Page 1


Phases present L

Reactions

δ Bcc structure Paramagnetic

α ferrite Bcc structure Ferromagnetic Fairly ductile

γ austenite Fcc structure Non-magnetic ductile

Fe3C cementite Orthorhombic Hard brittle

Peritectic L + δ = γ Max. solubility of C in ferrite=0.022% Eutectic L = γ + Fe3C Eutectoid γ = α + Fe3C

Max. solubility of C in austenite=2.11%


Steel is an interstitial solid solution of carbon in iron. Theoretically steel has a maximum of 2.11% carbon. In practice, the amount of carbon rarely exceeds 0.8%

Classification/Nomenclature Low carbon steels up to 0.2%C Medium carbon steels 0.2-0.4%C High carbon steels >0.4% C

AISI 1020: Last two numbers indicate Amount of carbon :0.2%C 10 indicates plain carbon steel AISI 4340: 0.4%C 43 indicates alloy steel


IRON-CARBON (Fe-C) PHASE DIAGRAM

-Eutectoid (B): γ ⇒ α + Fe 3 C

1600

δ L

1400

γ+L γ (austenite)

1200

γ α+

800

α

6 00

120 µm

B

γ+Fe 3 C

727°C = T eutectoid

R

4 00 0 (Fe)

Result: Pearlite = alternating layers of α and Fe 3 C phases. (Adapted from Fig. 9.24, Callister 6e. (Fig. 9.24 from Metals Handbook, 9th ed., Vol. 9, Metallography and Microstructures, American Society for Metals, Materials Park, OH, 1985.)

L+Fe 3 C S

R

γ γ γ γ

1000

A

1148°C

S α+Fe 3 C 1

0.77

2

3

4

5

6

Fe 3 C (cementite)

L ⇒ γ + Fe 3 C

T(°C)

C eutectoid

• 2 important points -Eutectic (A):

6.7

4.30 C o , wt% C Fe 3 C (cementite-hard) α (ferrite-soft)

Adapted from Fig. 9.21,Callister 6e. (Fig. 9.21 adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)


IRON-CARBON (Fe-C) PHASE DIAGRAM BCC crystal structure

FCC crystal structure

Note: δ phase is also called δ ferrite and is a BCC phase

1


ALLOYING STEEL WITH MORE ELEMENTS • Ceutectoid changes:

Si

Mo

W

C eutectoid

10 00 Cr 8 00 Mn 600

Ni 0

4

(wt%C)

Ti

1200

T Eutectoid

(°C)

• Teutectoid changes:

0.8

0.4

12

wt. % of alloying elements

Si

0.2 0

8

Ni Cr

0.6

Ti Mo 0

4

W 8

Mn

12

wt. % of alloying elements


Above, a syntectic reaction lurks within the diagram. This is a rare beast among alloy systems, occurring in systems like K-Zn, Na-Zn, K-Pb, Pb-U and Ca-Cd. The pertinent reaction is: L1 + L2 -> alpha


Week12- Phase Diagram @ WWW.07MET.TK  

γ austenite Fcc structure Non-magnetic ductile α ferrite Bcc structure Ferromagnetic Fairly ductile Eutectic L = γ + Fe 3 C Eutectoid γ = α...

Read more
Read more
Similar to
Popular now
Just for you