kiau-materials dep

Dr. A. Salemi Golezani

Department of Materials Eng., Islamic Azad University-Karaj Branch

Mechanical Behavior of CrMo Steel with Tempered Martensite and Ferrite-Bainite-

Martensite Microstructure

From Low Carbon to Advanced High Strength

Conventional Steels Low Carbon Bake Hardenable Solid Solution Strengthened High Strength Low Alloy

elAdvanced High Strength Ste Dual PhaseComplex Phase Transformation Induced Plasticity Martensitic

Complexe phase steels which contain martensite, bainite,

ferrite and/or retained austenite in quantities sufficient to

produce unique mechanical properties. These steels

exhibit an excellent combination of high strength and high

ductility results.

Definition of Complex Phase

Application of Complex Phase

Automotive

Pressure Vessels

Pipelines

Materials

C Cr Mo Mn Si P S

0.35 1.10 0.23 0.52 0.36 0.014 0.006

Heat Treatment

Quench Temparing

Step Quenching

Quench Tempering

850oC, 1hr

650oC, 1hr

Time

Tem

pera

ture

Oil-quenched

Step Quenching

F+B+M

F+(B+M)

F+(B+M)

F1+(B+M)F2+(B+M)

F1+(BL+M) F2+(BL+M)

Step Quenching

850oC, 1hr

650oC

4, 8, 12 min

430, 400oC

4min

Water-quenched

Microstructure

35µm

T: 850OC, t: 1 hr o.q

T: 6

50OC

, t: 1

hr

35µm

650OC, 4 min 650OC, 8 min

650OC, 12 min

T:850 OC, t: 1 hr

Color Tint Etching

F

B

M

Strength

400

500

600

700

800

900

1000

UTSYS

Stre

ngth

(MPa

)

Ductility

20

30

40

50

60

70

Reduction of areaElongation

Microstructure

Duc

tility

(%)

CVN

0

15

30

45

60

75

90

Microstructure

Impa

ct E

nerg

y (J

)

Yield Drop Effect

%25

%31

%39FBM-1

FBM-2

FBM-3 200X

Ferrite Volume Fraction

Yield Drop Effect

Pinning of dislocation by:

Solute atoms

Formation of short range order region near dislocation Electrical charge of solute atoms and depositing on

dislocations

Dislocation generation

Fractography

Tempered Martensite Ferrite-Bainite-Martensite

20µm

Fractography

2µm

Conclusions

Mechanical properties of tempered martensite, is more favorable than F-B-M microstructure.

Decreasing bainite transformation temperature from 430 to 400oC, increases yield and tensile strength but decreases in impact energy.

Conclusions

Fractography of the Charpy impact specimens indicates ductile and transgranular fracture mechanisms in quench tempered and step quenched specimens, respectably.

Yield drop effect was observed in F-B-M microstructure with ferritic matrix.

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