The Science of Forming
Things That Bother Me
This month and next, I am going to deviate from my own formal writing style to become part of my column. Why? This month is column number 119 and December is number 120, which allows me to celebrate 10 short years of writing Science of Forming. I have enjoyed writing every single column because the editors of MetalForming have given me complete freedom to write about anything and everything.
Over the many years in this field, I have kept a log of Things That Bother Me. The log contains statements about sheetmetal forming made by seminar speakers and classroom lecturers, sentences in technical papers, and comments made in magazine articles or ads that make no sense, are completely wrong or show a lack of understanding by their creator. The log now has 81 entries. If I really was vigilant in searching for entries, the log easily could contain hundreds of entries. This month and next, I will share some of the more interesting log entries. For better reading, I have grouped entries that address the same general topic. While a few entries are humorous, all are instructive and their topics worth studying.
Cold Working, Strain Hardening and Work Hardening
• “Cold working causes the steel to become harder and brittle.”
• “Work hardening coefficient (n-value) describes how the metal becomes harder.”
• “Batch annealing steel: Metal that has been heated and slowly cooled as batches of coils to reduce brittleness and toughen the steel.”
All these phrases contain an incorrect concept, that cold working makes the steel harder and more brittle. Metalforming creates the shape established by the part designer through deformation of a blank. Most commonly achieved through cold working, this deformation increases the strength of most alloys. The ability of sheetmetal to undergo these large amounts of stretch is dependent on the strain or work-hardening capacity of the material as measured by the n-value.
In contrast, hardness is neither a measure nor predictor of formability. Hardness indicates the wear resistance of dies, coatings and surface hardening. Yes, there usually is an increase in hardness with increasing strength, but that is not the normal goal of cold working.
Likewise, strengthening does not mean that an alloy becomes more brittle. With few exceptions, sheetmetal undergoes stretching, then local necking and finally failure. Examination of the failure surface reveals a cup-and-cone type of deformation seen in ductile fractures. The material may have a very high strength and a low amount of stretchability, but the final fracture mode is not brittle.
• “With AHSS (advanced high strength steel), you must create all of the part shape in the first die because the metal hardens and gets brittle. You can not restrike the part.”
Some of the AHSS undergo significantly greater work hardening than the traditional HSLA (high-strength low-alloy) steels for the same amount of strain. This means that a restrike of the part will require a much higher force that often creates tears in other locations of the stamping—especially over tight punch radii or sharp character lines. However, the AHSS has not become brittle. As long as the total strain is below the forming limit, one can restrike the part.
• “I do not want my steel to work-harden because it stalls my press.”
The solution to the stalled press is proper sizing of the press to the job—not zero work hardening. With zero work hardening, almost all parts requiring stretching or bending will fail with little increase in length of line.
The root cause of this major misunderstanding is the improper terms used. ‘Work hardening’ and ‘strain hardening’ naturally lead the metalformer to think in terms of hardness and, subsequently, brittle behavior. To correct this problem, better terms are ‘work strengthening’ and ‘strain strengthening’ with the strengthening exponent still being the n-value. The switching of terms would be easy. The difficult task is erasing a century of misuse.
• “Strain-rate hardening coefficient (m-value) shows how much harder the metal becomes as the deformation speed increases.”
An increase in deformation speed can affect the yield and tensile strengths in two different ways. For most steels, the m-value is positive, which means that the yield and tensile strengths increase with increased deformation speed. It certainly does not increase the hardness. Some alloys of aluminum and other nonferrous metals have a negative m-value. Now the yield and tensile strengths decrease with an increase in deformation speed. Why would one expect the material to decrease in hardness (become softer) as forming speed increases?
• “I can make good parts at 12 strokes/min. but not at 16 strokes/min. The problem is the change in material properties with speed.”
Measurements have shown that the increase in yield and tensile strengths for low-strength steels is about 5 ksi for each 10-fold increase in deformation speed. The n-value does not change with deformation speed. So an increase from 12 to 16 strokes/min. (33 percent increase) would proportionally increase the yield strength only about 0.2 ksi. Recent studies with AHSS have shown the speed effect to be even less for these steels. However, the heat generated for a speed increase from 12 to 16 strokes/min. would grow by 33 percent and could cause major differences in lubricant viscosity, coefficients of friction and binder flow patterns.
Next month’s column continues with more Things That Bother Me. MF
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