Page 131 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

I I0 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment

4.6 Cast Irons

The term cast iron refers to a family of ferrous alloys composed of iron, carbon
(ranging from 2.11% to about 4.5%), and silicon (up to about 3.5%). Cast irons
are usually classified according to their solidification morphology from the eutectic
temperature (see also Section 12.3):
a. Gray cast iron, or gray iron;
b. Ductile cast iron, nodular cast iron, or spheroidal graphite cast iron,

c. White cast iron;
d. Malleableiron;
e. Compacted graphite iron.
Cast irons are also classified by their structure: ferritic, pearlitic, quenched and
tempered, or austempered.
The equilibrium phase diagram relevant to cast
1600 /¢' irons is shown in Fig. 4.12, in which the right bound-
} Li uid ary is 100% C-that is, pure graphite. The eutectic

A y + iiqui 430% " temperature is 1154°C; thus, cast irons are com-
,' Liquid
pletely liquid at temperatures lower than those re-
gg 1200 _ “MOC / + graphite
E’ y (austenite) 211% quired for liquid steels. Consequently, iron with high
carbon content can be cast (see Part II) at lower tem-
Q 1000
peratures than can steels.
Q y + graphite
Cementite is not completely stable; it is
E 800 0.77% 738°o K metastable, with an extremely low rate of decompo-
sition. It can, however, be made to decompose into
Cl (ferrite) Oz + graphite alpha ferrite and graphite. The formation of graphite
600
(graphitization) can be controlled, promoted, and
O 1 2 3 4 5 |%, 90 100 Graphite accelerated by modifying the composition and the
rate of cooling, and by the addition of silicon.
Composition (%)
Gray Cast Iron. In this structure, graphite exists
FIGURE 4.I2 Phase diagram for the iron-carbon system with largely in the form of flakes (Fig. 4.13a). It is called
graphite (instead of cementite) as the stable phase. Note that gray cast iron, or gray iron, because, when it is
this figure is an extended version of Fig. 4.8. ;>f Kr . 1
/,`5;,_ ,.¢~~~;..;'#~ ‘lil if 1

/ii ff, .ff
/
\ px,-5
y
\
4
lo
Q 0
yfgtw
.. . .i a

G Bw
<4 > if i _ » 9' ~ ` 1 . »
Q - ':s.
==/’ i
r
g .f 1 ... »...</ .', . .. ag,
.< ,
1.
~
If
-~
,
J/ / "~._ w
_
(8) (D) (C)
FIGURE 4.13 Microstructure for cast irons. Magnification: 100><. (a) Ferritic gray iron with
graphite flakes. (b) Ferritic ductile iron (nodular iron), with graphite in nodular form.
(c) Ferritic malleable iron; this cast iron solidified as white cast iron, with the carbon present
as cementite, and was heat treated to graphitize the carbon.