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530.352 Materials Selection. Lecture #21 Steels - part II Wednesday November 2 nd , 2005. Transformations :. Equilibrium transformations (slow cooling)  ->  “ferrite”  -> Fe 3 C “iron carbide”   ->  + Fe 3 C “pearlite”

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530 352 materials selection
530.352 Materials Selection

Lecture #21 Steels - part IIWednesday November 2nd, 2005

transformations
Transformations :
  • Equilibrium transformations (slow cooling) ->  “ferrite” -> Fe3C “iron carbide” ->  + Fe3C “pearlite”
  • Nonequlibrium (fast cooling / quenching) -> ’ “martensite”
pearlite
Pearlite :
  • Two phases ( + Fe3C)
normalized slow cooled steels mechanical properties
Normalized (slow cooled) steels;mechanical properties :

2,500

50

Strength (MPa)

Ductility (%)

f

UTS

500

YS

0

0 1%

w/o Carbon

martensite
Martensite :
  • Quenched in distortions :

+ C

C

Fe

martensite mechanical prop
Martensite mechanical prop. :
  • very hard and
  • very brittle (too brittle !!!)
quench and temper
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 .
quenched and tempered
Quenched and tempered :

2,500

50

UTS

f

YS

Strength (MPa)

Ductility (%)

f

UTS

• normalized

• tempered

500

YS

0

0 1%

w/o Carbon

heat treatments and cooling
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
quench rates
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-.6%C + 2-7% (Mo, Mn, Cr, Ni)
    • CCR < 1 C/sec
alloying elements are added to
Alloying elements are added to :
  • improvehardenabilityof 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)
alloying of steels
Alloying of steels :

Type of steel:Fe + ...Typical uses:

Low-alloy .2%C + pressure vessels, aircraft .8Mn,1Cr,2Ni parts, high  applications.

High-alloy .1% C High T and anti-corrosion,

Stainless-steels .5Mn,18Cr,8Ni silverware, medical, etc.

steel terminology sae aisi
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

other steel standards
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