MST_PPT_7
description
Transcript of MST_PPT_7
UMA.S.N.Assistant Professor
Department of Metallurgy and Material ScienceCollege of Engineering, Pune.
UMA.S.N/MST/COEP/2014
Overview of the lecture
Transformation Products of Austenite
Pearlite
Bainite
Martensite
TTT/IT diagram
Critical cooling rate
CCT diagram
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Isothermal transformationfrom Austenite to Pearlite
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Incubation period
(For Fe-C Eutectoid alloy)
As the transformation temperature decreases (727 - 540)
–transformation time decreases
- incubation period decreases
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(For Fe-C Eutectoid alloy)
Transformation of Austenite to Pearlite
Diffusion controlled transformation – Nucleation (of Cementite) and growth.
By diffusion of carbon and Iron atoms
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Interlamellar spacing Vs under cooling
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20 60 100 140
T, undercooling
Inte
rlam
ella
rsp
acin
g l
og
of
, A0
2.8
3.2
3.6
4.0
Transformation of Austenite to Bainite
Austenite
Pearlite
Bainite *
T > 540 C
215 <T < 540 C
* The study was first conducted by Scientist Bain
•As the transformation temperature decreases(540-215)–transformation time increases- incubation period increases- Bainite formation
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Bainite Bainite = extremely fine mixture of ferrite and carbide
Diffusion controlled mechanism – Nucleation (of Ferrite) and growth and shear mechanism.
1. Upper Bainite – feathery structure
2. Lower Bainite – acicular structure
Subunit/ ferrite plate – 0.2μm thick and about 10μm long
Sheath – group of sub units
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Martensitic transformation
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Austenite (FCC) Martensite (BCT)
(0.8 %C) (0.8 %C)
Supersaturated solid solution of carbon trapped in BCT
Mechanism of Martensitictransformation
Diffusion less (no long rage diffusion) transformation
No change in the composition
shear transformation
Independent of time
Function of temperature only – rapid cooling -Athermaltransformation
Ms temperatures – independent of cooling rate –function of alloying elements.
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* Names after German Metallurgist Martens.
Ms&Mf temperature Vs Carbon Ms (oC) = 539 - 423 (%C) - 30.4(%Mn) -12.1(%Cr)
- 17.7(%Ni) -7.5(%Mo)
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Tem
per
atu
re
Carbon, Wt%
0.2 0.4 0.6 0.8 1.2
Ms
Mf
0
Habit Plane Model –inhomogeneous or lattice invariant deformation
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Shear Parallel to Habit plane
Undistorted lattice
Dilatation perpendicular to habit plane
Atom shuffle within the unit cell
Habit Planes
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(111) Low carbon steels
(225) Medium carbon steels
(259) High carbon steels
TTT DiagramFor Iron-carbon alloy
with eutectoid composition.
A: Austenite
P: Pearlite
B: Bainite
M: Martensite
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Continuous Cooling Transformation Diagrams
Isothermal heat treatments are not the most practical due to rapidly cooling and constant maintenance at an elevated temperature.
Most heat treatments for steels involve the continuous cooling of a specimen to room temperature.
TTT diagram (dashed curve) is modified for a CCT diagram (solid curve).
For continuous cooling, the time required for a reaction to begin and end is delayed.
The isothermal curves are shifted to longer times and lower temperatures. (to rightwards and downwards)
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Critical cooling rate Rate which just by-passes the nose of the IT/TTT
diagram
Controlling factors
Carbon content (Shifts the nose to right)
Alloying elements (Will be studied later)
Less critical cooling rate more hardenability
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10
Fe3C particles within an a-ferrite matrix diffusion dependent heat bainite or pearlite at temperature just below eutectoid for long times driving force – reduction of a-ferrite/Fe3C interfacial area
Spheroidite: Non equilibrium Transformation
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Str
engt
h
Duc
tility
MartensiteLower bainite
Fine pearlite
Coarse pearliteSpheroidite
General Trends
Upper bainite
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For Eutectoid Steel
Coarser Pearlite Rc 15
Finer Pearlite Rc 35-40
Upper Bainite Rc 40
Lower Bainite Rc 60
Martensite Rc 64