In conventional heat treatments or rolling schedules, the microstructure can be properly identified by its mean attributes. These parameters can properly predict the ductile-brittle (DB) transition temperatures measured by Charpy tests. However, if the austenite grain size distribution prior to transformation remains heterogeneous, after transformation, wider distributions of grain sizes will be obtained. In this context, the classical approaches do not properly predict the DB regime. This study analyzes the behavior of several ferrite-pearlite microstructures with different local heterogeneities. Grain size distributions, EBSD analysis identifying high angle misorientation boundaries and cleavage facet measurements were performed. These parameters have been incorporated in previous empirical expressions in order to quantify the contribution of these heterogeneities to the DB transition temperature.

1. 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

2. 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.

3. Objectives
– Analysis of the behavior of several ferritepearlite 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.

4. Steel composition and Techniques
EXPERIMENTAL

5. Material and Heat Treatments
• CMn steel
C
0.1
Heat treatment #
1
2
3
4
Mn
0.48
Si
Al
N
0.006 0.041 48 ppm
Thermal cycle
As-wrought microstructure
910ºC for 30 minutes and air cooling at 1.5ºC/s
980ºC for 30 minutes and furnace cooling at 0.1ºC/s
1000ºC for 30 minutes and furnace cooling at 0.1ºC/s

6. 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

7. Austenite and Transformed Structures
MICROSTRUCTURAL
CHARACTERIZATION

8. 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.

9. Austenite Grain Sizes
HT 2: 910ºC + 1.5ºC/s
Area Fraction
0.3
HT 3: 980ºC + 0.1ºC/s
0.2
0.1
HT 4: 1000ºC + 0.1ºC/s
0
5
0.3
35
50
65
80
0.2
0.3
Area Fraction
Area Fraction
20
Austenite Grain Size (mm)
0.1
0.2
0.1
0
0
20
80
140
200
260
320
380
Austenite Grain Size (mm)
20
80
140
200
260
320
380
Austenite Grain Size (mm)

10. Transformed Microstructures
Sample 1: As-wrought
Dα = 28.3 µm
HT 3: 980ºC + 0.1ºC/s
Dα = 25.4 µm
HT 2: 910ºC + 1.5ºC/s
Dα = 10.3 µm
HT 4: 1000ºC + 0.1ºC/s
Dα = 30.6 µm

11. Transformed Microstructures
Accumulated Area Fraction
1
0.8
0.6
0.4
Treatment 1
Treatment 2
0.2
Treatment 3
Treatment 4
0
0
50
100
Ferrite Grain Size (mm)
Treatment #
1
2
3
4
Proeutectoid ferrite
fraction %
88
90
89
88
Ferrite mean
size (µm)
28.3
10.3
25.4
30.6
150

12. Mechanical Properties
CHARPY TESTS

13. Charpy Tests
400
Sample 1: As-wrought
(a)
350
Treatment #1
300
Absorbed Energy (J)
Absorbed Energy (J)
350
400
250
200
150
100
300
250
200
150
100
50
0
HT 2: 910ºC + 1.5ºC/s
(b)
50
0
Treatment #2
-80
-60
-40
-20
0
20
40
-80
60
-60
-40
HT 3: 980ºC +400
0.1ºC/s
0
20
40
60
400
350
300
300
Absorbed Energy (J)
350
Absorbed Energy (J)
-20
Temperature (ºC)
Temperature (ºC)
250
200
150
100
50
(c)
Treatment #3
0
250
200
150
100
HT 4: 1000ºC + 0.1ºC/s
(d)
50
Treatment #4
0
-80
-60
-40
-20
0
20
40
60
-80
-60
Temperature (ºC)
Treatment #
1
2
3
4
50% ITT (ºC)
28
-30
-4.9
-6.5
-40
-20
0
20
Temperature (ºC)
27J (ºC)
12
-39
-8
-15
54 J (ºC)
18
-36
-7
-12
40
60

14. Fractography
HT #3: Test @ -20ºC
No inclusions detected
in the origin

15. Fractography
HT #4: Test @ -40ºC

16. 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.

17. Fractography
Etched Fracture Surface:
Grain boundary Carbides revealed as initiators
GB
carbides
Treatment
Pearlite
Grain Boundary
Cementite Thickness
(mm)
1
0.6
2
0.5
3
0.54
4
0.55

18. Facet Size Distribution Measurements
Sample 1: As-wrought
0.7
(a)
Frequency
0.6
Treatment #1
0.5
Facets
0.4
Grains
0.3
0.2
0.1
0
10
30
50
70
90
110
130
150
170
Size (mm)
Ni
Secondary Crack stopped
at a grain boundary

19. Facet Size Distribution Measurements
Sample 1: As-wrought
HT 2: 910ºC + 1.5ºC/s
0.7
0.7
(a)
Treatment #1
0.5
Facets
0.4
Grains
0.3
0.2
0.1
(b)
0.6
Frequency
Frequency
0.6
Treatment #2
0.5
Facets
0.4
Grains
0.3
0.2
0.1
0
0
10
30
50
70
90
110
130
150
170
10
30
50
Size (mm)
90
110
130
150
170
Size (mm)
HT 4: 1000ºC + 0.1ºC/s
HT 3: 980ºC + 0.1ºC/s
0.7
0.7
(c)
0.6
Treatment #3
0.5
Facets
0.4
Grains
0.3
0.2
(d)
0.6
Frequency
Frequency
70
Treatment #4
0.5
Facets
0.4
Grains
0.3
0.2
0.1
0.1
0
0
10
30
50
70
90
Size (mm)
110
130
150
170
10
30
50
70
90
Size (mm)
110
130
150
170

20. Microstructural Characterization by EBSD
Sample 1: As-wrougth
—2º~12º
— >12º

21. Microstructural Characterization by EBSD
HT 2: 910ºC + 1.5ºC/s
—2º~12º
— >12º

22. Crystallographic Measurements by EBSD
Treatment
Mean ferrite
size OM (mm)
Low angle
boundary fraction
(<12º)
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
5º mean 12º mean
size (mm) size (mm)
Dc20%
(mm)
90
80
70
0.8
Dc20% (mm)
Accumulated Area Fraction
1
0.6
0.4
Treatment 1
60
50
40
30
Treatment 2
0.2
20
Treatment 3
Treatment 4
0
50
100
Ferrite Grain Size (mm)
𝐷𝑐~3𝐷 𝑚𝑒𝑎𝑛
10
0
150
0
0
10
20
Ferrite 12º Grain Size (mm)
30

23. 50% ITT
DUCTILE BRITTLE
TEMPERATURE PREDICTION

24. Ductile-Brittle Temperature Prediction


50%ITT  19  44%Si  700 % N f  2.2% pearlite  11.5 Dmean
80
Predicted 50%ITT (ºC)
60
40
20
0
-20
-40
Equation 1
-60
-60
-40
-20
0
20
40
Experimental 50%ITT (ºC)
60
80

0.5
 112t 0.5

25. Ductile-Brittle Temperature Prediction
50%ITT  87  44%Si  700 % N f  2.2% pearlite  11.5Dc20%
0.5
80
Predicted 50%ITT (ºC)
60
40
20
0
-20
-40
Equation 2
-60
-60
-40
-20
0
20
40
Experimental 50%ITT (ºC)
60
80
 112t 0.5

26. Extension to Nb-Mo Microalloyed Steels.
Ductile-Brittle Temperature Prediction
Predicted 50%ITT (ºC)
50
-50
3NbMo0
-150
3NbMo31
6NbMo0
6NbMo31
-250
-250
-150
-50
Experimental 50%ITT (ºC)
50
50%ITT(ºC)  Composition  Secondary Phases(%pearl  %M/A  D M/A ) 
 Precipitation  Dmean  Dc20%

27. Extension to Nb-Mo Microalloyed Steels.
Ductile-Brittle Temperature Prediction
Predicted 50%ITT (ºC)
50
-50
3NbMo0
-150
3NbMo31
6NbMo0
6NbMo31
CMn
-250
-250
-150
-50
Experimental 50%ITT (ºC)
50
50%ITT(ºC)  Composition  Secondary Phases(%pearl  %M/A  D M/A ) 
 Precipitation  Dmean  Dc20%

28. Final Remarks
CONCLUSIONS

29. 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

30. Acknowledgements
• Financial support by:
– Spanish Ministry of Economy and
Competitiveness (MAT2009-09250)
– Basque Government (PI2011-17)

31. 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

32. Extension to Nb-Mo Microalloyed Steels.
Ductile-Brittle Temperature Prediction
Predicted 50%ITT (ºC)
50
-50
3NbMo0
-150
3NbMo31
6NbMo0
6NbMo31
CMn
-250
-250
-150
-50
Experimental 50%ITT (ºC)
0.5
50%ITT(ºC)  11M n  42Si  700(N free )
 0.5Δ y  14(D
50
 15(%pearl  %M /A)1/3 
) -0.5  1.4( Dc20% )1.5  23.9(D M/A ) 0.5
mean_15º
D mean_15º