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Complex Widmanstätten plates consisting of cementite andferrite, product phases of a eutectoid reaction, in anFe–C–Mn alloy

Wei-Chun Cheng⁎, Jung ChangDepartment of Mechanical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4,Taipei, 106, Taiwan

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +886 2 27376241E-mail address: weicheng@mail.ntust.edu

1044-5803/$ – see front matter © 2012 Elseviehttp://dx.doi.org/10.1016/j.matchar.2012.12.0

A B S T R A C T

Article history:Received 15 July 2012Received in revised form8 November 2012Accepted 24 December 2012

Keywords:Widmanstätten plates

Complex Widmanstätten plates of cementite and ferrite, product phases of a eutectoidreaction, in an Fe–10.6Mn–0.64 C steel have been investigated. Different forms of proeutectoidM3C carbide appear in the austenite as grain boundary precipitates orWidmanstättenplates inthe austenite at temperatures below 1048 K. Some Widmanstätten plates may consist oflamellar cementite and ferrite as complex Widmanstätten plates at temperatures below1023 K. Aside from the complex plates, pearlite nodules exist at the austenite grain boundariesat temperatures below 973 K. Therefore, the lamellar cementite and ferrite in both complexWidmanstätten plates and pearlite nodules are all product phases from the eutectoid reaction.The upper temperature limit for the eutectoid reaction is between 1023 K and 973 K. Thecomplex Widmanstätten plates have some characteristics similar to inverse bainites, butappear at much higher temperatures, near the eutectoid temperature of the steel.

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M3C carbideEutectoid reactionFe–C–Mn alloy

1. Introduction

A well-known eutectoid reaction takes place in steels. Whenbinary Fe–C alloys containing about 0.77 wt.% C are cooledbelow the eutectoid temperature, the eutectoid reaction takesplace involving the decomposition of austenite into ferrite andcementite. The product phases of the eutectoid reaction featurepearlite, comprising lamellar ferrite and cementite [1]. Thepearlite colonies nucleate most easily at the original austenitegrain boundaries. The lattice parameters of M3C carbide areadopted as a=0.4524 nm, b=0.5089 nm, and c=0.6743 nm.Orientation relationships (ORs) between the ferrite and cement-ite have been well documented in the literature. For example,Bagaryatsky's OR between the ferrite (α) and M3C (C) is [110]α//[100]C, [111]α//[010]C and (112)α//(001)C. Isaichev's OR is [111]α//[010]C and (011)α//(103)C [2–4].

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When steels with more or less carbon than the eutectoidcomposition are cooled slowly or held isothermally below theeutectoid temperature, the appearance of the pearlite is usuallypreceded by the precipitation of proeutectoid cementite andferrite, respectively, at the grain boundaries prior to theeutectoid reaction [1]. For austenite being cooled well belowthe eutectoid temperature, for example, at the bainite formationregion in a time–temperature–transformation (TTT) diagram,different eutectoid products of bainite are produced. Bainite is amixture of ferrite and carbide like pearlite, but it is micro-structurally quite distinct frompearlite andcan be characterizedby its own C curve on the TTT diagram. In Fe–C binary alloys,this curve overlaps with the pearlite curve so that at tempera-tures around 773 K both pearlite andbainite form competitively.Themicrostructure of bainite dependsmainly on the isothermalholding temperatures. Upper bainite appears at higher temper-atures between773 Kand 573 Kand consists of ferrite lathswithcementite precipitates between the laths. Usually, bainite formswith the leading phase of Widmanstätten ferrite in the maingrowth direction. Thus, bainite is favored in low carbon contentsteels. At temperatures below 573 K the microstructure of

Fig. 1 – (a) OM and (b) XRD analyses of the Fe–10.6 Mn–0.64 Csteel in the as-quenched condition. The Miller indexes ofFCC matrix are underlined in (b).

Fig. 2 – The OMs of the high manganese steel after solutiontreatment at 1373 K and isothermal holding at (a) 1048 K and(b) 1023 K, respectively, for 100 h. Note that grain boundaryprecipitates and Widmanstätten plates appear in theaustenite in (b).

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bainite changes from laths to plates and the carbide dispersionbecomesmuch finer. Thismicrostructure is called lower bainite[5–10].

In hypereutectoid steels the proeutectoid cementite inaustenite exhibits differentmorphologies. Cementite precipitatesat the grain boundaries as separate grains, continuous films andallotriomorphs, and appears in the matrix as Widmanstättenside-plates, intragranular Widmanstätten plates and intra-granular idiomorphs [11–13]. The allotriomorphic cementiteprecipitates at the grain boundaries at high temperatures. BothWidmanstätten side-plates and intragranular Widmanstättenplates exist in the austenite at low temperatures. Forthe co-existence of grain boundary precipitates and theWidmanstätten plates at the intermediate temperature range,Widmanstätten carbide plates normally form before pearlite andafter the grain boundary precipitates [11]. Grain boundarycarbides exist in all hypereutectoid steels. Continuous carbide

films develop along the grain boundaries after proeutectoidcarbide grains precipitate. When carbide films thicken, grainboundary allotriomorphs appear. The growth ofWidmanstättenplates proceeds by either protrusions of the grain boundaryallotriomorphs or by separate nucleation and growth of cement-ite plates in the austenite. The former producesWidmanstättenside-plates and the latter intragranular Widmanstättenplates [11]. Inverse bainites formwith the ferrite plates attachedto M3C Widmanstätten plates, and are the major structure inthe austenite of hypereutectoid steels at sufficiently lowtemperatures [5–10].

We have investigated the phase transformations of qua-ternary Fe–C–Mn–Al alloys subjected to various heat treat-ments, for example, cooling from high temperatures [14–18]and holding isothermally at low temperatures [19–22]. Somepreliminary results have been acquired. For example, anotherpearlite with lamellae of ferrite and M23C6 carbide, called

Fig. 3 – The TEM analysis of the M3C carbide in the austenite of the steel after 100-h isothermal holding at 1023 K. (a) The BFimage revealing the grain-boundary M3C carbide (γ: austenite; C: M3C), (b) another BF image showing a dark Widmanstättencementite in the austenite, (c) the accompanying SAD pattern from the [111] direction of M3C carbide in (a), and (d) the [011] SADpattern of the light colored austenite adjacent to the M3C carbide grain in (a). (T: transmitted beam).

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M23C6 pearlite, has been found in Fe–C–Mn–Al steels. Insteadof M3C plates, M23C6 sheets embed in the ferrite of the M23C6

pearlite [19,20]. However, we found some phase transforma-tions of the quaternary Fe–C–Mn–Al alloys are extended fromthe ternary Fe–C–Mn alloys. Therefore, we have furtherstudied the phase transformations of high Mn steels to verifythe origin of the phase transformations of the quaternary Fe–C–Mn-Al alloys. For example, we discovered that the FCCmicro-twins occur along with HCP ε-martensite in Fe–C–Mnalloys after cooling from high temperature [18]. The followingare some interesting findings concerning high Mn steel afterisothermal holding at low temperatures.

2. Experimental Procedures

Slabs with a composition of Fe–10.6 Mn–0.64 C (wt.%) wereinitially prepared by induction melting. Commercial 1020

steel, pure carbon and electrolytic manganese were meltedtogether and cast into 3-kg ingots. After being homogenized at1473 K for 4 h under a protective argon atmosphere, theingots were hot forged and annealed to assure the uniformityof the composition in the steel plates. The steel plates werecold rolled to thin plates with a thickness of 2 mm, and cutinto specimens with dimensions of 15 mm×10 mm. Themeasured composition of the minor elements in wt.% of thesteel plates is (0.05 Si–0.01 P–0.01 S–0.01 Al). Note that the siliconcontent in the steel is very low because the melting of the 1020steel first burned out the original Si content of the 1020 steel.The steel samples were heated at 1373 K for 1 h in a protectiveargon atmosphere, and quenched in room-temperature waterfor solution heat treatment. The specimens in the as-quenchedcondition were sealed in vacuum quartz tubes, reheated, andheld isothermally at low temperatures ranging from 1123 K to873 K for 100 h. Two additional isothermal holding periods ofthe steel samples, 1 h and 10 h, were carried out at 873 K.

Fig. 4 – An OM of the high Mn steel after isothermal holdingat 973 K for 100 h.

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Samples were sectioned, mechanically polished and etchedin a 5% nital solution for observation in an optical microscopeand a Jeol JEM 6500F high resolution field-emission scanningelectron microscope (SEM). Some of the samples were alsoexamined by X-ray diffraction (XRD) in a Rigaku DMAX-B X-raydiffractometer operated at a power of 12 kW. Samples used forobservation by transmission electron microscope (TEM) werepolished mechanically into thin foils about 80 μm in thickness,punched into circles with diameters of 3 mm, and thenelectro-polished using a twin jet polisher in a 10% HClO4 and90% CH3COOH solution. The TEM samples also underwent ionmilling in an ion miller to polish the thin areas of thespecimens. The samples were examined in a JEOL JEM 2010transmission electronmicroscope equippedwith a Link ISIS 300energy dispersive X-ray analyzer (EDS) operated at 200 kV. Thetilting angles of the specimens inTEMare ±45 and ±34° in the x-and y-axis, respectively.

3. Results and Discussion

Anopticalmicrograph (OM) is shown in Fig. 1(a), of the Fe–C–Mnalloy after solution heat treatment. Similar grains withannealing twins indicate that the steel is single phase at1373 K. There are irregular thin plates distributed in the matrixgrains. Fig. 1(b) shows the XRD analysis of alloy steel whichreceived the same heat treatment as in Fig. 1(a). Some peaks,with Miller indices underlined, came from the FCC phase andthe others from theHCPphase.Wealso performed aTEM study,not shown here, to investigate the constituent phases of theas-quenched steel. We found that the matrix is FCC and theirregular thinplateswhich formedduring cooling are eitherHCPε-martensite or FCCmicro-twin [18]. ε-martensite is common inhigh Mn alloy steels [23,24].

Fig. 2(a) and (b) shows OMs of the steel after being solutiontreated, reheated, and held isothermally at 1048 K and 1023 K,respectively, for 100 h. Fig. 2(a) shows themicrostructure of the

alloy steel at 1048 K. The alloy is full of austenite grains alongwith irregular thin plates. We performed another TEM analysisfor the steel and found it had the same phases as those in theas-quenched steel. Therefore, the steel is single austenite attemperatures from 1373 K to 1048 K. The OM in Fig. 2(b) revealsthat at 1023 K, precipitates appear either at the grain bound-aries as continuous films or in the matrix as Widmanstättenplates. The continuous films almost completely outline thegrain boundaries. Widmanstätten side-plates develop from thegrain boundary films and grow into the austenite matrix. Otherintragranular Widmanstätten plates appear in the austenite.Thus, other phases have precipitated in the Fe–C–Mn alloy attemperatures below 1048 K. Because of the small amount of theprecipitates, the signals of the XRD from the precipitates are tooweak to analyze. Therefore, a TEM study to identify theprecipitates was performed.

The TEM analysis, focused on the grain boundary pre-cipitates and Widmanstätten plates, is shown in Fig. 3. Theisothermal holding temperature of the high Mn steel in Fig. 3is 1023 K and the holding period is 100 h. A bright-field (BF)image in Fig. 3(a) illustrates the appearance of precipitates inthe austenite (γ). Three second-phase grains (C) connecttogether along the austenite grain boundary. Another BFimage in Fig. 3(b) shows a dark Widmanstätten plate (C)existing in the austenite matrix. Fig. 3(c) shows the corre-sponding selected area diffraction (SAD) pattern taken fromthe grain boundary precipitate in the middle of the three Cgrains in Fig. 3(a). After tilting the C-grain to some other SADpatterns, we identified that the grain boundary precipitate isM3C carbide, the cementite. The zone-axis of the SAD patternin Fig. 3(c) is along the [111] direction of the cementite. Wealso investigated the Widmanstätten plate in Fig. 3(b), andconfirmed that the Widmanstätten plate is also M3C carbide.Because of the large lattice parameters of the M3C carbidecompared with the lattice parameter of austenite, somereflections of the cementite in the SAD patterns are too closeand are not easy to identify. In addition, some SAD patternsfrom the M3C carbide are similar and easily confused. Toidentify the SAD patterns of the M3C carbide is arduous work,and special efforts must be put into the SAD patterns withzone-axis directions in [010] and [111]. These two SADpatterns are similar at first sight. However, due to thesymmetry of the real space lattice of the cementite, the [010]zone-axis has two-fold rotational symmetry and the [111]zone-axis does not have any rotational symmetry. Because ofthe diffraction of parallel planes in both opposite orientations,the inversion symmetry appears in the TEM SAD patterns.Thus, to investigate the symmetry of the reciprocal latticespace, wemust consider the additional inversion symmetry ofthe SAD patterns. Therefore, besides the two-fold symmetryaxis, the [010] SAD pattern has mirror symmetry planes withplane normal vectors as the (200) and (002) reciprocal latticevectors, and the [111] SAD pattern only has two-fold symme-try along the [111] zone axis, as reported previously [20].Thus, we confirmed that grain boundary precipitates andWidmanstätten plates are all M3C carbide, but exhibitingdifferent morphologies. The upper temperature limit for theprecipitation of proeutectoid M3C carbide in the high Mn steelis between 1048 K and 1023 K. It is noteworthy that a layer,light colored, adjacent to the grain boundary M3C grains, is

Fig. 5 – Another TEM analysis of the Mn steel after isothermal holding at 973 K for 100 h. (a) The BF image showing a section ofWidmanstätten plate, focusing on the cementite plate with a dark contrast, in the austenite matrix, and (b) the correspondingSAD pattern covering [101] M3C carbide in (a). (c) Similar BF image at the same location as (a), but with a different tilting angle ofthe specimen, revealing a dark lamellar ferrite attached to M3C plate, and (d) the accompanying SAD pattern of the [001] ferritegrain in (c).

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austenite as the BF image shows in Fig. 3(a). The SAD patternfrom the light [011] austenite layer is shown in Fig. 3(d).

The chemical compositions (wt.%) of the phases of thesteel were analyzed by EDS which is standard equipment inTEM. Because of carbon contamination on the surface of theTEM specimen, all the carbon contents of the constituentphases measured by EDS are neglected. The chemical com-positions of the phases at 1023 K are as follows. The grainboundary M3C carbide is Fe–17.3 Mn and the austenite matrixis Fe–10.6 Mn. Note that cementite has high carbon content asindicated in the EDS analysis. Thus, the cementite contains ahigh concentration of Mn and C. The Mn content of the lightaustenite layer in Fig. 3(a) is Fe–7.2 Mn, which is lower thanthat of the austenite matrix. This can be explained by thefollowing. M3C precipitates at the austenite grain boundaries.During the growth of carbide, Mn and C solute atoms must be

transported from the matrix to the grain boundaries. Thediffusion of Mn and C solute atoms from the austenite toM3C carbide results in the depletion of the Mn and C atoms inthe austenite adjacent to carbide. The austenite layer nearthe cementite grains shows distinguishing characteristicsof lower Mn content and a lighter color than the austenitematrix.

Fig. 4. shows another OM observation of the alloy steelafter solution heat treatment at 1373 K, reheating, andisothermal holding at 973 K for 100 h. As the OM shows,quite a few grain boundary precipitates, Widmanstättenside-plates and intragranular plates exist in the austenite.The major grain boundary precipitates are in the formof continuous films which may thicken and appear asallotriomorphs and completely outline the grain boundaries.Widmanstätten side-plates proceed by extrusions from the

Fig. 6 – The isopleths phase diagram of an Fe–C–13 Mn system [25].

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allotriomorphs and the intragranular Widmanstätten platesdevelop by separate nucleation and growth of the cementitein the austenite [11]. Most of these precipitates have beenidentified by TEM as M3C carbide. Therefore, we have ob-served that the proeutectoid carbide exhibits various mor-phologies. There are at least five forms of M3C carbide. 3 formsare at grain boundaries: separate grains, continuous filmsand allotriomorphs. 2 forms are in the austenite matrix:Widmanstätten side-plates and intragranular plates. All thesefive morphologies of M3C carbide in the Mn steels have beeninvestigated in the literature [11–13].

The microstructures of the Widmanstätten plates at 973 Kwere identified by TEM, and the results are revealed in Fig. 5.The BF image in Fig. 5(a) shows a section of a Widmanstättenplate in the austenite. ThemajorWidmanstätten plate, markedC, is M3C carbide and appears dark because of being viewedalong an exact zone-axis of the [101] cementite for which thecorresponding SAD pattern is shown in Fig. 5(b). Note that inFig. 5(a) a thin white lamellar grain attaches to the left-side ofthe Widmanstätten cementite. The white layer has a differentcrystal structure from theM3C carbide and the austenitematrix.The identification of the thin lamellar phase by TEM is shown inFig. 5(c) and (d). Fig. 5(c) illustrates a similar BF image to that inFig. 5(a), but viewed from a different orientation. The BF imagedisplays a dark thin lamellar plate, labeled as α, attached tothe M3C Widmanstätten plate (C) light by contrast. The thinlamellar α plate appears dark also because of being viewedalong an exact zone-axis of the layer phase. It is the same layeras the white layer on the left side of the M3C plate in Fig. 5(a).The light C plate in Fig. 5(c) is the same dark Widmanstättencementite shown in Fig. 5(a) but at a different viewing angle. Thecrystal structure of the thin layer was identified as BCC ferrite.The zone-axis of the SAD pattern in Fig. 5(d) is along the [001]direction of the ferrite phase. Consequently, the thin layer grainattached to the Widmanstätten cementite is ferrite. Both M3C

carbide and ferrite appear simultaneously in Widmanstättenplates. These plates may be called complex Widmanstättenplates to distinguish them from M3C Widmanstätten plates.Note that M3C and ferrite are product phases from the eutectoidreaction featuring the decomposition of the austenite below theeutectoid temperature. However, instead of forming pearlite inthe Fe–C–Mn alloy, cementite and ferrite appear in the austeniteas complex Widmanstätten plates. The chemical compositionsof the Widmanstätten cementite, ferrite and austenite matrixare Fe–14.1 Mn, Fe–6.9 Mn and Fe–10.7 Mn, respectively. Thegrain boundary M3C carbide is Fe–17.3 Mn. The upper tempera-ture limit for the special eutectoid reaction is between 1023 Kand 973 K. According to the isopleths diagram of an Fe–C–13 wt.%Mnsystem, as shown in Fig. 6 [25], thephases in thehighmanganese steel from high temperature to low temperature areaustenite, (austenite+M3C carbide), and (ferrite+austenite+M3Ccarbide). The phases of the steel are quite consistent with theprediction of the isopleth diagram in Fig. 6, butwith 2.4 wt.% lessMn.

Further investigations of the phase transformations ofhigh Mn steel at lower temperatures were executed. We foundthat pearlite nodules appear in the austenite of the steel attemperatures below 948 K. Fig. 7 shows other OM and TEManalyses of the steel after 100-h isothermal holding at 923 K.The OM in Fig. 7(a) reveals that in addition to Widmanstättenplates, pearlite nodules appear at the grain boundaries.Pearlite nodules nucleate at the grain boundaries and growinto the surrounding austenite with radial shapes. Almostall the grain boundaries are covered by the pearlite nodules.The micro-structural study of the Widmanstätten plate andpearlite nodule by TEM is shown in Fig. 7(b) through (d). TheBF image in Fig. 7(b) shows a section of complex (α+C)Widmanstätten plates in the austenite. The lamellar ferriteis at the top position while the M3C plate is at the lower part.Another BF image in Fig. 7(c) reveals the lamellar structure of

Fig. 7 – The OM and TEM analyses of the steel after 100-h isothermal holding at 923 K. (a) OM revealing Widmanstätten platesand pearlite nodules in the austenite. (b) BF image showing a section of a (C+Α) Widmanstätten plate in the austenite matrix.(c) Another BF image revealing the partial pearlite colony in the same TEM specimen as that in (b) but at a different location.(d) The corresponding SAD pattern from [010] cementite in the pearlite lamellae in (c).

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a partial pearlite colony in a different location of the sameTEM specimen. Dark colored lamellar grains embedding in thematrix are vividly seen in Fig. 7(c). The crystal structures ofdark lamellar grains and the light matrix were confirmed byTEM as orthorhombic M3C carbide and BCC ferrite, respec-tively. For example, Fig. 7(d) shows the corresponding SADpattern from [010] cementite in the pearlite lamellae. Thecompositions of the Widmanstätten cementite, ferrite andaustenite matrix are Fe–11.3 Mn, Fe–1.6 Mn and Fe–10.7 Mn,respectively. The grain boundary M3C carbide is Fe–14.4 Mn.

Other OM and TEM analyses of the steel at 873 K are shownin Fig. 8. Note that the isothermal holding periods in Fig. 8(a)and (b) are 1 h and 10 h, respectively, which are different fromthe common 100-h holding in Fig. 8(c) and (d). The OMs inFig. 8(a) through (c) show that Widmanstätten plates andpearlite colonies appear in the austenite. A portion of thecomplex (C+α) Widmanstätten plate is shown as the BF image

in Fig. 8(d). The dark lamellar ferrite exists in the lower partand the light M3C plate occupies the top position of theside-plate. From the OM observation of the steel samples atvarious temperatures below the eutectoid temperature, grainboundary precipitates may appear first, then Widmanstättenplates, and pearlite nodules in the austenite. As we have seenfrom the OMs with various isothermal holding periods at 873 Kin Fig. 8(a) through (c), theWidmanstätten plates occur in all theaustenite grains after the isothermal holding of the steel at873 K. The volume fraction of the Widmanstätten plates in thesteel with 1-h holding at 873 K is the highest among the threespecimens. Thus, when M3C Widmanstätten plates develop inthe austenite, the growth rate of the plates is very rapid. Notethat the volume fraction of theWidmanstätten plates decreasesas the holding time increases. However, the volume fraction ofthe pearlite nodules increases as the isothermal holding timeincreases. This shows that the pearlite is the final stable

Fig. 8 – (a) to (c) OM and (d) TEM analyses of the steel at 873 K. The isothermal holding periods are 1 h in (a), 10 h in (b), and100 h in (c) and (d). The OMs in (a) to (c) illustrate Widmanstätten plates and pearlite nodules in the austenite. (d) The BF imageshows a portion of a (C+α) Widmanstätten plate in the austenite matrix.

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structure in the austenite at 873 K. The chemical compositionsof the phases analyzedby EDSare indicated: theWidmanstättencementite, ferrite and austenite are Fe–10.7 Mn, Fe–2.7 Mn andFe–10.5 Mn, respectively.

Widmanstätten plates occupy almost the entire austenitematrix after short holding times at 873 K, for example, 1-hholding, as the OM shows in Fig. 8(a). The Widmanstättenplates grow very rapidly. The requirement to build up the fastgrowing Widmanstätten cementite plates in the austenite isthat all the solute atoms constituting the carbide plates mustbe transported from the austenite matrix to the plates veryrapidly. According to the chemical compositions of the phasesmeasured by EDS, the Mn content of Widmanstätten cement-ite is slightly higher than that of the austenite matrix, i.e. themetallic portion of the M3C plate is very similar to that of theaustenite. The major composition difference between theWidmanstätten cementite and the austenite is the carbon

content. During the rapid growth of the Widmanstättenplates, the carbide plates received, almost exclusively, carbonatoms from the surrounding austenite, and few Mn atoms. Itis evident that the diffusion rate of carbon atoms in theaustenite via interstitial diffusion is several orders higher thanthat of Mn atoms in the austenite through substitutionaldiffusion. This satisfies the requirement for rapid supply ofcarbon atoms to the growingWidmanstätten plates. During thegrowth of Widmanstätten plates, few of the Mn atoms migrateto the Widmanstätten plates. The carbon atoms are the majorsolute atoms fluxing from the austenite matrix to the plates.This allows thehigh growth rate of theWidmanstätten plates inthe austenite. However, as carbon atoms diffused rapidly intoWidmanstätten plates, the neighboring austenite became shortof C atoms. The austenite adjacent to theWidmanstätten plateswas depleted of carbon and low in Mn, and became unstable.Thus, the condition for the unstable austenite to transform into

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ferrite has been met. When the austenite transformed intolamellar ferrite, a special eutectoid reaction took place, fea-turing the decomposition of the austenite into Widmanstättenplates of (cementite+ferrite). A similar situation for the de-pletion of Mn and C solute in the austenite layer was alsoobserved near the grain boundary M3C grains as the BF imageshows in Fig. 3(a); however, the light layer remained as stableaustenite at 1023 K.

Fig. 9. shows two secondary electron images (SEI) fromSEM for the steel after 100-h isothermal holding at 873 K.Fig. 9(a) displays the grain boundary allotriomorphs,Widmanstätten plates, and pearlite nodules in the austenite.Some Widmanstätten plates connect to the grain boundaryallotriomorphs, and some do not. However, this may beattributable to the viewing conditions. They actually connectedto the grain boundary allotriomorphs, however, not at theviewing level. After examining quite a few samples, we

Fig. 9 – (a) and (b) the SEM analysis of the steel after 100-hisothermal holding at 873 K. (a) SEI shows the grainboundary allotriomorphs with Widmanstätten plates andpearlites in the austenite matrix. (b) SEI reveals that pearlitesmay develop from the Widmanstätten plates.

discovered that most of the Widmanstätten plates are fromthe grain boundary allotriomorphs. The SEI in Fig. 9(a) alsoshows that some pearlite nodules develop from the grainboundary allotriomorphs and cause some Widmanstättenplates to merge into the pearlite nodules. In Fig. 9(b), the SEIshows the co-existence of pearlite nodules andWidmanstättenplates, and illustrates that some pearlite colonies may developfrom the Widmanstätten plates. Some pearlite nodules maystart from other places and cease to grow when contactingWidmanstätten plates. Therefore, both SEIs in Fig. 9 show thesimultaneous occurrence ofWidmanstätten plates and pearlitecolonies in the austenite.

Complex Widmanstätten plates comprising M3C and ferriteplates have been discovered in the austenite at temperaturesbelow 1023 K. They are product phases from the eutectoidreaction, however, in the form of Widmanstätten plates ratherthan pearlite. Pearlite colonies also appear in the steel attemperatures below 973 K. Thus, the product phases from theeutectoid reaction include twodifferent structures; i.e., complexWidmanstätten plates and pearlite colonies. However, thereaction formula for the complex Widmanstätten platesmay be modified as follows: γ→Wid (M3C+α). The complexWidmanstätten plates are similar to (cementite+ferrite)Widmanstätten plates of inverse bainites in hypereutectoidsteels [5–10]. However, the complex Widmanstätten platesappear in the region for the formation of pearlite, notbainites, and are much larger than those in inverse bainites.In addition, the complexWidmanstätten plates appear in theaustenite matrix at higher temperatures close to the eutec-toid temperature.

4. Conclusions

The eutectoid reaction of a hypereutectoid high manganesesteel was investigated. The steel samples underwent solutionheat treatment at 1373 K, followed by isothermal holding atlow temperatures. Various forms of proeutectoid cementiteexist in the austenite as either grain boundary precipitates orWidmanstätten plates at temperatures below 1048 K. TheWidmanstätten plates feature either side-plates from thegrain boundary allotriomorphs or intragranular plates inthe austenite. At temperatures below 1023 K, in addition totheproeutectoid carbide, lamellar ferrite plates accompanyM3CWidmanstätten plates to form complex Widmanstätten plates.Complex Widmanstätten plates are product phases from theeutectoid reaction. Thus, the upper temperature limit for theeutectoid reaction of the Fe–C–Mn alloy is between 1023 K and973 K. Besides the complex Widmanstätten plates, pearlitenodules composed of lamellar ferrite and cementite areinvolved in the eutectoid reaction at temperatures below973 K. Normally Widmanstätten plates appear before pearlitenodules. Complex Widmanstätten plates are similar to theWidmanstätten (cementite+ferrite) in inverse bainites ofhypereutectoid steels. However, Complex Widmanstättenplates appear just below the eutectoid temperature. Thus, twotypes of product phases from the eutectoid reaction arediscovered in the austenite. One is pearlite of lamellar ferriteand cementite, and the other is complexWidmanstätten platesof (cementite+ferrite).

62 M A T E R I A L S C H A R A C T E R I Z A T I O N 7 7 ( 2 0 1 3 ) 5 3 – 6 2

Acknowledgments

The authors are pleased to acknowledge financial support forthis paper by the National Science Council, Taiwan, underGrant No. NSC-101-2221-E-011-044.

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