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Rolling (metalworking)
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Rolling is a fabricating process in which the metal, plastic, paper, glass, etc. is passed through a pair (or pairs) of rolls. There are two types of rolling process, flat and profile rolling.
In flat rolling the final shape of the product is either classed as sheet (typically thickness less than 3 mm, also called "strip") or plate (typically thickness more than 3 mm).
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Rolling is a fabricating process in which the metal, plastic, paper, glass, etc. is passed through a pair (or pairs) of rolls. There are two types of rolling process, flat and profile rolling.
In flat rolling the final shape of the product is either classed as sheet (typically thickness less than 3 mm, also called "strip") or plate (typically thickness more than 3 mm). In profile rolling the final product may be a round rod or other shaped bar, such as a structural section (beam, channel, joist etc).
Rolling is also classified according to the temperature of the metal rolled. 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.
Another process also termed as 'hot bending' is induction bending, whereby the section is heated in small sections and dragged into a required radius.
Heavy plates tend to be formed using a press process, which is termed forming, rather than rolling.
Hot Rolling
Hot rolling is a hot working metalworking process where large pieces of metal, such as slabs or billets, are heated above their recrystallization temperature and then deformed between rollers to form thinner cross sections. Hot rolling produces thinner cross sections than cold rolling processes with the same number of stages. Hot rolling, due to recrystallization, will reduce the average grain size of a metal while maintaining an equiaxed microstructure where as cold rolling will produce a hardened microstructure.
Hot Rolling Process
A slab or billet is passed or deformed between a set of work rolls and the temperature of the metal is generally above its recrystallization temperature, as opposed to cold rolling, which takes place below this temperature. Hot rolling permits large deformations of the metal to be achieved with a low number of rolling cycles. As the rolling process breaks up the grains, they recrystallize maintaining an equiaxed structure and preventing the metal from hardening. Hot rolled material typically does not require annealing and the high temperature will prevent residual stress from accumulating in the material resulting better dimensional stability than cold worked materials.
Hot rolling is primarily concerned with manipulating material shape and geometry rather than mechanical properties. This is achieved by heating a component or material to its upper critical temperature and then applying controlled load which forms the material to a desired specification or size.
Hot Rolling Applications
Hot rolling is used mainly to produce sheet metal or simple cross sections such as rail road bars from billets.
Mechanical properties of the material in its final 'as-rolled' form are a function of:
- material chemistry,
- reheat temperature,
- rate of temperature decrease during deformation,
- rate of deformation,
- heat of deformation,
- total reduction,
- recovery time,
- recrystallisation time, and
- subsequent rate of cooling after deformation.
Types of Hot Rolling Mills
Prior to continuous casting technology, ingots were rolled to approximately thick in a slab or bloom mill. Blooms have a nominal square cross section, whereas slabs are rectangular in cross section.
Slabs are the feed material for hot strip mills or plate mills and blooms are rolled to billets in a billet mill or large sections in a structural mill.
The output from a strip mill is coiled and, subsequently, used as the feed for a cold rolling mill or used directly by fabricators. Billets, for re-rolling, are subsequently rolled in either a merchant, bar or rod mill.
Merchant or bar mills produce a variety of shaped products such as angles, channels, beams, rounds (long or coiled) and hexagons. Rounds less than in diameter are more efficiently rolled from billet in a rod mill.
Cold rolling
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. Another term for this process is called plate roll bending. This is defined as a cold forming process where plate and steel metal is formed into cylindrical shapes by a combination of three rolls arranged in a pyramid formation. Two of the rolls are power driven, in a fixed position, and the third is adjustable to suit the desired bend radius and workpiece thickness.
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.
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).
Plate Roll Bending
"Plate Roll Bending is a cold forming process. Plate or steel metal is formed into cylindrical shapes by a combination of three rolls arranged in a pyramid formation. Two of the rolls are power driven, in a fixed position, and the third is adjustable to suit the desired bend radius and workpiece thickness."
Workpiece Geometry
"Shapes produced range from simple cylindrically shaped parts to more complex parts, such as conical and flattened from 1/16in to 6in, and lengths of up to 20in, or more, can be formed by this method."
Progressive Roll Forming
Roll forming, also spelled rollforming, is a continuous bending operation in which a long strip of metal (typically coiled steel) is passed through consecutive sets of rolls, or stands, each performing only an incremental part of the bend, until the desired cross-section profile is obtained. Roll forming is ideal for producing parts with long lengths or in large quantities.
Geometric Possibilities
The geometric possibilities can be very broad and even include enclosed shapes so long as it is the same cross-section throughout. Typical sheeting thicknesses range from 0.025in. to 0.25in., but they can exceed that. Length is almost unaffected by the rolling process. The part widths typically aren't smaller than 1in. however they can exceed 20in.
See also
External links
Bibliography
- Reed-Hill, Robert, Et. Al. "Physical Metallurgy Principles", 3rd Edition, PWS publishing, Boston, 1991. ISBN 978-0534921736.
- Callister Jr., William D., "Materials Science and Engineering - an Introduction", 6th Edition, John Wiley & Sons, New York, Ny, 2003. ISBN 0-471-13576-3
- Todd, Robert H., Allen, Dell K., Alting, Leo, "Manufacturing Processes Reference Guide", 1st Edition, Industrial Press Inc., New York 1994, ISBN 0-8311-3049-0
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