Influence of Annealing Treatment on the Formation of Nano/Submicron Grain Size AISI 301 ...

14 Pages·2006·0.67 MB·English

Influence of Annealing Treatment on the Formation
of Nano/Submicron Grain Size AISI 301 Austenitic
Stainless Steels
D.L. JOHANNSEN, A. KYROLAINEN, and P.J. FERREIRA
Nano/submicronausteniticstainlesssteelshaveattractedincreasingattentionoverthepastfewyears
duetofinestructuralcontrolfortailoringengineeringproperties.Atthenano/submicrongrainscales,
grainboundarystrengtheningcanbesignificant,whileductilityremainsattractive.Toachieveanano/
submicrongrainsize,metastableausteniticstainless steelsare heavilycold-worked,andannealedto
convert the deformation-induced martensite formed during cold rolling into austenite. The amount
of reverted austenite is a function of annealing temperature. In this work, an AISI 301 metastable
austeniticstainlesssteelis90pctcold-rolledandsubsequentlyannealedattemperaturesvaryingfrom
600°C to900 °C for adwellingtime of30 minutes. The effects ofannealing onthe microstructure,
average austenite grain size, martensite-to-austenite ratio, and carbide formation are determined.
Analysis of the as-cold-rolled microstructure reveals that a 90 pct cold reduction produces a combi-
nation of lath type and dislocation cell-type martensitic structure. For the annealed samples, the
average austenite grain size increases from 0.28 mm at 600 °C to 5.85 mm at 900 °C. On the other
hand,theamountofrevertedausteniteexhibitsamaximumat750°C,whereaustenitegrainswithan
averagegrainsizeof1.7mmcomposeapproximately95pctofthemicrostructure.Annealingtemper-
aturesabove750°Cshowanincreaseintheamountofmartensite.Uponannealing,(Fe,Cr,Mo) C
23 6
carbides form within the grains and at the grain boundaries.
I. INTRODUCTION teniticSS.Inthisfashion,theundeformedregionsmaintain
A the high strength and hardness of the martensite phase,
USTENITIC stainless steels (SS) are frequently
whereas the deformed regions acquire the high strength
selected in applications where good corrosion properties
and hardness associated with the martensite phase. How-
and aesthetic considerations are important. However, in
ever, due to the poor formability of the martensitic struc-
cases where an austenitic SS sheet needs to be subjected to
ture, products manufactured in this fashion may fracture.
coldforming,suchasdeep-drawingorstamping,itexhibits
To overcome the aforementioned limitations, a research
alargevariationinyieldstrength(above50 pct)andunde-
planhasbeendesignedtodevelop,incold-rolledSSsheets,
sirableyieldstrengthinregionsabsentofdeformation.The
regions(10to20mm)withnano/submicronaustenitegrain
main cause of this variation is related to the cold-working
size, exhibiting high strength and high formability proper-
process,duringwhichthedeformedregionstransformfrom
ties, while the sheet remains in the martensite phase, out-
thesoftparentface-centered-cubic(fcc)austenite(g)phase
side the narrow regions. Using this concept, a single SS
into the hard martensite (a9) phase. In low-carbon auste-
sheetcan beproducedsuchthatundeformedregionsmain-
nitic SS, the tetragonality of the martensite is basically
tain the strength and hardness of the martensite, while
absent, and thus the martensite phase can be considered
deformed regions have enhanced strength and hardness
as exhibiting a body-centered-cubic (bcc) structure.
duetotherefinedaustenitegrainstructureproducedwithin
Afeasiblesolutiontoresolvethevariationinyieldstrength
the narrow regions (Figure 1). In particular, the idea con-
associated with the cold-working process has not been
sistsofheavilycold-rollinganAISI301austeniticSSsheet
achieved,mainlybecauseintheformingprocesses,deforma- (;90pctreductioninthickness)toinducetheformationof
tionandhardmartensiteformationoccurinareaswherehigh
martensite(Figure1,step1),followedbyalocalannealing
strengthisnotrequired,whileregionswherehighstrengthis treatmentwithinthe600°Cto900°Ctemperaturerangeto
necessaryarenotsubjectedtodeformation,leavingsoftaus-
nucleate and reform the austenite phase in narrow specific
teniticregions. In these soft areasthematerial can be easily
regions (Figure 1, step 2) before subjecting the samples to
scratched,dented,anddeformedinservice,andthusitscor-
coldforming(Figure1,step3).Grainsizesachievedbythis
rosion properties and appearance strongly deteriorate.
methodarebelow1mm.[1–6]Inthismanner,itispossibleto
A tempting solution to this problem is to use a fully
locally control the nano/submicron grain size and conse-
martensiticSS for theformingoperationinsteadofanaus-
quently the strength of the material before forming.
Within the wider scope of the aforementioned idea, the
D.L. JOHANNSEN (now D.L. Rittermann), formerly a Graduate
initialresearchobjective,describedinthispaper,istoiden-
ResearchAssistantintheMaterialsScienceandEngineeringProgramat
the University of Texas at Austin, Austin, TX 78712,is now withIntel, tify, during step 2, the influence of annealing temperature
Oregon.A.KYROLAINEN,SeniorResearcher,iswithResearchLabora- on the microstructure of a metastable austenitic AISI 301
tories, Outokumpu Stainless Steel Oy, Tornio, Finland. P.J. FERREIRA, SS. Typical factors affecting the reversion from martensite
Assistant Professor, is with the Materials Science and Engineering Pro-
to austenite are the chemical composition, amount of cold
gram, Universityof Texas atAustin, Austin,TX78712.Contact e-mail:
working,annealingtemperature,anddwellingtime.Although
[email protected]
ManuscriptsubmittedOctober29,2005. previous experiments on Fe-Cr-Ni steels and plain carbon
METALLURGICALANDMATERIALSTRANSACTIONSA VOLUME37A,AUGUST2006—2325
Table I. Chemical Compositionof AISI301Stainless Steel
(WeightPercent)
C Cr Ni N Mn Si Mo
0.096 16.7 6.6 0.0635 1.23 1.18 0.72
P Cu Co S D Y Fe
0.26 0.17 0.11 0.001 0.0031 0.005 Balance
perature of 18 °C[8,9] and a calculated M temperature of
S
–118 °C.[10,11] The material was produced by continuous
casting and subjected to hot rolling, heat treatment, and a
final 90 pct cold-rolling reduction at Outokumpu Stainless
Oy.Theamountofretainedd-ferritewasfoundtobeappro-
ximately 0.2 vol pct. As-received 90 pct cold-rolled sheets
werecutintorectangularspecimens,subsequentlysubjected
to a heating rate of 100 °C/min, and annealed isothermally
for30minutesatthetemperatures600°C,650°C,700°C,
750 °C, 800 °C, 850 °C, and 900 °C, followed by forced
air-cooling. For comparison, one specimen was left in the
as-cold-rolled (CR) state.
B. Methods
1. X-ray diffraction
Rectangular sections approximately 5 3 7 3 0.8 mm3
werecutwiththerollingdirection(RD)paralleltothelong
dimension.Thesesamplesweresubsequentlymeasuredina
Fig. 1—Novelmethodusedtoproducenano/submicron-grainedaustenite PhillipsPW1720X-RayDiffractometer(XRD)operatingat
withgoodformabilityproperties.
avoltageof40keVandacurrentof40mA.Thetestswere
performed at ambient temperature using CuKa radiation.
steels have shown correlations between amount of cold 1
Peak measurements were taken from 30 to 90 deg (2u) in
work, annealing temperature, and dwelling time on the
steps of 0.1 deg with a dwell time of 4 seconds. From the
microstructure achieved, very limited work has been done
resulting intensity vs 2u plots, MDI Jade 6.5 software was
on commercial austenitic SS alloys containing carbon and
used to index the peaks. The results were compared to
nitrogen.[1,2,7] In addition, most of the research performed
published XRD data for an austenitic SS.[12]
so far has dealt with the effects of annealing on the rever-
Inaddition,theX-rayspectrumoftheCRspecimenwas
sionmechanisms,butastrongcorrelationbetweenthegrain
used tocalculatethevolumefraction ofausteniteand mar-
size and the microstructure achieved has not been investi-
tensite in this sample, based on the method published by
gated. In particular, a fundamental understanding of the
Dickson.[13]Accordingtothismethod,thevolumefractions
effects of the annealing temperature on the microstructure,
of austenite C and martensite C were evaluated on the
martensite-to-austenite(a9/g)ratio,austenitereversion,and g a
basis of the first three reflexions for the austenite ((111),
average grain size has not yet been accomplished.
(200),(220))andthemartensitephase((110),(200),(211)),
The goal of this paper is thus to correlate the annealing
in the form:[13]
temperature with the reverted austenite grain size, phase
(cid:2) (cid:3)
fraction,andmicrostructureachievedinacommercialAISI 1 ng I
301 SS via magnetic measurements, X-ray diffractometry, + g
C n R
and transmission electron microscopy (TEM) experiments. g 5 g 0 (cid:2) g(cid:3) [1]
A commercial AISI 301 metastable SS grade was selected Ca 1 +na Ia
because its metastable austenitic structure facilitates the n R
a 0 a
formation of stress-induced martensite and subsequently
promotes an efficient austenite reversion. In addition, AISI (cid:4) (cid:5) (cid:4) (cid:5)
301 SS grades are commercially available, thereby widen- whereCa¼1= 11ðCg=CaÞ ;Cg¼ðCg=CaÞ=11ðCg=Ca ,
ing the potential impact of this research work. naandngarethenumberofmartensiteandaustenitereflex-
ions considered, I and I are the integrated intensities of
a g
martensite and austenite for the reflexions considered, and
II. EXPERIMENTAL PROCEDURE R and R are the relative intensities of martensite and
a g
austenite for the reflexions considered. On the basis of
A. Materials
Eq. [1], the volume fraction of martensite present in the
The AISI 301 SS used in this work was provided by CR(90pctreductioninthickness)samplecanbecalculated
Outokumpu Stainless Oy, Finland, with an alloy composi- as97.4pct.Thissamplewasselectedasthereferencevalue
tion (wt pct) given in Table I. This SS has an M tem- forthedeterminationofphasefractioninallthesamplesby
d30
2326—VOLUME37A,AUGUST2006 METALLURGICALANDMATERIALSTRANSACTIONSA
correlating The obtained X-ray results with The SQUID 4. Hardness measurements
magnetic measurements described below. Hardness measurements were performed in the CR and
annealed samples using the Rockwell B hardness test
2. SQUID method. In this technique, a hardened steel ball indenter
To determine the fractional amount of martensite (ferro- is forced into the test material under a preliminary minor
magnetic phase) and austenite (paramagnetic phase), each loadof10kgf,andwhilethepreliminaryminorloadisstill
sample was measured in a Quantum Design’s Supercon- applied, the sample is subjected to an additional load of
ducting Quantum Interference Device (SQUID). This sys- 100kgf.Tenmeasurementswereobtainedfromeachsample
tem uses the extreme sensitivity of the SQUID to detect andanaveragevaluewascalculated.HardnessRockwellB
minute changes, as small as 1 3 10(cid:2)14 T, in a specimen’s numbers (HRB) were subsequently converted to hardness
response to an applied field. Due to its high sensitivity, Vickers numbers (HV) using the ASTM E140 standard.
measurement errors are typically below 0.01 pct. For the Finally, HV values were converted to yield strength values
SQUID tests, specimens with dimensions approximately using the linear relation HV 5 3s /9.8.[15]
y
1 3 1 3 5 mm were cut with the long dimension parallel
to the rolling direction and polished to remove irregular
edgesandsurfaceoxidation.Themagneticfieldwasapplied III. RESULTS
parallel to the rolling direction of the specimen. The tests
A. X-ray Diffractometry
wereperformedat27°Cinincreasingstepsof0.0250Tesla
uptoamagneticfieldof0.2T,andtheninincreasingsteps Figure2showstheX-ray spectrafor theCRsampleand
of0.1Tuntilafieldof1Twasreached.Eachdatapointwas the samples annealed at various temperatures. For each
measured five times and averaged. The magnetic response of spectrum, the intensity of each peak was normalized with
the stainless steel as a function of the applied magnetic field respect to the total spectrum intensity. As shown in Figure
was measured and normalized with respect to the specimen 2,theCRsampleexhibitsprimarilymartensitepeakswitha
volume.Amaximumappliedfieldof1Twaschosentoensure small (220) austenite peak. As the annealing temperature
the saturation of the magnetic phases present in the alloy. increases from 600 °C to 800 °C, the austenite peaks
To determine phase fraction and to establish a firm con- increase in intensity with respect to the martensite peaks,
fidenceintheSQUIDmeasurements,areferencevaluewas revealing an increase in the volume fraction of austenite.
selectedbasedonthevolumefractionofmartensitepresent This trend is reversed above 800 °C, for which the austen-
in the CR (90 pct reduction in thickness) sample, which ite-to-martensite peak ratios decrease, indicating an
was calculated by X-ray as 97.4 pct (see Section II–B–1). increase in the volume fraction of martensite.
Asthemeasured magnetizationperunitvolumeofthisCR A comparison between the obtained spectra (Figure 2)
sample is 633 emu/cm3, the saturation magnetization per andpublishedresultsforrandomlyorientedpolycrystalline
unit volume mSa91Tof a fully 100 pct martensitic structure austeniteandmartensiteallowsustodeterminequalitatively
can be calculated as approximately 650 emu/cm3, if we the existence of preferred orientation. This analysis shows
assume a linear behavior. However, as the CR sample that the CR sample has a definitive preferred orientation
exhibited0.2volumepercentofd-ferrite(dhasasaturation along the {211}a9 planes of martensite and the {220}g
magnetization of971 emu/cm3[14]), weneed tosubtract the planes of austenite. However, upon annealing at 600 °C, a
contribution of d-ferrite from the measured 677 emu/cm3. change occurs in the martensite phase, with the preferred
Under these conditions, a CR sample containing 100 pct orientation shifting toward the {110}a9 planes. This trend
martensiteshouldcorrespondtoasaturationmagnetization
of 653 emu/cm3.
3. Transmission electron microscopy
A JEOL 200CX anda JEOL 2010TEM operating at 200
kVandequippedwithsingleanddoubletiltstageswereused
to identify the overall microstructure, austenite grain size,
and dislocation substructure. For each annealing condition,
three samples were observed. Representative images were
taken at each annealing temperature. The JEOL 2010 TEM
is equipped with an Oxford energy dispersive spectroscopy
(EDS) system, which was used for chemical composition
analysis of second-phase particles present in some of the
samples.EDSspectrawerealsocollectedfromareasadjacent
and away from the second-phase particles for comparison.
Electron transparent disks were made using a Struers
TenuPol-5 Twin-Jet electro-polisher using the following
electrolyte and parameters: 590 mL m-Butanol, 350 mL
methanol, and 60 mL perchloric acid performed at –10 °C,
at a voltage of 25/30 V, and a current of 24 mA.
AustenitegrainsizeswerecalculatedfromscannedTEM
negatives using standard ASTM E112. The average grain
Fig. 2—XRD spectra for the cold-rolled (CR) and annealed samples at
sizes were calculated from a measurement of 100 grains. variousannealingtemperatures.
METALLURGICALANDMATERIALSTRANSACTIONSA VOLUME37A,AUGUST2006—2327
Fig. 4—The volume fraction of the martensite and austenite phases
determinedbymagneticmeasurements.
Fig. 3—Saturationmagnetizationasafunctionofannealingtemperature.
Allsampleswereannealedfor30minutesattherespectivetemperatures.
becomesmoreevidentabove650°C(Figure2).Ontheother
hand, the austenite phase maintains the {220}g preferred
orientation up to 650 °C, while at 700 °C and above the
austenite reversion becomes more pronounced and gradu-
ally acquires a more random orientation (Figure 2).
Forthedeterminationofthevolumefractionofaustenite
and martensite, X-rays were used and correlated with
SQUID measurements in the CR sample to establish a
reference value. On the basis of this reference sample,
andduetothesuperioraccuracyoftheSQUID,thevolume
fraction of austenite and martensite was calculated for all
samples using the latter technique (see next section).
B. SQUID Magnetic Measurements
BecauseoftheaccuracyoftheSQUIDmethod,magnetic
measurementswereusedtodeterminethevolumefractions
of martensite and austenite. The saturation magnetization
per unit volume of the CR specimen (no annealing) was Fig. 5—The average austenite grain size as a function of annealing
temperature.
foundtobe633emu/cm3.Themeasuredsaturationmagneti-
zation per unit volume for all annealed samples (measured
error around 610(cid:2)4) is shown in Figure 3. a drastic increase in average grain size is evident. Figure 5
UsingthesaturationmagnetizationdataofFigure3,itis also shows that wider grain sizevariations are found above
thuspossibletocalculatethevolumefractionofmartensite 800 °C. However, a calculation of the ratio between the
and austenite, as discussed in Section II–B–2. The phase largestandthesmallestgrainsizesfoundamong100grains
fractions for each annealing temperature, converted from showsthatthemostsignificanteffect(ratio(cid:3)40)isobserved
the saturation magnetization data, are shown in Figure 4. at 750 °C; all other temperatures exhibit a ratio below 20.
Thedatashowthatthemaximumaustenitereversionoccurs
foranannealingtemperatureof750°C;abovethistemper- 2. Microstructural analysis
ature, the amount of martensite increases.
a. CR specimen
The CR sample shows a microstructure composed of
C. TEM Observations
twodistinctmartensitestructures:regionsoflathmartensite
1. Austenite grain size and regions of dislocation cell-type martensite. These two
Figure 5 provides the average grain size diameter regions can be clearly identified by the diffraction patterns
obtained at each annealing temperature. The average grain obtained (Figure 6): the regions of lath martensite produce
sizewas calculated from measurements of 100 grains. The adiffractionspotpattern,whereastheregionsofdislocation
error bars represent the deviation from the mean grain size cell-type martensite produce a diffraction ring pattern. The
given a 95 pct confidence limit. As shown in Figure 5, the ring-like diffraction pattern exhibits brighter portions in
average grain size starts to increase above 650 °C and some regions, indicating that some preferred orientation
suffersaslight decreaseat 800 °C;abovethistemperature, ispresent.Forthepurposesofgrainrefining,thedislocation
2328—VOLUME37A,AUGUST2006 METALLURGICALANDMATERIALSTRANSACTIONSA
Fig. 6—(a) TEM micrograph showing the two distinct martensitestruc-
turesintheCRspecimen.(b)Diffractionspotpatternofthelathmartensite
takenfromtheregionrepresentedbythewhitecircle,B5½111(cid:4)a0:Resid-
ual austenite is also identified in the spot pattern. (c) Diffraction-ring
patternofthedislocationcell-typemartensitetakenfromtheregionrep-
resentedbytheblackcircle.fccspotscorrespondingtotheaustenitephase
arealsoshown.
cell-type martensite is ideal because it contains a much
higher dislocation density than the lath-like martensite,
which can act as nucleation sites for the austenite during
annealing.[6,16,17]
Also important is the fact that some retained austenite
is present in the CR sample, as shown by the diffraction
pattern in Figure 6(b), which confirms the SQUID results
obtained. Fig. 7—(a)Microstructureofthe600°Csample.Thewhitearrowshowsthe
coldrollingdirection.(b)Highermagnificationof(a),showingrecrystallized
b. 600 °C annealing fine-grainedaustenite.(c)Highermagnificationof(b),showingthepresence
oflathmartensiteadjacenttoaustenitegrains,severalofwhichareoutlined
Figure 7 shows the microstructure of the samples anne-
inblack.Thepresenceofcarbidescanbeseenwithinthesmallblackcircle.
aled at 600 °C at different magnifications. In Figure 7(a), (d)Electrondiffractionpatterntakenfromthecircledareain(a),indicating
the cold-rolling direction is still readily apparent (white thepresenceofaustenite,B5[001]g andquasi-ringsofthemartensite.
,
arrow). Figure 7(b) shows a higher-magnification image of
Figure7(a),wherethepresenceofausteniteisvisible.Thisis (Figure5),whichindicatesthatthesampleisgoingthrough
confirmed by Figure 7(c), which shows elongated grains of the very initial process of phase conversion. Furthermore,
austenite(outlinedinblack)adjacenttoregionsoflathmar- second-phase particles, approximately 20 nm in size and
tensite,andbytheelectrondiffractionspotpatternshownin identified as carbides (see Section III–C–3 for a thorough
Figure7(d),takenfromthewhitecircledareainFigure7(a). analysis), were detected on large reverted austenite grains
Many of the austenite grains have low dislocation density (black circle in Figure 7(c)).
associated with the nucleation and growth of new crystals.
A careful examination of the microstructure revealed an c. 650 °C annealing
average austenite grain size of approximately 0.28 mm The microstructure of the samples annealed at 650 °C is
(Figure 7(c) shows an example of two austenite grains showninFigure8.Figures8(a)through(c)showrecrystal-
delineated by dark lines) and a narrow grain size variation lizedultrafinegrainsofaustenite,aswellassignsoflath-type
METALLURGICALANDMATERIALSTRANSACTIONSA VOLUME37A,AUGUST2006—2329
martensite.Theultrafinescaleoftheaustenitegrainscanbe
confirmedbythestilllargenumberofspotsconfiguredina
ring-like pattern (Figure 8(b)), whereas the presence of
martensiteisrevealedbythespotdiffractionpatternshown
in Figure 8(c). Figure 8(d) shows a higher-magnification
image of the austenite grain structure evident in Figure
8(a). A careful examination revealed the presence of equi-
axedgrainswith anaveragegrainsizeof0.28mm.Similar
tothesamplesannealed at600°C, theannealingtreatment
at650°Cproducedanarrowgrainsizevariation(Figure5).
This result indicates that at 650 °C the conversion process
from martensite to austenite is still underway. Carbides
were also detected on some ofthe largerreverted austenite
grains (black circles in Figure 8(d)).
d. 700 °C annealing
The microstructure of the sample annealed at 700 °C
differed significantly from the samples annealed at lower
temperatures.Figure9showsregionsoflargerevertedaus-
tenitegrains(regionLinFigure9(a)) surroundedbypock-
ets of small austenite grains (region S in Figure 9(a)). The
presence of areas with fine-grained austenite can be con-
firmedbythering-likeelectrondiffractionpatternshownin
the insert of Figure 9(b). As expected, this sample exhibits
a large deviation in grain sizes from the measured 83 mm
average grain size (Figure 5).
Further TEM observations (Figure 10) confirmed the
presence of martensite in the sample annealed at 700 °C,
as also detected via SQUID and XRD. In Figure 10(a), a
slip band separating two different regions of austenite is
observed. In addition, carbides were most noticeable on
the larger austenite grains.
e. 750 °C annealing
Similar to the sample annealed at 700 °C, the micro-
structure of the 750 °C sample shows regions of large
and small austenite grains (Figure 11(a)). However, as
depictedinFigure11(a),fewerpocketsofultrafine-grained
austenite and wider areas of larger austenite grains are
observed. TEM observations revealed an average austenite
grain size of approximately 1.74 mm, with a significant
deviation from the average (Figure 5). Thislarge deviation
from the average is a clear indication of the simultaneous
phase conversion and grain growth processes.
In addition, as shown in Figure 11(a), a large number of
carbides with an average diameter of 110 nm were present
in the 750 °C sample. In Figure 11(a), the white arrow
indicates strings of carbides consistent with precipitation
along previously existing martensite–lath boundaries.
As expected from the SQUID and X-ray results, the
TEMobservationsconfirmedthepresenceofsomeretained
martensite, as shown in Figure 11(b). Fig. 8—(a)TEMmicrostructureofthe650°Csampleshowingthenewly
nucleatedausteniteandlath-typemartensite.(b)Ring-likediffractionpat-
f. 800 °C annealing tern of austenite grains taken from the region in (a) represented by the
AsshowninFigure12,themicrostructureofthesamples blackcircle.(c)Spot-likepatternoflathmartensitetakenfromtheregion
annealed at 800 °C looks very different from the samples in(a)representedbythewhitecircle,B5½111(cid:4)a0.(d)Highermagnifica-
tionof(a),showingthepresenceofcarbides.
annealed at lower temperatures. This apparent difference
is due mainly to a sharp increase in the number of defects
present in the austenite grains. Due to the presence of A careful examination of the austenitic regions (Figure
reverted austenite with a high density of defects, the mar- 13) shows austenite grains with a high density of defects,
tensitephaseisvisuallymoredifficulttodistinguish.How- andsmallequiaxeddefect-freeaustenitesubgrains(labeled
ever,asshowninFigure12,lathmartensitecanbeidentified, s.g in Figure 13). These subgrains are the byproduct of
confirming the results from SQUID and XRD testing. a shear-type reversion mechanism by which the reverted
2330—VOLUME37A,AUGUST2006 METALLURGICALANDMATERIALSTRANSACTIONSA
Fig. 10—(a)TEMmicrographofthesampleannealedat700°C,showing
anareawheremartensiteandaustenitephasescoexist.Aslipband(arrow)
separatestworegionsofaustenitewithdifferentgrainsizes.(b)Electron
diffractionpatterntakenfromtheregionin(a)circumscribedbythebot-
tomleftwhitecircle,B5½111(cid:4)a0kB5½011(cid:4)g:(c)Electrondiffractionpat-
terntakenfromtheregionin(a)circumscribedbytheupperrightwhite
circle,B5½111(cid:4)a0kB5½011(cid:4)g:
Fig. 9—(a) TEM micrograph of sample annealed at 700 °C for 30
minutes, showing large (L) and small (S) austenite grains. (b) Higher
magnificationof(a),showingmoreclearlythevariationinaustenitegrain
size. The ring-like electron diffraction pattern shown in the insert and increase in average grain size (5.5 mm). In addition, there
takenfromtheregionin(b)representedbythewhitecircleconfirmsthe are significantly fewer defects than in the 800 °C sample.
presence of ultrafine-grained austenite. Carbides can be identified in the
Thecarbides,approximately115nminsize,aredistributed
microstructurewithintheblackcircles.
alongextendedregionsandareseenmainlyinthecenterof
the grains.
austenite microstructure maintains microstructural features Althoughmartensitewasnotfoundwithintheaforemen-
similar to the martensite. tionedausteniteareas,thesamplesannealedat850°Cshow
Although grain size measurements are very difficult to the presence of martensite (Figure 15), which confirm the
assess in the 800 °C sample, a conservative approach indi- results obtained by SQUID and XRD.
catesanaverageaustenitegrainsizeof1.26mm(Figure5), Inaddition,carbidesarevisiblewithinthelath-typemar-
whichissimilartothe750°Csample.However,incontrast tensite, as indicated by the small white circles in Figure
to the 750 °C sample, the 800 °C sample displays a much 15(a). As in the other samples, the carbides are located
smaller ratio between the largest and the smallest grain within the grains and not at the grain boundaries.
sizes.Furthermore,the800°Csampleexhibitssignificantly
lesscarbideprecipitationthanthe750°Csample,although h. 900 °C annealing
thecarbidespresentwereapproximatelyofthesamesizeas The microstructure of the samples annealed at 900 °C
those found in the 750 °C sample. show large reverted equiaxed austenite grains containing
carbides (Figure 16). The average austenite grain size was
g. 850 °C annealing measuredas5.85mm,althoughtheratiobetweenthelargest
The microstructure of the samples annealed at 850 °C and the smallest grain sizes measured decreased with
shows the presence of a large number of defect-free equi- respect to the 850 °C sample. Moreover, the austenite
axedaustenitegrainsandahighdensityofcarbides(Figure grains are relatively free of defects when compared with
14).Whencomparedtothesamplesannealedatlowertem- thesamplesannealedat800°Cand850°C.Figure16also
peratures, the samples at 850 °C exhibit a significant showsaheterogeneousdistributionofcarbides,withextended
METALLURGICALANDMATERIALSTRANSACTIONSA VOLUME37A,AUGUST2006—2331
Fig. 12—(a) TEM micrograph of the 800 °C sample, showing reverted
austenite grains and lath martensite. (b) Electron diffraction pattern of
austenitetakenfromtheregionwithinthewhitecircle.(c)Electrondif-
fractionpatternofmartensitetakenfromtheregionwithintheblackcircle,
B5½111(cid:4)a0:
Fig. 11—(a) TEMmicrographofthesampleannealedat 750°C for30
minutes,showingareascomposedbysmall(S)grainsizesandlarge(L)
grain sizes and severe carbide precipitation within the large grains. The
whitearrowindicatesastringofcarbidesthatnucleatedatlath–martensite
boundaries.(b)Regionshowingthepresenceoflath-typemartensiteinthe
sampleannealedat750°C.Theelectrondiffractionpatternwastakenfrom
thecircledregion,confirmingthepresenceofmartensite,B5½111(cid:4)a0:
austeniteareasfreeofcarbides,whileotherregionsexhibit
severe carbide precipitation.
The presence of martensite in this sample, which was
detected by SQUID and XRD, is also confirmed by TEM.
Figure 17(a) shows a region where both lath-type marten-
site and reverted austenite can be seen. As was the case in
the 850 °C sample, the regionsof lath-type martensite also
contain the presence of carbides (the small white circle in
Fig. 13—Microstructureoftheausteniteregionforthesampleannealedat
Figure 17(b)).
800°C,showingthepresenceofdefect-freesubgrains(s.g)andadjacent
grainswithahighdensityofdefects.
3. Second-phase particles
As discussed inthe previous sections,second-phasepar-
ticles were found in all annealed samples. Upon annealing formed, and in the respective size of the particles. The
at600°Cand650°C,thenumberofsecond-phaseparticles 800°Csamplewasanexceptiontothistrend,withvirtually
was relatively low and constant, and their size were rela- nosecond-phaseparticlesvisible(Figure12).However,for
tively small, of the order of 20 nm (Figure 18). Annealing the second-phase particles found, the size of the existing
at temperatures between 750 °C and 900 °C led to a sig- particles was similar to those present in samples annealed
nificant increase in the number of second-phase particles between 700 °C and 900 °C.
2332—VOLUME37A,AUGUST2006 METALLURGICALANDMATERIALSTRANSACTIONSA
Fig. 14—TEM micrograph of the sample annealed at 850 °C, showing Fig. 16—TEM micrograph of the sample annealed at 900 °C, showing
largerevertedaustenitegrainsandcarbideprecipitation.
largeequiaxedgrainsofausteniteandsevereprecipitationofcarbides.
Fig. 15—(a)TEMmicrographofthesampleannealedat850°C,showing
thepresenceofmartensiteandcarbides(withinthesmallwhitecircles).(b)
Electron diffraction pattern of martensite taken from the region corre-
spondingtothelargewhitecirclein(a).B5½012(cid:4)a0:
Thecharacterizationofthesesecond-phaseparticleswas
carried out by EDS and TEM observations. A comparison
oftheEDSspectrafromthematrixandtheparticlesshows
an increase in iron, chromium, and molybdenum content
in the second-phase particles (Figure 19). In addition, a
carbonpeakcanbeidentifiedintheEDSspectrumobtained
from the particles (Figure 19(b)). To conclusively identify
the second-phase particles, TEM images and diffraction
patterns were obtained from the particles (Figure 20). Fig. 17—(a)TEMmicrographofsampleannealedat900°C,showingan
Basedontheseobservations,theparticles’crystalstructure areawherebothausteniteandmartensitearepresent.(b)Highermagnifi-
was determined to be FCC, with a lattice parameter of cationof(a),revealingthecarbidesamongthelathmartensite(areawithin
approximately 10.6 A˚. These results, in conjunction with the small white circle). (c) Electron diffraction pattern taken from the
region corresponding to the white circle in (a), confirming the presence
the EDS analysis, confirm that the second-phase particles
ofbothausteniteandmartensite.
are iron-chromium-molybdenum carbides of the type
M C . In practically all cases, these observed carbides
23 6 Vickers hardness values and corresponding yield strength
exhibit the shape of globular particles (Figure 20) and are
values are shown in Figure 21. As depicted in Table II and
seen to precipitate predominantly in the austenite matrix.
Figure 21, samples annealed at 600 °C and 650 °C show
very high hardness/yield strength values. As the annealing
D. Mechanical Properties
temperature is increased from 650 °C to 750 °C, there is
RockwellBhardnessmeasurementsperformedintheCR a significant reduction in hardness/yield strength. Above
and annealed samples are shown in Table II. Converted 750 °C, the hardness/yield strength values are essentially
METALLURGICALANDMATERIALSTRANSACTIONSA VOLUME37A,AUGUST2006—2333
IV. DISCUSSION
A. Influence of Annealing Temperature on a9/g Ratio
As can be seen in the results from XRD (Figure 2),
increasing the annealing temperature causes an increase
in the amount of reverted austenite up to 800 °C. Above
800 °C, the XRD data show an increase in the amount of
martensite produced, as depicted by an increase of the
{211}a9 peak at 850 °C, and {110}a9, {211}a9 peaks at
900°C.Themaximumamountofrevertedausteniteshown
by XRD is thus different from the results obtained by
the SQUID method (Figures 3 and 4). In this case, the
maximum amount of reverted austenite achieved occurs
at 750 °C, which is in agreement with the work by
Di Schino et al.[4] At annealing temperatures above
750 °C, an increase in the SQUID magnetic response is
observed, which indicates an increase in the amount of
martensite formed.
ThisdiscrepancybetweentheXRDandtheSQUIDdata
Fig. 18—Size of second-phase particles as a function of annealing
is not yet fully understood. However, as shown in Figure
temperature.
14,thespecimenannealedat800°Cexhibitsaverydiffer-
ent microstructure from the samples annealed at 750 °C
(Figure 12), despite their similarity in terms of marten-
site-to-austenite ratio. As will be discussed later, this sig-
nificant difference in microstructure is attributed to a
change in the mechanism for austenite reversion whereby
austeniteformsbyshearreversionat800°C.Asaresult,it
can be argued that at 800 °C, the texture produced by
annealing could be rather different from the samples
annealed at 750 °C.
One other interesting observation, shown by both XRD
and SQUID measurements, is that the amount of reverted
austenite goes to a maximum and then decreases for an
additionalincreaseinannealingtemperature.Althoughthis
phenomenonhasbeenreportedinseveralworks,[2,5,6,18,19,20]
ithasyettobefullyexplained.Theargumentthathasbeen
presented several times is that higher annealing tempera-
tures cause an increase in the austenite grain size, thus
raising the M temperature.[2,5,6,18,19,20] As a consequence,
S
upon cooling, thermally induced martensite is more likely
toformforsamplesannealedathighertemperatures.While
this statement may sound plausible, the AISI 301 SS used
inthisresearchhasacalculatedM temperatureof–118°C
S
for an average grain size above 40 mm. Since in this study
thelargestrevertedaustenitegrainsizeachievedis;5.8mm
at900°C,weshouldthenexpectanM temperaturelower
S
than (cid:2)118 °C based on thegrain sizeargument.[2,5,6,18,19,20]
Thus, for annealed samples cooled to room temperature, it
is unlikely for thermally induced martensite to form as a
result of larger austenite grains.
A more likely argument for the increase in martensite
contentforannealingtemperaturesabove750°C(assuming
the results obtained by the SQUID data) is that carbon
Fig. 19—EDSspectrumobtainedfrom(a)theaustenitematrixand(b)the depletion from the matrix due to the formation of the (Fe,
second-phase particles. The second-phase particles exhibit a significant Cr,Mo) C carbidesoccurs.Thisphenomenontakesplace
23 6
increaseincarbon,chromium,andmolybdenumwithrespecttothematrix. when, upon annealing, the martensite starts to release car-
bon, leading to the formation of carbides. For an SS with
very similar, except for the sample annealed at 800 °C. carboncontentsabove0.08wtpct,precipitationofcarbides
Evenforsamplesannealedat850°Cand900°C,theyield is expected to occur in less than 1 minute for temperatures
strengthis considerably higher than for conventionalstain- in the range of 700 °C to 900 °C.[21] This means that the
less steels (Figure 21). revertedaustenitephasewillbedepletedincarbon,leading
2334—VOLUME37A,AUGUST2006 METALLURGICALANDMATERIALSTRANSACTIONSA