Engineering Materials: Metallic vs. Non-Metallic

• Engineering materials are classified into metallic and non-metallic materials

Metallic Materials

• Metallic materials are divided into ferrous (containing iron) and non-ferrous materials
  • Ferrous materials include steels and cast iron
    • Steels are further classified into plain, carbon, and alloy steels
    • Cast iron has types like grey, white, malleable, ductile, and nodular
  • Non-ferrous materials include aluminum, copper, magnesium, tin, zinc, lead, nickel, and their alloys

Non-Metallic Materials

• Non-metallic materials are classified into organic and inorganic materials
  • Organic materials include plastics, wood, paper, rubber, and petroleum
  • Inorganic materials include minerals, cement, glass, ceramics, and graphite

Ferrous Materials

• Steels are iron-carbon alloys and may contain other alloying elements
• Several grades are available

Low Alloy Steels

• Low Alloy steels contain less than 10 wt% alloying elements
  • Low Carbon steels have less than 0.25 wt% C
  • Medium Carbon steels have 0.25 to 0.60 wt% C
  • High Carbon steels have 0.6 to 1.4 wt% C

High Alloy Steels

• High Alloy Steels include stainless steel containing greater than 11 wt% Cr, and tool steel

Low Carbon Steel

• Plain carbon steels have very low alloying elements and small amounts of Mn
• Low carbon steel is the most abundant and least expensive grade
• It is not responsive to heat treatment and requires cold working to improve strength
• It has good weldability and machinability
• High Strength, Low Alloy (HSLA) steels contain alloying elements like Cu, V, Ni, and Mo, up to 10 wt%

Medium Carbon Steel

• Carbon content ranges from 0.3 - 0.6%
• Can be heat treated through austenitizing, quenching, and tempering
• It is most often used in tempered condition as tempered martensite
• Medium carbon steels have low hardenability
• Adding Cr, Ni, Mo improves heat treating capacity
• Heat-treated alloys are stronger but have lower ductility
• Typical applications include railway wheels and tracks, gears, and crankshafts

High Carbon Steel

• High carbon steels have a carbon content of 0.6 – 1.4%
• High carbon content provides high hardness and strength
• It is the hardest and least ductile.
• Used in hardened and tempered condition
• Strong carbide formers like Cr, V, W are added as alloying elements to form carbides
• High carbon steel is used as tool and die steels because of the high hardness and wear resistance

Effects of Alloying Elements on Steel

• Manganese increases strength and hardness but decreases ductility and weldability
• Phosphorus increases strength and hardness but decreases ductility and notch impact toughness
• Sulfur decreases ductility and notch impact toughness and weldability, found in sulfide inclusions
• Silicon is a principal deoxidizer in steel making, detrimental to surface quality in low-carbon steels
• Copper is detrimental to hot-working steels but beneficial to corrosion resistance (Cu>0.20%)
• Nickel is a ferrite strengthener and increases hardenability and impact strength of steels
• Molybdenum increases hardenability and enhances the creep resistance of low-alloy steels

Stainless Steel

• Stainless steels contain at least 11% Cr, exhibiting corrosion resistance due to a thin layer of Cr2O3 on the surface

Categories of Stainless Steels

• Ferritic Stainless Steels: Composed of α ferrite (BCC)
• Martensitic Stainless Steels: Can be heat treated
• Austenitic Stainless Steels: Austenite (γ) phase field is extended to room temperature, the most corrosion resistant
• Precipitation-Hardening (PH) Stainless Steels: Ultra high-strength due to precipitation hardening
• Duplex Stainless Steels: Ferrite + Austenite

Cast Irons

• Cast irons contain carbon 2.1-4.5 wt% and Si (normally 1-3 wt%)
• Lower melting points approximately 300 °C lower than pure iron
• Low shrinkage and good fluidity, for easy casting
• Types of cast iron include: grey, white, nodular, malleable and compacted graphite.

Gray Cast Iron

• Contains graphite in flake form with 3.0 – 4.0 wt% C and 1.0 – 3.0% Si
• Microstructure is graphite flakes in ferrite or pearlite matrix
• Weak & brittle in tension, stronger in compression
• Excellent damping capacity and wear resistance
• Microstructure modified by varying silicon content and cooling rate
• Exhibits low casting shrinkage

White Cast Iron

• Contains 2.5 – 3 wt.% C and 0.5 - 1.5% Si
• Most of the carbon is in form of cementite
• Named after its white fracture surface.
• Results from faster cooling, it contains pearlite + cementite but no graphite
• Thickness variation may result in non-uniform microstructure from variable cooling
• Very hard and brittle
• Used as an intermediate to produce malleable cast iron

Malleable Cast Iron

• Has Carbon: 2.3 – 2.7 wt%, Si: 1.0 – 1.75%
• Obtained by heat treating white iron for an extended duration, which decomposes cementite into graphite
• Heat treatment involves two stages - isothermal holding at 950 °C and then holding at 720 °C
• Graphite forms in rosettes in a ferrite or pearlite matrix
• Offers reasonable strength and improved ductility

Nodular/Ductile Iron

• Addition of Mg and/or Cerium converts graphite flakes to nodules
• Has a pearlite matrix
• Castings are stronger and much more ductile than gray iron

Non-Ferrous Materials

• Copper, aluminum, and titanium are commonly used non-ferrous materials

Copper Alloys

• Copper alloys include:
  • Brass: Cu-Zn alloy that is corrosion resistant and used in costume jewelry and coins
  • Bronze: Cu with Sn, Al, Si, Ni
  • Cu-Be: precipitation hardened materials used for landing gear

Copper

• Most plumbing used in Pyramids was found in serviceable condition after more than 5,000 years
• Copper is a ductile metal when pure.
• Copper is difficult to machine, as pure copper is soft and malleable
• It has very high electrical conductivity
• Refined to a high purity for many electrical applications
• It has excellent thermal conductivity
  • Copper cookware most highly regarded - fast and uniform heating.
• Electrical and construction industries are the largest users of Cu
• The second largest use of Cu is probably in coins
• The U.S. nickel is actually 75% copper
• The dime, quarter, and half dollar coins contain 91.67%
• The Susan Anthony dollar is 87.5% copper
• Copper alloys include:
  • Brasses and Bronzes are most commonly used alloys

Aluminum

• Properties include:
  • Low density: 2.7 g/cm³
  • Added elements can strenghten through solid solution or precipitation
• Very reactive and readily combustible in air
• Easily ignites aircraft and missiles
• Aluminum alloys include:
  • Al-Li alloys: Widely used in aerospace

Titanium

• Excellent oxidation/corrosion resistance

Aluminum

• Light metal (p = 2.7 g/cc) that has a wide variety of surface finishes and good electrical and thermal conductivity
• Corrosion resistant and reflective to heat and light
• Can be riveted, welded, brazed, or resin bonded
• Has a high strength-to-weight ratio
• Used in aerospace and automotive applications
• Al-Li alloys are among the lightest Al alloys with wide applications in the aerospace industry
• Aluminum alloys are classified into cast and Wrought alloys

Wrought Alloys

• Can be either heat-treatable or non-heat treatable
• Designated by a 4-digit number, with the first digit representing the major alloying element
• Minimum 99.00% Aluminum: 1xxx Series
• Copper: 2xxx Series
• Manganese: 3xxx Series
• Silicon: 4xxx Series
• Magnesium: 5xxx Series
• Magnesium and Silicon: 6xxx Series
• Zinc: 7xxx Series
• Other Elements: 8xxx Series

Titanium

• Pure titanium melts at 1670 °C and has a low density of 4.51 g/cc which is 40% lighter than steel and 60% heavier than aluminum
• Has high affinity to oxygen - strong deoxidiser
  • Can cause severe damage
• Ti is stronger than Al and high strength and low weight are very useful as a structural metal
• Has excellent corrosion resistance due to a protective thin oxide surface film
• Used as biomaterial and for elevated temperature components
• Limitation of pure Ti is its lower strength. Alloying is done to improve strength Oxygen, nitrogen, and hydrogen can cause titanium to become more brittle, requiring care during processing
• Titanium can also be cast using a vacuum furnace
• Uses include:
  • Aircraft body structure and engine parts
  • Sporting equipment, chemical processing, and desalination
  • Turbine engine parts, valve and pump parts, marine hardware, and medical implants
  • Use of Ti in bikes and automotives is increasing

Titanium Alloys

• Pure Ti exhibits two phases which is hexagonal α-phase at room temperature and BCC β-phase above 882 °C
• Strength of Titanium is improved by alloying with either an alpha or beta stabilizer α stabilizer (Al, O, Ga), a neutral (Sn, Zr) and a β stabilizer (V, Mo, Ta, W) Alpha () alloys have lower density, reasonable strength, reasonable ductility, and good creep resistance

Nickel

• High density, high strength metal with excellent corrosion resistance and high temperature properties
• Ni has may unique properties including its excellent catalytic property, acts as a Nickel Catalyst for Fuel Cells
• Nickel-cobalt is seen as a low-cost substitute for platinum catalysts
• Uses include:
  • Two-thirds of all nickel produced goes into stainless steel production
  • Extensively used in electroplating various parts
  • Ni-base super alloys are a unique class of materials having exceptionally good high temperature strength, creep and oxidation resistance
    • Used in many high temperature applications like turbine engines
    • Building and infrastructure, chemical production, communications, energy batteries, environmental
• Shape Memory Alloys: Ni base and Ni containing shape memory alloys that can go back to original form, are an important class of engineering materials finding widespread use in many applications

Magnesium

• Lightest among commonly used metals (p 1.7 g/cm³). Melting point is 650 °C and it has HCP structure
• Magnesium Is very reactive and readily combustible in air. Can be used as igniter or firestarter
• Very reactive and readily combustible in air
• Can be used as igniter or firestarter
• Pure Mg has adequate atmospheric resistance and moderate strength
• Thermal conductivity is less than Al while their CTE is almost same
• Properties of Mg can be improved substantially by alloying
• Alloy with Al, Zn, Mn and Zr
• Mg alloys are cast, wrought

Ceramics Materials

• Used for impact and dent resistance, good damping capacity - effective for high-speed applications
• Due to its light weight, superior machinability and ease of casting, Mg and its alloys are used in many applications
• Automotive applications include gearboxes, valve covers, alloy wheels, clutch housings, and brake pedal brackets
• Classified into:
  • Ceramics
    • Refractory Materials
    • Abrasives
    • Glass
    • Advanced Ceramics

Refractory Materials

• Retain their strength at high temperatures > 500°C
• Chemically and physically stable at high temperatures
• Resistant to thermal shock and chemically inert
• Used in linings for furnaces, kilns, incinerators, crucibles and reactors
• Include Aluminium oxide (alumina), silicon oxide (silica), calcium oxide (lime)
• Can also use magnesium oxide (magnesia) and fireclays for manufacturing refractory materials
• Zirconia - extremely high temperatures
• SiC and Carbon – also used in some very severe temperature conditions, but oxidizes and burns when exposed to oxygen

Abrasive Ceramics

• Used to cut, grind and polish other softer materials
• Diamonds (natural and synthetic) are used as abrasives, though relatively expensive
• Common abrasives
  • SiC, WC, Al2O3 (corundum) and silica sand
• Either bonded to a grinding wheel or made into a powder and used with a cloth or paper

Glass

• Inorganic, non-crystalline (amorphous) material
• Can use soda-lime silicate glass for soda bottles or the extremely high-purity silica glass for optical fibers
• Used in windows, bottles, glasses to drink from and transfer piping and receptacles for highly corrosive liquids and for nuclear applications
• The main constituent of glass is silica (SiO2). The most common form of silica used in glass is sand.
• Sand fusion temp to produce glass - 1700 °C
• Additives that significantly reduce the fusion temperature include:
  • chemicals to sand can considerably reduce the fusion temperature
  • Sodium carbonate (Na2CO3) or soda ash which is 75% SiO2 + 25% Na2O - will reduce the fusion temperature to 800 °C

Polymers

• Chain of H-C molecules whose repeat unit of H-C is a monomer
  • e.g. ethylene (C2H4), Polyethylene – (-CH2 –CH2)n
• Thermosets Soften when heated and harden on cooling
  • Totally reversible
• Thermoplasts do not soften on heating
• Plastics – moldable into many shape and have sufficient structural rigidity
• Among the most commonly used class of materials
• Uses include clothing, housing, automobiles, aircraft, packaging, electronics, signs, recreation items, and medical implants.
• Natural plastics include hellac, rubber, asphalt, and cellulose

Elastomers

• A polymer with rubber-like elasticity
• Each of the monomers that link to form the polymer is usually made of carbon, hydrogen, oxygen and/or silicon
• Cross-linking in the monomers provides the flexibility
• Glass transition temperature, Tg, is the temperature at which transition from rubbery to rigid state takes place in polymers
• Elastomers are amorphous polymers existing above their Tg with considerable segmental motion in them
• Primary uses are in seals, adhesives and molded flexible parts

Advanced Ceramics for Automobile Engine parts

Advantages

• High operating temperatures results in high efficiencies
• Operate without a cooling system
• Yields lower weights than current engines
• Creates Low frictional losses

Disadvantages

• Difficult to remove internal voids
• Ceramic parts are difficult to form and machine
• The high brittleness of Ceramic materials

Basics of Heat Treatment

• Heating and cooling process of a metal or an alloy in the solid state
• The purpose is to change properties
• Also a process of heating and cooling of ferrous metals where some special properties like softness, hardness, tensile-strength, toughness etc, are induced

Steps for Success

• Consists of three main phases: heating of the metal, soaking of the metal, and cooling of the metal
• The theory is based on the fact that a change takes place in internal structure by heating and cooling, which induces desired properties.
• The rate of cooling is the major controlling factor with rapid cooling resulting in a hard structure

Important Factors in Heat Treatment

• The rate of heating, machining properties, annealing.

Objectives of Heat Treatment

• Relieves internal stresses induced hot or cold working
• Changes or refines grain size
• Increases resistance to heat and corrosion
• Improves mechanical properties such as ductility, strength, hardness, toughness, etc
• Helps to improve machinability
• Increases wear resistance
• Improves electrical and magnetic properties
• Changes the chemical composition
• Helps improves shock resistance
• Improves weldability

Constituents of Iron and Steel

• Figure (a) shows the microstructure of mild steel
  • Has low free carbon in iron
  • White constituent is very pure iron or having very low free carbon in iron in form of ferrite
  • Dark patches contain carbon in iron combined form known as carbide Cementite
• Cementite is very hard and brittle
• Pearlite is composed of - 87% ferrite and 13% cementite
• With a specific carbon content in steel which leads to a structure of steel
• As steel is further increased, such steel will be classified as high carbon steel

Allotropy of Iron

• A substance that exists in one or more different physical forms
• Can be affected by temperature
  • Includes first changing occurs at 1539°C at which formation of delta iron starts
  • Second changing takes place at 1404°C and where delta iron starts changes into gamma iron or austenite structure
  • Third changing occurs at 910°C and where gamma iron structure starts changes into beta iron structure
  • Fourth changing takes place at 768°C and where beta iron structure starts changes into alpha iron form

Transformation During Heating and Cooling of Steel

• If heat is extracted, the temperature falls unless there is change in state or a change in structure
• Change of structure does not occur at a constant temperature
• Sufficient time is required for transformation, known as transformation range

Lower Critical Temperature

• Internal grain structured changed through changing temperature with about 700 degrees with carbon steels

Upper level Critical Temperature

• Structural changes continue until a full change in internal structure
• Steel is heated until a change has occurred
• Temperature range can known as the critical range

Iron-Carbon Equilibrium Diagram

• Austenite
  • Solid solution of carbon (ferrite) and iron in gamma iron
  • Hard, ductile, nonmagnetic
  • Steel formed at contains 1.8% carbon at 1130 degrees C
  • Transforms into pearlite and ferrite below 723 degrees C
• Ferrite contains very little carbon in iron - it’s very magnetic, soft and ductile
  • Does not harden when cooled
• Cementite contains carbon with iron
  • Used in cast iron and in possessing complete structure of cementite
• Pearlite contains mixture of alloy of ferrite and cementite
  • Is usually strong, hard and ductule

Common Heat Treatment Processes

• Normalizing
• Annealing
• Hardening
• Tempering
• Case hardening
  • Carburizing
  • Cyaniding
  • Nitriding

Normalizing

• Carried out in normal structure for steel
• For example, steel which has been forged with distorted grain structure due to hot working.
• Requires grains to to their normal un distorted to be in best condition
• Differing from annealing occurs only in rate of cooling - for soak required heat

Annealing

• Soften steel to may be machined or so that additional cold forming operations
• Then heats the steel to temperature dependent on carbon, and thickness
• Then allows steel to reach cool slowly with the furnace itself switching of
• Achieves a slow rate with machining and easy to be shaped

Hardening

• Inducing a kind of heat resistant with it be heated to a required temp
• Time is important for quenching in a bath
• It also some quick reaching of carbon and held for a time
• Low carbon steels increased object like for the manufacture of machine parts carbon

Tempering

• After steel hardness has been increased temperature
• And it happens with reheating so that will happen which means raising the change
• It’s also the highest in the hardening steel to which has lower and brittle

Case Hardening

• The hard layer of outer hardening while maintaining a soft internal core
• And it normally increased object which is manufacture

Cyaniding

• An iron based process which heated with salt so carbon and nitrogen increases.

Nitriding

• Heat increased and it is treated by duffising nitrogen which is created by surface and it increase carbon