47.2 SHEET ROLLING

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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,

Sheets - up to 1/4" thick

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

Rods

Bars

Beams

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,

- waviness

- 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