Composition axis only extends to 6.70wt% where the intermediate compound Fe_3C (cementite) is formed and is represented by a vertical line in the diagram. Fe-C alloys usually have <6.70wt% C.
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Describe the structural change that occurs upon heating
Why does adding carbon to Fe increase its strength?
It forms a solid solution with all regions and causes local strain.
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Explain how interstitial alloying works?
The introduction of local stress/local strain due to the distortion of the crystal structure gives it strength as when the material is subjected to a load the dislocation motion is made more difficult.
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Describe α - ferrite
BCC. Only small conc of C are soluble (max 0.022wt%). It is relatively soft and magnetic at temps below 768(degrees celsius)
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Describe δ -ferrite
BCC structure.
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Describe γ - austenite
FCC.non-magnetic. max solubility: 2.14wt% as interstitial positions are larger (so more C can be added)
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What is cementite?
Intermetallic compound. Forms when solubility limit of carbon in α - ferrite is exceeded below 727. Coexists with γ - austenite below 727 and 1147. Hard and brittle. It's presence enhances the strength of steels.
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Give the Eutectic reation
Liquid solidifies to form γ - austenite and Fe_3C phases.
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What is the Eutectoid reaction?
γ - austenite is transformed to α - ferrite and cementite.
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State the Peritectic reaction
δ -ferrite and liquid into γ - austenite
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What are Ferrous alloys?
Alloys where the primary component is Fe. Fe:<0.008wt% C; Steel:2.14>C>0.08wt%; Cast irons: 2.14<C<6.70 wt%
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Describe the microstructure of eutectoid steel
Called Pearlite.Alternating lamellar structure. Thick light layers =α - ferrite ,thin dark =cementite; It has properties intermediate btw soft,ductile α - ferrite and hard,brittle cementite.
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Describe the deveopment eutesctoid steel's microstructure
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How/Why does the pearlite structure form?
Phase transformation requires a redistribution of C by diffusion. It diffuses from the ferrite region to the cementite layers. As C atoms need only to diffuse minimal distances the lamellar structure forms.
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What are hypoeutectoid alloys?
When C<C_E, γ transforms to and (γ+α) before the eutectoid reaction.. α-ferrite that existed before the eutectoid is present as continuous matrix phase surrounding the isolated pearlite colonies.
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Define 'eutectoid ferrite'
α-ferrite present in the peralite
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Define Proeutectoid ferrite
α-ferrite formed before the eutectoid reaction.
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What are hypereutectoid alloys?
When C>C_E. γ transforms to (γ +Fe-3C) before eutectoid reaction.
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Define proeutectoid cementite
Fe_3C that forms before the eutectoid reaction
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Why is there difficulty distinguishing between hypoeutectoid and hypereutectoid steels?
As proeutectoid cementite looks similar to proeutect
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Describe the structural change that occurs upon heating
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