METAL PROPERTIES, CHAR, USES, AND CODES - OD1643 - LESSON 1/TASK 1
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.
molten state, the carbon completely dissolves in the iron.
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
(f) M a l l e a b l e C a s t I r o n .
Malleable cast iron
made by heating white cast iron to between