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