The Science of Forming
Correct Terminology Helps Troubleshooting
Slang, jargon, ancient expressions, personal opinions and press-shop war stories are not beneficial when attempting to troubleshoot problems. Some terms may be meaningful to one person but cause another to completely misunderstand the problem. An analogy: Texting abbreviations that are well known to teenagers but appear to be a foreign language to others.
In the workplace, use of incorrect terminology can waste excessive and often critical time as employees try to uncover root causes, increase production speed or improve product quality. The following case studies highlight these problems.
1) “The steel stamping is beginning to strain.” A visually critical area of the stamping surface shows a series of narrow lines or depressions. If this area was just beginning to strain, Lüder’s lines or yield-point elongation in the incoming steel would be high on the probable-cause list. These Lüder’s lines initiate at very low strain. However, ultrasonic thickness measurements show high thinning in that area, indicating that Lüder’s lines are not a probable cause. The next step: Grid a stamping to allow measurement and plotting of the two surface strains (50 percent major and 35 percent minor) on the forming-limit diagram. The strains are on the edge of deformation cliff (zero safety margin). The surface markings are shear bands associated with the onset of the localized thickness neck—the precursor to failure. Some companies in Asia use surface-roughness measurements to detect these lines and use them as red flags highlighting incipient failure sites. Seeing these surface marks, one should have said, “The stamping is beginning to fail.” This requires completely different troubleshooting solutions.
A critical amount of cold work can trigger formation of a few extremely large sheetmetal grains that cause early failure during forming.
3) “Some grades of the new advanced high-strength steels (AHSS) are so hard that we can only form them once, because they become brittle and fracture in the next die.” Calling these grades “hard” sends the wrong message. While hardness is an excellent measure of a material’s or die surface’s resistance to wear, it does not describe formability, strength does. Strength is a measure of resistance to deformation. While hardness generally increases as strength increases, the various mechanical properties controlling formability correlate to material strength. Some AHSS are designed to be more formable than HSLA steels of the same strength. Others have the same formability as HSLA steels but at a higher strength. Forming any steel creates work hardening that further increases its strength. Within formability limits, the sheet steel still can undergo additional forming. However, a greater force is required to generate the next forming operation. This higher force often causes increased deformation in other areas of the stamping and not at the desired location. Eventually the useful formability is exhausted and the stamping undergoes local necking and fracture. However, when these steels fail, the failure mode is still ductile, not brittle.
4) “We interstage anneal the stamping to increase toughness and reduce brittleness.” The purpose of annealing sheetmetal is to generate the correct microstructure from work-hardened material. The mill cold rolls the steel to attain the correct thickness. The resulting steel grains are small, elongated in the rolling direction and strong. The annealing process changes the cold-worked grains back to lower-strength equiaxial grains of the proper size to achieve the required ductility needed for forming. Toughness and brittleness are not typical goals of the annealing process.
Interstage annealing of stampings must be done with good metallurgical knowledge and extreme care. Annealing activates the crystallographic renewal when the energy of work hardening combines with the energy of the heat to reach the critical energy level needed for the transformation. The steel mill knows exactly the amount of cold work in the coil and annealing temperature to create the ideal microstructure.
However, attempts to anneal a formed stamping in the press shop must overcome one major obstacle (see graph). The amount of cold work generally varies from near zero in flat areas to excessive in highly deformed locations. When the stamping reaches the annealing temperature, some areas with low work hardening will not change, while areas with high work hardening will have sufficient energy to duplicate the annealing process at the steel mill. Somewhere in between these two extremes, an area will exist in the stamping with just the minimum amount of work hardening needed to create a new grain structure. At that location a few grains will grow to extreme size. The material surface will be grainy and rough, and the grains will only tolerate a small amount of deformation before failure.Annealing partially formed stampings creates a high risk of failure. In contrast, sometimes heavily worked, very thick stampings are given interstage anneals to reduce the strength of the intermediate stamping. The problem here is not to increase formability but to reduce the press load so that the press will not stall during forming. MF
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