By Mr. Horst M. Glaser, Product Manager, The TD Center, Columbus, Indiana.
— Reprinted from meeting notes with permission from the SME (Society of Manufacturing Engineers). Rosemont (Chicago), Illinois, Fabtech International ‘93.
This paper describes the Thermal Diffusion (TD) Process and its uses in the roll form industry.
TD is a surface modification technology long used in Japan but applied only since 1988 in the United States. The TD Process is briefly described to show how a diffusion layer is formed on TD-treated materials, making the process superior to a simple hard coating.
Case studies are provided to demonstrate actual results of TD applications in the roll forming industry. The resulting performance and economic benefits are noted.
Thermal Diffusion (TD) is a high temperature surface modification process that forms a carbide layer on carbon-containing materials (.3% min.) 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 has shown to be superior to other coating processes.
The diffused layer is thin, 2–20 µm (.00008–.0008 in.) but extremely dense and metallurgically bonded to the substrates.
TD-processed materials exhibit properties of carbides and nitrides; high hardness and excellent resistance to wear, seizure, and corrosion. These properties impart substantial life improvement to all wear-related applications, e.g. sheet metal dies, forging tooling, tooling for pipe and tube manufacturing, roll form tooling, etc. In many cases tool life improvement of 30 to 50 times has been achieved after TD treatment. Significant increases in machine uptime, reduced maintenance costs, and reductions in lubricating costs are realized as well.
Surface modification by salt bath immersion (TD Process) was developed in Japan and has been used in their industries for over 20 years. Developed by Toyota Central Research and Development Laboratories in the early 1970's, TD was little more than a laboratory curiosity at first. But the Japanese soon realized its potential and moved it from the lab into practical industry applications.
In 1987 Arvin Industries, Inc. signed a license agreement to use and offer the process throughout the United States. Arvin built its own TD treating center, which became operational in 1988. The furnaces can accept parts measuring up to 17 inches in diameter and 20 inches in length.
The TD Process is performed by immersing parts in a fused salt bath kept at temperatures of 871 to 1037° Centigrade (1600–1900° F) 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% or greater. A carbide layer is formed into and onto the surface of the substrate by diffusion of carbon and nitrogen from the substrate. The carbide layer produced has a fine, non-porous composition and is metallurgically bonded into the surface through diffusion rather than by coating.
Parts to be processed are pre-heated 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 1900° F may be post-heat treated in vacuum or 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 exhibit superior peel strength and resistance to wear, corrosion, and oxidation when compared to other processes. Chromium carbide has lower wear resistance; however, it has higher resistance to oxidation.
Since most tooling applications require the hardest surface possible, the TD Center uses the Vanadium carbide application. TD-treated substrates with Vanadium carbide will show surface hardness in the range of 3200 to 3800 on the Vickers hardness scale. For comparison, most cemented carbide used in tooling applications will register only in the range of 1800 on the Vickers scale.
The TD Center has treated a variety of air-hardening tool steels. These include AISI-A2, AISI-D2, AISI-H13, and many of the high speed steels, including most of the new powdered particle high performance steels. Other materials, such as cemented carbides, have also been TD treated with great success.
Case Studies as Applicable to The Roll Forming Industry
The TD Process has shown outstanding wear life improvement on many tooling applications, such as metal stamping, aluminum die casting, cold and warm forging, pipe and tube manufacturing, etc. This part of the paper will specifically address the application of the TD Process in roll forming applications.
In most cases, the objective was to reduce wear, galling, and pickup. In some cases, it was to reduce or eliminate the use of any type of lubricant. The main objective was always the improvement of the quality of the parts produced with a minimum of downtime.
Case Study #1
The application is roll forming of Series 300 Stainless Steel.
Prior surface treatment of the rolls was chrome plating. Tool life for the chrome plating was approximately 16 hours. Failure occurred and manifested itself by peeling of the plating and consequent galling on the rolls, giving unacceptable product.
After TD treatment, tool life was extended to six months.The rolls to date have been re-treated four times.
Roll material is AISI-D2. Rolls are 4” diameter × 1” thick × 1” bore.
Case Study #2
The application is roll forming of Series 300 Stainless Steel.
The part is decorative automotive trim. The part must be free of any surface blemishes. Consequently, the roll must remain mirror polished.
The rolls are TD treated, and are capable of producing the parts without using lubricant in large quantities.
To maintain required quality needs and production schedules, these rolls were TD treated when built.
The rolls have been re-treated approximately five times.
The roll material is AISI-D2. The rolls vary from 3” to 5” in diameter, 3/4” to 1” thick, and the bores are 1” diameter.
Case Study #3
The application is roll forming of automotive steel rims (wheels) made of high strength, low alloy steel.
Prior to TD treatment, tool life was four to six weeks before severe galling occurred. Now tool life has been extended to six months or longer.
Roll material is AISI-D2. The rolls vary from 8” to 12” in diameter, 3” to 5” in thickness, and the bores vary from 3” to 5” diameter.
Case Study #4
The application is production of welded Series 439 Stainless Steel tubing. Tube diameters vary from 2” to 2.5” with .080” wall.
Prior to TD treatment, a certain amount of polishing of the rolls was necessary after only approximately 10,000 linear feet of tubing was produced. After TD treatment, the rolls produced in excess of 1,250,000 linear feet with only one polishing operation.
Roll material is cemented carbide (13% cobalt). The rolls are inserts, held in place by carbon steel casings.
All of the above are considered severe wear applications. It should be noted that the substrate material in all cases is either AISI-D2 tool steel, or in the most severe application, cemented carbide.
For the TD Process to perform to its optimum, it is important that the vanadium carbide layer has sufficient support from the substrate material (hardness). Since the TD Process is a high temperature process (austenizing temperature of the tool steel), consideration must be given to tolerances and dimensional integrity of the tooling treated. For instance, close tolerance bores can be finish machined after TD treatment.
Proper heat treatment and tempering of the tool steels prior to TD treatment will substantially reduce possible growth, shrinkage, or distortion during TD treatment. When using cemented carbide as substrate material, very close tolerances can be maintained during TD processing (tenths).
Lubricants, even though reduced or eliminated in certain applications, should be used when roll forming materials prone to gall, pickup, or flake severely. Some of these materials are aluminum, aluminized steel, and galvanized steel. In these cases, very light oils or spray-type water-based lubricants should be used to prevent galling of the rolls.
In conclusion, the TD Process has proven to be very effective in extending tool life, reducing downtime, and increasing quality of the product in the roll forming industry.