PCV EXPO MOSCOW October 2009

Magnetic properties of stainless steels

Dr M.MantelR&D Center Ugitech Fr. Schmolz&Bickenbach Group

1. How an electro-valve functions

2. Magnetism and ferromagnetism : physical principles

3. Steel microstructure and ferromagnetism

4. Magnetic stainless steels – Chemical analysis

5. Manufacturing process

6. Conclusion

SUMMARY

WHERE ARE ELECTRO-VALVES USED ?

REFRIGERATIONREFRIGERATION

CHEMICALS, CHEMICALS, PETROCHEMICALSPETROCHEMICALS

FOOD FOOD DISTRIBUTIONDISTRIBUTIONcoffee machines, beverage machines, etc.

HYDRAULICSHYDRAULICSvalves, transmission of fluids, etc.

HOUSEHOLD HOUSEHOLD APPLIANCESAPPLIANCES

4

spring

fixed core

mobile core

Inlet pipe

Current I in the coil- creation of a field strongly amplified by the inductor (fixed core)- the mobile core moves to find an equilibrium position

• Transient phase, eddy currents occur in the magnetic components with negative actions heating (Joule effect)opposed to the initial field(Lenz's law)

electrical resistivity must be increased

An electrovalve is a valve (faucet) electrically controlled with electromagnetic activation

• An iron monocrystal : no spontaneous natural magnetization :• Why ? Division into magnetic domains

MAGNETIZATION

distribution of magnetic domains

B : induction

direction of H

magnetic excitation H

2

3

8

9

5

4Br

Hc

76

Induction B

1 : demagnetized state

2 : the well "oriented" domain grows to the detriment of the others

3 : this process continues

4 : a single domain

5 : rotation of the moments

6 : rotation of the moments

7 : formation of new domains

8 : H=0 --> B#0=Br

9 : H=Hc --> B=0

Bs

1

HYSTERESIS LOOP

THE HYSTERISIS CYCLE

-3000 -2000 -1000 0 1000 2000 3000

-1.0

-0.5

0.0

0.5

1.0

H (A/m)

B (T

)

cH

rBrμμ0

hystW

-3000 -2000 -1000 0 1000 2000 3000

-1.0

-0.5

0.0

0.5

1.0

H (A/m)

B (T

)

-3000 -2000 -1000 0 1000 2000 3000

-1.0

-0.5

0.0

0.5

1.0

H (A/m)

B (T

)

cH

rBrμμ0

hystW

1. Softening and recrystallization2. Grain size 3. Mechanical deformation, internal stresses

RELATIONSHIP BETWEEN MICROSTRUCTURE AND MAGNETISM

Microstructural defects as grain boundaries, precipitates ans dislocations are obstacles to the the movement of magnetic domain walls.

Softening kinetic followed by hardness / coercive force

Deformed stateHc = 500 A/m

Recovered stateHc = 300 A/m

Nucleation and growth of ReX « phase »

Hc = 200 A/mReX complete

Hc = 120 A/m

Annealing treatment : New grains (recrystallization)Grain growthReduced straightening Avoid oxidization (N2,H2, vac.)

Influence of softening and recrystallization

Coe

rciv

e fie

ld (O

e)

1/grain size (mm-1)

Influence of grain size on magnetic properties

Influence of cold working

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

-1.0

-0.5

0.0

0.5

1.0

H (A/m)

B (T

)

Deformed State

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

-1.0

-0.5

0.0

0.5

1.0

H (A/m)

B (T

)

Annealed State

Hc µmax

Coer

cive

For

ce (A/m

)

Saturation Induction (T)0 0.5 1 1.5 2

106

105

104

103

102

10

1

0.1

Hard

Hard ferrites Al Ni Co

1/2 hard Fe-Co

T-R Magnets

Soft ferrites

Fe-Ni

Soft Fe-Co

Fe-SiSoft

Main properties of ferromagnetic stainless steels

StainlessSteels

CO

RR

OSI

ON

RE

SIST

AN

CE

ULTIMATE TENSILE STRENGTH MPa

-

+

250 500 750 1000 1250 1500

Various stainless steels for magnetism

Austenitic FeCrNi

para

ferroMartensitic

FeCrC

AnnealedHardened+tempered

ferro

ferro

Formation CEDRAT 12 mars 2004

Saturation Induction Cr, Mo, Si : Bs

1,4

1,6

1,8

2

2,2

0 5 10 15 20

%Cr+%Si+%Mo

Bs

(T)

Magnetism is only due to iron atomsBs decreases when %Cr or % alloying elements increases

Chemical Analysis

pure iron : Bs ≈ 2.1Twith 12%Cr: Bs ≈ 1.7 Twith 17%Cr : Bs ≈ 1.6T

Chemical Analysis

• Electrical resistivity must be increased in order to reduce the eddy currents

• By the addition of Si and Al

Ferritic stainless steel grades

4105/430F

4105Si/430FR 4106 (IMRE)

UGIPERM 12FM

4016/430

+Si

-Cr

+S +Mo

Cr 17 % + C (<0,1%) + Fe

4511/430LNb

+Nb

Steel maker transformations Machining Annealing

recuit MachiningAnnealingSteel maker transformations

960

482

960

643

494

246200

300

400

500

600

700

800

900

1000Hc (A/m)

960

482

960

643

494

246

Just machined After annealing

Bars

Rough

Annealed

PartsComparison :

700 parts measuredsame final valueconfirmed with customer testFull annealed solution is the best logistic and costs reduction route

Route 1

Route 2

Example of dedicated solutions

Stainless steels offer a large range of magnetic and non magnetic parts with high electric resistivity and high corrosionresistanceFerritic steels for soft parts with fast polarization Martensitic steels for compromise mechanical / magnetism Duplex steels are somewhere between martensitic and austenitic steels with high corrosion resistanceIt is almost always possible to modify desirable magnetic properties with a specific transformation’s route including thermal treatmentA good partnership between steel maker and customer is necessary

Conclusion

Thank youfor your attention