Correlation of microstructure to macroscopic magnetic measurements on electrical steels

Polykseni Vourna1,a and Aphrodite Ktena2,b 1Laboratory of Metallurgy, NTUA, Zografos 15773, Greece

2TEI of Chalkida, Psahna, Evia 34400 Greece

axvourna@metal.ntua.gr, baktena@teihal.gr

Keywords: electrical steels, barkhausen noise, magnetic properties, microstructure

Abstract. Results of an experimental study of electrical steel annealed at 500, 600, 700 oC and

subsequently cooled via quenching or air, are presented. The samples have been characterized with

respect to their magnetic properties using Magnetic Barkhausen Noise (MBN) and major and minor

loop (B-H) measurements. MBN increases slightly with the annealing temperature especially in the

quenched samples. The B-H loops suggest that the prevalent magnetization reversal mechanism in

the air cooled samples is domain wall propagation, while in the quenched samples non 180o domain

rotation seems to be significant approaching the high induction region. Scanning Electron

Microscopy studies show a more homogeneous texture after annealing which in the case of the

quenched samples is accompanied by not fully formed grain boundaries and orientation along he

easy axis

Introduction

Magnetic effects have been used for sensing applications in the field of non destructive testing

[1]. Surface techniques have mainly been used for this purpose. Hall sensor measurements may

realize surface flux leakage in steel [2]. Eddy currents techniques may be used for ferrous but also

non ferrous materials [3, 4]. The magnetostrictive delay line technique has been implemented in

surface non destructive characterization in the past [5-7]. The need for determination of the surface

plastic deformation was facilitation by a particular magnetoelastic device able to measure the

dependence of magnetic permeability on surface plastic deformation in steels [8, 9]. Recently,

Barkhausen noise (BHN) measurements allowed for the correlation of mechanical and magnetic

properties [10, 11].

Electric steel, also known as silicon steel, contains carbon (0.005% or lower) and silica (0.05%

to 3.5%) and is widely used in laminated form as core material in transformers, motors and

generators. It is classified in two categories: grain oriented (GO) steel with a predefined

crystallographic orientation following the rolling direction and non-oriented (NO) steel which is

magnetically isotropic with random crystallographic orientations.

The efficiency of a core depends on the total core losses separated into three components: the

hysteresis loss, due to atomic level eddy currents, related to Barkhausen jumps and independent of

frequency, the classical loss, due to eddy currents at the macroscopic level and proportional to the

operating frequency, and the excess loss due to eddy current in the vicinity of domain wall motion

[12]. In order to increase the efficiency of a core it is therefore important to control all three types of

losses.

Silica as an alloying element increases the electrical resistivity of the steel, keeping at bay eddy

currents, while improving the properties of hardness and elasticity. Thermal treatment greatly

improves the efficiency of the core because it leads to more homogeneous microstructure, increased

permeability and decreased coercivity and therefore lower excess and hysteresis losses.

In the present work, magnetic properties resulting from Magnetic Barkhausen Noise (MBN) and

B-H loop measurements have been determined for commercially available electric steel in its

original form and after being annealed at various temperatures. The effect of quenching and air

cooling on these properties has been examined. Finally, Scanning Electron Microscopy (SEM) is

used to correlate the macroscopic magnetic properties with the microstructure.

Key Engineering Materials Vol. 495 (2012) pp 257-260Online available since 2011/Nov/15 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.495.257

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Experimental Procedure

The samples are rectangular strips, 140mm long, 30mm wide and 0.5mm thick. They have been

annealed for one hour at 500, 600, 700oC, and subsequently air cooled or quenched. The samples

were characterized before and after anealling with respect to their magnetic properties, using

Barkhausen noise and hysteresis loops measurements and their in-plane and cross-sectional

microstructure was studied using Scanning Electron Microscopy.

MBN measurements. MBN is measured using the Laboratory of Metallurgy’s device ΜΕΒ-2C.

A triangular shaped magnetic field produced by a generator is applied on the surface of the sample

by an electromagnet in the shape of a probe. The discrete changes in the resulting local

magnetization (MBN) are sensed at the ends of a receiving coil as voltage pulses. The number of

pulses above a given potential threshold (COUNTS) is related to the Barkhausen jumps in the area

of the measurement. All MBN measurements were taken at a constant threshold at different

positions on either side of the sample. The RMS value of MBN yields the parameter V1, which is a

more stable metric than COUNTS, and is preferred for comparisons between samples. A third

parameter, V2, is recorded to supply information on the degree of coupling between the sample’s

surface and the probe.

Hysteresis loop measurements. An in-house AC hysteresiograph was used to measure

hysteresis loops on the samples at various frequencies and excitation fields. An excitation solenoid

ensures the generation of a homogeneous magnetic field at its center where the sample is placed.

The voltage pulse induced at the ends of a sensing coil around the sample is proportional to the

sample’s permeability in the longitudinal direction. The integration of the voltage pulse yields the

dependence of the magnetic induction B with respect to the field H. The hysteresiograph is

controlled via a MATLAB GUI allowing the input of excitation field and frequency and showing

the output pulse and resulting B-H loop. Numerical integration is used for the B-H loop and a

digital Savitzky-Golay filter is used to suppress the noise. B-H loops down to 0.1 Hz have been

measured. However, the hysteresiograph is not calibrated and the input and output units of the

voltage pulses and B-H loops are arbitrary (a.u.).

Microstructural studies. After Barkhausen and hysteresis loop measurements, all specimens

were prepared for optical microscopy by standard grinding and polishing methods followed by

etching in 10% Νital solution. SEM was performed on the cross section and surface of the samples

in order to observe the changes in microstructure depending on the thermal treatment.

Results and discussion

SEM images show an intense technical topographic relief as the result of the different orientation

of the grains. After etching in 10% Nital solution, grains, depending on their crystallographic easy

axis, react at different rates, hence the intense technical topographic relief. The electrical steel is

mainly ferrite (single phase electrical steel) with low carbon content and consequently no eutectoid

transformation is observed when annealed. The steel is following the solvus curve of the phase

diagram for room temperature and for specific carbon composition. During annealing up to 700oC,

the alloy enters the two-phase region a+Fe3C and a small number of carbons is removed from the

supersaturation ferrite and forms small quantities of cementite at the grain boundaries (tertiary

cementite). Before thermal treatment, the sample grains are inhomogeneous and uneven. After

annealing, the grains become polyhedral and the grain size distribution is considerably narrower

leading to a more homogeneous texture. In the quenched samples, grain boundaries are not fully

formed which is probably due to the fast rate of cooling. This is more evident in the grains along the

easy axis (`lower level` grains are more resposive to Nital etching). Also the relief is less,

suggesting that most grains, though not yet fully frmed, are along the easy direction.

The B-H loops (Fig. 1) are measured along the longitudinal axis of the laminate. The field is

applied at an angle to the rolling direction as the slanting of the loops suggests. The samples are

measured before the annealing and after annealing at 600oC for rapid and slow cooling. The

respective permeabilities, as derivatives of the B-H loop, are shown on Fig.2. Annealing and

subsequent air cooling is known to relieve samples of built-in stresses and lead to a narrower grain

258 Materials and Applications for Sensors and Transducers

size distribution (Fig. 3). However, in the case of quenching cooling is done very fast and the grains

do not have the time to relax along their preferred crystallographic axes, as suggested by the SEM

image in Fig. 3b.

Qualitatively, the B-H loop and the permeability of the air-cooled sample (Figs. 1b, 2b) is not

significantly different than that of the reference sample (Fig. 1a, 2a). However, loop squareness

increases and the apparent decrease in saturation magnetization and the permeability underlines the

fact that the B-H loop is measured off easy axis and corroborates the conclusion of a narrower grain

size distribution. In the case of the quenched sample, both the major loop and the corresponding

permeability measurement are quite different. The permeability envelope varies smoothly with the

applied field, its peak value is significantly lower and so is remanent magnetization.

Fig. 3a: Surface image of sample

with no thermal treatment

Fig. 3b: Surface image of

sample annealed at 600oC and

air cooled

Fig. 3c: Cross sectional image of

sample annealed at 600oC and

quenched

Fig. 1a: B-H loop before the

annealing, f=0.1Hz

Fig. 2a: B-H loop after air-

cooling; f=0.1Hz; annealing

temperature 600oC

Fig. 3a: B-H loop after quenching;

f=0.1Hz; annealing temperature

600oC

Fig. 1b: Permeability before the

annealing, f=0.1Hz

Fig. 2b: Permeability after air-

cooling; f=0.1Hz; annealing

temperature 600oC

Fig. 3b: Permeability after

quenching; f=0.1Hz; annealing

temperature 600oC

MBN measurements suffer from instability partly because of the stochastic nature of the noise

measurements and partly because they are localized. The latter problem is countered by taking

several measurements at the same point, along the same sample as well as on different samples of

similar processing. The results are summarized on Table 1. V1, rather than COUNTS, is used for

comparison because it is the result of averaging and exhibits more stability. The values shown for

each sample are the average values obtained from measurements on several similar samples and the

respective standard deviations. There is no clear and indisputable relationship between the various

cases and a more extensive study needs to be carried out before sound conclusions are drawn.

Eventhough MBN of the air cooled samples (A) is not significantly different than that of the

reference sample (REF), it can be argued that it increases slightly with the annealing temperature.

Annealing leads to a small reduction in the density of dislocations and air cooling allows their

rearrangement. Pinning is therefore reduced and MBN increases. In the case of the quenched

samples (Q), MBN is even higher. SEM images (Fig. 3) show that grain boundaries are not well

Key Engineering Materials Vol. 495 259

formed which may explain the hysteresis loop shape (Fig. 1). Then, the increased MBN may be

explained by the decrease in pinning sites which also facilitates the non-180o domain wall

movement [13] favoring domain rotation.

TABLE 1: Average values and standard deviations of Magnetic Barkhausen Noise parameters V1, V2 REF 500A 600A 700A 500Q 600Q 700Q

Average V1 [mV] 376,13 349,60 458,08 409,40 461,65 474,51 447,10

V2 [mV] 426,31 423,77 422,20 290,24 429,25 382,76 372,12

St. Dev. V1 [mV] - - 98,37 95,53 56,80 68,45 25,41

V2 [mV] - - 76,34 16,97 96,64 45,44 70,59

Conclusions

Magnetic and structural characterization of electric steel samples before and after annealing has

been carried out in order to study the effect of thermal treatment and subsequent cooling on the

macroscopic magnetic parameters as well as on the microstructure. SEM images reveal finer and

more homogeneous texture in the case of annealed samples while grain boundaries are not fully

formed in the case of the quenched samples. The microstructural changes agree with the hysteresis

loop and MBN measurements which suggest less pinning in the annealed samples and more domain

rotation in the case of the quenched sample. A more detailed study, accompanied by applied stress

studies, is necessary to corroborate these preliminary findings.

Acknowledgements

The authors wish to thank Prof E. Hristoforou for the helpful discussions and advice on the

subject of magnetism, magnetic materials and their correlation with microstructure and mechanical

properties.

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