Greater Life for Forming Tools

The process, referred to as TD, for thermal diffusion, was developed at Toyota Central Research & Development (Japan) in the early 1970s, and has been licensed to many firms in Japan and several in Europe and Australia. TD Center, the sole licensee for North and South America, acquired its license in 1987 and has been operating its TD center since 1988. In the first nine months of this year, it has processed some 30,000 lb of tool-steel forming tools, compared with 24,000 lb for all of 1990, says James K. Derby, Marketing Services. With forecast growth indicating that demand will exceed plant capacity in 1994, a second plant is now in the planning stage.

In general, the process is applicable to ferrous and nonferrous alloys containing at least 0.3% carbon, and various elements can be dispersed in the salt bath and diffused into the substrate to combine with the carbon. TD Center, however, limits the process to the formation of vanadium carbide (VC) and, as to substrates, air-hardening tool steels, largely A2 and D2. That second plant, though, will be equipped to process oil-hardening steels as well.

The exceptional wear resistance of the treated tools stems of course from the extreme hardness of vanadium carbide—Vickers 3200-3800. Moreover, however, the thin carbide surface layer is metallurgically bonded to the tool so that it will not easily peel off, as some coatings may, says Horst Glaser, product manager. About 20% of the layer, which averages .0.0004-0.0005 in., penetrates the tool surface. As shown, the wear resistance of D2 forming punches coated with the vanadium carbide far exceeds that of those simply hardened and tempered or chromium-plated.

Preheating, to about I000°F in either a chlorine-salt or atmosphere furnace, minimizes tool distortion. From there, the rack enters the TD furnace, where the temperature of the borax-salt bath is maintained at 1700-1900°F, depending on the austenitizing temperature of the particular steel being processed. Immersion time ranges from one-eight hours, depending on the size and quantity of tools in the load. After TD, the rack is immersed in the chlorine-salt bath of the next furnace to quench the tools to about 1000°F. On leaving that furnace, the tools are air-cooled, and then enter the last furnace for double tempering.

Prices are based largely on tool size, weight, quantity, and critical dimensions that must be maintained. Thus, firm prices can only be established by submitting a tool drawing, noting critical dimensions and tolerances, and providing a few details as to quantity, work conditions, etc.

Except for long thin tools, which make size a major cost factor because of rack space or fixturing requirements, some rough price approximations can be made based on tool weight and quantity: $125/tool for those weighing less than 5 lb, $30/lb/tool for heavier ones. Prices decrease with increasing tool weight to about 200 lb, then stabilize at about $21/lb/tool. Delivery time is typically ten days from receipt of tools.