Martensitic Transformations in steels

MP Advanced metallic materials
Diffusionless Martensitic Steels
Hitesh Sharma
Awais Qadir
Supervisor: Igor S. Golovin

Phase Transformation
• Alteration of one or more phases to other phase(s)
• Most phase transformations begin with the
formation of numerous small particles of the new
phase that increase in size until the transformation is
complete.
Fe
g
(Austenite)
Eutectoid
transformation
C FCC
Fe3C
(cementite)
a
(ferrite)
+
(BCC)

Types of Phase Transformation
 Diffusion-independent with no change in composition or
number of phases present
(melting/solidification of pure metal,
allotropic transformations, recrystallization)
 Diffusion-dependent but changes in composition or
number of phase
( eutectoid transformations)
 Diffusionless  metastable phase by small displacements
of atoms in structure
(martensitic transformation discussed later)

Strength
Ductility
Martensite
T Martensite
bainite
fine pearlite
coarse pearlite
spheroidite
General Trends
Possible Transformations

Diffusionless Transformation
• A diffusionless transformation is a phase change that
occurs without the long-range diffusion of atoms but
rather by some form of cooperative, homogeneous
movement of many atoms that results in a change in
crystal structure.
• These movements are small, usually less than the
interatomic distances, and the atoms maintain their
relative relationships.
• The ordered movement of large numbers of atoms lead
some to refer to these as military transformations in
contrast to civilian diffusion-based phase changes.

Martensitic Transformation
• Martensite: austenite quenched to room T
• Austenite martensite does not involve diffusion  no activation: athermal
transformation
• Each atom displaces small (sub-atomic) distance to transform FCC g-Fe (austenite) to
martensite, a Body Centered Tetragonal (BCT) unit cell (like BCC, but one unit cell axis
longer than other two).
• Martensite is metastable - persists indefinitely at room T: transforms to equilibrium
phases on at elevated temperature
• Since martensite is a metastable phase, it does not appear in phase Fe-C phase
diagram.
• The amount of martensite formed is a function of the temperature to
which the sample is quenched and not of time.
• The shear changes the shape of the transforming region:
→ results in considerable amount of shear energy
→ plate-like shape of Martensite

MP Advanced metallic materials 7
• Martensite:
-- g(FCC) to Martensite (BCT)
Adapted from Fig. 10.21, Callister &
Rethwisch 8e. (Fig. 10.21 courtesy
United States Steel Corporation.)
Adapted from Fig. 10.20,
Callister & Rethwisch 8e.
Martensite: A Nonequilibrium Transformation
Product
Martensite needles
Austenite
60m
x
x x
x
x
x
potential
C atom sites
Fe atom
sites
Adapted from
Fig. 10.22,
Callister &
Rethwisch 8e.
• Isothermal Transf. Diagram
• g to martensite (M) transformation..
-- is rapid! (diffusionless)
-- % transf. depends only on T to
which rapidly cooled
10 103
105
time (s)10-1
400
600
800
T(ºC)
Austenite (stable)
200
P
B
TEA
A
M + A
M + A
M + A
0%
50%
90%

Martensite
FCC
Austenite
FCC
Austenite
Alternate choice of
Cell
Tetragonal
Martensite
Austenite to Martensite → 4.3 % volume increase
Possible positions of
Carbon atoms
Only a fraction of
the sites occupied
20% contraction of c-axis
12% expansion of a-axis
In Pure Fe after
the Matensitic transformation
c = a
C along the c-axis
obstructs the contraction
C
BCT
C
FCC Quench
%8.0
)('
%8.0
)( ag
 

MP Advanced metallic materials10
TTT Diagram including Martensite
Austenite-to-martensite is diffusionless and fast.
Amount of martensite depends on T only.
A: Austenite P: Pearlite
B: Bainite M: Martensite

Austenite
Austenite
Pearlite
Pearlite + Bainite
Bainite
Martensite
100
200
300
400
600
500
800
723
0.1 1 10 102 103 104
105
Eutectoid temperature

Not an isothermal
transformation
Ms
Mf
Coarse
Fine
t (s) →
T→
Time- Temperature-Transformation (TTT) Curves – Isothermal Transformation
Eutectoid steel (0.8%C)

MP Advanced metallic materials12
Tempered Martensite
Martensite is so brittle it needs to be modified for
practical applications. Done by heating to 250-650
oC for some time: (tempering) 
tempered martensite, extremely fine-grained, well
dispersed cementite grains in a ferrite matrix.
 Tempered martensite is more ductile
 Mechanical properties depend upon
cementite particle size: fewer, larger
particles means less boundary area and
softer, more ductile material - eventual
limit is spheroidite.
 Particle size increases with higher
tempering temperature and/or longer
time (more C diffusion).

 Tempered martensite is less brittle than martensite; tempered at 594 °C.
 Tempering reduces internal stresses caused by quenching.
 The small particles are cementite; the matrix is a-ferrite. US Steel Corp.
Tempered Martensite
4340 steel

Hardness as a function of carbon
concentration for steels

Hardness versus tempering time for a water-quenched eutectoid plain carbon steel (1080) that
has been rapidly quenched to form martensite.
Rockwell C and Brinell Hardness

 Other elements (Cr, Ni, Mo, Si and
W) may cause significant changes
in the positions and shapes of the
TTT curves:
 Change transition temperature;
 Shift the nose of the austenite-to-
pearlite transformation to longer
times;
 Shift the pearlite and bainite noses
to longer times (decrease critical
cooling rate);
 Form a separate bainite nose;
Effect of Adding
Other Elements
4340 Steel
plain
carbon
steel
nose
 Plain carbon steel: primary
alloying element is carbon.

Effect of Alloying Elements
• Most alloying elements which enter into solid
solution in austenite lower the martensite start
temperature (Ms), with the exception of Co and Al.

Effect of Alloying Elements

Thanks
Спасибо
धन्यवाद
‫شکریہ‬

Martensitic Transformations in steels

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