Greater Life for Forming Tools

By John A. Vaccari, Senior Editor
— Reprinted with permission from American Machinist, December 1991.

Part runs can go from the thousands to the hundreds of thousands by imparting a thin vanadium-carbide layer into and onto the tool surface

Thermal diffusion of vanadium from a borax-salt bath containing the carbide-forming element is markedly increasing the wear resistance, thus life, of a variety of tool-steel forming tools, as the six case histories from the TD Center (Columbus, IN) illustrated on the following page indicate.

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.

The full treatment, involves preheating, TD, quenching, air cooling, and double tempering in a series of in-line furnaces. The tools, selected to be of the same steel and roughly similar in size per load, are held in place and kept from contacting each other in an open wire-frame-style multi-tier cylindrical rack measuring 17 in. dia × 22 in. long, which is conveyed furnace to furnace by overhead crane.

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.

Surface Hardness

The process treats the entire surface of the tools. Afterwards, we diamond polish the tool work areas. When wear necessitates retreatment, the process can be repeated typically five times, perhaps seven or eight, Glaser says, until all of the free carbon—that which has not already been combined in carbides—has been used.

Although the treatment will anneal hardened and tempered tools, tools having critical dimensions or tight tolerances should never-the-less be hardened and double-tempered by manufacturers for dimensional control. Also, tool work areas should be polished. TD Center will only process “soft” tools if tolerances are quite liberal. All incoming tools are inspected to insure they are crack-free, then reinspected after processing to see that critical dimensions have been maintained.

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.


Case Histories of Tool-Life Improvement resulting from Thermally Diffused Vanadium-Carbide Coating

Annualized Savings

$9,600Increased Uptime:

$1,300Tool Maintenance:

$10,900Total:

23 DaysPayback Period:

Tool:
bracket die, 55 lbs, D2 steel (formerly PVD TIN-coated) for forming 0.05-in.-thick 3XX-type stainless steel in progressive die.

Performance (parts between servicing):
before TD, 4000; after TD, 110,000.

Annualized Savings

$21,000Increased Uptime:

$9,500Tool Maintenance:

$9,100Reject Reduction:

$8,500Tool Replacement:

$3,400Elimination of Die Lubricant:

$28,200Other:

$79,700Total:

85 DaysPayback Period:

Tool:
wear-prone sections of 6- × 12-ft nine-station transfer die (not shown), 673 lb, A2 and D2 steels, for diesel-engine valve cover (shown).

Performance (parts between servicing):
before TD, 4200; after TD, 272,000.

Annualized Savings

$6,000Increased Uptime:

$600Tool Maintenance:

$4,700Reject Reduction:

$11,300Total:

11 DaysPayback Period:

Tool:
boss-extrusion punch (foreground), 3.25 lb, D2 steel.

Performance (parts between servicing):
before TD, 6000; after TD, 121,000.

Annualized Savings

$11,700Increased Uptime:

$8,100Tool Maintenance:

$3,900Reject Reduction:

$23,000Total:

40 DaysPayback Period:

Tool:
draw caps (not shown), 254 lb, D2 steel for double-drawn microwave-oven liner (shown) made of drawing-quality steel.

Performance (parts between servicing):
before TD, 650; after TD, 58,000.

Annualized Savings

$6,600Increased Uptime:

$14,800Tool Maintenance:

$16,900Reject Reduction:

$8,300Elimination of Die Lubricant:

$2,300Other:

$48,900Total:

112 DaysPayback Period:

Tool:
form blocks, 542 lb, 4340 and D2 steels.

Performance (parts between servicing):
before TD, 600; after TD, 7378

Annualized Savings

$5,300Increased Uptime:

$1,000Tool Maintenance:

$1,400Reject Reduction:

$1,400Tool Replacement:

$9,100Total:

24 DaysPayback Period:

Tool:
punches (not shown), 20 lb, A2 and D2 steels, for piercing and notching double thickness (0.25 in.) of bent and seam-welded front flange of mild-steel part (shown).

Performance (parts between servicing):
before TD, 6000; after TD, 260,000.

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