Mechanical Engineering Materials-I

Mr.H.J.AHIREProfessor, Dept of Mechanical Engg.,Late Julalsign Mangtu College of Engg., Diploma Chalisgaon

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Classification of Steels & Cast iron Iron Carbon Phase diagram

Contains

Classification of Engineering Materials

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Solid Metals Classified as Amorphous:- Material have no regular

arrangement of their molecules. Crystalline:- The atoms are arranged in

a three dimension array called a lattice Ferrous:- These material contain iron as

their main contain Non-Ferrous:-These material contain

other than ferrous material

Classification of Engineering Material

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Amorphous Crystalline

Classification of Engineering Material

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A crystal is defined as an orderly array of atom in space.

Crystalline form of solid has periodically reputed arrangement of atoms Polymorphism:-It is ability of solid material to

exist in more than one form or crystal structure. Types :- (a) BCC; (b) FCC (c) HCP

Crystal Structure

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Body Centered Cubic (BCC) A Unit Cell Contains 8 atoms at corner X 1/8 =01 1 Center atom =01 Total atoms = 02

Examples: chromium, tungstenAlpha iron, delta iron, vanadium

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Face Centered Cubic (FCC) A Unit Cell Contains 8 atoms at corner X 1/8 = 01 6 atoms at face X 1/2 =03 Total atoms = 04

Examples: aluminum, nickel, Copper, gold, silver, lead, platinum

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Hexagonal-Close Packed (HCP) A Unit Cell Contains 12 atoms at corner X 1/8 =1.5 2 atoms at face X 1/2 =01 3 Center atom =03 Total atoms = 5.5

Examples: Magnesium, Beryllium,Zinc, cadmium

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Lattice :- Unit Cell is the smallest part of the lattice which when repeated in three directions produces the lattice.

Unit Cell :-Unit Cell is the smallest part of the lattice which represents the lattice.

Lattice and Unit Cell

Atomic Packing Efficiency Atomic Packing Efficiency is the

fraction of volume occupied by atoms in a unit cell.

APE = vol. of atomic spheres in unit cell total unit cell vol.

Atomic Packing Efficiency for BCC

Geometry: 2 atoms/unit cell

68.0834

3342

3

3

a

a

VVAPE

cell

atoms

4R a 3

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APE = vol. of atomic spheres in unit cell total unit cell vol.

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Density:- It is defined as the mass per unit volume of the material

Melting Point:- It is the fixed and constant temperature at which pure metal or non-metal changes from solid to liquid form

Specific Heat:- The amount of heat required to raise the temperature of material by 1◦C

Physical Properties

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Thermal expansion:-When thermal energy is added to a material, the change in its dimension is known as thermal expansion

Thermal Conductivity:- It is the mode of transmission from one substance to other in direction of fall of temperature

Physical Properties

Mechanical Properties of Metals

Stress and Strain Stress: When an external load is applied to a

material the material resist the deformation force upon cross section area

Strain When deformation is caused per unit length

or volume Change in dimension to the original

dimension

Strength“Ability of a material to resist deformation.” or“The strength of material is its ability to withstand external forces applied on it”Tensile strength: Measure of level of tensile stress required to make material fail.Compressive strength: Maximum compressive stress that a material can resist without being crushed.

Tensile Strength

Compressive Strength

Ductility & Brittleness Ability of a

material by which it can be drawn into wires.

Opposite to ductility.

Tendency of a body to break without being distorted.

Malleability Ability of a body to be plastically extended in all

directions without breaking under compressive forces only.

Property by which metals drawn into sheets.

HARDNESSResistance to the plastic deformation.Hardness of a material indicates the strength of material to resist penetration ,abrasion and wear

Toughnesso Measure of amount of energy that a material

can absorb before fracturing.o Work done to propagate a crack.

Stiffness“Ability of a material to resist bending.”“It is defined as resistance of material to

elastic deformation”

Creep• Progressive deformation of a

material under constant load with time.

• Important for some type of engineering design particularly those operating on high temperature.

• Tertiary creep > Primary creep > Secondary creep.

Fatigue It occurs due to repeated loading and unloading. It is defined as behavior of a material when

exposed to fluctuating or periodic loads

Elasticity and Plasticity Elasticity: ability of a material to return to its original

shape after applied load is removed

Plasticity: Property of a material to its permanent

deformation of material after applied load is removed

Metals

Ferrous metals Non-ferrous metals

Steels Cast Irons

Plain carbon steels

Low alloy steels

High alloy steelsStainless & Tool steels

Grey Iron

White Iron

Malleable Irons

Low carbon steels

Medium carbon steels

High carbon steels

Ferrous-Carbon alloy classification

Ductile Irons

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Pure Metal :- A pure metal is defined as an element only one single element

Alloy :- It is the mixture of two or more elements

Equilibrium Diagrams

Metal A+ Metal B Metal C

Equilibrium Diagrams for Isomorphous system

Heating

Metal A Metal B

Solid

Liquid

Liquids Liquid + solid

Solidus

T2

T1

Temp.

Solid A+ Solid B Liquid

Equilibrium Diagrams for Eutectic system

Heating

Cooling

Metal A Metal B

Solid

Liquid

M-A+Liquid

Solid Metal A+B

T2 T1

Temp.

E Liquid + Metal B

Solid α+ Solid β Solid γ

Equilibrium Diagrams for Eutectoid system

Heating

Cooling

Metal A Metal B

Solid

Solid Solution γ

Solid Metal α + β

T2 T1

Temp.

E α + γ

β + γ

Iron Carbon Equilibrium Diagram

Allotropes of Iron

If the change in structure is reversible, then the polymorphic change is known as allotropy.

Five individual phases a–ferrite (BCC) Fe-C solid solution g-austenite (FCC) Fe-C solid solution d-ferrite (BCC) Fe-C solid solution Fe3C (Iron Carbide) or cementite –

an inter-metallic compound Pearlite

a–ferrite (BCC) Fe-c Solid Solution

Known as a –iron a–ferrite is solid solution of carbon in iron. It is BCC structure Maximum solubility of Carbon in Iron is 0.02% at 723◦C Pure iron at room temperature Soft & ductile and imparts these properties to

the steel.

g-austenite (FCC) Fe-C solid solution

g–austenite is solid solution of carbon in iron. It is FCC structure Maximum solubility of Carbon in Iron is 2.08% at 1148◦C Known as g –iron Much softer than ferrite Not present at room temperatures. More easily hot worked

d-ferrite (BCC) Fe-C solid solution

d-ferrite is solid solution of carbon in iron. It is BCC structure Maximum solubility of Carbon in Iron is 0.09% at 1195◦C

Fe3C (Iron Carbide) or cementite

Maximum solubility of Carbon in Iron is 6.67% at 1147◦C and Iron 93.3% It is hard , brittle and crystal structure is

orthorhombic Hard, brittle, white melts at 1837°C , density of 7.4 g/cc Its presence in steels causes an increase in

hardness and a reduction in ductility and toughness

PearlitePearlite is not a phase.It is a microconstituent and is a mixture of

two phases a- Ferrite and Fe3C.Pearlite is eutectoid steelA laminated structure formed of alternate

layers of ferrite and cementite with average composition 0.83% carbon

It combines the hardness and strength of cementite with the ductility of ferrite and is the key to the wide range of the properties of steels.

This gives it toughness

Three invariant reactions

Peritectic reaction at 1495˚C and 0.18%C, d-ferrite + L↔ g-iron (austenite)

 Eutectic reaction at 1147˚C and 4.3 %C, L ↔ g-iron + Fe3C (cementite)

 Eutectoid reaction at 727˚C and 0.77%C, g-iron ↔ a–ferrite+Fe3C (cementite) [pearlite]

Fe-C alloy classification Fe-C alloys are classified according to wt.% C

present in the alloys Commercial pure irons % C < 0.008 Low-carbon steels 0.008 - %C - 0.3  Medium carbon steels 0.3 - %C - 0.8 High-carbon steels 0.8- %C - 2.14  Cast irons 2.14 < %C 

Cast irons Cast irons that were slowly cooled to room

temperature consists of cementite, look whitish – white cast iron.

If it contains graphite, look grayish – gray cast iron.

It is heat treated to have graphite in form of nodules – malleable cast iron.

If inoculants are used in liquid state to have graphite nodules – spheroidal graphite (SG) cast iron.

Time-Temperature Transformation Diagram for Austenite To Pearlite

Time-temperature Transformation Diagram For Plain Carbon Steel

Time-Temperature Transformation Diagram on Rapid Cooling

Pearlite 727 - 540°C Bainite 540 - 210°C Martensite below 210°C

Transformation of Austenite in Eutectoid steel

Transformations involving austenite

Heat Treatment of Steels

Heat Treatment process is a series of operations involving the heating and cooling of  metals in the solid state.  Its purpose is to change a mechanical property or combination of mechanical properties so that the metal will be more useful, serviceable, and safe for definite purpose. By heat treating, a metal can be made harder, stronger, and more resistant to impact,

Classification Heat Treatment Processes

1) Annealing2) Normalizing 3) Hardening4) Tempering5) Surface Hardening

Heat Treatment Purposeand Application

Purpose Harden and strengthen metals Reliving internal stresses Improve machinability change in grain size Improve ductility and

toughness Improve electrical and

magnetic property

Applications Hate treatment of forgings of

shaft and axels, drills, cutting tools, taps, dies

Measuring instruments etc.

1.Annealing Process

Purpose Refining structure Reliving internal stresses Improve machinability Reducing hardness Producing desirable

microstructure Improving mechanical,

physical and electrical property

Applications Steel used in sheet and wire

drawing Casting of carbon and alloy steels High carbon tool steels Ball bearing steels

Types a) Stress relieving b) Process annealingc) Spheroidise annealingd) Full annealing

Process Process of heating a metal which is in a metastable or

distortion state. Temperature which remove the distortion and cooling in furnace

for slow cooling process.

A. Process Annealing

It is also called as subcritical annealing The steel is heated below lower critical temperature 500◦ to

700 ◦ C Holding time periods 2 to 4 hours Process annealing is the continuous or batch type in furnace

cooling method As slow cooling process It is applied for low carbon steel used to draw the wires and

deep drawing operation

B.Spheroidise Annealing

Heat treatment used to produce spheroidal form of cementite from of plates of cementite in steel is called spheroidise annealing

It is applied for High carbon steels The steel is heated below lower critical temperature 650◦ to

700 ◦ C Holding prolonged time period. This resulting steel has improved machinability, ductility

and toughness

C.Full Annealing

It is also called as conventional annealing The steel is heated 30 ◦ C to 50 ◦ C above the upper critical

temperature. Holding time periods 2 to 4 hours Rate of cooling 30 ◦ C to 200 ◦ C / Hrs The process is used mainly to remove the internal stresses. It is applied for casting carbon and alloy steel

D. Stress relieving Annealing

Stress relieving Annealing Relieves or eliminates stresses induced by casting, machining, cold working

It is special type of annealing applied for the purpose of stress reliving

The cold working steel is heated about temperature 500◦ Below its recrystallisation temperature Holding time periods 1 to 2 hours As slow cooling process

2. Normalizing Process

Process of heating a steel to about 40◦ C to 50◦ C above the upper critical temperature

Cooling in air type because of faster cooling compared to annealing

Desirable temperature of steel shall maintained for a time period more than 2 min/ mm of section thickness

Temperature shall not be exceed more than 50◦ C above the upper critical temperature

The structure produced by this process is pearlite or pearlite in ferrite matrix

Because the steel is cooled in air to produced the fine peralite with improved mechanical properties

2. Normalizing Purpose

Uniform structure Refines the grain size of steel Improve machinability

Reducing internal stresses Produces harder and stroner steel Improve structure in welds Improves engineering property of

steel

Applications Normalizing is usually

performed on rolled and cast steel to refine grain structure

Improve microstructure Applied for low and mediuum

carbon steel It is applied on welded

structure to improve homogeneity

Advantages Refines the grain size of steel structure To encourage reduced grain segregation in casting and forgings\ Provide moderate hardening

3. Hardening Process

Hardening is that heat treatment of steel which increases its hardness Tools of machine and machine parts having heavy duty are required

often hardness