Copyright © 1998 ASM International®
All rights reserved.
www.asminternational.org
Metals Handbook Desk Edition, Second Edition
J.R. Davis, Editor, p 153-173
Structure/Property
Relationships in Irons and Steels
Bruce L. Bramfitt, Homer Research Laboratories, Bethlehem Steel Corporation
Basis o f Material Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Role o f Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ferrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pearlite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ferrite-Pearl ite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bainite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Martensite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
......................
Austenite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ferrite-Cementite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ferrite-Martensite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ferrite-Austenite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Graphite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cementite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
153
155
156
158
160
162
164
169
170
171
171
172
172
This Section was adapted from Materials 5election and Design, Volume 20, ASM Handbook, 1997,
pages 3 5 7 - 3 8 2 . Additional information can also be found in the Sections on cast irons and steels w h i c h
immediately f o l l o w in this H a n d b o o k and by consulting the index.
THE PROPERTIES of irons and steels are
linked to the chemical composition, processing
path, and resulting microstructure of the material;
this correspondence has been known since the
early part of the twentieth century. For a particular
iron and steel composition, most properties depend
on microstructure. These properties are called
" "o" - grade 50). 2% nital + 4% picral etch. 200x
structure-sensitive properties, for example, yield
strength and hardness. The structure-insensitive
properties, for example, electrical conductivity,
are not discussed in this Section. Processing is a
means to develop and control microstructure, for
example, hot rolling, quenching, and so forth. In
this Section, the role of these factors is described
Fig. :2
in both theoretical and practical terms, with particular focus on the role of microstructure.
Basis of Material Selection
In order to select a material for a particular
c o m p o n e n t , the designer m u s t have an intimate
Microstructu
r e p e a r linterlamellar°f
ite
a typicalspacing.fUllY2%pearlitiC
+ni4%rai
tal steel
l picralShowingetch.
500xthecharacteristic fine
154/Structure/Property Relationships in Irons and Steels
k n o w l e d g e o f w h a t p r o p e r t i e s are r e q u i r e d . C o n s i d e r a t i o n m u s t b e g i v e n to the e n v i r o n m e n t
( c o r r o s i v e , h i g h t e m p e r a t u r e , etc.) a n d h o w the
c o m p o n e n t w i l l be f a b r i c a t e d ( w e l d e d , b o l t e d ,
etc.). O n c e t h e s e p r o p e r t y r e q u i r e m e n t s are est a b l i s h e d the m a t e r i a l s e l e c t i o n p r o c e s s c a n beg i n . S o m e o f the p r o p e r t i e s to b e c o n s i d e r e d
are:
Mechanical properties
Strength
Tensile strength (ultimate
strength)
Yield strength
Compressive strength
Hardness
Toughness
Notch toughness
Fracture toughness
Ductility
Total elongation
Reduction in area
Fatigue resistance
Other properties/
characteristics
Formability
Dmwability
Stretchability
Bendability
Wear resistance
Abrasion resistance
Galling resistance
Sliding wear resistance
Adhesive wear resistance
Machinability
Weldability
Table 1 lists m e c h a n i c a l properties of selected s t e e l s
in v a r i o u s h e a t - t r e a t e d or c o l d - w o r k e d c o n d i t i o n s .
In the s e l e c t i o n p r o c e s s , w h a t is r e q u i r e d for
one a p p l i c a t i o n m a y be t o t a l l y i n a p p r o p r i a t e for
a n o t h e r a p p l i c a t i o n . For e x a m p l e , steel b e a m s for
a r a i l w a y b r i d g e r e q u i r e a t o t a l l y d i f f e r e n t set o f
p r o p e r t i e s than the s t e e l r a i l s that are a t t a c h e d to
the w o o d e n ties on the b r i d g e deck. In d e s i g n i n g
the b r i d g e , the steel m u s t h a v e s u f f i c i e n t s t r e n g t h
to w i t h s t a n d s u b s t a n t i a l a p p l i e d l o a d s . In fact,
the d e s i g n e r w i l l g e n e r a l l y s e l e c t a s t e e l w i t h
h i g h e r s t r e n g t h than a c t u a l l y r e q u i r e d . A l s o , the
d e s i g n e r k n o w s that the s t e e l m u s t h a v e f r a c t u r e
t o u g h n e s s to r e s i s t the g r o w t h and p r o p a g a t i o n o f
c r a c k s a n d m u s t be c a p a b l e o f b e i n g w e l d e d so
that s t r u c t u r a l m e m b e r s can be j o i n e d w i t h o u t
sacrificing strength and toughness. The steel
b r i d g e m u s t a l s o be c o r r o s i o n r e s i s t a n t . T h i s can
be p r o v i d e d b y a p r o t e c t i v e l a y e r o f p a i n t . I f
p a i n t i n g is not a l l o w e d , s m a l l a m o u n t s o f c e r t a i n
a l l o y i n g e l e m e n t s s u c h as c o p p e r and c h r o m i u m
can be a d d e d to the s t e e l to i n h i b i t or r e d u c e
c o r r o s i o n rates. Thus, the s t e e l s e l e c t e d for the
b r i d g e w o u l d be a h i g h - s t r e n g t h l o w - a l l o y
( H S L A ) s t r u c t u r a l s t e e l s u c h as A S T M A 5 7 2 ,
g r a d e 50 or p o s s i b l y a w e a t h e r i n g s t e e l s u c h as
A S T M A 5 8 8 . A t);pical H S L A s t e e l h a s a f e r r i t e p e a r l i t e m i c r o s t r u c t u r e as s e e n in Fig. 1 and is
m i c r o a l l o y e d w i t h v a n a d i u m a n d / o r n i o b i u m for
s t r e n g t h e n i n g . (Microalloying is a t e r m u s e d to
d e s c r i b e the p r o c e s s o f u s i n g s m a l l a d d i t i o n s o f
carbonitride forming elements--titanium, vanad i u m , and n i o b i u m - - t o s t r e n g t h e n s t e e l s by g r a i n
r e f i n e m e n t and p r e c i p i t a t i o n h a r d e n i n g . )
On the o t h e r hand, the s t e e l r a i l s m u s t h a v e
high strength coupled with excellent wear resistance. M o d e m rail s t e e l s c o n s i s t o f a f u l l y p e a r l i tic m i c r o s t r u c t u r e w i t h a fine p e a r l i t e i n t e r l a m e l l a r s p a c i n g , as s h o w n in Fig. 2. P e a r l i t e is u n i q u e
b e c a u s e it is a l a m e l l a r c o m p o s i t e c o n s i s t i n g o f
88% soft, d u c t i l e ferrite a n d 12% hard, b r i t t l e
c e m e n t i t e (Fe3C). The h a r d c e m e n t i t e p l a t e s provide excellent wear resistance, especially when
e m b e d d e d in soft ferrite. P e a r l i t i c s t e e l s h a v e
h i g h s t r e n g t h and are f u l l y a d e q u a t e to s u p p o r t
h e a v y a x l e l o a d s o f m o d e m l o c o m o t i v e s and
f r e i g h t cars. M o s t o f the l o a d is a p p l i e d in c o m pression. Pearlitic steels also have relatively
p o o r t o u g h n e s s and c a n n o t g e n e r a l l y w i t h s t a n d
i m p a c t l o a d s w i t h o u t f a i l u r e . T h e rail s t e e l c o u l d
not m e e t the r e q u i r e m e n t s o f the b r i d g e b u i l d e r ,
Table I Mechanical properties of selected steels
Steel
Condition
Carbon steel bar(a)
1006
Hot rolled
Colddrawn
1008
Hot rolled
Colddrawn
1010
Hot rolled
Cold drawn
1012
Hot rolled
Colddrawn
1015
Hot rolled
Cold drawn
1016
Hot rolled
Cold dmwn
1017
Hot rolled
Cold drawn
1018
Hot rolled
Cold drawn
1019
Hot rolled
Cold drawn
1020
Hot rolled
Cold drawn
1021
Hot rolled
Colddrawn
1022
Hot rolled
Colddrawn
1023
Hot rolled
Cold drawn
1524
Hot rolled
Cold drawn
1025
Hot rolled
Colddrawn
1026
Hot rolled
Colddrawn
1527
Hot rolled
Colddmwn
1030
Hot rolled
Cold drawn
1035
Hot rolled
Colddrawn
1536
Hot rolled
COlddrawn
1037
Hot rolled
Cold drawn
1038
Hot rolled
Colddrawn
1039
Hot rolled
Cold drawn
1040
Hot rolled
Colddrawn
1541
Hot rolled
Cold drawn
Annealed, cold drawn
1042
Hot rolled
Colddrawn
Normalized, cold drawn
1043
Hot rolled
Cold drawn
Normalized, cold drown
1044
Hot rolled
1045
Hot rolled
Colddmwn
Annealed, cold drawn
1046
Hot rolled
Cold drawn
Annealed, cold drawn
1547
Hot rolled
Cold drawn
Annealed, cold drawn
1548
Hot rolled
Colddrawn
Annealed, cold drawn
Tensile
strength
MPa
ksi
Yield
strength
MPa
kd
295
330
305
340
325
365
330
370
345
385
380
420
365
405
400
440
405
455
380
420
420
470
425
475
385
425
510
565
400
440
440
490
515
570
470
525
495
550
570
635
510
565
515
570
545
605
525
585
635
705
650
550
6!5
585
565
625
600
550
565
625
585
585
650
620
650
710
655
660
735
645
165
285
170
285
180
305
185
310
190
325
205
350
200
340
220
370
225
380
205
350
230
395
235
400
215
360
285
475
220
370
240
415
285
485
260
440
270
460
315
535
280
475
285
485
300
510
290
490
350
600
550
305
515
505
310
530
515
305
310
530
505
325
545
515
360
605
585
365
615
540
43
48
44
49
47
53
48
54
50
56
55
61
53
59
58
64
59
66
55
61
61
68
62
69
56
62
74
82
58
64
64
71
75
83
68
76
72
80
83
92
74
82
75
83
79
88
76
85
92
102.5
94
80
89
85
82
91
87
80
82
91
85
85
94
90
94
103
95
96
106.5
93.5
(continued)
24
41
24.5
41.5
26
44
26.5
45
27.5
47
30
51
29
49
32
54
32.5
55
30
51
33
57
34
58
31
52.5
41
69
32
54
35
60
41
70
37.5
64
39.5
67
45.5
77.5
40.5
69
41
70
43.5
74
42
71
51
87
80
44
75
73
45
77
75
44
45
77
73
47
79
75
52
88
85
53
89.5
78.5
Elongation
iaS0muma, ReductionHardness,
30
20
30
20
28
20
28
19
28
18
25
18
26
18
25
15
25
15
25
15
24
15
23
15
25
15
20
12
25
15
24
15
18
12
20
12
18
12
16
12
18
12
18
12
16
12
18
12
15
10
10
16
12
12
16
12
12
16
16
12
12
15
12
12
15
10
10
14
10
10
55
45
55
45
50
40
50
40
50
40
50
40
50
40
50
40
50
40
50
40
48
40
47
40
50
40
42
35
50
40
49
40
40
35
42
35
40
35
40
35
40
35
40
35
40
35
40
35
40
30
45
40
35
45
40
35
45
40
40
35
45
40
35
45
30
28
35
33
28
35
86
95
86
95
95
105
95
105
101
111
110
121
105
116
116
126
116
131
l 1l
121
116
131
121
137
111
121
149
163
116
126
126
143
149
163
137
149
143
163
163
187
143
167
149
163
156
179
149
170
187
207
184
163
179
179
163
179
179
163
163
179
170
170
187
179
192
207
187
197
217
192
(a) All values are estimated minimum values; type 1100 series steels are rated on the basis of 0.10% max Si or coarse-grain melting practice; the mechanical properties shown are expected minimums for the sizes ranging from 19 to 31.8 mm (0.75 to 1.25
in.). (b) Most data are for 25 mm (1 in.) diam bar. Source: Ref 1
Structure/Property Relationships in Irons and Steels / 155
Table I (continued)
Tensile
strength
Steel
Condition
Yield
strength
MPa
ksi
MPa
ksi
Elongation
in 50 ram,
%
600
670
635
620
690
655
745
675
650
660
675
620
670
615
690
635
705
640
725
650
690
650
770
675
820
690
835
695
770
670
840
695
825
680
380
515
385
540
385
540
395
540
345
385
345
385
425
475
450
495
425
475
570
635
605
675
545
605
650
725
670
745
585
650
585
650
635
705
87
97
92
90
100
95
108
98
94
96
98
90
97
89
100
92
102
93
105
94
1130
94
112
98
119
100
121
100.5
112
97
122
101
120
99
55
75
56
78
56
78
57
78
50
56
50
56
62
69
65
72
62
69
83
92
88
98
79
88
94
105.1
97
108
85
94
85
94
92
102
330
560
530
340
580
550
410
570
355
560
370
485
370
475
380
490
385
495
400
505
380
500
425
515
450
530
460
540
425
510
460
540
455
525
230
400
230
415
230
415
235
415
190
325
190
325
235
400
250
420
235
400
315
530
330
565
300
510
355
605
365
620
325
550
325
550
350
595
48
81.5
77
49.5
84
80
59.5
83
51.5
81
54
70
53.5
69
55
71
56
72
58
73
55
72.5
61.5
75
65.5
77
66.5
78
61.5
74
67
78
66
76
33
58
33.5
60
33.5
60
34
60
27.5
47
27.5
47
34
58
36
61
34
58
45.5
77
48
82
43.5
74
51.5
88
53
90
47
80
47
80
50.5
86
15
10
10
15
10
10
12
10
12
10
12
10
12
10
12
10
12
10
12
10
12
10
10
10
10
10
10
10
10
10
10
10
10
10
25
10
25
10
25
10
22
10
30
20
30
20
23
15
23
15
23
15
16
12
15
10
16
12
15
10
15
10
15
12
15
12
15
10
Reduction Hardness,
~a area, %
HB
C a r b o n steel bar(a) (continued)
1049
1050
1552
1055
1060
1064
1065
1070
1074
1078
1080
1084
1085
1086
1090
1095
1211
1212
1213
12L14
1108
1109
11i7
1118
1119
1132
~1137
1140
1141
1144
1145
1146
1151
Hot rolled
Cold drawn
Annealed, cold drawn
Hot roned
Cold da'awn
Annealed, cold drawn
Hot rolled
Annealed, cold drawn
Hot rolled
Annealed, cold drawn
Hot rolled
Spheroidized annealed, cold drawn
Hot rolled
Spheroidized annealed, cold drawn
Hot rolled
Spheroidized annealed, cold drawn
Hot rolled
Spheroidized annealed, cold drawn
Hot rolled
Spheroidized annealed, cold drawn
Hot rolled
Spheroidized annealed, cold drawn
Hot rolled
Spheroidized annealed, cold drawn
Hot rolled
Spheroidized annealed, cold drawn
Hot rolled
Spheroidized annealed, cold drawn
Hot rolled
Spheroidized aimealed, cold drawn
Hot rolled
Spheroidized annealed, cold drawn
Hot rolled
Spheroidized annealed, cold drawn
Hot rolled
Colddrawn
Hot rolled
Cold drawn
Hot rolled
Cold drawn
Hot rolled
Cold drawn
Hot roUed
Colddrawn
Hot rolled
Cold drawn
Hot roned
Colddrawn
Hot rolled
Colddrawn
Hot roned
Colddrawn
Hot roUed
Cold drawn
Hot roiled
Colddrawn
Hot rolled
Colddrawn
Hot roned
Colddrawn
Hot rolled
Colddrawn
Hot rolled
Colddrawn
Hot roUed
Cold drawn
Hot rolled
Colddrawn
35
30
40
35
30
40
30
40
30
40
30
45
30
45
30
45
30
45
30
40
30
40
25
40
25
40
25
40
25
40
25
40
25
40
45
35
45
35
45
35
45
35
50
40
50
40
47
40
47
40
47
40
40
35
35
30
40
35
35
30
35
30
40
35
40
35
35
30
179
197
187
179
197
189
217
193
192
197
201
183
201
183
207
187
212
192
217
192
207
192
229
192
241
192
248
192
229
192
248
197
248
197
121
163
121
167
121
167
121
163
101
121
101
121
121
137
131
143
121
137
167
183
179
197
156
170
187
212
197
217
170
187
170
187
187
207
(continued)
(a) All values are estimated minimum values; type 1100 series steels are rated on the basis of 0.10% max Si or coarse-grain melting practice; the mechanical properties shown are expected minimums for the sizes ranging from 19 to 31.8 mm (0.75 to 1.25
in.). (b) Most data are for 25 mm (1 in.) diam bar. Source: Ref 1
and the HSLA structural steel could not meet the
requirements of the civil engineer who designed
the b r i d g e o r the r a i l s y s t e m .
A similar case can be made for the selection of
cast irons. A cast machine housing on a large
lathe requires a material with adequate strength,
r i g i d i t y , a n d d u r a b i l i t y to s u p p o r t t h e a p p l i e d
l o a d a n d a c e r t a i n d e g r e e o f d a m p i n g c a p a c i t y in
o r d e r to r a p i d l y a t t e n u a t e ( d a m p e n ) v i b r a t i o n s
f r o m the r o t a t i n g p a r t s o f t h e l a t h e . T h e c a s t i r o n
jaws of a crusher require a material with substantial w e a r r e s i s t a n c e . F o r t h i s a p p l i c a t i o n , a c a s t i n g is r e q u i r e d b e c a u s e w e a r - r e s i s t a n t s t e e l s a r e
v e r y d i f f i c u l t to m a c h i n e . F o r the m a c h i n e h o u s i n g , g r a y c a s t i r o n is s e l e c t e d b e c a u s e it is r e l a tively inexpensive, can be easily cast, and has the
a b i l i t y to d a m p e n v i b r a t i o n s as a r e s u l t o f t h e
g r a p h i t e f l a k e s p r e s e n t in its m i c r o s t r u c t u r e .
These flakes are dispersed throughout the ferrite
a n d p e a r l i t e m a t r i x ( F i g . 3). T h e g r a p h i t e , b e i n g a
m a j o r n o n m e t a l l i c c o n s t i t u e n t in the g r a y i r o n ,
p r o v i d e s a t o r t u o u s p a t h f o r s o u n d to t r a v e l
t h r o u g h t h e m a t e r i a l . W i t h so m a n y f l a k e s , s o u n d
w a v e s a r e e a s i l y r e f l e c t e d a n d the s o u n d d a m p ened over a relatively short distance. However,
f o r t h e j a w c r u s h e r , d a m p i n g c a p a c i t y is n o t a
r e q u i r e m e n t . In this c a s e , a n a l l o y w h i t e c a s t i r o n
is s e l e c t e d b e c a u s e o f its h i g h h a r d n e s s a n d w e a r
resistance. The white cast iron microstructure
s h o w n in F i g . 4 is g r a p h i t e f r e e a n d c o n s i s t s o f
m a r t e n s i t e in a m a t r i x o f c e m e n t i t e . B o t h o f t h e s e
constituents are very hard and thus provide the
r e q u i r e d w e a r r e s i s t a n c e . T h u s , in t h i s e x a m p l e
the g r a y c a s t i r o n w o u l d n o t m e e t t h e r e q u i r e ments for the jaws of a crusher and the white cast
i r o n w o u l d n o t m e e t the r e q u i r e m e n t s f o r t h e
lathe housing.
Role of Microstructure
In s t e e l s a n d c a s t i r o n s , t h e m i c r o s t r u c t u r a l
constituents have the names ferrite, pearlite,
b a i n i t e , m a r t e n s i t e , c e m e n t i t e , a n d a u s t e n i t e . In
m o s t a l l o t h e r m e t a l l i c s y s t e m s , the c o n s t i t u e n t s
a r e n o t n a m e d , b u t a r e s i m p l y r e f e r r e d to b y a
G r e e k l e t t e r (ct, 13, Y, e t c . ) d e r i v e d f r o m t h e l o c a tion of the constituent on a phase diagram. Ferr o u s a l l o y c o n s t i t u e n t s , o n the o t h e r h a n d , h a v e
b e e n w i d e l y s t u d i e d f o r m o r e t h a n 1 0 0 y e a r s . In
the e a r l y d a y s , m a n y o f t h e i n v e s t i g a t o r s w e r e
petrographers, mining engineers, and geologists.
Because minerals have long been named after
t h e i r d i s c o v e r e r o r p l a c e o f o r i g i n , it w a s n a t u r a l
to s i m i l a r l y n a m e the c o n s t i t u e n t s in s t e e l s a n d
cast irons.
It c a n b e s e e n t h a t t h e f o u r e x a m p l e s d e s c r i b e d
above have very different microstructures: the
structural steel has a ferrite plus pearlite microstructure; the rail steel has a fully pearlitic mic r o s t r u c t u r e ; the m a c h i n e h o u s i n g ( l a t h e ) h a s a
ferrite plus pearlite matrix with graphite flakes;
and the jaw crusher microstructure contains
m a r t e n s i t e a n d c e m e n t i t e . In e a c h c a s e , the m i c r o s t r u c t u r e p l a y s the p r i m a r y r o l e in p r o v i d i n g
the properties desired for each application. From
t h e s e e x a m p l e s , o n e c a n see h o w m a t e r i a l p r o p e r ties c a n b e t a i l o r e d b y m i c r o s t r u c t u r a l m a n i p u l a tion or alteration. Knowledge about microstruct u r e is t h u s p a r a m o u n t in c o m p o n e n t d e s i g n a n d
a l l o y d e v e l o p m e n t . In the p a r a g r a p h s t h a t f o l l o w ,
e a c h m i c r o s t r u c t u r a l c o n s t i t u e n t is d e s c r i b e d
w i t h p a r t i c u l a r r e f e r e n c e to the p r o p e r t i e s t h a t
can be developed by appropriate manipulation of
the m i c r o s t r u c t u r e t h r o u g h d e f o r m a t i o n ( e . g . , h o t
and cold rolling) and heat treatment. Further de-
156 / Structure/Property Relationships in Irons and Steels
t a i l s a b o u t these m i c r o s t r u c t u r a l c o n s t i t u e n t s can
be f o u n d in R e f 2 to 6.
]'able 1 (continued)
Tensile
Ferrite
A w i d e v a r i e t y o f s t e e l s and c a s t i r o n s f u l l y
e x p l o i t the p r o p e r t i e s o f ferrite. H o w e v e r , o n l y a
f e w c o m m e r c i a l s t e e l s are c o m p l e t e l y f e r r i t i c . A n
e x a m p l e o f the m i c r o s t r u c t u r e o f a f u l l y f e r r i t i c ,
u l t r a l o w c a r b o n s t e e l is s h o w n in Fig. 5.
F e r r i t e is e s s e n t i a l l y a s o l i d s o l u t i o n o f iron
c o n t a i n i n g c a r b o n or o n e or m o r e a l l o y i n g elem e n t s s u c h as s i l i c o n , c h r o m i u m , m a n g a n e s e ,
a n d n i c k e l . T h e r e are t w o t y p e s o f s o l i d solutions: i n t e r s t i t i a l and s u b s t i t u t i o n a l . In an i n t e r stitial solid solution, elements with small atomic
d i a m e t e r , for e x a m p l e , c a r b o n and n i t r o g e n , occ u p y s p e c i f i c i n t e r s t i t i a l sites in the b o d y - c e n t e r e d c u b i c (bcc) i r o n c r y s t a l l i n e l a t t i c e . T h e s e
sites are e s s e n t i a l l y the o p e n s p a c e s b e t w e e n the
l a r g e r iron a t o m s . In a s u b s t i t u t i o n a l s o l i d s o l u tion, e l e m e n t s o f s i m i l a r a t o m i c d i a m e t e r r e p l a c e
or s u b s t i t u t e for iron a t o m s . The t w o t y p e s o f
s o l i d s o l u t i o n s i m p a r t d i f f e r e n t c h a r a c t e r i s t i c s to
ferrite. For e x a m p l e , i n t e r s t i t i a l e l e m e n t s l i k e
c a r b o n and n i t r o g e n can e a s i l y d i f f u s e t h r o u g h
the o p e n bcc l a t t i c e , w h e r e a s s u b s t i t u t i o n a l elements like manganese and nickel diffuse with
g r e a t d i f f i c u l t y . T h e r e f o r e , an i n t e r s t i t i a l s o l i d
s o l u t i o n o f iron and c a r b o n r e s p o n d s q u i c k l y during heat t r e a t m e n t , w h e r e a s s u b s t i t u t i o n a l s o l i d
s o l u t i o n s b e h a v e s l u g g i s h l y d u r i n g h e a t treatm e n t , s u c h as in h o m o g e n i z a t i o n .
A c c o r d i n g to the i r o n - c a r b o n p h a s e d i a g r a m
(Fig. 6a), v e r y l i t t l e c a r b o n ( 0 . 0 2 2 % C) can diss o l v e in ferrite (ctFe), e v e n at the e u t e c t o i d t e m p e r a t u r e o f 727 °C ( 1 3 3 0 °F). (The i r o n - c a r b o n
p h a s e d i a g r a m i n d i c a t e s the p h a s e r e g i o n s that
e x i s t o v e r a w i d e c a r b o n and t e m p e r a t u r e r a n g e .
The d i a g r a m r e p r e s e n t s e q u i l i b r i u m c o n d i t i o n s .
F i g u r e 6(b) s h o w s an e x p a n d e d i r o n - c a r b o n diag r a m w i t h b o t h the e u t e e t o i d and e u t e c t i c reg i o n s . ) At r o o m t e m p e r a t u r e , the s o l u b i l i t y is an
o r d e r o f m a g n i t u d e l e s s ( b e l o w 0 . 0 0 5 % C). H o w ever, e v e n at t h e s e s m a l l a m o u n t s , the a d d i t i o n o f
c a r b o n to p u r e iron i n c r e a s e s the r o o m - t e m p e r a ture y i e l d s t r e n g t h o f i r o n by m o r e t h a n five
t i m e s , as s e e n in Fig. 7. If the c a r b o n c o n t e n t
e x c e e d s the s o l u b i l i t y l i m i t o f 0 . 0 2 2 % , the carbon f o r m s a n o t h e r p h a s e c a l l e d c e m e n t i t e (Fig.
8). C e m e n t i t e is a l s o a c o n s t i t u e n t o f p e a r l i t e , as
s e e n in Fig. 9. The r o l e o f c e m e n t i t e and p e a r l i t e
on the m e c h a n i c a l p r o p e r t i e s o f s t e e l is d i s c u s s e d
below.
The i n f l u e n c e o f s o l i d - s o l u t i o n e l e m e n t s on the
y i e l d s t r e n g t h o f ferrite is s h o w n in Fig. 10. H e r e
one c a n c l e a r l y see the s t r o n g e f f e c t o f c a r b o n on
i n c r e a s i n g the s t r e n g t h o f ferrite. N i t r o g e n , a l s o
an i n t e r s t i t i a l e l e m e n t , has a s i m i l a r effect. P h o s p h o r u s is also a f e r r i t e s t r e n g t h e n e r . In fact, there
are c o m m e r c i a l l y a v a i l a b l e s t e e l s c o n t a i n i n g
p h o s p h o r u s (up to 0 . 1 2 % P) for s t r e n g t h e n i n g .
T h e s e s t e e l s are the r e p h o s p h o r i z e d s t e e l s ( t y p e
1211 to 1215 series). M e c h a n i c a l p r o p e r t y data
for t h e s e s t e e l s can be f o u n d in T a b l e 1.
In Fig. 10, the s u b s t i t u t i o n a l s o l i d s o l u t i o n elements of silicon, copper, manganese, molybden u m , n i c k e l , a l u m i n u m , and c h r o m i u m are s h o w n
to h a v e far l e s s e f f e c t as ferrite s t r e n g t h e n e r s
than the i n t e r s t i t i a l e l e m e n t s . In fact, c h r o m i u m ,
n i c k e l , and a l u m i n u m in s o l i d s o l u t i o n h a v e very
l i t t l e i n f l u e n c e on the s t r e n g t h o f ferrite.
In a d d i t i o n to c a r b o n ( a n d o t h e r s o l i d - s o l u t i o n
e l e m e n t s ) , the s t r e n g t h o f a f e r r i t i c s t e e l is a l s o
Steel
Condition
Low-alloy steels(b)
1340
Normalized at 870 °C (1600 °F)
Annealed at 800 °C (1475 °F)
3140
Normalized at 870 °C (1600 oF)
Annealed at 815 °C (1500 °F)
4130
Normalized at 870 °C (1600 °F)
Annealed at 865 °C (1585 °F)
Water quenched from 855 °C (1575 °F)
and tempered at 540 °C (1000 °F)
4140
Normalized at 870 °C (1600 oF)
Annealed at 815 °C (1500 °F)
Water quenched from 845 °C ( 1550 °F)
and tempered at 540 °C (1000 °F)
4150
Normalized at 870 °C ( 1600 °F)
Annealed at 830 °C (1525 °F)
oil quenched from 830 °C (1525 °F)
and tempered at 540 °C (1000 °F)
4320
Normalized at 895 °C (1640 oF)
Annealed at 850 °C (1560 °F)
4340
Normalized at 870 °C (1600 oF)
Annealed at 810 °C (1490 oF)
Oil quenched from 800 °C (1475 °F)
and tempered at 540 °C (1000 °F)
4419
Normalized at 955 °C (1750 oF)
Annealed at 915 °C (1675 °F)
4620
Normalized at 900 °C (1650 oF)
Annealed at 855 °C (1575 oF)
4820
Normalized at 860 °C (1580 oF)
Annealed at 815 °C (1500 °F)
5140
Normalized at 870 °C (1600 oF)
Annealed at 830 °C (1525 °F)
Oil quenched from 845 °C (1550 °F)
and tempered at 540 °C (1000 °F)
5150
Normalized at 870 °C (1600 oF)
Annealed at 825 °C (1520 oF)
Oil quenched from 830 °C (1525 °F)
and tempered at 540 °C (1000 °F)
5160
Normalized at 855 °C (1575 oF)
Annealed at 815 °C (1495 oF)
Oil quenched from 830 °C (1525 °F)
and tempered at 540 °C (1000 oF)
6150
Normalized at 870 °C (1600 oF)
Annealed at 815 °C (1500 oF)
Oil quenched from 845 °C (1550 °F)
and tempered at 540 °C (1000 oF)
8620
Normalized at 915 °C 0675 °F)
Annealed at 870 °C (1600 oF)
8630
Normalized at 870 °C (1600 oF)
Annealed at 845 °C (1550 °F)
Water quenched from 845 °C (1550 °F)
and tempered at 540 °C (1000 °F)
8650
Normalized at 870 °C (1600)
Annealed at 795 °C ( 1465 °F)
oil quenched from 800 °C (1475 °F)
and tempered at 540 °C ( 1000 °F)
8740
Normalized at 870 °C (1600 oF)
Annealed at 815 °C (1500 oF)
Oil quenched from 830 °C ( 1525 °F)
and tempered at 540 °C (1000 oF)
9255
Normalized at 900 °C ( 1650 oF)
Annealed at 845 °C (1550 oF)
Oil quenched from 885 °C (1625 °F)
and tempered at 540 °C ( 1000 oF)
9310
Normalized at 890 °C (1630 °F)
Annealed at 845 °C (1550 oF)
Ferritie stainless steels(b)
405
Annealed bar
Cold draw n bar
409
Annealed bar
430
Annealed bar
Yield
Elongatba
inSOnma, l~lt~tion Hardm~
%
~aarea, %
lib
strength
MPa
ksi
strength
MPa
ksi
834
703
889
690
670
560
1040
121
102
129
100
97
81
151
558
434
600
420
435
460
979
81
63
87
61
63
67
142
22.0
25.5
19.7
24.5
25.5
21.5
18.1
63
57
57
51
59.5
59.6
63.9
248
207
262
197
197
217
302
1020
655
1075
148
95
156
655
915
986
95
60
143
17.7
25.7
15.5
46.8
56,9
56,9
302
197
311
1160
731
1310
168
106
190
731
380
1215
106
55
176
11.7
20.2
13.5
30,8
40,2
47.2
321
197
375
793
580
1282
745
1207
115
84
186
108
175
460
425
862
470
1145
67
62
125
68
166
20.8
29.0
12.2
22.0
14.2
51
58
36.3
50.0
45.9
235
163
363
217
352
515
450
570
510
758
685
793
570
972
75
65
83
74
110
99
115
83
141
350
330
365
370
485
460
470
290
841
51
48
53
54
70
67
68
42
122
32.5
31.2
29.0
31.3
24.0
22.3
22.7
28.6
18.5
69.4
62.8
66.7
60.3
59.2
58.8
59.2
57.3
58.9
143
121
174
149
229
197
229
167
293
869
675
1055
126
98
159
530
360
1000
77
52
145
20.7
22.0
16.4
58.7
43.7
52.9
255
197
311
1025
724
1145
149
105
166
650
275
1005
94
40
146
18.2
17.2
14.5
50.7
30.6
45.7
285
197
341
938
670
1200
136
97
174
615
415
1160
89
60
168
21.8
23.0
14.5
61.0
48.4
48.2
269
197
352
635
540
650
565
931
92
78
94
82
135
360
385
425
370
850
52
56
62
54
123
26.3
31.3
23.5
29.0
18.7
59.7
62.1
53.5
58.9
59.6
183
149
187
156
269
1025
715
1185
149
104
172
690
385
1105
100
56
160
14
22.5
14.5
45.0
46.0
49.1
302
212
352
931
696
1225
135
101
178
605
415
1130
88
60
164
16.0
22.2
16.0
47.9
46.4
53.0
269
201
352
931
779
1130
135
113
164
580
485
924
84
70
134
19.7
21.7
16.7
43.4
41.1
38.3
269
229
321
910
820
132
119
570
450
83
65
18.8
17.3
58.1
42.1
269HRB
241HRB
276
483
240
310
40
70
35
45
30
20
25
30
60
60
150
185
75HRB
155
483
586
450
517
70
85
65
75
(confnued)
--65"
(a) All values are estimated minimum values; type 1100 series steels are rated on the basis of 0.10% max Si or coarse-grain melting practice; the mechanical properties shown are expected minimums for the sizes ranging from 19 to 31.8 mm (0.75 to 1.25
in.). (b) Most data are for 25 mm (1 in.) diam bar. Source: Ref I
Structure/Property Relationships in Irons and Steels / 157
d e t e r m i n e d b y its g r a i n size a c c o r d i n g
Hall-Petch relationship:
Table 1 (continued)
Steel
Ccmdition
Ferritic stainless steels(b) (continued)
430 (cont'd) Annealed and cold drawn
442
Annealed bar
Annealed at 815 °C (1500 °F) and cold
worked
446
Annealed bar
Annealed at 815 °C (1500 °F) and cold
drawn
Martensilic stainless steels(b)
403
Annealed bar
Tempered bar
410
Oil quenched from 980 °C ( 1800 °F);
tempered at 540 °C (1000 °F);.16 nun
(0.625 in.) bar
Oil quenched from 980 °C (1800 °F);
tempered at 40 °C (104 °F); 16 mm
(0.625 in.) bar
414
Annealed bar
Cold drawn bar
Oil quenched from 980 °C (1800 °F);
tempered at 650 °C (1200 oF)
420
Annealed bar
Annealed and cold drawn
431
Annealed bar
Annealed and cold drawn
Oil quenched from 980 °C (1800 °F);
tempered at 650 °C (1200 oF)
Oil quenched from 980 °C (1800 °F);
tempered at 40 °C (104 °F)
440C
Annealed bar
Annealed and cold drawn bar
Hardened and tempered at 315 °C
(6OO°F)
Austenitle stainless steels(b)
201
Annealed
50% hard
Full hard
Extra hard
202
Annealed bar
Annealed sheet
50% hard sheet
301
Annealed
50% hard
Full hard
302
Annealed strip
25% hard strip
Annealed bar
303
Annealed bar
Colddrawn
304
Annealed bar
Annealed and cold drawn
Cold-drawn high tensile
305
Annealed sheet
308
Annealed bar
309
Annealed bar
310
Annealed sheet
Annealed bar
314
Annealed bar
316
Annealed sheet
Annealed bar
Annealed and cold-drawnbar
317
Annealed sheet
Annealed bar
321
Annealed sheet
Annealed bar
Annealed and cold-drawn bar
330
Annealed sheet
Annealed bar
347
Annealed sheet
Annealed bar
Tensile
strength
MPa
ksi
Yield
strength
MPa
ksi
Elongation
in 50ram, Reduction Hardness,
%
in area, %
HB
586
515
545
85
75
79
483
310
427
70
45
62
20
30
35.5
65
50
79
185
160
92HRC
550
607
80
88
345
462
50
67
25
26
45
64
86HRB
96HRB
515
765
1085
75
111
158
275
585
1005
40
85
146
35
23
13
70
67
70
82HRB
97HRB
...
1525
221
1225
178
15
64
45HRB
795
895
1005
115
130
146
620
795
800
90
115
116
20
15
19
60
58
58
235
270
...
655
760
860
895
831
95
110
125
130
121
345
690
655
760
738
50
100
95
110
107
25
14
20
15
20
55
40
55
35
64
195
228
260
270
...
1435
208
1140
166
17
59
45HRC
760
860
1970
110
125
285
450
690
1900
65
100
275
14
7
2
25
20
10
97HRB
260
580
380
760
965
1480
275
310
760
275
655
1330
275
515
240
240
415
235
415
655
260
205
275
310
275
345
290
240
415
275
275
240
240
415
260
290
275
240
55
ll0
140
215
40
45
110
40
95
193
40
75
35
35
60
34
60
95
38
30
40
45
40
50
42
35
60
40
40
35
35
60
38
42
40
35
52
12
8
1
40
40
10
60
54
6
55
12
60
50
40
60
45
25
50
55
45
45
45
45
50
60
45
45
50
45
55
40
40
45
45
50
...
...
...
...
......
......
_
70'
61
...
...
_
70"
55
53
70
...
...
_
65'
65
_
65'
60
_
70"
65
...
...
_
65'
60
...
...
_
65"
87HRB
32HRC
41HRC
43HRC
760
1035
1275
1550
515
655
1030
725
1035
1415
620
860
585
620
690
585
690
860
585
585
655
620
655
689
580
550
620
620
585
620
585
655
550
585
655
620
110
150
185
225
75
95
150
105
150
205
90
125
85
90
100
85
100
125
85
85
95
90
95
100
84
80
90
90
85
90
85
95
80
85
95
90
(continued)
...
...
...
80HRB
25HRC
80HRB
160
228
149
212
275
80HRB
150
83HRB
85HRB
160
180
79HRB
149
190
85HRB
160
80HRB
150
185
...
80HRB
85HRB
160
(a) All values are estimated minimum values; type 1100 series steels are rated on the basis of 0.10% max Si or coarse-grain melting practice; the mechanical properties shown are expected minimums for the sizes ranging from 19 to 31.8 mm (0.75 to 1.25
in.). (b) Most data are for 25 mm (1 in.) diam bar. Source: Ref 1
Gy = Go + kyd -1/2
to t h e
(Eq 1)
w h e r e Oy is the y i e l d s t r e n g t h (in M P a ) , ~ o is a
c o n s t a n t , ky is a c o n s t a n t , a n d d is t h e g r a i n d i a m e t e r (in m m ) .
T h e g r a i n d i a m e t e r is a m e a s u r e m e n t o f s i z e o f
the f e r r i t e g r a i n s in the m i c r o s t r u c t u r e , f o r e x a m p l e , n o t e the g r a i n s in the u l t r a l o w c a r b o n s t e e l in
F i g . 5. F i g u r e 11 s h o w s t h e H a l l - P e t c h r e l a t i o n s h i p f o r a l o w - c a r b o n f u l l y f e r r i t i c steel. T h i s
r e l a t i o n s h i p is e x t r e m e l y i m p o r t a n t f o r u n d e r standing
structure-property
relationships
in
steels. Control of grain size through thermomechanical treatment, heat treatment, and/or
m i c r o a l l o y i n g is vital to the c o n t r o l o f s t r e n g t h
a n d t o u g h n e s s o f m o s t steels. T h e r o l e o f g r a i n
s i z e is d i s c u s s e d in m o r e d e t a i l b e l o w .
T h e r e is a s i m p l e w a y to s t a b i l i z e f e r r i t e ,
t h e r e b y e x p a n d i n g the r e g i o n o f f e r r i t e i n t h e
iron-carbon phase diagram, namely by the addit i o n o f a l l o y i n g e l e m e n t s s u c h as s i l i c o n , c h r o mium, and molybdenum. These elements are
called ferrite stabilizers because they stabilize
f e r r i t e at r o o m t e m p e r a t u r e t h r o u g h r e d u c i n g t h e
amount of y solid solution (austenite) with the
f o r m a t i o n o f w h a t is c a l l e d a y - l o o p a s s e e n at the
f a r l e f t in F i g . 12. T h i s i r o n - c h r o m i u m p h a s e d i a gram shows that ferrite exists up above 12% Cr
a n d is s t a b l e u p to t h e m e l t i n g p o i n t ( l i q u i d u s
temperature). An important fully ferritic family
o f s t e e l s is the i r o n - c h r o m i u m f e r r i t i c s t a i n l e s s
s t e e l s . T h e s e s t e e l s a r e r e s i s t a n t to c o r r o s i o n , a n d
a r e c l a s s i f i e d as t y p e 4 0 5 , 4 0 9 , 4 2 9 , 4 3 0 , 4 3 4 ,
436, 439, 442, 444, and 446 stainless steels.
T h e s e s t e e l s r a n g e in c h r o m i u m c o n t e n t f r o m 11
to 3 0 % . A d d i t i o n s o f m o l y b d e n u m , s i l i c o n , n i o bium, aluminum, and titanium provide specific
properties. Ferritic stainless steels have good
d u c t i l i t y ( u p to 3 0 % t o t a l e l o n g a t i o n a n d 6 0 %
r e d u c t i o n in a r e a ) a n d f o r m a b i l i t y , b u t l a c k
strength at elevated temperatures compared with
austenitic stainless steels. Room-temperature
y i e l d s t r e n g t h s r a n g e f r o m 1 7 0 to a b o u t 4 4 0 M P a
( 2 5 to 6 4 k s i ) , a n d r o o m - t e m p e r a t u r e t e n s i l e
s t r e n g t h s r a n g e f r o m 3 8 0 to a b o u t 5 5 0 M P a (55
to 8 0 ksi). T a b l e 1 lists t h e m e c h a n i c a l p r o p e r t i e s
of some of the ferritic stainless steels. Type 409
s t a i n l e s s s t e e l is w i d e l y u s e d f o r a u t o m o t i v e e x haust systems. Type 430 free-machining stainless
s t e e l h a s the b e s t m a c h i n a b i l i t y o f all s t a i n l e s s
steels other than that of a low-carbon, free-mac h i n i n g m a r t e n s i t i c s t a i n l e s s s t e e l ( t y p e 41.6).
Another family of steels utilizing a ferrite stabilizer (y-loop) are the iron-silicon ferritic alloys
c o n t a i n i n g u p to a b o u t 6 . 5 % Si ( c a r b o n - f r e e ) .
These steels are of commercial importance because they have excellent magnetic permeability
and low core loss. High-efficiency motors and
transformers are produced from these iron-silicon electrical steels (aluminum can also substit u t e f o r s i l i c o n in t h e m ) .
O v e r t h e p a s t 2 0 y e a r s o r so, a n e w b r e e d o f
very-low-carbon fully ferritic sheet steels has
emerged for applications requiring exceptional
f o r m a b i l i t y ( s e e F i g . 5). T h e s e a r e the i n t e r s t i t i a l - f r e e (IF) s t e e l s f o r w h i c h c a r b o n a n d n i t r o g e n a r e r e d u c e d in t h e s t e e l m a k i n g p r o c e s s to
very low levels, and any remaining interstitial
c a r b o n o r n i t r o g e n is t i e d u p w i t h s m a l l a m o u n t s
of alloying elements (e.g., titanium or niobium)
that form preferentially carbides and nitrides.
158/Structure/Property Relationships in Irons and Steels
Table I (continued)
Sted
Cand~laa
qI~mBe
st~ngth
MPa
k~
Elongation
inS0mm, ReductionHardness,
%
in area, % liB
Yield
strength
MPa
I~
Austenite ~ cementite + ferrite
Austenilic stainless steels(b) (continued)
347 (eont'd) Annealedandcolddrawnbar
384
Annealed wire 1040 °C (1900 °F)
Maraging steels(b)
18Ni(250) Annealed
Aged bar 32 mm (1.25 in.)
Aged sheet 6 mm (0.25in.)
18Ni(300) Annealed
Aged bar 32 mm (1.25 in.)
Aged sheet 6 mm (0.25 in.)
18Ni(350) Annealed
Aged bar 32 mm (l.25 in.)
Aged sheet 6 mm (0.25 in.)
pearlite forms. Pearlite is formed by cooling the
steel through the eutectoid temperature (the temperature o f 727 °C in Fig. 6) by the following
reaction:
690
515
100
75
450
240
65
35
965
1844
1874
1034
2041
2169
1140
2391
2451
140
269
272
150
296
315
165
347
356
655
1784
1832
758
2020
2135
827
2348
2395
95
259
266
110
293
310
120
341
347
40
55
60
72
17
11
8
18
11.6
7.7
18
7.6
3
75
56.5
40.8
72
55.8
35
70
33.8
15.4
212
70HRB
30 HRC
51.8 HRC
50.6HRC
32HRC
54.7 HRC
55.1HRC
35 HRC
58.4 HRC
57.7 HRC
(a) All values are estimated minimumvalues; type 1100 series steels ate rated on the basis of 0.10% max Si or coarse-grain melting practice; the mechanical properties shown are expected minimums for the sizes ranging from 19 to 31.8 mm (0.75 to 1.25
in.). (b) Most data are for 25 mm (1 in.) diam bar. Some: Ref 1
The cementite and ferrite form as parallel plates
called lamellae (Fig. 13). This is essentially a
c o m p o s i t e m i c r o s t r u c t u r e c o n s i s t i n g o f a very
hard carbide phase, cementite, and a very soft and
ductile ferrite phase. A fully pearlitic microstructure is formed at the eutectoid composition of
0.78% C. As can be seen in Fig. 2 and 13, pearlite
forms as colonies where the lamellae are aligned
in the s a m e orientation. The properties of fully
pearlitic steels are determined by the spacing between the ferrite-cementite lamellae, a dimension
called the interlamellar spacing, X, and the colony
size. A simple relationship for yield strength has
been developed by Heller (Ref 10) as follows:
fly = -85.9 + 8.3 (X-t/2)
T h e s e steels have very low strength, but are used
to produce c o m p o n e n t s that are difficult or impossible to form from other steels. Very-low-carbon, fully ferritic steels (0.001% C) are n o w being m a n u f a c t u r e d for automotive c o m p o n e n t s
that harden during the paint-curing cycle. These
steels are called bake-hardening steels and have
controlled a m o u n t s o f carbon and nitrogen that
c o m b i n e with other elements, s u c h as titanium
and niobium, during the baking cycle (175 °C, or
350 °F, for 30 min). The process is called aging,
and the strength derives from the precipitation o f
t i t a n i u m / n i o b i u m carbonitrides at the elevated
temperature.
Another form of very-low-carbon, fully ferritic
steel is motor lamination steel. The carbon is rem o v e d from these steels by a process known as
decarburization. The decarburized (carbon-free)
ferritic steel has good permeability and sufficiently low core loss (not as low as the iron-silicon alloys) to be used for electric motor lamina-
Fig, 3
tions, that is, the stacked steel layers in the rotor
and stator o f the motor.
As noted previously, a n u m b e r of properties
are exploited in fully ferritic steels:
•
Iron-silicon
steels:
Exceptional
electrical
properties
•
I r o n - c h r o m i u m steels: Good corrosion resis-
tance
•
Interstitial-free
steels:
Exceptional
forma-
steels:
Strengthens
during
bility
•
Bake-hardening
ffXl2)
(Eq 3)
where fly is the 0.2% offset yield strength (in
MPa) and X is the interlamellar spacing (in mm).
Figure 14 s h o w s Heller's plot of strength versus
interlamellar spacing for fully pearlitic eutectoid
steels.
It has also been s h o w n by Hyzak and Bernstein
( R e f 11) that strength is related to interlamellar
spacing, pearlite colony size, and prior-austenite
grain size, according to t h e following relationship:
paint cure cycle
•
L a m i n a t i o n s t e e l s : Good electrical properties
PearlRe
As the carbon content of steel is increased beyond the solubility limit (0.02% C) on the ironcarbon binary phase diagram, a constituent called
Microstructure of a gray cast iron with a ferrite-pearlite matrix. Note the graphite
flakes dispersed throughout the matrix. 4% picral etch. 320x. Courtesy of A.O.
Benscoter, Lehigh University
Fig. 4
YS = 52.3 + 2.18 (~-1/2) - 0 . 4 (de-L'2) - 2 . 8 8 (d-1/2)(Eq 4)
where YS is the yield strength (in MPa), d e is the
pearlite colony size (in mm), and d is the prioraustenite grain size (in mm). From Eq 3 and 4, it
can be seen that the steel composition does not
have a major influence on the yield strength of a
fully pearlitic eutectoid steel. There is s o m e solid-
Microstructure of an alloy white cast iron. White constituent is cementite and the
darker constituent is martensite with some retained austenite. 4% picral etch.
250x. Courtesy ofA.O. Benscoter, Lehigh University
Structure/Property Relationships in Irons and S t e e l s / 1 5 9
steel will typically have a total elongation of
more than 50%, whereas a fully pearlitic steel
(e.g., type 1080) will typically have a total elongation of about 10% (see Table 1). A low-carbon
fully ferritic steel will have a room-temperature
Charpy V-notch impact energy of about 200 J
(150 f t . lbf), whereas a fully pearlitic steel will
have room-temperature impact energy of under
10 J (7 f t . lbf). The transition temperature (i.e.,
the temperature at which a material changes from
ductile fracture to brittle fracture) for a fully
pearlitic steel can be approximated from the following relationship (Ref 11):
TT = 217.84 - 0.83 (de-1/2) - 2.98(d -1"~)
Microstructure of a fully ferritic, ultralow carbon
steel. Marshalls etch + HF, 300x. Courtesy of
A.O. Benscoter, Lehigh University
solution strengthening of the ferrite in the lamellar structure (see Fig. 10).
The thickness of the cementite lamellae can
also influence the properties of pearlite. Fine cementite lamellae can be deformed, compared
with coarse lamellae, which tend to crack during
deformation.
Although fully pearlitic steels have high
strength, high hardness, and good wear resistance, they also have poor ductility and toughness. For example, a low-carbon, fully ferritic
1180
2
I
(Eq5)
where TT is the transition temperature (in °C).
From Eq 5, one can see that both the prioraustenite grain size and pearlite colony size control the transition temperature of a pearlitic steel.
Unfortunately, the transition temperature of a
fully pearlitic steel is always well above r o o m
temperature. This m e a n s that at room temperature the general fracture mode is cleavage, which
is associated with brittle fracture. Therefore,
fully pearlitic steels should not be used in applications where t o u g h n e s s is important. Also, pearlitic steels with carbon contents slightly or moderately higher than the eutectoid c o m p o s i t i o n
(called hypereutectoid steels) have even poorer
toughness.
From Eq 4 and 5, one can see that for pearlite,
strength is controlled by interlamellar spacing,
colony size, and prior-austenite grain size, and
t o u g h n e s s is controlled by colony size and prior-
Fig. 5
1
I
austenite grain size. Unfortunately, these three
factors are rather difficult to measure. To determ i n e interlamellar spacing, a scanning electron
m i c r o s c o p e (SEM), or a t r a n s m i s s i o n electron
m i c r o s c o p e (TEM) is needed in order to resolve
the spacing, Generally, a magnification of
10,000x is adequate, as seen in Fig. 13. Special
statistical procedures have been developed to determine an accurate m e a s u r e m e n t o f the spacing
( R e f 12). The colony size and especially the
prior-austenite grain size are very difficult to
m e a s u r e and require a skilled metallographer using the light microscope or SEM and special
etching procedures.
B e c a u s e of poor ductility/toughness, there a r e
only a few applications for fully pearlitic steels,
including railroad rails and wheels and highstrength wire. By far, the largest tonnage application is for rails. A fully pearlitic rail steel provides excellent wear resistance for r a i l r o a d
wheel/rail contact. Rail life is m e a s u r e d in millions of gross tons (MGT) of travel and current
rail life easily exceeds 250 MGT. The wear resistance of pearlite arises from the unique morphology of the ferrite-cementite lamellar composite
where a hard constituent is embedded into a softductile constituent. This m e a n s that the hard cem e n t i t e plates do not abrade away as easily as the
rounded cementite particles found in other steel
microstructures, that is, tempered martensite and
bainite, which is discussed later. Wear resistance
o f a rail steel is directly proportional to hardness.
This is s h o w n in Fig. 15, which indicates less
weight loss as hardness increases. Also, w e a r resistance (less weight loss) increases as interlamellar spacing decreases, as s h o w n in Fig. 16.
3
I
Carbon, at.%
5
I
4
I
6
I
7
8
9
- ~...~
1154°C
1140
I
Fe-C equilibrium (experimental)
- -
1100
2.08 ~
Fe-Fe3C equilibrium (experimental)
.o"Y
"'"
2125
J., ~1,,/8 °C-'~ 2050
211
-- 1975
1060
• .~
1020
• *' Y
-- 1900
-- 1825
(~Fe)
980
auatenite
-- 1750
940
¢D
O.
E
900 ~ 9 1 2 °C
~
~0¢F.)
86O
820
AUS tenite + cementite
, / "
ferrite
%~
.
770 °C (Curie temperature)
-- 1700
E
,-~
-- 1625
-- 1550
-*°~
.../
-- 1475
780
740
. . . . . . . . . . . . . . . . . . ~-- - ' - ~
I ~
0.0206
700 /
66O
Fe 0.1
~
7
0.68
~,
.'°"
738 °C
-
1400
--
1325
-
1250
I
727 °C
0.0218
I
0.2
I
0.3
0.4
0.5
0.6
0.7
0.8
0.9
uo
1.0
Ferrite + cementite
I
I
I
I
1.1 1.2 1.3 1.4
I
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
Carbon, wt%
Fig. 6 ( a ) Iron-carbon phasediagram showing the austenite (y Fe)and ferrite (ocFe)phase regions and eutectoid composition and temperature. Dotted lines representiron-graphite equilibrium conditions and solid lines representiron-cementite equilibrium conditions. Only the solid lines are important with respect to steels.Source: Ref 2
160/Structure/Property Relationships in Irons and Steels
Thus, the m o s t important microstructural parameter for controlling hardness a n d wear resistance is the pearlite interlamellar spacing. Fortunately, interlamellar spacing is easy to control
and is d e p e n d e n t solely on transformation temperature.
Figure 17 shows a continuous cooling transformation (CCT) d i a g r a m for a typical rail steel. A
C C T d i a g r a m is a time versus temperature plot
s h o w i n g the regions at which various constituc n t s - - f e r d t e , pearlite, bainite, and m a r t e n s i t e - form during the continuous cooling of a steel
component. U s u a l l y several cooling curves are
s h o w n with the associated start and finish transformation temperatures of each constituent.
T h e s e diagrams should not be confused with isothermal transformation (IT or T T T ) diagrams,
which are derived by rapidly q u e n c h i n g very thin
s p e c i m e n s to various temperatures, and maintaining that temperature (isothermal) until the specim e n s begin to transform, partially transform, and
fully transform, at which time they are quenched
to room temperature. An IT d i a g r a m does not
represent the transformation behavior in m o s t
processes where steel parts are continuously
cooled, that is, air cooled, and so forth.
As s h o w n in Fig. 17, the p e a d i t e transformation temperature (indicated by the pearlite-start
curve, Ps) decreases with increasing cooling rate.
The hardness of peaflite increases with decreasing transformation temperature. Thus, in order to
provide a rail steel with the h i g h e s t hardness and
wear resistance, one m u s t cool the rail from the
austenite at the fastest rate possible to obtain the
lowest transformation temperature. This is done
in practice by a process known as head hardening, which is simply an accelerated cooling process u s i n g forced air or water sprays to achieve
the desired cooling rate (Ref 15). Because only
the head of the rail contacts the wheel of the
railway car and locomotive, only the head requires the higher hardness and wear resistance.
A n o t h e r application for a fully pearlitic steel is
h i g h - s t r e n g t h wire (e.g., piano wire). Again, the
composite m o r p h o l o g y of lamellar ferrite and cementite is exploited, this time during wire drawing. A fully pearlitic steel rod is heat treated by a
process k n o w n as patenting. During patenting,
1~M
3270
1~
3090
Ferrite-Pearlite
2910
1!
2730
GFe
1~
2550
2370
2190
E
i~
11
2010
lC
1830
E
1470
7
1290
~
rILE[
-~
930
4
750
3
570
the rod is transformed at a temperature of about
540 °C (1000 °F) by passing it through a lead or
salt bath at this temperature. This develops a
microstructure with a very fine pearlite interlamellar spacing because the transformation
takes place at the n o s e of the C C T diagram, that
is, at the lowest possible pearlite transformation
temperature (see Fig. 17). The rod is then cold
drawn to wire. B e c a u s e o f the very fine interlamellar spacing, the ferrite and cementite lamellae b e c o m e aligned along the wire axis during
the deformation process. Also, the fine ccmentite
lamella tend to bend and deform as the wire is
elongated during drawing. The resulting wire is
one of the strongest commercial products available; for example, a commercial 0.1 m m (0.004
in.) diam wire can have a tensile strength in the
range of 3.0 to 3.3 GPa (439 to 485 ksi), and in
special cases a tensile strength as h i g h as 4.8
G P a (696 ksi) can be obtained. These wires are
used in m u s i c a l instruments because of the sound
quality developed from the high tensile stresses
applied in stringing a piano and violin and are
also used in wire rope cables for suspension
bridges.
The m o s t c o m m o n structural steels produced
have a m i x e d ferrite-pearlite microstructure.
Their applications include b e a m s for bridges and
high-rise buildings, plates for ships, and reinforcing bars for roadways. These steels are relatively inexpensive and are produced in large tonnages. They also have the advantage o f being
able to be produced with a wide range of properties. The microstructure of typical ferrite-pearlite
steels is s h o w n in Fig. 18.
In m o s t ferrite-pearlite steels, the carbon content and the grain size determine the microstructure and resulting properties. For example,
Fig. 19 s h o w s the effect of carbon on tensile and
impact properties. The ultimate tensile strength
steadily increases with increasing carbon content. This is caused by the increase in the volume
fraction o f pearlite in the microstructure, which
has a strength m u c h higher than that of ferrite.
Thus, increasing the volume fraction o f pearlite
has a profound effect on increasing tensile
strength.
However, as seen in Fig. 19, the yield strength
is relatively unaffected by carbon content, rising
from about 275 MPa (40 ksi) to about 415 MPa
(60 ksi) over the range of carbon content shown.
This is because yielding in a ferrite-pearlite steel
is controlled by the fcrrite matrix, which is generally considered to be the continuous phase (maO3
"~ 35
I
P_~
Fe
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
30
7.0
Carbon, wt%
Fig. 6(b)
Expanded iron-carbon phase diagram showing both the eutectoid (shown in Fig. 6a) and eutectic regions.
Dotted lines represent iron-graphite equilibrium conditions and solid lines represent iron-cementite equilibrium conditions. The solid lines at the eutectic are important to white cast irons and the dotted lines are important to gray
cast irons. Source: Ref 2
241 ~:
-'~ 25
~'
.....
"N,
103=
/
0
o~
o.
o
172
10 ~
0
Fig, 7
0.001
0.002
0.003
Carbon, wt%
0.004 0.005
o
6
Increase in room-temperature yield strength of
iron with small additions of carbon. Source: Ref 7
Structure/Property Relationships in Irons and Steels / 161
Fig. 8
Fig. 9
Photomic.rograph of an annealed low-carbon sheet steel with grain-boundary cementite. 2% nital + 4% picral etch. 1000x
trix) in the microstructure. Therefore, pearlite
plays only a minor role in yielding behavior.
From Fig. 19, one can also see that ductility, as
represented by reduction in area, steadily decreases with increasing carbon content. A steel
with 0.10% C has a reduction in area of about
75%, whereas a steel with 0.70% C has a reduction in area of only 25%. Percent total elongation
would show a similar trend, however, with values
m u c h less than percent reduction in area.
Much work has been done to develop empirical
equations for ferrite-pearlite steels that relate
strength and t o u g h n e s s to microstructural features, for example, grain size and percent of
pearlite as well as composition. One such equation for ferrite-pearlitc steels under 0.25% C is as
follows (Ref 16):
YS = 53.9 + 32.34 (Mn) + 83.2(Si)
+ 354.2(Nf) + 17.4(d-U2)
(Eq 6)
where Mn is the m a n g a n e s e content (%), Si is the
silicon content (%), Nf is the free nitrogen content
(%), and d is the ferrite grain size (in mm). Equation 6 shows that carbon content (percent pearlite)
4-375
Photomicrograph of pearlite (dark constituent) in a low-carbon steel sheet. 2% nital + 4% picral etch. 1000x
pact energy versus test temperature, the shelf energy decreases from about 200 J (150 ft • lbf) for
a 0.11% C steel to about 35 J (25 f t . lbf) for a
0.80% C steel. Also, the transition temperature
increases from about - 5 0 to 150 °C ( - 6 0 to 300
°F) over this s a m e range o f carbon content. The
effect of carbon is due mainly to its effect on the
percentage of pearlite in the microstructurc. This
is reflected in the regression equation for transition temperature below (Ref 16):
has no effect on yield strength, whereas the yield
strength in Fig. 19 i n c r e a s e s somewhat with carbon content. According to Eq 6, m a n g a n e s e , silicon, and nitrogen have a pronounced effect on
yield strength, as does grain size. However, in
most ferrite-pearlite steels nitrogen is quite low
(under 0.010%) and thus h a s m i n i m a l effect on
yield strength. In addition, as discussed below,
nitrogen has a detrimental effect on impact properties.
The regression equation for tensile strength for
the s a m e steels is as follows (Ref 16):
TT = - 1 9 + 44(Si) + 700(N~/2)
(F-47)
It can be seen in all these relationships that
ferrite grain size is an important parameter in
i m p r o v i n g both strength and toughness. It can
also be seen that while pearlite is beneficial for
increasing tensile strength and nitrogen is beneficial for increasing yield strength, both are harmful to toughness. Therefore, m e t h o d s to control
the grain size of ferrite-pearlite steels have rapidly evolved over the past 25 years. T h e two m o s t
important m e t h o d s to control grain size are controlled rolling and microalloying. In fact, these
where TS is the tensile strength (in MPa) and P is
pearlite content (%). Thus, in distinction to yield
strength, the percentage o f pearlite in the micros t r u c t u r e h a s an i m p o r t a n t e f f e c t on t e n s i l e
strength.
T o u g h n e s s of ferrite-pearlite steels is also an
important consideration in their use. It has long
been k n o w n that the absorbed energy in a Charpy
V-notch test is decreased by increasing carbon
content, as seen in Fig. 20. In this graph of im-
I
(F_.q8)
+ 2.2(P) - 11.5 (d -1/2)
TS = 294,1 + 27.7(Mn) + 83.2(Si)
+ 3.9(P) + 7.7(d -lt2)
600
80
C and N
500
+225
Si
.--~_m+150
"~
o
+75
400
~
300
/
80
200
y
0
-75
&
- - Ni and AI
20
100
0
0.5
1.0
1.5
2.0
2.5
3.0
Alloy content, wt%
0
Fig, 1 0 Influence of solid-solution elements on the
changes in yield stress of low-carbon ferritic
steels. Source: Ref 5
Fig. 11
I
I
I
1
2
3
I
I
I
I
I
4
5
6
7
8
Grain diameter (d-l~), mm -1~
Hall-Petch relationship in low-carbon ~mtic steels, souse: Ref 8
I
I
I
I
9
10
11
12
"N.
|
162 / Structure/Property Relationships in Irons and Steels
Chromium, at.%
0
20OO
10
20
30
40
50
60
70
80
I
I
I
i
I
I
I
I
90
100
I
1863 °C
1800
1600
oo
1538 °C
1516 °:
......
21
1400 - 1394 °C
1200 -
~
~
(~Fe,Cr)
(9
¢:L
E
1000 _ ( ~ F e ) / / _ 1 2 . 7
oc/I
oc
8001:-- -.7
I
-/
~nn I Magnetic
" ~ • "---- . . . . .
Itransformabon./
o.'. . . . . . . . . . . . . . . . . . .
400 i
r'1°°
I
I
0
10
20
30
(I o I,
,,
"-..
".,..
I
I
I
I
I
40
50
60
70
80
Fe
Fig. 12
"* ".
:
475 o C
=.." . . . . . . . . . . . . . . . . . . . . . . . . . . .
t "'~
90
Chromium, wt%
100
Cr
Iron-chromium phase diagram. Source: Ref 9
methods are used in conjunction to produce
strong, tough ferrite-pearlite steels.
Controlled rolling is a thermomechanical
treatment in which steel plates are rolled below
the recrystailization temperature of aastcnite.
This process results in elongation of the austenite
grains. Upon further rolling and subsequent cooling to room temperature, the austenite-to-ferrite
transformation takes place. The ferrite grains are
restricted in their growth because of the "pancake" austeaite grain morphology. This produces
the fine ferrite grain size required for higher
strength and toughness.
Microalloying is the term applied to the addition of small amounts of special alloying elements (vanadium, niobium, or titanium) that aid
in retarding austenite recrystallization, thus allowing a wide window of rolling temperatures
for controlled rolling. Without retarding recrystallization, as in normal hot rolling, the pancaketype grains do not form and a fine grain size
cannot be developed. Microalloyed steels are
used in a wide variety of high tonnage applications including structural steels for the construction industry (bridges, multistory buildings,
etc.), reinforcing bar, pipe for gas transmission,
and numerous forging applications.
Bainite
Like pearlite, bainitc is a composite of ferrite
and cementitc. Unlike pearlite, the ferritc has an
acicular morphology and the carbides are discrete particles. Because of these morphological
differences, bainite has much different property
characteristics than pearlite. In general, bainitic
steels have high strength coupled with good
toughness, whereas pearlitic steels have high
strength with poor toughness.
Another difference between baiaite and pearlite is the complexity of the bainite morphologies
compared with the simple lamellar morphology
of pearlite. The morphologies of bainite are still
being debated in the literature. For years, since
the classic work of Bain and Davenport in the
1930s (Ref 18), there were two classifications of
bainite: upper and lower bainite. This nomenclature was derived from the temperature regions at
which bainite formed during isothermal (constant
temperature) transformation. Upper bainite
formed isothermally in the temperature range of
400 to 550 °C (750 to 1020 °F), and lower
bainite formed isothermally in the temperature
range of 250 to 400 °C (480 to 750 °F). Examples of the microstructure of upper and lower
bainite are shown in Fig. 21. One can see that
both types of bainite have an acicular morphology, with upper bainite being coarser than lower
bainite. The true morphological differences between the microstructures can only be determined by electron microscopy. Transmission
electron micrographs of upper and lower baiaite
are shown in Fig. 22. In upper bainitc, the iron
carbide phase forms at the lath boundaries,
whereas in lower bainite, the carbide phase forms
on particular crystallographic habit planes within
the laths. Because of these differences in morphology, upper and lower bainite have different
mechanical properties. Lower bainite, with a fine
acicular structure and carbides within the laths,
has higher strength and higher toughness than upper bainite with its coarser structure.
Because during manufacture most steels undergo continuous cooling rather than isothermal
holding, the terms upper and lower baiaite can
become confusing because "upper" and "lower"
are no longer an adequate description of morphology. Bainite has recently been reclassified
by its morphology, not by the temperature range
in which it forms (Ref 19). For example, a recent
classification of bainite yields three distinct
types of morphology.
Class 1 (B1): Acicular ferrite associated with
intralath (plate) iron carbide, that is, cemcntite (replaces the term "lower bainite")
Interlamellar spacing (Sp), nm
300
200
100
I
900
80
60
I
f
I
S
. j ¢ ,-
O.
8O0
~ 7oo
.~>6OO
~.500
0
400
60
80
100
120
140
Reciprocal root of
Interlamellar spacing (Sp-1/2), mm-1/2
Fig. 14
Fig, 13
SEM micrograph of pearlite showing ferrile and cementite lamellae. 4% picral etch. 10, O00x
Relationship behveen peadite interlamellar
spacing and yield strength for eutectoid steels.
Source: Ref I0
Structure/Property Relationships in Irons and Steels/163
1.6
1.6
1.2
1,2
0.8
z 0.8
/
o~
o
J~
o=
/~
o
0
2OO
Fig. 15
•
i<
0.4
0.4
225
250
275
300
325
Brinell hardness, HB
350
0
0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26
375
Pearlite spacing, pm
Relationship between hardnessand wear resistance(weight loss)for rail steels.
Source: Ref 13
Class 2 (B2): Acicular ferrite associated with
i n t e r l a t h (plate) particles or films of cementite
and/or austenite (replaces the term "upper
bainite")
• Class 3 (B3): Acicular ferrite associated with a
constituent consisting of discrete islands of
austenite and/or martensite
The bainitic steels have a wide range of mechanical properties depending on the microstructural m o r p h o l o g y and composition; for example, yield strength can range from 450 to 950
MPa (65 to 140 ksi), and tensile strength from
530 to 1200 M P a (75 to 175 ksi). Another aspect
of a bainitic steel is that a single composition,
I/2Mo-B steel for example, can yield a bainitic
microstructure over a wide range o f transformation temperatures. The C C T diagram for this
steel is shown in Fig. 23. Note that for this steel
the bainite start (Bs) temperature is almost constant at 600 °C (1110 °F). This flat t r a n s f o r m a tion region is important because transformation
temperature plays an important role in the development of microstructure. A constant transformation temperature permits the d e v e l o p m e n t of a
similar microstructure and properties over a wide
range of cooling rates. This has m a n y advantages
in the m a n u f a c t u r i n g of bainitic steels and is particularly advantageous in thick sections where a
wide range in cooling rates is found from the
surface to the center of the part.
In designing a bainitic steel with a wide transformation region, it becomes critical that the
pearlite and ferrite regions are pushed as far to
the right as possible on the C C T diagram; that is,
pearlite and ferrite form only at slow cooling
rates. Alloying elements such as nickel, chrom i u m , and m o l y b d e n u m (and m a n g a n e s e ) are selected for this purpose.
For low-carbon bainitic steels, the relationship
between transformation temperature and tensile
strength is s h o w n in Fig. 24 (martensite is discussed in the next section). Note the rapid increase in tensile strength as the transformation
temperature decreases. For these steels, a regression equation for tensile strength has been developed as follows (Ref 21):
Fig. 16
Relationship between pearlite interlamellar spacing and wear resistance
(weight loss)for rail steels.Source: Ref 13
ties (for example, 0.003%) has a pronounced effect on retarding the ferrite transformation. Thus,
in a boron-containing steel (e.g., l/2Mo + B), the
ferrite nose in the C C T diagram is p u s h e d to
slower cooling rates. Boron retards the nucleation of ferrite on the austenite grain boundaries
and, in doing so, permits bainite to be formed
(Fig. 23). W h e n e v e r boron is added to steel, it
m u s t be prevented from combining with other
elements such as oxygen and nitrogen. Generally,
a l u m i n u m and titanium are added first in order to
lower the o x y g e n and nitrogen levels o f the steel.
E v e n when adequately protected, the effectiven e s s of boron decreases with increasing carbon
content and austenite grain size.
A t t e m p t s have been made to quantitatively relate the microstructural features o f bainite to mechanical properties. One such relationship is (Ref
22):
YS = -194 + 17.4(d -1/2) + 15(nl/4)
(Eq 10)
where YS is the 0.2% offset yield strength (in
MPa), d is the bainite lath size (mean linear inter-
cept) (in ram), and n is the n u m b e r o f carbides per
m m 2 in the plane of section.
With bainitic steels, the lath width of the
bainite obeys a Hall-Petch relationship as shown
in Fig. 25. The lath size is directly related to the
austenite grain size and decreases with decreasing bainite transformation temperature. B e c a u s e
of the fine microstructure of bainite, the m e a s u r e m e n t of lath size and carbide density can only
be done by SEM or TEM.
In low-carbon bainitic steels, type B 2 (upper)
bainite has inferior t o u g h n e s s to type B 1 (lower)
bainite. In both cases, strength increases as the
transition temperature decreases. In type B 2 (upper) bainite, the carbides are m u c h coarser than
in type B 1 (lower) bainite and h a v e a tendency to
crack and initiate cleavage (brittle) fracture. In
type B l bainite, the small carbides h a v e less tendency to fracture. One can lower the transition
temperature in type B l bainitic steels by providing a finer austenite grain size through lowertemperature t h e r m o m e c h a n i c a l treatment and
grain refinement.
Bainitic steels are used in m a n y applications
including pressure vessels, b a c k u p rolls, turbine
9oo
700
?
600
¢tz
E
500
_I o.n,r
~_
400
300
200
100
T S = 246.4 + 1925(C) + 231(Mn + Cr) + 185(Mo)
+ 92(W) + 123(Ni) + 62(Cu) + 385(V + 11) (Eq9)
In addition to the elements carbon, nickel,
chromium, m o l y b d e n u m , vanadium, and so forth,
it is well k n o w n that boron in very small quanti-
j
0
Fig. 17
.e
°Clmln
1 -643
2 - 600
0-545_
4 - 500
5 - 450
6 400
7 - 352
8 - 300
9 - 253
10 - 225
11 - 1 8 9
12 - 50
I
10
~
\\1
~s
~ ~
( % ~ .
'Bf
Ms \ "~
Martensite
I
I
I
lO0
Time, s
I
I
1000
^ ccT diagram of a typical rail steel (composition: 0.77% C, 0.95% Mn, 0.22% Si, 0.014% P, 0.017% S, 0.10%
Cr). Source:Ref 14
164 / Structure/Property Relationships in Irons and Steels
(a)
(h)
Fig. 18 Microstructureof typical ferrite-pearlitestructuralsteelsat two differentcarboncontents.(a)0.10%C. (b)0.25%C. 2% nital + 4% picraletch.200x
rotors, die blocks, die-casting molds, nuclear reactor c o m p o n e n t s , and e a r t h m o v i n g equipment.
O n e m a j o r advantage of a bainitic steel is that an
optimal s t r e n g t h / t o u g h n e s s combination can be
produced without e x p e n s i v e heat treatment, for
example, q u e n c h i n g and tempering as in martensitic steels.
by rapid quenching. Most all the conventional
alloying elements in steel promote hardenability.
For example, type 4340 steel s h o w n in Fig. 26
has significant levels o f carbon, m a n g a n e s e ,
nickel, copper, and m o l y b d e n u m to promote hardenability. More details about hardenability can
be found in R e f 2.
2O0
(271)
Martensite
Martensite is essentially a supersaturated solid
solution o f carbon in iron. The a m o u n t of carbon
in martensite far exceeds that found in solid solution in ferrite. B e c a u s e of this, the normal bodycentered cubic (bcc) lattice is distorted in order
to a c c o m m o d a t e the carbon atoms. The distorted
lattice b e c o m e s body-centered tetragonal (bet).
In plain-carbon and low-alloy steels, this supersaturation is generally produced through very
rapid cooling from the austenite phase region
(quenching in water, iced-water, brine, icedbrine, oil or aqueous p o l y m e r solutions) to avoid
forming ferrite, pearlite, and bainite. Some
highly alloyed steels can form martensite upon
air cooling (see the discussion o f m a r a g i n g steels
later in this section). D e p e n d i n g on carbon content, martensite in its q u e n c h e d state can be very
hard and brittle, and, because of this brittleness,
martensitic steels are usually tempered to restore
s o m e ductility and increase toughness.
Reference to a C C T diagram s h o w s that
martensite only forms at high cooling rates in
plain-carbon and low-alloy steels. A C C T diag r a m for type 4340 is s h o w n in Fig. 26, which
indicates that martensite forms at cooling rates
exceeding about 1000 °C/rain. Most commercial
martensitic steels contain deliberate alloying additions intended to suppress the formation o f
other c o n s t i t u e n t s - - t h a t is ferrite, peariite, and
b a i n i t e - - d u r i n g continuous cooling. This m e a n s
that these constituents form at slower cooling
rates, allowing martensite to form at the faster
cooling rates, for example, during oil and water
quenching. This concept is called hardenability
and is essentially the capacity o f a steel to harden
The martensite start temperature (Ms) for type
4340 is 300 °C (570 °F). Carbon lowers the M s
temperature, as s h o w n in Fig. 27, and alloying
elements such as carbon, m a n g a n e s e , chromium,
nickel, and m o l y b d e n u m also lower M s temperature. M a n y empirical equations have been developed over the past 50 years relating M s tempera160
Notched impact tests
oo
/
120
"~ (217)
,~
>:, 120
(163)
=
~
0
~ e
If energy
j ~
,"
Transitiontemperature
80
8
40
80
(106)
~
(54)
~
~
'------
o
-4O
100
80
~
~'~UItimate:
~ ' ~strength
I t~
=0=
.¢
Yield strength
60
~
=m
03
Reductionin area
20 - - Smooth tensile testa
0
Fig. 19
,==
CO
==
40
0
.cg
~
E
e
0
0.1
0.2
0:3
I
0.4
0.5
Carbon,wt%
0.6
0.7
0.8
0.9
Mechanicalpropertiesof ferrite-pearlitesteelsasa functionof carboncontent.Source:Ref2
Structure/Property Relationships in Irons and Steels / 165
Temperature, °F
250
-1 O0
I
0
I
100
I
200
F
300
400
I
0.11%C
150
200
¢=
•
175
125 a=
>~
150
f
t~
¢x
100
g
_E
100
/
0.20% C .." ....................... 0.31 YoC
__
0.41%C
060%C."".""" • .- s ~ ~ ",. . . . . . . . ~0.49%C •2
~~........~
50
,.:
0
-100
- 75
oO."
... s
,'°1
•
.o.°
"~ .."
, , .-" " ' . t . . - "
.. "
--.-'.'.
..........
0.80%C 25
~'~
•
-50
50
0
0
50
100
150
200
250
Temperature, °C
Fig. 20
Effect Of carbon content in ferrite-peadite steels on Charpy V-notch transition temperature and shelf energy.
Source: Ref 17
ture to composition. O n e recent equation by Andrews (Ref 24) is:
M s (°C) = 539 - 423(C) - 30.4(Mn) - 12.1(Cr)
- 17.7(Ni) - 7_5(Mo)
(Eq 11)
With sufficient alloy content, the M s temperature can be below r o o m temperature, which
m e a n s that the transformation is incomplete and
retained austenite can be present in the steel.
The microstructure of martensitic steels can be
generally classed as either lath martensite, plate
martensite, or mixed lath and plate martensite. In
plain carbon steels, this classification is related
(a)
Fig. 21
to carbon content, as s h o w n in Fig. 27. Lath
martensite forms at carbon contents up to about
0.6%, plate martensite is found at carbon contents greater than 1.0%, and a m i x e d martensite
microstructure forms for carbon contents between 0.6 and 1.0%. An example of lath martensite is s h o w n in Fig. 28 and plate martensite in
Fig. 29. Generally, plate martensite can be distinguished from lath martensite by its plate morphology with a central mid-fib. Also, plate
martensite m a y contain n u m e r o u s microcracks,
as s h o w n in Fig. 30. These form during transformation when a growing plate impinges on an existing plate. Because of these microcracks, plate
martensite is generally avoided in most applica-
(b)
Microstructure of (a) upper bainite and (b) lower bainite in a Cr-Mo-V rotor steel.2% nital + 4% picral etch. 500x
tions. The important microstructural units m e a s ured in lath martensite are lath width and packet
size. A packet is a grouping o f laths having a
c o m m o n orientation.
Plain-carbon and low-alloy martensitic steels
are rarely used in the a s - q u e n c h e d state because
of poor ductility. To increase ductility, these
martensitic steels are tempered (reheated) to a
temperature below 650 °C (1200 °F). During
tempering, the carbon that is in supersaturated
solid solution precipitates on preferred crystallographic planes (usually the octahedral {111}
planes) of the martensitic lattice. B e c a u s e of the
preferred orientation, the carbides in a tempered
martensite have a characteristic a r r a n g e m e n t as
seen in Fig. 31.
Tempered martensite has similar morphological features to type B) (lower) bainite. However,
a distinction can be m a d e in terms o f the orientation differences of the carbide precipitates. This
can be seen by comparing type B l bainite in Fig.
22 with tempered martensite in Fig. 31. However,
unless the carbide m o r p h o l o g y is observed it is
very difficult to distinguish between B] bainite
and tempered martensite.
The hardness of martensite is determined by its
carbon content, as s h o w n in Fig. 32. Martensite
attains a m a x i m u m hardness o f 66 H R C at carbon
contents of 0.8 to 1.0%. The reason that the hardness does not m o n o t o n i c a l l y increase with carbon
is that retained austenite is f o u n d when the carbon content is above about 0.4% (austenite is
m u c h softer than martensite). Figure 33 s h o w s
the increase in volume percent retained austenite
with increasing carbon content. Yield strength
also increases with increasing carbon content as
seen in Fig. 34. This empirical relationship between the yield strength and carbon content for
u n t e m p e r e d low-carbon martensite is (Ref 25):
YS (MPa) = 413 + 17.2 x 10P(C1/2)
(Eq 12)
Lath martensite packet size also has an influence
on the yield strength, as s h o w n in Fig. 35. The
166 / Structure/Property Relationships in Irons and Steels
(a)
Fig, 22
(b)
TEM micmgraphs of (a) upper bainite and (b) lower bainite in a Cr-Mo-V rotor steel
linear behavior follows a Hall-Petch type relationship of (d-l/2).
M o s t martcnsitic steels are used in the tempered condition where the steel is reheated after
q u e n c h i n g to a temperature less than the lower
critical temperature (Act). Figure 36 s h o w s the
decrease in hardness with tempering temperature
for a n u m b e r o f carbon levels. Plain-carbon or
low-alloy martensitic steels can be tempered in
lower or higher temperature ranges, depending
on the balance of properties required. Tempering
between 150 and 200 °C (300 and 390 °F) will
1
0
0
maintain m u c h of the hardness and strength of
the q u e n c h e d martensite and provide a small imp r o v e m e n t in ductility and toughness (Ref 26).
This treatment can be used for bearings and gears
that are subjected to compression loading. Tempering above 425 °C (796 °F) significantly improves ductility and toughness but at the expense
of hardness and strength. The effect of tempering
temperature on the tensile properties of a typical
oil-quenched low-alloy steel (type 4340) is
s h o w n in Fig. 37. These data are for a 13.5 m m
(0.53 in.) diam rod quenched in oil. The as-
0
~
1800
Ac3 = 930 °C
900
Fs
.0o
,oo
°o
1!0
1600
?--
6oo
"
-
500
~'E
~
l
I
I "l\
~
\1
I
I \
. . . .
~l~""r I I I ~
+
--
~ I
I
I X I I
I II
quenched rod has a hardness of 601 HB. Note
that by tempering at 650 °C (1200 °F), the hardn e s s (see x-axis) decreased to 293 HB; or to less
than half the a s - q u e n c h e d hardness. The tensile
strength has decreased from 1960 MPa (285 ksi)
at a 200 °C (400 °F) tempering temperature to
965 M P a (141 ksi) at a 650 °C (1200 °F) tempering temperature. However, the ductility, represented by total elongation and reduction in area,
increases dramatically. The tempering process
can be retarded by the addition of certain alloying elements such as vanadium, m o l y b d e n u m ,
m a n g a n e s e , c h r o m i u m , and silicon. Also, for
tempering, temperature is m u c h more important
than time at temperature.
Temper embrittlement is possible during the
t e m p e r i n g of alloy and low-alloy steels. This embrittlement occurs when quenched-and-tempered
steels are heated in, or slow cooled through the
340 to 565 °C (650 to 1050 °F) temperature
range. Embrittlement occurs when the embrittling elements, antimony, tin, and phosphorus,
concentrate at the austenite grain boundaries and
create intergranular segregation that leads to intergranular fracture. T h e element m o l y b d e n u m
-1800
1200
"'"q'e
~-. 1050
200 ~
~
~ ' ~
400
"
100
900
200
0
10
Seconds
I
102
'
'
' ' I
103
'
'
10
1
Minutes
104
~
' ' I
'
' ~I
102
I
'
1
Time
32
105
103
.==
¢/)
i.~
750
600
450
I
' ' I
' I
4
10
30
Hours
Fig. 23 A CCT diagram of a I/2Mo-B steel. Composition: 0.093% C, 0.70% Mn, 0.36% Si, 0.51% Mo, 0.0054% B.
Austenitized at Ac 3 + 30 °C for 12 rain. Bs, bainite start; Bo bainite finish; Fs, ferrite start; Fo ferrite finish. Numbers in circles indicate hardness (HV) after cooling to room temperature. Source: Ref 20
~1
eel==,
Martensites
Bainites
I
I
I
I
3OO
4OO
500
600
Ferrite +
peadite
', ~* "/45
~'~
~-'- i
7OO
i
1-
8O0
Transformation temperature, °C
Fig. 2 4
Relationship between transformation temperature and tensile strength of ferrite-pearlite, bainitic, and martensitic steels. Source: Ref 5
Structure/Property Relationships in Irons and Steels / 167
t~
IL
/
900
to
750
~
"
QII)
600
1600
870
1400
760
1200
650
1000
540
~
o
800
425
E
600
315
400
205
•
•
°
450
15
Fig. 25
20
25
Grain size (d-1/2), mm-1/2
g
Relationship between bainite lath width (grain
size) and yield strength. Source: Ref 5
E
has been shown to be beneficial in preventing
temper embrittlement.
The large variation in mechanical properties o f
q u e n c h e d - a n d - t e m p e r e d martensitic steels provides the structural designer with a large n u m b e r
of property combinations• Data, like that shown
in Fig. 37, are available in the Section "Carbon
and Alloy Steels" in this Handbook as well as
Volume 1 of the ASM Handbook and the ASM
Specialty Handbook: Carbon and Alloy Steels.
Hardnesses of q u e n c h e d - a n d - t e m p e r e d steels can
be estimated by a m e t h o d established by Grange
et al. (Ref 27). The general equation for hardness
is:
HV = HV C + AHVMn + AHVp + AHVsi + AHVNi
+ AHVcr + AHVMo + AHVv
(Eq 13)
where HV is the estimated hardness value (Vickers).
In order to use this relationship, one m u s t determine the hardness value of carbon (HVc) from
Fig. 38. For example, if one a s s u m e s that a tempering temperature of 540 °C (1000 °F) is used
and the carbon content of the steel is 0.2% C, the
HV c value after tempering will be 180 HV. Second, the effect o f each alloying e l e m e n t m u s t be
determined from a figure such as Fig. 39. This
graph represents a tempering temperature of 540
°C (1000 °F). Graphs representing other tempering temperatures can be found in R e f 27.
To illustrate the u s e of the Grange et al.
method, the s a m e type 4340 steel s h o w n in Fig.
37 is used. The composition of the steel is 0.41%
C, 0.67% Mn, 0.023% P, 0.018% S, 0.26% Si,
1.77% Ni, 0.78% Cr, and 0.26% Mo. A s s u m i n g a
540 °C (1000 °F) tempering temperature, the estimated h a r d n e s s value for carbon is 210 HV.
From Fig. 38, the hardness values for each of the
other alloying elements are:
1
Carbon
Manganese
Phosphorus
Silicon
Nickel
Chromium
Molybdenum
Total hardness
Ccetent, %
Hardaess,HV
0.41
0.67
0.023
0.26
1.77
0.78
0.26
210
38
7
15
12
43
55
5
2
10
20
50
100
200
500
Cooling timel s
Fig, 26
The CCT diagram for type 4340 steel austenitized at 845 °C (I 550 °F). Source: Ref 23
e x a m p l e s of hardness conversion tables for
steels, which can be found in the Section "Glossary of Terms and Engineering Data" in this
Handbook), a Brinell hardness of 363 HB equates
to a Vickers hardness of 383 HV. The calculated
value of 380 HV (in the table above) is very
close to the actual m e a s u r e d value of 383 HV.
Thus, this m e t h o d can be used to estimate a specific hardness value after a q u e n c h i n g - a n d - t e m pering heat treatment for a low-alloy steel. Also,
as a rough approximation, the derived Brinell
h a r d n e s s value can be used to estimate tensile
strength by the following equation (calculated
from A S T M E 140 conversion table):
TS (MPa) = - 42.3 +3.6 HB
(Eq 14)
For the above example, a type 4340 quenchedand-tempered (540 °C, or 1000 °F) steel with a
calculated hardness of 363 HB would have an
estimated tensile strength f r o m Eq 14 of 1265
M P a (183 ksi). F r o m Table 1, this m e a s u r e d tensile strength of a type 4340 q u e n c h e d - a n d - t e m pered (540 °C, or 1000 °F) steel is 1255 M P a
(182 ksi).
It is seen that q u e n c h e d - a n d - t e m p e r e d martensitic steels provide a wide range of properties.
The design engineer can c h o o s e from a large
n u m b e r of plain-carbon and low-alloy steels. In
.8o
1600
870
~o
cff
Element
1400
650
1200
540
lOOO =d
ii
o
425
~""~.,,~
800
Q_
E
315
~o
205
400
95
380
l%~: ~
Lath
....
.........
?~Mixea
/ , ~ ~:,:
Plate
D20
According to Fig. 37, the hardness value after
tempering at 540 °C (1000 °F) was 363 HB (see
Brinell hardness values along x-axis). F r o m the
A S T M E 140 conversion table (included in the
95
1000
200
0
0.2
0.4
0.6
0.6
1.0
Carbon, wt%
Fig. 27
Effect of carbon content on M s temperature in steels. Source: Ref 6
1.2
1.4
0
1.6
ff
168 / Structure/Property Relationships in Irons and Steels
Fig. 28
Microstructure of a typical lath martensite. 4% picral + HCI. 200x
addition to this large list of steels, there are two
other commercially important categories o f fully
martensitic steels, namely, martensitic stainless
steels and m a r a g i n g steels.
Like the ferritic stainless steels, martensitic
stainless steels (e.g., type 403, 410, 414, 416,
420, 422, 431, and 440) are h i g h - c h r o m i u m iron
alloys (12 to 18% Cr), but with deliberate additions of carbon (0.12 to 1.2% C). T h e s e steels
use carbon in order to stabilize austenite in ironc h r o m i u m alloys (Fig. 12). The expanded region
of austenite is called the y-loop. In the Fe-Cr
phase diagram (without C), the y-loop e x t e n d s to
about 12% Cr (see Fig. 12). With carbon additions, austenite can exist up to 25% Cr. T h e s e
steels can be heat treated m u c h like those of the
low-alloy steels. However, martensitic stainless
steels, with s u c h high c h r o m i u m contents, can
form martensite on air cooling, even in thick sections. Martensitic stainless steels are considered
Fig. 30
Fig. 29
Microstructure of a typical plate martensite. 4% picral + HCI. 1000x
h i g h - s t r e n g t h stainless steels because they can be
treated to achieve a yield strength between 550
MPa (80 ksi) and 1725 MPa (250 ksi), as seen in
Table 1. On the other hand, ferritic stainless
steels, which do not contain carbon, are not considered h i g h - s t r e n g t h steels because their yield
strength range is only 170 to 450 MPa (25 to 64
ksi). B e c a u s e of their high strength and hardness,
coupled with corrosion resistance, martensitic
stainless steels are used for knives and other applications requiring a cutting edge as well as
s o m e tool steel applications (for example, molds
for producing plastic parts).
Maraging steels are a separate class of martensitic steels and are considered ultrahigh-strength
steels with yield strength levels as high as 2500
MPa (360 ksi), as seen in Table 1. In addition to
extremely high strength, the maraging steels
have excellent ductility and toughness. These
very-low carbon steels contain 17.5 to 18% Ni,
Microcracks formed in plate martensite. 4% picral + HCl/sodium metabisulfite
etch. 1000x
Fig. 31
8.5 to 12.5% Co, 4 to 5% Mo, 0.20 to 1.8% Ti,
and 0,10 to 0.15% A1. Because of the high alloy
content, especially the cobalt addition, they are
very expensive. Their high strength is developed
by austenitizing at 850 °C (1560 °F), followed by
air cooling to r o o m temperature to form lath
martensite. However, the martensitic constituent
in m a r a g i n g steels is relatively s o f t - - 2 8 to 35
H R C - - w h i c h is an advantage because the component can be m a c h i n e d to final form directly
upon cooling. The final stage o f strengthening is
through an aging process, carried out at 480 °C
(900 °F) for 3 h. During aging, the hardness increases to about 51 to 58 HRC depending on the
grade o f m a r a g i n g steel. The aging treatment promotes the precipitation of a rodlike intermetallic
c o m p o u n d Ni3Mo. These precipitates can only be
observed at high magnification (e.g., by TEM).
The precipitates strengthen the surrounding matrix as they form during aging. Full hardening
Ttransmission electron micrograph showing carbide morphology in tempered
martensite
Structure/Property Relationships in Irons and Steels / 169
L
900
65
800
>o
700
>
"I-
6o
ot~
-.r
of
600
¢=
~
so
500
t~
"1-
•->¢
=o
100
"~
75
8
40
300
30
o
]
M s temperature
500
100
Lath marten ' l i t e , ' ~ ~
relative volh°/° - - - ~ ~
t~
03
~"~1..........~
~
/
0
J
I
0.2
0.4
I
I
I
0.6 0.8 1.0
Carbon, wt%
.t
0.4
40
1.2
o
p-
Retained 7, vol%
I
I
I
1.2
1.6
~
0.8
Carbon, wt%
I
Fig. 33 Effectof carbon content on the volume percent of retained austenite (7) in as-quenched martensite. Source:
Ref 4
Fig. 3 2 Effect of carbon content on the hardness of
martensite. Source: Ref4
can be developed, even in very thick sections.
Maraging steels are used for die-casting molds
and aluminum hot-forging dies as well as numerous aircraft and missile components.
Austenite
Austenite does not exist at room temperature
in plain-carbon and low-alloy steels, other than
as small amounts of retained austenite that did
not transform during rapid cooling. However, in
certain high-alloy steels, such as the austenitic
stainless steels and Hadfield austenitic manganese steel, austenite is the microstructure. In
these steels, sufficient quantifies of alloying elements that stabilize austenite at room temperature are present (e.g., manganese and nickel).
The crystal structure of austenite is face-centered
cubic (fee) as compared to ferrite, which has a
(bcc) lattice. A fcc alloy has certain desirable
characteristics; for example, it has low-temperature toughness, excellent weldability, and is nonmagnetic. Because of their high alloy content,
austenitic steels are usually corrosion resistant.
Disadvantages are their expense (because of the
260
<
20
1o
0
100
--
25
20
200
E
3OO
E
400
700
alloying elements), their susceptibility to stresscorrosion cracking (certain austenitic steels),
their relatively low yield strength, and the fact
that they cannot be strengthened other than by
cold working, interstitial solid-solution strengthening, or precipitation hardening.
The austenitic stainless steels (e.g., type 301,
302, 303, 304, 305,308, 309, 310, 314, 316, 317,
321, 330, 347, 348, and 384) generally contain
from 6 to 22% Ni to stabilize the austenite at
room temperature. They also contain other alloying elements, such as chromium (16 to 26%) for
corrosion resistance, and smaller amounts of
manganese and molybdenum. The widely used
type 304 stainless steel contains 18 to 20% Cr
and 8 to 10.5% Ni and is also called 18-8 stainless steel. From Table 1, the yield strength of
annealed type 304 stainless steel is 290 MPa (40
ksi), with a tensile strength of about 580 MPa (84
ksi). However, both yield and tensile strength can
be substantially increased by cold working as
shown in Fig. 40 (see Table 1). However, the
increase in strength is offset by a substantial decrease in ductility, for example, from about 55%
2
i
1200
4
i
elongation in the annealed condition to about
25% elongation after cold working.
Some austenitic stainless steels (type 200, 201,
202, and 205) employ interstitial solid-solution
strengthening with nitrogen addition. Austenite,
like ferrite, can be strengthened by interstitial
elements such as carbon and nitrogen. However,
carbon is usually excluded because of the deleterious effect associated with precipitation of chromium carbides on austenite grain boundaries (a
process called sensitization). These chromium
carbides deplete the grain-boundary regions of
chromium, and the denuded boundaries are extremely susceptible to corrosion. Such steels can
be desensitized by heating to high temperature to
dissolve the carbides and place the chromium
back into solution in the austenite. Nitrogen, on
the other hand, is soluble in austenite and is
added for strengthening. To prevent nitrogen
from forming deleterious nitrides, manganese is
added to lower the activity of nitrogen in the
austenite, as well as to stabilize the austenite.
For example, type 201 stainless steel has composition ranges of 5.5 to 7.5% Mn, 16 to 18% Cr,
ASTM grain size
8
10
i
i
6
=
12
=
120
a.
oo-~,~11700
E
,50
J~-
"o
220
t300
o,°
"~ 140
~. 100
o
60
o-o~
600
"~
t oo
Y
I
800
80
"o
so
0
"~
-lltOO
180
I
I
0.10
0.20
0.30
Carbon content, wt%
~
-r.,
400 ~ . ~
I
0
0.40
Fig. 3 4 Relationshipbetween carbon content and the
yield strengthof martensite.Source: Ref 4
~
40
2
4
6
8
10
12
14
o
16
Lath martensite packet size (d-1/2), mm -1/2
Fig. 3 5
Relationship between lath martensite packet size (dl and yield strength of Fe-0.2%C (upper line) and Fe-Mn
(lower line) martensites. Source: Ref 2
170 / Structure/Property Relationships in Irons and Steels
Tempering temperature,°F
200
70
400
600
1000
1200
555
300
(2070)
1400
477
Hardness, HB
415
363
50 ~
~
~
~
\
250
(172o)
0 ,0,,o
~-o.~c
%
I1.
~
o
N
Tensilestrength
200
'\ - N N N
(13oo) - - 'Yield' point
J::
=
0=00=0;
o}
~=
"lm
40 ~
~
0.10"0.
~20
~/oC ~
NiX
~
100
(690)
I
I
200
I
I
I
300
400
500
600
Tempering temperature, °C
\
Fig. 37
I
700
6O
Elongation
' i ~ '
800 1 0 0 0 1200
600
tic, these steels can be work hardened to provide
higher hardness and wear resistance. A workhardened Hadfield manganese steel has excellent
resistance to abrasive wear under heavy loading.
Because of this characteristic, these steels are
ideal for jaw crushers and other crushing and
grinding components in the mining industry.
Also, Hadfield manganese steels have long been
used for railway frogs (components used at the
junction point of two railroad lines).
Ferrite-Cementite
When plain-carbon steels are heated to temperatures just below the lower critical tempera-
900
As-quenched
800
!i
5ILl
Effect of tempering temperature on the mechanical properties of type 4340 steel. Source:
Ref 2
Decreasein the hardnessof martensitewith temperingtemperaturefor various carbon contents.Source: Ref 2
3.5 to 5.5% Ni, and 0.25% N. The other type 2xx
series of steels contain from 0.25 to 0.40% N.
Another important austenitic steel is austenitic
manganese steel. Developed by Sir Robert Hadfield in the late 1890s, these steels remain
austenitic after water quenching and have considerable strength and toughness. A typical Hadfield
manganese steel will contain 10 to 14% Mn, 0.95
to 1.4% C, and 0.3 to 1% Si. Solution annealing
is necessary to suppress the formation of iron
carbides. The carbon must be in solid solution to
stabilize the austenite. When completely austeni-
ture (Ac]), the process of spheroidization takes
place. Figure 41 shows a fully spheroidized steel
microstructure. The microstructure before spheroidization is pearlite. During spheroidization,
the cementite lamellae of the pearlite must
change morphology to form spheroids. The process is controlled by the diffusion rate of carbon
and portions of the lamellae must "pinch-off"
(dissolve) and that dissolved carbon must diffuse
to form a spheroid from the remaining portions
of lamellae. This process takes several hours.
Spheroidization takes place in less time when the
starting microstructure is martensite or tempered
martensite. In this process, the spheroidized carbides are formed by growth of carbides formed
during tempering.
A fully spheroidized structure leads to improved machinability. A steel in its fully sphe-
65
As-quenched /
hardiness " ~ - / / I
/
0=
~0~°F I
400°F
8O
°°
], /
..,°
/b'/''
"f"
z ~ " l " - ~.c.
"O
/
70
=
n-
600 °F..,@
,6
50 ="
>
"1-
60
~
50
~
4o
~
o
3o
il
300
--~
(200) (320) (430) (540) (650)
Tempering temperature, °F(°C)
As100
quenched
400
70
4O
400
10
"
t~
- - Reductionin area
20
'=
¢5
150
(1030)
30
Fig. 36
293
\
60
n-°
800
• -.
; ; ~ o ~
800 °F"°"
~,,,,, ~o- I
45
40 ~
.E
~.,,,~ ~ , - , 1000OF.o" 30
j
• Mo
e•
•tw
.S/
p
!° Cr
o
Si
20
200
~....,~1200 °F=o=
1300 °F
100
0.2
0.4
0.6
I
0.8
lO
1.0
0
0.02
Carbon, %
Fig. 38
Relationship between hardness of tempered
martensite with carbon content at various tempering temperatures.Source: Ref 2
j
0.04
o.o6
0.1
0.2
0.4
0.6
f
1
2
Element content, %
Fig. 39
E ~ of alloying elementson the retardationor softeningduring tempering at 540 °C (1000 °F) relative to ironcarbon alloys. Source:Ref 2
Strudure/Property Relationships in Irons and Steels / 171
1200
0
/
~+,
, + , . . + .,yff/S+~ ++
z9
O,o,:"
Q
~
-
*o "
/ f
100o
-- Tensile/~stStrength
-- /
"x
s~t"
~,
~
•~,
o+u t z
,
.o0.
o
DO
°%0
v
# =
",-,~',
.
.
:':,~,
-
,,~.
=
o
o-
~
o
o
~©
o
~ mo
=
600 /
I
/ '
.
•
rength
(0.2*/.offset)
O . .++
~200
~~
0
Fig. 40
0
10
20 30 40
Cold work. %
• ~
40 =m
o
[]
20
~
50
o,,o
o +°
,"
~.
t)
•
o o " +' (/)1 (7 * . , ~ ,,,,. ~ ====, I=, .',,a
¢'~'~
oo o oo
v
. ,0 I=
~F' =
,~,=p
"
OC~ 06
0
"
"o. o
0+,o+
.o
.~l..~,,o.:
~>/2 + .~.,o,'-." • ,
~:~ ,e-~a ~, ~" 0,//~,,.. ,,o 2+ :+" '
•
~,
.= . . . .
."1 ".
~
==
%
~
+
^+L
~+)1
.. oOo..~:" 0~"o o:.- "'+,~" +o~"~'~ ~o'+<3c
. oo ,,? a : - - , £ . 0v++o
.
¢
~',,*"
o
o
~'
60
°e:+
Influence of cold work on mechanical properties of type 304 stainlesssteel.Source:Ref 4
roidized state is in its softest possible condition.
Some steels, such as type 1020, are spheroidized
before cold forming into tubing because
spheroidized steels have excellent formability.
Ordinary low-carbon, cold-rolled, and annealed sheet steels have ferritic microstructures
with a small amount of grain-boundary cementite, as shown in Fig. 8. These carbides nucleate
and grow on the ferrite grain boundaries during
the annealing process, which takes place in the
lower portion of the intercritieal temperature region (i.e,, the region between the A 3 and A 1 temperatures shown in the iron-carbon diagram, Fig.
6). Many modern-day automotive sheet steels are
produced with very low carbon levels to avoid
these grain-boundary carbides because they degrade formability.
Ferrite-Martensite
A relatively new family of steels called dualphase steels consists of a microstracture of about
Fig, 41
e"
ooV+
'~ d'-" o~;~, +
"
\
Microstructureof a fullyspheroidizedsteel.4 % picraletch.1000x
15 to 20% martensite in a matrix of ferrite. The
microstructure of a typical dual-phase steel is
shown in Fig. 42. In most plain-carbon and lowalloy steels, the presence of martensite in the
microstructure is normally avoided because of
the deleterious effect that martensite has on ductility and toughness. However, when the martensite is embedded in a matrix of ferrite, it imparts
desirable characteristics. One desirable characteristic is that dual-phase steels do not exhibit a
yield point. Figure 43 compares the stress-strain
behavior of four steels: plain carbon, SAE 950X,
and SAE 980X, which exhibit a yield point with
the fourth, a dual-phase steel (GM 980X). This
means that the cosmetically unappealing Ltiders
bands that form during the discontinuous yielding (i.e., yield point) are absent in a dual-phase
steel. Also note in Fig. 43 that the dual-phase
steel has much more elongation than the SAE
980X of similar tensile strength. These characteristics are especially important in formability.
A unique characteristic of a ferrite-martensite
dual-phase steel is its substantial work hardening
capacity. This allows the steel to strengthen
while being deformed. By proper design of the
stamping dies, this behavior can be exploited to
produce a high-strength component. Most conventional high-strength steels have limited formability because their high strength is developed
prior to the forming process.
FerriteoAustenite
High-alloy steels having approximately equal
proportions of fcc austenite and bcc ferrite, with
ferrite comprising the matrix, are referred to as
duplex stainless steels. The microstructure of a
typical duplex stainless steel is shown in Fig. 44.
Although the exact amount of each phase is a
function of composition and heat treatment, most
alloys are designed to contain about equal
amounts of each phase in the annealed condition.
100
~" (552)(690)80~
~v
/
40
038)
0
Fig. 43
Microstructureof a typical dual-phasesteel.2% nital etch. 250x
........
60 t" ~ J ~
(414)
~ (276) /
Fig. 42
/
0
/
SAE 950X
~',1'IGM 980X'
~'~
Plainc a / ~ n ~
I
10
20
30
40
Strain in two-inch gage length, %
Comparisonof the stress-strainc u r v e s of three
discontinuouslyyielding sheetsteels(plain carloon,5AE 950X, and SAE980X) and a dual-phasesteel(GM
980X). in addition to the differences in yielding behavior,
note the higher percentageof uniform elongation in the
dual-phase steel compared with the conventional SAE
980X of similar tensilestrength.Source:Ref 2
172 / Structure/Property Relationships in Irons and Steels
The duplex structure results in i m p r o v e d stresscorrosion cracking resistance, c o m p a r e d with
austenitic stainless steels, and i m p r o v e d toughn e s s and ductility, compared with the ferritic
stainless steels. Duplex stainless steels are capable of tensile yield strengths ranging f r o m 400 to
550 MPa (60 to 80 ksi) in the annealed condition,
which is approximately twice the strength of
either phase alone.
The principal alloying e l e m e n t s in duplex
stainless steels are c h r o m i u m and nickel, but nitrogen, m o l y b d e n u m , copper, silicon, and tungsten m a y be added to control structural balance
and to impart certain corrosion-resistance characteristics. Four commercial groups o f duplex
stainless steels, listed in order o f increasing corrosion resistance, are:
Fe-23Cr-4Ni-0.1N
• Fe-22Cr-5.5Ni-3Mo-0.15N
• Fe-25Cr-5Ni-2.5Mo-0.17N-Cu
• Fe-25Cr-7Ni-3.5Mo-0.25N-W-Cu
Fig. 44
B e c a u s e o f their excellent corrosion resistance,
ferrite-austenite duplex stainless steels have
found widespread use in a range o f industries,
particularly the oil and gas, petrochemical, pulp
and paper, and pollution control industries. They
are c o m m o n l y used in aqueous, chloride-containing e n v i r o n m e n t s and as r e p l a c e m e n t s for
austenitic stainless steels that have suffered
stress-corrosion cracking or pitting during service.
Graphite
W h e n carbon contents o f iron-carbon alloys
exceed about 2%, there is a tendency for graphite
to form (see Fe-C diagram in Fig. 6b). This is
especially true in gray cast iron in w h i c h graphite flakes are a p r e d o m i n a n t microstructural feature (Fig. 3). Gray cast iron has been used for
centuries because it melts at a lower temperature
than steel and is easy to cast into various shapes.
Also, the graphite flakes impart good machinability, acting as chip breakers, and they also
provide excellent d a m p i n g capacity. D a m p i n g capacity is important in m a c h i n e s that are subject
to vibration. However, gray cast iron is limited to
applications that do not require t o u g h n e s s or ductility, for example, total elongation o f less than
1%. The flake m o r p h o l o g y of the graphite provides for easy crack propagation under applied
stress.
Gray cast irons usually contain 2.5 to 4% C, 1
to 3% Si, and 0.1 to 1.2% Mn. T h e graphite
flakes can be p r e s e n t in five different morpholoType A
Microstructure of a typical mill-annealed duplex stainless steel plate showing elongated austenite islands in the
ferrite matrix. Etched in 15 mL HCI in 100 mL ethyl alcohol. 200x
gies as seen in Fig. 45. Type A, because o f its
r a n d o m orientation and distribution, is preferred
in m a n y applications, for example, cylinders of
internal combustion engines. The matrix of a
typical gray cast iron is usually pearlite. However, ferrite-pearlite or martensitic microstructures can be developed by special heat treatments. As a structural material, gray cast iron is
selected for its high compressive strength, which
ranges from 572 to 1293 MPa (83 to 188 ksi),
although tensile strengths of gray iron range only
from 152 to 431 MPa (22 to 63 ksi). Gray cast
irons are used in a wide variety of applications,
including automotive cylinder blocks, cylinder
heads and brake drums, ingot molds, m a c h i n e
h o u s i n g s , pipe, pipe fittings, manifolds, compressors, and pumps.
A n o t h e r form of graphite in cast iron is
spheroidal graphite found in ductile cast irons
(also called nodular cast irons). The microstructure of a typical ductile cast iron is shown in
Fig. 46. This form of graphite is produced by a
process called inoculation, in which a m a g n e s i u m or cerium alloy is thrust into molten cast
iron immediately prior to the casting operation.
T h e s e elements form intermetallic c o m p o u n d s
that act as a nucleating surface for graphite. With
a spherical morphology, the graphite no longer
renders the cast iron brittle as do graphite flakes
in gray cast iron. Ductile irons have m u c h higher
ductility and toughness than gray iron and thus
expand the use of this type of ferrous alloy. M o s t
ductile iron castings are used in the as-cast form.
However, heat treatment can be employed to al-
Type B
ter the matrix microstructure to obtain desired
properties. The matrix can be fully ferritic, fully
pearlitic, fully martensitic, or fully bainitie, depending on composition and heat treatment. The
yield strength o f typical ductile cast irons ranges
from 276 to 621 MPa (46 to 76 ksi), and their
tensile strengths range from 414 to 827 MPa (60
to 120 ksi). Total elongation ranges from about 3
to 18%. Heat treated, austempered ductile irons
have yield strengths ranging from 505 to 950
MPa (80 to 138 ksi), tensile strengths ranging
from 860 to 1200 MPa (125 to 174 ksi), and total
elongations ranging from 1 to 10%. Uses for duetile iron include gears, crankshafts, paper-mill
dryer rolls, valve and p u m p bodies, steering
knuckles, rocker arms, and various machine components.
Cementite
A m a j o r microstructural constituent in white
cast iron is cementite. The microstructure of a
typical white cast iron is s h o w n in Fig. 47. The
cementite f o r m s by a eutectic reaction during solidification:
T h e e u t e c t i c c o n s t i t u e n t in w h i t e cast iron is
called ledeburite and has a two-phase morphology
shown as the smaller particles in the white matrix
in Fig. 48. The eutectic is s h o w n in the Fe-C
Type D
Type C
./T.¢,i!
/
,'~..,,~,,,
i:t"
~,~.<.'.,..~ .~
,,. --'~,1
Uniform distribution,
random orientation
Fig. 45
E
I/ / ; [_,~'~"
72,:
•
Type
.~>..'
•
,/
(Eq 15)
Liquid ~-~ Cementite + Austeuite
'~.
'
~.:
•
Rosette grouping,
random orientation
Classificationof different graphite flake morphology
Superimposed flake size,
random orientation
Interdendritic segregation,
random orientation
Interdendritie segregation,
preferred orientation
Structure/Property Relationships in Irons and Steels / 173
Fig, 46
Microstructure of a typical ductile (nodular) cast
iron showing graphite in the form of spheroids.
2% nital etch. 200x. Courtesy of A.O. Benscoter, Lehigh
University
Fig, 47
Microstructure of a typical white cast iron. 4% picral etch. 100x. Courtesy of A.O. Benscoter, Lehigh University
binary diagram in Fig. 6(b). The austenite in the
eutectic (as well as the austenite in the primary
phase) transforms to pearlite, ferrite-pearlite, or
martensite, depending on cooling rate and composition. Because of the high percentages of cementite, white cast irons are used in applications
requiring excellent wear and abrasion resistance.
These irons contain high levels of silicon, chromium, nickel, and molybdenum and are termed
alloy cast irons. Such applications include steel
mill rolls, grinding mills, and jaw crushers for the
mining industry. Hardness is the primary mechanical property of white cast iron and ranges
from 321 to 400 HB for pearlitic white iron and
400 to 800 HB for alloy (martensitic) white irons.
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Fig. 48
Microstructure of the eutectic constituent ledebutite in a typical white cast iron. 4% picral etch. 500x. Courtesy
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