Things That Bother MeNovember 1, 2008
A technical writer normally avoids describing what the writer has accomplished and let’s the part, die, press, material or even unidentified forming personnel tell their story through graphs, tables, photographs and a story line. A technical article that begins with “I arrived at the plant, looked at the stamping and immediately knew how to solve the problem” does not convey any description of the problem or useful data from which one can learn how things work, or more accurately, why things do not work. The writer must be the storyteller, not the main character of the story.
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.