Effect of integrated extrusion–equal channel angular pressing temperature on microstructural characteristics of low carbon steel

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Effect of integrated extrusion–equal channelangular pressing temperature onmicrostructural characteristics of low carbonsteel

M. Shaban Ghazani and B. Eghbali*

In the present research, a combined forward extrusion–equal channel angular pressing was

developed and executed for the deformation of a plain carbon steel. In this method, two different

deformation steps, including forward extrusion and equal channel angular pressing, take place

successively in a single die. The deformation process was performed at different deformation

start temperatures (800, 930 and 1100uC). Three-dimensional finite element simulation was used

to predict the strain and temperature variations within the samples during deformation. With

microstructural observations and the results of finite element simulation, the main grain refinement

mechanisms were studied at different deformation temperatures. The results show that the

forward extrusion–equal channel angular pressing is effective in refining the ferrite grains from an

initial size of 32 mm to a final size of ,0?9 mm. The main mechanisms of grain refinement were

considered to be strain assisted transformation, dynamic strain induced transformation and

continuous dynamic recrystallisation, depending on the deformation temperature.

Keywords: Extrusion–equal channel angular pressing, Steel, Finite element simulation, Grain refinement

IntroductionUltrafine grained (UFG) microstructures lead to greatermechanical properties of the bulk material.1–3 In recentyears, severe plastic deformation (SPD) has beenproposed as the most attractive method for the fabrica-tion of UFG metallic materials.4–6 So far, different SPDmethods have been suggested to produce UFG steels.7,8

Although equal channel angular pressing (ECAP)9 andsome other SPD methods have been used in someinvestigations, the deformation temperatures of pro-cessed steels have been always under critical transforma-tion temperatures. Therefore, plastic deformation hasalways been imposed on ferrite or ferrite plus pearlitephases, depending on the chemical composition ofprocessed steel. Moreover, in the last years, it has beenconfirmed that the combination of ECAP with otherconventional deformation processes results in improvedmechanical properties of the material.10 However, thereare a few attempts to investigate the applicability ofcombined SPD processes, including extrusion and ECAPon steels.11 Accordingly, in the present research, anintegrated forward extrusion–ECAP (EX-ECAP) wasdeveloped and executed for the deformation of a plaincarbon steel. The deformation process was performed at

different deformation start temperatures. The mainobjective was to investigate the effect of EX-ECAPtemperature and strain on the ferrite grain refinementmechanisms.

Experimental

Materials and deformation processThe material used in the present study was a plain lowcarbon steel with chemical composition (wt%) of 0.033C–0.12Si–0.8Mn–0.008S–0.007P–0.024Al–0.0038N, and ba-lance Fe. The initial microstructure of the as received steelconsisted of 95% ferrite with an average grain size of32 mm and remaining pearlite (Fig. 1). Initially, adifferential scanning calorimeter (DSC) with a coolingrate of 1uC min21 was used to measure the criticaltransformation temperatures. The Ar1 and Ar3 tempera-tures were found to be 745 and 835uC, respectively.

Cylindrical samples were machined out from the asreceived hot rolled plate. The samples were 40 mm inlength and 14 mm in diameter, with the longitudinalaxis aligned in the rolling direction of the plate. Dieand ram were designed and manufactured from hotworked tool steel. Figure 2 shows the two-dimensionalschematic representation of the die used in the presentresearch. As can be seen, the deformation processconsists of two different successive steps. At first, theforward extrusion process takes place in a verticalchannel, and the billet diameter is reduced from 14 to

Department of Materials Science Engineering, Sahand University ofTechnology, PO Box 51335-1996, Tabriz, Iran

*Corresponding author, email [email protected]

1802

� 2011 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 22 September 2010; accepted 19 November 2010DOI 10.1179/1743284710Y.0000000035 Materials Science and Technology 2011 VOL 27 NO 12

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7 mm. Therefore, the extrusion ratio is about 4 : 1,which corresponds to the true strain of 1?39. Then, thedeformed material passes the vertical channel, andsubsequently, the ECAP process executes at theintersection of two channels with equal cross-sections.When the sample passes through the intersection of twochannels, the value of the equivalent strain is dependentupon the values of two angles denoted by W and Y,where W is the die intersection angle, and Y is the outercurvature angle of two intersecting channels. The totaleffective strain in the pressed workpiece after a singlepass ECAP was given by Iwahashi et al.12

e~1

31=22cot

WzY

2

� �zYcosec

WzY

2

� �� (1)

The W and Y angles of the present die are 120 and 0urespectively, and the imposed strain is ,0?8 at theECAP step. The deformation process was conducted athot and warm deformation temperatures by preheatingworkpieces to the different deformation start tempera-tures and then inserting the sample in a cold die. Threedifferent deformation start temperatures, just beforeprocessing, were selected to fulfil the requirements forthe deformation at different phase regions. Thedeformation start temperatures of 1100, 930 and800uC were chosen to deform the material within stableaustenite, metastable austenite and ferrite region,respectively. In addition, the elapsed time between theremoval of billet from furnace and the start ofdeformation was 10 s. The process was performed witha ram speed of 10 mm s21. Also, a lubricant on thebasis of ultrafine graphite was used to reduce thefriction between workpiece and die channel wall.

Microstructure characterisationThe extrusion–equal channel angular pressed sampleswere cut into pieces using wire cut. Optical microscopywas used for microstructural investigations. For thispurpose, the pieces of samples were mounted so that themicrostructure on a selected surface can be studied. Thespecimens were mechanically polished and etched in 2%nital solution. Optical micrographs were taken just froman area after extrusion (point A) and from an area justafter extrusion plus ECAP (point B) on a planeperpendicular and parallel to the material flow direction.The location of points A and B are shown in Fig. 2. Theaverage equiaxed ferrite grain sizes were measured usingthe linear intercept method.

Finite element simulationThe deformation parameter (strain and temperature)variations were defined using three-dimensional finiteelement simulation. For this purpose, the commercialcode Abaqus 6?7 software was used. In the simulationprocedure, a cylindrical specimen having dimensions of14 mm diameter and 40 mm length was meshed with31 832 linear hexahedral elements of type C3D8T. Hotand warm flow curves obtained at different temperaturesranging from 600 to 1100uC were used to import themechanical properties of material to software. The otherphysical and heat transfer properties of the materialused in finite element method (FEM) simulation arelisted in Table 1. In the simulation, it was assumed that,90% of the work carried out in the deformationprocess is converted to heat and transforms equally todie and workpiece. The coefficient of friction betweenspecimen and die wall is selected as 0?05.

Results and discussion

Plastic strain distribution within sampleFigure 3a shows the three-dimensional plastic straindistribution in the sample at the end of EX-ECAPprocess. As can be seen, the plastic strain is notuniformly distributed throughout the sample. Strain atthe central regions is always lower than that at the nearsurface regions. This is mainly due to the existence offriction between specimen and die channel walls. Straindistribution at the centreline of the specimen is alsoshown in Fig. 3b. It is seen that the extrusion–equalchannel angular pressed sample can be divided into fourdifferent regions according to the development of plasticstrain at the centreline. The first region (I) denotes metalthat does not realise any plastic strain. The secondregion (II) denotes metal in the extrusion zone, which isbeing subjected to plastic deformation, and straingradually increases during plastic flow in this region.

2 Two-dimensional schematic view of EX-ECAP die used

in present investigation

1 Initial microstructure of as received steel

Shaban Ghazani and Eghbali Effect of extrusion–equal channel angular pressing temperature on low carbon steel

Materials Science and Technology 2011 VOL 27 NO 12 1803

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The third region (III) denotes metal that was previouslysubjected to plastic deformation by extrusion but doesnot realise ECAP. As can be seen, the equivalent plasticstrain is constant in region III. Finally, the fourth region(VI) denotes that the extruded metal is being subjectedto ECAP process. These results are in agreement withthose reported by other workers.13,14

Hydrostatic pressure and Ae3 temperaturevariationsFigure 4 shows the hydrostatic pressure variations in thesample at point A (in Fig. 2) during extrusion atdifferent deformation start temperatures. As can beseen, hydrostatic pressure changes continually duringdeformation and reaches the maximum value at the endof ECAP, which is in agreement with what has beenpreviously observed by other authors.11,13 Oscillations in

hydrostatic pressure value are related to the differentconstraints in the way of material during flow within thechannel. In addition, it is seen that with increasingdeformation start temperature, hydrostatic pressuredecreases.

It has been demonstrated that the imposed hydro-static pressure on austenite can alter the austenite toferrite transformation temperatures (Ae3 and Ar3).15 Ithas been reported that in low carbon steel equilibriumtransformation (cRa), temperature variations withpressure can be calculated by the following equation16

dT=dP~{110 K GPa{1 (2)

Accordingly, in the present research, the effect ofhydrostatic pressure on the equilibrium transformationtemperature of austenite to ferrite (Ae3) has beendetermined by the calculation of hydrostatic pressurevariations against time using FEM simulation andequation (2).

Figure 5 shows the temperature and strain variationsin the sample at point A (in Fig. 2) during EX-ECAPprocess at different deformation start temperatures. TheAe3 and Ar3 temperature variations are seen as well. Ascan be seen, at 800uC (Fig. 5a), the temperature of thespecimen decreases during extrusion from 750 to 710uC.Therefore, in this temperature range, warm deformationhas been executed on the ferrite phase and therebycontinuous dynamic recrystallisation (CDRX) canoccur.17 At 930uC (Fig. 5b), the temperature of thespecimen is between Ae3 and Ar3 during deformation.

a three-dimensional distribution of equivalent plasticstrain; b developments of plastic strain on centreline ofsample after completion of plastic deformation

3 Finite element method simulation results

4 Hydrostatic pressure and Ae3 temperature variations of

sample (corresponds to point A in Fig. 2) during extru-

sion at different deformation start temperatures

Table 1 Physical and heat transfer properties used in FEM simulation

Heat transfer by radiation Emissivity factor 1Ambient temperature/uC 13

Heat transfer by convection Film coefficient/W m22 uC21 10Sink temperature/uC 13

Heat transfer by conduction13 Interface pressure/MPa Conductivity/W m22 uC21

0 5000.03 9000.85 400014 650085 7500

Other physical properties Density/kg m23 7800Specific heat 460Conductivity/W m2-2 uC21 51


1804 Materials Science and Technology 2011 VOL 27 NO 12

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As reported,18,19 at this temperature range, the dynamicstrain induced transformation (DSIT) of austenite toferrite phase can be the main grain refinement mechan-ism. At 1100uC (Fig. 5c), plastic deformation is imposedabove Ae3 on austenite phase during continuous coolingfrom 980 to 920uC.

Evolution of microstructureFigure 6 shows the typical micrographs taken frompoint A (in Fig. 2) on a plane parallel and perpendicularto the flow direction of the deformed sample. As can beseen, at 800uC on a plane parallel to the flow direction(Fig. 6a), the microstructure consisted of elongatedferrite grains with fine grains near their boundaries.The average grain size of these fine grains is calculated tobe ,0?9 mm. On a plane perpendicular to the flowdirection (Fig. 6d), a curly structure was obtained.Furthermore, a very fine equiaxed grain consisted atdeformed ferrite boundaries. At 930uC, on a plane

parallel to the flow direction (Fig. 6b), the microstruc-ture can be divided in two distinct regions. The whiteregion consisted of ferrite grains with carbide particles attheir boundaries. The average grain size of ferrite at thisregion is calculated to be ,3 mm. The second regionconsisted of elongated ferrite grains with fine grains attheir boundaries (average grain size of 1?5 mm). It isworth noting that there is no carbide particle at theferrite grain boundaries in this region. On a planeperpendicular to the flow direction (Fig. 6e), thedeformed microstructure is uniform with the averagegrain size of 1?2 mm. Finally, at 1100uC (Fig. 6c and f),the microstructure on both planes is quite uniform andconsists of equiaxed ferrite grains with average size of4 mm.

Figure 7 shows the microstructures of the deformedsample at point B (in Fig. 2) on a plane parallel to theflow direction. This region of sample was subjected totwo successive deformations by extrusion and ECAP. Ascan be seen, the macroscopic shear bands in Fig. 7a andb demonstrate that the second deformation by ECAP isimposed below Ar1 temperature on the extrudedmaterial where ferrite phase is dominant. On the otherhand, one cannot see any macroscopic shear bandobtained in the microstructure at 1100uC (Fig. 7c). Thisis due to the fact that in this deformation starttemperature, both deformations (extrusion and ECAP)are imposed successively on austenite phase and thenferrite transforms from deformed austenite.

Grain refinement mechanism at 800uCAccording to the simulation results shown in Fig. 5a, itis seen that at the deformation start temperature of800uC, the specimen is deformed within the ferriteregion. It has been reported20 that dynamic recrystalli-sation (DRX) in ferrite is the main grain refinementmechanism during heavy deformation at elevatedtemperatures. However, the mechanism of DRX in lowcarbon steels is strongly influenced by the Zener–Hollomon Z parameter.21 At low Z conditions, conven-tional DRX, apparently similar to that seen in hotworked austenite, has been demonstrated to occur ininterstitial free steels.22 Ferrite grains .5 mm have beenreported to be obtained as a result of conventionalDRX.23 At moderate Z conditions (1012,Z,1014),CDRX occurs by clustering dense dislocation walls,which led to the progressive increase in misorientationangles among subgrains. Much smaller ferrite grainswith a size of 1 mm are obtained as a result of CDRX. Athigher Z conditions, CDRX does not occur becausethere is no sufficient time for dislocations to migrate intosub-boundaries. Therefore, only the substructure isdeveloped.24 As shown in Fig. 8, the results of FEMsimulation show that at 800uC, the Z parameter ismainly between 1012 and 1?261014 s21. Hence, thepossible grain refinement mechanism is CDRX. It is seenfrom Fig. 6a and d that the ultrafine grains mainlyconcentrated at deformed ferrite grain boundaries. Thisultrafine ferrite with average grain size of 0?9 mm wasdeveloped as the result of CDRX.

Grain refinement mechanism at 930uCAs illustrated in Fig. 5b, at deformation start tempera-ture of 930uC, the specimen temperature varies from 860to 785uC during deformation. It is mentioned in thesection on ‘Materials and deformation process’ that the

5 Strain and temperature variations in specimen (corre-

sponding to point A in Fig. 2) during 10 s elapsed

time and extrusion at different deformation start

temperatures



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Ar3 temperature of the specimen has been measured byDSC with a cooling rate of 1uC min21. However, thecooling rate of the specimen during deformation in colddie and under hydrostatic pressure was calculated to be50uC s21. Therefore, it is realised that the Ar3 tempera-ture during deformation is lower than the Ar3 tempera-ture measured by DSC. In addition, considering theeffect of hydrostatic pressure on critical transformationtemperatures, as illustrated in Fig. 4, it can be concludedthat the deformation has been applied within themetastable austenite region (Ae3,T,Ar3) during extru-sion. In this condition, the DSIT of austenite to ferrite islikely to occur.19 However, the critical amount ofdeformation energy is required to store in austenitebefore the start of DSIT. The critical strain for initiationof DSIT is about 0?1–0?8 for low carbon steels.18

Effect of strain on development of deformationinduced ferriteFigure 9 shows the effect of strain on the developmentof deformation induced ferrite grains in the extrusion

zone at the deformation start temperature of 930uC. Thebright phase is ferrite, and the dark phase is martensite,which has been transformed from austenite duringcooling.

As can be seen, ferrite grains nucleate preferably onprior austenite grain boundaries at the early stage ofDSIT (Fig. 9a). With increasing strain, new ferrite layersoriginate close to the c/a interface and then grow into theaustenite interior (Fig. 9b–d). In addition, it is apparentthat the strain rate increases continuously duringextrusion. As reported,25 at low strain rate, ferrite grainsnucleate mainly at prior austenite grain boundaries andappear as continuous bands. However, with increasingstrain rate, deformation induced ferrite grains becomefine and more equiaxed. This trend is also seen in Fig. 9.

Grain refinement mechanism at 1100uCThe results shown in Fig. 10 correspond to the evolutionof ferrite grain refinement at the extrusion zone when thespecimen deformation start temperature is 1100uC.According to the simulation results shown in Fig. 5c,

6 Optical micrographs of deformed microstructures (corresponds to point A in Fig. 2) on plain parallel to pressing direc-

tion at a 800uC, b 930uC and c 1100uC and perpendicular to pressing direction at d 800uC, e 930uC and f 1100uC

7 Optical micrographs of deformed microstructures (corresponds to point B in Fig. 2) on plain parallel to pressing direc-

tion at a 800uC, b 930uC and c 1100uC



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at the deformation start temperature of 1100uC, thespecimen temperature varies from 980 to 910uC duringdeformation. Therefore, it is apparent that at thisdeformation temperature range, steel is deformed inthe austenite single phase region, and thereby, ferritegrains nucleate from the deformed austenite. It is clearlyvisible that with increasing strain, the volume fraction offine ferrite grains is noticeably increased, whereas that ofaustenite (martensite) is reduced. In addition, it is seenthat the ferrite grain boundaries are serrated at lowerstrains (Fig. 10a and b) as a consequence of high coolingrate at the deformation zone during transformation.However, with increasing strain, the ferrite grainboundaries become smoother (Fig. 10c and d).

The effect of strain amount on the ferrite averagegrain sizes produced during extrusion is shown inFig. 11. As can be seen, the grain size at the beginningof deformation (e50?2) is ,17 mm. With increasing

strain, ferrite grain size noticeably decreases and reachesto ,4 mm at the end of extrusion (e51?4). It isknown26,27 that in undeformed austenite, ferrite nucle-ates only at austenite grain boundaries. However, indeformed austenite, there are additional nucleation sitessuch as deformation bands, twin bands and dislocationarrays that increase the number of possible ferritenucleation sites and accelerate the austenite to ferritetransformation. This transformation is the so calledstrain assisted transformation.

ConclusionsIn the present research, a plain carbon steel was sub-jected to EX-ECAP at different deformation starttemperatures (1100–800uC). The results obtained aresummarised as follows.

The mechanisms of microstructure refinement duringEX-ECAP processing are considered to be strainassisted transformation, DSIT and CDRX, dependingon the deformation temperature. For specimens sub-jected to EX-ECAP at 1100uC, the strain assistedaustenite to ferrite transformation is the main grainrefinement mechanism, and the minimum ferrite grainsize of ,4 mm is obtained at the end of deformation. At930uC, finer ferrite with grain size of ,1?2 mm isobtained as a result of DSIT of austenite to ferrite. At800uC, ultrafine ferrite with average grain size of,0?9 mm is obtained as a result of CDRX. Most ofthe ferrite grains are elongated and inclined to the ECAPdirection, which resulted from shear deformation duringdeformation at 800uC.

Acknowledgements

The authors would like to acknowledge the financialsupport provided for the present research by Sahand

8 Variations in Zener–Hollomon parameter during extru-

sion with deformation start temperature of 800uC

a e50?2; b e50?6; c e51; d e51?49 Development of strain induced ferrite grains with increase in strain in extrusion zone at deformation start temperature

of 930uC



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University of Technology. In addition, they are indebtedto Iran Tractor Forging Company for designing andmanufacturing EX-ECAP die. Thanks are also due toMr A. Alipour Jahani and Mr Naghizadeh for theirtechnical help.

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a e50?2; b e50?6; c e51; d e51?410 Evolution of ferrite grain refinement in extrusion zone at deformation start temperature is 1100uC

11 Effect of strain on ferrite average grain size, produced

during extrusion, at deformation start temperature of

1100uC



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Effect of integrated extrusion–equal channel angular pressing temperature on microstructural characteristics of low carbon steel