By Donald B. Dobbins, Editor
— Reprinted with permission from Metalforming Magazine, May 1995.
Block (lower left) shows tooling after salt bath treatment, before cleaning and buffing (upper, right).
The Thermal Diffusion (TD) Process is a surface modification technology – long used in Japan but only applied in the United States since 1988. A diffusion layer is formed on TD-treated materials, making the process superior to a simple hard coating.
Thermal Diffusion (TD) is a high temperature surface modification process that forms a carbide layer on carbon-containing materials (0.3 percent minimum carbon), such as steels, nickel alloys, cobalt alloys and cemented carbides-dramatically hardening the surface of the materials treated. The diffused layer is thin, measuring from 0.0001 in. to 0.0008 in. It is extremely dense and thoroughly bonded to the substrate.
All parts are checked twice for dimensions and substrate hardness – once upon receipt and a second time after treatment.
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 forming tooling. Significant increases in machine uptime, reduced maintenance costs and reductions in lubrication costs may be realized.
Pre-heat, TD salt bath and tempering furnaces.
Surface modification by salt bath immersion (known as the Toyota diffusion process) was developed in Japan and has been used by their industries for more than 20 years. Developed by Toyota Central Research and Development Laboratories in the early 1970s, 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., Columbus, IN, signed a license agreement to use and offer the process throughout the United States. Arvin built its own TD treatment center, which became operational in 1988. The furnaces can accept parts measuring up to 17 in. dia. by 20 in. long. In many cases, tool life improvement of 30 to 50 times have been achieved after TD treatment.
An overview of the pack-out area shows the variety of parts shipped each day.
The TD process is performed by immersing parts in a fused (molten) salt bath at temperatures of 1600°F to 1900°F (871°C to 1037°C) for one to eight hours. This temperature range is suitable for quench hardening many grades of low alloy steels and tool steels.
Successful drawing and forming depends upon the condition of working surfaces. Here, a final buff is given to working surfaces prior to return of the tooling.
Carbide constituents dispersed in the salt bath combine with carbon atoms contained in the tooling substrate. 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. 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.
A close-up look at section lines in an oil pan draw ring. The die was sectioned to enable it to fit into TD salt bath equipment. Note the high polish of the working area.
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 also can 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 neighborhood 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 been TD treated with success.
An assortment of some of the smaller tooling parts that have been TD treated.
An extrude punch weighing 2.85 lbs. is made from AISI-D2 steel. As manufactured, it was able to produce just 4000 parts before servicing was required. After TD coating, the same punch produced 202,000 parts before servicing was needed. This represents an increase of 5000 percent resulting in annualized savings of $34,000.
A rectangular draw die weighing 254 lbs. is made of AISI-D2 steel. It produced 650 parts before service was required. After a TD treatment, it provided 58,000 parts before service was needed. This resulted in annualized savings of $23,700, equivalent to a payback period of 40 days.
A nine-station transfer die with treated details weighing 673 lbs. is made from AISI-A2 and D2 steel. Before treatment, the die produced only 4200 parts before servicing was necessary. After TD treatment 272,000 parts were produced, an improvement of 5429 percent. In addition the stamper now runs the parts without lubricant to satisfy the customer's need for clean parts. Annualized savings amounted to $79,700, equivalent to a payback of 141 days.
A vector bender die weighs 47 lbs. It produced 13,750 parts before galling and wear were evident. The tool bends 400-series stainless steel tubing. Aluminum-bronze inserts were replaced with TD-treated D2 inserts. After this change, the die produced 256,000 parts-an improvement of 1860 percent. Annualized savings are estimated at $12,000, equivalent to a 31-day payback.
The TD process is undergoing continued advancement and improvements. The latest advances being studied for implementation include:
- A surface modification (TD) that uses the combination of niobium and vanadium. This process produces an attractive silver-colored surface that has an improved hardness of 4000 vickers. This surface will have enhanced wear and galling resistance.
- Applying existing fluidized bed technology for the purpose of TD treatment. Advantages gained are excellent temperature control, reduced distortion and a relatively cleaner method of surface modification. This process also may make possible the TD treatment of larger parts and reduce turn-around time.
This article is based on information supplied by TD Center, Columbus, IN, and various users of the TD process as presented during seminars and symposiums sponsored by PMA at various locations since June 1993. Much of this material was taken from a paper authored by Horst M. Glaser, product manager, TD Center.