Page 55 - MetalForming July 2013

reformed because the steel becomes too hard and brittle. While work hardening does increase the hardness of the cold- worked steels, the change in formabil- ity parameters does not correlate with any increase in hardness.
Since the 1930s, the easiest materi- al-property test to perform in the press shop has been the hardness test. In 1940, Professor J. Dudley Jevons, of the University of Sheffield, England, stated in his book The Metallurgy of Deep Drawing and Pressing:
“Hardness and cupping tests are best regarded by suppliers and consumers as an indication of the probability that some particular sequence of mill oper- ations has been carried out, not as an indication of true deep drawing and pressing properties.”
Since then, good correlation between formability and hardness has not been achieved. Actually, the term “work hardening” should be changed to “work strengthening.” We have learned that the hardness test correlates well with the resistance of sheetmetal sur- faces to wear and scoring.
Being required to complete all form- ing in the first hit is a good suggestion, but not because the steel becomes brit- tle and fails. Steel work hardens so much that the areas formed in the first hit are very much stronger than the rest of the stamping. If the deformation in these areas falls below the forming- limit curve, the material’s residual formability can be used. However, attempting to create any additional forming most likely will cause other areas of the stamping, with less defor- mation and less strength, to react to the increased forces—not the desired area. Finally, when failure initiates, the frac- ture mode is not brittle but the usual ductile cup-and-cone.
Another example of outdated press- shop practices: Some shops still obey a maximum press linear-velocity table published in 1939 by E.V. Crane. His table shows a maximum ram velocity for steel of 50 ft./min. for single action, and 35 to 50 ft./min. for double action. Presses run at much higher speeds, yet the same data has been republished in
die handbooks for decades. Going back to the initial study, the onset of galling (a lubrication problem) established the limiting press speed, back when lubri- cation was 15-percent lard oil and 5- percent wool fat in a mineral-oil base.
Today we have specific information on forming strain rates for yield and tensile strengths of higher strength steels. Fig. 1 shows an approximate yield-strength increase of only 3 ksi
over a strain-rate increase of 1000 from tensile-test speed to press speed. This is lower than strain-rate increases for mild steels. Tensile strengths exhibit the same rate increase. The dramatic result is shown in Fig. 2: no reduction in the work-hardening exponent (n-value) over the same strain-rate increase of 10,000. Therefore, the forming-limit curves and gradient control are strain- rate independent. MF
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