COERCIMETER FOR NON-DESTRUCTIVE MEASUREMENT OF THE COERCIVITY OF STEEL SHEETS

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COERCIMETER FOR NON-DESTRUCTIVE

MEASUREMENT OF THE COERCIVITY OF STEEL SHEETS

J. Billan

To cite this version:

J. Billan. COERCIMETER FOR NON-DESTRUCTIVE MEASUREMENT OF THE COERCIV- ITY OF STEEL SHEETS. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-965-C1-968.

�10.1051/jphyscol:19841197�. �jpa-00223674�

JOURNAL DE PHYSIQUE

Colloque C I , supplement a u n° 1, Tome 4 5 , Janvier 1984 page CI-965

COERCIMETER FOR N O N - D E S T R U C T I V E M E A S U R E M E N T OF T H E C O E R C I V I T Y OF STEEL SHEETS

J. Billan

LEP Division, C.E.B.N., 1211 Geneva 23, Switzerland

Résumé - Cet instrument spécialement conçu pour mesurer la coercivité de tôles d'acier sans découper d'échantillon, sera u t i l i s é pour f a i r e les 8000 mesures de coercivité nécessaires au contrôle de la production des ll'OOO tonnes de tôles d'acier pour les dipôles LEP. Le coercimètre effectue des mesures loca- les et directionnelles de l a coercivité avec une précision de 3 % et une re- p r o d u c t i b i l i t é de 1 %. La perméabilité peut aussi être mesurée avec une préci- sion comparable. Description et performances sont présentées.

Abstract - A special instrument has been built to measure the coercivity di- rectly on steel sheets without having to cut samples. It will be used to per- form the 8000 coercivity measurements which will be needed during the produc- tion of the ll'OOO ton of steel sheets for the LEP dipole magnets. The coerci- meter allows local and directional coercivity measurements to be made on a steel sheet with an accuracy of 3 % and a reproducibility of better than 1 %.

The permeability can also be measured with similar precision.

I - INTRODUCTION

The 3304 dipole magnets of LEP require the production of ll'OOO tons of 1.5 mm thick decarburized steel sheet. As explained in a previous paper [ 1 ] , t h i s s t e e l , besides having a low coercivity and a high permeability, must also have a very low spread of coercivity. The maximum dispersion of ± 11 A m"

, guaranteed by the manufacturer, is s t i l l too large and a controlled mixing of the steel is necessary. To this end, about 8000 coercivity measurements w i l l have to be performed during production. For such a large number of measurements, classical permeameters (ring or Epstein frame) requir- ing preparation of samples are excessively time- and steel-consuming, hence the idea to build a "coercimeter" measuring the coercivity d i r e c t l y on the steel sheets with- out having to cut samples.

II - PRINCIPLES

The steel sheets are of rectangular dimensions, 1.0 x 0.5 m. Magnetization is provid- ed by an excitation coil and is concentrated by using a pair of flux return yokes placed on each side of the sheet. The yokes also serve to minimize the excitation.

The flux in the sheet is measured by means of a detection coil as shown in Fig. 1.

Also shown in this figure are the auxiliary excitation and detection coils placed on the yokes, which are used to estimate the mean air gap of the yoke contacts by meas- uring the magnetic flux circulating only between yokes.

The basic principle of the measurement is as follows: considering a closed flux line passing through the steel sheet and one of the return yoke, by definition the coer- cive field is calculated from the number of ampere-turns {N I

) which give a zero in- duction along this flux line. According to Ampere's law:

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841197

C1-966 JOURNAL DE PHYSIQUE

a

I -

@ FLUX RETURN YOKE @ W I N DETECTION COIL @ AUXILIARY EXCITATION COIL

'

Fig. 1 - Schematic cross-section Fig. 2 - Typical v a r i a t i o n o f Hc

o f t h e coercimeter vs. maximum i n d u c t i o n

( a t zero i n d u c t i o n , t h e r e i s o b v i o u s l y no drop of amp6re-turns due t o t h e r e l u c t a n c e o f sheet, yoke and a i r gap), hence

N Ic - ^{Hcy x ly}

"cs =

where: 1s

= c o e r c i v e f i e l d o f t h e s t e e l sheet,

= c o e r c i v e f i e l d o f t h e yoke, lSY = f l u x l i n e l e n g t h i n t h e sheet, ly = f l u x l i n e l e n g t h i n t h e yoke.

From eq. 2 i t can be seen t h a t f o r Hey<< Hcs, t h e measuring p r e c i s i o n i s l a r g e l y de- termined by t h e p r e c i s i o n of 1,. The d e t e c t i o n c o i l measures t h e t o t a l f l u x i n t h e sheet, b u t due t o t h e w i d t h o f t h e yoke contacts, each f l u x l i n e has a d i f f e r e n t length. Nevertheless, a mean l e n g t h has been computed under t h e c o n d i t i o n t h a t t h e f l u x l i n e s do n o t wander i n t h e sheet b u t pass from one c o n t a c t t o t h e o t h e r i n s t r a i g h t 1 in e s i n t h e plane o f a cross-section o f t h e coercimeter (two-dimensional problem hypothesis). For t h i s reason, t h e f o l l o w i n g c o n s i d e r a t i o n s guided t h e design:

a) The e x c i t a t i o n f i e l d must be as homogeneous as possible, hence t h e choice o f an e x c i t a t i o n c o i l o f one l a y e r placed i n c l o s e c o n t a c t w i t h t h e sheet and c o v e r i n g a l l t h e f r e e space between yoke contacts.

b ) The yokes should be good magnetic s h o r t - c i r c u i t s , hence t h e a i r gap o f t h e con- t a c t s must be as small as p o s s i b l e and t h e m a t e r i a l o f t h e yoke must have t h e h i g h e s t p o s s i b l e p e r m e a b i l i t y .

C) The yokes should cover completely one o f t h e dimensions o f t h e sheet i n order t o a v o i d "end e f f e c t s " i n c r e a s i n g t h e mean l e n g t h o f t h e f l u x l i n e s i n t h e sheet.

d) The yokes should be laminated i n o r d e r t o prevent t h e f l u x i n one o f t h e yoke l a -

minations from passing i n t o an adjacent one: t h i s helps t o 1 i m i t t h e i n f l u e n c e o f

v a r i a t i o n s o f t h e air-gap c o n t a c t s (e.g. due t o sheet surface i r r e g u l a r i t i e s ) .

Another i m p o r t a n t c o n s i d e r a t i o n i n designing a coercimeter i s t h a t i t i s n o t necessa-

r y t o magnetize t h e s t e e l sheets up t o complete s a t u r a t i o n : t o reach t h e maximum va-

l u e of Hc, 1.5 T i s s u f f i c i e n t (see F i g 2). T h i s l i m i t s t h e maximum f i e l d t o

approximately 1200 A m-1, thus s i m p l i f y i n g t h e design o f t h e e x c i t a t i o n c o i l .

Some r e s u l t s o f t h e computations o f t h e mean l e n g t h o f t h e f l u x l i n e s i n t h e sheet

a r e given i n Fig. 3, and t h e c o e r c i v e f i e l d v a r i a t i o n i n t h e sheet under t h e yoke

contacts, deduced from those computations, i s g i v e n i n Fig. 4 f o r a 1.5 mm t h i c k

s t e e l sheet.

A

A

-

-- length (mm~ ^air- gap ( x 10 '1 alr - g o p =

o

--.

108 - '\ \

nom~nal values

107 --

'

_{. '}

106 -- sheet t h ~ c k n e s s I x 10 'I

w d t h of contact 105 -

104 - 103

0 10 15 20 I m m l 10 x(mml

Fig. 3 - V a r i a t i o n o f t h e e f f e c t - Fig. 4 - V a r i a t i o n o f Hc under t i v e f l u x l i n e l e n g t h yoke c o n t a c t

I 1 1 - DESCRIPTION 1. Main components

The yokes are t h e most important component o f t h e coercimeter. They are made by s t a c k i n g 0.9 mm t h i c k mumetal l a m i n a t i o n s having a f i l l i n g f a c t o r o f 0.95. A t t h e maximum i n d u c t i o n i n t h e yoke (0.12 T), t h e mumetal c o e r c i v e f i e l d i s smaller than

1 A m-1 and i s o f t h e o r d e r o f 105. The o v e r a l l l e n g t h o f t h e stack i s s l i g h t l y more than t h e w i d t h o f t h e sheet (520 mm), the l a m i n a t i o n and c o n t a c t w i d t h a r e 10 mm and the f r e e space between c o n t a c t s 100 mm.

The e x c i t a t i o n c o i l s , made o f 2.5 mm2 f l e x i b l e copper wire, have 24 t u r n s f o r t h e main c o i l and f o u r t u r n s i n t o t a l f o r the a u x i l i a r y c o i l s . The d e t e c t o r c o i l s have 50 t u r n s f o r t h e main c o i l and a l s o 50 t u r n s f o r each o f t h e a u x i l i a r y c o i l s . The l a t t e r a r e connected i n such a way as t o be i n s e r i e s f o r a f l u x c i r c u l a t i n g o n l y i n t h e yokes and i n o p p o s i t i o n f o r t h e r e t u r n f l u x o f t h e sheet.

The yokes a r e pressed a g a i n s t t h e sheet w i t h o f f o r c e o f 3000 N by use o f a pneumatic jack. A system o f r o l l e r s guides t h e sheet f o r an easy and safe i n t r o d u c t i o n i n t o t h e coercimeter. This i s shown i n Fig. 5.

2. E l e c t r o n i c and c o n t r o l equipment

Once the sheet i s i n p l a c e i n the coercimeter, t h e sequence o f measurements i s auto- matic, c o n t r o l l e d by microcomputer. The r e s u l t s a r e displayed on t h e screen, logged on paper and s t o r e d on a mini-cassette tape f o r s t a t i s t i c a l treatment.

Fig. 5 - I n t r o d u c t i o n o f a sheet i n the coercimeter

Fig. 6 - F l u x measurement on

a h y s t e r e s i s c y c l e

Cl-968 JOURNAL DE PHYSIQUE

E x c i t a t i o n c u r r e n t s a r e supplied by a p r e c i s e b i p o l a r power supply w i t h a r e s o l u t i o n o f 0.3 mA (= 10-3 o f I c ) , and f l u x v a r i a t i o n s i n d e t e c t o r c o i l s are measured by i n t e - g r a t o r s having a r e s o l u t i o n o f 1 ~ V S , a maximum d r i f t o f 5 uV and a maximum non- l i n e a r i t y o f 0.015 %.

I V - MEASUREMENT PROCEDURE AND PERFORMANCE

F i r s t , an average value o f the f o u r a i r gaps i s c a l c u l a t e d by measuring t h e f l u x i n the yokes when magnetized by t h e a u x i l i a r y e x c i t a t i o n c o i l s . This value i s used t o c a l c u l a t e t h e e f f e c t i v e f l u x p a t h l e n g t h i n the s t e e l , 1,: A f t e r demagnetizing t h e yokes, t h e power supply i s commutated on t h e main excitation c o i l and a s t a b l e hyste- r e s i s c y c l e i s established. The f o u r f l u x v a r i a t i o n s along t h i s c y c l e a r e measured as i n d i c a t e d i n Fig. 6 and t h e remanent f l u x , @r, i s c a l c u l a t e d :

Then, t h e c u r r e n t , I,, necessary t o cancel @r i s measured on b o t h sides o f t h e c y c l e and Hc i s c a l c u l a t e d from t h e mean absolute value o f I , t a k i n g i n t o account t h e coer- c i v e f i e l d o f the yoke according t o eq. 2. One measurement l a s t s about f o u r minutes.

The coercimeter accuracy has been evaluated a n a l y t i c a l l y and r e s u l t s have been com- pared w i t h measurements performed on a r i n g permeameter [21. The d i f f e r e n c e s were o f t h e o r d e r o f t y p i c a l l y 1 % and maximum 3 %.

The r e p r o d u c i b i l i t y i s w e l l w i t h i n 1 % as l o n g as t h e parameters i n f l u e n c i n g the c o e r c i v i t y a r e k e p t reasonably constant, p a r t i c u l a r l y t h e temperature (10-3/"C).

V - PERMEABILITY MEASUREMENT

P e r m e a b i l i t y can be deduced from the measurements o f t h e maximum f l u x i n the sheet,

@,

x, and t h e c a l c u l a t i o n o f t h e drop o f amp&-e-turns i n t h e f r e e p a r t o f t h e sheet between the yoke c o n t a c t s (e.g. 96 % f o r 1200 A m-1 e x c i t a t i o n f i e l d and 0.01 mn a i r gaps). A t such a f i e l d l e v e l , t h e p e r m e a b i l i t y o f t h i s s t e e l i s s l i g h t l y s u p e r i o r t o 1000 and t h e measuring accuracy i s about 2 %.

CONCLUSION

Designed t o be operated i n a simple and r e l i a b l e way by non-specialists, two c o e r c i - meters have been i n s t a l l e d a t t h e s t e e l works. Since t h e beginning o f 1983, about 1000 c o e r c i v i t y measurements have been performed s a t i s f a c t o r i l y .

ACKNOWLEDGEMENTS

T h i s work was i n i t i a t e d by J.P. Gourber who launched t h e idea o f a s i n g l e sheet t e s t - er. Thanks a r e due t o K.N. Henrichsen, H. Depierre and G. Turcato who designed and b u i l t most o f t h e e l e c t r o n i c equipment, t o C. Sanz f o r h i s s k i l l f u l work i n t h e yoke f a b r i c a t i o n and t o G. Erskine f o r h i s h e l p f u l p a r t i c i p a t i o n i n program computations.

REFERENCE

I 1 1 GOURBER J.P., BILLAN J., LAEGER H., PERROT A., RESEGOTTI L., CERN LEP-IA/83-9 -

presented a t 1983 Part. Acc. Conf., Santa Fe, N.M (March 1983).

[ 2 1 HENRICHSEN K., CERN AR/Int. SG/65-7 (19651.