Yudong Zhang 1,2 , Xiang Zhao 1 , Changshu He 1 ,

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


Heat treatment arrangementHeat treatment arrangement


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


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%



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


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))


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


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


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


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


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


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-

-


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.


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!


[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.

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Yudong Zhang 1,2 , Xiang Zhao 1 , Changshu He 1 ,

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