Isothermal Transformation Diagrams (Time-Temperature-Transformation (TTT) Diagrams) Plot temperature...
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Transcript of Isothermal Transformation Diagrams (Time-Temperature-Transformation (TTT) Diagrams) Plot temperature...
Isothermal Transformation Diagrams(Time-Temperature-Transformation (TTT) Diagrams)
• Plot temperature on the y-axis
• Plot time on the x-axis (typically logarithmic scale)
• Maps of phase creation as a function time at temperature
• These are ONLY valid for isothermal (constant temperature) transformation
• Each diagram is ONLY valid for a specific composition
Consider Eutectoid Transformation … Eutectoid transformation (Fe-C): + Fe3C
0.76 wt% C0.022 wt% C
6.7 wt% C
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+Fe3C
+Fe3C
L+Fe3C
(Fe) Co , wt%C
1148°C
T(°C)
ferrite727°C
Eutectoid:
Equil. Cooling: Ttransf. = 727ºCT
Undercooling by Ttransf. < 727C
0.7
6
0.0
22
Isothermal Transformation Diagrams
• Fe-C system, Co = 0.76 wt% C• Transformation at T = 675°C.
100
50
01 102 104
T = 675°C
y,
% tr
ansf
orm
ed
time (s)
400
500
600
700
1 10 102 103 104 105
0%pearlite
100%
50%
Austenite (stable) TE (727C)Austenite (unstable)
Pearlite
T(°C)
time (s)
isothermal transformation at 675°C
• Eutectoid composition, Co = 0.76 wt% C• Begin at T > 727°C• Rapidly cool to 625°C and hold isothermally.
Effect of Cooling History in Fe-C System
400
500
600
700
0%pearlite
100%
50%
Austenite (stable)TE (727C)
Austenite (unstable)
Pearlite
T(°C)
1 10 102 103 104 105
time (s)
Hypoeutectoid TTT Diagram
Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.
Fe 3
C
0 1 2 3 4 5 6 6.7
L
+L
+Fe3C
+Fe3C
L+Fe3C
(Fe) Co , wt%C
1148°C
T(°C)
727°C
Isothermal transformation diagram for 0.35% C, 0.37% Mn
• Af represents highest temperature ferrite can form
• As is the eutectoid temperature
• MS is the martensite start temperature
Hypereutectoid TTT Diagram
Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.
Fe 3
C
0 1 2 3 4 5 6 6.7
L
+L
+Fe3C
+Fe3C
L+Fe3C
(Fe) Co , wt%C
1148°C
T(°C)
727°C
Isothermal transformation diagram for 1.13%C, 0.30% Mn
• Af represents highest temperature ferrite can form
• As is the eutectoid temperature
• MS is the martensite start temperature
Metastable Phase Transformations
• Where on this diagram is martensite shown?
• How about bainite?• How about spheroidite?
This is the EQUILIBRIUM Phase Diagram for Fe-C system
Metastable phases are temporary phase which are intermediate between the initial and equilibrium states
Spheroidite: -- grains with spherical Fe3C
Spheroidite
60 m
(ferrite)
(cementite)
Fe3C
Fe 3
C
0 1 2 3 4 5 6 6.7
L
+L
+Fe3C
+Fe3C
L+Fe3C
(Fe) Co , wt%C
1148°C
T(°C)
727°C
• Equilibrium phase diagram tells us that the stable phase distribution in the two phase field is:
+ Fe3C – typically as lamellar microstructural constituent pearlite
• Is pearlite the lowest energy state – NO! With lamellar structure pearlite has a lot of interfacial energy
• If we anneal a pearlite microstructure we will get a transformation to a new phase distribution that minimizes the energy of the system when atomic mobility is activated
Martensite Transformation• Formed by rapid quenching of an alloy
• Occurs in several alloy systems (indium-thallium, titanium, nickel-iron, gold-cadmium, and Steel)
• Shear driven atomic realignment – similar to deformation twinning only more complex
• Militaristic transformation – diffusionless transformation
• Only driven by changes in temperature -- G
• New lattice is formed around a habit plane (plane shared between parent and daughter phases)
• Form lens-shaped shear plates during transformation – speed of transformation can approach speed of sound in the material
• Congruent phase change
Martensitic Transformation
Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.
Bain distortion: conversion of one lattice into another by expansion or contraction along crystallographic axes
Lattice parameters change as we increase the amount of carbon in solution
Martensite Effects
• Change in volume associated with formation of martensite
• For a 1% carbon steel we see a volume increase of 4%
• The shear transformation and the volume change combine to create a high density of dislocations
• Lath martensite has internal dislocation density on the order of 1015 – 1016 /m2
• Very fine microstruture of cell boundaries and laths
Mechanical Properties
Source: H. K. D. H. Bhadeshia, Bainite in Steels, 2nd Edition, Cambridge Press, 2001.
Bainite• Bainite: -- lathes (strips) with long
rods of Fe3C --diffusion controlled.• Isothermal Transf. Diagram
Fe3C
(cementite)
5 m
(ferrite)
10 103 105
time (s)10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%
50%
pearlite/bainite boundary
A
A
100% bainite
100% pearlite
Tempering Martensite• reduces brittleness of martensite,• reduces internal stress caused by quenching.
Adapted from Fig. 10.33, Callister 7e. (Fig. 10.33 copyright by United States Steel Corporation, 1971.)
• decreases TS, YS but increases %RA• produces extremely small Fe3C particles surrounded by
9 m
YS(MPa)TS(MPa)
800
1000
1200
1400
1600
1800
30
40
50
60
200 400 600Tempering T (°C)
%RA
TS
YS
%RA
Alloying Additions
Effect of adding other elementsChange transition temp.
Cr, Ni, Mo, Si, Mn
retard + Fe3C
transformation
4340 steel (alloyed steel)
Cooling Curveplot temp vs. time