By Horst M. Glaser
— Reprinted with permission from The Fabricator, October 1993.
How to Use the Technology in Roll Forming
This article describes how a diffusion layer is formed on Thermal Diffusion (TD)-treated materials. Case studies are provided to demonstrate actual results of TD applications in the roll forming industry.
The TD process is a surface modification technology that has been used since 1972 in Japan, and since 1988 in the U.S. This high-temperature process forms a carbide layer on carbon-containing materials (.3 percent minimum) such as steels, nickel alloys, cobalt alloys, and cemented carbides, hardening the surface of the materials treated.
The diffused carbide layer formed by TD processing is thin — 2 to 20 µm (.00008 to .0008 inch) — but very dense and metallurgically bonded to the substrates.
TD-processed materials exhibit properties of carbides and nitrides: high hardness, and resistance to wear, seizure, and corrosion. In wear-related applications, these properties can help improve the life of such tooling as sheet metal dies, forging tooling, tooling for pipe and tube manufacturing, roll form tooling, etc.
In most cases, tool life improvement of 30 to 50 times and higher has been achieved after TD treatment. Increased machine uptime, reduced maintenance costs, and reduced lubricating costs have also been realized.
TD increases machine uptime by reducing the tool maintenance required to prevent galling or spalling caused by tool wear. Because of the higher surface hardness, the tool retains its original polished finish.
This chart compares the surface layer hardness of different treating processes.
Lubricants can be reduced for the same reason. The coefficient of friction between a part and the tooling is directly related to the finish or polish on the tool surface.
In other words, the tool finish remains as slippery as the initial polish for a lot longer. Think about an ice rink before and after it is used by skaters. If TD-processed, the rink would look the same after as before skating started.
Lubricants are used to prevent scratching or surface tool wear, and they overcome the added friction of a worn surface.
In the TD process, parts are immersed in a fused salt bath kept at temperatures of 871 to 1,037 degrees Celsius (1,600 to 1,900 degrees Fahrenheit) for one to eight 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 .3 percent or greater.
A carbide layer is formed into and onto the surface of the substrate by diffusion of carbon and nitrogen from the substrate. This layer is fine, nonporous, and metallurgically bonded into the surface through diffusion rather than by coating.
Parts to be processed are preheated to minimize distortion. They are then TD processed at the austenizing temperature recommended for the grade of steel being treated. After processing, the parts are quenched in air or salt to produce the hardened substrate. The parts then receive the proper tempering cycle.
Steels that have austenizing temperatures greater than 1,900 degrees F may be post-heat-treated in a vacuum or a protective salt bath to achieve full substrate hardness after TD treatment.
The TD process produces layers of vanadium carbide, niobium carbide, and chromium carbide, depending on the carbide-forming elements used in the salt bath. Tantalum, titanium, tungsten, and molybdenum can also be used.
Vanadium and niobium usually exhibit better peel strength and resistance to wear, corrosion, and oxidation than other processes. Chromium carbide has lower wear resistance, but higher resistance to oxidation.
Substrates TD-treated with vanadium carbide can show surface hardness in the range of 3,200 to 3,800 on the Vickers hardness scale. For comparison, most cemented carbide used in tooling applications registers in the range of 1,800 on the Vickers scale.
Vanadium carbide can be used on a variety of air-hardening tool steels, including AISI-A2, AISI-D2, AISI-H13, and many high-speed steels, including most of the new powdered particle high-performance steels. Other materials, such as cemented carbides, have been successfully treated with TD.
The TD process has been used on tooling applications such as metal stamping, aluminum die casting, cold and warm forging, pipe and tube manufacturing, etc. The case studies cited here involve roll forming applications.
These are all considered severe wear applications. The substrate material in all cases is either AISI-D2 tool steel or, in the most severe application, cemented carbide.
Case Study 1. This application involves roll forming of Series 300 stainless steel. The rolls are made of AISI-D2 and measure 4 inches diameter by 1 inch thick by 1 inch bore.
Surface treatment of the rolls was originally chrome plating. Tool life for the chrome plating was approximately 16 hours. However, some peeling of the plating occurred, causing galling on the rolls and giving unacceptable product.
After TD treatment, tool life has been extended to six months. The rolls have been retreated four times.
Case Study 2. As in the first case study, this roll forming application uses Series 300 stainless steel and AISI-D2 roll material. The rolls vary from 3 inches to 5 inches diameter, 3/4 inch to 1 inch thick, with 1-inch-diameter bores.
The part is decorative automotive trim. It must be free of any surface blemishes. Consequently, the roll must remain mirror polished.
The rolls are TD treated and can produce the parts without using large quantities of lubricant. These rolls were TD treated when built and have been retreated approximately five times.
Case Study 3. This application is roll forming of automotive steel rims (wheels) made of high-strength, low-alloy steel.
Roll material is AISI-D2. The rolls vary from 8 inches to 12 inches diameter, 3 inches to 5 inches thick, and the bores vary from 3 inches to 5 inches diameter.
Before TD treatment, tool life was four to six weeks before galling occurred. Tool life has now been extended to six months or longer.
Case Study 4. The application is the production of welded Series 439 stainless steel tubing. Tube diameters vary from 2 inches to 2.5 inches with a .080-inch wall.
Roll material is cemented carbide (13 percent cobalt). The rolls are inserts held in place by carbon steel casings.
Before TD treatment, a certain amount of polishing of the rolls was necessary after approximately 10,000 linear feet of tubing was produced. After TD treatment, the rolls produced more than 1,250,00 linear feet with one polishing operation.
Helpful Hints When Using TD
- If you note “pick off” on the tooling, clean it with Scotch Brite™. It is not necessary to grind the surface with a power tool which could remove the TD.
- If for some reason the tool is damaged, the tool can be welded without removing the TD. Weld it, polish the welded area, finish the production run, and then send it back for to be TD treated again.
- Eventually, you will wear through the TD-treated surface. At that point, you will begin to see part galling. The sooner you can pull the tool for retreatment, the smaller the amount of prepolishing that will be required to prepare the tooling for retreatment.
- Reduce the lubricant in increments — such as 25 percent, then another 25 percent, and so forth. In many cases, TD is run dry, but a 75 percent lubricant reduction is more the norm.
- If you roll form coated materials, like galvanize, a small amount of lubricant will always be needed to avoid coating “pick off”.
- During the tool design process, call the treating center for assistance in selecting a tool substrate. The proper substrate combined with TD can provide tool design improvements. Remember, not all substrates can be treated.
- Remember, TD does not improve the finish of a tool surface — it just improves the life of that finish.
The disadvantages of TD or any other coating processes are the time and expense added to the tooling process. Also, some risk is involved anytime your tooling is not in your possession. Risk of loss or damage exists in transit or treating.
Also, If you wreck treated tooling, you will also wreck the coating, which could offset any potential savings.
High-temperature surface treatments can cause some movement or distortion in tooling. This can be overcome by proper tool steel selection, care in tooling heat treatment, and working closely with the treating center.
Today, many fabricators use tool coatings as a last resort. For productivity and quality improvements, the U.S. metalworking industry should consider making tool surface treatment a part of its tooling preventative maintenance.
If you are scrapping parts, pulling tools for polishing, polishing tools in the equipment, or flooding parts with lubricants to prevent marks on the finished part, you should investigate a tool surface treatment.
With treatment, part cosmetics improve, and scrap, tool maintenance, and equipment downtime are reduced, along with waste treatment expenses.
Horst Glaser is Product Manager with TD Center, Columbus, Indiana. TD is a Registered Trademark of Toyota Central Research and Development Laboratories, Inc., Japan.