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Carbon and Alloy Steels
Content:
–Introduction,
–Carbon steel
–Classification of alloy steel
–Effect of alloying elements.
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Steel - Introduction
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
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Steel – Introduction ….. Contd.
• The deoxidation practice,
– such as killed, semi-killed – capped, and 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
thermomechanical processing
• Quality descriptors,
– such as forging quality and commercial quality
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Carbon Steel
• The American Iron and Steel Institute
(AISI) defines carbon steel as follows:
– Steel is considered to be carbon steel when
no minimum content is specified or required
for chromium, cobalt, columbium [niobium],
molybdenum, nickel, titanium, tungsten,
vanadium or zirconium, or any other element
to be added to obtain a desired alloying effect.
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Carbon steels
• Steels whose alloying elements do not
exceed the following limits:
Element Max weight %
C 1.00 (2%)
Cu 0.60
Mn 1.65
P 0.40
Si 0.60
S 0.05
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L + Fe3C
2.14 4.30
6.70
0.022
0.76
M
N
C
P
E
O
G
F
H
Cementite Fe3C
The Iron–Iron Carbide Phase Diagram
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Carbon steels
• Effects of carbon in the carbon steel,
increased hardness
increased strength
decreased weldability
decreased ductility
Machinability - about 0.2 to 0.25%
C provides the best machinability
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Carbon steel
• increasing carbon content leading to,
– increased hardness and strength
– increases brittleness and reduces weldability .
• Carbon steels ( Max 2% C) are generally
categorized according to their carbon content.
– low-carbon steels ( < 0,30 % C)
– medium-carbon steels ( 0,30% – 0,45% C)
– high-carbon steels( 0,45% - 0,75% C)
– ultrahigh-carbon steels ( Up to 1,5 % C)
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Classification of carbon steel-Designation system:
• American Iron and Steel Institute (AISI) together with
Society of Automotive Engineers (SAE) have
established four-digit (with additional letter prefixes)
designation system:
• SAE 1XXX
• First digit 1 indicates carbon steel (2-9 are used for alloy steels);
• Second digit indicates modification of the steel.
– 0 - Plain carbon, non-modified
– 1 - Resulfurized
– 2 - Resulfurized and rephosphorized
– 5 - Non-resulfurized, Mn over 1.0%
• Last two digits indicate carbon concentration in 0.01%.
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Designation system - modification of the steel
XX :0.xx% average carbon
content
AISI
10
60
10
:Nonresulfurized grades
11
:Resulfurized grades
12
:Resulfurized and rephosphorized grades
15
:Nonsulfurized grades; max Mn content > 1%
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Classification of carbon steel-Designation system:
• A letter prefix before the four-digit number
indicates the steel making technology:
– A - Alloy, basic open hearth
– B - Carbon, acid Bessemer
– C - Carbon, basic open hearth
– D - Carbon, acid open hearth
– E - Electric furnace
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Example:Designation system SAE 1040 ?
SAE 1040
Indicates whether is a carbon steel or alloy
steel ( 1 indicates carbon steel, 2 and above
indicates alloy steel)
Modification in alloy(none) : plain carbon
Carbon content(0.40 %)
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Example:Designation system:
• SAE 1030
– means non modified carbon steel( Plain carbon),
– containing 0.30% of carbon.
• AISI B1020
– means non-modified carbon steel,
– produced in acid Bessemer and
– containing 0.20% of carbon
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Alloy Steel - Introduction,
Alloying
Changing chemical composition of steel by adding elements
with purpose to improve its properties as compared to the
plane Carbon steel.
Alloy Steels are irons where other elements (besides
carbon) can be added to iron to improve:
Mechanical property - Increase strength, hardness,
toughness (a given strength & hardness),
creep, and high temp resistance.
Increase wear resistance,
Environmental property [Eg: corrosion].
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Classification of metal
alloys
Ferrous Non - ferrous
Cast Iron Steels
Low Alloy High Alloy
Low
Carbon Med.
Carbon
High
Carbon Stainless
Steel
Tool
Steel
White
Grey
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Alloy steels grouped into low, medium and
high alloy steels.
High-alloy steels would be the stainless steel groups.
Most alloy steels in use fall under the category of low alloy
Alloy steels are, in general, with elements as:
> 1.65%Mn, > 0.60% Si, or >0.60% Cu.
The most common alloy elements includes:
Chromium, nickel, molybdenum, vanadium,
tungsten, cobalt, boron, and copper
Classification of alloy steel
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Low Alloys: Low Carbon
•Composition:
• less than ~ 0,25% C ( 0,30%)
•Microstructure:
•ferrite and pearlite
•Properties:
•relatively soft and weak, but possess high ductility and toughness
•Other features: machinable and weldable, not responsive to heat
treatment - Plain carbon steels
Applications: auto-body components, structural shapes, sheets etc.
• High-strength low alloy (HSLA) steels:
• up to 10 wt% of alloying elements, such as Mn, Cr, Cu, V, Ni, Mo –
can be strengthened by heat-treatment
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Low Alloys: Medium Carbon Steels
• Composition:
– 0.25< C <0.6 C wt.%
• Microstructure:
– typically tempered martensite
• Processing: Increasing the carbon content to approximately
0.5% with an accompanying increase in manganese allows
medium carbon steels to be used in the quenched and tempered
condition.
• Properties: stronger than low-carbon steels, but in expense of
ductility and toughness
• Applications: couplings, forgings, gears, crankshafts other
high-strength structural components. Steels in the 0.40 to
0.60% C range are also used for rails, railway wheels and rail
axles.
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Low Alloys: High&Ultra High - Carbon Steels
• High-carbon steels
0.60 to 1.00 % C with manganese contents ranging
from 0.30 to 0.90%.
Application: High-carbon steels are used for spring
materials, high-strength wires, cutting tools and etc.
Ultrahigh-carbon steels are experimental alloys
containing 1.25 to 2.0% C. These steels are thermo-
mechanically processed to produce microstructures
that consist of ultra-fine, equiaxed grains of spherical,
discontinuous proeutectoid carbide particles.
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High-Alloy Steels: Stainless Steels (SS)
• The primarily-alloying element is Cr (≥11 wt.%)
• Highly resistance to corrosion;
– Nickel and molybdenum additions INCREASE
corrosion resistance
• A property of great importance is the ability of alloying
elements to promote the formation of a certain phase or to
stabilize it.
– These elements are grouped as four major classes:
1. austenite-forming,
2. ferrite-forming,
3. carbide-forming and
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Distribution of alloying elements in steels.
• Alloying elements can influence the equilibrium
diagram in two ways in ternary systems Fe-C-X.
1. Expanding the γ -field, and encouraging the
formation of austenite over wider compositional
limits. These elements are called γ -stabilizers.
2. Contracting the γ-field, and encouraging the
formation of ferrite over wider compositional limits.
These elements are called α-stabilizers.
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Classification of iron alloy phase diagrams: a. open γ -field; b. expanded γ -field;
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Classification of iron alloy phase diagrams: c. closed γ -
field d. Contract γ - field
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Nickel and manganese
depress the phase
transformation from γ to α to
lower temperatures
both Ac1 and Ac3 are lowered.
It is also easier to obtain
metastable austenite by
quenching from the γ-region to
room temperature
A. Open - field: austenitic steels.
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B. Expanded -field : austenitic steels
Carbon and nitrogen (Copper,
zinc and gold)
The γ-phase field is expanded
Heat treatment of steels,
allowing formation of a
homogeneous solid solution
(austenite) containing up to
2.0 wt % of carbon or 2.8 wt
% of nitrogen
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C. Closed -field : ferritic steels
Silicon, aluminium, beryllium and
phosphorus (strong carbide forming
elements - titanium, vanadium,
molybdenum and chromium )
γ-area contract to a small area referred
to as the gamma loop
encouraging the formation of BCC
iron (ferrite),
Not amenable to the normal heat
treatments involving cooling through the
γ/α-phase transformation
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D. Contracted -field : ferritic steels
• Boron is the most significant element of
this group (carbide forming elements -
tantalum, niobium and zirconium.
• The γ-loop is strongly contracted
• Normally elements with opposing
tendencies will cancel each other out at
the appropriate combinations, but in
some cases irregularity occur. For
example, chromium added to nickel in a
steel in concentrations around 18%
helps to stabilize the γ-phase, as shown
by 18Cr8Ni austenitic steels.
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High-Alloy Steels: Stainless Steels (SS)
(a) The austenitic SS:
• -Fe FCC microstructure at room temperature.
Typical alloy Fe-18Cr-8Ni-1Mn-0.1C
• Stabilizing austenite – increasing the
temperature range, in which austenite exists.
• Raise the A4 point (the temperature of
formation of austenite from liquid phase) and
decrease the A3 temperature.
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High-Alloy Steels: Stainless Steels (SS)
• Austenite-forming elements
•The elements Cu, Ni, Co and Mn
Disadvantage: work harden rapidly so more
difficult to shape and machine
• Advantages of ALL fcc metals and alloys
toughness;
ductility;
creep resistance
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High-Alloy Steels: Stainless Steels (SS)
(b) The ferritic SS:
– α−Fe BCC structure.
– Not so corrosion resistant as austenitic SS, but less
expensive magnetic steel;
An alloy Fe-15Cr-0.6C, used in quench and tempered
condition
Used for: rust-free ball bearings, scalpels, knives
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High-Alloy Steels: Stainless Steels (SS
lower the A4 point and increase the A3 temperature.
Ferrite-forming elements
The most important elements in this group are Cr,
Si, Mo, W, V and Al.
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High-Alloy Steels: Stainless Steels (SS)
(c) The martensitic SS this fine magnetic bcc structure is
produced by rapid quenching and possesses high yield
strength and low ductility.
Applications: springs.
(d) The precipitation hardening SS – producing multiple
microstructure form a single-phase one, leads to the
increasing resistance for the dislocation motion.
– (a) and (b) are hardening and strengthening by cold work
• Microstructure - martensitic, ferritic or austenitic based on
microstructure, and precipitation hardening based on
strengthening mechanism
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High-Alloy Steels: Tools steels
• Wear Resistant, High Strength and Tough BUT low ductility
• High Carbon steels modified by alloy additions
AISI-SAE Classification
Letter & Number Identification
Classification
Letters pertain to significant characteristic
W,O,A,D,S,T,M,H,P,L,F
– E.g. A is Air-Hardening medium alloy
Numbers pertain to material type
1 thru 7 (E.g. 2 is Cold-work )
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High-Alloy Steels: Tools steels
• Provide the necessary hardness with simpler
heat-treatment and retain this hardness at
high temperature.
• The primary alloying elements are:
– Mo, W and Cr
•Examples:
I. HSS – Turning machine tools
II. High carbon tool steels – Drill
bits/Milling tools/punches/saw blade
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SAE-AISI system - Classification of alloy steel
• Low alloy steels (alloying elements =< 8%);
• High alloy steels (alloying elements > 8%).
• According to the four-digit classification SAE-AISI system -
SAE 1XXX (X)
– First digit: 1 indicates carbon steel (2-9 are used for alloy
steels);
– Second digit indicates concentration of the major element
in percents (1 means 1%).
– Last two ( three) digits indicate carbon concentration in
0.01% ( 0,001%).
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SAE-AISI system - SAE 1XXX(X)
• First digit indicates the class of the alloy
steel:
2- Nickel steels;
3- Nickel-chromium steels;
4- Molybdenum steels;
5- Chromium steels;
6- Chromium-vanadium steels;
7- Tungsten-chromium steels;
9- Silicon-manganese steels.
Other reference materials
7. Tungsten
8. Nickel, Chromium and
Molybdenum
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Stainless steels – ANSI designation:
AISI has established three-digit system for the stainless steels:
• 2XX series – chromium-nickel-manganese austenitic
stainless steels;
• 3XX series – chromium-nickel austenitic stainless steels;
• 4XX series – chromium martensitic stainless steels or
ferritic stainless steels;
• 5XX series – low chromium martensitic stainless steels;
Stainless steel contains a
maximum of 1.2 % carbon, a
minimum of 10.5% chromium
(standard EN 10088-1) and
other alloying elements
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Example: SAE 5130
• Alloy chromium steel,
• containing 1% of chromium and
• 0.30% of carbon.
– SAE 2515:
• Indication for carbon or alloy steel ( 2 = Nickel steel)
• Major alloying element (5% Nickel)
• Carbon content (0.15%)
– SAE 5120
• Indication for carbon or alloy steel ( 5 = Chromium steel)
• Major alloying element (1% Chromium)
• Carbon content (0.20%)
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Effect of alloying elements
• Rule of thumb: Chromium (Cr) makes steel
hard whereas Nickel (Ni) and Manganese
(Mn) make it tough.
• Note that:
– 2% C, 12% Cr tool steel grade - very hard and
hard-wearing
– 0,10% C and 12% Cr - Modest hardening
– 13% manganese steel, so-called Hadfield steel
- increases steel toughness
– Mn between l% and 5%, however - toughness
may either increase or decrease
48. Effect of Alloying Elements
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Aluminum
Ferrite hardener
Graphite former
Deoxidizer
Chromium
Mild ferrite hardener
Moderate effect on hardenability
Graphite former
Resists corrosion
Resists abrasion
Cobalt
High effect on ferrite as a hardener
High red hardness
Molybdenum
Strong effect on hardenability
Strong carbide former
High red hardness
Increases abrasion resistance
49. Effect of Alloying Elements…cont
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Manganese Strong ferrite hardener
Nickel
Ferrite strengthener
Increases toughness of the
hypoeutectoid steel
With chromium, retains austenite
Graphite former
Copper
Austenite stabilizer
Improves resistance to corrosion
Silicon
Ferrite hardener
Increases magnetic properties in steel
Phosphorus
Ferrite hardener
Improves machinability
Increases hardenability
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Effect of alloying elements
2XXX Nickel steels
5 % Nickel increases the tensile strength without reducing ductility
8 to 12 % Nickel increases the resistance to low temperature impact
15 to 25 % Nickel (along with Al, Cu and Co) develop high magnetic
properties. (Alnicometals)
25 to 35 % Nickel creates resistance to corrosion at elevated temperatures.
3XXX Nickel-chromium steels
These steels are tough and ductile and exhibit high wear resistance
, hardenability and high resistance to corrosion.
4XXX Molybdenum steels
Molybdenum is strong carbide former. It has a strong effect on
hardenability and high temperature hardness. Molybdenum also
increases the tensile strength of low carbon steels.
5XXX Chromium steels
Chromium is a ferrite strengthener in low carbon steels. It
increases the core toughness and the wear resistance of the case in
carburized steels.
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Effect of alloying elements on the properties of steel
Carbon has a major effect on steel properties.
Hardness and
tensile strength
increases as carbon
content increases
up to about 0.85% C
Ductility and
weldability decrease
with increasing
carbon