Page 58 - MetalForming October 2009

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
The explanation of why work harden- ing of most metal alloys is required for stretchability and bendability was discussed last month. One key role of work hardening is to minimize the development of strain gradients, which are highly localized peak tensile strains accompanied by localized sheetmetal thinning. A later column will show that increased work hardening usually increases the maximum allowable stretchability defined by the forming limit diagram. Work hardening is a very powerful contributor when forming in the common stretching mode.
Unfortunately, the term work hard- ening was coined many decades ago when the only press-shop measure of formability was the hardness test. The more the material was worked, the hard- er it became—thus the term work hard- ening. However, our understanding today of sheetmetal characteristics indi- cates that hardness relates primarily to the resistance to wear. The real benefit of deformation is an increase in strength of the material. Therefore, deforma- tion really creates work strengthening. Unfortunately, to make that change in terminology would be more difficult then convincing press shops to substi- tute a series of tensile-test properties for current hardness readings.
One can bypass the work hardening/ work strengthening controversy by uti- lizing the numeric value of work hard- ening—the work-hardening exponent or n-value. The n-value is a measure of the slope of the stress-strain curve between the yield strength and the ten- sile strength. A steeper slope means more work hardening and a higher n- value. For low-carbon steels, n-value is nearly constant over the range from 10-percent strain to the end of uni-
form elongation (strain at the load max- imum or tensile strength). This strain range and the 10- to 20-percent strain range are traditionally used by auto- matic tensile test computers to fit a straight regression line to determine the n-value. In addition, a quick quali- tative n-value comparison between two samples can be made by comparing either the tensile strength/yield strength ratio or the uniform elongation. High- er ratios or elongations mean higher n-values. In contrast, many aluminum and brass alloys have an n-value that decreases with strain. Therefore, a reported n-value of 0.30 (0.18) means an initial n-value of 0.30 and an average n-value of 0.18.
Unfortunately, most low-carbon steels and many other alloys have n-values
56 METALFORMING / OCTOBER 2009
www.metalformingmagazine.com
THE SCIENCE OF FORMING
More Understanding About Work Hardening
STUART KEELER
0.3
0.2
0.1
HSLA
0
0 50 100
Yield Stress (Ksi)
Fig. 1—The n-value decreases as yield strength increases for lightly temper- passed low-carbon steels. Shown is the range of properties for three common types of steel.
n-value
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