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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


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