Page 68 - MetalForming April 2013

Tooling by Design
By Peter Ulintz
Stainless Steel Light-Weighting Challenges
Light-weighting in the automotive industry typically has been associated with materials such as aluminum, magnesium, advanced high-strength steels (AHSS) and composites. Now stainless steels are getting in on the act, not only for use in exhaust systems and applications such as body trim, windshield-wiper arms, cylinder-head gaskets and fuel-filler necks, but also for structural components. The 2010 Audi A6, for example, features eight highly stressed components in the body structure comprised of ultra-high- strength austenitic stainless steel (Nirosta H400). Fuel cells also offer new opportunities for stainless steel—heat exchang- ers, humidifiers and combustors, for example.
So what challenges will light-weighting with stainless steels pose for metal-stamping companies? Metal stampers and tool shops experienced with plain carbon steels often experience problems making parts from stainless steel. Poor- ly designed manufacturing processes often produce inade- quate results that can fuel misconceptions and myths regard- ing these corrosion-resistant materials.
For example, most stainless steels have work-hardening rates greater than those for plain carbon steels. Many stam- pers wrongly believe that stainless steels are less formable than low-carbon steels because they work-harden exces- sively. However, many grades of stainless steel have sub- stantially higher ductility than mild steels, and metalformers frequently deep-draw stainless steels into very complex shapes without performing intermediate annealing. Even the less-formable ferritic grades have outstanding ductility (see illustration).
Fighting Surface Friction
Stainless steels contain more than 10.5-percent chromi- um, which reacts naturally with oxygen in the air to create a passive chromium-oxide film on the surface of the steel, meaning the surface of the steel no longer reacts chemically
Peter Ulintz has worked in the metal stamping and tool and die industries since 1978. He has been employed with the Anchor Manufacturing Group in Cleveland, OH, since 1989. His background includes tool and die making, tool engineering, process engi- neering, engineering management and product devel- opment. He is vice-president of the North American Deep Drawing Research Group. Peter speaks regularly at PMA seminars and conferences and maintains the website, www.ToolingbyDesign.com. The site serves as a web-based source for the transfer of modern metal- forming technology and the advancement of “Perfor- mance-Based Die Engineering Strategies.”
Peter Ulintz pete.ulintz@toolingbydesign.com www.toolingbydesign.com
to its surrounding environment. It is this passive film that pro- vides stainless steel with its superior corrosion resistance.
The chromium-oxide surface film, even though very thin, significantly increases the level of friction between the stamping tool and the workpiece. The combination of high forming pressures and surface friction results in signifi- cantly higher tool-wear rates compared to stamping low-car- bon steels. These higher wear rates increase requirements for tool maintenance and cause added downtime and produc- tion costs. As a result, metalformers must carefully select pro- cessing parameters, tool coatings and lubricants to opti- mize tooling performance.
Due to the high work-hardening rate of stainless steels, more press power and press energy (as much as 100-percent more) is required for forming. Tooling must be of sufficient hardness and have a highly polished surface. Stampers should avoid using D2 tool steel with stainless steel in severe forming and deep-drawing applications, since D2 also con- tains chromium. When sliding occurs between two materi- als under high pressure and heat, the components in contact should be of dissimilar materials to avoid galling.
Extreme-pressure lubricants are essential for forming and deep-drawing stainless steel. Standard practice should include lubricating both sides of the blank to minimize fric- tion and reduce galling tendency. Stampers often apply polyethylene (PE) or polyvinylchloride (PVC) plastic films to the sheet surface to aid forming and drawing and to also pro- tect the surface finish. These films provide excellent lubri- cation with friction coefficient values below that of oil. PE usually suffices, but PVC may be required for severe draws
Tooling Technology
600
500
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0
Ferritic Stainless Steels Type 430
Martensitic Stainless Steels
HSLA Steels
Austenitic Stainless Steels Type 304/316
4
8 12 16 20 24 28 32 36 40 44 48 52 Strain %
Carbon Steels
A36
66 MetalForming/April 2013
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Stress (MPa)