— Reprinted with permission from Tooling & Production, November 1993.
Producing heavy-gage steel parts with tight radii, sharp corners, and other demanding features is a challenge for stampers. The heavy stamping forces needed for part forming can create enormous friction, resulting in die wear, galling, and inability to hold part tolerances.
Many stamping plants pull dies for polishing, or polish the die while it's still in the press. Either method results in lost production time. Polishing eventually will result in a die that's out of dimensional tolerance.
Thermal diffusion is a salt bath surface treatment process that increases die surface hardness and lubricity. Die life and part quality have both improved at Dana's Parish Div since the plant began using TD-treated dies for stamping heavy-gage steel parts.
Parish Div, Dana Corp, Hopkinsville, KY, is familiar with these problems. The plant uses presses ranging from 25 to 1000 tons capacity to produce more than 200,000 vehicle structural parts annually in some dies. Until recently, the only way around the galling problem was die polishing, according to Dana tooling coordinator David Reynolds, “Someone climbed into a die once or twice a week to polish,” he says. Eventually, the thin layer of metal removed with each polishing led to parts running out of tolerance.
Sticking was also a problem with the heavily formed parts. The plant used nitrogen cylinders to strip parts from the die as well as large amounts of lubricant on some parts.
A work team at Dana wanted to resolve the galling, sticking, and die wear problems, and evaluated several potential solutions before choosing thermal diffusion, a tool surface treatment process from TD Center, Columbus, IN. The process both increases die surface hardness and reduces the coefficient of friction, resulting in improved die life and less galling and seizure.
Thermal diffusion is a salt bath surface treatment process in which parts to be treated are immersed in vanadium-containing molten salts at 1900°F for one to eight hours. Vanadium atoms or ions in the salt bath combine with carbon atoms in the die steel, resulting in a thin layer of vanadium carbide on the die surface. The vanadium carbide surface layer has a Vickers hardness of 3200 to 3800, resists impact and corrosion, and has a low coefficient of friction. The process works particularly well with cold- and hot-working die steels, hss, powder metallurgy (PM) steels, and carbides.
To test the effectiveness of the TD process, the team chose the die that gave them the most trouble. Over the next few months, they documented production improvements and trained other Dana personnel in handling the TD-treated dies. The TD-treated die, which previously had required polishing once a week, was polished only once in nine months, and then using only Scotch-Brite.
Lubrication use dropped by as much as 75%. In one case, lubrication sprays were taken off the die, and compound was used only in the bottom of the draw cavity. Operators ran 200 pieces before they realized the nitrogen cylinders were not hooked up. The die had such a low coefficient of friction that the weight of the stripper plate alone was enough to remove the part from the die. Other benefits noted include better part consistency over time because polishing was reduced, improved part appearance, and reduced die and press maintenance.
As a result of the tests, the Dana team prioritized other tooling to receive TD treatment. Several more dies have been treated, and others are planned.