Martensitic Transformations in steels

MP Advanced metallic materials

Diffusionless Martensitic Steels

Hitesh SharmaAwais QadirSupervisor: 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.

Feg

(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)


c11f37

Stre

ngth

Duc

tility

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 needlesAustenite

60

m

xx x

xx

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

TE

0%100%50%

A

A

M + AM + A

M + A

0%50%90%


Martensite

FCCAustenite

FCCAustenite

Alternate choice of Cell

Tetragonal Martensite

Austenite to Martensite → 4.3 % volume increase

Possible positions of Carbon atoms

Only a fraction ofthe sites occupied

20% contraction of c-axis12% expansion of a-axis

In Pure Fe after the Matensitic transformation

c = a

C along the c-axis obstructs the contraction

CBCT

CFCC Quench

% 8.0)( '

% 8.0)( ag


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

Martensite100

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)


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.

c11f34 Tempered Martensite

4340 steel


c11f33Hardness 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.

c11f36Rockwell C and Brinell Hardness


c11f24

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 carbonsteel

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


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Martensitic Transformations in steels

Engineering

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