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


Isothermal transformationfrom Austenite to Pearlite


Incubation period

(For Fe-C Eutectoid alloy)

As the transformation temperature decreases (727 - 540)

–transformation time decreases

- incubation period decreases


(For Fe-C Eutectoid alloy)

TTTDiagram

for Eutectoid composition steel.


TTT diagram for Eutectoid composition steel


Transformation of Austenite to Pearlite

Diffusion controlled transformation – Nucleation (of Cementite) and growth.

By diffusion of carbon and Iron atoms


Lamellar structure


Interlamellar spacing Vs under cooling


20 60 100 140

T, undercooling

Inte

rlam

ella

rsp

acin

g l

og

of

, A0

2.8

3.2

3.6

4.0


Coarser Pearlite Finer Pearlite


IT/ TTT diagram (For Fe-C Hyper Eutectoid alloy)

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


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


Microstructures of Bainite


Kinetics


Martensitic transformation


Austenite (FCC) Martensite (BCT)

(0.8 %C) (0.8 %C)

Supersaturated solid solution of carbon trapped in BCT

Complete IT/TTT diagram for eutectoid steel


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.


* 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)


Tem

per

atu

re

Carbon, Wt%

0.2 0.4 0.6 0.8 1.2

Ms

Mf

0


Bain Model (Homogeneous lattice deformation)


(b)Greninger – Troiano relationship(c)Kurdjurmov – sach relationship

Habit Plane Model –inhomogeneous or lattice invariant deformation


Shear Parallel to Habit plane

Undistorted lattice

Dilatation perpendicular to habit plane

Atom shuffle within the unit cell

Habit Planes


(111) Low carbon steels

(225) Medium carbon steels

(259) High carbon steels

Morphology of microstructures

Lath martensite


Plate martenite


Tem

per

atu

re

Carbon, Wt%

0.2 0.4 0.6 0.8 1.2

MsMf

Lath Mixed Plate

TTT DiagramFor Iron-carbon alloy

with eutectoid composition.

A: Austenite

P: Pearlite

B: Bainite

M: Martensite


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)



>140 C/sec

140 C/sec

35 C/sec

3 C/sec

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


Transformation product of austenite


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



Str

engt

h

Duc

tility

MartensiteLower bainite

Fine pearlite

Coarse pearliteSpheroidite

General Trends

Upper bainite


For Eutectoid Steel

Coarser Pearlite Rc 15

Finer Pearlite Rc 35-40

Upper Bainite Rc 40

Lower Bainite Rc 60

Martensite Rc 64

mst_ppt_7

Documents

diffusion of carbon

transformation temperature

pearlite uma

eutectoid steeluma

eutectoid composition

nmstcoep2014bainite

c215 uma

nmstcoep2014ttt diagram