METALS

• Hard, shiny, and tough—metals are the macho poster boys of the material world. Learning how to extract these substances from the Earth and turn them into all kinds of useful materials was one of the most important developments in human civilization, spawning tools, jewelry, engines, machines, and giant static constructions like bridges and skyscrapers.
• What exactly are metals and what makes them so useful?
• Over three quarters of the chemical elements that occur naturally on our planet are metals, so it's almost easier to say what metal isn't.
• Metals


  Chemical elements that are solid (with relatively high melting points), hard, strong, durable, shiny, silvery gray in color, good conductors of electricity and heat, and easy to work into various different shapes and forms (such as thin sheets and wires)
• Classification of elements


  • Metals
  • Nonmetals
  • Semimetals or metalloids

  
• Types of metals


  • Alkali metals
  • Alkaline earth metals
  • Transition metals
  • Lanthanides
  • Actinides
• Metal physical properties


  • Shine (lustrous) in nature
  • Good conductors of heat and electricity
  • High melting point
  • High density (heavy for their size)
  • Malleable (mouldable; can be hammered)
  • Ductile (can be drawn into wires)
  • Usually solid at room temperature (except mercury)
  • Opaque as a thin sheet (can't see through metals)
  • Metals are sonorous or make a bell-like sound when struck
• Metal chemical properties

  • Have one to three electrons in the outer shell of each metal atom and lose electrons readily
  • Corrode easily (e.g., damaged by oxidation such as tarnish or rust)
  • Lose electrons easily and changes to positive ion and are called electropositive elements
  • Have lower electronegativities
  • Are good reducing agents
  • Form oxides that are basic
• Metal + Dioxygen gas


  Metal Oxide
• Metals forming oxides


  • Al (s) + 3 O2 (g) → 2 Al2O3 (s)
  • 2 Zn (s) + O2 (g) → 2 ZnO (s)
  • 2 Mg (s) + O2 (g) → 2 MgO (s)
• Classification of Metals

  • Ferrous Metals
  • Non-Ferrous Metals

  
• Ferrous Metals


  • Steels
  • Cast Irons
• Steels


  Alloys of iron and carbon (may contain other alloying elements)
• Types of Steels


  • Low Alloy
  • High Alloy
• Low Alloy Steels


  • Low Carbon (<0.25 wt% C)
  • Medium Carbon (0.25 to 0.60 wt% C)
• High Alloy Steels


  • Stainless Steel (> 11 wt% Cr)
  • Tool Steel
• Low Carbon Steel


  • Plain carbon steels - very low content of alloying elements and small amounts of manganese
  • Not responsive to heat treatment; cold working needed to improve the strength
  • Good weldability and machinability
• Typical Applications of Plain Low-Carbon Steels

  • Automobile panels, nails and wire
  • Pipe; structural and sheel steel
  • Structural (bridges and buildings)
  • Low-temperature pressure vessels
• Typical Applications of High-Strength, Low-Alloy Steels

  • Structures that are bolted or riveted
  • Structures used at low ambient temperatures
  • Truck frames and railway cars
• Medium Carbon Steel


  • Carbon content in the range of 0.3% to 0.6%
  • Can be heat treated - austenitizing, quenching and then tempering
  • Most often used in tempered condition - tempered martensite
  • Addition of Cr, Ni, Mo improves the heat treating capacity
  • Heat treated alloys are stronger but have lower ductility
• Typical applications of Medium Carbon Steel


  • Railway wheels and tracks, gears, crankshafts
• High Carbon Steel


  • High carbon content of 0.6% - 1.4% provides high hardness and strength
  • 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 of these metals
• Typical Applications of Some Tool Steels

  • Drills, saws, lathe and planer tools
  • Punches, embossing dies
  • Cutlery, drawing dies
  • Shear blades, cutting tools
  • Pipe cutters, concrete drills
• Stainless Steel


  A group of steels that contain at least 11% Cr, exhibits extraordinary corrosion resistance due to formation of a very thin layer of Cr2O3 on the surface
• Categories of Stainless Steel

  • Ferritic Stainless Steels
  • Martensitic Stainless Steels
  • Austenitic Stainless Steels
  • Precipitation-Hardening (PH) Stainless Steels
  • Duplex Stainless Steels
• Effects of Alloying Elements on Steel

  • Manganese - strength and hardness; decrease ductility and weldability; affects hardenability of steel
  • Phosphorous - increases strength and hardness and decreases ductility and notch impact toughness of steel
  • Sulfur - decreases ductility and notch impact toughness, weldability decreases; found in the form of sulfide inclusions
  • Silicon - one of the principal deoxidizers used in steel making; in low-carbon steels, silicon is generally detrimental to surface quality
  • Copper - detrimental to hot-working steels; beneficial to corrosion resistance (Cu > 0.20%)
  • Nickel - ferrite strengthener; increases the hardenability and impact strength of steels
  • Molybdenum - increases the hardenability; enhances the creep resistance of low-alloy steels
• Cast Irons


  • Carbon 2.1 - 4.5 wt% and Si (normally 1 - 3 wt%)
  • Lower melting point (about 300 oC lower than pure iron) due to presence of eutectic point at 1153 oC and 1.2 wt% C
  • Low shrinkage and good fluidity and casting ability
• Types of Cast Iron

  • Grey
  • Nodular or Ductile
  • White
  • Malleable
  • Compact Graphite Iron (CGI)

  
• Grey Cast Iron


  • Contains graphite in the form of flakes; named after its grey fractured surface; C: 3.0 - 4.0 wt%, Si: 1.0 - 3.0 wt%
  • Microstructure: graphite flakes in a ferrite or pearlite matrix
  • Weak and brittle in tension (the graphite flake tips act as stress concentration sites); stronger in compression
  • Excellent damping capacity, wear resistance
  • Microstructure modification by varying silicon content and cooling rate
  • Casting shrinkage is low
• Nodular or Ductile Iron


  • Addition of magnesium and/or cerium to grey iron converts the graphite flakes to nodules
  • Normally a pearlite matrix
  • Castings are stronger and much more ductile than grey iron as the stress concentration points existing at the flake tips are eliminated
• White Cast Iron


  • C: 2.5 - 3 wt%, Si: 0.5 - 1.5 wt%; most of the carbon is in the form of cementite; names after its white fracture surface
  • Results from faster cooling; contains pearlite + cementite, not graphite; thickness variation may result in nonuniform microstructure from variable cooling
  • Very hard and brittle
  • Used as intermediate to produce malleable cast iron
• Malleable Cast Iron


  • C: 2.3 - 2.7 wt%, Si: 1.0 - 1.75 wt%
  • Obtained by heat treating white iron for a prolonged period that causes decomposition of cementite into graphite
  • Heat treatment: two stages - isothermal holding at 950 oC and then holding at 720 oC
  • Graphite forms in the form of rosettes in a ferrite or pearlite matrix
  • Reasonable strength and improved ductility (malleable)
• Compact Graphite Iron (CGI)


  • CGI graphite occurs as blunt flakes or with a worm-like shape (vermicular); C: 3.1 - 4.1 wt%, Si: 1.7 - 3.0 wt%
  • Microstructure and properties are between grey and ductile iron
  • Alloying addition may be needed to minimize the sharp edges and formation of spheroidal graphite; matrix varies with alloy additions or heat treatment
  • As castable as grey iron, has a higher tensile strength and some ductility
  • Relatively high thermal conductivity, good resistance to thermal shock, lower oxidation at elevated temperatures
• Applications of Cast Iron

  • Car parts - cylinder heads, blocks and gearbox cases
  • Pipes, lids (manhole lids)
  • Foundation for big machines (good damping property)
  • Bridges, buildings
  • Cook wares - excellent heat retention
• Copper is one of the earliest metals discovered by man
• The boilers on early steamboats were made from copper
• The copper tubing used in water plumbing in pyramids was found in serviceable condition after more than 5,00 years
• Copper


  • Ductile metal; pure Cu is soft and malleable, difficult to machine
  • Very high electrical conductivity - second only to silver
  • Excellent thermal conductivity - copper cookware most highly regarded - fast and uniform heating
• Uses of Copper


  • Coins
  • Electrical applications (refined to high purity)
• Copper


  One of the earliest metals discovered by man