Metals composition and Microstructure Ferrous Metals and Alloys



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METALS

  • Composition and Microstructure

  • Ferrous Metals and Alloys

  • Non-Ferrous Metals and Alloys

  • Specifications and Proof Testing

  • Corrosion


Composition and Microstructure

  • Metal: element that readily loses electrons to form positive ions, characterized by high electrical conductivity and malleable

  • Alloy: combinations of metals in a crystalline structure



Structure of Metals

  • Microstructural properties determine all of the material properties of metals and alloys.



Alloying Structure

  • 3-D lattice in metalic bonds provides

    • opportunity for other element to occupy some of the positions.
    • or for small element to enter the lattice


Interstitial Alloy

  • Between atomic lattice location

  • < 60% of the size of the lattice atoms

  • only a small % can fit interstitially

  • For Transition metals only a few fit

  • H, B, C, N



Substitutional Alloy

  • Replacing elements in the lattice

  • + 15% radius of lattice atoms

  • large percentage is possible

  • Alloys may contain both interstitial and substitutional elements



Forming a Crystalline Structure

  • Liquid: large degree of disorder

  • Freezing Point: order begins to form

  • Grain Initiation: initiation energy

  • Solidification: ordered lattice structures form

  • Grain Boundary: separate lattices collide

  • FCC:BCC or FCC:FCC with different angle



Forming a Crystalline Structure

  • Grain Structure: each grain has its own lattice structure (FCC, BCC, HCP).



Introduction to Steel

  • Production

  • Commercial Forms

  • Applications

  • Microstructure

  • Strengthening Mechanisms

  • Corrosion



Metal Processing

  • Crushing and Calcining, or Separation

  • Extraction

    • Smelting
      • Ore is melted and separated in solution
    • Electrolytic processing
      • electric furnace or process is used to separate metal
    • Leaching (liquid processing)
      • metal is recovered from leachate


Ferrous Metals

  • principle element is iron, cast iron, steel, wrought iron.

  • Metals come from ore, "minerals" ore consists of metal and gangue (valueless extra)

  • Mining

    • open pit
    • underground


Refining the Metal

  • Refining the Metal

    • oxidizing impurities
    • distillation
    • chemical agents
    • electrolysis


Iron Production

  • Blast Furnace

  • Molten Iron

  • Slag



Processing of Virgin Steel

  • 1) first step in reducing iron ore,

  • 2) separates impurities

  • 3) absorbs carbon (leaves 2.5 - 4.5 % carbon)

  • End product is cast in bars, "pigs".



Ferrous Metals

  • Pig Iron

    • Iron ore is combined with coke, and limestone (fluxing agent). Blasts of hot air are forced through the material to ignite the coke and melt the iron ore. The impurities in the iron are absorbed by the limestone and forms blast furnace slag.


Forms of Ferrous Alloys

  • Cast Iron

    • cast iron is pig iron is any other shape. Remelted and cast into desired shape.
  • Malleable Cast Iron

    • annealed (heating then slow cooling to encourage refined grains and soften mechanical properties, removes internal stresses, removes gases) cast iron that has been made more ductile and formable.


Forms of Ferrous Alloys

  • Wrought Iron

    • a form of iron that contains slag, and virtually no carbon. making it workable when it is hot but hardens very rapidly when cooled rapidly.
  • Ingot Iron

    • low carbon steel or iron cast from a molten state.


Forms of Ferrous Alloys

  • Steel

    • Iron - Carbon alloy which is cast from a molten mass in a form which is malleable. Carbon steel is steel with less than 1.5% carbon. Alloy steel is steel which has properties controlled by elements other than carbon.
    • Steel has the best structural properties of these materials


Carbon Steels

  • Carbon steels have between .008 and 1.7 percent C (most are between 0.1 and 0.8%)

  • Carbon may be substitutional or interstitial depending upon the amount present

  • Alloys with greater than 1.7 percent carbon become very brittle and hard, i.e. cast iron properties.



Phase Diagrams

  • Phase Diagrams relate the

    • composition & temperature
    • to the
    • crystalline structure (“phase”)
  • Inverse Lever Law

    • determines the percentage of each crystalline phase


Two Component (Binary) Phase Diagram for completely soluble elements or compounds



Two Component (Binary) Phase Diagram: Ni - Cu



Binary Phase Diagram for insoluble elements or compounds



Definitions

  • Eutectic Reaction –

  • Eutectic Point –

  • Eutectic Solid –



Water - NaCl Phase Diagram



Binary Phase Diagram for partially soluble elements or compounds



Lead-Tin Phase Diagram



Definitions

  • Eutectoid Reaction –

  • Eutectoid Point –

  • Eutectoid –



Steps to Analyzing a Phase Diagram

  • Determine the phase/phases present at the point (composition vs. temperature)

  • The mass percentage composition of each phase at the point can be determined by the drawing a horizontal through the point for the length of the entire region.

  • The intersection of the horizontal line and a line on the phase diagram defines the composition of the solution.



A Point with 2 Phases

  • If the point is located in a region with more 2 phases, the mass percentage of each phase within the region can be determined by the inverse lever law.



Inverse Lever Law

    • Inverse Lever Law (Derivation on pgs 56 + 57 of text)
    • The mass percentage of a phase present in a two phase region is the length along the “tie line” portion from the state point to the other phase region divided by the total “tie line” length. Compositions are used as a measure of length.


Example: Ni-Cu

  • For a 1000 kg block of Ni-Cu metal at a defined state point of 53% Nickel and 47% Copper at 1300 oC, determine the following:

  • Compositions (%) of both the liquid and solid phases

  • Mass percentages of the liquid and solid phases

  • The mass of Nickel in the Liquid Phase



Example: Ni - Cu



Phase diagram for Fe-C

  • Cementite:

    • above 4.35 to 6.67
    • very hard and brittle alloy forms
    • 6.67% Carbon 93.33% Iron "iron carbide"
  • Ferrite:

    • iron which contains very little carbon. this is soft ductile material


Phase diagram for Fe-C

  • Pearlite:

    • combination of ferrite and cementite structures
    • intermediate property structure
  • Austinite:

    • solid state gamma phase iron-carbon combination.


Phase Diagram for C-Fe



Microstructure

  • Phases of Steel

    • Ferrite (BCC)
    • Austenite (FCC)
    • Cementite (Orthorhombic)
    • Delta Iron (BCC)
  • Grain Size





Time-Temperature-Transition Curves



Heat Treatments

  • Annealing

    • heated above critical temperature and
    • cooled slowly
    • softens structure
  • Quenching

    • heated above critical temperature and
    • cooled rapidly in water or oil
    • improves hardness and strength


Heat Treatments

  • Tempering

    • heated below critical temperature,
    • held and
    • quenched
    • improves ductility and toughness
    • while retaining hardness






Mild Steel Grades

  • A992 “Low Alloy” Carbon Steel

    • <0.23% Carbon
    • Common Structural Sections
    • Replaced A36 steel
  • A 572 “High-Strength Low-Alloy Columbium-Vanadium Steel”

    • Grades 42, 50, 60, 65
    • Structural sections and bolts.....


Mild Steel Grades

  • A 615 Billet Reinforcing Steel

    • low alloy, high ductility steel
    • reinforcing bars
  • A588 Weathering Steel

    • should not be used in Cl water environments
    • Free from moisture 40% of the time; avoid extreme humid environments


Corrosion

  • Oxidation of metal requires

    • oxygen,
    • water,
    • two different metals connected electrically
    • electrolyte


Corrosion

  • Major problem with steel

  • Control Methods

    • Protective Coatings
    • Galvanic Protection
    • Cathodic Protection
    • Corrosion-resistant Steels


S-N Curve



Strengthening Mechanisms



Alloying

  • Forming Solid Solution with Iron

    • Carbon, Chromium, Manganese, Nickel, Copper, and Silicon
  • Formation of Carbide

    • Titanium, Vanadium, and Molybdenum
  • Formation of an Undissolved, second phase

    • Lead, Sulfur, and Phosphorus


Heat Treatments

  • Full Annealing

  • Process Annealing

  • Normalizing

  • Quenching



Cold Working

  • Plastic deformation

  • Done below recrystallization temperature



Other Properties of Steel

  • Impact

    • resistance to dynamic loadings (toughness)
  • Creep

    • time dependent deformation due to sustained loads
  • Ductility

    • mild steels may yield at  = 0.002 and
    • fracture at  > 0.200


Forms of Steel

  • Structural Shapes

    • Wide flange sections,
    • Channels,
    • Tubing,
    • Plate
  • Reinforcing Steel

  • Cold Rolled forms, pans, sheet

  • Pipe



Structural Grades

  • ASTM

    • A36 & A 572 (being phased out)
    • A992 Structural Shapes
    • A325 Bolts
  • AISI - SAE

    • 10XX
      • XX defines Carbon content
    • 13XX
      • 13 defines a manganese alloy steel


Applications

  • Structural Members

  • Bolts, Connectors

  • Reinforcement

  • Tools

  • Machines



Steel Grades



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