Journal of Magnetism and Magnetic Materials 330 (2013) 147–151

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Thermomagnetic characterization in AISI 316L austenitic stainless steelfibers with various true strains

Shun-Tung Yang a, Tien-Wei Shyr b, Weng-Sing Hwang a,n, Cheng-Nan Ko c

a Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan, R.O.C.b Department of Fiber and Composite Materials, Feng Chia University, Taichung 40724, Taiwan, R.O.C.c Department of Operations Management, Kang Ning University, Tainan 70970, Taiwan, R.O.C.

a r t i c l e i n f o

Article history:

Received 2 May 2012

Received in revised form

31 October 2012Available online 16 November 2012

Keywords:

Austenitic stainless steel

Thermomagnetic analysis

Phase transformation

53/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jmmm.2012.11.008

esponding author. Tel.: þ886 6 275 7575; fa

ail address: wshwang@mail.ncku.edu.tw (W.-

a b s t r a c t

This paper investigated the effect of temperature on the magnetic properties of AISI 316L stainless steel

wire drawn to micron-size fibers with various true strains using thermomagnetic analysis (TMA). The

volume fraction of martensite induced by deformation was determined using a superconducting

quantum interference device and a Ferritescope. During the heating process, the content of a0martensite in the samples with a true strain of above 2.31 increased at around 460 1C. A shoulder of

reverse transformation from a0 to g was observed at around 625 1C in the TMA heating curves for

samples with a true strain above 1.61. The shoulder shifted slightly to a higher temperature as the true

strain increased. The relatively small increase in magnetization above approximately 760 1C was

related to the ferrite content, as confirmed by electron probe microanalysis-wavelength-dispersive

spectrometry. During the cooling process, the Ms temperature decreased as the true strain of fibers

increased.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Stainless steel can be drawn into fibers with a diameter below10 mm using the bundle-drawing process [1]. Stainless steel fibersare extensively applied in the textile industry. Micron-sized stainlesssteel fibers have an inherent electric conductivity, heat resistance,and corrosion resistance, while possessing a softness and flexibilitythat are similar to those of natural and synthetic textile fibers. Ingeneral, the strain-induced martensite transformation that occurs inmetastable austenitic stainless steels improves the strength whilesubstantially retaining the ductility [2,3]. A thermomechanicaltreatment that reverses strain-induced martensite (a0) to ultra-fine-grain austenite (g) after heavily cold working metastableaustenitic stainless steels was proposed by Tomimura et al. [4]. Thisapproach has been used to produce grains on a nano/submicronscale in austenitic stainless steels [5–8].

To precisely control the grain size of austenitic stainless steelduring the reversion process, the a0-g transformation tempera-ture, soaking time, and heating and cooling rates are importantfactors. Soaking can be performed above the reversion tempera-ture, if the time is being properly controlled [6–8]. This phasetransformation has been studied extensively by examining themagnetic and structural differences between ferromagnetic a0and paramagnetic g using saturation magnetization [4–6,8,10],

ll rights reserved.

x: þ886 6 234 4393.

S. Hwang).

magnetic balance [11,12], a Ferritescope [6,9,13], and X-rayanalysis [5,6,8,10]. However, the volume fraction identificationof these phases was generally analyzed at room temperature afterthe materials had been annealed at a fixed temperature. It isdifficult to determine the exact temperature of the phase reversalbecause the a0 and g content levels are functions of the annealingtemperature [4–13]. Moreover, the in situ phase transformationduring the cooling process cannot be observed [4,5].

Traditionally, dilatometry has been employed to investigatesolid–solid phase transformations in steels [14–16]. However, it isdifficult to measure the dimensional changes that occur in thesample during the thermal cycle since in this study the fiber is fineand soft. Thermomagnetic analysis (TMA), which can be used torecord the magnetization response as a function of the temperatureduring thermal treatment, has been employed to study the phasetransformation in some stainless steels [17–22]. TMA allows theferromagnetic and paramagnetic volume fractions to be measuredduring the heating and cooling processes. The present studydetermined the temperature at which the a0/g phase transformsin deformed AISI 316L austenitic stainless steel fibers with variousstrains during the heating and cooling processes.

2. Experimental

AISI 316L austenitic stainless steel wire with a diameter of170 mm was used as the starting material. The chemical composition

Table 1Chemical composition (wt%) of the tested AISI 316L austenitic stainless steel.

Element C Si Mn P S Ni Cr Mo Fe

wt% 0.024 0.381 1.570 0.030 0.002 11.899 17.864 2.011 Balance

0 1 2 3 4 5 6 70

10

20

30

40

50

60

70 SQUIDFS reading

True strain

Ferr

omag

neitc

pha

se c

onte

nt (%

)

0

5

10

15

20

25

30

35

Ferritescope reading

Fig. 1. Ferromagnetic phase content and the Ferritescope reading as functions of

the true strain.

100 200 300 400 500 600 700 800 9000

2

4

6

8

10

12

14

16

627°C

630°C

635°C

638°C

625°C

Mag

netiz

atio

n (a

.u.)

ε = 6.16 ε = 5.38 ε = 4.02 ε = 2.97 ε = 2.31 ε = 1.61 ε = 0.80

Temperature (°C)

Fig. 2. TMA heating curves of the samples with various true strains.

The temperature of the shoulder is shown in each curve.

S.-T. Yang et al. / Journal of Magnetism and Magnetic Materials 330 (2013) 147–151148

measured by inductively coupled plasma mass spectrometry(ICP-MS) is listed in Table 1. This material has an Md30 temperatureof �103 1C, at which a 30% true strain induces 50% martensite inannealed stainless steel, as calculated using the Nohara equation[23], and an Ms temperature of �229 1C based on the Angel formula[24]. The wire was treated at 1020 1C for 10 min followed by waterquenching. Subsequently, the wire was drawn at room temperatureby a multi-pass bundle drawing process to obtain fibers that werereduced in diameter to 113.7 mm (true strain e¼0.80, reductionr¼55.3%), 75.8 mm (e¼1.61, r¼80.1%), 53.6 mm (e¼2.31, r¼90.1%),38.4 mm (e¼2.97, r¼94.9%), 22.8 mm (e¼4.02, r¼98.2%), 11.5 mm(e¼5.38, r¼99.5%), and 7.8 mm (e¼6.16, r¼99.8%), respectively.

To investigate the phase transformation between a0 and g, aTMA was carried out using a Perkin Elmer TAC 7/DX measure-ment system. Thermo-gravimetric analysis was performed usinga microbalance with a magnetic field positioned below thesample in a high-temperature oven to obtain the TMA curve. Thisallowed us to determine the relationship between magnetic forceand temperature. The analysis was performed in a temperaturerange of 50–900 1C with an applied magnetic field of 110 Oe. Thesamples were tested using a heating and cooling rate of 10 1C/minunder an argon atmosphere.

Two methods were used to determine the a0 martensite volumefraction. The saturation magnetization at room temperature wasmeasured by a Quantum Design MPMS7 superconducting quantuminterference device (SQUID) with a maximum applied magnetic fieldof 10 kOe. The values of the saturation magnetization measuredfrom the SQUID can be considered as being proportional to the a0martensite content. The a0 martensite content was calculated as thesaturation magnetization divided by an intrinsic saturation magne-tization of 127 emu/g, which corresponds to 100% a0 martensite inAISI 316L [25]. A Fischer Feritscope FMP30, which is a commerciallyavailable device for measuring the ferrite content according to themagnetic permeability of the material, was used to measure theferrite reading for comparison with the martensite volume fractionobtained from the SQUID data. The tests were repeated six times toprovide an average value. To obtain comparable results, these twotechniques were performed on the same sample. An electron probemicro analyzer (JXA-8500F Fe-EPMA) equipped with a wavelength-dispersive X-ray spectrometer (WDS) was used to investigate themorphology and composition of the phases. The samples with truestrains of 2.31 and 5.38 were heated to 800 1C and 900 1C respec-tively, at a rate of 10 1C/min, which was immediately followed bywater quenching in order to observe the precipitated phases.Metallographic specimens were mounted in resin and ground using2500 grit emery paper followed by polishing with alumina powder(0.05 mm), in preparation for microscopic analysis.

3. Results and discussion

The a0 martensite content and the Ferritescope readings asfunctions of true strain are shown in Fig. 1. The stimulation of thea0 formation at lower deformation (1.61 and 2.31 true strains) andthe suppression of the a0 formation at the middle deformation(2.97 and 4.02 true strains) [26] are shown in the curve measuredby the SQUID. With an increase in the cold work up to 6.16 truestrain, the volume fraction of a0 calculated by SQUID was 68.9%.There seems to be a close relationship between the Ferritescope

readings and the a0-martensite contents measured by the SQUID.The linear fitting gives the equation of Ca0(mass%)¼2.079� F

with R2¼0.967. The correlation factor differs slightly from the

factor determined by Talonen et al. [27].In a previous study, with the temperature in the range of 25–

460 1C, an increase in magnetization was observed for austeniticstainless steel fibers after the cold-drawing process [21]. Fig. 2shows the TMA heating curves obtained from samples withvarious true strains. It is evident that the magnetization doesnot change below 460 1C for samples with true strains of 0.80,1.61, and 2.31. However, for the samples with true strains ofabove 2.31 the magnetization increases and reaches its maximumbefore 460 1C (see Fig. 2). The magnetization significantlyincreases with the increasing level of deformation. The criticalthreshold of the true strain for phase transformation is between2.31 and 2.97. Talonen [3] found that the percolation threshold ofa0-martensite phase at a strain rate of 3�10�4 s�1 was about30% irrespective of the chemical composition or deformationtemperature. Above the critical fraction, the percolating clustersof the a0 martensite phase formed and extended throughout thewhole body, contributing to the work-hardening rate andstrength. The percolation threshold of the a0 martensite phaseshifted to a lower volume fraction with the increase in strainrate [3]. In the present study, the volume fractions of the

S.-T. Yang et al. / Journal of Magnetism and Magnetic Materials 330 (2013) 147–151 149

a0-martensite were 15% and 20% for samples drawn to a truestrain of 2.31 and 2.97 at a strain rate of about 50 s�1, respec-tively. Below the percolation threshold, the harder a0 martensiteparticles are not plastically deformed, but the softer austenitephase experiences plastic deformation, generating prismatic dis-location loops in the austenite matrix [3]. When the temperatureincreases, these prismatic dislocation loops can disappear, and thestress will relax in the g matrix. This leads to a negligible stress

750 800 850 9000.0

0.1

0.2

0.3

0.4

0.5

Mag

netiz

atio

n (a

.u.)

ε = 6.16ε = 5.38ε = 4.02ε = 2.97ε = 2.31ε = 1.61ε = 0.80

Temperature (°C)

Fig. 3. Magnetic response of the samples with various true strains in the TMA

heating curves in the range of 730–900 1C.

σ

Fig. 4. Back-scattered electron images of a 316L stainless steel fiber (e¼2.31) heated to

σ (A)

α (C)

γ (B)

Fig. 5. Higher-magnification view of back-scattered electron images of 316L stainle

immediately followed by water quenching.

gradient acting on the a0/g interfaces. Consequently, no increasingmagnetization is noticeable during the heating period. However,the a0 martensite phase is plastically co-deformed with theaustenite matrix and generates dislocation loops at the vicinityof the interface between both phases at a volume fraction overthe threshold value. According to the work of Gauzzi et al. [28],the stresses were more relaxed in martensite, when a cold-rolledAISI 304 steel containing 94% a0 was heated to 300–400 1C.Therefore, a net stress gradient will act on the a0/g interfaceand force it to move towards the phase where the stress is lower,i.e., towards the austenitic phase. In this work a marked incre-ment in magnetization was observed prior to the reverse trans-formation of a0-g at the above 2.31 true strain. This suggestedthat a net gradient of stress acted on the a0/g interface, whichresulted in an increase of the a0 phase.

The mechanisms of reverse transformation from a0 to g inregions with various temperatures in hard drawn and lightlydrawn austenitic stainless steel fibers have been discussed andillustrated in the literature [22]. As shown in Fig. 2, the startingtemperature for the magnetization to decrease is between 460 1Cand 470 1C irrespective of the true strain. A shoulder of reversetransformation from a0 to g appears in the heating curve around625 1C for samples with a true strain above 1.61. According to thecalculation of the modified WGa�g equation proposed by Kauf-man et al. [29], the starting transformation temperature will shiftto a higher temperature if the Nieq value [30] of the phase islower. It was confirmed by electron microprobe analyzer that theNieq value was lower for the reformed a0-phase than for theexisting one due to the 10.7% Ni which resulted in the Nieq

decreasing its value [22]. In this work the shoulder shifts slightlyto a higher temperature with the increase in true strain. We

σ

α

(a) 800 1C and (b) 900 1C at 10 1C/min immediately followed by water quenching.

α

σ

ss steel fibers with (a) e¼2.31 and (b) e¼5.38 heated to 900 1C at 10 1C/min

S.-T. Yang et al. / Journal of Magnetism and Magnetic Materials 330 (2013) 147–151150

therefore argue that this shift could be caused by a lower value ofthe Nieq in the reformed martensite phase for a sample at a higherstrain, as a result of the higher dislocation density and the effectof pipe diffusion [31].

Fig. 3 shows a magnification of the magnetic response in theTMA heating curves in the range of 730–900 1C. It shows acomplete austenite transformation and an increasing magnetiza-tion after reversion transformation can be observed. The tem-perature at which the a0-g transformation ends decreases with aconcurrent increase in deformation, except for the true strain of0.80. Above 760 1C, there is a relatively small increase in magnet-ism response with the increase in temperature. There is anincrease in the ferromagnetic response and a decrease in thestarting temperature of the re-magnetization with an increasingtrue strain. Fig. 4 (a) and (b) shows the back-scattered electronimages (BEI) from a scanning electron microscope of a 316L fiber

Table 2The normalized chemical composition (wt%) of the sigma (s), austenite (g), and

ferrite (a) phases, which are marked in Fig. 5(a).

Position Phase wt%

Fe Cr Ni Mo Mn Si

A s 59.8 25.6 5.4 7.1 1.6 0.5

B g 68.1 17.0 10.6 2.0 2.0 0.3

C a 67.1 17.5 11.0 2.1 2.0 0.3

100 200 300 400 500 600 700 800 9000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

98°C

223°C

297°C

Mag

netiz

atio

n (a

.u.)

Temperature (°C)

ε = 5.38ε = 2.31ε = 0.80

Fig. 6. TMA cooling curves of samples deformed with various true strains. The Ms

temperature is shown in each curve.

Fig. 7. Back-scattered electron images of 316L stainless steel fibers with (a) e�2.31 an

immediate water quenching.

with a true strain of 2.31 heated to 800 and 900 1C at 10 1C/minimmediately followed by water quenching respectively. Fig. 5(a) shows a higher-magnification image of Fig. 4 (b), where thechemical compositions of sigma (s), austenite (g), and ferrite (a)phases given in Table 2 were determined by EPMA-WDS. The sphase (in the form of white spots) that precipitated on thestainless steel fiber is below 200 nm for the specimen treated at800 1C. The s phase grew when the specimen was heated to900 1C (see Fig. 4). The a-ferrite phase content at 900 1C washigher than at 800 1C. Therefore, the small re-magnetization after800 1C can be attributed to the partial transformation of the non-ferromagnetic austenite matrix to ferromagnetic ferrite by adiffusive nucleation and growth process [32]. The early startingtemperature of re-magnetization in the higher strain fiber resultsfrom a faster g-a transformation.

The magnetization of the samples with different true strainsduring the cooling period as a function of temperature is shown inFig. 6. It is worth noting that the value of magnetization for eachsample increases slowly with the decrease in temperature andthen rises abruptly at a low temperature. Several studies found anincreasing martensite content after annealing [4,5,26,28,33,34],which was attributed to a locally raised Ms above room tempera-ture due to the depletion zones of the alloying elements inaustenitic stainless steels. In general, there is a so-called sensiti-zation zone at 500–800 1C in austenitic stainless steel during thecooling process [35]. The time required for the precipitation ofchromium-rich carbides and other intermetallic phases can beprolonged by reducing the carbon content to below 0.03% in 316Lstainless steel [35]. However, increasing the prior deformation[36–39] and reducing the grain size [38,39] can accelerate therate of sensitization and lower the sensitization temperature.Compared to the 24 h required for a pure austenitic sample, thetime required to produce sensitization is just 0.6 min for 304Lstainless steel containing 55% a0 [40]. In this study, the heatingand cooling rates used in the TMA measurements were 10 1C/min,and as a result the elapsed time of the sensitization temperaturezone during the thermal cycle was about 1 h. More s phase (inthe form of white spots) precipitated in the fibers for 2.31 and5.38 true strains at 700 1C during the cooling period (see Fig. 7).Therefore, the inflection point in the TMA cooling curves can beconsidered the Ms temperature.

In Fig. 6, the Ms temperature of the samples decreased with theincreasing true strain. A likely explanation for this tendency isthat the grain refinement results in g stabilization, leading to alower Ms temperature [4]. For metastable austenitic stainlesssteel, the grain size after annealing decreases with the increasein prior deformation [4,5]. Consequently, upon cooling, smallergrains cause the thermally induced martensite to transform at alower temperature.

d (b) e�5.38 heated to 900 1C and then cooled to 700 1C at 10 1C/min followed by

S.-T. Yang et al. / Journal of Magnetism and Magnetic Materials 330 (2013) 147–151 151

4. Summary

The AISI 316L stainless steel wire in this study underwentvarious deformations to produce micron-size fibers. The magneticevolution of the material with various true strains during athermal cycle of 900 1C was studied in thermomagnetic balance.During the heating process, the magnetization increased ataround 460 1C for samples with a true strain of above 2.31. Theincrease in magnetization with the increase in true strain was dueto the formation of the a0-phase, which resulted from consider-ably non-equilibrium conditions at the a0/g interfaces. A shoulderof reverse transformation from a0 to g was found at around 625 1Cfor samples with a true strain above 1.61. The shoulder shifted toa slightly higher temperature as the true strain increased due to alower Nieq value in the reformed martensite phase of the sample.We found a re-magnetization of the samples beyond �760 1C,which was attributed to the transformation of g-a.

During the cooling stage, the Ms temperature decreaseddistinctly with increasing true strain. The effect of true strain onMs temperature is significant.

Acknowledgments

The authors are grateful to the National Science Council ofTaiwan for financially supporting this research under grants NSC97–2221-E-006-006-MY3 and NSC 98–2221-E-006–066-MY3.The authors also would like to thank the Top-Tier Universityand Elite Research Center Development Plan (D101-23012) ofTaiwan supporting this work.

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