engineering material lec#1

7/30/2019 Engineering Material LEC#1

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It is the field of engineering that encompasses the spectrum of materials types and how to use

them in manufacturing. Materials engineering is different from Materials science. Materials science involvesinvestigating the relationships that exist between the structures and properties of materials,

whereas, Materials engineering, on the basis of these structureproperty correlations, design orengineer the structure of a material to produce a predetermined set of properties. From a

functional perspective, the role of a materials scientist is to develop or synthesize new materials,

whereas a materials engineer is called upon to create new products or systems using existing

materials, and/or to develop techniques forprocessing materials.What is a Material? Everything we see and use is made of materials Engineers make things. They make them out of materials.Why Study Materials Engineering?

In order to be a good designer, an engineer must learn what materials will be appropriate to use indifferent applications.

Any engineer can look up materials properties in a book or search databases for a material that

meets design specifications, but the ability to innovate and to incorporate materials safely in a

design is rooted in an understanding of how to manipulate materials properties and functionality

through the control of the materials structure and processing techniques.

WHAT IS MATERIALS ENGINEERING?


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EngineeringMaterials

Metals &

Alloys

Advanced

MaterialsPolymers

Ceramics

& Glasses Composites

Metals and AlloysAtoms in metals and their alloys are arranged in a very orderly manner, and in comparison

to the ceramics and polymers, are relatively dense . Metals have High electrical

conductivity, good formability, Castable, machinable. An alloy is a metal that contains

additions of one or more metals or non-metals. Metals and alloys have relatively high

strength, high stiffness, ductility or formability, and shock resistance.

CeramicsThermally insulating, Refractories. Ceramics can be defined as inorganic crystalline

materials. Beach sand and rocks are examples of naturally occurring ceramics. Traditional

ceramics are used to make bricks, tableware, toilets, bathroom sinks, refractories (heat-

resistant material), and abrasives. In general, due to the presence of porosity (small

holes), ceramics do not conduct heat well; they must be heated to very high temperatures

before melting. Ceramics are strong and hard, but also very brittle. Advanced ceramics

are materials made by refining naturally occurring ceramics and other special processes.

Glasses

Optically transparent. Glass is an amorphous material, often, but not always, derived from

a molten liquid. The term amorphous refers to materials that do not have a regular,periodic arrangement of atoms.


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Polymers (Greek; Polys + mers = many + parts)

Polyethylene Food packaging Easily formed into thin, flexible, airtight film. Electrically insulating and moisture-resistant.

Polymers are typically organic materials. They are produced using a process known as polymerization. Polymeric

materials include rubber (elastomers), PE, nylon, PVC, PC, PS, and silicone rubber. and many types of adhesives.

Polymers typically are good electrical and thermal insulators although there are exceptions such as the semiconducting

polymers

Composites

The main idea in developing composites is to blend the properties of different materials. The design goal of a composite

is to achieve a combination of properties that is not displayed by any single material, and also to incorporate the best

characteristics of each of the component materials.

Advanced Materials

Semiconductors

Used in Silicon Transistors and integrated circuits. Unique electrical behaviour, converts electrical signals to light,

lasers, laser diodes, etc.

Biomaterials

Biomaterials are employed in components implanted into the human body to replace diseased or damaged body parts.

These materials must not produce toxic substances and must be compatible with body tissues (i.e., must not cause

adverse biological reactions).

Smart MaterialsThese materials are able to sense changes in their environment and then respond to these changes in predetermined

manners. Smart material include some type of sensors, and actuators. Piezoelectric actuators expand and contract in

response to an applied electric field .

Nanomaterials

Nanomaterials may be any one of the four basic types; metals, ceramics, polymers, & composites. the term is limited to

dealing with particles whose dimensions range in size from a few nanometres up to around 100 nm.


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One common item that presents some interesting material property requirements is

the container for carbonated beverages. The material used for this application must

satisfy the following constraints: (1) provide a barrier to the passage of carbon

dioxide, which is under pressure in the container; (2) be nontoxic, non-reactive with

the beverage, and, preferably, recyclable; (3) be relatively strong and capable ofsurviving a drop from a height of several feet when containing the beverage; (4) be

inexpensive, including the cost to fabricate the final shape; (5) if optically transparent,

retain its optical clarity; and (6) be capable of being produced in different colours

and/or adorned with decorative labels. All three of the basic material typesmetal(aluminium), ceramic (glass), and polymer (PE plastic)are used for carbonated

beverage containers. All of these materials are nontoxic and un-reactive withbeverages. In addition, each material has its pros and cons.


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For example, the aluminium alloy is relatively strong (but easily dented), is a very good

barrier to the diffusion of carbon dioxide, is easily recycled, cools beverages rapidly, and

allows labels to be painted onto its surface. On the other hand, the cans are optically

opaque and relatively expensive to produce. Glass is impervious to the passage ofcarbon dioxide, is a relatively inexpensive material, and may be recycled, but it cracks

and fractures easily, and glass bottles are relatively heavy. Whereas plastic is relatively

strong, may be made optically transparent, is inexpensive and lightweight, and is

recyclable, it is not as impervious to the passage of carbon dioxide as the aluminium and

glass. For example, you may have noticed that beverages in aluminium and glass

containers retain their carbonization (i.e., fizz) for several years, whereas those in two-litre plastic bottles goflat within a few months.


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The properties of some materials are directly related to their structures. The structure of

a material usually relates to the arrangement of its internal Components. There are four

levels of materials structure:

Subatomic structure involves electrons within the individual atoms and interactionswith their nuclei.

Atomic structure involves arrangement of atoms in materials and defines interaction

among atoms (interatomic bonding).

Microscopic structure involves arrangement of small grains of material that can beidentified by microscopy.

Macroscopic structure relates to structural elements that may be viewed with thenaked eye.

Materials Structure

Subatomic levelAtomic level

Microscopic structure

Macroscopic

structure


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Ionic bonds: When one or more electrons from an outer orbit aretransferred from one material to another, a strong attractive force

develops between the two ions.

All matter is made up of atoms containing a nucleus of protons and neutrons and

surrounding clouds, or orbits, of electrons.

Atoms can transfer or share electrons; in doing so, multiple atoms combine to form

molecules. Molecules are held together by attractive forces called bonds

Metallic bonds: Metals have relatively few electrons in their outerorbits; thus, they cannot complete the outer shell of other self-mated

atoms. Instead, metals and alloys form metallic bonds, whereby the

available electrons are shared by all atoms in contact. The resultant

electron cloud provides attractive forces to hold the atoms together andresults in generally high thermal and electrical conductivity.

Covalent bonds: ln a covalent bond, the electrons in outer orbits areshared by atoms to form molecules. The number of electrons shared

is reflected by terms such as single bond, double bond, etc.Polymers consist of large molecules that are covalently bonded

together. Solids formed by covalent bonding typically have low

electrical conductivity and can have high hardness.

Atomic Structure (Arrangement of Atoms)


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The properties of some materials are directly related to their crystal structures. The

crystalline structure of a material usually relates to the arrangement of its internal

Components. In metals, the knowledge of atomic structure guides us in controlling and

predicting the behaviour and performance of metals in various manufacturing processes.

Subatomic structure involves electrons within the individual atoms and interactions with

their nuclei.

Atomic structure involves arrangement of atoms in materials and defines interaction

among atoms (interatomic bonding).

Microscopic structure involves arrangement of small grains of material that can beidentified by microscopy.

Macroscopic structure relates to structural elements that may be viewed with the naked

eye.

Crystalline Structure Of Metals

Subatomic levelAtomic level

Microscopic structure

Macroscopic

structure


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why we study the crystal structure of metals?By studying the crystal structure of metals, information about various properties can be

inferred. By relating structure to properties, one can predict processing behaviour or

select appropriate applications for a metal. Metals with face-centered cubic structure, for

example, tend to be ductile whereas hexagonal close-packed metals tend to be brittle.

Difference between Unit Cell and Single Crystal

A unit cell is the smallest group of atoms showing the characteristic lattice structure of a

particular metal. It is the building block of a crystal,

A single crystal consists of a number of unit cells. All unit cells interlock in the same way

and have the same orientation For a crystalline solid, when the periodic and repeated

arrangement of atoms is perfect or extends throughout the entirety of the specimen

without interruption, the result is a single crystal. Single crystals exist in nature, but they

may also be produced artificially. Examples include; turbine blades and electronic

microcircuits, which employ single crystals of silicon and other semiconductors.

Crystalline Structure Of Metals

Polycrystalline Materials

Most crystalline solids are composed

of a collection of many small crystals

or grains; such materials are termedpolycrystalline..


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The Figure shows three thin disk specimens placed over some printed matter. It is

obvious that the optical properties of each of the three materials are different; the one on

the left is transparent, whereas the disks in the center and on the right are, respectively,

translucent and opaque. All of these specimens are of the same material, aluminum

oxide, but the:

Leftmost one is what we call a single crystalthat is, it is highly perfectwhich givesrise to its transparency.

The center one is composed of numerous and very small single crystals that are all

connected; the boundaries between these small crystals scatter a portion of the light

reflected from the printed page, which makes this material optically translucent.

The specimen on the right is composed not only of many small, interconnected crystals,but also of a large number of very small pores or void spaces. These pores also

effectively scatter the reflected light and render this material opaque.

Thus, the structures of these three specimens are different in terms of crystal

boundaries and pores, which affect the optical transmittance properties. Furthermore,

each material was produced using a different processing technique.


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Face-Centered Cubic Crystal Structure

Body-Centered Cubic Crystal Structure

Hexagonal Close-Packed Crystal Structure


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APF

s

Volume of atoms in unit cell

Volume unit cell

Total number of atom in unit cell Volume of unit atoms

Volume unit cell

ATOMIC PACKING FACTOR

For BCC APF 0.68

For FCCAPF 0.74

For HCP APF ?

Show that the atomic packing factor for the BCC crystal structure is 0.68.

Show that the atomic packing factor for the FCC crystal structure is 0.74.

What is the atomic packing factor for the HCP crystal structure?.Show that for HCP the c/a ratio is 1.633

23 3

2

aArea of a Hexagon

1. Calculate the volume of an BCC unit cell in terms of the atomic radius R.

2. Calculate the volume of an FCC unit cell in terms of the atomic radius R.

3. Calculate the volume of an HCP unit cell in terms of the atomic radius R.

Numerical Problems


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Allotropy is the ability of an element to exist in different structural forms while in the

same state of matter. The allotropes depend on both the allotropy temperature andthe external pressure. For example, the allotropes of carbon include diamond (where

the carbon atoms are bonded together in a tetrahedral lattice arrangement),

graphite (where the carbon atoms are bonded together in sheets of a hexagonal

lattice). Graphite is the stable polymorph at ambient conditions, whereas diamond is

formed at extremely high pressures. Also, pure iron has a BCC crystal structure at

room temperature, which changes to FCC iron at 912 C

Allotropy


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Crystallographic Directions, and Planes

Deformation under loading (slip) occurs on certain crystalline planes and in certain

crystallographic directions. Before we can predict how materials fail, we need to know

what modes of failure are more likely to occur.

It is often necessary to be able to specify certain directions and planes in crystals.

Many material properties and processes vary with direction in the crystal. Directions and planes are described using three integers; Miller Indices

Method of describing Miller indices for Directions

Draw vector, and find the coordinates of the head, h1,k1,l1and the tail h2,k2,l2.

Subtract coordinates of tail from coordinates of head Remove fractions by multiplying by smallest possible factor Enclose in square brackets


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(a) The position coordinates for the direction indices [100] and [110] direction are (1, 0,0) and (1, 1, 0), respectively (Fig. a).

(b) The position coordinates for the [112] direction are obtained by dividing the

direction indices by 2 so that they will lie within the unit cube. Thus the position

coordinates are (1/2,1/2,1) (Fig. b).

(c) The position coordinates for the [10] direction are (-1, 1, 0) (Fig. c). Note that the

origin for the direction vector must be moved to the lower-left front corner of the cube.(d) The position coordinates for the direction are obtained by first dividing

all the indices by 3, the largest index. This gives -1,-2/3,-1/3 which are shown in Fig. d.

[321]

Draw the following direction vectors in cubic unit cells:

(a) [100] and [110] (b) [112]

(c) (d)[110] [321]


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The procedure for determining the Miller indices for a cubic crystal plane is as follows:

1. Choose a plane that does not pass through the origin at (0, 0, 0).

2. Determine the intercepts of the plane in terms of the crystallographic x, y, and z

axes for a unit cube. These intercepts may be fractions.

3. Form the reciprocals of these intercepts.

4. Clear fractions and determine the smallest set of whole numbers that are in the

same ratio as the intercepts. These whole numbers are the Miller indices of the

crystallographic plane and are enclosed in parentheses without the use of commas.

The notation (hkl) is used to indicate Miller indices in a general sense, where h, k, and

l are the Miller indices of a cubic crystal plane for the x, y, and z axes, respectively.

Method of describing Miller indices for Planes

Draw the following crystallographic planes in cubic unit cells:

(a) (101) (b) ( ) (c) (221)110


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Point defects: (a) vacancy, (b) interstitial atom, (c) small substitutional atom,(d) large substitutional atom, (e) Frenkel defect, and (f) Schottky defect.

V i A i d d h t i i i i f it


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Vacancies: A vacancy is produced when an atom or an ion is missing from itsnormal site in the crystal structure. When atoms or ions are missing (i.e., when

vacancies are present), the overall randomness or entropy of the material increases,

which increases the thermodynamic stability of a crystalline material. All crystalline

materials have vacancy defects. Vacancies are introduced into metals and alloys

during solidification, at high temperatures, or as a consequence of radiation damage.

Interstitial Defects: An interstitial defect is formed when an extra atom or ion is

inserted into the crystal structure at a normally unoccupied position. Interstitial atoms

such as hydrogen are often present as impurities, whereas carbon atoms are

intentionally added to iron to produce steel.

A substitutional defect is introduced when one atom or ion is replaced by a differenttype of atom or ion.

A Frenkel defect is a vacancy-interstitial pair formed when an ion jumps from anormal lattice point to an interstitial site.

A Schottky defect, is unique to ionic materials and is commonly found in manyceramic materials. When vacancies occur in an ionically bonded material, a

stoichiometric number of anions and cations must be missing from regular atomic

positions if electrical neutrality is to be preserved. For example, one Mg+2 vacancy

and one O-2 vacancy in MgO constitute a Schottky pair.


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Iron ores are rocks and minerals from which metallic iron can be economicallyextracted. The ores are usually rich in iron oxides and vary in colour from dark grey,

bright yellow, deep purple, to rusty red. The iron itself is usually found in the form of

magnetite (Fe3O4), hematite (Fe2O3), goethite (FeO(OH)), limonite (FeO(OH)n(H2O))

or siderite (FeCO3).

SmeltingTo convert it to metallic iron it must be smelted or sent through a direct reduction

process to remove the oxygen. Oxygen-iron bonds are strong, and to remove the iron

from the oxygen, a stronger elemental bond must be presented to attach to the

oxygen. Carbon is used because the strength of a carbon-oxygen bond is greaterthan that of the iron-oxygen bond, at high temperatures. Thus, the iron ore must be

powdered and mixed with coke, to be burnt in the smelting process.


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Iron Pure iron rarely exists outside of the laboratory. Iron is produced by reducing iron ore to pigiron through the use of a blast furnace. From pig iron many other types of iron and steel are

produced by the addition or deletion of carbon and alloys. The following paragraphs discuss the

different types of iron and steel that can be made from iron ore.

PIG IRON. Pig iron is composed of about 93% iron, from 3% to 5% carbon, and variousamounts of other elements. Pig iron is comparatively weak and brittle; therefore, it has a limited

use and approximately ninety percent produced is refined to produce steel. Cast-iron pipe and

some fittings and valves are manufactured from pig iron.

Carbon 3.04.5%Manganese 0.152.5%

Phosphorus 0.12.0%Silicon 1.03.0%Sulphur 0.050.1%

WROUGHT IRON. Wrought iron is made from pig iron with some slag mixed in duringmanufacture. Almost pure iron, the presence of slag enables wrought iron to resist corrosion and

oxidation. The chemical analyses of wrought iron and mild steel are just about the same. The

difference comes from the properties controlled during the manufacturing process. Wrought iron

can be gas and arc welded, machined, plated, and easily formed; however, it has a low hardness

and a low-fatigue strength.

CAST IRON.Cast iron is any iron containing greater than 2% carbon alloy. Cast iron has ahigh compressive strength and good wear resistance; however, it lacks ductility, malleability, and

impact strength.


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WroughtIron

Pig Iron

Iron Ore


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Conventional Blast Furnace

Modern Blast Furnace


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1: Iron ore + Calcareous sinter

2: coke

3: conveyor belt

4: feeding opening, with a valve

that prevents direct contact with the

internal parts of the furnace

5: Layer of coke

6: Layers of sinter, iron oxide

pellets, ore,

7: Hot air (around 1200C)

8: Slag

9: Liquid pig iron10: Mixers

11: Tap for pig iron

12: Dust cyclon for removing dust

from exhaust gasses before

burning them in 13

13: Air heater

14: Smoke outlet (can beredirected to carbon capture &

storage (CCS) tank)

15: feed air for Cowper air heaters

16: Powdered coal

17: cokes oven

18: cokes bin19: pipes for blast furnace gas

Modern Blast Furnace


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Iron is the fourth most plentiful element in the earths crust, it is rarely found in themetallic state. Instead, it occurs in a variety of mineral compounds, known as ores, the

most attractive of which are iron oxides coupled with companion impurities. To produce

metallic iron, the ores are processed in a manner that breaks the ironoxygen bonds.Ore, limestone, coke (carbon), and air are continuously introduced into specifically

designed furnaces and molten metal is periodically withdrawn.

The production of iron in a blast furnace is a continuous process. The furnace is heated

constantly and is re-charged with raw materials from the top while it is being tapped from

the bottom. Iron making in the furnace usually continues for about ten years before the

furnace linings have to be renewed.

Blast furnace is a furnace for smelting of iron from iron oxide ores (hematite, Fe2O3 or

magnetite, Fe3O4). Coke, limestone and iron ore are poured in the top, which would

normally burn only on the surface. The hot air blast to the furnace burns the coke andmaintains the very high temperatures that are needed to reduce the ore to iron. The

reaction between air and the fuel generates carbon monoxide. This gas reduces the iron

oxide in the ore to iron.

Fe2O3(solid) + CO(gas) Fe(solid) + CO2(gas)

PRODUCTION OF IRON

BLAST FURNACE CHEMISTRY FOR THE PRODUCTION OF IRON


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At 500o C3Fe2O3 +CO -> 2Fe3O4 + CO2

Fe2O3 +CO -> 2FeO + CO2

At 850o CFe3O4 +CO -> 3FeO + CO2

At 1000o

CFeO +CO -> Fe + CO2

At 1300 oCCO2 + C -> 2CO

At 1900o CC+ O2 -> CO2

FeO +C -> Fe + CO

BLAST FURNACE CHEMISTRY FOR THE PRODUCTION OF IRON

The significant reactions occurring within the Blast Furnace can be described as follows:

1. Iron is extracted from its ores by the chemical reduction of iron oxides with carbon in

a furnace at a temperature of about 800C - 1900C.

2. Coke, the source of chemical energy in the blast furnace, is burnt both to release heat

energy and to provide the main reducing agent:3. Calcium oxide, formed by thermal decomposition of limestone, reacts with the silicon

oxide present in sand, a major impurity in iron ores, to form slag (which is less dense

than molten iron). Overall, the chemical processes can be summarized by these

equations:

All of the phosphorus and most

of the manganese will enter the

molten iron. Oxides of silicon

and sulphur compounds are

partially reduced, and these

elements also become part of

the resulting metal. Other

contaminant elements, such as

calcium, magnesium, and

aluminium, are collected in the

limestone-based slag and are

largely removed from the

system.

PRODUCTION OF STEEL


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PRODUCTION OF STEEL

When iron is smelted from its ore by commercial processes, it contains more carbon

than is desirable. In order to convert the pig iron into steel, it must be melted and

reprocessed to reduce the carbon to the correct amount, at which point other elements

can be added. This liquid is then continuously cast into long slabs or cast into ingots.

Approximately 96% of steel is continuously cast, while only 4% is produced as cast steelingots. The ingots are then heated in a soaking pit and hot rolled into slabs, blooms,

or billets. Slabs are hot or cold rolled into sheet metal or plates. Billets are hot or cold

rolled into bars, rods, and wire. Blooms are hot or cold rolled into structural steel, such

as I-beams and rails. In modern foundries these processes often occur in one assembly

line, with ore coming in and finished steel coming out. Sometimes after steels final

rolling it is heat treated for strength, however this is relatively rare.

Iron as obtained from blast furnace

contains from 3-4% of Carbon, and

variable amount of silicon,

manganese sulphur and

phosphorus.

1. Dead mild steel up to 0.15% carbon2. Low carbon or mild steel 0.15% to 0.45% carbon3. Medium carbon steel 0.45% to 0.8% carbon

4. High carbon steel 0.8% to 1.5% carbon


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STEEL MAKING PROCESSES

Bessemer Process

Crucible Process

Open Hearth Process

Electric Process (Arc, Induction)

Duplex Process

Linz Donnawitz Process

Kaldo Process

Modern Process

B P


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Bessemer Process

Bessemer process was invented in1875 by Thomas Gilchrest. In Bessemer process

the molten pig iron from blast furnace is poured into converter. The converter is

made of steel plates lined inside with refractory material. In the bottom of converter

vessels, a no of holes are introduced through which air is blown at a pressure of

200-250KN/m2. Based on full capacity, the converter is charged with 100-150 ton,and this charge is carried out from 10 to 15 ton at different time intervals. Their first

oxidizes silicon, and manganese which together with iron oxide rise to the top from

slag. During this air blowing process the carbon begins to burn and blowing

continued, until 0.25% of carbon is eliminated. In Bessemer process acids are used

to burn and eliminated silicon and phosphorus. The finished steel is then poured into

ladles and from ladle it is poured into ingot moulds for subsequently rolling and

forging process.

C ibl P


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Crucible Process

In Crucible process wrought iron together with a small amount of pig iron, necessary

alloying metals and slagging materials are placed in a clay or clay-graphite crucible,

covered with an old crucible bottom and melted in a gas or coke-fired furnace. After

the charge is entirely molten, with sufficient time allowed for the gases and impurities

to rise to the surface, the Crucible is withdrawn, the slag removed with a cold ironbar, and the resulting steel poured into a small ingot which is subsequently forged to

the desire shape.

There are three types of Crucible Furnaces:

(a) lift-out crucible,

(b) stationary pot, from which molten metal must be ladled, and(c) tilting-pot furnace

Open Hearth Process


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Open Hearth Process

The open-hearth furnace is rectangular and rather low, holding from 15 to 200 ton of

metal in a saucer-like shallow pool. It is heated either by producer gas, oil, tar, mixed

blast furnace and coke oven gas, pitch, mixture of creosote and pitch and heavy fuel

oil. Flames come from first one end and then the other. Waste gases pass through

regenerators. The furnace is charged with ore and limestone. The lime stone beginsto decompose in carbon di-oxide and calcium oxide.

A. gas and air enter

B. pre-heated chamberC. molten pig iron

D. hearth

E. heating chamber

F. gas and air exit.

Electric Furnace


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Electric Furnace

An Electric Arc Furnace (EAF) is a furnace that heats charged material by meansof an electric arc. An electric arc furnace used for steelmaking consists of

a refractory-lined vessel, usually water-cooled in larger sizes, covered with a

retractable roof, and through which one or more graphite electrodes enter the

furnace. Arc furnaces differ from induction furnaces in that the charge material isdirectly exposed to an electric arc, and the current in the furnace terminals passes

through the charged material.

An induction furnace is an electrical furnace in which the heat is appliedby induction heating of metal. The advantage of the induction furnace is a clean,

energy-efficient and well-controllable melting process compared to most othermeans of metal melting.

The one major drawback to Electric furnace usage in a foundry is the lack of refining

capacity; charge materials must be clean of oxidation products and of a known

composition and some alloying elements may be lost due to oxidation.Induction furnace is based on the principle of heating by induced currents. If a conductor is

placed within a coil through which an alternating current is flowing, a current is induced in theconductor. By the normal la of electricity this conductor is heated. The magnitude of the current

generated depends on:

_the physical dimensions of the coil;

_the resistivity of the conductor and

_the frequency of the current.

Linz-Donawitz (LD) or Basic oxygen steel making (BOS) Process


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( ) yg g ( )

The basic oxygen steel-making process is as follows:

1. Molten pig iron from a blast furnace is poured into a large refractory-lined container

called a ladle. Besides the BOS vessel is one-fifth filled with steel scrap.

2. The metal in the ladle is sent directly for basic oxygen steelmaking or to a pre-

treatment stage. Pre-treatment of the blast furnace metal is used to reduce therefining load of sulphur, silicon, and phosphorus. In desulfurising pre-treatment,

several hundred kilograms of powdered magnesium are added. Sulphur impurities

are reduced to magnesium sulphide in a violent exothermic reaction. The sulphide is

then raked off. Similar pre-treatment is possible for desiliconisation and

dephosphorisation using lime as reagents. The decision to pretreat depends on the

quality of the blast furnace metal and the required final quality of the BOS steel.3. Fluxes (lime or dolomite) are fed into the vessel to form slag, which absorbs

impurities of the steelmaking process. During blowing the metal in the vessel forms

an emulsion with the slag, facilitating the refining process.

Kaldo Process:


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Kaldo Process:

The Kaldo process, is a modification of LD process. It was originally developed in Sweden by Prof.

Kalling. This process is based on the advantage of evolution of heat by high phosphorus(2%) pig

iron to as low as 0.02% P.

The converter in Kaldo Process is inclined at 150 to 200 with the horizontal, and rotated at a speed

of 25-30 r.p.m. The oxygen lance is introduced through the open end of the vessel, which also actsas the outlet for the exhaust gases. The use of oxygen allows simultaneous removal of carbon and

phosphorus from the (p 1.85%) pig iron. The rotation of the converter ensures better slag-metal

reaction.

MODERN STEEL MAKING PROCESS


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Vacuum Induction Melting process:This process is similar to the induction melting process with suitable arrangement for creating a

vacuum. This process is used for making super alloys containing nickel and cobalt as base metals.

It is very suitable process for further remelting for investment casting. Due to vacuum prevailing in

the chamber , non-metallic inclusions can be minimized and composition of chemically reactiveelements like titanium , boron and aluminium can be controlled accurately. New alloys of steel

possessing greater uniformly and reproducibility of properties accompanied by greater strength,

creep resistance, etc can be produced.

Consumable Electrical Vacuum Arc Melting Process:It is direct arc steel melting process in which the electrode is consumed during melting. This

process was originally used for titanium. Since this process eliminates hydrogen, oxygen, andvolatile materials, it is extensively used for special-purpose steels, as in moving parts of aircraft

engines, due to need of high strength, uniformity of properties, greater toughness and freedom from

tramp and volatile elements.

Electric slag refining (ESR) Process:This process is commonly known as ESR. It is a larger form of the original welding process . It is the

electrical resistance heating process that remelts the preformed electrode into a water-cooled

crucible. Due to resistance to flow of current, the metal melts and drops onto the crucible through alayer of slag around the ingot. The process is used for making high alloy, high quality steels for

obtaining superior properties normally not achieved in conventional processing. For example, ultra

high strength weldable steel.

MODERN STEEL MAKING PROCESS


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EQUILIBRIUM PHASE DIAGRAM

An equilibrium phase diagram is a graphic mapping ofthe natural tendencies of a material or a material

system, assuming that equilibrium has been attained

for all possible conditions. There are three primary

variables to be considered: temperature, pressure,

and composition. The simplest phase diagram is a

pressuretemperature (PT) diagram for a fixed-composition material. Areas of the diagram are

assigned to the various phases, with the boundaries

indicating the equilibrium conditions of transition.

PT phase diagrams are rarely used for engineering applications. Most engineering

processes are conducted at atmospheric pressure, and variations are more likely tooccur in temperature and composition.

One-component or Unary Phase Diagram(P-T Diagram)

A phase may be defined as a homogeneous portion of a

system that has uniform physical and chemical

characteristics.

COMPLETE SOLUBILITY IN BOTH LIQUID AND SOLID STATES


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The upper line is the liquidus line, the lowest temperature for which the material is 100%

liquid. Above the liquidus, the two materials form a uniform-chemistry liquid solution. The

lower line, denoting the highest temperature at which the material is completely solid, is

known as a solidus line. Below the solidus, the materials form a solid-state solution in

which the two types of atoms are uniformly distributed throughout a single crystallinelattice. Between the liquidus and solidus is a freezing range, a two-phase region where

liquid and solid solutions coexist.

CONDITIONS FOR UNLIMITED SOLID SOLUBILITY

1. Size factor: The atoms or ions must be of similar size, with no more than a 15%

difference in atomic radius.

2. Crystal structure: The materials must have the same crystal structure; otherwise,

there is some point at which a transition occurs from one phase to a second phase with

a different structure.

3. Valence: The ions must have the same valence; otherwise, the valence electron

difference encourages the formation of compounds rather than solutions.

Binary Phase Diagram


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INTERPRETATION OF PHASE DIAGRAMS


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In a phase diagram, for each point of temperature and composition, following three

pieces of information can be obtained:

1. The phases present: The stable phases can be determined by simply locating thepoint of consideration on the temperaturecomposition mapping and identifying the

region of the diagram in which the point appears.2. The composition of each phase: If the point lies in a two-phase region, a tie-line

must be constructed. A tie-line is simply an isothermal (constant-temperature) line drawn

through the point of consideration, terminating at the boundaries of the single phase

regions on either side. The compositions where the tie-line intersects the neighbouring

single-phase regions will be the compositions of those respective phases in the two-

phase mixture.3. Amount of each phase:1. The tie line is constructed across the two-phase region at the temperature of the alloy.

2. The overall alloy composition is located on the tie line.

3. The fraction of one phase is computed by taking the length of tie line from the overall

alloy composition to the phase boundary for the other phase, and dividing by the total tie

line length.

4. The fraction of the other phase is determined in the same manner.


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PARTIAL SOLID SOLUBILITY


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INSOLUBILITY

If one or both of the components are totally insoluble in the other, the diagrams will also

reflect this phenomenon. The following Figure illustrates the case where component A is

completely insoluble in component B in both the liquid and solid states.

Many materials do not exhibit complete solubility in the solid state. Each is often soluble

in the other up to a certain limit or saturation point, which varies with temperature.


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The three-phase reaction that occurs upon cooling through 183C can be written as:

The leadtin phase diagram

Figure given below summarizes the various forms of three-phase reactions that may


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occur in engineering systems, along with the generic description of the reaction shown

below the figures. These include the eutectic, peritectic, monotectic, and syntecticreactions, where the suffix -ic denotes that at least one of the three phases in the

reaction is a liquid. If the same prefix appears with an -oid suffix, the reaction is of a

similar form but all phases involved are solids. Two such reactions are the eutectoid and

the peritectoid. The separation eutectoid produces an extremely fine two-phase mixture,

and the combination peritectoid reaction is very sluggish since all of the chemistry

changes must occur within (usually crystalline) solids.

If components A and B form a compound, and the compound cannot tolerate any

deviation from its fixed atomic ratio, the product is known as a stoichiometric

intermetallic compound and it appears as a single vertical line in the diagram


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IRONCARBON PHASE DIAGRAM


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Steel, composed primarily of iron and carbon, is the most important of the engineering

metals. For this reason, the ironcarbon equilibrium diagram assumes specialimportance. We normally are not interested in the carbon-rich end of the Fe-C phase

diagram and this is why the full ironcarbon (Fe-C) diagram is not normally encountered,

but we examine the Fe-Fe3C diagram as part of the Fe-C binary phase Diagram.

In the Figure, stoichiometric intermetallic

compound, Fe-Fe3C, is used to terminate

the carbon range at 6.67 wt% carbon.

Immediately after solidification, iron forms

a BCC structure called -ferrite. Onfurther cooling, the iron transforms to a

FCC structure called , or austenite.

Finally, iron transforms back to the BCC

structure at lower temperatures; this

structure is called , or ferrite. Both of the

ferrites ( and ) and the austenite aresolid solutions of interstitial carbon atoms

in iron.

The fourth single phase is the stoichiometric intermetallic compound which goes by the


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name cementite, or ironcarbide. Like most intermetallics, it is quite hard and brittle, andcare should be exercised in controlling the structures in which it occurs. Alloys with

excessive amounts of cementite, or cementite in undesirable form, tend to have brittle

characteristics. Because cementite dissociates prior to melting, its exact melting point is

unknown, and the liquidus line remains undetermined in the high-carbon region of thediagram.

The main microscopic constituents of iron and steel are as follows:


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The main microscopic constituents of iron and steel are as follows:

1. Austenite 2. Ferrite 3. Cementite 4. Pearlite

AUSTENITE is a solid solution of free carbon (ferrite) and iron in gamma iron. Onheating the steel, after upper critical temperature, the formation of structure completes

into austenite which is hard, ductile and non-magnetic. It is able to dissolve large amountof carbon. It is formed when steel contains carbon up to 1.8% at 1130C. On cooling

below 723C, it starts transforming into pearlite and ferrite. Austenitic steels cannot be

hardened by usual heat treatment methods and are non-magnetic.

FERRITE contains very little or no carbon in iron. It is the name given to pure ironcrystals which are soft and ductile. The slow cooling of low carbon steel below the

critical temperature produces ferrite structure. Ferrite does not harden when cooled

rapidly. It is very soft and highly magnetic.

CEMENTITE is a chemical compound of carbon with iron and is known as iron carbide

(Fe3C). Cast iron having 6.67% carbon is possessing complete structure of cementite.

Free cementite is found in all steel containing more than 0.83% carbon. It increases with

increase in carbon % as reflected in Fe-C Equilibrium diagram. It is extremely hard. Thehardness and brittleness of cast iron is believed to be due to the presence of the

cementite. It decreases tensile strength. This is formed when the carbon forms definite

combinations with iron in form of iron carbides which are extremely hard in nature. The

brittleness and hardness of cast iron is mainly controlled by the presence of cementite in

it. It is magnetic below 200C.


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PEARLITE is a eutectoid alloy of ferrite and cementite. It occurs particularly in mediumand low carbon steels in the form of mechanical mixture of ferrite and cementite in the

ratio of 87:13. Its hardness increases with the proportional of pearlite in ferrous material.

Pearlite is relatively strong, hard and ductile, whilst ferrite is weak, soft and ductile. It is

built up of alternate light and dark plates. These layers are alternately ferrite andcementite. When seen with the help of a microscope, the surface has appearance like

pearl, hence it is called pearlite. Hard steels are mixtures of pearlite and cementite while

soft steels are mixtures of ferrite and pearlite.

As the carbon content increases beyond 0.2% in the temperature at which the ferrite is

first rejected from austenite drop until, at or above 0.8% carbon, no free ferrite is

rejected from the austenite. This steel is called eutectoid steel, and it is the pearlite

structure in composition.

P-135, Introduction to Basic Manufacturing Processes and Workshop Technology

SUBATOMIC STRUCTURE


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All matters are made up of atoms containing a nucleus composed of protons and

neutrons and surrounding clouds, or orbits, of electrons. Both electrons and protons are

electrically charged, the charge magnitude being 1.62*10^-19 C, which is negative in

sign for electrons and positive for protons; neutrons are electrically neutral. Masses for

these subatomic particles are infinitesimally small; protons and neutrons have

approximately the same mass, 1.67*10^-27 kg, which is significantly larger than that of

an electron, 9.11*10^-31 kg.

The atomic mass of a specific atom may be expressed as the sum of the masses of

protons and neutrons within the nucleus.

The number of protons in the nucleus determines whether an atom will be metallic, non-metallic, or semi-metallic. An atom with a balanced charge has the same number of

electrons as protons; when there are too many or too few electrons, the atom is called

an ion. An excess of electrons results in a negatively charged atom, referred to as an

anion, while too few electrons results in a positively charged atom, called a cation. The

number of electrons in the outermost orbit of an atom determines the chemical affinity of

that atom for other atoms. These outer-shell electrons are called valence electrons.

engineering material lec#1

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