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Alloy Steel - Introduction,

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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

HighCarbon Stainless

Steel

Tool Steel

White

Grey

Classification of alloy steel

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Classification of alloy steelAlloy 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

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Low Alloys: Low Carbon

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• 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 steelsApplications: 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 martensiteProcessing: 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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Phase change- SS

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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.

B. Expanded -field : austenitic steelsCarbon 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 steelsSilicon, 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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18Cr8Ni austenitic steels.

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opposing tendencies

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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Fe-Ni equilibrium diagram

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A4 increase

A3 Decrease

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 machineAdvantages 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 conditionUsed for: rust-free ball bearings, scalpels, knives

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Cr-Fe equilibrium diagram

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Lower A4Increase the A3

High-Alloy Steels: Stainless Steels (SS)lower the A4 point and increase the A3 temperature. Ferrite-forming elementsThe 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

ductilityHigh Carbon steels modified by alloy additions      

AISI-SAE ClassificationLetter & Number Identification 

ClassificationLetters pertain to significant characteristic

W,O,A,D,S,T,M,H,P,L,F –  E.g. A is Air-Hardening medium alloyNumbers pertain to material type 1 thru 7 (E.g. 2 is Cold-work )

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High-Alloy Steels: Tools steelsProvide 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 toolsII.High carbon tool steels – Drill

bits/Milling tools/punches/saw blade

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THANK YOU

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