Cold rolling
Published:2013-05-02 04:47:08    Text Size:【BIG】【MEDIUM】【SMALL
Summary:Rolling mill for cold rolling metal sheet like this piece of brass sheet.

Cold rolling is a metalworking process in which metal is deformed by passing it through rollers at a temperature below its recrystallization temperature. Cold rolling increases the yield strength and hardness of a metal by introducing defects into the metal's crystal structure. These defects prevent further slip and can reduce the grain size of the metal, resulting in Hall-Petch hardening.

Cold rolling is most often used to decrease the thickness of plate and sheet metal.

 Physical metallurgy of cold rolling

Cold rolling is a method of cold working a metal. When a metal is cold worked, microscopic defects are nucleated throughout the deformed area. These defects can be either point defects (a vacancy on the crystal lattice) or a line defect (an extra half plane of atoms jammed in a crystal). As defects accumulate through deformation, it becomes increasingly more difficult for slip, or the movement of defects, to occur. This results in a hardening of the metal.

If enough grains split apart, a grain may split into two or more grains in order to minimize the strain energy of the system. When large grains split into smaller grains, the alloy hardens as a result of the Hall-Petch relationship. If cold work is continued, the hardened metal may fracture.

During cold rolling, metal absorbs a great deal of energy. Some of this energy is used to nucleate and move defects (and subsequently deform the metal). The remainder of the energy is released as heat.

While cold rolling increases the hardness and strength of a metal, it also results in a large decrease in ductility. Thus metals strengthened by cold rolling are more sensitive to the presence of cracks and are prone to brittle fracture.

A metal that has been hardened by cold rolling can be softened by annealing. Annealing will relieve stresses, allow grain growth, and restore the original properties of the alloy. Ductility is also restored by annealing. Thus, after annealing, the metal may be further cold rolled without fracturing.

 Degree of cold work

Cold rolled metal is given a rating based on the degree it was cold worked. "Skin-rolled" metal undergoes the least rolling, being compressed only 0.5-1% to harden the surface of the metal and make it more easily workable for later processes. Higher ratings are "quarter hard," "half hard" and "full hard"; in the last of these, the thickness of the metal is reduced by 50%.

 Cold rolling as a manufacturing process

Cold rolling is a common manufacturing process. It is often used to form sheet metal. Beverage cans are closed by rolling, and steel food cans are strengthened by rolling ribs into their sides. Rolling mills are commonly used to precisely reduce the thickness of strip and sheet metals.

Foil rolling

Foil rolling is a continuous deformation process compressing metal between a pair of rollers called work rolls.[1]

Foil is produced for several applications:

  • Thermal insulation for the construction industry
  • Fin stock for air conditioners
  • Electrical coils for transformers
  • Capacitors for radios and televisions
  • Insulation for storage tanks
  • Decorative products
  • Containers and packaging

Foil stock is reduced in thickness by a rolling mill, where the material is passed several times through metal work rolls. As the sheets of metal pass through the rolls, they are squeezed thinner and extruded through the gap between the rolls. The work rolls are paired with heavier rolls called backup rolls, which apply pressure to help maintain the stability of the work rolls. The work and backup rolls rotate in opposite directions. As the foil sheets come through the rollers, they are trimmed and slitted with circular or razor-like knives installed on the rolling mill. Trimming refers to the edges of the foil, while slitting involves cutting it into several sheets.[2]

Aluminium alloys are most commonly produced in the foil rolling process because the raw materials necessary for its manufacture are abundant. Aluminium foil is inexpensive, durable, non-toxic, and greaseproof. Iron, silicon, and manganese are all major alloying elements. Sheet metals with a thickness below 200 micrometers are considered foils (Some foils may be as thin as 6.3 micrometers).[3]

Foil rolling is a commonly used method of metal compression. Ingots cast from molten metal begin the thinning process as they are set between a series of rollers with one set rolling across the top and one set rolling across the bottom. These ingots are handled by forklifts due to their extreme weight. Small work rolls are supported by larger rolls that apply pressure, helping to maintain the work rolls. Each set of rollers rotates in opposite directions. The intensity is carefully monitored by technicians who adjust the pressure as necessary. The ingots pass through the rollers back and forth several times, becoming slimmer with each run. [4]

In many cases the rollers are heated to assist in the workability of the metal. The temperature of the rollers usually sits around 455 to 540 degrees Celsius. To keep the ingots from sticking to the rollers everything is flushed often with a cooling fluid composed of 95% water, 5% oil. [5] According to final product specifications the metal deformation can be adjusted by changing the rpm of the rolls, the roller resistance and the temperature of the rollers. [6] Certain personal hazards should be identified such as moving work pieces, scorching rollers and metal, sharp razors and heavy materials.

Foil rolling is used in the manufacturing of many different products including but not limited to:

- aluminum foil
- candy wrappers
- art and decoration
- insulation
- packaging materials[7]

Once the sheets of metal have been pressed thinly enough they are trimmed and cut to appropriate size.

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