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Tooling Up for High-Strength Steel

By: Lou Kren

Wednesday, April 01, 2009
 
Advanced high-strength steels (AHSS) require the use of much higher forming pressures than traditional carbon steels, and can do quite a number on tool surfaces. Those high forming pressures often lead to use of different tool steels than those typically chosen for work on traditional carbon steels. And, forming brings high frictional forces as the tougher steels are forced to flow through the tooling. All of these issues carry huge implications in the consideration of an ideal tool coating.

To examine AHSS forming from a coating perspective, MetalForming interviewed Bernard Janoss, segment manager for forming and molding at IonBond LLC, Madison Heights MI, a provider of thin-film PVD, PA-CVD and CVD coatings. Read on as Janoss details the factors that metalformers as well as tool designers and builders should consider.

Tool Material

Know the heattreat history of the tool, in particular the tempering temperature. Tempering temperature influences the type of coating that can be deposited on the tool. “In most cases, especially assuming a PVD coating, you don’t want the coating temperature to exceed the tempering temperature of the tool steel,” Janoss says. “That leads to tool softening and size change.”

Also, the choice of substrate material influences coating choice.

“You must determine if the substrate can support the coating sufficiently,” says Janoss. “For example, A2 tool material, even with a coating, cannot handle the pressure in forming DP 600. We would tell such tool users that, at a minimum they need to consider nitriding in addition to the coating. Just placing TiN on A2 to form DP 600 will just wear out the tooling almost right a, because the substrate cannot support the tooling.”

Tool Tolerances

Tolerances play a role in choosing whether to employ a high-temperature coating process, as such a process can change tool size.

“Trends point to manufacturers, especially among manufacturers in Europe and Japan, and increasingly automakers in North America, not having the core capabilities, or interest, in refitting tools back into dies,” explains Janoss. “Very simply, tolerances dictate whether you can use high-temperature coating processes such as CVD or thermal diffusion. Lower-temperature coating processes ease refitting.”

Tool Function

Look at how tooling fails. Blanking or trim steels typically fail because cutting edges become dull, resulting in burring on parts. Draw steels tend toward friction-based failure: galling, scoring and tearing of the part due to inadequate material flow in the die.

“Harder coatings are ideal for trimteels 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 als 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 unanticipated areas. The result: less clearance than intended, leading to higher forming pressure. This places extreme lubricity and toughness demands on the coating so that it will not crack under the pressure. So we ask customers if they intended to iron material and understand why that occurred. If not, that issue must be addressed before a coating 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

Factors that Affect Tool-Coating Choice
Material—Know the heattreat history of the tool; determine if the substrate can support the material.Tolerances—A high-temperature coating process changes tool dimensions, so refitting may be needed. Function—Look at how tooling fails. Design—Make sure tooling aligns properly in the press, and be aware of clearances and other tool attributes. Lubrication—Want to cut costs here? Don’t
Lubrication often is the target of efforts to cut costs or reduce cleaning prior to secondary processes such as welding, but Janoss stresses that forming 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 lubrication characteristics, cannot reduce that heat. In such cases, lubricant is almost as important from a cooling standpoint as it is from a friction-reduction 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 continues, “economics plays a big role. With coatings comes a tradeoff of productivity 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

 

See also: IonBond, Inc.

Related Enterprise Zones: Lubrication, Materials/Coatings, Tool & Die


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