The characterisation of work-hardened austenitic stainless steel by NDT

micro-magnetic techniques

D. O’Sullivana,*, M. Cotterella, I. Meszarosb

aDepartment of Mechanical and Manufacturing Engineering, Cork Institute of Technology, Bishopstown, Room A243, Cork, IrelandbDepartment of Materials Science and Engineering, Budapest University of Technology and Economics, H-1111 Goldmann ter 3,

(V2/159), Budapest, Hungary

Received 15 July 2003; revised 30 September 2003; accepted 15 October 2003

Abstract

It is accepted that the work-hardening of austenitic stainless steels during machining or cold-working results in two main products: the

appearance of a0-martensite and increased dislocation densities within the host material. In machining, this results in many difficulties (poor

surface finish, poor machinability and high tool wear). Non-destructive sensing is essential in today’s high volume production environments

because of its ease of use, speed and non-invasive sensing. Non-destructive magnetic measurement techniques have been employed to

characterise the work-hardening of an austenitic stainless steel grade (SS404) due to room-temperature plastic tensile loading. These

techniques include the use of magnetic Barkhausen noise, ferromagnetic phase measurement and coercivity measurement. It was found that

the dislocation density, rather than the a0-martensite phase, to be the cause of material work-hardening. It is suggested that the use of

coercivity measurement is a useful quantitative and non-destructive method for characterising work-hardening of the studied alloy in relation

to the amount of its plastic deformation (work-hardening).

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Coercivity; Work-hardening; Magnetic Barkhausen noise

1. Introduction

It is widely accepted that when austenitic stainless steels

are deformed or strained, there is a martensite formation and

the dislocation density (or crystalline defects), within the

host material, increases in accordance with the amount of

deformation. The gradual transformation of austenite to

strain induced martensite increases the work-hardening of

these steels. The fine a0-martensite grains which appear

inside the austenite grains mainly at the intersection of shear

bands, make the movement of dislocations more difficult.

Martensite may form in austenitic stainless steels due to

the working of the material (mechanical) or due to

temperature effects (thermal). Two types of martensite can

form: 1-martensite, which forms on close-packed (111)

planes in the austenite, has a hexagonal close-packed (hcp)

crystal structure; and body-centred cubic (bcc) a0-marten-

site, which forms as plates with (225) habit planes in groups

bounded by faulted sheets of austenite on (111) planes [1].

All austenitic stainless steels are paramagnetic in the

annealed, fully austenitic condition. The hcp 1-martensite is

paramagnetic in contrast to the bcc a0-martensite which is

strongly ferromagnetic (hard-magnet) and the only mag-

netic phase in the low carbon austenitic stainless steels [2].

Therefore, the cold worked austenitic stainless steels have

detectable magnetic properties that can be eliminated by

annealing.

A device that would lead to the fast detection of work-

hardening in a workpiece material during machine operation

is desirable. Thus, any machine or material changes that

need to be made to combat this problem can be completed,

eliminating consequential production problems. This can

lead to the optimised machining of the material. An

investigation into work-hardening detection techniques is

presented.

0963-8695/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ndteint.2003.10.001

NDT&E International 37 (2004) 265–269

www.elsevier.com/locate/ndteint

* Corresponding author. Tel.: þ353-21-432-6507; fax: þ353-21-432-

6627.

E-mail address: dosullivan@cit.ie (D. O’Sullivan).

2. Experimental

Tensile tests were completed on SS304 samples with

dimensions of 100 mm £ 10 mm £ 3 mm. These samples

were deformed to various degrees of strain within the plastic

range (i.e. from the yield point (0.98%) of the material to the

ultimate tensile strength point (48%)). The amount of strain

was determined by the percentage of plastic deformation to

be completed to samples. All tests were completed at room

temperature on a Hounsfield tensile tester in accordance

with BS EN 10 002-1: 1990 (Tensile testing of metallic

materials). A commercial magnetometer, based on an eddy

current excitation in the specimen (Fischer, Feritscope), was

employed in order to measure the percentage of ferromag-

netic phase in the samples after tensile loading.

2.1. Magnetic Barkhausen noise measurements

The magnetic Barkhausen noise (MBN) was investi-

gated by using sinusoidal (10 Hz) excitation magnetic

field produced by a function generator and a power

amplifier. The applied measuring head contained a

U-shaped magnetising coil and a pick-up coil, which is

perpendicular to the surface of the specimen. The signal

of the pick-up coil was processed by a 0.3–38 kHz band

pass filter and amplified with a gain of 50. KRENZ TRB

4000 (Krenz-Eckelmann Industrieautomation GmbH,

Wiesbaden, Germany) computer controlled signal

analysing device was used for processing the noise. The

applied sampling frequency was 100 kHz and the

maximum magnetic field induction of the excitation was

74 G. With this sampling frequency, only the relatively

low frequency components of the noise (under 10 kHz)

was sampled and evaluated. The Root Mean Square

(RMS) value of the noise was determined and was used to

characterise the microstructural changes in the material.

2.2. Coercivity measurement

For the coercivity measurements, a Forster coercimeter

(Type 1.093) for precision coercivity measurement was

used. A schematic of the apparatus is given in Fig. 1.

2.3. Microhardness testing

The Vickers hardness was measured with a load of

98.1 N.

3. Results and discussion

The RMS of the MBN and volume fraction of the a0-

(Alpha Prime) martensite phase (also known as the %ferrite

content) of the samples was measured. Fig. 2 shows the

variation of the RMS value of the noise with %plastic strain

during tensile deformation. It has been observed that above

30% plastic strain, as the %ferrite content of the samples

increase, the RMS of the MBN also increases nearly linearly

showing that the MBN can used as an effective means to

determine the amount of a0-martensite content in strained

samples. Before 30% plastic strain, the MBN signals are

considered to be insignificant as the signal-to-noise ratio is

too small. This is attributed to the lower volume of ferrite in

these specimens, which results in reduced Barkhausen

activity.

The relationship between the two sensing devices can be

seen in Fig. 3. After approximately 5% ferrite content,

Fig. 1. Schematic for coercivity measurement.

Fig. 2. Dependence of RMS of magnetic Barkhausen noise and the amount of a0-martensite formed in relation to plastic strain% for AISI 304 samples.

D. O’Sullivan et al. / NDT&E International 37 (2004) 265–269266

the Barkhausen signal-to-noise ratio improves and a nearly

linear relationship is observed between the two sensing

devices.

The RMS and the Vickers hardness values were

measured. Fig. 4 shows that the hardness of the sample

increase linearly to about 30% plastic strain and then begins

to saturate. But in contrast to that, the MBN signals increase

linearly after 30% plastic strain. This would suggest that the

a0-martensite phase formed within the strained sample, does

not contribute fully to the work-hardening of these

specimens.

This saturation in hardness level could be a consequence

of a phenomena called work softening. In his papers,

Mullner [5–7] discusses this phenomenon and explains it in

terms of the dislocation reaction at twin boundaries. The

main physics is the splitting of a perfect 1/2k110l dislocation

into two Shockley partials of type 1/6k211l. In terms of a

single moving dislocation, the first (leading) partial

produces a stacking fault whereas the second (trailing)

partial closes the stacking fault. This discussion can be

directly applied to 1-martensite because twining

dislocations and transformation dislocations of 1-martensite

are identical, namely 1/6k211l Shockley dislocations. The

role of dislocation–twin interaction (dislocation–epsilon

interaction) or twin–twin interaction is to produce a group

Fig. 3. Dependence of MBN (RMS) on the %ferrite content of the AISI 304 samples.

Fig. 4. Variation in Barkhausen noise and hardness as a function of plastic strain for AISI 304 samples.

Fig. 5. Dependence of RMS of magnetic Barkhausen noise and hardness on

the temperature of heat treatment of AISI 304 samples [2,3].

D. O’Sullivan et al. / NDT&E International 37 (2004) 265–269 267

of trailing (‘untwining’) dislocations, that ‘close’ the twin

(or 1-martensite), hence softening.

To consider this in terms of a0-martensite is much more

difficult, and the dislocation reactions at martensite habit

planes would have to be considered. More research looking

into the deformation structure with transmission electron

microscopy needs to be completed.

The dislocation density rather than the a0-martensite

phase as a cause of material work-hardening has also

been shown in the past by Meszaros [2,3]. From Fig. 5,

the reverse transformation of the a0-phase back to

austenite occurs between 350 and 600 8C. But this does

not lead to the reduced hardness of the material, and this

recovery only begins after all the a0-phase has dis-

appeared. This can be explained by the precipitation of

carbides (M23C6) [4]. Microstructural examination by this

author on AISI 304 samples annealed at 700 8C shows

precipitation is evident along grain boundaries, within the

grains, especially along shear bands (slip lines) because

the nucleation rate is higher at these irregularities (Fig. 6).

The increase in carbide particles makes movement

of the dislocations more difficult, thus interfering with

the normal annealing process. This phenomenon can be

explained according to the Orowan hardening

mechanism.

The coercivity, Hc, of the deformed specimens was also

determined. Fig. 7 shows that the coercivity and the

hardness measurements are comparable. Both increase

linearly to about 30% plastic strain and then saturate. The

coercivity is seen to increase more strongly than the hardness

before saturation, which can be explained by the effect of

internal stresses that can modify the magnetic behaviour of

a0-martensite.

Coercivity is affected by material structural change.

During plastic deformation—the increase in dislocation

densities and resultant structural changes lead to increased

material coercivity. The suggested dislocation–twin inter-

actions could result in more magnetic domain wall move-

ment hence reducing the coercivity of the material at high

deformation.

Fig. 8 compares the RMS of MBN and the coercivity

signal. Work-hardening leads to two distinct products in

the affected material, the formation of a0-martensite and

increased dislocation densities. From this graph, it can be

concluded that the appearance of the a0-phase, above a

certain volume fraction, can be effectively measured

using MBN. In this case it was not possible to detect a0-

martensite below 30% plastic strain. For increased MBN

sensitivity to a0-martensite formation, it is suggested to

use a higher magnetic field strength for excitation,

which would allow for increase magnetic domain

activity, even at low levels of deformation. The increase

in dislocations with deformation can be effectively

examined using coercivity measurement at all levels

of material deformation. But, for the overall non-

destructive testing of work-hardened specimens,

Fig. 7. Dependence of hardness and coercivity on plastic strain% of AISI 304.

Fig. 6. Carbide precipitation in AISI 304 at 700 8C.

D. O’Sullivan et al. / NDT&E International 37 (2004) 265–269268

coercivity measurement is suggested to be the best

characterisation technique.

4. Conclusions

This study examines the effects of work-hardening in

plastically deformed and strained austenitic stainless steel

specimens using various non-destructive sensing tech-

niques.

† It is proved that the a0-martensite forms continuously

during plastic strain.

† It is believed that increased dislocation density has a

much stronger affect on material work-hardening than

the appearance of a0-martensite.

† MBN is an effective tool for studying the transform-

ation mechanisms of austenite to a0-martensite during

deformation but is not an effective means to

characterise the work-hardening of austenitic stainless

steel.

† For non-destructive testing of work-hardening; coer-

civity measurement is seen as the most effective

method.

Acknowledgements

The authors gratefully acknowledge the financial assist-

ance provided for this project by Enterprise Ireland under

the International Collaboration Programme.

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Fig. 8. Variation in Barkhausen noise and coercivity as a function of %plastic strain for AISI 304 samples.

D. O’Sullivan et al. / NDT&E International 37 (2004) 265–269 269