Page 41 - MetalForming August 2013

Carbides can play havoc, resulting in bad parts and underperforming short-life tooling. The slide on the left shows carbides exposed to the tool surface, while the slide on the right shows carbides pulled out of tool steel due to an improper polishing technique. Photo courtesy Phygen Coatings.
for example, that our customers use upgraded D2 materials. These tool steels have smaller carbides in their grain structure and exhibit increased tough- ness. When you polish standard D2, the carbides in the steel stick up and you end up polishing around those harder car- bides. Hence, the roughness of the tool affects part quality and tool life.
“A systems approach,” Bell contin- ues, “includes better tool design, mate- rials, surface preparation and coatings to give customers total value—long life and high performance. Placing a good coating on an inferior substrate is a waste of money.
“Tool polishing, by performing the last simple steps adequately, can cam- ouflage tool problems,” continues Bell. “Buffing the tool gives a shiny appear- ance, but a look under a microscope can tell a different story. When examin- ing tool surfaces microscopically, we have noticed that scratches or machine marks often are present yet covered up with buffing. The thought is, ‘I’ve already paid for the tool, I just want to get it coated and start making parts.’ That is the wrong approach. It is like having open-heart surgery, then having the sur- geon use the wrong needle and thread to sew you up. Scratches must be removed before applying a final tool finish.”
Needed: More Vanadium and Chromium
With part materials tending toward the exotic as new recipes come online for specific applications, so too are tooling materials evolving. Tool-steel substrates designed to tackle tough tasks in ever-tougher part materials do not come cheap, and consistent avail- ability at any price is not guaranteed.
Consider vanadium and chromium, two key ingredients in tool steels. Only about 75,000 tons of vanadium is pro- duced each year worldwide. Luckily, only tiny amounts of vanadium are required to double the strength of tool steels and provide corrosion and abra- sion resistance. And while vanadium often is employed in steelmaking, a significant share finds use in tool-steel production.
This part, following the DLC-Si coating process at Teikuro Corp., is ready for action. The process draws interest from automotive-parts makers as aluminum content increases in vehicles.
high enough to deform the substrate more than the coating can with- stand, the coating will crack. And when coating fragments release from the substrate, they act as an abrasive in the con- tact area, speeding wear. The coating cannot hide an inferior substrate.
“We take the ‘moun- taintops’ (the rough sur- faces of a tool substrate as seen through a micro- scope) down,” Bell explains. “This process does not create pockets
and thermal-diffusion ( TD) coatings usually are applied at temperatures greater than 1800 F. Phygen’s coating requires significantly lower tempera- tures, claims Bell, lessening the chance of distortion that can play havoc with tolerances and minimizing tool-hard- ness concerns.
Match Coating to Substrate to Avoid Delamination
Regardless of the coating used, experts insist that the tool substrate must be sufficient for the application. If contact during metalforming is par- ticularly severe, even the toughest coat- ing wears away.
Most frequently, however, coatings fail by delamination, separating from the substrate. Extremely hard coatings are more susceptible to separation if they are brittle. If the forming load is
to hold lubricant but provides a higher load-bearing surface for the coating to perform. The tooling is given a super- polished, super-slick film with a very low coefficient of fiction—less than 0.1 in typical lubricated conditions.
“With these coatings,” Bell contin- ues, “the tooling is less reliant on lubricant to provide a high-perform- ance system that consists of a superi- or substrate, surface finish and coat- ing, and requires less lubricant. That combination optimizes tool life in the most difficult metal-stamping appli- cations.”
More than the Coating
That description hints at a major piece of advice from Bell: Tool coating requires a systematic approach.
“A tool coating does not work by itself,” he says. “We strongly recommend,
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Tooling Technology