Page 24 - MetalForming February 2010

Tooling Technology
Stuart Keeler (Keeler Technologies LLC) is best known worldwide for his discovery of forming limit diagrams, development of circle grid analysis and implementation of other press shop analysis tools. Stuart’s sheetmetal forming experience includes 24 years at National Steel Corporation and
12 years at The Budd Company Technical Center, enabling him to bring a very diverse background to this column and the many seminars he teaches for PMA. His most recent project is technical editor of the AHSS Application Guidelines—Version 4.1, which now is available for downloading free from www.worldautosteel.org. Keeler Technologies LLC
P.O. Box 283
Grosse Ile, MI 48138
Fax: 734/671-2271
E-mail: keeltech@comcast.net
As soon as man discovered that heat softened metal, the application of heat became part of forming metal. Blacksmiths quickly learned that ham- mering a room-temperature chunk of metal accomplished very little useful change in its shape—only a rebounding of the hammer. In contrast, heating the metal white hot allowed it to flow into a multitude of shapes, from swords to horseshoes. Heat still is a major player in bulk forming or forging.
Application of heat has been extend- ed to sheetmetal. A current example is hot forming of boron-treated steels. The blank is heated to a target temper- ature of 1650 F and formed. The yield strength is extremely low, allowing cre- ation of complex shapes. If quenched in the die, the microstructure changes to martensite with the attendant 150-ksi minimum yield strength. After reaching room temperature, issues with spring- back and residual stresses are absent.
Both examples describe forming after the entire starting piece has been heated. This month the important topic of differential heating is analyzed. Many decades ago, the press shop rule for producing slightly more depth in
a deep-drawn cylindrical cup was
to heat the binder portion of the
die and chill the punch. The intent was to heat the outer ring
of the blank (decrease the strength of the deforming cup flange) while chilling the central
area of the blank (increasing the strength of the cup bottom that
pulls the flange into the cup wall).
This process worked best with
very slow presses that allowed
time to heat and chill the respec-
tive zones. In reality, this process
STUART KEELER
was still too close to the edge of the deformation cliff and was not a viable or robust solution. However, it did demon- strate that localized or gradient heating (or cooling) can change the mechanics of the forming process.
Simply deforming a workpiece cre- ates an increase in temperature in the deformation zone. Press shops often attribute this heating to internal friction. Whatever causes the temperature increase, localized heating accelerates the severity of a strain gradient (see schematic). Assume that deformation begins at zone A. Deformation heating causes a reduction in strength that results in more deformation than in the surrounding material. The increased deformation in zone A creates more heat—and the cycle repeats. The result is an ever-increasing peak strain accom- panied by an increasing localized thin- ning at zone A (to satisfy the constancy of volume rule).
Three or four decades ago, Dr. Har- mon Nine of GM Research conducted thermal imaging studies of steel stretched over a 4-in.-dia. hemispheri- cal punch. Shortly after onset of defor-
THE SCIENCE OF FORMING Deformation Heating—Friend or Foe?
Stamping Location
Zone A
22 METALFORMING / FEBRUARY 2010
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Schematic of a strain gradient showing both an increase in strain magnitude and localization with increasing deformation.
Percent Major Strain