METALS Composition and Microstructure 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 + 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
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
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
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 - 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
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 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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