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8/12/2019 Chapter 3 - Alloying
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Revision 1 - February 2011 1
CHAPTER 3
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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.
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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
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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.
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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
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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.
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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 .
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EQULIBRIUM DIAGRAM
Bismuth-cadmium alloy
(Materials for Engineering Technician – R.A. Higgins)
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LAMELLAR STRUCTURE(ARRANGEMENT OF ATOMS FOR BISMUTH-CADMIUM ALLOY)
(Materials for the Engineering Technician – R.A. Higgins)
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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.
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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
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FACTORS FAVOURING THE FORMATION
OF SUBSTITUTIONAL SOLID SOLUTIONS
Constituent elements must have:
similar properties,
similar crystal structure and
similar atomic size.
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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
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SUBSTITUTIONAL/INTERSTITIAL
SOLID SOLUTIONS
(Materials for the Engineering Technician – R.A. Higgins)
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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 .
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CORING
(Materials for the Engineering Technician – R.A. Higgins)
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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 .
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DIFFUSION
(Materials for the Engineering Technician – R.A. Higgins)
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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 .
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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.
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(Materials for the Engineering Technician – R.A. Higgins)
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EQUIL IBRIUM DIAGRAM FOR LEAD-TIN ALLOY
(Materials for the Engineering Technician – R.A. Higgins)
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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)
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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.
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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 .
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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.
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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 ).
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EFFECT OF COOLING ON CORING
(Materials for Engineering Techn ician – R.A. Higgins)
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EQUIL IBRIUM DIAGRAM FOR NI-CU ALLOY
Fig. 9.4: The nickel-copper thermal diagram
(Materials for Engineering Techn ician – R.A. Higgins)
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CD-BI EQUIL IBRIUM DIAGRAM
(Materials for Engineering Techn ician – R.A. Higgins)
Completely uniform liquid
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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.
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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)
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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.
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White Cast Iron (Low Silicon)
(Materials for the Engineering Technician – R.A. Higgins)
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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.
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Fine Grey Iron
Materials for the Engineering Technician – R.A. Higgins)
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Coarse Grey Iron
(Material for the Engineering Technician – R.A. Higgins)
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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
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HIGH-DUTY CAST IRONS
Spheroidal-Graphite (SG) Cast Iron
Compact-Graphite (CG) Cast Iron
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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
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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
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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)
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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
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Blackheart Malleable Iron
(Materials For Engineering Technician – R.A. Higgins)
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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.
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Whiteheart Malleable Iron
(Materials For The Engineering Technician – R. A. Higgins)
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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.
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Spheroidal-Graphite Iron
(Materials For The Engineering Technician – R.A. Higgins)
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TILL WE MEET
AGAIN NEXT WEEK