Temper Pass or Skin Pass Rolling
Material can be reduced accurately while skin passing. Hot rolled steel coming from a steel mill or pickle line may need to be reduced 1% to 2% in thickness. This can be done on a coil-to-coil temper pass mill or in line on a cut-to length line. Many automobile and truck blanks made from hot rolled steel are from laser and plasma burnouts. After the material is temper passed 1% to 2% and leveled on a backed up corrective leveler, the material will lay out flat and hold flat after plasma or laser cutting. It is important to have the skin passed material delivered to the laser or plasma burnout operation because while it is cutting, the material will not bend upward and destroy the plasma torch or laser head during high-speed cutting.
A temper mill is also used to make cold rolled after annealing to assure a consistent quality. After the material is made hot rolled, it needs to be pickled and cold reduced. Cold reducing increases the yield strength and the material can be too hard to be used for any type of deep draw work or accurate bending. Therefore, most cold rolled after reducing must be annealed. The annealed product can be too soft for the next operation. Above all, when uncoiling without tension, very soft material such as AISI 1008 can create coil breaks.
Temper passing or skin passing material will eliminate coil breaks (hairline wrinkles in the surface of the soft metal). Once there are wrinkles in the material, they are difficult to remove. Skin passing eliminates wrinkles or coil breaks. Coil breaks can show up on low carbon, hot rolled, as well as any soft material. When you skin pass, you are only increasing the yield strength of the surface. You have not changed the physical properties inside of the core of the metal. Skin passing can be done on any alloy carbon steel including aluminum. Skin passing and temper passing improves shape as well. Once steel is annealed, distortion can be a problem. Many steel coils are substantially improved by skin passing.
Skin passing can also improve surface finish and accuracy. Some materials have very close tolerances of ± .0002”. The final pass may be considered a skin pass to hold the tolerance of precision cold rolled material.
