steel and effect of alloying elements

4/27/2015 Engineering Materals II (MEng 2122) 1

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

Engineering Materals II (MEng 2122)

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Steel – Introduction ….. Contd.• The deoxidation practice,

– such as killed, semi-killed – capped, and rimmed

• 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

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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Classification scheme for ferrous alloys

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Classification of ferrous alloys

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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

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

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].

Classification of metal

alloys

Ferrous Non - ferrous

Cast Iron Steels

Low Alloy High Alloy

Carbon Med.

Carbon

CarbonStainless

Classification of alloy steel

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

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

Low Alloys: Medium Carbon Steels• Composition:

– 0.25< C <0.6 C wt.%

• Microstructure:

– typically tempered martensite

• Processing: Increasing the carbon content to approximately0.5% with an accompanying increase in manganese allowsmedium carbon steels to be used in the quenched and temperedcondition.

• Properties: stronger than low-carbon steels, but in expense ofductility and toughness

• Applications: couplings, forgings, gears, crankshafts otherhigh-strength structural components. Steels in the 0.40 to0.60% C range are also used for rails, railway wheels and railaxles.

Low Alloys: High&Ultra High - Carbon Steels

• High-carbon steels

0.60 to 1.00 % C with manganese contents rangingfrom 0.30 to 0.90%.

Application: High-carbon steels are used for springmaterials, high-strength wires, cutting tools and etc.

Ultrahigh-carbon steels are experimental alloyscontaining 1.25 to 2.0% C. These steels are thermo-mechanically processed to produce microstructuresthat consist of ultra-fine, equiaxed grains of spherical,discontinuous proeutectoid carbide particles.

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

Distribution of alloying elements in steels.

• Alloying elements can influence the equilibriumdiagram in two ways in ternary systems Fe-C-X.

1. Expanding the γ -field, and encouraging theformation of austenite over wider compositionallimits. These elements are called γ -stabilizers.

2. Contracting the γ-field, and encouraging theformation of ferrite over wider compositional limits.These elements are called α-stabilizers.

Phase change- SS

Classification of iron alloy phase diagrams: a. open γ -field; b. expanded γ -field;

Classification of iron alloy phase diagrams: c. closed γ -

field d. Contract γ - field

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.

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

Engineering Materals II (MEng 2122) 30

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

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.

18Cr8Ni austenitic steels.

opposing tendencies

(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.

Fe-Ni equilibrium diagram

A4 increase

A3 Decrease

• Austenite-forming elements

•The elements Cu, Ni, Co and Mn

Disadvantage: work harden rapidly so moredifficult to shape and machine

• Advantages of ALL fcc metals and alloys

toughness;

ductility;

creep resistance

(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

Cr-Fe equilibrium diagram

Lower A4

Increase the A3

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.

(c) The martensitic SS this fine magnetic bcc structure isproduced by rapid quenching and possesses high yieldstrength and low ductility.

Applications: springs.

(d) The precipitation hardening SS – producing multiplemicrostructure form a single-phase one, leads to theincreasing resistance for the dislocation motion.

– (a) and (b) are hardening and strengthening by cold work

• Microstructure - martensitic, ferritic or austenitic based onmicrostructure, and precipitation hardening based onstrengthening mechanism

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 )

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

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%).

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

Stainless steels – ANSI designation:

AISI has established three-digit system for the stainless steels:

• 2XX series – chromium-nickel-manganese austeniticstainless 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

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%)

Low Alloys - Summary

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

Effect of Alloying Elements

AluminumFerrite hardener Graphite former Deoxidizer

Chromium

Mild ferrite hardener Moderate effect on hardenability Graphite former Resists corrosion Resists abrasion

CobaltHigh effect on ferrite as a hardener High red hardness

Molybdenum

Strong effect on hardenability Strong carbide former High red hardness Increases abrasion resistance

Effect of Alloying Elements…cont

Manganese Strong ferrite hardener

Nickel

Ferrite strengthener Increases toughness of the hypoeutectoid steel With chromium, retains austenite Graphite former

CopperAustenite stabilizer Improves resistance to corrosion

Silicon Ferrite hardener Increases magnetic properties in steel

Phosphorus Ferrite hardener Improves machinability Increases hardenability

4/27/2015 Engineering Materals II (MEng 2122)

Effect of alloying elements2XXX Nickel steels

5 % Nickel increases the tensile strength without reducing ductility8 to 12 % Nickel increases the resistance to low temperature impact15 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 steelsThese steels are tough and ductile and exhibit high wear resistance , hardenability and high resistance to corrosion.

4XXX Molybdenum steelsMolybdenum 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 steelsChromium is a ferrite strengthener in low carbon steels. It increases the core toughness and the wear resistance of the case in carburized steels.

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

Effect of alloying elements on the properties of steel

steel and effect of alloying elements

carbon steelseffects

carbon steels max

carbon steelssteels

carbon concentration

plain carbon

cmediumcarbon steels

lowcarbon steels

c highcarbon steels

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