— Reprinted with permission from Tooling & Production, September 1993.
The Arvinyl Division uses mechanical and hydraulic presses up to 1000 tons to deep draw oil pans for diesel truck engines. To use the TD process, dies would have to be sectioned, since they are too large to fit into current salt bath equipment.
For TD Center, Columbus, IN, seeing the value of a salt bath coating process developed in Japan for extending the life of dies has taken on a business life of its own. Introduced a few years ago to this country, the Thermal Diffusion (TD) process has attracted a word-of-mouth following among stampers and other metalworkers who have experienced dramatic improvement in die wear from this method of hardening steel surfaces to reduce galling, seizure, and corrosion.
“It's hard for us to quote the magnitude of tool wear improvement from TD process users without raising some eyebrows. The results are that good,” explains Jon B Knapp, vice president and general manager, TD Center. Here are examples of improvements attributed to the TD process:
- Tube roll forming. Tubing made from 439 stainless steel with a diameter of 2½" and a 0.078" wall is produced using forming rolls made from pre-formed cemented carbide and mounted in steel retainers. Prior to TD treatment, severe galling and pickup occurred after relatively short runs of 10,000 linear feet with excessive downtime polishing. After TD treatment, the forming rolls have run in excess of 1 million linear feet with a single polishing.
- Swaging. An automotive exhaust resonator made from 409 stainless is produced with tooling made from cemented carbide encased in steel retainers. The swaging operation is done in four stages. Prior to TD treatment, it was necessary to polish the tooling after each 100 pieces. After TD treatment, production was increased to over 3000 pieces between polishings.
- Tube bending. A 47 lb vector bender die made from D2 tool steel was used to bend tubing made from 400 stainless. Prior to TD treatment, 13,750 parts were produced, and the tooling had to be inserted with aluminum bronze to prevent galling; after TD treatment, 256,000 parts were produced, an improvement of 1860%.
How TD works
The thermal diffusion process is a high-temperature surface modification process that it forms a carbide layer on carbon-containing materials (having a minimum of 0.3% carbon) such as steels, nickel alloys. cobalt alloys, and cemented carbides, dramatically hardening the surface of the materials treated. The diffused carbide layer formed by TD processing is a thin coating of vanadium carbide 0.0001" to 0.0008" thick with hardness ranging from 3200 to 3800 Vickers.
The process involves immersing parts in a fused salt bath kept at temperatures of 871 to 1037°C (1600 to 1900°F) for one to eight hours. This range is suitable for quench hardening many grades of low-alloy steels and tool steels, explains product manager Horst M Glaser.
Since most tooling applications require the hardest surface possible, the TD Center uses vanadium carbide, says Mr Glaser. Most cemented carbide used in tooling applications will register only in the range of 1800 on the Vickers scale, he says.
But other carbide forming elements may be used including niobium and chromium, as well as tantalum, titanium, tungsten, and molybdenum, Vanadium and niobium exhibit superior peel strength and resistance to wear, corrosion, and oxidation when compared to other processes; chromium carbide has lower wear resistance but higher resistance to oxidation.
Tooling treated with the TD process exhibits characteristic darkening of metal. Wear surfaces are polished to a bright finish.
Typical applications for the TD process include roll forming, extruding and bending tooling as well as blank dies, form punches and blocks, swaging dies, core pins for aluminum, expanding dies, draw dies, mandrels, cold forging dies, flange dies, and pierce and notch dies. Currently. there is a size limitation. Parts that fit into a 22" × 17" cylinder can be treated.
The TD coating can be mechanically removed with silicon sand blasting, or it can be ground off only by getting under the coating, not grinding through it. It can also be coated on welding as long as stainless rod, which is too low in carbon, is not used.
Cost of the coating process ranges from about $30 per lb down to $21 per lb, except for carbides. Size and configuration may also figure in the cost equation. Improvements in the TD process include surface modification using the combination of niobium and vanadium. This process produces an attractive silver-colored surface that has an improved hardness of 4000 Vickers and enhanced wear and galling resistance.
Another possibility is using fluidized-bed technology for the purpose of TD treatment. Fluidized bed treatment provides excellent temperature control, lower distortion, relatively clean processing, and the possibility of treating larger parts than can now be handled with the salt bath.
Both of these projects are planned for implementation in the foreseeable future, says Mr Glaser.