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Producing flat material--that stays flat--for cutting operations: stretcher leveling steel coils relieves trapped internal stresses and prevents harmful springback during laser or plasma cutting jobs.

Article Excerpt
[ILLUSTRATION OMITTED]

All too often steel that appears to be flat doesn't stay that way after parts are cut using laser or plasma technology. This springback is primarily due to the existence of randomly trapped internal stresses. While a flat piece of material may appear to be relaxed and at rest, in reality there is often a "tug-of-war" of epic proportions being waged right before our eyes.

Trapped internal stresses can be introduced in a host of areas. Typically these random stresses are initially created at the mill during the rolling process as some portions of the strip are worked more than others. The outer wraps of the hot coil will cool at different rates than the inner wraps, adding more stresses. Subsequent exposure to temperature changes can also induce additional changes in the material, such as coils that are stored in a heated environment, shipped during the cold winter months and then allowed to warm in the next facility. Because the material expands and contracts at different rates, additional stresses result.

The leveling process itself, while having the ability to produce flat material, can also induce randomly trapped stresses.

Because no one single process or condition is solely responsible, trapped internal stresses and their consequences are inevitable.

While there is a general understanding and acceptance regarding the common principles of leveling, there is considerable debate over how or if one particular type of leveler is really better than another at addressing the problem of trapped internal stresses and producing flat material. Understanding this tug-of-war will ultimately reveal clear reasons why some systems are more effective than others.

Flatness and stress

While flatness is frequently referred to as a side-to-side length...

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