Heterogeneity and Microstructural Features

Intervening in the Ductile-Brittle Transition of

Ferrite-Pearlite Steels

October 29, 2013 – Montreal, Quebec Canada

R. Zubialde, P. Uranga, B. López and J.M. Rodriguez-Ibabe

puranga@ceit.es

(CEIT and TECNUN, Univ. Navarra)

San Sebastian, Basque Country, Spain

Introduction

• Mechanical strength is properly described by mean

grain sizes in ferrite-pearlite structures.

• Toughness prediction is not straightforward with

average grain sizes.

• Classical equations include dα and %pearlite to

predict the ductile-brittle (DB) transition temperatures.

• However, if austenite distribution is not properly

controlled

– Austenite heterogeneity → heterogeneous ferrite

distributions.

– Weakest link behavior: Coarsest grains will trigger brittle

fracture.

Objectives

– Analysis of the behavior of several ferrite-

pearlite microstructures with different local

heterogeneity.

• Grain size distributions, EBSD analysis

identifying low/high angle misorientation

boundaries and cleavage facet measurements.

– Incorporation in previous empirical

expressions to quantify the contribution of the

heterogeneity to the ductile-brittle (DB)

transition temperature.

EXPERIMENTAL

Steel composition and Techniques

Material and Heat Treatments

C Mn Si Al N

0.1 0.48 0.006 0.041 48 ppm

• CMn steel

Heat treatment # Thermal cycle

1 As-wrought microstructure

2 910ºC for 30 minutes and air cooling at 1.5ºC/s

3 980ºC for 30 minutes and furnace cooling at 0.1ºC/s

4 1000ºC for 30 minutes and furnace cooling at 0.1ºC/s

Experimental Procedure

• Optical Microscopy

• Philips XL30CP Scanning Electron Microscope (SEM). TSL

(TexSEM laboratories) MSC 2002 equipment.

• Field Emission Scanning Electron Microscope (FEG-SEM)

Jeol JSM-7000F. HKL Channel5 EBSD

• Charpy tests

MICROSTRUCTURAL

CHARACTERIZATION

Austenite and Transformed Structures

Austenite Grain Sizes

HT 2: 910ºC + 1.5ºC/s HT 3: 980ºC + 0.1ºC/s HT 4: 1000ºC + 0.1ºC/s

• HT #2: fine and homogeneous austenite (Dγ = 15 μm)

• HT #3 and 4: heterogeneous austenite (Dγ = 37 and 25 μm).

− HT #3: coarse austenite grains (400 μm approx.) within a fine matrix.

− HT#4: coarse austenite structure (200 μm approx.) with fine austenite

grains decorating the grain boundaries.

Austenite Grain Sizes

HT 2: 910ºC + 1.5ºC/s

HT 3: 980ºC + 0.1ºC/s HT 4: 1000ºC + 0.1ºC/s

0

0.1

0.2

0.3

20 80 140 200 260 320 380

Are

a F

rac

tio

n

Austenite Grain Size (mm)

0

0.1

0.2

0.3

20 80 140 200 260 320 380

Are

a F

racti

on

Austenite Grain Size (mm)

0

0.1

0.2

0.3

5 20 35 50 65 80

Are

a F

racti

on

Austenite Grain Size (mm)

Transformed Microstructures

HT 2: 910ºC + 1.5ºC/s

HT 3: 980ºC + 0.1ºC/s HT 4: 1000ºC + 0.1ºC/s

Sample 1: As-wrought

Dα = 28.3 µm Dα = 10.3 µm

Dα = 25.4 µm Dα = 30.6 µm

Transformed Microstructures

0

0.2

0.4

0.6

0.8

1

0 50 100 150

Accu

mu

late

d A

rea F

racti

on

Ferrite Grain Size (mm)

Treatment 1

Treatment 2

Treatment 3

Treatment 4

Treatment # Proeutectoid ferrite

fraction %

Ferrite mean

size (µm)

1 88 28.3

2 90 10.3

3 89 25.4

4 88 30.6

CHARPY TESTS

Mechanical Properties

0

50

100

150

200

250

300

350

400

-80 -60 -40 -20 0 20 40 60

Ab

so

rbed

En

erg

y (

J)

Temperature (ºC)

(c)Treatment #3

Charpy Tests

0

50

100

150

200

250

300

350

400

-80 -60 -40 -20 0 20 40 60

Ab

so

rbe

d E

ne

rgy (

J)

Temperature (ºC)

(a)Treatment #1

0

50

100

150

200

250

300

350

400

-80 -60 -40 -20 0 20 40 60

Ab

so

rbed

En

erg

y (

J)

Temperature (ºC)

(b)Treatment #2

0

50

100

150

200

250

300

350

400

-80 -60 -40 -20 0 20 40 60

Ab

so

rbed

En

erg

y (

J)

Temperature (ºC)

(d)Treatment #4

Treatment # 50% ITT (ºC) 27J (ºC) 54 J (ºC)

1 28 12 18

2 -30 -39 -36

3 -4.9 -8 -7

4 -6.5 -15 -12

HT 2: 910ºC + 1.5ºC/s

HT 3: 980ºC + 0.1ºC/s

HT 4: 1000ºC + 0.1ºC/s

Sample 1: As-wrought

Fractography

HT #3: Test @ -20ºC

No inclusions detected

in the origin

Fractography

HT #4: Test @ -40ºC

Fracture Initiation Ductile-Brittle Transition

#1: Test @ 27ºC #4: Test @ -7ºC

• Energy absorbed by plastic deformation until brittle fracture happens.

• Brittle fracture initiation areas isolated by a ductile region.

• Crack energy lower than the matrix/matrix interface energy.

• First facet size 2-3 times bigger than average grain size.

Fractography

Etched Fracture Surface:

Grain boundary Carbides revealed as initiators

Treatment

Grain Boundary

Cementite Thickness

(mm)

1 0.6

2 0.5

3 0.54

4 0.55

Pearlite

GB

carbides

Facet Size Distribution Measurements

Sample 1: As-wrought

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

10 30 50 70 90 110 130 150 170

Fre

qu

en

cy

Size (mm)

Facets

Grains

(a)

Treatment #1

Ni Secondary Crack stopped

at a grain boundary

Facet Size Distribution Measurements

HT 2: 910ºC + 1.5ºC/s

HT 3: 980ºC + 0.1ºC/s HT 4: 1000ºC + 0.1ºC/s

Sample 1: As-wrought

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

10 30 50 70 90 110 130 150 170

Fre

qu

en

cy

Size (mm)

Facets

Grains

(a)

Treatment #1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

10 30 50 70 90 110 130 150 170

Fre

qu

en

cy

Size (mm)

Facets

Grains

(b)

Treatment #2

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

10 30 50 70 90 110 130 150 170

Fre

qu

en

cy

Size (mm)

Facets

Grains

(c)

Treatment #3

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

10 30 50 70 90 110 130 150 170

Fre

qu

en

cy

Size (mm)

Facets

Grains

(d)

Treatment #4

Microstructural Characterization by EBSD

—2º~12º

— >12º

Sample 1: As-wrougth

Microstructural Characterization by EBSD

—2º~12º

— >12º

HT 2: 910ºC + 1.5ºC/s

Crystallographic Measurements by EBSD

Treatment Mean ferrite

size OM (mm)

Low angle

boundary fraction

(<12º)

5º mean

size (mm)

12º mean

size (mm)

Dc20%

(mm)

1 28.3 8.4% 21.0 26.1 77

2 10.3 7.5% 10.7 11.9 30

3 25.4 9.4% 22.0 22.3 57

4 30.6 9.4% 25.0 25.1 75

0

0.2

0.4

0.6

0.8

1

0 50 100 150

Accu

mu

late

d A

rea F

racti

on

Ferrite Grain Size (mm)

Treatment 1

Treatment 2

Treatment 3

Treatment 4

0

10

20

30

40

50

60

70

80

90

0 10 20 30

Dc2

0%

(m

m)

Ferrite 12º Grain Size (mm)

𝐷𝑐~3𝐷𝑚𝑒𝑎𝑛

DUCTILE BRITTLE

TEMPERATURE PREDICTION

50% ITT

Ductile-Brittle Temperature Prediction

5.05.01125.11%2.2%700%4419%50 tDpearliteNSiITT meanf

-60

-40

-20

0

20

40

60

80

-60 -40 -20 0 20 40 60 80

Pre

dic

ted

50

%IT

T (

ºC)

Experimental 50%ITT (ºC)

Equation 1

Ductile-Brittle Temperature Prediction

5.05.0112%205.11%2.2%700%4487%50 tDcpearliteNSiITT f

-60

-40

-20

0

20

40

60

80

-60 -40 -20 0 20 40 60 80

Pre

dic

ted

50

%IT

T (

ºC)

Experimental 50%ITT (ºC)

Equation 2

-250

-150

-50

50

-250 -150 -50 50

Pre

dic

ted

50

%IT

T (ºC

)

Experimental 50%ITT (ºC)

3NbMo0

3NbMo31

6NbMo0

6NbMo31

Extension to Nb-Mo Microalloyed Steels.

Ductile-Brittle Temperature Prediction

%20DcDmeanionPrecipitat

)D%M/AarlPhases(%peSecondary nCompositioC)50%ITT(º M/A

Extension to Nb-Mo Microalloyed Steels.

Ductile-Brittle Temperature Prediction

%20DcDmeanionPrecipitat

)D%M/AarlPhases(%peSecondary nCompositioC)50%ITT(º M/A

-250

-150

-50

50

-250 -150 -50 50

Pre

dic

ted

50

%IT

T (ºC

)

Experimental 50%ITT (ºC)

3NbMo0

3NbMo31

6NbMo0

6NbMo31

CMn

CONCLUSIONS

Final Remarks

Final Remarks

• Toughness of ferrite-pearlite microstructures:

– importance of microstructural heterogeneity.

– contribution of the largest grains in the

toughness of the material is one of the key

factors controlling brittle behavior.

– a modified equation has been proposed to

accurately predict ductile-brittle transition

temperature.

• Strategy extension to microalloyed steels with

complex microstructures

Acknowledgements

• Financial support by:

– Spanish Ministry of Economy and

Competitiveness (MAT2009-09250)

– Basque Government (PI2011-17)

Heterogeneity and Microstructural Features

Intervening in the Ductile-Brittle Transition of

Ferrite-Pearlite Steels

October 29, 2013 – Montreal, Quebec Canada

R. Zubialde, P. Uranga, B. López and J.M. Rodriguez-Ibabe

puranga@ceit.es

(CEIT and TECNUN, Univ. Navarra)

San Sebastian, Basque Country, Spain

Extension to Nb-Mo Microalloyed Steels.

Ductile-Brittle Temperature Prediction

0.5

M/A

1.5

mean_15º

0.5-

mean_15ºy

1/30.5

free

)23.9(D)DDc20%1.4()14(D0.5Δ

%M/A)15(%pearl)700(N42Si11MnC)50%ITT(º

-250

-150

-50

50

-250 -150 -50 50

Pre

dic

ted

50

%IT

T (ºC

)

Experimental 50%ITT (ºC)

3NbMo0

3NbMo31

6NbMo0

6NbMo31

CMn