Page 36 - MetalForming April 2009

Tooling Technology Tooling Up for
High-Strength
Like your mom always said, put that coat on.
Steel
And when considering a tool coating, keep in mind the many ways that forming high- strength steel affects the decision.
BY LOUIS A. KREN, SENIOR EDITOR
Advanced high-strength steels (AHSS) require the use of much higher forming pressures than traditional carbon steels, and can do quite a num- ber 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 inter- viewed Bernard Janoss, segment man- ager for forming and molding at Ion- Bond 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 tem- perature. Tempering temperature influ- ences 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 materi- al influences coating choice.
“You must determine if the substrate can support the coating sufficiently,”
says Janoss. “For example, A2 tool mate- rial, even with a coating, cannot handle the pressure in forming DP 600. We would tell such tool users that, at a min- imum 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 away, 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 hav- ing 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 ther- mal diffusion. Lower-temperature coat- ing processes ease refitting.”
Tool Function
Look at how tooling fails. Blanking or trim steels typically fail because cut- ting 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 trim
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 clear-
ances and other tool attributes. Lubrication—Want to cut costs here? Don’t.
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METALFORMING / APRIL 2009
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