Heterogeneity and Microstructural Features Intervening in the Ductile-Brittle Transition of...
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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 [email protected] (CEIT and TECNUN, Univ. Navarra) San Sebastian, Basque Country, Spain
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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.
Transcript of Heterogeneity and Microstructural Features Intervening in the Ductile-Brittle Transition of...
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
(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
(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
