Rolling
Classification:
If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling
If the temperature of the metal is below its recrystallization temperature, the process is termed as cold rolling.
Hot Rolling:
Hot rolling is a metalworking process that occurs above the recrystallization temperature of the material. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening. The starting material is usually large pieces of metal, like semi-finished casting products, such as slabs, blooms, and billets. If these products came from a continuous casting operation the products are usually fed directly into the rolling mills at the proper temperature. In smaller operations the material starts at room temperature and must be heated. This is done in a gas- or oil-fired soaking pit for larger workpieces and for smaller workpieces induction heating is used.
While the finished product is of good quality, the surface is covered in mill scale, which is an oxide that forms at high-temperatures. It is usually removed via pickling or the smooth clean surface process, which reveals a smooth surface.[8] Dimensional tolerances are usually 2 to 5% of the overall dimension
Cold Rolling:
Cold rolling occurs with the metal below its recrystallization temperature (usually at room temperature), which increases the strength via strain hardening up to 20%. It also improves the surface finish and holds tighter tolerances. Commonly cold-rolled products include sheets, strips, bars, and rods; these products are usually smaller than the same products that are hot rolled. Because of the smaller size of the workpieces and their greater strength, as compared to hot rolled stock, Cold rolling cannot reduce the thickness of a workpiece as much as hot rolling in a single pass.
Other shapes can be cold-rolled if the cross-section is relatively uniform and the transverse dimension is relatively small; approximately less than 50 mm (2.0 in). This may be a cost-effective alternative to extruding or machining the profile if the volume is in the several tons or more. Cold rolling shapes requires a series of shaping operations, usually along the lines of: sizing, breakdown, roughing, semi-roughing, semi-finishing, and finishing.
Technologies
Technologies