Page 35 - MetalForming January 2011
P. 35
Tooling Technology
a tensile test? The terms load, stress and strength are used and not hardness. Stretching is tensile elongation of sheetmetal in one or more directions that result in a reduction in the sheet thickness. Bending creates a deformation on the convex surface of the sheetmetal similar to a tensile test specimen.
In contrast, most hardness tests force an indenter of a specific shape and load into the material to create a crater whose diam- eter is then measured. A harder steel creates a smaller crater. While the hardness test does not correlate well with stretching and bending, it is an excellent predictor of wear resistance.
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Why then is increased strength during cold working called strain hardening? The early days of blacksmiths, press shop workers and metallurgists needed to measure how cold working changed the characteristics of the steel. Different types of hardness tests were readily available and easy to use. Therefore, work hardening became the term to use. With the current science of forming, the term really should be work strengthening. Changing the term work hardening probably is not going to happen. However, we should always think work strengthening and forget the hardness test.
• All forming of AHSS must be made in the first hit because the steel work hardens, becomes brittle and fractures.
This statement has two problems. First, designing the AHSS forming process to maximize the amount of deformation in the first die is an excellent goal. However, the statement implies that further deformation in a second or later die is not pos- sible because the steel work hardens too much in the first die. The material can withstand multiple forming operations until the sum of the deformation reaches the forming limit curve. The first problem occurs when the part is removed from the first die. Springback can be excessive and variable, which makes proper fitting into the second die difficult. A much larger force is required to resume deformation in the previously highly deformed zone because of the greatly increased yield strength. Other less deformed areas with lower yield strengths now may initiate unwanted deformation. The forming problems with AHSS are similar to those found with HSLA steels but usually are a much greater magnitude.
The second problem with the statement is the AHSS work hardens and become brittle. The amount of total elongation may be small for some of the higher-strength AHSS (Fig. 1). The local necking alongside the fracture line may be minimal. However, a metallurgical evaluation of the fracture zone will show a cup-and-cone fracture surface of ductile steel. The actual failure mode at the fracture surface determines duc- tile or brittle failure—not the amount of deformation.
Most of the technical literature is accurate and prepared by well-qualified experts. However, errors do occur. The reader has to carefully judge if statements are correct or in error. If the statement is important to your work or project, double checking with one or more sources is always a good invest- ment of time. Then tell your associates about the errors. MF
Stuart Keeler, Pete Ulintz and Ed Tarney will present the Deep Draw seminar on February 16 in Cleveland, OH. Check www.pma.org for this seminar.
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