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Title: 530.352 Materials Selection


1
530.352 Materials Selection
Lecture 21 Steels - part IIWednesday
November 2nd, 2005
2
Transformations
  • Equilibrium transformations (slow cooling)
    ? -gt ? ferrite ? -gt Fe3C iron
    carbide ?? -gt ? Fe3C pearlite
  • Nonequlibrium (fast cooling / quenching) ?
    -gt ? martensite

3
Pearlite
  • Two phases (? Fe3C)

4
Normalized (slow cooled) steelsmechanical
properties
2,500
50
Strength (MPa)
Ductility ()
?f
UTS
500
YS
0
0 1
w/o Carbon
5
Martensite
  • Quenched in distortions

C
C
Fe
6
Martensite mechanical prop.
  • very hard and
  • very brittle (too brittle !!!)

7
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
    ?.

8
Quenched and tempered
2,500
50
UTS
?f
YS
Strength (MPa)
Ductility ()
?f
UTS
normalized tempered
500
YS
0
0 1
w/o Carbon
9
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

10
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

11
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)

12
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.
13
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
14
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
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