Chapter 3 - Alloying

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CHAPTER 3




WHAT IS AN ALLOY?

An alloy is a material that has metallic properties.and it is made up of two or more chemical elementsof which at least one is a metal

Objectives of alloying:

(i) To improve the properties of one of these

metals,

(ii) To produce new properties which are notpossessed by either metal in its pure state.




EXAMPLES OF ALLOYS

Pure Copper is a good conductor. Theaddition of 40% nickel to copper will

produce Cu-40%Ni alloy which has high

electrical resistance.

Pure iron is soft and ductile. With the

addition of 0.5% carbon, it becomes steel

which is strong and hard.

Stainless steels which are made up of

iron and carbon, chromium and sometimes

nickel




PRE-REQUISITE OF ALLOYING

In order to produce a useful alloy, the two

metals (elements) must be completelydissolved into each other in the molten

state.




TYPES OF ALLOYS

Upon solidification, the metals may:

separate from each other to form pure

crystals oreutect ic

, remain soluble in each other to form a

sol id solut ion or

combine chemically with each other to

form in ter-metal lic compounds




EUTECTICS

When two metals in an alloy system are

soluble in each other in all proportions in

the liquid state but are completely insoluble

in the solid state. They separate fromeach o ther to form i ts ow n ind iv idual

crystals .

Eutectics usually have a lamellars t ructure , namely, they exist as alternate

layers of the individual crystals.




EUTECTIC COMPOSITION

The eutectic composition alloy is thelowest melt ing point alloy of any givenalloy system and i t melts at a f ixed

temperature .

At eutectic composition, the crystals of thetwo metals will oc cu r as alternate layers (e.g pure cadmium - malleable and purebismuth - brittle).

Eutectic alloy is stronger than the

cons t i tuent pure metals and it is alsotough .




EQULIBRIUM DIAGRAM

Bismuth-cadmium alloy

(Materials for Engineering Technician – R.A. Higgins)




LAMELLAR STRUCTURE(ARRANGEMENT OF ATOMS FOR BISMUTH-CADMIUM ALLOY)

(Materials for the Engineering Technician – R.A. Higgins)




SOLID SOLUTIONS

In solid solution, the micros t ruc ture consists of sim i lar crys tals and there isno evidence of separate existence ofeither alloying element or inter-metalliccompounds.

The two metals are completely solub le into each other in liquid state, and remainso even after sol id i f icat ion .

Stronger than pure metals and retainmuch toughness and ductility of theoriginal pure metal.

Types: (a) Substitutional (b) Interstitial.




SUBSTITUTIONAL SOLID SOLUTIONS

The two metals will form a single space lattice

structure with atoms of one metal replacing

atoms of the other metal in the lattice structure.

Examples:(i) Complete solid solution - Copper/Nickel,

Silver/Gold,

Cr/Iron.

(ii) Partial solid solution –

Copper/Tin,Copper/Zinc,

Copper/Aluminium




FACTORS FAVOURING THE FORMATION

OF SUBSTITUTIONAL SOLID SOLUTIONS

Constituent elements must have:

similar properties,

similar crystal structure and

similar atomic size.




FACTOR FAVOURING THE FORMATION

OF INTERSTITIAL SOLID SOLUTION

Interstitial solid solution is formed when

the atom ic sizes o f the const i tuent

elements are markedly di f ferent . Thesolute atoms are small enough to lie in

between the solvent atoms.

Examples: Carbon/F.C.C. Iron




SUBSTITUTIONAL/INTERSTITIAL

SOLID SOLUTIONS





CORING OF SOLID SOLUTION ALLOY

Due to the difference in meltingtemperatures of the constituent metals,the one with higher melting point will

solidify first. As a result in the castcondition, the cores of the crystals containmore atoms of the metal with highermelting point. The outer fringes aretherefore richer in atoms of metal withlower melting point.

This phenomenon is called CORING .




CORING





AVOIDING CORING

Coring can be avo ided by extremely slow

cooling during the solidification of the alloy.

Coring in the casting can be removed by

anneal ing or ho t work ing. The main

purpose is to enable di f fus ion of the

atoms to be complete .




DIFFUSION





INTERMETALLIC COMPOUNDS

These alloys consist of two or more

metals in a def ini te (f ixed) com posi t ion .Example: Cementite (Fe3C) in steel.

One element is strong ly electroposi t ive while the other is weakly electropo sit ive.

Physical/chemical properties are differentfrom that of the constituent metals.

They are of little use because they arehard and b ri t t le .




THERMAL EQUIL IBRIUM DIAGRAMS

A diagram that shows the metal lurgical

phases that exist under equ i libr ium

condi t ions (i.e. extremely slow cooling) forany temperature and composi t ion of any

alloy.








EQUIL IBRIUM DIAGRAM FOR LEAD-TIN ALLOY





COOLING CURVE FOR LEAD-TIN ALLOY

Liquid & Solid

Cooling over a temperature range Cooling at a fixed temperature

(Materials for Engineering Technician – R.A. Higgins)




PHASE

Refers to any chemically stable homogeneous

constituent in an alloy.

In a solid alloy,a phase may be a solid

solution, an inter-metallic compound or a puremetal.

Any of the solid phases form the basic units

of which metallic alloys are composed. Liquid solution from which an alloy is

solidifying also constitutes a phase.




THERMAL EQUIL IBRIUM DIAGRAMS

(FEATURES)

Lines (phase boundaries) divide the diagram intoa number of areas or fields.

In a binary system, the fields may be eithersingle phased or two phased.

Liquidus LineThis line on the thermal equilibrium diagrammarks the temperature-composition relationshipabove which the only stable phase is l iquid.

Solidus LineThis is the line on the thermal equilibriumdiagram below which the only stable phase(s)is/are sol id .




INFORMATION OBTAINABLE FROM

A THERMAL EQUIL IBRIUM DIAGRAM

Freezing range of a given composition.

The state (types of phases) a particularcomposition exists at a given temperature.

The compositions of the phases which existin equilibrium with one another at anytemperature.

Note: Their relat ive proport io ns can be

ob tained by the Lever Rule wh ich is

no t covered in this modu le.




TYPES OF THERMAL EQUIL IBRIUM DIAGRAM

An alloy system in which two metals are completely

soluble in each other in all proportion in both liquid

and solid states (e.g. copper-nickel)

An alloy system in which two metals are soluble in

each other in the liquid state but completely

insoluble in the solid state (e.g. bismuth-cadmium)

An alloy system in which two metals are soluble in

the liquid state but only partially soluble in each

other in the solid state. (Note: This al loy system is n ot

co vered in th is sy l labus ).




EFFECT OF COOLING ON CORING

(Materials for Engineering Techn ician – R.A. Higgins)




EQUIL IBRIUM DIAGRAM FOR NI-CU ALLOY

Fig. 9.4: The nickel-copper thermal diagram





CD-BI EQUIL IBRIUM DIAGRAM


Completely uniform liquid




CAST IRONS

These are ferrous metals that contain

more than 2% carbon.

They are produced from crude pig iron in a

cupola or line frequency induction furnace.

Should there be a necessity to make any

adjustment to the composition, this should

be done during the melting process.




CAST IRON – cont’d

Important Features

Cheap materials

- Produced by simple adjustments to the pig iron by

addition of selected scrap-iron and scrap steel.

High rigidity, high compressive strength and

good wear resistance (referring to white castirons)

Easy to machine (except white cast iron)

High fluidity – easy to cast into complex shape.

Lower melting temperature (compared to mild steel).

High duty cast iron can be produced

(e.g. spheroidal grey cast iron, malleable cast iron)




DIFFERENT TYPES OF CAST IRONS,

PROPERTIES AND APPL ICATIONS

White Cast Iron

- This type of cast iron produces massiveamount of cementite rather than graphite duringsolidification.

- Structure consists of cementite + pearlite.

- Properties include high wear resistant, hard andbrittle. Hence difficult to machine.

- Applications:

(a) rolls for steel making

(b) stone and ore crushing mills(c) low carbon equivalent white cast iron is an

intermediate product for producingmalleable cast iron.




White Cast Iron (Low Silicon)





DIFFERENT TYPES OF CAST IRONS,

PROPERTIES, AND APPLICATIONS (cont’d)

Ordinary Cast Irons

Engineering Grey Cast Iron- Cast iron that permits graphite flakes to grow during

solidification.

- Structures

Coarse grey iron – large graphite flakes in a matrix of

ferrite.Fine grey iron – small graphite flakes in a matrix of pearlite

- Properties – low tensile strength, poor ductility and highcompressive strength

- Applications: engine blocks, machine tool bases andheavy machine castings.

Fluid Irons- These are metals which have high fluidity during casting and

it can be achieved by using high silicon content and highphosphorus content in the melt.

- Used to produce ornamental castings of intricate design.




Fine Grey Iron

Materials for the Engineering Technician – R.A. Higgins)




Coarse Grey Iron

(Material for the Engineering Technician – R.A. Higgins)




Table:15.1 Composition and uses of some

ordinary cast irons

Composition % Uses

C Si Mn S P

3.30 1.90 0.65 0.08 0.15 Motor brake drums

3.25 2.25 0.65 0.10 0.15 Motor cylinders and pistons

3.25 2.25 0.50 0.10 0.35 Light machine castings

3.25 1.75 0.50 0.10 0.35 Medium machine casting

3.25 1.25 0.50 0.10 0.35 Heavy machine castings

3.60 1.75 0.50 0.10 0.80 Water pipes

3.50 2.75 0.50 0.10 0.90 Low-strength ornamental

castings of yesteryear




HIGH-DUTY CAST IRONS

Spheroidal-Graphite (SG) Cast Iron

Compact-Graphite (CG) Cast Iron




SPHEROIDAL-GRAPHITE CAST IRON

Spheroidal-Graphite (SG) Cast Iron

- This cast iron is produced by adding small amount of

magnesium or cerium to ordinary grey cast iron to causegraphite to precipitate as spheres during solidification.

- Structures consist of spheroidal graphite in a matrix of

ferrite/or pearlite.

- Properties when compared to grey iron:

(i) Excellent strength, ductility and toughness

(Ductility/strength are higher than malleable irons)

(ii) High fatigue resistance

- Applications:

It replaces the steel forgings used for highly stressedcomponents in the automobiles and other industries.

Examples: camshafts, crankshafts, differential gear carriers,

pistons, cast gears, pumps and ship propellers




COMPACT-GRAPHITE CAST IRON

Before being cast, molten iron is treated with alloy

containing magnesium, cerium and titanium

which prevents the graphite from being

completely spherical such as in SG cast iron.

Applications:

(i) gear pumps

(ii) vehicle brake parts

(iii) eccentric gears

(iv) fluid and air cylinders




MALLEABLE CAST IRONS

These are cast irons that are produced from

white iron (Si < 1%) by a lengthy heat treatment

during which cementite decomposes to produce

rounded clumps of graphite. The types of

malleable cast irons are:

(i) Black-heart malleable cast iron (see microstructure)

(ii) White-heart malleable cast iron

(iii) Pearlitic malleable iron (see microstructure)

http://localhost/var/www/apps/Temp/Blackheart%20malleable.ppt

http://localhost/var/www/apps/Temp/PearliticMalleable.ppt

http://localhost/var/www/apps/Temp/PearliticMalleable.ppt

http://localhost/var/www/apps/Temp/Blackheart%20malleable.ppt




BLACK-HEART MALLEABLE CAST IRON

Process:- Heat white cast iron to 850oC – 950oC for 50 – 70 hours

in air-tight boxes

- Prolonged heating will break down the iron carbide intosmall rosettes of graphite.

Structure is of ferrite (soft and ductile) androsettes of graphite.

Applications:(i) brake shoes in automobiles

(ii) pedals

(iii) wheel hubs

(iv) door hinges




Blackheart Malleable Iron

(Materials For Engineering Technician – R.A. Higgins)




WHITE-HEART MALLEABLE IRON

Process:- Heat white cast iron to about 1000oC for 70 – 100

hours in air-tight boxes packed with iron oxides.

- The ore oxidises and draws out the carbon in thecastings.

Structure is of ferritic structure near the surfaceand pearlitic structure with some fine rosettes ofgraphite at the core of the casting.

Applications:

(i) pipe fittings(ii) fitting for bicycle and motorcycle frames.




Whiteheart Malleable Iron

(Materials For The Engineering Technician – R. A. Higgins)




PEARLITIC MALLEABLE IRON

Process:- Similar to black-heart process except thatrapid cooling will cause the austenite tochange into fine pearlitic structure instead.

Properties:- Harder, tougher and has higher tensile

strength than black-heart malleable iron.- Marked reduction in malleability and ductility.

Structure is pearlitic matrix with rosettesof graphite.

Applications:(i) gears, (ii) couplings, (iii) camshafts,(iv) axle (v) housings,(vi) differential housings and components.




Spheroidal-Graphite Iron

(Materials For The Engineering Technician – R.A. Higgins)




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Chapter 3 - Alloying