Lecture Problem 11'22

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Lecture Problem 11.22

• Eric Burkhart
• 13 April 2007

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Problem Statement

• Sketch and label time-temperature paths to
  produce the following microstructures

a.) 6.2 proeutectoid cementite and 93.8 coarse
pearlite
b.) 50 fine pearlite and 50 bainite
c.) 100 martensite
d.) 100 tempered martensite
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What is an Isothermal Transformation Diagram?
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What is an Isothermal Transformation Diagram?

• Time indicates time at that temperature, when
  temperature is changed starts back at zero
• Once austinite changes to another phase, that
  phase will stay unless temperature is raised
  again to reform austinite
• Dotted line indicates 50 of transition has
  occurred

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6.2 Proeutectoid Cementite and 93.8 Course
Pearlite

• Start with austinite at about 760C
• Rapidly cool to around 660C
• Hold at that temperature at least until you cross
  the 100 solid line
• Cool to room temperature

Why this works Lever Rule! In crossing through
the AC region, you create an amount of Cementite
equal to the wt Cementite just above the
eutectoid line.
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Calculation

• wc(ca-c0)/(ca-cc)
• (0.76-1.13)/
• (0.76-6.70)
• 0.0623
• 6.23 Proeutectoid Cementite!

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50 Fine Pearlite and 50 Bainite

• Cool from about 760C to about 600C, hold at
  that temperature until you reach dotted 50 line
• Rapidly quench to about 500C and hold until you
  surpass the 100 B line
• Cool to desired temperature
• IMPORTANT If you stop at the 50 B line and
  quench, you will have 50 pearlite and 25
  bainite!

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100 Martensite

• Quench from initial temperature (about 760C) to
  room temperature very quickly!
• The faster the more brittle, but too slow and
  youll form a mixture of phases

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100 Tempered Martensite

• Same as regular martensite, except once
  martensite is formed, reheat to 250-650C and
  hold for about 103 -104 seconds.
• Putting this on a transformation diagram is
  misleading and doesnt make sense

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Explanation of Heat Treating in Tools

• Remember heating your screwdriver blade at a high
  temperature, then quenching in oil, then heating
  at a lower temperature and letting it air-cool?
• You made tempered martensite!

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Continuous Cooling Diagrams

• Can you really cool 700C in less than a second?
  Then why are these diagrams actually useful?
• Continuous cooling transformation diagrams are
  more useful for actual heat treatments. These
  show how the final microstructure is affected by
  different rates of cooling.