44 Journal of Magnetism and Magnetic Materials 28 (1982) 44-50 North-Holland Publishing Company

M A G N E T I C A G E I N G C H A R A C T E R I S T I C S O F L O W S I L I C O N E L E C T R I C A L S T E E L S

San tanu K u m a r RAY

Product Laboratory Group, R&D Centre, SAIL, Ranchi-834 002, India

and

O.N. M O H A N T Y

Metallurgical Engineering Department, HT, Kharagpur-721 302, India

Received 18 January 1982

Increase is core-loss during the service life of soft magnetic materials is known as 'magnetic ageing'. This is essentially due to the enhancement of coercive force owing to interaction of domain walls with precipitated carbide particles. Although the effect of silicon at 3 wt% is known, magnetic ageing studies in low-silicon electrical steels are few. This investigation studies through coercive force changes during isothermal ageing the magnetic ageing behaviour of three grades of steel with 0.3, 1.1 and 1.5 wt% silicon, respectively. For steels with 1.1 and 1.5 wt% silicon coercive force increase was slight even after prolonged ageing at low temperatures and initially at higher temperatures. Appreciable magnetic ageing was detected in these two grades at higher temperatures and in the steel with 0.3% silicon at all ageing temperatures. In all three steels the peak increase was larger at lower temperatures and the time required for the peak was longer. While the 0.3% Si-steel indicated sharp coercive force peaks at all ageing temperatures, the behaviour was considerably different for the other two grades. The observations are rationalized on the basis of the type, density and growth rate of precipitated carbides.

I. Introduction

Soft magnet ic steels find use in electromagnets, motors, generators and transformers. The qual i ty

of steel for such appl icat ions is usually determined by its core loss. Since the quant i ty of energy equivalent to core loss is dissipated as heat, there is a major incentive in reducing such losses. Dur-

ing the useful service life of motors, generators or t ransformers a slow deter iorat ion in performance

may occur, associated with an increase in core loss. While at ambient temperatures it may take from a few months to a few years for such degradat ion to be perceptible, exposure to slightly elevated tem- peratures can accelerate the deter iorat ion and hence can be a cause of concern. This phenome- n o n is termed 'magnet ic ageing'. Increase in core loss with time is essentially due to the increase in coercive force. Owing to a sizeable material re- qu i rement and longer t ime for direct measurement

of core loss, coercive force has alternatively been

used to follow magnet ic ageing processes in

laboratory investigations.

Magnet ic ageing is due to precipi tat ion of

carbon and ni t rogen when their amoun t in solu- t ion exceeds the solid solubili ty limit at the tem- perature of use. In practice, the amoun t of nitro- gen in electrical steels is usually not sufficient to make any significant contr ibut ion. Dur ing the processing of electrical steels the rate of cooling

after mi l l -anneal ing is usually not sufficiently slow for the excess carbon atoms to precipitate. As a consequence, they can precipitate as a fine disper- sion of carbide particles when the material is in service. These precipitates interact with and pin the domain walls, which results in an increase in coercive force [1-4]. It is now recognized that the increase in coercive force depends upon the mag- netic characteristics and volume fraction of carbides, their state of dispersion and size.

0304 -8853 /82 /0000 -0000 /$02 .75 © 1982 Nor th -Ho l l and

S.K. Ray, "O,N. Mohanty / Low Si steel and magnetic ageing 45

Substitutional alloying elements affect the in- crease in coercive force of steels. Manganese [1,5] has no perceptible influence on coercive force dur- ing isothermal ageing. Aluminium [6,7] has been reported to delay the increase and when present in amounts of about 5 wt% the increase in coercive force during isothermal ageing is completely in- hibited [7]. Phosphorus, when present only to the extent of 0.17 wt%, effects both lesser and delayed increase [8,9]. Silicon, when present in amounts of 3 wt% or more, restricts magnetic ageing [1,10-12]. However, the effect of silicon in lesser concentra- tions does not appear to have been studied. Thus, the minimum amount of silicon required to control magnetic ageing has not been established. Since most of the varieties of non-oriented electrical steels contain only up to 1.8 wt% silicon, it was appropriate to study the magnetic ageing char- acteristics of three steels having silicon contents in the range 0.3 to 1.5 wt%.

2. Experimental

Specimens 5 0 × 4 × 0.5 mm 3 were cut from strips of the three commercial low-carbon steels in the mill-annealed condition. The chemical com- positions are listed in table 1. Although in mill practice low-carbon steels are furnace cooled or air cooled, water quenching helps retain the maximum amount of carbon in solid solution and hence can indicate the maximum potential of the steels for subsequent magnetic ageing. The specimens were solution treated at 750°C for 40 min in vacuum (5 X 10 -5 Torr), water quenched, then isotherm- ally aged at 125, 170, 210, 250 and 300°C for

Table 1 Chemical composition of the investigated steels

Designation Weight(%)

C Si Mn P S

0.3 Si-steel 0.013 1.1 Si-steel 0.020 1.5 Si-steel 0.015

0.30 0.25 0.03 0.02 1.11 0.28 0,03 0.02 1.52 0.23 0.03 0.02

various periods. Coercive force was measured after magnetisation to 1 T with an accuracy of 0.02 Oe.

3. Results and discussion

In the water-quenched condition the coercive force values for the 0.3 Si, 1.1 Si and 1.5 Si-steels were respectively 0.85, 1.26 and 0.98 Oe. It is apparent that the magnitude of the coercive force depends on the amount of carbon in solid solu- tion.

Figs. 1 and 2 show the increase in coercive force with isothermal ageing for the 0.3 Si-steel. At 125 and 170°C (fig. 1), the increase was marginal (0.2 Oe) in the early stage of ageing. On continued ageing coercive force increased sharply. Further ageing caused a peak and then a decrease. The maximum increase at 125°C was slightly higher than at 170°C, 1.28 Oe as compared to 1.22 Oe, but the time required to reach the peak was greater at 125°C.

In contrast to the behaviour at lower tempera- tures, the coercive force increase was appreciable even during the early stage of ageing at higher temperatures. Fig. 2 indicates that at 210°C the increase was about 0 . 4 0 e after 30 min and at 250 ° and 300°C the rise was about 1 0 e after only 2 min (the shortest ageing time investigated). In general, the peak was larger at lower temperature and the time required to attain the peak was greater.

An initial small rise and a subsequent sharp increase in coercive force during isothermal ageing in certain temperature ranges have been observed in commercial low-carbon steels and pure Fe-C, Fe - Mn - C , Fe-A1-C alloys [5-9, 13,14]. Earlier [13] such behaviour was explained by the precipi- tation and subsequent growth of cementite par- ticles. However, later investigations involving both magnetic studies and microstructural observations have clearly shown that the initial small rise is associated with the precipitation of epsilon carbide and coercive force increases only when cementite precipitates [8,9,14,15]. The configuration of cementite particles in ferrite is highly anisotropic and its direction of easy magnetisation [001] . . . . . tire lies at an angle of about 55 ° to the axes of easy

46 S.K. Rap', O.N. Mohanty / Low Si steel and magnetic ageing

~ _ i I r I l l l i j i i i f J i l l I - - I i r I i I I I [ - - - - - T - I r I l i l l 1 I I I [ I I I

14 0 3 % Si, 0.013 % C - o

• 170 C O ~ LJ 12 OG

W 2 10 (D O2 O L 013 W > ~D

~ 06 O

Z - 0 A k J U3 <

_z

n _ _ t I l J l l l J i i I u 10 2 5 102 2 5 103 2 5 10" 2 5 105 2 5

AGEING TIME(MINUTE)

Fig. 1. Increase in coercive force during 125 and 170°C isothermal ageing of 0.3 Si-steel.

112

- -96

- - 80

- -64

_ 4 8 <

- - 32

- -16

10

magne t i sa t ion of ferri te (100)rerrite [16,17]. This together with the high magnet ic an i so t ropy [18] of cement i te poss ib ly expla in the larger in terac t ion of

d o m a i n b o u n d a r y with cement i te part icles. The increase in coercive force cont inues with

the growth of the cement i te par t ic les up to a

14 ~ T ~ r [ - - • q - ~ I q T r T T ~ r x - F ~ - r r [ • ~ T - ~ q q T T - - ~ - - f

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U ~D

_z 06

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02

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AGEING TIME (MINUTE)

Fig. 2. Increase in coercive force dtlring 210, 250 and 300°C isothermal ageing of 0.3 Si-steel.

r I I 1 1 1 2

- 9 6

- 80

- 64

<~ - -48

- -32

- -16

I I I I I 0 5 105

S.K. Ray, O.N. Mohanty / Low Si steel and magnetic ageing 47

critical size. Domain wall observation through Lorentz microscopy has revealed [1] that at the ageing time corresponding to the peak in coercive force domains of closure are usually found to form around precipitate particles of such a critical size. This reduces the magnetostatic energy associated with the cementite particles and lessens interaction with domain boundaries [19]. Moreover, addi- tional ageing causes individual precipitate particles to become more isotropic in shape and to decrease in number. All these factors lead to a net reduc- tion in coercive force.

High supersaturation of carbon and low ageing temperatures induce high density of nuclei. Hence a larger number of cementite particles of optimum size is instrumental in causing a greater increase in coercive force at lower ageing temperature. The longer ageing time required for the peak at lower temperature is obviously due to the slower growth of carbide particles at lower temperature.

Figs. 3, 4, 5 and 6 represent the change in coercive force with ageing time for the 1.1 Si and 1.5 Si-steels. For the 1.1 Si-steel at 125°C the

increase was only 0 . 5 0 e even after ageing for about 105 min. Likewise, for the 1.5 Si-steel the increase was slight at 125 and 170°C. Transmis- sion electron microscopy has revealed [20] that this stage is associated with metastable carbide precipi- tation. With the appearance of cementite particles in the 1.1 Si-steel coercive force took a sharp turn upwards after 8 min at 250°C, after 120 min at 210°C (fig.4) and after about 3× 103 min at 170°C (fig. 3). Similar increases for the 1.5 Si-steel at 210, 250 and 300°C are also associated with cementite formation.

When cementite formed in the 1.1 and 1.5 Si-steels the maximum increases were higher at lower temperature and the maximum values of coercive force were attained after longer ageing. A comparison of maximum increases for the three steels at 210, 250 and 300°C, when cementite precipitates, shows that for the 1.1 Si-steel (0.02 wt% C) the peaks are maximum and those for the 0.3 Si-steel (0.013 wt% C) are minimum. A higher supersaturation of carbon in the former caused a larger amount of cementite precipitation. For the

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168

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0 ~ [ i i I i i i ii l i I l i i i ii i I I I I I I

10 2 3 5 7 10 2 2 3 S 7 10 3 2 3 5 7 ~0 z' 2 3 S 7 10 5

A G E I N G T I M E ( M I N U T E )

F i g . 3. I n c r e a s e i n c o e r c i v e f o r c e d u r i n g 125 a n d I 7 0 ° C i s o t h e r m a l a g e i n g o f l . l S i -s teel .

120

p,

96 ~-

<

72 ~-~

48

24

o 3 0 0 °C • 2 5 0 °C • 2 1 0 °C

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o (,..)

0 ' 9 Z

Ld

0"6 U.I no.

N 0.3

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S.K. Ray, O.N. Mohanty / Low Si steel and magnetic ageing

t I I I I ' ' ' 1 I I l I I [ I I ] I

si, o.oz % c

0 1 I I i I I I III l I I I I I ill I J l I I I Lil l t

1 2 3 5 7 I0 2 3 5 7 lO 2 2 3 5 7 10 3 2 3

AGEING TIME (MINUTE)

I r I m m i m m I

Fig. 4. Increase in coercive force during 210, 250 and 300°C isothermal ageing of 1.1 Si-steel.

l ' Z. 8

2 4

[ I i i I i

5 7 10 4

68

z,z,

120

96 ~

<~

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0

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W

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( . )

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I I i l , i i ,i I 1 i I I I # l r l i ' ' II 12B

48

0 ~.5 °/o st ,o Om/o c

® 210 °C

• 170 °C

o 125 °C

- v - 0 L J I I : J l i t

lO 2 3 5 7 I0 2 "2 3 5 7 I0 3 2 3 5 7 lO 4 2 ]

AGEING TIME (MINUTE)

Fig. 5. Increase in coercive force during 125, 170 and 210°C isothermal ageing of 1.5 Si-steel.

e0

6z. r ~

<~

32

16

5 7 10 5

1"6

S.K. Ray, O.N. Mohanty / Low Si steel and magnetic ageing

l [ I , i i i I I I I i l [ I i [ T ' ' ' I l ~ ' I I l

I ' - '

ILl I'-" (.,0 L~J 1'2

0

h J 1 . 6 (.9

0

O ' !

t.~ 0-6 0

Z 0 . 4

W

< ~ 0 . 2

Z - - 0

1

1"5 % Si, 0"015% C

• 300 °C

® 250 °C

I l f I t t l I I I I I I I I I I I I ] I I I I ] I I

3 5 7 10 . 2 3 S 7 102 2 3 5 7 103 2 3

Fig. 6. Increase in coercive force during 250 and 300°C isothermal ageing of 1.5 Si-steel.

, , , , , , 128

-1112

96

80

164 e~.

48 v

32

16

I [ ] I ~ 0

5 7 104

49

coercive force change associated with metastable carbide precipitation as well, higher supersatura- tion can explain the relatively larger increase in the 1.1 Si-steel as compared to the other two.

In sharp contrast to the appreciable magnetic ageing in these three steels, Leslie and Stevens [1] reported that in an Fe-3.3% Si-0.016% C steel the maximum increases were less than 0 . 4 0 e at ageing temperatures in the range of 100 to 500°C. This was explained by the stabilisation of the metasta- ble carbide and inhibition of cementite precipita- tion in presence of 3.3 wt% silicon. In the present study, in the 0.3 Si-steel cementite formation and consequent sharp increase in coercive force was detected even at 125°C. Lower magnetic ageing due to stabilisation of metastable carbide was ob- served only at 125°C for the 1.1 Si-steel and at 125 and 170°C for the 1.5 Si-steel. It is thus apparent that the amount of silicon governs the temperature up to which cementite formation can be inhibited and magnetic ageing can be restricted.

A comparison of the magnetic ageing character- istics of the three steels indicate that unlike in the

0.3 Si-steel, the coercive force changes in the 1.1 and 1.5 Si-steels did not register a sharp peak even in presence of cementite. In these two steels the increase towards the maximum value was much slower and the peak was attained much later. The subsequent fall in coercive force was insignificant even after prolonged ageing. Transmission electron microscopy indicated that in these two steels the growth of the carbide particles during isothermal ageing was much slower than in the 0.3 Si-steel and that eventually growth of the carbide particles virtually stopped [20]. Thus, the critical size of the carbide particles associated with peak increase in coercive force is reached only on pronlonged age- ing and with the stabilisation of the size of the particles overaging is slight.

A slower growth rate of carbide particles was reported earlier in presence of 3.3 wt% silicon [21]. It appears that although up to 1.5 wt% silicon cannot prevent appreciable magnetic ageing, its presence in amounts greater than about 1 wt% nevertheless delays the coercive force enhance- ment.

50 S.K. Ray, O.N. Mohanty / Low Si steel and magnetic ageing

Acknowledgements

The authors are grateful for the encouragement received from Drs. G. Mukherjee, S.K. Gupta, V. Ramaswamy and Sanak Mishra of the R&D Centre, Steel Authority of India.

References

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dison-Wesley, Reading, MA, 1972) p. 317. [3] S. Titto, Acta Polytech. Scand. Phys. Nucl. Ser. 119 (1977). [4] S. Mishra, S.K. Ray, V. Ramaswamy and P.R. Bapat, Steel

India 2 (1979) 66. [5] J.F. Enrietto, M.G.H. Wells and E.R. Morgan, Met. Soc.

Conf. AIME (1965) p. 141. [6] W. Jaeniche, J. Brauner and W. Heller, Arch. Eisenhiit-

tenw. 37 (1966) 719.

[7] W.C. Leslie and G.C. Rauch, Met. Trans. 9A (1978) 343. [8] S.K. Ray, S. Mishra and O.N. Mohanty, IEEE Trans.

Magn. MAG-17 (1981) 2881. [9] S.K. Ray, S. Mishra and O.N. Mohanty, Scripta Met. 15

(I98I) 97I. [10] G.S. Gardner, in: Conf. on Magn. and Magn. Materials

(AIEE, New York, 1955) p. 100. [11] D.A. Leak and G.M. Leak, J. Iron Steel Inst. (London)

187 (1957) 190. [12] R. Wagner, Acta Met. 7 (1959) 523. [13] M. Nacken and W. Heller, Arch. Eisenhiittenw. 31 (1960)

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(1982) 43. [16] W.C. Leslie, R.M. Fisher and N. Sen, Acta Met. 7 (1959)

632. [17] P. Blam and R. Pauthenet, Compt. Rend. 237 (1963) 1501. [18] A.S. Keh and C.A. Johnson, J. Appl. Phys. 34 (1963) 2670. [19] H.J. Williams, Phys. Rev. 71 (1947) 646. [20] S.K. Ray, Ph. D. Thesis, liT Kharagpur, India (198I). [21] A.S. Keh and W.C. Leslie, Mater. Sci. Res. 1 (1963) 208.