort. The larger a part, the greater the chance of expansion or shrinkage. For example, 0.25-in. diameter punches will change less than 0.0005 in. during the TD treatment.

By proper heat treatment and tempering, closer tolerances can be obtained, even for larger tooling sections.

For the best dimensional control of the tool, follow suggested stress relieving, heat-treating, and tempering cycles. To minimize dimensional change, the TD process should be done to parts that have been hardened and finish ground.

Additional steps to minimize distortion is to minimize variations in cross-section, use air-hardenable tool-steel grades that can be slow cooled, machine tools so that critical dimensions are transverse to the rolling direction of the raw material, use powdered-metal steels, and relieve residual stresses that arise from machining and grinding. Because cemented carbides do not harden during the TD cycle, it sees little dimensional change.

The edge preparation of cutting and piercing tools that undergo coating is important. Sharp edges and burrs can break. Finish the tool's cutting edges so they are rounded to a radius of 0.002 to 0.010 in. Because performance of the coated tool depends on the carbide layer on the side surface of the cutting edge, worn cutting edges can be resharpened.

The coating process, when applied to an already coated but worn surface, does not significantly alter the carbide thickness layer on the tool. Because of the slow growth rate of the carbide layer on previously coated areas, variations in layer thickness are insignificant. This allows tooling to be retreated several times after use.

The surface finish and polishing direction of forming dies can affect the load necessary to cause seizure. TD-processed tooling having a rough surface finish will experience seizure at lower loads than tooling having finer finishes before undergoing TD treatment. Best results are with surface finishes having a maximum peak-to-valley roughness height, Rmax, of 3µm. For tooling made from plated steel, stainless-steel, high-strength steel, and aluminum, the substrate should have finish of 0.1 to 1µm Rmax. Tooling cut using EDM, the white layer of material left on the cut surfaces should be removed before undergoing TD treatment.

Because TD processing does not alter surface finish, for best results, parts should be prepared with the microfinish required by the customer for his application. Tools that will see sliding, forming, or drawing operations are typically diamond polished after coating.

Lubricants, even though reduced or eliminated in certain applications, should be used when roll forming materials prone to galling, pickup, or severe flaking. These materials can include aluminum, aluminized steel, and galvanized steel. Lubrication can be the application of very light oils or spray-type water-based lubricants to prevent galling of the rolls.

Continued developments to the TD process include surface modification with a combination of niobium and vanadium. The coating gives the part an attractive silver-colored surface with an improved hardness of 4,000 VHN. This surface will have greater wear and galling resistance. Another development includes the use of fluidized bed processing for TD treatment.

Advantages of this technology include excellent temperature control, lower distortion, and a relatively cleaner method of surface modification. This may also make possible the TD treatment of larger parts. Currently, parts up to 17×22 in. can be TD processed.