Page 62 - MetalForming October 2010

Tooling Technology
THE SCIENCE OF FORMING
n-Value Q&A
Long-term readers of The Science of Forming column and attendees at PMA metalforming seminars are acutely aware of the emphasis given to the workhardening of metals and the workhardening exponent or n-value. This exposure to n-value generates a variety of questions about its impor- tance, measurement, source, applica- tion and other sometimes fine details. A number of these issues are reviewed this month. They provide a quick check- list to those very familiar with the n- value, and an introduction for those less active in the field.
• Why is n-value so important?
Excessive stretching leads to local neck- ing and tearing of the stamping. The n- value is the one property of sheetmetal that helps the most in evaluating its relative stretchability.
• Is n-value a super-property that combines different material proper- ties to assess formability? An example might be:
n = (T.S. – Y.S.) x Total elongation Hardness number
I have never heard this type of ques- tion asked before last month. Perhaps the person was familiar with equations such as that used to calculate carbon equivalent (CE) used to predict the rel- ative difficulty of welding.
CE = %C + (%Mn + %Si)/6 + etc.
Fortunately, the n-value is not that complicated, but is a primary metal property measured directly during a tensile test.
• What physically is the n-value?
When metal alloys are cold worked, their yield strength increases. The n- value is the amount of strengthening for each increment of straining. The high- er the n-value, the steeper the stress- strain curve in the uniform elongation
STUART KEELER
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
region of the tensile test.
• How much additional tensile test- ing is required to obtain the n-value? Usually none. If the property data are acquired automatically during a tensile test and processed by computer soft- ware, most of the new property calcu- lation codes automatically compute n-value. The n-value is measured over a strain range from 10 to 20 percent, 7 to 15 percent or 10 percent to UTS. The n-value will print with the rest of the data if the n-value switch is turned on.
• Our tensile test machine does not compute the n-value. Is there some other way of estimating n-value? There is a complex equation based on the ten- sile strength/yield strength ratio. If the exact value of n is not needed, two or more material samples can be com- pared using just the TS/YS ratio if none of the samples have any yield point elongation (YPE). A higher ratio means more stretchability.
• Some higher strength steel speci- fications require a spread of 10,000 or 20,000 PSI between the yield strength and tensile strength. Is this related to the n-value? Absolutely. The greater the spread between the yield and tensile strengths, the steeper will be the stress-strain curve and the higher the n-value. Likewise, the TS/YS ratio will increase with the increased spread. This shows the steel with the 20,000 PSI spread will have more stretchability than the steel with only a 10,000 PSI spread.
• We order by yield strength and only that value is reported to us from our supplier. Is there a way of obtain- ing the n-value from only that single piece of data? For low-carbon steels that have a single microstructure of
60 METALFORMING / OCTOBER 2010
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