Page 39 - MetalForming February 2015

process of cold working therefore was named workhardening; we know today the correct term should be work strengthening. Unfortu- nately, the term workhardening is so common in our culture that it will remain the phrase of choice.
Today’s knowledge of deforma-
tion teaches us that a pointed
indenter under a standard load
will compress the workpiece sur-
face until a crater forms. We meas-
ure the diameter of the crater and
give it a hardness number. A small-
er crater with a higher hardness
number signifies a stronger sur-
face that resists indentations from
rough dies or in-service applica-
tions. However, in most metal-
forming applications, we see
stretch deformation through the entire thickness of the blank and not just a single surface crater. We measure the capacity for stretch forming using a tensile test, the FLC, stretch-bend and other formability tests.
4) Some metalforming profes- sionals complain that the FLC is flawed, because the actual meas- ured edge stretch before fracture is much less than predicted by the FLC. However, the real problem lies with improper application of the FLC, never intended as a tool for predicting the allowable stretch in sheared edges. The height of the FLC depends on the n-value and thickness of the as-received sheet- metal. Cold working the blank via cutting, shearing or punching greatly reduces the n-value of the edge, and the FLC. Fig. 2 shows a relatively undamaged milled edge with a hole-expansion ratio of 280 percent, and a traditional punched hole with a hole-expansion ratio of
only 80 percent. The severe reduction in hole-expansion ratios can be even more severe for AHSS DP and TRIP steels, because the islands of martensite at the cut edge can initiate
Fig. 2—This hole-expansion study documents the tremendous reduction in edge stretchability for punched holes. R.R. HilsonMicroalloying 75.
earlier fractures.
MF
The Science of Forming
300
200
100
Condition of Punched Hole
A = Milled
B = 10% Clearance Removed C = 10% Clearance
D = 40% Clearance
30 Ksi YS
0
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MetalForming/February 2015 37
Hole Expansion, %