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Yudong Zhang1,2, Xiang Zhao1, Changshu He1, Weiping Tong1, Liang. Zuo1, Jicheng He1 and Claud. Esling2
Effects of a High Magnetic Field on the Microstructure Formation in 42CrMo Steel
during Solid Phase Transformations
Sino-German Workshop on EPM, Oct.11-12, 2004, Shanghai Univ. Shanghai, ChinaSino-German Workshop on EPM, Oct.11-12, 2004, Shanghai Univ. Shanghai, China
11 22
Outline
• Summary
• Introduction
• Part II - Tempering Behaviors in High magnetic field
• Part I - Characteristics of Phase Transformation from Austenite to ferrite in High magnetic field
Introduction
• Theoretical simulation of the effect of magnetic field on ferrite/austenite and austenite/ferrite phase equilibrium;
• Morphological features appearing during ferrite to austenite and austenite to ferrite transformation ;
• Kinetic characteristics of proeutectoid ferrite transformation under magnetic field.
The introduction of magnetic field to solid phase transformations in steels has been a subject of much attention in materials science. If the parent and product phases are different in saturation magnetization and are allowed to transform under the magnetic field, the transformation temperature and transformed amount can be considerably affected, as the Gibbs free energy of a phase can be lowered by an amount according to its magnetization. Previous work has focused on the influence of a magnetic field on the martensitic phase transformation in some materials with lower martensitic transformation start temperatures. Quite recently, attention has been shifted to the high temperature diffusional transformations. Research on this topic is mainly on following aspects:
Now, the research on these issues is on its initial stage!Now, the research on these issues is on its initial stage!
•Part I - under high magnetic field (1)
Materials: Materials: Hot Rolled 42CrMo SteelHot Rolled 42CrMo Steel
C Cr Mo Si Mn P S Fe
0.38-0.45 0.90 -1.20
0.15 -0.250.20 -0.40
0.50 -0.80
0.04 0.04
balance
Chemical composition(wt.Chemical composition(wt.%)%)
Cooling-jacket
Magnets
Pt-Rh thermocouple
Pt heater
Ar
Cooling water
Ar Furnace
Magnetic field
direction
Specimen
Hot-rolling direction
Zero force area
•Part I - under high magnetic field (2)
Heat treatment arrangementHeat treatment arrangement
•Part I - under high magnetic field (3)
Heat treatmentHeat treatment
Temp.
880°C
33min
Time
10°C/min
Slow cooling
Fast coolingTemp.
880°C
33min 46°C/min
Temp.
Time
B0=6; 10; 14T
880°C
33min10°C/min
Temp.
Time
B0=14T
880°C
33min46°C/min
Time
MicrostructureMicrostructure
•Part I - under high magnetic field (4)
Heated at 880C for 33min and cooled at 10C/min
50m
0TRD
50 m
6T
50 m
10T
50 m
14T
RD//MFD
%100)10%(
%100)0%(
cb
abTFerrite
bc
baTFerrite
Magnetic field increases the amount of product ferrite
6 8 10 12 14
26
27
28
29
30
31
Are
a p
erce
nta
ge o
f fe
rrit
e, %
Intensity of magnetic field, Tesla
Image analysisImage analysis (for the slow cooling group)
0.0 0.2 0.4 0.6 0.8 1.0 1.2727
777
827
877
927
c
Steel
b'ba
B0=10 T B0=0 T
Te
mp
era
ture
, °C
carbon content, wt%
•Part I - under high magnetic field (5)
•Part I - under high magnetic field (6)
50m50m
0T, bainite 14T, ferrite+pearlite
MicrostructureMicrostructureHeated at 880C for 33min and cooled at 46C/min
Ferrite Fraction:2%Ferrite Fraction:2% Ferrite Fraction 23.1%Ferrite Fraction 23.1%
RD RD//MFD
•Part I - under high magnetic field (7)
xx
xxE
GGC
RT
QB
fAt
MV
ln
)()
1
1ln(lnln
2
3
t ---- transformation timeA, B, C, & E ---- constantsT----absolute temperatureR----gas constantQ ---- activation energy for diffusion T ---- absolute temperature ---- interfacial energy GV ---- Gibbs volume free energy difference between the product and the parent phasex, x ---- solubility values of austenite and ferrite at Tx ---- carbon content of the material
When a high magnetic
field is applied
xx
xxE
GC
RT
QB
fAt
V
ln)1
1ln(lnln 2
3
The magnetic- field induced extra energy difference between the ferrite and the austenite
(1(1))
(2(2))
According toAccording to Johnson-Mehl equation,Johnson-Mehl equation, The The kinetic equationkinetic equation of proeutectoid fe of proeutectoid ferrite transformation from austenite can be expressed as Eq.(1)rrite transformation from austenite can be expressed as Eq.(1)
As a consequence, t for As a consequence, t for transformation is reducedtransformation is reduced
High temperature nucleation
nucleation on Grain boundaries
2V
3*
)G(CG
2MV
3*M )GG(
CG
50m
Original austenite grains
RD
The magnetic- field induced extra energy difference between the ferrite and the austenite
C---- constantsT----absolute temperatureR----gas constantQ ---- activation energy for diffusion T ---- absolute temperature ---- interfacial energy GV ---- Gibbs volume free energy difference between the product and the parent phase
)RT
Gexp()
RT
Qexp(NN
*
0
(3(3
))
the nucleation barrier:the nucleation barrier: (4(4))
•Part I - under high magnetic field (8)
How and Why the band structure formed during slow-cooling under magnetic field
Magnetic
field
S pole
N pole
Austenite
Ferrite
Dipolar interaction between ferrite nuclei
The schematic illustration of nucleation of ferrite at austenite grain boundary triple junctions along magnetic field direction
•Part I - under high magnetic field (9)
Hot-working ……Annealing Machining Quenching+Tempering
Formation of banded structure in conventional full annealing Formation of banded structure in conventional full annealing • Hot working history• nucleation on grain boundaries due to slow cooling
Eliminating methodEliminating methodNormalizing+high temperature tempering
Complicated processesNot satisfactory !
Rapid annealing under high magnetic field(1)
50m
Banded structure obtained by conventional annealing
50m
Original austenite grain structure by special etching
RD
Conventional annealing
860°C
30min
Temp.
Time
Furnace cooling1°C/min
General processing proceduresGeneral processing procedures
hardness
hardness
Conventionally:
Rapid:
Optimum hardness for machining :
HV192.75~211 (HB 197~210)
HV164.8~174 (HB 170~179)
HB160~230
cooling rate cooling rate
24.4%
23.1%
50m
RD//MFD
Advantages of rapid annealing Advantages of rapid annealing
Rapid annealing under high magnetic field
B0=14T
880°C
33min46°C/min
• Effectively avoiding the formation of banded microstructure;• improving microstructure (refining and homogenizing)
• simplifying processes by shortening treatment time leaving out subsequent additional treatments
Rapid annealing under high magnetic field(2)
ferrite% ferrite%
1°C/min
46 °C /min
A potential alternativeA potential alternative Y.D. Zhang et al. Adv.Eng. Mater., 2004,6(5):310
RD//MFD
•Part II - Tempering Behaviors in High magnetic field
Quenching
High Temperature Tempering
650°C60min
Temp.
Time
Temp.
Time
860°C20min Water
cooling
B0=14T
650°C60min
Temp.
Time
200°C60min
Temp.
Time
B0=14T
200°C60min
Temp.
Time
Carbide PrecipitationCarbide PrecipitationMatrix RecoveryMatrix Recovery
Low Temperature Tempering
Carbide PrecipitationCarbide Precipitation
•Part II - Tempering Behaviors in High magnetic field
High Temperature Tempering (650°C×60min)
1m1m
0T 14T
Cementite precipitated during tempering (650C for 60 min)---bright areas
Magnetic field effectively prevents cementite from growing directionally along boundaries and shows spheroidization effect.
Carbide PrecipitationCarbide Precipitation
• Spherical cementite has the lowest magnetostrictive strain energy
• Magnetic field increases the cementite/ferrite interfacial energySphere and particle like cementite has minimum interface area, which is advantageous to minimize the final total interfacial energy
•Part II - Tempering Behaviors in High magnetic field
High Temperature Tempering(650°C×60min)
interface
0 T
14 T
Cementite
Ferrite
Vol
umE
nerg
y
D istance
0
MfG
McG
M
interface
0 T
14 T
Cementite
Ferrite
Vol
umE
nerg
y
D istance
0
MfG
McG
M
Schematic illustration of cementite/ferrite interfacial energy
Why magnetic field can influence the Why magnetic field can influence the morphologymorphology and and distributiondistribution of carbide of carbide
)(2
)(2
0 00 0cMfM
Mc
Mf
M
MdBMdB
GG
cf
5m5m
0T 14T
The magnetic field obviously retards the recovery of the matrix
•Part II - Tempering Behaviors in High magnetic field
0 14
Pe
rce
nta
ge
, %
0
1
2
3
4
5
6
7
8
Induction of the magnetic field, Tesla
RecoveredTempered at 650°C
7.24%
5.42%
EBSD maps(blue area are recovered regions)
Matrix RecoveryMatrix Recovery
High Temperature Tempering(650°C×60min)
Y.D. Zhang et al. Acta. Mater., 52 (2004), p3467-3474
TransformationTransformation
Martensite Precipitation of
transition carbides
°C
-Fe2C or -Fe2C
-Fe5C2
Fe3C
Precipitation sequence
For a given time
duration
•Part II - Tempering Behaviors in High magnetic field
Low Temperature Tempering (200°C×60min)
For low temperature tempering, the main change in microsturcture is the precipitation of transition carbides. They are metastable at different temperatures and change their form when tempering temperature rises.
’’[133] zone axis pattern-
–Fe5C2[536] zone axis pattern-
021--
312-
310
301
310--
011-
--312
-312
011
--301
-331
313021
313-
312--
331--
---
--
(b)
-’’[133] zone axis pattern-
–Fe5C2[536] zone axis pattern-
021--
312-
310
301
310--
011-
--312
-312
011
--301
-331
313021
313-
312--
331--
---
--
(b)(b)(b)
-
110
200101002
101-
101- -
002-
101-
200-
301
-211
--110
-211
121-
301--
121- -(a)
’’[113] zone axis pattern--
-Fe2C [020] zone axis pattern-
-
110
200101002
101-
101- -
002-
101-
200-
301
-211
--110
-211
121-
301--
121- -(a)(a)
’’[113] zone axis pattern--
-Fe2C [020] zone axis pattern-
-
•Part II - Tempering Behaviors in High magnetic field
Low Temperature Tempering (200°C×60min)Carbide PrecipitationCarbide Precipitation
1m
-Fe5C2
monoclinic
1m
-Fe2C
orthorhombic
0T
14T
(a) -Fe2C formed during non-magnetic tempering
(b) -Fe5C2 formed during magnetic tempering
Diffraction patterns and their indexing
Magnetic field has an obvious effect on changing the precipitation sequence by skipping the precipitation of -Fe2C.
•Part II - Tempering Behaviors in High magnetic field
Low Temperature Tempering (200°C×60min)Carbide PrecipitationCarbide Precipitation
Temperature variations of magnetization of -Fe2C, -Fe5C2 and -Fe at 14 T
- 200 - 100 0 100 200 300 4000
10
15
-Fe
Mag
net
izat
ion
, JT
-1m
ol-1
- 0 100 200 300 4000
5
Temperature, °C
-Fe2C
-Fe5C2
Gibbs free energy vs. carbon concentration for ’ martensite, -Fe5C2 and - Fe2C at 200C
14T
Carbon content
0T
Gib
bs
free
en
ergy
Fe C
Gib
bs
free
en
ergy
- Fe2C-Fe5C2
’
The thermodynamic and kinetic effects of the high magnetic field on the austenite to ferrite transformation show that it can obviously increase the amount of the product ferrite and accelerate the transformation by enhancing the Gibbs free energy difference between the parent and product phases.
Magnetic field can effectively prevent the cementite from growing directionally along the plate and twin boundaries and retard the recovery process of the ferrite matrix when high temperature tempering is conducted.
In the case of low temperature tempering, magnetic field can change the precipitation sequence of transition carbides, distribution and size of carbides and improve the impact toughness
SummaryA high magnetic field was applied to the austenite to ferrite transformation and tempering processes in a 42CrMo steel:
This work was supported by the National Science Fund for Distinguished Young Scholars (Grant No. 50325102), the National Natural Science Foundation of China (Grant No.50234020) and the National High Technology Research and Development Program of China (Grant No. 2002AA336010).
We also gratefully acknowledge the support obtained in the frame of the Chinese-French Cooperative Research Project (PRA MX00-03) and the Key International Science and Technology Cooperation Program (Grant No. 2003DF000007). The authors would like to thank the High Magnetic Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University, for the access to the magnetic field experiments.
Acknowledgement
Thank you for your attention!
•Part I - under high magnetic field (5)
[111]
[001] [101]
[111]
[001] [101]
(a) Longitudinal direction (b) Normal direction
EBSD AnalysisEBSD Analysis
Inverse pole figures of aligned ferrite grains
formed in 14T at a cooling rate of 10℃/min.