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Japanese technology finds a home in Indiana: Toyota Diffusion process greatly increases hardness and extends tool life in stamping operations.

Japanese technology finds a home in Indiana

Toyota Diffusion process greatly increases hardness and extends tool life in stamping operations.

While visiting stamping facilities on a trip to Japan in 1981, Arvin Industries, Columbus, IN, saw presses running without lubricant. After difficult attempts at communicating their awe upon witnessing these operations, Arvin was finally told about the Toyota Diffusion (TD) process (also called thermal diffusion). Since then, Arvin has become the sole North and South American licensee of the process.

The process has been widely used in Japan since the late 70s, with Japanese automakers being some of the largest users. Successful applications of the process include surface treatments of tooling for sheet metal, cold forging, and powdered metals.

While it may initially appear to be another coating process, similar to chemical vapor deposition (CVD) or physical vapor deposition (PVD), it is actually quite different. While other coating processes leave a layer of coating material on the surface, TD diffuses vanadium carbide into and onto the metal.

Altered structure

"It is a thermal reactive process that uses the carbon content in the steel to form a layer of pure vanadium carbide," says Horst Glaser, product manager of Arvin TD Center. "The vanadium in the salt bath reacts with the carbon in the steel to form a layer of smooth vanadium carbide with a surface hardness of 3200 to 3800 Vickers. For comparison, that would be equivalent to about 90 Rockwell C."

The TD process uses a borax salt bath that is brought to the austenizing temperature of the steel being treated. The object is dipped into the bath, up to a maximum of about eight hours. When the material is brought out of the bath, there is a surface layer of vanadium carbide approximately 0.000 08" to 0.0008" thick.

Size and weight determine how long a piece is left in the bath and how thick the vanadium carbide layer will be. Because an object may be immersed for a relatively long period of time, there will be some structural movement in the steel. To minimize movement of steel elements, Arvin requests that the steel be properly heat treated before it is sent to them.

"We have suggested heat-treat cycles that we supply our regular customers," says James Derby, marketing services. "On a piece of D2 that has been double high tempered and stress relieved, we can hold tolerances in tenths, depending on parts size."

Any air-hardened tool steel can be subjected to thermal diffusion as long as it contains at least 0.3% carbon. A2, D2, M2, CPMs, H Series, T Series, and ASPs have all been successfully treated.

"Of the 8000 lb of tool steel we have treated this year, 70% has been D2, 20% has been A2, and the remainder is made up of the other tool steels on the market," reports Horst Glaser. Mr Glaser attributes the high percentage of D2 in the mix to the fact that it is the most popular steel for die applications.

One of the limitations Arvin faces is bath size. Because TD is a hot process, to achieve a stable temperature throughout the bath requires limiting its size. The bath in the Columbus, IN, facility is 22" deep by 17" dia.

Superior wear resistance

A TD coated die can be used in six to eight stamping runs of 50,000 hits each without needing to be recoated, depending on the application.

"It's not a cure all," concedes Horst Glaser, "but in the right applications it can't be beat." Arvin TD Center has had about a 98% success rate since they began offering TD treatment in 1988.

"If you're doing a piercing application where you're running a 3/4" punch through 3/4" sheet, I would not recommend TD," Mr Glaser continues. Because of the extreme hardness created by the process, it creates a very brittle surface that is not suitable for heavy piercing applications. But that is not to say it can't be used for punching.

"For piercing applications, we attempt to not have the vanadium layer exceed 0.0002" or 0.0003" thickness," Mr Glaser explains. "Then we encourage customers to sharpen the punch before using it."

When a TD-treated punch is sharpened, the vanadium carbide layer is removed from the punch face, leaving extremely hard sides. As the most severe wear takes place on the sides of a punch, TD-treated punches can provide exceptional tool life.

Because only 0.001" of the tool has to be ground off of the face, you only remove about 0.0002" of vanadium carbide. Grinding can be done with an aluminum oxide wheel. In the event a TD treatment does need to be removed, Arvin uses a silicon carbide blasting process that can be applied to very selective areas.

In addition to being highly wear resistant, TD-treated tools offer excellent peel strength. In a test, steel pieces were repeatedly struck with a pointed hammer on the same spot. Steel coated with TiC (titanium carbide) by CVD and PVD processes cracked after 50,000 strikes and peeled after 100,000 strikes. The TD processed vanadium carbide layer did not crack or peel.

Technological reluctance

As product manager, Horst Glaser visits stamping plants all over the country. He often sees dies being polished in the middle of runs because they aren't producing quality parts. He has a hard time understanding the reluctance of American industry to try thermal-diffusion technology.

"In Japan, there are over 30 TD treatment centers. Every Japanese automaker who comes over here has his tools TD treated," says Mr Glaser, referring to tooling used for under-the-skin brackets, hinges, etc. Because of the bath-size limitation, the tooling for skin parts such as hoods, fenders, and trunk lids is just too large.

"Because of the structural movement in the steel, we don't recommend TD for extremely tight tolerance work. But for things like brackets and hinges, it significantly increases tool life," says Mr Glaser. That is why Arvin recommends using the process on tools with tolerances of 0.001" or 0.002".

Mr Glaser says that Australia, Great Britain, France, and Spain all used TD technology before US manufacturers tried it. "We are losing ground to other industrial countries because they are willing to implement advanced technologies."

As Arvin TD Center enters the 1990s, their plans include expansion. The firm will open new treatment centers around the country, and hopes to find new applications for the TD process.

"In Japan, the process is used in the textile industry, glass making industry, the forging and stamping industries, in addition to some product applications," Mr Glaser says.

Mr Glaser mentioned he is looking at treating a rotor in an application where very abrasive fluids are being pumped. Pondering this application, he tells us that "pure vanadium is not only very hard and wear resistant, it's also corrosion resistant."

"We are going to try to treat the rotor to alleviate some of their corrosion problems," Mr Glaser explains. "As we learn more about the TD process, we expect to find many new and useful applications."

PHOTO : Cut away view of a TD treated surface shows the diffused layer of vanadium carbide.

Rich Arter Associate Editor
COPYRIGHT 1990 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Title Annotation:thermal diffusion
Author:Arter, Rich
Publication:Tooling & Production
Date:Oct 1, 1990
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