530.352 Materials Selection

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530.352 Materials Selection
Lecture 21 Steels - part IIWednesday
November 2nd, 2005
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Transformations

• Equilibrium transformations (slow cooling)
  ? -gt ? ferrite ? -gt Fe3C iron
  carbide ?? -gt ? Fe3C pearlite
• Nonequlibrium (fast cooling / quenching) ?
  -gt ? martensite

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Pearlite

• Two phases (? Fe3C)

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Normalized (slow cooled) steelsmechanical
properties
2,500
50
Strength (MPa)
Ductility ()
?f
UTS
500
YS
0
0 1
w/o Carbon
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Martensite

• Quenched in distortions

C
C
Fe
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Martensite mechanical prop.

• very hard and
• very brittle (too brittle !!!)

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Quench and Temper

• Temper heat treat at intermediate T (300-600
  C)
• Mechanical properties
• regains toughness (critical) with only a moderate
  drop in hardness
• Microstructurally
• C comes out of lattice a precipitates as Fe3C,
  and distortion decreases with decrease amounts
  of dissolved C.
• Loss of distortion leads to bcc structure and
  ductility.
• Fe3C precipitates - precipitation strengthen the
  ?.

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Quenched and tempered
2,500
50
UTS
?f
YS
Strength (MPa)
Ductility ()
?f
UTS
normalized tempered
500
YS
0
0 1
w/o Carbon
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Heat treatments and cooling

• Normalizing (heating to form ?)
• T 1,000 C
• much easier to roll / forge / form at this
  temperature
• Quench - or - Slow cooling
• martensite pearlite
• Tempering (heating to soften martensite)
• carbides form, distortions relax

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Quench rates

• To form martensite in pure Fe
• Critical Cooling Rate (CCR) 100,000 C/sec
• To form martensite in mild steel Fe - 0.8 C
• CCR 200 C/sec
• To form martensite in alloy steels Fe -
  0.2-.6C 2-7 (Mo, Mn, Cr, Ni)
• CCR lt 1 C/sec

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Alloying elements are added to

• improve hardenability of the steel
• aides nucleation of martensite
• solution strengthen and precipitation hardening
• MxCy carbides form
• give corrosion resistance
• especially Cr which forms Cr203
• stabilize FCC austenite at RT
• especially Ni
• tougher, more ductile and easier to form
• non-magnetic and creep resistant (diffusion is
  slower in FCC)

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Alloying of steels
Type of steel Fe ... Typical
uses Low-alloy .2C pressure vessels,
aircraft .8Mn,1Cr,2Ni parts, high ?
applications. High-alloy .1 C High T and
anti-corrosion, Stainless-steels .5Mn,18Cr,8Ni s
ilverware, medical, etc.
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Steel terminology (SAE-AISI)

• Plain carbon
• 10xx
• Manganese steels
• 13xx
• Nickel steels
• 23xx
• Ni-Cr-Mo steels
• 43xx
• HSLA
• 9xx

Note xx indicates carbon content in hundredths
of a percent
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Other steel standards

• SAE-AISI
• Society of Automotive Engineers American Iron
  and Steel Institute
• ASTM
• American Standards for Testing and Materials
• AMS
• Aerospace Materials Specifications
• DIN
• Deutsches Institut fur Normung
• JIS
• Japanese Industrial Standards Committee
• UNS
• Unified Numbering System