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Materials Science and Engineering A 527 (2010) 1986–1996

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l homepage: www.e lsev ier .com/ locate /msea

icrostructure evolution in nano/submicron grained AISI 301LN stainless steel

. Rajasekharaa, L.P. Karjalainenb, A. Kyröläinenc,1, P.J. Ferreiraa,∗

Materials Science and Engineering Program, The University of Texas at Austin, TX 78712, USADepartment of Mechanical Engineering, The University of Oulu, 90014 Oulu, FinlandOutokumpu Stainless Oy, 95400 Tornio, Finland

r t i c l e i n f o

rticle history:eceived 1 September 2009eceived in revised form2 November 2009ccepted 13 November 2009

a b s t r a c t

The phase and microstructure evolution of a heavily cold-rolled AISI 301LN stainless steel (SS), beforeand after annealing is discussed. AISI 301LN SS has been cold-rolled to 63% rolling reduction and subse-quently annealed from 600 to 1000 ◦C for short annealing durations (1–100 s). Phase analysis indicatesthat the cold-rolled sheet comprises almost 100% martensite, while transmission electron microscopyexamination reveals its morphology to be of dislocation cell- and heavily deformed lath-type martensite.

eywords:ransmission electron microscopyustenitic stainless steelartensite

The martensite → austenite reversion upon annealing at 600 ◦C for 1 and 10 s is negligible, but nanoscaleaustenite grains are formed in the martensitic matrix. Partial reversion to nano/submicron austenitegrains is observed for samples annealed at 600 ◦C for 100 s, and 700 ◦C for 1 s. Samples annealed at highertemperatures exhibit a complete reversion to submicron/nano-austenite grains with a large grain sizevariation, as well as secondary phase chromium nitride precipitates.

icrostructure evolutionano/sub-micron grains

. Introduction

The corrosion resistance of austenitic stainless steels (SSs)akes them desirable candidates for structural applications, such

s railcar and truck bodies. However, the lack of high strength pre-ludes their wide spread use in constructional applications [1–2].he strength in conventional austenitic SS grades may be improvedy temper rolling, where austenite (�) is strain hardened andome strain-induced martensite (�′) is formed, but the ductilityecreases. Nitrogen alloying also increases the strength by solidolution strengthening [3], but nitrogen solubility is limited in con-entional austenitic SS grades. In this regard, a route for improvinghe strength of SS, without degrading ductility, that is gaining widecceptance is to anneal a heavily cold-rolled metastable austeniticS to produce nano/submicron austenite grain structures [4–7].n understanding of this two-step process; namely cold-rolling

nd annealing, is key for the development of these high strengthSs.

So far, extensive research has been performed to understandhe origins and morphology of martensite in cold-rolled Fe–Cr–Ni

∗ Corresponding author at: Materials Science and Engineering Program, The Uni-ersity of Texas at Austin, 1 University Station, MC 2201 Room ETC 9.176, Austin,X 78712, USA. Tel.: +1 512 471 3244; fax: +1 512 471 7681.

E-mail address: ferreira@mail.utexas.edu (P.J. Ferreira).1 The author is currently with Raahe Regional Technology Center, 92100 Raahe,

inland.

921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2009.11.037

© 2009 Elsevier B.V. All rights reserved.

alloys, low-carbon steels, and specialty steels formation of marten-site during cold-rolling of austenitic SS. Olson et al. [8] andTamura [9] discussed a phenomenological model to explain strain-induced martensite formation due to cold-rolling, while severalresearch groups demonstrated that the martensite morphologyin cold-rolled low-carbon steel, specialty steels, and Fe–Cr–Nialloys comprises dislocation forests, dislocation walls and tangles,incidental dislocation boundaries, geometrically necessary disloca-tions, heavily deformed lath martensite and dislocation cell-typemartensite [6,10–17].

Researchers have also discussed the morphological and phasechanges that take place upon annealing cold-rolled Fe–Cr–Ni alloys.Smith and West [18] reported an increase in strength due to theformation of austenite with high dislocation density, Singh [19]carried out measurements to determine the �′ → � phase rever-sion temperature, Jana and Weyman [20], and Coleman and West[21] discussed the �′ → � reversion mechanism based on magneticmeasurements performed on a Fe–Cr–Ni alloy, Guy et al. [22] stud-ied the �′ → � reversion from a crystallographic perspective, whileBreedis and Krauss [23,24] reported the �′ → � reversion to resultin micro-twins and stacking faults. More recently, several papershave demonstrated the formation of ultra-fine austenite grains inmetastable austenitic Cr–Ni steels, while others demonstrated this

concept in commercial metastable austenitic AISI 301 and AISI 304SS grades [4–7,25–27]. The development of ultra-fine grain struc-tures in commercial metastable austenitic SS grades is importantbecause it opens up the possibility of using high strength austeniticSS in structural applications. In particular, considerable research

S. Rajasekhara et al. / Materials Science and Engineering A 527 (2010) 1986–1996 1987

Table 1Chemical composition (in wt%) of AISI 301LN SS used in this work.

ha3pas

sc3hCsctcoc

2

2

caaifsorhtif2as

2

l∼gatfPo

TR

C N Ni Cr Mn Si Cu Mo

301LN 0.017 0.15 6.5 17.3 1.29 0.52 0.2 0.15

as been conducted to understand phase evolution, microstructure,nd mechanical properties of heavily cold-rolled and annealed AISI01 SS [25–27]. However, it was determined that heavy Cr23C6recipitation accompanies ultra-fine grain formation in cold-rollednd annealed AISI 301 SS, rendering this SS grade susceptible toensitization issues.

To overcome this problem, the present authors have thus con-idered the commercial AISI 301LN SS grade, which is deficient inarbon but rich in austenite-stabilizing nitrogen, instead of AISI01, which is rich in carbon. The reported results show a SS withigh strength and excellent ductility [28–34], while the problem ofr23C6 precipitation may be avoided. However, an in-depth under-tanding of phase and microstructure evolution of this alloy, duringold-rolling and annealing is still lacking, which forms the basis forhis work. To our knowledge, this investigation provides the firstomprehensive analysis of the microstructure and phase changesccurring during the initial few seconds of annealing of cold-rolledommercial austenitic stainless steels.

. Experimental

.1. Materials and methods

A 165 mm thick AISI 301LN-type stainless steel slab, with theomposition shown in Table 1, was produced by continuous castingt Outokumpu Stainless Oy, Tornio, Finland. The slab was solutionnnealed at 1100 ◦C for 1 h to dissolve any secondary phase precip-tates and subsequently hot rolled to ∼1.5 mm thickness. Samplesrom this 1.5 mm thick sheet were then cold-rolled through 19 con-ecutive passes in a four-calendar rolling mill at the Research Centerf Outokumpu Stainless Oy to achieve a cold-reduction of 63%. Cold-olled samples of dimensions ∼100 mm × 5 mm × 0.8 mm wereeat-treated in a Gleeble-1500 thermo-mechanical simulator athe University of Oulu, Oulu. The samples were heated at a heat-ng rate of 200 ◦C/s and annealed at 600, 700, 800, 900 and 1000 ◦Cor 1, 10 and 100 s and forced-air-cooled at a cooling-rate of about00 ◦C/s. Subsequently, X-ray diffraction, SQUID magnetic tests,nd TEM studies were performed on cold-rolled and annealedamples.

.1.1. X-ray diffractionX-ray diffraction studies were performed on rectangu-

ar sections of cold-rolled AISI 301LN SS with dimension5 mm × 5 mm × 0.8 mm. Cut sections were polished using 600-rit silicon carbide grinding paper to remove surface oxidation

nd cleaned in acetone to remove contaminants. Subsequently,he samples were glued onto a glass slide, which ensured properocusing of X-ray beam on the sample. X-rays were generated by ahillips PW1720 X-ray diffractometer, which operated at a voltagef 40 keV, a current of 40 mA and used CuK� radiation. The X-ray

able 2elative intensities for the measured reflections in martensite and austenite.

Martensite Austenite

Bragg reflections R�′ Bragg reflections R�

1 1 0 1628 1 1 1 1196.82 0 0 251.4 2 0 0 566.72 1 1 457.95 2 2 0 326.7

Fig. 1. (a) X-ray diffraction analysis of 63% cold-rolled AISI 301LN SS, (b) satura-

tion magnetization of 63% cold-rolled AISI 301LN SS, and (c) volume percentageof austenite obtained after annealing a 63% cold-rolled AISI 301LN SS at differenttemperatures and times.

source was kept fixed and peaks corresponding to various Braggreflections were recorded by rotating the sample from 10◦ to 90◦

(2-theta) in steps of 0.1◦ with a dwell time of 4 s at each angle. Theobtained peaks were plotted as function of 2-theta and the MDI

Jade 6.5 software was used to index the peaks, as well as obtain theintensities for those peaks. Intensities corresponding to �′ and �phases were used to estimate phase fractions through a procedurefirst described by Dickson [35], and recently applied to stainless

1988 S. Rajasekhara et al. / Materials Science and Engineering A 527 (2010) 1986–1996

Fcm

stm

wpf

ig. 2. TEM images of 63% cold-rolled AISI 301LN SS: (a) region with dislocationell martensite and corresponding SADP and (b) region with heavily deformed lathartensite and corresponding SADP.

teels by Johannsen et al. [27]. According to this method, the diffrac-ion intensity obtained from the two-phase material (austenite and

artensite) is given by [35]:

C� = [1/n� ] ×∑n�

0 (In� /Rn� )∑n˛′ , (1)

C˛′ [1/n˛′ ] × 0 (In˛′ /Rn˛′ )

here C� and C˛′ are the concentrations of austenite and martensitehases, respectively, while n� and n˛′ are the numbers of reflectionsor the � and �′ phases. The relative intensities Rn,� and Rn,˛′ for

Fig. 3. TEM images of 63% cold-rolled AISI 301LN SS annealed at 600 ◦C for (a) 1 s,(b) 10 s, and (c) 100 s. Diffraction patterns corresponding to these images are shownin the image insets.

three h k l selected reflections of the � and �′ phases are shown inTable 2. Thus, by estimating the C� /C˛′ ratio and assuming that C� +

C˛′ = 1, C� and C˛′ are calculated from the following relationships:

C� = 11 + (C� /C˛′ )

, (2)

S. Rajasekhara et al. / Materials Science and Engineering A 527 (2010) 1986–1996 1989

F N SS a1

C

2

fiuapvva

5cbwfiswod0me

ig. 4. TEM images and diffraction patterns corresponding to cold-rolled AISI 301L00 s.

˛′ = C� /C˛′

1 + (C� /C˛′ ). (3)

.1.2. SQUID magnetic measurementsUsing the SQUID, a sample’s response to an applied magnetic

eld is determined, from which the saturation magnetization pernit volume of the sample is estimated. The fractional amount ofustenite (paramagnetic) versus martensite (ferromagnetic) that isresent in a sample can be obtained from this data. The results areery accurate (error of ±0.01%), since the SQUID can detect minuteariations (of the order of 1 × 10−14 T) of a sample’s response to anpplied magnetic field.

For the current work, samples of dimensionsmm × 1 mm × 0.8 mm were placed in the SQUID with theirold-rolling direction parallel to the applied magnetic fieldecause the magnetic response of samples positioned in this wayas the strongest. Samples were subjected to one applied magneticeld cycle with maximum and minimum fields of ±1.0 T. The cycletarted at 0 T, increased to 0.2 T in increments of 0.025 T, beyondhich the maximum field of 1.0 T was attained in increments

f 0.1 T. Thereafter, the applied field was decreased to 0.2 T inecrements of 0.1 T, further decreased to −0.2 T in decrements of.025 T, and finally decreased in decrements of 0.1 T to reach ainimum field of −1.0 T. A maximum and minimum of all samples

xhibited saturation magnetization at ±1.0 T. Five measurements

nnealed at 700 ◦C for (a) 1 s, (b) 10 s, (c) 100 s, and 800 ◦C for (d) 1 s, (e) 10 s, and (f)

were done at each increment of the applied field path, and theaverage data point was noted.

2.1.3. Transmission electron microscopyFor transmission electron microscopy (TEM) observations, the

annealed samples were first thinned to ∼100 �m by mechani-cal polishing. Subsequently, these samples were thinned using aStruers Tenupol-5 jet-polisher, where the sample was maintainedat a potential of ∼32 V, a current of ∼50–60 mA, and −10 ◦C. Underthese conditions, a continuous electrolyte jet composed of a mix-ture of 59 vol.% methanol, 35 vol.% m-butanol and 6 vol.% perchloricacid was used to thin the sample down to electron transparency.Two different TEMs, a JEOL 200CX and a JEOL 2010F, were usedin this work to identify (i) the martensite morphology in the cold-rolled sample, (ii) the morphology of the austenite formed afterreversion, (iii) the austenite grain size and (iv) secondary phaseprecipitates. The JEOL 200CX is ideal for viewing large areas at lowmagnification, whereas the JEOL 2010F is ideal for studying themicrostructure at high resolutions. In both microscopes, the mag-nification was calibrated using a Mag*I*cal® sample—an ion-milledsingle crystal silicon containing epitaxially grown SiGe interlayers

of known widths.

2.1.4. Grain size measurementsImage J software (National Institute of Health, Bethesda, MD)

was used on the scanned TEM negatives to estimate the austenite

1990 S. Rajasekhara et al. / Materials Science and Engineering A 527 (2010) 1986–1996

900 ◦

g1wpage(

atcnuoitb

3

3

3ia

Fig. 5. TEM images of 63% cold-rolled AISI 301LN SS samples annealed at

rain size of annealed samples. A statistically significant sample of00 grains was located in the scanned images and measurementsere taken along the longest direction within the grain and its per-endicular, from which the mean was calculated and used as theverage grain size d̄ for a particular sample. The nano/submicronrains (30–200 nm) measured through this technique exhibit anrror of approximately 10%, while the larger austenite grains>1 �m) have a reduced error of approximately 4%.

The more typical ASTM E112 standard [36] to determine aver-ge grain size was not used in this work. As per this standard,hree concentric rings of fixed radii are superimposed on the imagesontaining the grain size data, by which the numbers of grain cor-ers that intersect the rings are determined. This number is thentilized to determine the average grain size. In our work, it wasften observed that the nano/submicron grains present in the TEMmages did not intersect the superimposed concentric rings, andhus their influence on the average grain size and grain size distri-ution could not be captured.

. Results

.1. X-ray diffraction

The X-ray diffraction spectrum of the 63% cold-rolled (CR) AISI01LN sample is shown in Fig. 1a. The sample consists of primar-

ly martensitic (�′) with strong (2 1 1), (1 1 0) and (2 0 0) �′ peaksnd a weak (2 2 0) austenite (�) peak, which may be due to the

C for (a) 1 s, (b) 10 s, (c) 100 s and 1000 ◦C for (d) 1 s, (e) 10 s and (f) 100 s.

retained austenite that failed to convert to martensite upon cold-rolling deformation. Knowing the peak intensities obtained fromthis X-ray diffraction and employing the procedure described inSection 3.2, the volume percentage of martensite in the CR samplewas approximately 95.7%.

3.2. Magnetic measurements

Using the method described in Section 2.1.2, the satu-ration magnetization of the CR sample with dimensions of∼5 mm × 1 mm × 0.8 mm was determined to be ∼ 903 emu/cm3

(Fig. 1b), which corresponds to 95.7% of martensite, as discussedin the preceding section. Thus, assuming a linear behavior, thesaturation magnetization for a sample containing 100% marten-site is determined to be approximately 944 emu/cm3. This valueis set as standard and designated, mS�′ against which the satu-ration magnetization of all subsequent annealed samples (m�′ )is normalized. Knowing m�′ and mS�′ values, the volume frac-tion of martensite in all annealed samples is obtained through theexpression:

V˛′ =[

m˛′

m ′

]× 100, (4)

S˛

where V˛′ is the volume percentage of martensite in the sam-ple. Assuming that the only paramagnetic phase present in thesample is austenite, the volume percentage of austenite may becalculated by subtracting V˛′ from 100%. The results for all the

S. Rajasekhara et al. / Materials Science and Engineering A 527 (2010) 1986–1996 1991

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microstructure of the CR sample. In fact, these annealing conditions

ig. 6. Grain size measurements for AISI 301LN SS samples as a function of time andemperature: (a) annealed at 600, 700, 800, 900 and 1000 ◦C, (b) enlarged view ofhe samples annealed at 600, 700 and 800 ◦C.

nnealed samples are shown as a function of temperature inig. 1c. At 600 ◦C, hardly any reversion to austenite occurs foramples annealed for 1 and 10 s. However, partial reversion toustenite is shown in samples annealed at 600 ◦C for 100 s and00 ◦C for 1 s. At higher annealing temperatures and longer anneal-

ng durations, almost complete reversion to austenite is observedFig. 1c).

.3. Transmission electron microscopy

The 63% cold-rolled (CR) AISI 301LN SS sample prior to anneal-ng shows regions of dislocation cell-type martensite and lath-type

artensite, which are characterized by the presence of disloca-ion forests and ultra-fine martensite [4,7,13,16,17], and confirmedy the ring-like selected area diffraction pattern (SADP) (Fig. 2a),hile lath-type martensite regions are confirmed by the spot-like

ADP (Fig. 2b). A detailed discussion of the cold-rolled structure isresented in Section 4.1.

Samples annealed at 600 ◦C for 1 and 10 s exhibit dislocation

ell-type and lath-type martensite, which is confirmed by the ring-ike and spot-like patterns (Fig. 3a and b). Martensitic regionsre bounded by dislocation walls and forests (Fig. 3b). Diffrac-ion streaks and spots corresponding to austenite (for instance

Fig. 7. Normalized grain size in AISI 301LN SS samples annealed at 600, 700, 800,900 and 1000 ◦C for 1, 10 and 100 s. �d = dmax − dmin.

(1 1 3)� in Fig. 3a, and (2 2 0)� in Fig. 3b) are also observed, whichmay indicate the formation of new austenite grains within themartensite matrix. Nevertheless, the overall microstructure of thesamples annealed at 600 ◦C for 1 and 10 s still resembles the overall

are still insufficient to significantly drive the �′ → � rever-sion. When the annealing duration is increased to 100 s, evenlydistributed equiaxed nano/submicron grains of austenite withinthe martensitic matrix are visible, which is confirmed by the

1992 S. Rajasekhara et al. / Materials Science and Engineering A 527 (2010) 1986–1996

F ite anp

at31

efidamisso1pi

ig. 8. TEM image of 63% cold-rolled AISI 301LN SS where regions of lath martensatterns obtained from each of the regions.

ustenitic ring-like reflections (Fig. 3c). Combining this result withhe phase fraction analysis (Fig. 1c), it is clear that cold-rolled AISI01LN SS partially reverts to austenite when annealed at 600 ◦C for00 s.

Samples annealed at a higher temperature of 700 ◦C for 1 sxhibit regions with equiaxed nano/submicron grains as well asne martensite, which is confirmed by the presence of ring-likeiffraction patterns from both phases (Fig. 4a). This morphologylong with the phase fraction results reveals partial reversion fromartensite to austenite (∼35% austenite formation) (Fig. 1b). TEM

mages of samples annealed for longer annealing times at 700 ◦Chow the formation of larger grains (Fig. 4b and c). The corre-

ponding SADP changes from being almost ring-like (in the casef 1 s anneal: Fig. 4a) to a more spot-like pattern for the 10 and00 s annealing (Fig. 4b and c) confirming this tendency. The sam-le annealed at 10 s shows the presence of a shear band, which

s likely to be an austenitic shear band, inherited from the cold-

d austenite shear bands can be observed. The insets correspond to the diffraction

rolled sample. For both 10 and 100 s annealing, there is a significantdistribution of grain sizes.

Relatively defect-free and equiaxed austenitic grains areobserved in samples annealed at 800 ◦C for all the three anneal-ing durations, which are also confirmed by corresponding SADPswhere diffraction spots and intermittent rings corresponding tothe austenite phase are observed (Fig. 4d–f). Samples annealedat 800 ◦C exhibit a large grain size distribution with a mixture ofnano/submicron grains and large grains, which is likely due to tworeasons: (i) presence of different types of martensite (dislocationcell-type and lath) in the cold-rolled sample leading to differentrates for austenite nucleation (as it happened at 700 ◦C) and (ii)

nucleation of austenitic grains at lower annealing temperatures,which grow in size as the annealing temperature is rapidly broughtto 800 ◦C.

Samples annealed at 900 and 1000 ◦C for all the three anneal-ing durations exhibit austenitic grain structures with apparently

S. Rajasekhara et al. / Materials Science and Engineering A 527 (2010) 1986–1996 1993

Fig. 9. (a) TEM image of 63% cold-rolled AISI 301LN SS sample annealed at 600 ◦C for 1 s, showing regions of dislocation forests and dislocation walls, (b) TEM image of samplea artensa ary php from

na

3

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sisnspgaos(

nnealed at 600 ◦C for 10 s, showing regions of dislocation forest, dislocation cell mt 600 ◦C for 100 s, showing regions of equiaxed austenitic grains, as well as secondresence of dislocations. The insets correspond to the diffraction patterns obtained

arrower grain size distribution, which rapidly grow in size as thennealing duration is increased from 1 to 100 s (Fig. 5).

.4. Grain size analysis

The average grain sizes, estimated from the procedure describedn Section 2.1.4, for all annealing treatments are shown in Fig. 6a.t was observed that while the samples annealed at 600 and 700 ◦Cardly exhibit any grain growth (Fig. 6b), samples annealed at 800,00 and 1000 ◦C show a dramatic increase in grain size as thennealing duration is increased to 100 s (Fig. 6a).

The normalized range of grain sizes present in a given annealedample (ratio of the difference between the maximum and the min-mum grain size in a sample size of 100 grains to the mean grainize) as a function of annealing conditions was also analyzed. Weote that for 1-s annealing duration, the samples at 600 and 700 ◦Chow a grain size variation that is considerably less than for sam-les annealed at 800, 900 and 1000 ◦C (Fig. 7a). Interestingly, the

rain size variation peaks at 800 ◦C and decreases with increasingnnealing temperature. At 10 s of annealing, the grain size variationf samples annealed at 600, 700, 900 and 1000 ◦C is comparable, butamples annealed at 800 ◦C exhibit the most grain size variationFig. 7b). However, when samples are annealed for longer duration

ite, lath martensite and recrystallized austenite, (c) TEM image of sample annealedase nitrides, and (d) TEM image from a different region of sample (c), showing the

each region.

of 100 s, the normalized grain size range values for all annealingtemperatures are similar (Fig. 7c).

4. Discussion

4.1. Morphology of martensite in the cold-rolled sample

As previously discussed, the CR sample exhibits regions oflath-type martensite, austenite shear bands and dislocation cellmartensite. A more detailed observation of the CR AISI 301LN SSsample is shown in Fig. 8, which shows the presence of elongatedgrains bounded by shear bands. Diffraction patterns, using a 2 �maperture, were acquired from these areas. The spot-like diffrac-tion pattern due to the elongated grains is consistent with lathmartensite (Fig. 8), while faint streaks in the pattern are likelydue to austenitic platelets that did not revert to martensite duringcold-rolling. Additionally, a diffraction pattern from the shear bandreveals that it is austenitic in nature. The presence of austenitic

shear bands in cold-rolled AISI 301LN SS is not surprising. Singh[19], and Shrinivas et al. [37] discussed that austenite shear bandsform in low stacking fault energy (SFE) alloys due to enhanced pla-nar slip upon deformation. The SFE in AISI 301LN SS was calculatedto be only ∼10–15 mJ/m2 from a stacking fault model proposed by

1 e and Engineering A 527 (2010) 1986–1996

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4f

saTtas7aTavaaAtart1tkrhaTpi

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Fig. 10. (a) TEM image of 63% cold-rolled AISI 301LN SS sample annealed at 700 ◦Cfor 1 s, showing regions of secondary phase nitrides, equiaxed austenitic grains,

994 S. Rajasekhara et al. / Materials Scienc

erreira and Muellner [38] and Talonen and Hänninen [39], andhus propensity to form shear bands upon cold-rolling and prioro martensite nucleation should be relatively high in this alloy.owever, we found relatively few shear bands of the type shown

n Fig. 8 in the CR samples, which might be due to the follow-ng sequence of events during heavy cold-rolling: first cold-rollingf austenitic SS sheet produces austenitic shear bands due to thenhanced planar slip. Subsequently, as the sheet is further cold-olled, the shear bands intersect and become nucleation centers forartensite embryos [37]. Further cold-rolling produces more shear

and intersections and nucleation and growth of strain-inducedartensite. Thus, upon heavy cold-rolling we expect most of the

hear bands and shear band intersections to be consumed and onlyartensite to be present in the sample.

.2. Influence of annealing conditions on the austenite phaseraction

Samples annealed at 600 ◦C for 1 and 10 s show a relativelymall austenite phase fraction (∼6%), which is comparable to theustenite phase fraction present in the CR sample (Section 4.2).he microstructure present in these samples is also similar tohat present in the CR sample. On this basis, we infer that thesennealing conditions are not sufficient to drive the �′ → � rever-ion. However, samples annealed for 100 s at 600 ◦C, or 1 s at00 ◦C exhibit partial �′ → � reversion leading to a nano/submicronustenite and martensite microstructure (Figs. 1c, 3c and 4a).his data indicates that the �′ → � reversion should be driven bydiffusion mechanism where an activation energy barrier pre-

ents austenite nucleation at low annealing temperature and shortnnealing durations, but higher annealing temperatures or longernnealing durations result in austenite nucleation and growth.

higher annealing temperature provides more thermal energyo overcome the activation energy for nucleation, while longernnealing durations promote diffusion driven kinetics allowing theeversion process to proceed towards completion [41]. Evidence ofhis concept is clearly visible in samples annealed at 700 ◦C, wheres of annealing may be just sufficient to overcome the activa-

ion energy barrier for austenite nucleation, while diffusion driveninetics at longer annealing durations of 10 and 100 s take �′ → �eversion to completion (Fig. 7c). As expected, samples annealed atigher annealing temperatures of 800, 900 and 1000 ◦C all showlmost complete reversion to austenite phase (∼93% austenite).he retained martensite in these samples is thought to be tem-ered martensite and the reason for its presence will be discussed

n greater detail in Section 4.4.

.3. Morphology of reverted austenite

The microstructures shown in Fig. 9a and b are typical of sam-les annealed at 600 ◦C for 1 and 10 s. Although there is a negligibleeversion to austenite, pockets of nanoscale austenite grains withinmartensitic matrix can be observed (Fig. 9a and b), which are

onfirmed by the respective SADPs. To a great extent, the sam-le retains the morphology of the 63% cold-rolled steel with largeegions of dislocation cell martensite (Fig. 9a). Keeping in mindhat the recrystallization rate is much lower than that of the �′ → �eversion [42], the nanoscale austenitic regions shown in Fig. 9a andare likely to be due to newly nucleated austenite grains. Accord-

ngly, after a sufficient incubation period, the samples annealedt 600 ◦C for 100 s show a relatively different morphology where

egions of equiaxed austenitic grains, secondary phase precipitatesFig. 9c), and regions of dislocations (Fig. 9d) are clearly visible. Weetermined the secondary phase particles to be chromium nitrideCrN) precipitates (Fig. 10). Precipitation of fcc CrN nitride at lowemperatures (600 and 700 ◦C) and short annealing (1, 10, 100 s)

martensite (SADP in the inset), as well as dislocations, (b) TEM image of 63% cold-rolled AISI 301LN SS sample annealed at 800 ◦C for 1 s, showing regions of secondaryphase nitrides, equiaxed austenitic grains, martensite (SADP in the inset), as well asdislocations.

time have not been reported before this work [40]. The existingliterature has reported that in high nitrogen steels (>0.1 wt% N)the formation of stable hcp Cr2N chromium nitrides takes place atannealing temperatures of 800 ◦C and above, and only after 600 sannealing [43–50]. Previous investigations, carried out by Machadoand Padilha [49] and Padilha and Rios [50], have shown that CrNnitrides are unstable in comparison to Cr2N precipitates. It can beargued that the short annealing used in this work may initially leadto the precipitation of fcc CrN nitrides, which revert to more sta-ble hcp Cr2N nitrides upon prolonged annealing. Finally, samplesannealed at 700 and 800 ◦C for 1 s exhibit a microstructure con-sisting of secondary phase nitrides, equiaxed austenitic grains, andregions of retained martensite (Fig. 11a and b).

4.4. Influence of annealing conditions on grain size variation

As reported earlier, samples annealed at 600 and 700 ◦C forall annealing durations show a narrow grain size variation, whileannealing at higher temperatures results in a broad variation in

grain sizes. The presence of different kinds of nucleation sites withdifferent morphologies, such as dislocation cell martensite, dislo-cation walls, and lath-type martensite seems to be responsible forsuch variation. Johannsen et al. [27] have shown that the nucleationof austenite on dislocation cell-type martensite is easier to occur

S. Rajasekhara et al. / Materials Science and

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ticas

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ig. 11. (a) Secondary phase precipitates observed in AISI 301LN SS annealed at00 ◦C for 100 s, and (b) electron nano-diffraction analysis of the precipitates shown

n (a) reveal the presence of CrN nitrides with a face-centered cubic structure and aattice parameter of 4.14 Å.

han on lath-type martensite. Thus, for a fixed annealing time dur-ng the �′ → � reversion, the austenite nucleates first on dislocationell martensite, followed by grain growth, whereas austenite nucle-tion on deformed lath-type martensite takes place later leading tomaller grains.

Moreover, according to the research done by Hansen [15], Uejit al. [16], and Tsuji et al. [17], the deformed lath-type marten-ite boundaries comprise incidental dislocation boundaries (IDBs)nd geometrically necessary boundaries (GNBs) and ultra-finerains preferentially nucleate at the GNBs. In the context of theesults shown herein, it can be proposed that upon annealing theeavily cold-rolled AISI 301LN SS, austenite nucleates on the dis-

ocation cell-type martensite. Once these nucleation centers areonsumed, the austenite grains preferentially nucleate at GNBsresent within the deformed lath-type martensite. The sequentialature of austenite nucleation and growth is reflected in the wide

◦

rain size variation in samples annealed at 800, 900, and 1000 C,ince the smallest austenite grains measured correspond to theustenite that nucleated last (Fig. 7a). However, annealing at longerurations allows the smaller grains to grow thus reducing the grainize variation (Fig. 7c).

Engineering A 527 (2010) 1986–1996 1995

Since extremely short annealing durations are studied in thiswork, we believe that 100 s of annealing may not be sufficient tocompletely consume the IDBs present in the deformed lath marten-site, which then leads to tempered martensite in the annealedsamples. Detailed observations of the microstructures of the sam-ples annealed at 700 and 800 ◦C confirm the presence of smallpockets of martensite, which is presumably tempered martensite(Fig. 11). In fact, Tomimura et al. [4] and Takaki et al. [6] have foundpockets of tempered martensite present at austenite grain triplejunctions in thermo-mechanically treated steels.

4.5. Mechanism for the martensite → austenite reversion

From the analysis of the microstructure, the morphology ofthe phases present and the phase fraction of all annealed AISI301LN SS samples (Fig. 3), we can infer that the �′ → � rever-sion is characterized by: (i) a wide annealing temperature range(600–1000 ◦C) where the reversion occurs, (ii) the formation ofdefect-free equiaxed austenitic grains which grow in size withtime, (iii) a sequential use of various types of martensite nucleationsites, leading to a wide grain size distribution, and (iv) forma-tion of secondary phase precipitates. These features are typicalof a diffusion-type reversion mechanism, in contrast to a shear-type reversion mechanism where: (i) reversion occurs over a smalltemperature range of ∼50 K [4,6,42], (ii) nucleation is time indepen-dent, (iii) the formation of defect-laden austenite grains retainingthe parent martensite morphology occurs [4,6,27,42], and (iv)absence of austenite grain growth is observed [4,6]. Accordingly,a diffusion-type reversion occurs in AISI 301LN SS, as concludedand demonstrated earlier [28,30].

5. Conclusions

Based on the examination of the microstructure and phase evo-lution in heavily cold-rolled and annealed AISI 301LN SS it can beconcluded that:

1. Cold-rolled samples consist of regions of dislocation cell-typemartensite, which are characterized by the presence of disloca-tion walls and forests, and regions of heavily deformed lath-typemartensite.

2. Diffusion driven �′ → � reversion kinetics is responsible for par-tial �′ → � reversion to nano/submicron austenite (∼200 nm) forsamples annealed at 600 ◦C for 100 s and 700 ◦C for 1 s, whilecomplete �′ → � reversion to submicron to ultra-fine grains insamples annealed at higher temperatures.

3. Austenite nucleation from martensite is sequential where dis-location cell-type martensite is consumed first, followed byaustenite nucleation on geometrically necessary grain bound-aries of heavily deformed lath-type martensite. This sequence ofevents is responsible for a narrow austenite grain size variationobtained at lower annealing temperatures and a broader grainsize variation at higher temperatures.

4. Short annealing duration of 1–100 s at 600–800 ◦C producesnanoscale (∼20 nm) fcc chromium nitride precipitates within theaustenite grains.

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