Governing Behavior: Phase Diagrams

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Governing Behavior Phase Diagrams

• Solid-Liquid 2 Phase Zones
• 2 Phase Regions
• Composition determined by tie lines at one temp2
  phases at equilibrium)
• Solid and Liquid Fraction also determined by tie
  line lengths

2
Gibbs Phase Rule

• 2 Phase regions are more constrained in terms of
  conditions where they form than one phase regions
• 3 phase regions would be even more limited
• Degrees of Freedom, DF, based on external
  variables..(T, P, Composition for a 2 component
  system)
• PF CN, (P of phases, F DF, C of
  components, N Noncompositional variables)

3
Iron-Carbon System

• Historically, the most studied phase diagram
• Steelmaking
• Model system in terms of the number and type of
  reactions forming.
• Forms
• Austinite
• Ferrite
• Cementite

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Iron-Carbon Phase Diagram

• Most interest is focused on low C content
• Eutectic 4.3 C by weight
• Euctectoid 0.76 C by weight
• Off-Eutectoid and Off-Eutectic reactions
• Hypereutectic, hypereutectoid
• Hypoeutectic, hypoeutectoid

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Iron Carbon

• So microstructure is again influenced not only by
  composition but also by processing condition.
• Alloying elements Cr, Ni, Si, Mo are most
  common for steel
• Recognize that ores are impure with Mn, S, P,
  that can lead to poisoning of a steel heat
  (material of poor quality, higher chance for
  rust, etc.)

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Designations

• 1000 Series of Iron Carbon Alloys
• 10XX, xx stands for weight of Carbon as .xx
• 1040, 0.4 carbon
• 1040 hypoeutectoid steel
• 10800.8 carbon, slightly hypereutectoid

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Iron Carbon Phase Diagrams

• Equilibrium Microstructures
• Hypereutectoid Steels
• Proeutectoid Cementite
• Pearlite
• Hypoeutectoid Steels
• Proeutectoid Ferrite
• Pearlite