Post on 26-Jan-2015
description
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