MODULE 4

ENGINEERING MATERIALS

STEELS�

• Plain carbon Steels
• Alloy steels

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Plain carbon Steels�

• Low carbon steels 0.1 to 0.3% C
• Medium carbon steels 0.3 to 0.6% C
• High carbon steels 0.6 to 1.2% C

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• Traces of other elements like Si (0.3%max), Mn (1% max), S (0.4% max), P (0.05% max) are considered as impurities

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Low Carbon steels

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Medium Carbon steels�

�High Carbon steels�

ALLOY STEELS

• Alloying elements are added to the steel so that the steel satisfies various requirements, such as:
• Environmental: corrosion resistance
• Mechanical: hardness, strength, and toughness
• Thermal: strength and durability of a metal at either sub zero or extremely high temperatures
• Commonly added elements are Mn, Ni, Cr, Mo, W, V, Cu, B, Al and Si

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Purpose of Adding Alloy Elements in Steels

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• In low alloy steels, the effect of alloy elements may be separated into two groups:

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• Elements that combine with carbon: Chromium, Manganese, Molybdenum, Titanium, Tungsten, Vanadium, etc.
• Elements that dissolve in ferrite: Aluminum, Copper, Nickel, Silicon, etc.

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Alloy elements are added to steel for some basic purposes, for example:

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• Aluminum: deoxides steel
• Cobalt: Improves mechanical properties at high temperatures.
• Copper: Increases corrosion resistance and improves machinability.
• Chromium: Improves resistance to corrosion and wear.
• Lead: aids machinability.
• Manganese: Deoxidizes steel and improves hardenability.
• Molybdenum: Improves strength, hardenability, and wear resistance.
• Titanium: Improves strength, hardenability, and wear resistance.
• Vanadium: Improves strength, hardenability, and wear resistance.
• Nickel: Increases hardenability and corrosion resistance.
• Phosphorus: Improves corrosion resistance in structural grades of steel.
• Silicon: improves strength
• Sulphur: improves machinability

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EFFECTS OF ALLOYING ELEMENTS

Dislocation Movement

• The presence of alloying atoms would develop lattice stresses.
• The alloying atoms would have different elastic modulus which would create a different terrain for moving dislocation.
• The dislocations will experience difficulty in mobility

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Polymorphic Transformation Temperature

• Alloying elements like Mo, Cr, W, Si, V, Ti will tend to contract austenite region
• Alloying elements Ni, Mn, Cu, Co etc will tend to enlarge the austenite region
• Alloying elements will change the mean temperature for α-γ and γ-δ transformation

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Strengthening of ferrite

• Most of the alloy elements form solid solution with ferrite.
• Though the solubility is limited, alloying elements increase hardness and strength

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Formation and stability of carbides

• Carbide formed with alloying elements are hard, brittle and provide wear resistance to steels.
• Carbides of Cr and V are having outstanding hardness and wear resistance.

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Displacement of Eutectoid Point

• A1temperature is lowered by the austenite-formers and raised by the ferrite-formers.

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Retardation of transformation rate

• The austenite transformation temperature is shifted up or down by alloying elements.
• Increasing Ni or Mn content will lower the austenite transformation temperature thereby postponing of austenite on slow cooling

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Lowering of critical cooling rates

• TTT curves are shifted to the right due to most of the alloying elements.
• Except Co this means that the critical cooling rate required for formation of martensite is decreased and hence leading to better hardenability

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Improvement in Corrosion Resistance

• Elements like Al forms thin oxide layers and protect it from further corrosion.
• Cr imparts corrosion resistance only when the composition is more than 13%

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Influence on Grain Growth

• Grain growth is retarded in the presence of Ni and V
• These elements are usually called as grain refiners

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FUNCTIONS OF ALLOYING ELEMENTS

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FUNCTIONS OF ALLOYING ELEMENTS

FUNCTIONS OF ALLOYING ELEMENTS

CLASSIFICATION OF ALLOY STEELS

Ni - Steels

• Nickel
• Unlimited solubility in γ-iron.
• Highly soluble in ferrite
• Does not form carbide.
• Retards transformation of austenite
• Lowers critical temperature
• Pearlite formed at lower temperature is finer and tougher than the pearlite of plain carbon steel.

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• Ni-steels have better toughness, ductility, fatigue resistance.

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Cr- Steels

• A strong carbide former
• Soluble upto 13% in γ-iron
• Unlimited solubility in α – ferrite
• A special type with 1%C and 2-4% Cr has excellent magnetic properties.

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• With more than 5% Cr, Chromium steels have improved high temperature properties and corrosion resistance.

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Ni-Cr Steels

• Ni to Cr ratio is about 5:2.
• This combination results in increased toughness, ductility, hardenability and wear resistance.

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Mo – Steels

• Limited solubility in γ and α iron.
• A strong carbide former.
• Used with Cr, Ni or both
• Mo- steels have good hardenability, high temperature hardness, wear resistance

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High Speed Steels (HSS)

• Wear resistant due to the presence of hard matrix carbides
• Maintain high hardness at high temperature upto 550 ^oC
• 18%W 4% Cr, 1% V and carbon 0.6%-0.8%
• Used for cutting tools

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• W based and Mo based HSS
• W + Cr, V, Co
• Mo + Cr, V, Co
• High temperature hardness is due to the formation of alloy carbides

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OTHER POPULAR ALLOY STEELS ….�

Free Cutting steels

• Higher machinability is the required property.
• Two types
• High sulpher steels:0.33% Sulphur and 0.12% phosphorus
• Leaded steels: 0.35% lead

Rail steels

• Requirement:
• Strength, ductility, high impact strength, fatigue resistance
• Addition of Mn and Cr (upto1%) improve these properties

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Spring steels

• Requirements:
• High elastic limit, good elongation, high fatigue resistance
• Mn and Si are used as main alloying elements in spring steels

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Tool steels

• Requirements:
• High hardness, wear resistance, good toughness, resistance to shock
• Used to make different type of tools like chisels, hammers, punches, dies shears etc

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HSLA Steels(Micro alloyed steels) High Strength Low Alloy

• They are designed to provide better mechanical properties than conventional carbon steels.
• The HSLA steels have low carbon contents (0.50 to 0.75%C) in order to produce adequate formability and weldability, and they have manganese contents up to 2.0 %.
• Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium, and zirconium are used in various combinations.
• The chemical composition of a specific HSLA steel may vary for different product thickness to meet mechanical property requirements.

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TMT Steels

• Thermo mechanically treated (TMT) steels have superior properties like weldability strength, ductility and toughness.
• 0.17 – 0.24% Carbon
• Sulphur:0.05%
• Phosphorus: 0.045%
• Under thermo mechanical treatment of bars, the steel bars are made to pass through a specially designed water cooling system where these are kept for such a period that outer surface of bars becomes colder while the core remains hot.
• This creates a temperature gradient in the bars. When the bars are taken out of the cooling system, the heat flows from the core to the outer surface causing further tempering of steel bars thereby helping them in attaining higher yield strength of steel.

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Stainless Steels

• Stainless Steels are iron-base alloys containing Chromium.
• They attain their stainless characteristics because of the formation of an invisible and adherent chromium-rich oxide surface film.
• Some other alloying elements added to enhance specific characteristics include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, and nitrogen.
• Corrosion resistance and mechanical properties are commonly the principal factors in selecting a grade of stainless steel for a given application.

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Stainless steels are commonly divided into 5 groups:

• Martensitic stainless steels
• Ferritic stainless steels
• Austenitic stainless steels
• Duplex (ferritic-austenitic) stainless steels
• Precipitation-hardening stainless steels.

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Martensitic stainless steels

• Martensitic stainless steels are essentially alloys of chromium (12-14%) and carbon (upto 0.15%) that possess a martensitic crystal structure in the hardened condition.
• They are ferromagnetic, hardenable by heat treatments, and are usually less resistant to corrosion than some other grades of stainless steel.
• Chromium content usually does not exceed 18%, while carbon content may exceed 1.0 %.
• The Cr and C contents are adjusted to ensure a martensitic structure after hardening.
• Excess carbides may be present to enhance wear resistance.
• Used un production of cutlery items, surgical instruments and ball bearings.

• Ferritic stainless steels
• Ferritic stainless steels are Cr (16-25%) and C (0.12 -0.2%) containing alloys with body centered cubic (bcc) crystal structures.
• The ferritic stainless steels are ferromagnetic.
• They may have good ductility and formability, but high-temperature mechanical properties are relatively inferior to the austenitic stainless steels.
• Toughness is limited at low temperatures and in heavy sections.
• Cannot be heat treated since austenite does not form at any temperature
• Used in parts in household appliances and transportation industry

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Austenitic stainless steels

• Austenitic stainless steels have a face centered cubic(fcc) crystal structure.
• Austenitic stainless steels are effectively nonmagnetic in the annealed condition and can be hardened only by cold working.
• Austenite is formed through the generous use of austenitizing elements such as nickel, manganese, and nitrogen.
• They have reasonable cryogenic and high temperature strength properties.
• Cr content typically is in the range of 16 to 26%; Ni content is commonly less than 35%.
• Some variety may contain Mo (upto 2%) for improved creep and corrosion properties.
• Used for household articles, sanitary fittings & storage and processing vessels in storage industry

Duplex stainless steels

• The primary alloying elements are chromium and nickel.
• Duplex stainless steels are a mixture of bcc ferrite and fcc austenite crystal structures.
• Most Duplex stainless steels are intended to contain equal amounts of ferrite and austenite in the annealed condition.
• Duplex stainless steels generally have better stress corrosion cracking resistance
• Duplex stainless steels also generally have greater tensile and yield strengths, but poorer toughness than austenitic stainless steels.

CAST IRONS

• Cast irons typically contain more than 2 wt% of carbon (2-5%C)

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• In this composition range liquid state is between 1150-1300 C, which is lower than that for steels.
• They are easily melted and cast
• Most cast irons are brittle. Hence casting is a convenient fabrication technique
• Cast irons have higher compressive strength, ability to absorb vibrations, better wear, abrasion, corrosion and heat resistance, rigidity and machinability
• With suitable composition and heat treatment, variety of microstructure could be developed with varying properties

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CLASSIFICATION OF CAST IRONS

• They are classified based on the metallurgical structure and appearance

Factors controlling structure & appearance

1. Carbon content
• Carbon is present either as Fe₃C or as free carbon in the form of graphite.
• Alloy carbides are also formed when other alloying elements are present

2. Cooling rate

• Higher cooling rate generally help the formation of carbides while slower cooling rates help carbon to be in the free form

3. Heat treatment

• Heat treatment helps in either
• (a) the formation of carbide or
• (b) decomposition of cementite into free carbon and
• (c) causes changes in the shape, size and distribution of graphite particles

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Depending upon the nature, shape, size and distribution of carbon, cast irons are classified as

• Grey cast iron
• White cast iron
• Ductile cast iron
• Malleable iron
• Wrought iron

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Grey cast iron

• Carbon exist in free form as graphite flakes embeded in a matrix of α-ferrite or pearlite
• Due to the presence of graphite flakes, a fracture surface appears grey in colour
• Have low strength and brittle in tension. But have better strength and ductility in compression
• Uses: Automobile parts like cylinder block, cylinder head, brake drum lining, underground piping, motor case, machine tool beds etc

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White cast iron

• Cast irons containing lower silicon content, upon rapid cooling results in a microstructure in which most of the carbon exists as cementite.
• The fracture surface of this alloy has a white appearance and thus it is called white cast iron.
• Composition of an typical white cast iron would be
• 2 to 3.5% carbon,
• 0.5 to 1.3% silicon,
• 0.2 to 0.8% manganese,
• 0.18% sulphur and
• 0.1% phosphorus.
• White cast irons are hard and brittle; they cannot easily be machined.
• Have good wear resistance, compressive strength
• Used for components requiring high abrasion resistance, pump liner, rollers for rolling mill etc.

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Ductile cast iron

• When magnesium or cerium is added in small quantities to the gray cast iron in the molten condition leads to a different microstructure on solidification.
• The carbon exists as graphite, but in the form of nodules or spheroids (sphere-like particles) instead of flakes.
• This variant of cast iron is called ductile or nodular cast iron.
• It is also known as spheroidal graphite iron (S.G.iron).
• A typical composition would be having
• 3 to 4.3% carbon,
• 1.0 to 3.5% silicon,
• 0.3 to 0.8% manganese,
• 0.03% sulphur and
• 0.08% phosphorus.
• The ductile iron is stronger and much ductile than gray iron.
• It has superior wear resistance as compared to other cast irons.
• Typical applications of this material are for manufacturing parts used in automobile, farm machinery, earth moving machinery, rolling mill equipments and rolls. valve and pump bodies, crank shafts, gears, rollers etc.

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Malleable iron

• Produced from white cast iron using heat treatment process known as melleabilising
• White cast iron is heated above 700 C and held for a longer period and then cooled.
• During this process cementite in white iron decomposes in the form of irregular shaped globules. The matrix is either pearlite or ferrite depending upon the cooling rate.
• These castings are widely used in automobile, electrical and railway industries due to good combination of strength, ductility, wear resistance and machinability
• Typical uses are brackets, hubs, break drums, cam shafts, crank shafts, housing, earth moving parts etc.

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Wrought iron

• It is an iron alloy with very low carbon content and impregnated with tiny fibrous inclusions of iron silicate known as slag
• The presence of slag changes the chemical properties of iron enough to create a new and beneficial metal.
• Have good strength, resistance to corrosion and malleability
• Wrought iron lacks the carbon content necessary for hardening through heat treatment

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Copper & Copper Alloys

• properties
• Good electrical conductivity
• Good corrosion resistance
• Good thermal conductivity
• Easily machinable
• Can be welded, brazed, soldered
• Lacks sufficient strength for structural applications

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ALLOYS OF COPPER

• Brasses (Cu-Zn)

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• Bronzes (Cu-Sn)

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• Cupronickels (Cu-Ni)

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• Nickel-silvers (Cu-Zn-Ni)

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Brasses (Cu-Zn alloy)

• Brass is an alloy of copper and zinc with the zinc content varying from 5 to 54%.
• Small amounts of lead, tin or aluminium also are added to impart specific properties to brass, in order to make it suitable for a given purpose.

Important properties of brasses are:

• Good strength, ductility, formability and machinability
• Good electrical and thermal conductivity
• Good wear and corrosion resistance
• Non magnetic
• Good aesthetic property

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α-brass

• Brass containing only α-phase is called α-brass
• Solubility of Zn (upto 38% at 500C)
• FCC structure
• Highly ductile at room temperature
• Have good corrosion resistance

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Variants of α-brass

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• Yellow α-brass (Contains 20-36% Zn)
• Colour is yellow
• Have good corrosion resistance

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• Cartridge brass (70% Cu-30% Zn)
• Used for production of cartridge and shell case for rifles
• Good combination of strength and ductility

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• Admiralty brass (71% Cu-28%Zn 1%Sn)
• More suitable for marine applications
• Also used for condensers, evaporaters and heat exchangers

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• Aluminum brass (76% Cu-22% Zn-2% Al)
• Better than admiralty brass
• Used for marine applications

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• Red α-brass (Zn content 5-20%only)
• Due to high Cu content, the alloy have a red colour
• Good corrosion resistance and workability
• Used for condenser, heat exchanger, plumbing pipes and radiators

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• Gliding Metal (95% Cu-5% Zn)
• Closely matches gold in colour
• Used for making coins, medals, jewelry etc

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• Leaded red brass (Cu and 5% Pb-5% Sn-5% Zn)
• Used for pressure valve, pipe fittings, pump castings
• Have fairly good strength and machinability

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Duplex brasses (α + β brasses)

• At Zn level more than 38%, a new solid solution resulting in a mixture of α and β is formed
• At lower temperature β phase changes to an ordered β’-phase making the alloy harder and more brittle.
• These alloys have good strength, but poor ductility and are more suitable for hot working

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Variants of Duplex brasses

1. Muntz Metal (60% Cu-40% Zn)
• Used for springs , chains etc
• Also used as a brazing alloy for steel
• It is also called yellow metal

2. Naval brass (60% Cu-39% Zn-1% Sn)

• Addition of Sn improves corrosion resistance making it suitable for sea water applications
• Used for propeller shafts, impeller for pumps, valve etc

3. Forging brass (60% Cu-38% Zn-2% Pb)

• Best hot working properties
• Used for hot forging for plumbing parts

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Variants of α-brass

BRONZES

• Bronzes represent alloys of Cu with other elements other than Zn
• Other alloying elements like Sn, Al, Si , Be are used to produce different bronzes

Bronzes Vs Brasses

• Bronzes have

1. lower coefficient of friction

2. higher strength and toughness

3. higher corrosion resistance

4. higher cost

Bronzes Vs Steel

• Bronzes have

1. better corrosion resistance

2. better heat and electric conductivity

• Used for bearings, springs, bells, statues and industrial castings

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1. Tin bronze (Phospher bronze)
1. 88-98%Cu,1-11%Sn, 0.1-0.5%P
• Sn – improves wear and corrosion resistance
• P – function as deoxidizer during melting. Also contribute to hardness and wear resistance.
2. Uses: springs, bellows, bushes, taps, clutch disc, electrical contacts etc.

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2. Gun metal (88% Cu, 10% Sn, 2% Zn)

• Zn replaces P in Tin bronze
• Used in marine components due to better corrosion resistance

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3. Aluminum bronze (Alloy of Cu and Al(4-11%)

• Other elements like Fe, Ni, Mn, Si may be added to obtain specific desired properties
• Good strength and corrosion resistance
• Of all cu alloys, it is having finest colour and often called as imitation gold
• Not suitable for casting

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4. Silicon bronze: Alloy: Cu – Si (1-4%)

• Small amounts of Mn, Zn, and Fe are alloyed
• Suitable for cold and hot working
• Typical uses: rivets, nuts and bolts, wood screw etc

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5. Beryllium bronze: Alloy Cu – Be(0.6 to 3%)

• High strength due to precipitation hardening
• Also called beryllium copper
• Used for springs due to high elasticity and fatigue resistance

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Variants of bronzes

Aluminum and its alloys

Characteristics of Aluminum:

• Low density, low strength
• Low melting point
• High electrical and thermal conductivity
• High ductility and malleability
• Good oxidation and corrosion resistance
• Good machinability, formability, workability and castability
• Non magnetic
• Used for lightly loaded structures, electrical cables etc

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• Strengthening methods of Al
• Cold work (Strain hardening)
• Solid solution hardening
• Age hardening
• Fiber reinforcement

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• This class of alloys contain small amounts of other alloying elements like Si, Fe, Mg, Cr and Zn

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Al-alloy groups

Duralumin (94Al-4Cu-0.5Mg-0.5Mn-0.5Si-0.5Fe)

• Have high strength and high electrical conductivity
• Used for sheets, tubes, forgings, rivets etc.
• Also used in aircraft industry, surgical equipment

Y- alloy (92.5%Al-4%Cu-2%Ni-1.5%Mg)

• Can be used upto 200C due to excellent strength and hardness
• Suitable for cold working and casting
• Used as cylinder heads and crank case for engines

Magnelium (Al, Mg,Cu alloy)

• Other small amounts of elements present are Ni, Sn, Fe, Mn and Si
• Have good strength and machinability, but is brittle
• Uses include vehicle door handles, luggage racks, ornamental fixtures
• Used in aircraft and automobile industries

Silumin alloys (88%Al-12%Si)

• Have good castability, corrosion resistance high ductility and low density

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Titanium and its alloys

Titanium

• Pure Ti is strong, ductile and light material
• Density is 60% of steel
• Below 880 C, HCP α-Ti, Above 880 C BCC β-Ti
• Have high strength and corrosion resistance at high temperature
• Suitable for cold and hot working and has good weldability
• Inferior machinability compared to that of steel

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• Ti and its alloys are used as a structural material

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Alloys of Titanium

• Important alloying elements for Ti are Al, Cr, Mn, V, Fe, Mo and Sn for increasing the strength.
• Among these Fe, Cr and Al provide highest strength
• Ti alloys exhibit better creep and fatigue strength and good corrosion resistance
• Respond to heat treatment by precipitation hardening
• Have highest specific strength (ratio of strength to weight)

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Uses of Ti- alloys

• Aircraft structural and turbine material due to high strength to weight ratio and high temperature corrosion resistance
• In chemical processing plants as vessels, valves and tanks due to higher corrosion resistance

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Ti-6Al-4V is the most widely used Ti alloy

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Magnesium and its alloys

Magnesium

• Lighter and less ductile than Aluminum
• Poor resistance to fatigue, creep and wear.

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Addition of:

• Al -> increases strength, hardness and castability
• Mn -> improves corrosion resistance
• Zr -> have grain refining effect (increases strength)

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Typical applications are in the area of

• Aerospace
• High speed machinery
• Transportation and material handling equipments

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Mg-Al-Zn alloys are suitable for casting, extrusion and forging operations

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Nickel and its alloys

Nickel

• A metal having good corrosion resistance
• Can be strengthened by strain hardening, age hardening or by precipitation hardening

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Most common alloying elements are Cu, Fe, Cr, Mo, Mn and Al

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Nickel and its alloys

Monel:

• Ni and Cu are in the ratio 2:1
• High corrosion resistance, good toughness and fatigue strength
• Better mechanical properties than brasses and bronzes
• When S is added to Monel (R-monel), machinability is improved
• When 3% Al added (K-monel) age hardening possible
• H-monel (3% Si added), S-monel (4% Si added) strength and corrosion resistance increased

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Cupro-nickel (Cu-30%Ni alloy)

• Suitable for service at elevated temperature
• Widely used for naval application and condenser tubes

Constantan (Cu-40%Ni-1.5%Mn)

• Have high electrical resistivity not affected by change in temperature
• Used for heating devices, thermocouples, rheostats

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Nickel-silvers/German silvers (Ni-Cu-Zn alloy)

• Cu alloy with 5-40% Zn and 5-30% Ni
• Have good strength ductility and low thermal conductivity
• Used as base metal for plumbing hardware and table ware which are silver plated

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Tin and Lead based alloys

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Babbitt Metal (White metal)

• Sn or Pb based
• Used to provide best bearing surface.
• Typical composition of babbitt metal are
• 90%Sn,10%Cu
• 89%Sn, 7% antimony, 4%Cu
• 80%Pb, 15% antimony, 5%Sn

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