Page 27 - MetalForming February 2010

generated at the tool-sheet interface releases chlorine and allows it to react with iron in the steel. This reaction forms an iron-chloride film which is an excellent EP lubricant. Sulfur and phos- phorus form sacrificial films similarly, each being effective within specific tem- perature ranges.
Die steels today are a quantum leap over what was available even 10 years ago. Most tool-steel suppliers now offer an array of powdered metals (PM) as a base material for premium tooling com- ponents. This type of steel has uniform and consistent microstructures that improves tool strength and wear resist- ance. PM materials also serve as good substrates for additional wear-resistant coatings. However, D2 may still be a very good tool-steel choice for many stainless-steel applications, especially if a tool surface coating is applied.
Regardless of your tool-steel selec- tion, of prime concern should be the
tooling surface finish. The micro-finish on the surface of the tool steel can be sur- prisingly rough, even when it appears smooth to the naked eye. Working, heat treating, cutting, and grinding can leave undesirable surface imperfections includ- ing pits, craters, scratches and micro- cracks. These surface flaws increase flow resistance during metalforming, which increases friction and heat buildup and promotes galling and tool wear.
Today’s grinding and polishing mate- rials provide surface finishes that were hard to come by a few years ago, but are commonplace today. These improved surface finishes reduce the coefficient of friction and allows many dies to run faster and last much longer.
Surface coatings increase tool-surface hardness and reduce friction. They often complement lubricants by providing a permanent barrier between the tool and the workpiece. Surface coatings also can help reduce the adhesion problems
common with uncoated D2 tool steel by providing a physical barrier between the chromium in the tool steel and the chromium in the stainless-steel sheet.
The most widely used coating processes are physical vapor deposition (PVD), chemical vapor deposition (CVD), and thermal reactive diffusion (TRD). The primary goal of each process is to protect the surface of the tool against abrasive, adhesive, and/or corrosive wear. The key is to match the correct coating type to the stamping application and tool steel selection.
One last thing to consider for deep- drawing processes is that the required clearances between the draw punch and die cavity wall are greater for stainless steels as compared to plain carbon steels. In general, austenitic grades will require a clearance equal to the sheet thickness plus an additional 35 to 40 percent, while ferritic alloys require thickness plus 10 to 15 percent. MF
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