Grey Cast Iron

METAL PROPERTIES, CHAR, USES, AND CODES - OD1643 - LESSON 1/TASK 1

(b) Capabilities.

Cast iron is commonly brazed or bronze

welded, but it can be gas or arc welded, hardened, or machined.

(c) Limitations. Cast iron must be preheated prior to welding;

it cannot be cold-worked.

(d) Grey Cast Iron.

If molten pig iron is permitted to cool

quite slowly, the chemical compound of iron and carbon breaks up to a

certain extent; much of the carbon separates out as tiny flakes of

graphite scattered throughout the metal. This graphitic carbon, so-

called to distinguish it from combined carbon, causes the gray

appearance of a fracture which characterizes ordinary gray cast iron.

Since graphite is an excellent lubricant, and since the metal is

filled with tiny flaky cleavages, it is not difficult to understand

why gray cast iron is so easy to machine, and why it cannot withstand

a heavy shock.

Gray cast iron consists of from 90 to 94 percent

metallic iron with varying proportions of carbon, manganese,

phosphorus, sulfur, and silicon.

Special high-strength grades

contain 0.75 to 1.5 percent nickel and 0.25 to 0.5 percent chromium,

or 0.25 to 1.25 percent molybdenum. Commercial gray iron has 2.5 to

4.5 percent carbon. Of this quantity, about 1 percent of the carbon

is combined with the iron, while about 2.75 percent remains in the

free or graphitic state.

In the production of gray cast iron,

silicon content is usually increased, since this facilitates the

formation of graphitic carbon.

The combined carbon (iron carbide),

which is a small percentage of the total carbon present in cast iron,

is known as cementite. In general, the more free (graphitic) carbon

present in cast iron, the lower the combined carbon content and the

softer the iron.

(e) White Cast Iron.

When gray cast iron is heated to the

molten state, the carbon completely dissolves in the iron.

If this

molten metal is cooled quickly, the two elements remain in the

combined state, and white cast iron is formed.

The carbon in this

type of iron is generally from 2.5 to 4.5 percent by weight and is

referred to as combined carbon.

White cast iron is very hard and

brittle, often impossible to machine, and has a silvery white

fracture surface.

(f) M a l l e a b l e C a s t I r o n .

Malleable cast iron

is

made by heating white cast iron to between

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