Diffusion Process Extends Tool Life

(Reprinted with permission from Stamping Quarterly, 1990)

The ultimate production run would be free of the downtime required to change, set-up and maintain dies, punches and tooling. There would be no seizure, wear, galling or corrosion of tools and dies. And there would be no need for lubricants on the line.

Although such an ultimate run remains in the future, one steel surface modification process has taken production a step forward. Arvin Industries, Inc. has tested a process called Thermal Diffusion (TD) in its plants for more than two years.

In that time, the company has documented cases of tool life being increased more than 50 times from treatments performed at its TD center in Columbus, Indiana.

One instance the company cites as an example of extending tooling life involved carbide tooling for 439 stainless steel tubing. Before treatment, the tooling could provide 50 pieces of tubing before galling and scratching took place. Then the tooling would have to be polished before continued use.

After treatment, more than 3,000 pieces could be turned out before polishing. And the need for lubricating oils was eliminated.

The Process

Thermal Diffusion is a surface modification process that forms a vanadium carbide layer on the surface of steel or carbides. This is done by immersing parts in a fused salt bath kept at temperatures of 1,600 to 1,900 degrees Fahrenheit for one to eight hours.

Vanadium carbide constituents dispersed in the salt bath combine with carbon atoms contained in the tooling substrate, which must contain a carbon content of .3 percent or greater. A vanadium carbide layer is formed into the surface of the substrate.

The resultant 2- to 20-micrometer-thick (.00008 - .0008 inch) vanadium carbide layer has a fine, nonporous composition metallurgically bonded into the surface through diffusion rather than by coating.

The advantages shown were obtained by applying the thermal diffusion process to dies, jigs and other tools.

Tests at the TD center show that the process creates a vanadium carbide layer that has superior peel strength and resistance to wear, corrosion, and oxidation when compared to other processes.

Treated materials show surface hardness in the range of 3,500 on the Vickers hardness scale.

Documented case histories have shown TD-processed components to have an extended life of up to 50 or 60 times longer than untreated components.

Arvin "discovered" the process in 1981 during a tour of Japanese stamping and automotive plants. One thing the Americans noticed with amazement was that a Toyota supplier ran extremely tough stamping jobs with little or no lubricants. Die maintenance was also noticeably lower.

Naturally, the Americans wanted to know how this was done. When the language and communications barriers were finally overcome, the TD process was introduced to the Americans.

Developed by Toyota Central Research and Development Laboratories in the 1970s, TD was little more than a laboratory curiosity at first. But the Japanese soon realized its potential and moved it from the lab to practical industry applications.

By 1987, Arvin had signed a license agreement to use and offer the process throughout the United States. The next year, the company had its own TD treating center.

The process has been used on tooling and dies for the following industries: sheet metal, cold forging, hot forging, powdered metals, glass, textile, and wire.

It has also been used on production parts having stringent wear resistance and corrosion requirements. Treated parts can even be retreated up to eight times.

Technicians say that parts with tolerances of plus or minus .04 millimeters (0.0015 inches) or greater make better candidates for treatment. Parts made from air-hardened steels requiring tight tolerances should be double high-tempered before using the process.

Presently, parts to be processed cannot exceed 17 inches in diameter by 20 inches in length.

Characteristics of Treated Materials

Hardness. Extreme surface hardness is obtained by the vanadium carbide layer produced. Vanadium carbide retains exceptional hardness of Hv 1,000 even at 800 degrees Centigrade.

Furthermore, hardness will be returned to previous levels once the layer is cooled to room temperature after exposure to high temperatures.

Wear resistance. Carbide layers produced by the process show remarkable wear resistance against any materials such as steel, nonferrous metal, plastics and rubber.

In results obtained by measuring the abrasion of the dies after continuous coining of cold rolled mild steel plates that were not TD-treated, hardened and tempered steel show considerable abrasion loss. Little abrasion is recognized on the Vanadium carbide-treated steels from the TD process.

Seizure resistance. Vanadium carbide coated steel resists seizing at any temperature.

In the case where the mating material is stainless steel, the seizure resistance of a TD-treated vanadium carbide layer is considerably better than that of cemented carbide. Vanadium carbide also shows superior score resistance, regardless of mating materials.

Impact resistance. In the lzod impact test, TD-treated steels are equivalent in impact values to hardened and tempered steels, regardless of the substrate.

Therefore, if a material having high impact resistance is selected for the substrate, it will be effective against breaking and chipping after TD treating.

Corrosion resistance. No corrosion is shown in test pieces immersed in a 36 percent hydrochloric acid solution that corrodes stainless steel.

Peeling resistance. Unlike plating, the treated layer will not easily peel off. The vanadium carbide layer is metallurgically bonded versus deposited or mechanically bonded.

In tests, various surfaces were repeatedly struck on the same spot with an acuminated hammer. A chromium-plated layer was cracked after a small number of strikes and peeled off after about 50,000 strikes. The titanium carbide layer produced by the CVD or PVD method is cracked after 50,000 strikes and peeled off after 100,000 strikes. The TD-treated vanadium carbide layer suffered neither cracks nor peeling after 200,000 strikes.

The information presented in this article was prepared by James V. Smith, Jr., an Indianapolis-based freelance technical writer.