Page 37 - MetalForming April 2009

steels as they will keep the tool edger sharper for a longer period,” Janoss explains. “For draw steels, we would recommend a coating with greater toughness and lubricity. With draw steels, harder is not always better.”
Tool Design
Without good tool alignment in the press, tool coating is not worth the cost, Janoss cautions. Address alignment first.
Tool clearance is a big issue. Does the tooling maintain enough punch-to-die clearance to make coating worthy of the investment?
“Stampers and tool builders may assume sufficient material clearance between the punch and die,” says Janoss, “but forming can thicken parts in unan- ticipated areas. The result: less clearance than intended, leading to higher form- ing pressure. This places extreme lubric- ity and toughness demands on the coat- ing so that it will not crack under the pressure. So we ask customers if they intended to iron material and under- stand why that occurred. If not, that issue must be addressed before a coat- ing can be used successfully.
“We look at other tool attributes, too,” he continues, “such as die radius, which provides an indication of relative pressure level. Very tight radii indicate the need for a tough coating due to high pressures as material has nowhere to flow except down the sides of the die.”
Lubrication
Lubrication often is the target of efforts to cut costs or reduce cleaning prior to secondary processes such as welding, but Janoss stresses that form- ing AHSS demands its use. “Forming AHSS imparts a lot of energy into the material,” he says, “which comes off as heat. Coatings, even those with lubri- cation characteristics, cannot reduce that heat. In such cases, lubricant is almost as important from a cooling standpoint as it is from a friction-reduc- tion standpoint. I would not advise forming dual-phase or TRIP steel, or even 54-ksi high-strength low-alloy material without lubrication.”
Optimize Coating Selection
Janoss summarizes tool-coating selection for high-strength steel work: “Design the right clearances, use the
right lubrication and choose the right mechanical support, referring to whether to nitride the substrate or select tool materials that can better handle compressive pressure. Such decisions allow a broader choice of tool coatings
to meet your needs.
“Clearly in today’s world,” he con-
tinues, “economics plays a big role. With coatings comes a tradeoff of produc- tivity versus expenditure.”
A good coating solution costs money, but attention to detail in developing the tools and means of forming AHSS ensures that you’ll get the best bang for the buck. MF
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