• While rolling a sheet the rollers will be under significant forces. This will lead to deflections at the centers of the rollers. To overcome this rollers are made with a slight barrel shape. therefore when rolling the deformed rollers will take the desired shape.
• When rolling sheets have a tendency to spread. This means that the width of the sheet increases when it is rolled.
• Input and output materials in rolling are,
Plates - between 1/4” to 12” thick
Billet - a square cross section of 6” or less per side
Bloom - a square cross section of 6” or more per side
• When rolling the material may be processed the following ways,
hot rolling is done above the recrystallization temperature (850°F for Al, and 1250°F for steel) and results in a fine grained wrought structure. The surface quality (500-1000 micro in.) and final dimensions are less accurate.
cold rolling is done near room temperature and produces better surface finishes (32-125 micro in.) and dimensional accuracy (0.004”-0.014”), but with strain hardening.
pack rolling involves rolling multiple sheets of material at once, such as aluminum foil.
• Defects in flat rolling include,
- tearing on the sides (edge), or in the middle (zipper), or between the top and bottom faces (alligatoring)
- spalling is cracking or flaking of surface layers results when improper material used in hot rolling
- heat checking is cracking caused by thermal cycling this results when improper material used in hot rolling
• Residual stresses are also built up in rolled materials. The two possible variations are,
tension outside, compression inside - the result of large rollers, or high reductions
compression outside, tension inside - the result of small rollers or small reductions per pass
• In commercial rolling mills some techniques are used,
two-high, three high, four-high, cluster mills - multiple rollers can be used to increase the stiffness of the contact rollers.
tandem rolling - a number of rollers are used in series. Each point reduces the material thickness a step.
lubricants - used with cold rolling
coolants - used with hot rolling to cool the rolls and break up scale