Metalurgia Basica

Metalurgia Basica

(Parte 1 de 7)

Engineering “C”–High Perfomance Alloys: HT2000S.G. Roberts2: 1

The region of most interest is around the eutectoid reaction γ γγγ⇒ ⇒⇒⇒α αα α+ Fe

Steels –the Basics Revisited

Ferrite α Fe BCC δFe BCC

Auste nite γFe FCC

Liqui d

Cement ite αFe + Cementite γFe + Cementite γ Fe + Liquid

(Liq uid r ap hi te) wt% CarbonFe at% Carbon

Most steels are based on the “metastable” phase diagram Fe –Fe 3 C.

Engineering “C”–High Perfomance Alloys: HT2000S.G. Roberts2: 2

Ferrite α Fe BCC α+ Fe 3 C

γ+ Fe 3 C

Auste nite γFe FCC wt% Carbon

T (ºC) Eutectoid (0.8wt%C) Carbon Steels

Engineering “C”–High Perfomance Alloys: HT2000S.G. Roberts2: 3

Plain Carbon Steels

Ferrite α Fe BCC α+ Fe 3 C γ+ Fe 3 C

Auste nite γFe FCC wt% Carbon

Engineering “C”–High Perfomance Alloys: HT2000S.G. Roberts2: 4

Plain Carbon Steels

Typical microstructures and properties of “normalised” (slow cooled) carbon steels.

10 µ µ µm

Engineering “C”–High Perfomance Alloys: HT2000S.G. Roberts2: 5

Quenched steels -Martensite

M s time (s)

Time –Temperature –Transformation diagram for “2340” steel: 0.37% C, 0.7% Mn, 3.4% Ni.

Slow cooling:

1: αnucleates at γgrain boundaries and grows into γgrains. 1

2: Cementitestarts to form: αand C grow together into γgrains as “pe arli te ”.

3: 50% of theγ has been transformed 3

4: Decomposition of theγinto αand pea rlite is co mplet e.

Engineering “C”–High Perfomance Alloys: HT2000S.G. Roberts2: 6

Quenched steels -Martensite

M s time (s)

Time –Temperature –Transformation diagram for “2340” steel: 0.37% C, 0.7% Mn, 3.4% Ni.

Rapid cooling (“quenching”):

1: γis supercooled past the diffusioncotrolled transformation “nose” . Too fast for αto nucleate.

2: γnow well below normal transformation T, but diffusion is very slow; carbon is “stuck” in supersaturated solution. 3

3: At M s temperature, γ⇒αfree energy change big enough to force rapid diffusionless transformation to near-α structure but with trapped carbon atoms –“martensite” 4

4: γ⇒martensite transformation is com plete.

Engineering “C”–High Perfomance Alloys: HT2000S.G. Roberts2: 7

Formation of Martensite

The face-centred cubic structure can distort to give a body centred structure.

Each BCC unit cell is directly related to the “parent” fcc unit cell s.

Engineering “C”–High Perfomance Alloys: HT2000S.G. Roberts2: 8

Formation of Martensite

The interstitial holes which house dissolved carbon and nitrogen are much smaller in the BCC structure than in the FCC structure. More C or N can dissolve in FCC than BCC.

“Quenched-in” supersaturated C or N distorts the martensite’s unit cells along three possible dire ction s.

Large locked-in stresses result.

Engineering “C”–High Perfomance Alloys: HT2000S.G. Roberts2: 9

(Parte 1 de 7)

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