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 6 1 ( 2 0 1 0 ) 6 5 – 7 2

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Evolution of cementite morphology in pearlitic steel wireduring wet wire drawing

Xiaodan Zhanga, Andrew Godfreya,⁎, Niels Hansenb, Xiaoxu Huangb, Wei Liua, Qing Liuc

aAdvanced Materials Laboratory, Department of Materials Science and Engineering, Tsinghua University, 100084, PR ChinabCenter for Fundamental Research: Metal Structures in Four Dimensions, Materials Research Division, Risø National Laboratory forSustainable Energy, DTU, DK-4000 Roskilde, DenmarkcSchool of Materials Science and Engineering, Chongqing University, Chongqing, 400030, PR China

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +86 10 62788317E-mail address: awgodfrey@mail.tsinghua

1044-5803/$ – see front matter © 2009 Elsevidoi:10.1016/j.matchar.2009.10.007

A B S T R A C T

Article history:Received 27 May 2009Received in revised form9 October 2009Accepted 19 October 2009

The evolution of the cementite phase during wet wire drawing of a pearlitic steel wire hasbeen followed as a function of strain. Particular attention has been given to a quantitativecharacterization of changes in the alignment and in the dimensions of the cementite phase.Scanning electron microscope observations show that cementite plates becomeincreasingly aligned with the wire axis as the drawing strain is increased. Measurementsin the transmission electronmicroscope show that the cementite deforms plastically duringwire drawing , with the average thickness of the cementite plates decreasing from 19 nm(ε=0) to 2 nm (ε=3.7) in correspondence with the reduction in wire diameter. Thedeformation of the cementite is strongly related to plastic deformation in the ferrite, withcoarse slip steps, shear bands and cracks in the cementite plates/particles observed parallelto either {110}α or {112}α slip plane traces in the ferrite.

© 2009 Elsevier Inc. All rights reserved.

Keywords:CementitePlastic deformationMicrostructurePearlitic steelWire drawing

1. Introduction

Cold drawn high-carbon steel wires have the highest strengthof all mass-produced steel materials and are widely used inindustry for a variety of applications, including cables forsuspension bridges, and steel cords for automobile tires andsprings. Much effort has been put into improving the tensilestrength of high-carbon steel wires, with a maximum exper-imentally obtained value of 5.7 GPa [1]. However, manyaspects related to the properties of high-carbon steel wiresare still unclear [2], and in the present paper the microstruc-tural evolution of the cementite phase during the wire-drawing process is considered in detail.

Studies have been carried out dating back to the 1960s withthe aim of understanding the details of microstructural

; fax: +86 10 62771160..edu.cn (A. Godfrey).

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evolution during cold drawing of pearlitic steel wires [3–6].More recently a number of studies have focussed on theimportance of understanding the evolution of the cementitephase during deformation [7–10]. These studies includeinvestigations into cementite dissolution based on 3D atomprobe measurements of carbon concentration in the ferrite,which is generally found to be lower than the expectedstoichiometric value of 25at.% in wires deformed to highstrains [7,10]. The process by which cementite deformationand decomposition takes place during the drawing process istherefore of both scientific and technological interest. In thisstudy the deformation of cementite over a wide range of cold-drawing strain is investigated, concentrating on a detailedcharacterization of the changes in the macroscopic orienta-tion and the dimensions of the cementite phase over the

.

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entire strain range from measurements in both the longitu-dinal and the transverse sections.

2. Experimental Methods

A high-strength near-eutectoid steel with a carbon content of0.7 wt.% supplied by NV Bekaert SA (Zwevegem, Belgium)Technology Center Laboratory was used for the study. Speci-mens taken at four cold-drawing strains were investigated.These were taken a various steps of the overall wire drawingprocess from as-patented wire (1.26 mm) to the final drawnwire (0.20 mm). The specific strains investigated in detailin this study are listed in Table 1. In the following these arereferred to as specimens 1 (lowest strain) to 4 (highest strain).

Standard grinding and electro-polishing (10% perchloric acidin ethanol) procedures were used for preparation of specimensfor investigation in the scanning electron microscope (SEM) andtransmission electron microscope (TEM). The wires were exam-inedboth in the longitudinal and transverse sections. For theSEMobservations specimens were additionally etched in 4% Nital.Crystal orientation (electron backscatter diffraction, EBSD) mea-surements were carried out using a FEI Nova400 field emissiongun (FEG) SEM equipped with an Oxford Instruments–HKLTechnology Nordlys EBSD system, using a step size of 0.5 µm.

Statistics on the macroscopic orientation and length in thelongitudinal section and on the length of cementite plates/particles in the transverse section were obtained from SEMmicrographs covering an area of at least 400 pearlite colonies.In the longitudinal section the pearlite colonies are classifiedinto three macroscopic orientation classes, based on the anglebetween the length direction of the cementite plates and thewire axis (0°–30°, 30°–60° and 60°–90°; hereafter called typeA, B,and C respectively), and into five length classes (0–1 µm, 1–2 µm, 2–3 µm, 3–4 µmand>4 µm).A small number of cementiteparticles (defined as cementite with a ratio of length tothickness smaller than 1) were also observed. These ‘particles’in fact represent a 2D section through a rod-like cementitephase. For analysis purposes thesewere included in the type Aclass. For analysis of the transverse section, the length rangesusedwere changed for themedium and high strain specimensin order to obtain a more detailed picture of the cementitedimensions. At ε=1.5 (specimen 3) length ranges of 0–0.5 µm,0.5–1 µm, 1–1.5 µm, 1.5–2 µm and >2 µm were used, at ε=2.7(specimen 4) length ranges of 0–0.25 µm, 0.25–0.5 µm, 0.5–0.75 µm, 0.75–1 µm and >1 µm were used; and ε=0 and 0.7(specimens 1 and 2), the five length ranges usedwere the sameas those specified for analysis of the longitudinal section. Inorder to make the measurements the SEM micrographs wereenlarged to 850×1200 mm2 (approx. A0 size) and themeasure-mentsmade by cutting out andweighing on amicrobalance allareas within each defined macroscopic orientation class. This

Table 1 – Summary of wire samples used in this study.

Sample 1 2 3 4

Diameter (mm) 1.26 0.899 0.591 0.332Strain 0.00 0.68 1.51 2.67Area reduction (%) 0 49 78 93

method was found to be superior to computer-based imageanalysis given the large variation in colony area.

Additionally, measurements of cementite plate thick-nesses were performed on thin-foil specimens made fromthe longitudinal section using a JEOL 200CX TEM at 200 kV anda JEOL 3000 FEG-TEM at 300 kV. For each specimen data werecollected from over 30 areas. The TEM specimens were madeusing a double-jet electropolisher [11].

3. Results

3.1. Cementite Alignment and Length

Strain 0: In the initial as-patented condition the cementiteplates within each pearlite colony is neither parallel nor conti-nuous (Fig. 1A and B). This morphology may be connected withthe patenting process used in the production. A high wire-drawing speed is used during the patenting process, such thatthe transformation from austenite to pearlite should becompleted quickly. As a result two types of pearlite are seen inthe microstructure, referred to here as plate-like pearlite andparticle-like pearlite (as explained previously the particle-likepearlite represents a 2D-section through cementite with a rod-likemorphology). In some places these two types exist inwithinone pearlite colony (indicated by arrows in Fig. 1A and B). Themajority of the cementite is, however, of the expected plate-likemorphology. Most of cementite plates are less than 4 µm inlength, with only around 12% larger than 4 µm. Around 40% ofthe cementite plates are less than 1 µm in length, with theremainder between 1 µm and 4 µm. The measurements ofcementite size fromthe longitudinal and transverse sectionsarein good agreement. No preferredmacroscopic orientation of thecementite plates is seen in the longitudinal section (Fig. 2A).

3.1.1. Strain 0.7At this strain the cementite plates/particles have begun torotate into the drawing direction (for example 65% of theplates/particles are of type A, with only 18% of type C).Twisting into the drawing direction of cementite plates/particles with type C orientation, and the occurrence of localshear bands are also observed in the longitudinal section(Fig. 1C). In the transverse section, the twisting (curling) issevere and break-up of the cementite plates/particles takesplace (Fig. 1D). In the longitudinal section the percentage sum(type A+type B+type C) of cementite plates/particles in eachof the five length ranges are almost the same as those seen forthe as-patented specimen (Fig. 2C). In the transverse section aclear shift to smaller sizes of the cementite plates/particles isseen (e.g. the percentage with length <1 µm is increased by27% from 40.5% to 67.5% as shown in Fig. 2D).

3.1.2. Strain 1.5At this strain almost all of the cementite plates/particles haverotated tonear thedrawingdirectionandhave lengthened, suchthat 97%of the cementite in the longitudinal section is classifiedas type A, and the fraction of cementite plates/particles longerthan 4 µm is doubled compared to the lower strains (Fig. 2E). Inthe transverse section continued break-up of the cementiteplates/particles is seen (86% less than 1 µm, with 63% less than

Fig. 1 – SEM images from longitudinal sections (A, C, E, G; the drawingdirection is horizontal to the page) and transverse sections(B, D, F, H) of wires deformed to strains of ε=0 (A and B), ε=0.7 (C and D), ε=1.5 (E and F) and ε=2.7 (G and H).

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500 nm— see Fig. 2F). In the longitudinal section it is interestingto note that some cracks are seen parallel to the drawingdirection across cementite plates/particles (Fig. 1E). In thetransverse section a typical “curled” structure can be clearlyseen (Fig. 1F).

3.1.3. Strain 2.7At this strain in the longitudinal section all the cementiteplates/particles have rotated to near the drawing direction,and the cementite plates are greatly lengthened comparedwith the lower strains (Fig. 2G). In the transverse section all

Fig. 2 – Bar charts summarizing the data for the alignment (in the longitudinal section) and length (in the longitudinal andtransverse sections) of the cementite plates/particles as a function of drawing strain: ε=0 (A and B), ε=0.7 (C andD), ε=1.5 (E and F),and ε=2.7(G and H). The data are grouped into orientation and size classes as described in themain text. Percentages are given asdetermined from area fractions.

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cementite plates/particles are less than 2.5 µm in length, withmost of the cementite plates/particles (90%) less than 1 µm inlength (52% less than 250 nm) as shown in Fig. 2H. Thetwisting of cementite is severe and the cementite plates aremuch thinner compared with those seen at strain 1.5 (Fig. 1G).The development of a microstructure where the pearlitestructure is repeatedly folded along an axis parallel to thedrawing direction resulting in a typical curled pattern isobvious in the transverse section. In some places where thecementite plates/particles are comparatively long (above1 µm) shear bands are seen to develop across the cementite(see inset in Fig. 1H, which is an enlarged view of the areamarked by the black frame).

The data for the observed change with increasing strain inmacroscopic orientation (alignment) of the cementite and forthe change in the cementite plate/particle length are summa-rized in Fig. 2. For the longitudinal section the data are shownas fifteen columns, representing the area percentage accord-ing to the previously defined five length scales and threemacroscopic orientations (type A, B and C). The figure clearlyshows that the cementite plates/particles rotate towards andare lengthened along the wire axis with increasing strain, asreflected by the movement of the highest column from thebottom-left corner first to the upper-left corner at smallstrains and then to the upper-right corner at the higheststrains. In the transverse section, the cementite plates/particles are noticeably shortened with increasing drawingstrains: the percentage of cementite plates/particles less than1 µm increases from around 40% at ε=0 to 90% at ε=2.7.

3.2. Cementite Plate Thickness

The change in the thickness of the cementite plates as afunction of drawing strain is illustrated in Fig. 3. These datawere obtained from TEM investigations of specimens takenfrom the longitudinal section (note that in this figure data are

Fig. 3 – Graph showing experimental data (from TEMinvestigations) of the variation in cementite plate thicknesswith increasing strain. The reduction in thickness assumingthe cementite deforms uniformly with the macroscopic wiredeformation is also given. The error bars represent thestandard deviation for the data at each strain.

also included for a specimen deformed to a strain of ε=3.7). Ineach case care was taken to tilt the cementite plates to anedge-on condition. Example TEM images for ε=0 and 0.7 areshown in Fig. 4. The average cementite plate thickness wasfound to vary from 19 nm (at ε=0) to 2 nm (at ε=3.7).

The experimentally measured thickness of the cementiteplates (T) is compared in the figure with that expected at eachstrain from the geometrical reduction due to the change inwire diameter (D). In this calculation it is assumed that thedeformation is homogeneous throughout the wire and thatthe cementite deforms in proportion to the drawing strain. Forthis the following equations were used:

D0

T0=Di

Tið1Þ

ε = lnD0

Di

� �2

; ð2Þ

where D0 and Di are the wire diameters at strain 0 and i, T0 andTi are the thicknesses of cementite plates/particles at strain 0and i, and where ε is the strain.

Combining the above equations, gives:

Ti = T0eð−ε=2Þ ð3Þ

It is seen that the experimentally measured cementite platethickness matches the calculated thickness within the given

Fig. 4 – TEM images showing cementite plates in an edge-oncondition at strains of ε=0 (A) and ε=0.7 (B) (longitudinal wiresections).

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experimental error, suggesting that the cementite on averagedeforms in proportion to the change in wire dimensions.

4. Discussion

The experimental measurements in both the longitudinal andtransverse sections show a systematic change in the cement-ite morphology with increasing strain. Between ε=0 and ε=1.5the percentage sum of the cementite plates/particles in eachlength range in the longitudinal section remains almost thesame, whilst the alignment changes. As a result of thisprocess, more than half of the cementite plates/particles arerotated towards the drawing axis in the longitudinal section atε=1.5. At higher strains the cementite plates/particles becomealigned close to the drawing direction (97% at strain 1.5, 100%at strain 2.7) and are lengthened as seen in the longitudinalplane (only 14% are longer than 4 µm at ε=0.7, whilst 35% and60% of the plates are longer than 4 µm at ε=1.5 and 2.7,respectively). In the transverse section a significant shorten-ing of the cementite plates/particles is seen. Additionally TEMmeasurements of the cementite plate thickness shows thatthis reduces in correspondence with the decrease in the wirediameter, from 19 nm at ε=0 to 2 nm at ε=3.7.

Fig. 5 – SEMmicrographs showing cementite morphology at strainplanes of ferrite are also shown, as determined from EBSD orienferrite colony boundaries with a misorientation angle higher tha

These observed changes in dimensions and alignmentsuggest that the thin cementite plates deform during thewhole drawing process, which is in accord with earlier reports[3,5,12]. In order to examine the deformation mechanismsbehind the changes in the cementite plate/particle morphol-ogy further SEM investigations were carried out. Localobservations of the deformation of cementite are shown inFig. 5. In this figure the traces of the {110} and {112} slip planesin ferrite in a longitudinal section in a wire deformed to astrain 0.7 are indicated. The trace analysis shows that incolony A and colony C local shear bands develop that areparallel to the (1̄01) slip plane trace in the ferrite, and that incolonies B, D and E shearing occurs inmore than one direction,though in each case still parallel to either a {110} or {112} ferriteslip plane trace. For example, in colony B coarse slip steps inthe cementite are seen along the ferrite (01̄1̄) plane trace, andcoarse slip steps and cracks in the cementite are observedparallel to the ferrite (1̄1̄0) plane trace. In colonies D and Ecoarse slip steps and cracks in the cementite are seen parallelto (112̄) and (11̄2) slip plane traces of ferrite, respectively, whilelocalized shearing is seen parallel to the (1̄01) and (112) slipplane traces of the ferrite phase.

Several previous investigations have reported that {110}<11̄1>Fe3C, and slip on {100}Fe3C can be activated in wire drawing

ε=0.7 (longitudinal section). Traces of the {110} and {112} sliptation measurements. Thick grey lines are added to indicaten 15°.

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[12–14]. Slip on (121̄)Fe3C//(1̄10)α−Fe was also observed in 10%cold-rolled pearlitic steel sheets [15]. These results, togetherwith the present data, indicate that slip initially takes place inthe ferrite lamellae. This slip is then transferred into thecementite lamellae, as seen from the observations of coarseslip, shear bands and cracks across cementite plates/particlesparallel to {110}α−Fe and {112}α−Fe plane traces in the presentdata.

Taking this slip-dominant mechanism for deformation ofthe cementite into account (and considering that in the initialstate both plate-like and particle-like cementite is present) sixtypes of routes for changes in the orientation and thickness ofthe cementite plates/particles, can be envisaged, as illustratedin Fig. 6. In the initial state the cementite plates/particlespossess one of four types of morphology/alignment in thelongitudinal section. Fig. 6a shows a favorable orientation inrelation to the drawing axis since plates/particles so orientedoffer low resistance for dislocation slip in the ferrite phase.Across cementite plates/particles inclined or perpendicular tothewire axis (Fig. 6b, c and d) the applied deformation causes arigid body rotation, resulting in twisting, and in some casesfracture of the cementite as the strain is increased. Accord-ingly at a strain of 1.5 almost all (97%) of cementite plates/particles are oriented close to the wire axis in the longitudinalsection and most of the cementite plates/particles areshortened in the transverse section. At still higher strains allthe cementite becomes aligned close to the drawing directionand the cementite plates are lengthened and thinned in thelongitudinal section.

The observations show that the change in cementitemorphology occurs by a combination of uniform deformation(lengthening) and by localized deformation (e.g. via shearingor twisting). In each case it is suggested that the deformationof the cementite is closely related to slip in the ferrite phase.Based on this idea it is suggested that shearing across thecementite should take place most easily in cementite platesinitially lying at 45° to the wire axis (type B cementite).Cementite plates with this alignment will quickly thoughrotate to lie parallel to the wire axis. Additionally Type Acementite is expected to be deformed by shearing at allstrains, but with the shearing becoming more uniform (i.e.

Fig. 6 – Schematic illustration of six routes for changes in the maplates/particles in the longitudinal section with cold drawing. Theat the top of the figure: (A) parallel, (B) inclined, (C) vertical to the

more finely spaced) as the cementite becomes thinner duringthe drawing process.

It is perhaps surprising that the cementite plate thicknessis observed to reduce approximately in proportion to theoverall drawing reduction. However, at low strains the mostimportant aspect of the plastic deformation of the cementite isthe alignment of the microstructure with the drawing direc-tion. At higher strains extensive work hardening is expectedin the ferrite phase, which together with a large extent oflocalized shearing due to the high drawing speed used (around800m per minute), results in lengthening and thinning of thecementite plates. It is worth noting in this regard that thatseveral authors have previously reported “fragmentation” ofthe cementite phase at high drawing strains [8,16] (as seenfrom dark field TEM images). This fragmentation reflectschanges in the crystal orientation within the still continuouscementite phase resulting from plastic deformation. Furtherdetailed investigations to characterize these plastic rotationswithin the cementite are underway.

5. Conclusions

The evolution in the characteristics of the cementite phase ina pearlitic wire has been followed by using SEM and TEMobservations as a function of strain. The conclusions are thefollowing:

• In the initial (as-patented) state the cementite plates/particles have no preferred alignment. Observations inlongitudinal sections show that drawing to low to mediumstrains (ε=0.7–1.5) results in a continuous change in thecementite to give a preferred alignment parallel to the wireaxis, with this transition complete at a strain of ε=1.5.

• Drawing to higher strains results in a lengthening andthinning of the cementite. Some fracture of the cementiteplates also takes place, related primarily to the curlingdeformation seen in the transverse cross section.

• Coarse slip steps, shear bands and cracks in the cementiteplates/particles are formed parallel to {110}α or {112}α slipplane traces in the ferrite.

croscopic alignment, morphology and thickness of cementitefour initial cementite plate/particle morphologies are shownwire axis and (D) a cementite particle.

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• The cementite plates/particles deform plastically withincreasing strain leading to a reduction in the averagethickness from 19 nm to 2 nm. This change corresponds tothe reduction in wire diameter.

Acknowledgements

The authors thank NV Bekaert SA Technology Center Labora-tory (Zwevegem, Belgium) for the supply of the pearlitic steelwires used in this investigation. Financial support from NVBekaert SA (XZ, AG, QL and WL), and from the Centre forFundamental Research: Metals Structures in Four Dimensions(XH, NH) is also gratefully acknowledged. The authors alsothank B. Ralph for his useful comments during preparation ofthe manuscript.

R E F E R E N C E S

[1] Ochiai M, Nishida S, Ohba H, Kawata H. Application ofhypereutectoid steel for development of high strength steelwire. Tetsu-to-Hagane 1993;79:1101–7.

[2] Tarui T, Moruyama N, Takahashi J, Nishida S, Tashiro H.Microstructure control and strengthening of high-carbonsteel wires. Nippon Steel Tech Rep 2005;91(1):56–61.

[3] Embury JD, Fisher RM. Structure and properties of drawnpearlite. Acta Metall 1966;14:147–59.

[4] Langford G. Study of the deformation of patented steel wire.Metall Trans 1970;1:465–77.

[5] Langford G. Deformation of pearlite. Metall Trans1977;8A:861–75.

[6] Zelin M. Microstructure evolution in pearlitic steels duringwire drawing. Acta Mater 2002;50:4431–47.

[7] Read HG, Reynolds WT, Hono K, Tarui T. APFIM and TEMstudies of drawn pearlitic wire. Scripta Mater 1997;37:1221–30.

[8] Languillaume J, Kapelski G, Baudelet B. Cementite dissolutionin heavily cold drawn pearlitic steel wires. Acta mater1997;45:1201–12.

[9] Danoix F, Julien D, Sauvage X, Copreaux J. Direct evidence ofcementite dissolution in drawn pearlitic steels observed bytomographic atom probe. Mater Sci Eng 1998;250A:8–13.

[10] Hono K, Ohnuma M, Murayama M, Nishida S, Yoshie A,Takahashi T. Cementite decomposition in heavily drawnpearlite steel wire. Scripta Mater 2001;44:977–83.

[11] Christiansen G, Bowen JR, Lindbo J. Electrolytic preparation ofmetallic thin foils with large electron-transparent regions.Mater Charact 2003;49:331–5.

[12] Porter DA, Easterling KE, Smith GDW. Dynamic studies of thetensile deformation and fracture of pearlite. Acta Metall1978;26:1405–22.

[13] Maurer K, Warrington DH. Deformation of cementite. PhilMag 1967;15:321–7.

[14] Gil Sevillano J. Room temperature plastic deformation ofpearlitic cementite. Mater Sci Eng 1975;21:221–5.

[15] Tagashira S, Sakai K, Furuhara T, Maki T. Deformationmicrostructure and tensile strength of cold rolled pearliticsteel sheets. ISIJ Int 2000;40:1149–56.

[16] Hong MH, Reynolds WT, Tarui T, Hono K. Atom probe andtransmission electron microscopy investigations of heavilydrawn pearlitic steel wire. Metall Mater Trans 1999;30A:717–27.