Lecture Problem 11'22

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Lecture Problem 11.22

• Ross Sclafani
• 4.13.2007

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The Problem
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The Figure
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The Background

• Phase Diagram of C iron-carbon alloy

Eutectoid
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Before We Begin

• How to read an Isothermal Transformation Diagram
• You are only transforming
• the remaining austenite
• You can only move in
• two directions
• Down OR
• Right

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Example 1

• We start with pure
• austenite
• T 400C
• Microstructures
• _at_t1s 100 austenite
• _at_t20s 50 austenite
• 50 bainite
• _at_t103s 100 bainite

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Example 2 Time Travel?

• Diagram only shows transformations over time at a
  fixed temp.
• We always start from the far left (t0s)
• However, we can also cool to a 2nd temperature
• T1400C, T225C

50 Bainite
50 Bainite 50 Martensite
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The Solution

• What exactly is proeutectoid cementite?
• Cementite (Fe3C)
• Lever Rule
• If we pass through this
• temp range Cem. will form

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The Solution

• Back to the isothermal transformation
• Cool (instantaneously) from 900C to 680C
• (forming 6.2 p.c.)
• Expose at 650C for 10s
• remaining 93.8 austenite ? pearlite)

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The Solution

• Pearlite transformations
• Coarse High T, High carbon diffusion rate,
  thick stripes
• Fine Low T (600C), Low diffusion, thin stripes
• Only expose to dashed line (50 line)
• Cool to bainite temp and cure
• Technically, impossible for 50/50 due to
  cementite formation (6.2C, 46.9P, 46.9B)

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The Solution

• Martensite ? very hard
• However, very brittle
• Formed simply by quenching (or cont. cool to room
  temp) austenite

Optional
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The Solution Last One!
We want our tempering time to be reasonable (not
105s 1 day) Pick 425C for this application to
achieve significant gains

• Benefits of tempering
• Sacrifice hardness
• Gain ductility (less brittle)
• How?
• Form martensite (from c)
• Reheat to tempering temp and expose
• Gains dependent on temper temp AND time

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Why Should You Care?

• When controlling materials properties (like Steel
  hardness and ductility) is critical to your
  application
• One example everyone is familiar with
• E8 Both your hammer head and screwdriver blade
  were heated, quenched (to form martensite), then
  tempered to a specified hardness
• Now your hammer wont shatter when you hit a nail

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Any Questions?