PHYSICAL PROPERTI ES
I
295
B. THERMAL EXPANSIO ... The coefficient of thermal expansion o£
iron depends primarily on the matrix structure, although it is influenced
to a minor ext ent by temperature and graphite structure and slightly ( with
the exception of a high nickel iron ) by ch anges in composition. For gray
iron, this coeffi cient is in the range of 5.6 to 7.0 X I0- 6 per degree Fahrenheit or 10.1 to 12.6 p er degree Centigrade. The influence of temperature
on the coefficient of thermal expansion for two pearlitic gray irons is shown
in Table II. The thermal expansion of irons with ferritic and martensitic
matrices is usually slightly higher than for pearlitic irons2 •6 . Austenitic
gray iron h as a considerably high er thermal expansion coefficient of about
10 X I0- 6 per degree F ahrenheit or 18 X I0- 6 per degree Centigrade. The
different types of flake graphite do not influence thermal expansion a
.
signi£.cant amount.
TABLE II
Coefficient of Thermal Expansion * for
Two Pearlitic Gray lrons <5 l
Temperature Range
68to212
20to100
68 to 392
20 to 200
68 to 572
20 to 300
68 to 75 2
20 to 400
68 to 932
20 to 500
68 to 1112
20 to 600
68 to 1292
20 to 700
- 13 to +32
- 25 to 0
- 58to - 13
- 50to - 25
- 103to - 58
- 75to - 50
- 148 to - 103 - 100to - 75
Chemical Analys is, Percent
Total Carbon
Combined Carbon
Silicon
Manganese
Nicke l
Chromium
Molybdenum
Copper
Phosphorus
Sulphur
Iron A
Iron B
10.0
10.7
11.9
12.3
13.0
13.3
13.6
5.6
5.9
6.6
6.8
7. 2
7.4
7.6
3.13
0 .80
2.31
0.69
0.12
0.12
none
none
0.67
0.14
* Example : A 1 00-inch long casti ng heated from 70 o
o=x 1 00°f (c hange) X 5.6 X
5.6
6.1
6.6
7.0
7 .3
7.5
7. 6
5.6
5.4
4.8
3. 7
10.0
11.0
11 .9
12.6
13. 1
13 .5
13 .7
10.1
9.7
8.7
6.6
3.02
0.77
1.57
0.87
0.16
296
I
PHYSICAL PROPERTIES
When the low temperature form of iron or alpha iron transforms to
the high temperature form or austenite, a contraction occurs because of
the greater density of the austenite. A corresponding expansion accompanies the transformation of austenite to ferrite. This expansion can contribute to cracking during rapid cooling from high temperatures, as discussed in the heat treatment section. The variation in the coefficient of
thermal expansion with temperature for four gray irons as compared w ith
steel' is shown in Fig. 1. At low temperatures, 12 to 68 degrees Fahrenheit,
the thermal expansion of a four percent carbon gray iron is reported 1 to
be 4.7 X 1Q-6 per degree Fahrenheit; a value appreciably lower than at
the higher temperature ranges .
The influence of composition on the thermal expansion coefficient is
relatively minor except for large percentages of nickel. Very low thermal
expansions can be obtained with high nickel cast irons of the Ni-Resist
type. This expansion can either be minimized or controlled to match other
metals of different characteristics 8 . By varying the composition, irons with
coefficients from 2.2 to 10.6 X 10- 6 per degree Fahrenheit can be produced.
The coefficient of expansion as a function of nickel content is illustrated
10
I
0
X
u. 6.6
0
(i;
(l_
6.4
I
c
0 6.2
·o;
f----+--- +----+--D""'--+-----+-----i
c
Fig. 1
Influence of temperatu re
on the coefficient of
therma l expansion of
steel and four different
gray irons of
compositions in the
following Table7 •
"'
w
6.0 1----+-----b-"'7''----+----.--t---:;;;.,....,'---+-----l
15
-8
o.
c
;gQ;"'
'r-
5.6 1-----::;.......'-----:;;;....."----+-'---+----+-----i
0
u 5.4
L __ __,___ __,___ __ . __ __,___ __ , __ ___,
100
200
400
300
500
Temperature - ° F.
Analysis of Gray Irons Shown in Figure 1
Chemical Analysis, Percent
Iron No.
TC
Si
Mn
p
s
Mo
1
2
3
4
3.25
3.32
3.26
3.14
2.22
1.93
1.89
1.99
0.59
0.44
0.43
0.40
0.093
0.127
0.116
0.126
0.091
0.065
0.064
0.080
0.37
0.73
Ni
1.41
PH YSICA L PROPERTIES
I
297
:::.
-..:::
-
2
Fig. 2
Effect of nickel content and
temperature on the coefficie nt
of thermal expansion of
gray iron•.
4
0
20
40
Nickel Content - Percent
60
in Fig. 2 for three different temperatures 8 . Silicon, aluminum, and copper
increase the expansion coefficient slightly in both pearlitic and ferritic gray
irons 6 •9 . Chromium raises the thermal expansion coefficient in a pearlitic
matrix iron somewhat when added in amounts up to 0.5%, but in larger
amounts has little additional influence. Small additions of nickel have no
appreciable effect6 •
The thermal expansion of high silicon irons at room temperature is
reported as 6.0 to 7.5 X 1Q-6 p er degree Fahrenheit depending on their
composition. The white irons, both unalloyed and the chromium and molybdenum alloy types, fall within the range of 4.5 to 5.5 X 10-6 per degree
Fahrenheit reflecting the reduced expansion of carbides . These values also
increase somewhat at higher temperatures 1 . As mentioned above, the austenitic irons such as the Ni-Resist have an appreciably higher thermal expansion value of 10.2 to 10.7 X 1Q- 6 per degree Fahrenheit or 18.3 to
19.3 X 10- 6 per degree Centigrade.
The thermal expansion of the unalloyed and alloy ductile irons is similar to the gray irons. The effect of temperature on the thermal expansion
coefficient for a group of irons including gray, ductile, and those with mixed
structures 10 is illustrated in Fig. 3. The alloy ductile irons are influenced
by composition changes in a similar way as the gray iron. Low expansion,
high nickel ductile iron is produced with an expansion coefficient as low
as 2.2' X 10-s per degree Fahrenheit. The data in Table III indicate some
typical values.
C . SPECIFIC HEAT .. . The specific heat of gray and ductile irons is
about 0. 13 BTU per pound per degree Fahrenheit or calories p er gram
per degree Centigrade for gray iron near room temperature and increases
298
I
PHYSICAL PROPERTI ES
TABLE Il l
Coeffic ie nt o f Th ermal Expansion for
Ductil e Iro n p e r Degree x 1 Q- 6 (11 , 12. 13)
Temperature
Ferritic
Pearli tic
A usten itic
Range
60 -45- 10
80-60 -03
20 -26% Ni
68 - 212
68- 39 2
68 - 572
68- 752
68- 932
68 -111 2
68 -1292
20 -100
20- 200
20 -300
20-400
20-500
20-600
20-700
6.4
6.5 -6.6
11 .5
11 .7- 11 .8
7 .5
13 .5
6.4
6.6-7 .0
7.0
7.3
7.4
7.5
7.7
11 .5
11.8 -1 2.6
1 2.6
13.2
13.4
13 .5
13.8
4- 19
2.2 -10. 5
2o r----r----, ---- ,----, ----,,----,----, ----,
0
Q;
a...
I
Fig. 3
Influence of tempe rat ure
on t he coeffici ent of
t herma l expansion of the
gray, ducti le, and mixed
structure i rons10 as
li sted in the fo llowing
table.
c
"'u
"'0
u
15
c
0
'iii
c.
X
w
400
0
800
Temp erat ure- ° F.
1200
1600
.19
1..1...
,x
0
-.......
..ci .1 7
Fig. 4
Effect of t emperature on
th e speci fic heat of
several irons as listed in
the fo llowi ng Tab le1o.
/ X
:::::.
:::>
iii
X
,/
x""' 1
I
"' .1s
:r:"'
_,
u
:.::
/ x"
·c::;
"' .13
c.
(f)
-" .•/.-<!
" ..,...
;
a
-c....... . . .......
X
.11
o I......
x' 'k
_,
• -od:
. ,. x ,
.--:: .......
-•,-: ; •
'-
X
0
•
- ,
tJ
l·
J....
e/
fj.c>- l.O
•6- <:t
o•oo
p•
,..ci
z
o..,.a'
c
e
'
-'
1
400
800
1200
Temperatu re- ° F.
(")
N
1600