Casting

sectional dimensions are less than 6 x 6 inches.

Rectangular

sections that have widths greater than twice their thicknesses are

called slabs.

The slab is the intermediate shape from which sheets

are rolled.

CASTING

Pouring a molten metal, or mixture of molten metals, into a mold

where it is allowed to solidify is called casting.

Two types of

molds are used: single-purpose and permanent molds.

Single-purpose

molds have to be specially prepared, sometimes by machines, from

patterns.

Using metal or permanent molds offers many advantages

where the process is applicable.

For example, the constantly

repeated cost of sand molding is eliminated; but the initial cost of

the metal mold or die is high. The cost is justified only when the

same casting is required in great numbers. However, for many metals,

the metal mold has a distinct advantage: the structure and strength

that result from the relatively rapid solidification in a metal or

chill mold.

For the nonferrous metals, chill casting is quite

practical, especially with some aluminum alloys.

HOT ROLLING

Blooms, billets, or slabs are heated above the critical range and

rolled into a variety of shapes of uniform cross section.

The more

common of these rolled shapes are sheet bars, channels, angles, and

I-beams.

Sheet bar and rods rolled from steel are used extensively

in aircraft work. Hot-rolled material is frequently finished by cold

rolling or drawing to obtain accurate finish dimensions and a bright,

smooth surface.

FORGING

Complicated sections that cannot be rolled or sections of which only

a small quantity is required are usually forged.

Forging is an

important hot-forming process. It is used in producing components of

all shapes and sizes from quite small items to large units weighing

several tons.

The metal, preheated to the appropriate forging

temperature, is shaped mainly by upsetting (compressive deformation)

between impact surfaces or pressure surfaces, that is, by hammering

or pressing the heated metal until the desired shape is obtained.

Hammering can be used only on relatively small pieces.

Because

hammering transmits its force almost instantly, its effect is

limited to a small depth. Therefore, it is necessary to

use a very heavy hammer or to subject the part to repeated blows

to ensure complete working of the section. If the force applied

is too weak to reach the center, the finished forged surface will

be convex or bulged. The advantage of hammering is that the

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