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STANDARD AISI and SAE STEELS
Definitions and explanations
Studies have been made in the steel
industry for the purpose of establishing certain “standard” steels
and eliminating as much as possible the manufacture of other steels
which vary only slightly in composition from the standard steels, These
standard steels are selected on the basis of serving the significant
metallurgical and engineering needs of fabricators and users of steel
products.
STANDARD CARBON STEELS
Definition. By common custom. steel is
considered to be carbon steel when no minimum content is specified or
required for aluminum, boron. chromium, cobalt, columbium, molybdenum.
nickel, titanium, tungsten, vanadium or zirconium, or for any other
element added to obtain a desired alloying effect; when the specified
minimum for copper does not exceed 0.40 per cent; or when the maximum
content specified for any of the following elements does not exceed the
percentages noted: manganese 1.65, silicon 0.60, copper 0.60.
Numbering System. In the AISI system of
identification. the prefix “B” is used to designate acid bessemer
steel. The letter “L’’ within the grade number is used to identify
leaded steels.
A four-numeral series is used to
designate graduations of chemical composition of carbon steel. The last
two numbers of which are intended to indicate the approximate middle of
the carbon range. For example, in the grade designation 1035, 35
represents a carbon range of 0.32 to 0.38 per cent.
It is necessary, however. to deviate
from this rule and to Interpolate numbers in the case of some carbon
ranges and for variations in manganese, phosphorus or sulphur with the
same carbon range.
The first two digits of the four-numeral
series of the various grades of carbon steel and their meanings are as
follows:
10xx Nonresulphurized carbon steel grades
11xx Resulphurized carbon steel grades
12xx Rephosphorized and resulphurized carbon steel grades
I5xx Nonresulphurized high manganese carbon steels.
STANDARD ALLOY STEELS
Definition. Steel is considered to be
alloy steel when the maximum of the range given for the content of
alloying elements exceeds one or more of the following limits:
manganese, 1.65 per cent; silicon, 0.60 per cent; copper, 0.60 per cent;
or in which a definite range or a definite minimum quantity of any of
the following elements is specified or required within the limits of the
recognized field of constructional alloy steels: aluminum, boron,
chromium up to 3.99 per cent, cobalt, columbium, molybdenum, nickel,
titanium, tungsten, vanadium, zirconium or any other alloying element
added to obtain a desired alloying effect.
Numbering System. In the AISI numbering
system, the prefix letter E is used to designate steels normally made
only by the basic electric furnace process. Steels without a prefix
letter are normally manufactured by the basic open hearth or basic
oxygen processes, but may be manufactured by the basic electric furnace
process with adjustments in phosphorus and sulphur limits.
The last two digits of the four-numeral
series are intended to indicate the approximate middle of the carbon
range. For example, in the grade designation 4142, 42 represents a
carbon range of 0.40 to 0.45 per cent. (Where a five-numeral series
occurs, the last three digits indicate the carbon content.) It is
necessary, however, to deviate from this rule and to interpolate numbers
in the case of some carbon ranges, and for variations in manganese,
sulphur, chromium, or other elements.
The first two digits indicate the type
of alloy according to alloying elements as follows:
13xx Manganese 1.75 per cent
40xx Molybdenum 0.20 or 0.25 per cent
41xx Chromium 0.50, 0.80 or 0.95 per cent — Molybdenum 0.12, 0.20 or
0.30 per cent
43xx Nickel 1.83 per cent — Chromium 0.50 or 0.80 per cent —
Molybdenum 0.25 per cent
44xx Molybdenum 0.53 per cent
46xx Nickel 0.85 or 1.83 per cent — Molybdenum 0.20 or 0.25 per cent
47xx Nickel 1.05 per cent Chromium 0.45 per cent
48xx Nickel 3.50 per cent Molybdenum 0.25 per cent
50xx Chromium 0.40 per cent
51xx Chromium 0.80, 0.88, 0.93, 0.95 or 1.00 per cent
5xxxx Carbon 1.04 per cent -- chromium 1.03 or 1.45 per cent
61xx Chromium 0.60 or 0.95 per cent -- Vanadium 0.13 per cent or 0.15
per cent min.
86xx Nickel 0.55 per cent --Chromium 0.50 per cent-- Molybdenum 0.25 per
cent
87xx Nickel 0.55 per cent -- Chromium 0.50 per cent -- Molybdenum 0.35
88xx Nickel 0.55 per cent --Chromium 0.50 per cent -- Molybdenum 0.35
92xx Silicon 2.00 per cent
EFFECTS OF COMMON
ALLOYING ELEMENTS IN STEEL
By definition, steel is a combination of
iron and carbon. Steel is alloyed with various elements to improve
physical properties and to produce special properties, such as
resistance to corrosion or heat. Specific effects of the addition of
such elements are outlined below:
Carbon (C), although not usually
considered as an alloying element, is the most important constituent of
steel. It raises tensile strength, hardness and resistance to wear and
abrasion. It lowers ductility, toughness and machinability.
Manganese (Mn) is a deoxidizer and
degasifier and reacts with sulphur to improve forgeability. It increases
tensile strength, hardness, hardenability and resistance to wear. It
decreases tendency toward scaling and distortion. It increases the rate
of carbon-penetration in carburizing.
Phosphorus (P) increases strength and
hardness and improves machinability. However, it adds marked brittleness
or cold-shortness to steel.
Sulphur (S) Improves machinability in
free-cutting steels, but without sufficient manganese it produces
brittleness at red heat. It decreases weldability, impact toughness and
ductility.
Silicon (Si) is a deoxidizer and
degasifier. It increases tensile and yield strength, hardness,
forgeability and magnetic permeability.
Chromium (Cr) increases tensile
strength, hardness, hardenability. toughness, resistance to wear and
abrasion. resistance to corrosion and scaling at elevated temperatures.
Nickel (Ni) increases strength and
hardness without sacrificing ductility and toughness. It also increases
resistance to corrosion and scaling at elevated temperatures when
introduced in suitable quantities in high chromium (stainless) steels.
Molybdenum (Mo) increases strength,
hardness, hardenability and toughness, as well as creep resistance and
strength at elevated temperatures. It improves machinability and
resistance to corrosion and it intensifies the effects of other alloying
elements. In hot-work steels, it increases red-hardness properties.
Tungsten (W) increases strength,
hardness and toughness. Tungsten steels have superior hot-working and
greater cutting efficiency at elevated temperatures.
Vanadium (V) increases strength,
hardness and resistance to shock impact. It retards grain growth,
permitting higher quenching temperatures. It also enhances the red
hardness properties of high speed metal cutting tools and intensifies
the individual effects of other major elements.
Cobalt (Co) Increases strength and
hardness and permits higher quenching temperatures. It also intensifies
the individual effects of other major elements in more complex steels.
Aluminum (Al) is a deoxidizer and
degasifier. It retards grain growth and is used to control austenitic
grain size. In nitriding steels it aids in producing a uniformly hard
and strong nitrided case when used in amounts 1.00% - 1.25%.
Lead (Pb), while not strictly an
alloying element, is added to improve machining characteristics. It is
almost completely insoluble in steel, and minute lead particles, well
dispersed, reduce friction where the cutting edge contacts the work.
Addition of lead also improves chip-breaking formations.
HEAT TREATMENT OF STEEL
By thermal treatment, steel may be made harder or softer, stresses
induced or relieved, mechanical properties increased or decreased,
crystalline structure changed, machinability enhanced, etc. The terms
used to describe such heat treatments and their effects are listed
below.
NORMALIZE
Normalizing consists of uniform heating to a temperature slightly above
the point at which grain structure is affected (known as the critical
temperature), followed by cooling in still air to room temperature. This
produces a uniform structure and hardness throughout.
ANNEAL
When not preceded by a descriptive adjective, annealing consists of
heating to and holding at a suitable temperature, then allowing to cool
slowly. Annealing removes stresses, reduces hardness, increases
ductility and produces a structure favorable for formability.
Full Anneal - This term is synonymous with annealing
and is used to differentiate anneal from bright anneal, stress relief
anneal, etc.
Spherodize Anneal - This treatment is similar to full
annealing except the steel is held at an elevated temperature for a
prolonged period of time, followed by slow cooling in order to produce a
microstructure where carbides exist in a globular or spheroidal form.
Soft Anneal - When maximum softness and ductility are
required without change in grain structure, steel should be ordered soft
annealed. This process consists of heating to a temperature slightly
below the critical temperature and cooling in still air.
Stress Relief Anneal - Stress relieving is intended
to reduce the residual stresses imparted to the steel in the drawing
operation. It generally consists of heating the steel to a suitable
point below the critical temperature followed by slow cooling.
Bright Anneal - This process consists of annealing in
a closely controlled furnace atmosphere which will permit the surface to
remain relatively bright.
QUENCH
Quenching consists of heating steel above the critical range, then
hardening by immersion in an agitated bath of oil, water, brine or
caustic. Quenching increases tensile strength, yield point and hardness.
It reduces ductility and impact resistance. By subsequent tempering some
ductility and impact resistance may be restored, but at some sacrifice
of tensile strength, yield point and hardness.
TEMPER
Tempering is the reheating of steel, after quenching, to the specified
temperature below the critical range, then air cooling. It is done in
furnaces, oil or salt baths, at temperatures varying from 300 to 1200°F.
Low tempering temperatures give maximum hardness and wear resistance.
Maximum toughness is achieved at the higher temperatures.
RELATIONSHIP OF HARDNESS TO
TENSILE
STRENGTH OF CARBON & ALLOY STEEL
|
Brinell
Indentation
Diameter
mm
|
Brinell Hardness
Number |
Rockwell
Hardness
Number |
Tensile
Strength
(Approx.
1000 psi)
|
|
Standard Ball
|
Tungsten
Carbide Ball
|
B
Scale |
C
Scale |
|
2.45
2.50
2.55
2.60
2.65
|
--
--
--
--
--
|
627
601
578
555
534
|
--
--
--
--
--
|
58.7
57.3
56.0
54.7
53.5
|
347
328
313
298
288
|
2.70
2.75
2.80
2.85
2.90 |
--
--
--
--
--
|
514
495
477
461
444
|
--
--
--
--
--
|
52.1
51.0
49.6
48.5
47.1
|
274
264
252
242
230
|
2.95
3.00
3.05
3.10
3.15 |
429
415
401
388
375
|
429
415
401
388
375
|
|
45.7
44.5
43.1
41.8
40.4
|
219
212
202
193
184
|
|
3.20
3.25
3.30
3.35
3.40
|
363
352
341
331
321
|
363
352
341
331
321
|
--
--
--
--
--
|
39.1
37.9
36.6
35.5
34.3
|
|
3.45
3.50
3.55
3.60
3.65
|
311
302
293
285
277
|
311
302
293
285
277
|
|
33.1
32.1
30.9
29.9
28.8
|
149
146
141
138
134
|
3.70
3.75
3.80
3.85
3.90 |
269
262
255
248
241
|
269
262
255
248
241
|
--
--
--
--
100.00
|
27.6
26.6
25.4
24.2
22.8
|
130
127
124
120
116
|
|
3.95
4.00
4.05
4.10
4.15
|
235
229
223
217
212
|
235
229
223
217
212
|
99.0
98.2
97.3
96.4
95.5
|
21.7
20.5
--
--
--
|
114
111
104
103
100
|
4.20
4.25
4.30
4.35
4.40 |
207
201
197
192
187
|
207
201
197
192
187
|
94.6
93.8
92.8
91.9
90.7
|
--
--
--
--
--
|
99
97
94
92
90
|
|
4.45
4.50
4.55
4.60
4.65
|
183
179
174
170
167
|
183
179
174
170
167
|
90.0
89.0
87.8
86.8
86.0
|
--
--
--
--
--
|
89
88
86
84
83
|
4.70
4.80
4.90
5.00
5.10 |
163
156
149
143
137
|
163
156
149
143
137
|
85.0
82.9
80.8
78.7
76.4
|
--
--
--
--
--
|
82
80
73
71
67
|
5.20
5.30
5.40
5.50
5.60
|
131
126
121
116
111
|
131
126
121
116
111
|
74.0
72.0
69.0
67.6
65.7
|
--
--
--
--
--
|
65
63
60
58
56
|
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* This table, which is based on ASTM A 370-68, Table lll,
lists the approximate relationship of hardness values to
corresponding approximate tensile strength values of steels.
Some compositions and processing histories may deviate from
these relationships. The data in this table do not represent
hardness-to-tensile strength conversions for austenitic,
ferritic, and martensitic stainless steel. If more precise
conversions are required, they should be developed for each
specific composition and heat treatment. Related Rockwell
superficial hardness numbers, if of interest, may be found
in ASTM A 370-68.
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