2. Steels can be classified by a variety of different systems
depending on: The composition, such as carbon, low-alloy
or stainless steel.
The manufacturing methods, such as open hearth, basic
oxygen process, or electric furnace methods.
The finishing method, such as hot rolling or cold rolling
The product form, such as bar plate, sheet, strip, tubing or
structural shape
The deoxidation practice, such as killed, semi-
killed, capped or rimmed steel
The microstructure, such as ferritic, pearlitic and
martensitic
The required strength level, as specified in ASTM
standards
The heat treatment, such as annealing, quenching and
tempering, and thermo mechanical processing
Quality descriptors, such as forging quality and
commercial quality.
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4. Classifications
►Steel is classified according to the alloying
elements it contains.
► Carbon is the most important element; therefore
all steels are classified according to carbon content.
► Plain carbon steels contain primarily iron &
carbon, and they are classified as10XX steels.
► The first two digits refer to plain carbon steel.
► The third & fourth digits refer to the carbon
content in hundredths of a percent.
► Thus, a 1035 steel is a plain carbon steel with 0.35
% carbon.
► There are varying amounts of other materials in
carbon steel, but their content is so small that they
do no affect physical properties.
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5. Alloy steels
• Alloy steels are classified by the society of Automotive
Engineers (SAE) & by the American Iron & Steel
Institute (AISI).
• Some of the designations accepted by them as
standard are shown in table 3.3.
• Often as many as 5 or more alloying elements may be
present, & it is impractical to describe the alloy
correctly by a simple numbering system.
• Steels may be more broadly classified as follows
A. Carbon steel
1. Low carbon - less than 0.30 %
2. Medium carbon - 0.30% to 0.70 %
3. High carbon – 0.70 % to 2.0% (Normally the upper
limit is 1.40 %)
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8. Table3.3 Classification of steel
Classification Number Range of Number
0.45 % Ni, 0.15 % Cr, 0.20 % Mo 97XX 9747-9763
1.0 % Ni, 0.80 % Cr, 0.25 % Mo 98XX 9840-9850
Boron (~0.005 % Mo) XXBXX
Boron is denoted by addition of B. Boron-Vanadium is denoted
by addition of BV. Example: 14BXX, 50BXX, 80BXX, 43 BV14. The
letters appearing before the no indicate the following: A, Alloy
basic open hearth: B, carbon-acid Bessemer; C, carbon basic
open hearth; D, carbon acid open hearth; E, electric furnace
Stainless and heat resisting steels:
2XX Chromium- Nickel-Manganese types
3XX Chromium- Nickel types
4XX Straight Chromium types
5XX Low Chromium types
All stainless steels are produced in electric furnace
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9. Classification Contd
B. Alloy Steel
1.low alloys-special alloying elements totaling
less than 8.0 %
2. High alloys- special alloying elements totaling
more than 8.0 %
Low carbon steel is used for wire, structural shapes,
and screw machine parts such as screws, & bolts.
Medium carbon steels are used for rails, axles, gears,
and parts requiring high strength & moderate to great
hardness. High carbon steels find use in cutting tools
such as knives, drills, taps, and for abrasion
resisting properties.
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10. Classification Contd
The Society of Automotive Engineers (SAE) has
established standards for specific analysis of steels. In
the 10XX series, the first digit indicates a plain carbon
steel. The second digit indicates a modification in the
alloys. 10XX means that it is a plain carbon steel
where the second digit (zero ) indicates that there is
no modification in the alloys. The last two digits denote
the carbon content in points. For example SAE 1040 is
a carbon steel where 40 points represent 0.40 %
Carbon content. Alloy steels are indicated by 2XXX,
3XXX, 4XXX, etc.. The American Iron and Steel
Institute (AISI) in cooperation with the Society of
Automotive Engineers (SAE) revised the percentages
of the alloys to be used in the making of steel, retained
the numbering system, and added letter prefixes to
indicate the method used in steel making. The letter
prefixes are:
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11. Classification of Steels
A = alloy, basic open hearth
B = carbon, acid Bessemer
C = carbon, basic open hearth
D = carbon, acid open hearth
E = electric furnace
If the prefix is omitted, the steel is assumed
to be open hearth. Example: AISI C1050
indicates a plain carbon, basic-open hearth
steel that has 0.50 % Carbon content.
Another letter is the harden ability or H-
value. Example: 4340H
General representation of steels:
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20. Classification Contd
• Red Hardness: This property , also called hot-
hardness, is related to the resistance of the steel to
the softening effect of heat. It is reflected to some
extent in the resistance of the material to
tempering.
• Hardenability: This property determines the depth
and distribution of hardness induced by quenching.
• Hot-shortness: Brittleness at high temperatures is
called hot-shortness which is usually caused by
sulfur. When sulfur is present, iron and sulfur form
iron sulfide (FeS) that is usually concentrated at the
grain boundaries and melts at temperatures below
the melting point of steel.
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21. Classification Contd
• Due to the melting of iron sulfide, the cohesion
between the grains is destroyed, allowing cracks
to develop. This occurs when the steel is forged
or rolled at elevated temperatures. In the
presence of manganese, sulfur tends to form
manganese sulfide (MnS) which prevents hot-
shortness.
• Cold-shortness: Large quantities of phosphorus
(in excess of 0.12%P) reduces the ductility,
thereby increasing the tendency of the steel to
crack when cold worked. This brittle condition at
temperatures below the recrystallization
temperature is called cold-shortness.
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