Page 43 - MetalForming December 2013

Stainless Steel
Compared to mild steels, forming stainless steels requires more press power and energy (often twice as much), due to their high workhardening rate. Blankholder (draw pad) pressure requirements will be much higher than with low- carbon drawing steel. Ferritic grades such as Type 430 stainless steel require about 50 percent more pressure, while the austenitic grades such as Type 304 and 316 may require as much as three times the force of low-carbon steel.
High forming pressures and frictional heat require slow- er drawing speeds than with low-carbon steel. A unique feature when cold-forming stainless steels: severe deforma- tion is possible when using slower forming speeds. This dif- fers from plain carbon steels, which form virtually the same regardless of forming speed. In general, when forming stain- less steel into difficult shapes, slow down.
Although stainless-steel draw-reduction ratios are similar to those for plain carbon steels, the required clearances between the draw post and die cavity wall may need to be greater for stainless steels. Austenitic grades require a clear- ance of sheetmetal thickness plus an additional 35 to 40
percent, while most ferritic alloys require sheetmetal thick- ness plus 10 to 15 percent additional clearance.
Copper and Copper Alloys
Formability of copper and copper alloys, such as bronzes and brasses, depends greatly on alloy and temper. Some alloys can display significant directionality after rolling, such as C51000 phosphor bronze. Bend directionality affects part layout in the strip.
Stampers achieve the deepest draws with alloy C52100, fol- lowed by the brasses, in order of decreasing zinc level. The LDR of cartridge brass increases with grain size.
Nickel and Cobalt Alloys
Generally, 30 to 50 percent more power is required to form nickel alloys compared to forming low-carbon steels. High-cobalt or tungsten alloys may require 50 to 100 percent more power.
For superior surface finish, stampers opt for soft die materials such as aluminum bronze, nickel-aluminum bronze and zinc alloys. Radii, fillets and clearances usually are larg- er than those for brass and steel. MF
Tooling by Design
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