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Materials Science and Technology

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Application of EBSD to analysis of microstructuresin commercial steels

L. Ryde

To cite this article: L. Ryde (2006) Application of EBSD to analysis of microstructuresin commercial steels, Materials Science and Technology, 22:11, 1297-1306, DOI:10.1179/174328406X130948

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Published online: 19 Jul 2013.

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Application of EBSD to analysis ofmicrostructures in commercial steels

L. Ryde*

Electron backscatter diffraction (EBSD) is a very powerful technique for microstructural

characterisation and analysis of crystalline materials. Many of the structural parameters that

control the properties and performance of the material can be derived from EBSD data, e.g. grain

size, phase constituents, mechanical anisotropy and residual strain. This should make EBSD a

valuable tool to control and develop microstructures of commercial metallic materials but

technique has mainly been used in basic research at universities and not to the same extent for

research and development in industry. The development in scanning electron microscopes and

EBSD equipment in recent years makes it possible to measure ‘difficult’ structures, e.g. with

higher dislocation content, but can it manage the complexity of the structures of commercial

materials and achieve reliable data? During 2004 an EBSD round robin test on industrial metallic

materials was coordinated by KIMAB to test the current status of the technique and most of the

results presented here are from that project and participants in that test. The results and

conclusions from the test will be published when it is accepted by the financing organisation but

the present paper will focus on common problems or difficult tasks that appear during EBSD

measurements of some complex or mixed structures in steel and how they can be solved. All

participants experienced difficulties despite being familiar with the EBSD technique, which

demonstrated the need for a comparative study of this sort and need for guidance in these topics.

Keywords: Electron backscatter diffraction, Steels, Martensite, Retained austenite, Grain size, Plastic strain

IntroductionElectron backscatter diffraction (EBSD) is very usefulfor microstructural characterisation and analysis ofcrystalline materials, mainly used in metallography andgeology. The development of the EBSD techniquestarted in 1973 by Venables and Harland1 introducinga new diffraction technique in a scanning electronmicroscope (SEM) and continued with semiautomaticindexing in 1983 by Dingley et al.2 At this stage,commercial EBSD systems became available and thedevelopment continued as a competition betweendifferent systems with fully automatic indexing, EBSDmapping and then the possibility to combine chemicalinformation from EDX with crystallography fromEBSD. Other important recent development steps thathave had a dramatic influence on the usefulness ofEBSD are the introduction of field emission gun SEMs(FEG SEMs) and the development of high resolutiondigital EBSD cameras.

Modern commercial metallic materials must have welldefined microstructures to obtain the required specific

properties or performance and are at the same timeoften difficult to analyse with conventional microscopy.Many of the structural parameters that control theproperties and performance of the material can bederived from EBSD data, e.g. grain size, phaseconstituents, misorientation distributions and microtex-ture which gives data for modelling and prediction ofe.g. mechanical anisotropy and residual strain. And yet,the use of EBSD has been limited in industrial researchand development. This is partly because of the complex-ity of the structures of many commercial products andthe difficulties that it poses to achieve reliable EBSDdata but also because the technique is rather timeconsuming initially and therefore more expensive to usethan conventional microscopy. Another problem whenEBSD is used at high resolution, which is often the casefor high strength steels, is the instabilities in thespecimen stage and electron beam during the longperiods of measurements. Poor diffraction means thatthe beam has to dwell for a longer period at the sameposition and any instability will make the results useless.It is important to reduce this by making sure that thestage and sample are electrically grounded, the sampledoes not charge and that the stage has stabilised beforethe measurements start. The increased acquisition ratewith digital cameras has reduced the problem but hasnot eliminated it completely.

Corrosion and Metals Research Institute, KIMAB (former Swedish Institutefor Metals Research), Drottning Kristinas v 48, SE 114 28 Stockholm,Sweden

*Corresponding author, email [email protected]

� 2006 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 11 November 2005; accepted 16 March 2006DOI 10.1179/174328406X130948 Materials Science and Technology 2006 VOL 22 NO 11 1297

The difficult tasks or problems that appear during theround robin tests, and which will be addressed are:

(i) analysing martensite in steel may be difficultowing to poor diffraction and there may also beconfusion on what crystal symmetry to matchthe pattern with

(ii) analysing retained austenite in martensiticstructures seldom gives the same high levels ase.g. X-rays

(iii) separating phases with the same crystal sym-metry in multiphase structures. Typical exam-ples of this are separation of martensite andbainite from ferrite

(iv) errors when indexing ferrite (pseudo symmetry),martensite and bainite results in checkerboardpatterns in the orientation maps

(v) analysing plastic strain qualitatively andquantitatively.

ExperimentalThe round robin test performed during 2004 included arange of specified measurements defined by the partici-pating industrial partners and made on their materials, acomplete list of the studied steel samples are shown inTable 1.

Ten samples of every material were prepared andchecked for diffraction quality by the coordinator. Thegoal was to have reasonable Kikuchi patterns withoutdifferences between samples of the same material. Allparticipants received samples from all materials andinstructions for the test including recommended resolu-tion, size of the scanned area and what parameters to

measure. Only some of the samples will be discussed inthe present paper as indicated in Table 1.

The EBSD packages used in this test were: twosystems with TSL OIM4, four systems with INCACrystal and three systems with Channel 5 (oneparticipant had both TSL OIM4 and INCA Crystal).Five of the nine participants used digital EBSD cameras(Digiview II, Nordlys II and Nordif CD 200). TheEBSD systems were mounted on both tungsten filamentSEMs and FEG SEMs.

The present paper also includes examples from otherEBSD investigations performed at KIMAB on steelsamples, e.g. the TRIP steel and analysis of retainedaustenite in a different martensitic material containingchromium carbides. All tests at KIMAB are performedon a LEO 1530 FEG SEM equipped with Channel 5and Nordlys II EBSD camera. The final step inspecimen preparation was usually 20 min of automaticsilica polishing if nothing else was stated. The partici-pants of the round robin exercise are listed in theacknowledgements.

Analysing martensite in steelsThe trend towards higher strength in structural steelsmeans that martensitic structures are starting to be usedin many more products than the traditional engineeringsteels. Examples of this are dual phase sheet steels andheavy plate products where strength levels haveincreased from 350 to 1000 MPa and even higher. Thisemphasise the need to be able to better characterise andcontrol the constituent phases.

When the high temperature phase austenite (fccsymmetry) is quenched, a tetragonal phase is created

Table 1 Five steel materials and tests included in round robin exercise

Material Measurements Comments

Ferritic powder sample in the shapeof a ring, pressed but not sintered

Grain size, grain shape, residualstrain and distribution of strain,particularly comparison of innerand outer diameters of the ring

An example of a strain mapand how strain can beanalysed is given in thesection ‘Analysing plasticstrain, qualitatively andquantitatively’.

A martensitic ball bearing steel withcementite particles, low temperaturetempered (160uC for 1 h)

Distribution of grain boundarymisorientations and the ‘grain’size based on certain misorientations,i.e. 15 and 2u (prior austenite grainsize, packet (block) size, lath size)Fraction of residual austenite andcarbides (Fe–3C)Texture

This was one of the moredifficult samples of the testbut several participantsshowed that it was possibleto obtain reliable data. Theresults are discussed under‘Analysing martensite in steel’,‘Analysing retained austenitein martensite’.

An experimental low carbon bainitic steel Distribution of grain boundarymisorientations and the ‘grain’ sizebased on certain misorientations, i.e.15 and 2u (prior austenite grain size,packet (block) size, lath size)

The analysis is similar to what isdiscussed under the section‘Analysing martensite in steels’and in ‘Analysing bainite in steels’.

A dual phase steel Fraction of ferrite and martensiteGrain size of ferrite and martensiteTexture

The difficulties in this material areto get good detection rate in themartensite and to separate thisphase from the ferrite (with thesame crystal symmetry). This isdiscussed in the section ‘Separatingmartensite from ferrite’.

Duplex stainless steel, partly recrystallised Phase constituents, grain size in ferriteand austenite, degree of recrystallisationin austenite and ferrite

The required data are easilyaccomplished with EBSD andtherefore not included in the presentpaper.

Ryde Application of EBSD to analysis of steels

1298 Materials Science and Technology 2006 VOL 22 NO 11

through a displacive phase transformation. This is ametastable phase and its structure is usually describedas having a body centred tetragonal symmetry. Thetetragonality (a/c ratio) of the crystal depends on thecarbon content,3 and the ratio is expected to be unityabove a critical temperature for superlattice ordering,which, for room temperature should be approximately,2.5 at.-%C for pure Fe with carbon, according toZener.4 This means that in most practical cases, themartensite is, in fact, cubic. This needs to be emphasisedsince one of the participants has tried to use other crystalsymmetries than bcc to evaluate the martensitic patternswith poor result.

The excess amount of carbon in solid solution isresponsible for distortion of the lattice, resulting in a highdislocation content and residual stresses in the crystal.This is the reason for the high strength and hardness ofthe phase and also for the poor diffraction propertieswhen the structure is studied by EBSD. The problem withpoor diffraction can be greatly reduced by tempering ofthe martensite, a treatment that also will solve anyproblem with the variation in lattice parameters owing tothe carbon content. The structure will then precipitatecarbides and return to the symmetry of ferrite (bcc).

The tempering treatment may, however, affect themisorientations in the structure, depending on theannealing temperature and time, as shown by Sureshet al.,5 and the chemical composition will also influencehow effective the tempering treatment is. Even with atempered martensite it is very difficult to obtain goodquality diffraction patterns in a tungsten filament SEM,and much better results are obtained in FEG SEMs witha smaller interacted volume. It is also a great advantageto have a digital camera and to use the binningpossibility to further improve the relation between thediffraction pattern and the background.

Microstructural parameters that are important to mea-sure in martensite are related to grain size, prior austenitegrain size, i.e. the grain size of the austenite when the phasetransformation occurred, packet size and lath size, as shownin Fig. 1. These parameters are considered to have animportant influence on the toughness of the material. Theaspect ratio of the laths may also be important.

It is also important to point out that any grain sizemeasured by EBSD will be affected by how themeasurements are made. It is obvious that it is necessaryto have a large number of grains in the map, and that theselected area should be representative of the structure,but it is also important to define what is accepted as thesmallest grain, given by a threshold in number of pixelsor mm.

All EBSD users should also be aware that theresolution of the map and estimated grain size is equalto the distance between adjacent measurements, unless astep size smaller than the resolution of the SEM hasbeen used. A single pixel should never be counted as agrain.

The prior austenite grain size is often defined in EBSDanalysis as being surrounded by high angle boundaries(.10 or 15u) and often excluding those with a S3relationship (60u rotation around a common [111] pole),as many of the boundaries created by the transformationhave this relationship.

An example of a grain map with this definition isshown in Fig. 2a, with the result that many smallmartensite laths are considered as separate prioraustenite grains, which skews the grain size distribution.The impression that the observer gets from this structureis that there are not more than 5–7 prior austenite grainsin the image, whereas if all the grains defined by theEBSD criterion are counted, the number will be over500. The grain boundary distribution in this martensiticstructure is shown in Fig. 3. This is the distributionusually found after a martensitic transformation in steel.The prior austenite grain boundaries are expected tofollow Mackenzie’s grain boundary distribution for annon-textured cubic structure, which indicates that mostof the boundaries created during the transformation areeither low angle boundaries (,15u) or high angleboundaries in the range from 50 to 63u (see Fig. 3).

If the misorientation axes of the grain boundaries inthe transformed structure are plotted in inverse polefigures (see Fig. 4), it is clear that the low angleboundaries are more randomly distributed than thehigh angle boundaries. It is also obvious that there areother preferred rotation axes than the S3 relationship,which makes the commonly selected definition ratherlimited. Several research groups6–8 are working withreconstructing mother grains from predictions of theorientation relationship between mother and daughtergrains during phase transformation. They are using boththe information on the angle and common axis, but thehigh number of possible transformation variants has sofar hindered the attempts from being a success in cubicsystems.6

Packets of laths are often defined as having amisorientation .5u, also excluding S3 boundaries, butfollowing the same argument as for the prior austenitegrains, this is not sufficient, although it is a betterapproximation. An example of the packet definition onthe same set of data as that in Fig. 2a is shown in Fig. 2b.

The most commonly used definition of a lathboundary is a boundary with a misorientation .2u.This works well as can be seen from the orientation mapin Fig. 2c. It is important to point out that the measuredvalues always should be accompanied by informationabout the resolution used in the map (distance betweenpixels) and the threshold for the smallest grain acceptedsince these values influence the results. In this case, theminimum size of an accepted grain is 4 pixels.

Analysing retained austenite in martensiticsteelsThe amount of retained austenite is crucial to themechanical behaviour of martensitic steels and is some-thing that can be difficult to analyse. A martensitic ball

1 Schematic picture of prior austenite grain after trans-

formation to martensite


Materials Science and Technology 2006 VOL 22 NO 11 1299

bearing steel was included in the round robin test andaccording to X-ray measurements made by the industrialpartner there should be 5–15% retained austenite in the

structure, depending on if the selected area was of highcarbon or low carbon type. After conventional samplepreparation, the sample was polished for 20 min withsilica as a final step before the EBSD investigation andvery little retained austenite was found by any of the sixparticipants, and then usually ,1%. It is possible thatthe retained austenite is unstable and may transform tostrain induced martensite owing to the deformationintroduced during specimen preparation, which wouldresult in an underestimate. It has been argued thatretained austenite should give fairly good diffraction butit is possible that the volume expansion during themartensite phase transformation increases the disloca-tion content also in the austenite. Alternative specimenpreparation methods to create an unstrained surface,free of topography may improve the results but it ispossible that merely the introduction of a free surfaceallows the austenite to transform, whereas it is hinderedfrom doing so when it is surrounded by martensite. X-ray measurements of retained austenite may suffer fromthe same problem but to a lesser extent since thepenetration of X-rays below the surface is considerablygreater. The poor diffraction often leaves the fewaustenite measurements as single pixels surrounded byunindexed points, and these are difficult to trust unlesstheir orientation is checked by comparing its orientationwith the neighbouring orientation of the martensite andwith other pixels from the same prior austenite grain. Bychecking their orientation, also single pixels of austenitecan be trusted if they fulfil the criterion of havinga ‘correct’ orientation relationship with the martensiteor other nearby austenite pixels, see the orientationmap and corresponding pole figure in Fig. 5. Notethat only the austenite pole figure is presented, showingfour orientations with two S3 relationships. Theorientation relation between the surrounding martensiteand the retained austenite should correspond tothe Kurdjumov–Sachs or Nishiyama–Wassermanrelationship.

Additional support to the theory that the problemsare caused by the strains from the phase transformationand the surrounding martensite, is given from an EBSDmap on a TRIP steel (see Fig. 6). This material consistsof ferrite and retained austenite and the austenitetransforms to strain induced martensite during deforma-tion, resulting in an excellent ductility. There is

3 Grain boundary distribution in martensitic steel (filled)

compared with Mackenzie distribution

a prior austenite grains defined as being surrounded by15u high angle boundaries but excluding S3’s; b packetboundaries defined as having larger misorientation than5u and excluding S3 boundaries, grain structure is given;c lath boundaries defined as having larger misorienta-tion than 2u

2 EBSD maps showing different grain size definitions

martensitic steel (colour in online version)



obviously no problem to identify the metastableaustenite in this case when it is stabilised throughalloying partitioning instead of by strain.

Separating martensite from ferriteAn important type of commercial sheet steels is thefamily of dual phase steels. These are ferritic steels withvarying amounts of low temperature tempered marten-site and their strength levels increase with the martensitecontent. The problem when these are analysed with

EBSD is to get good patterns from the martensite and toseparate this phase from the ferritic (with the samecrystal symmetry, as discussed earlier). Different labora-tories, participating in the round robin test, useddifferent approaches and properties of the measure-ments to separate the ferrite from the martensite. Twolaboratories used the difference in grain size between thetwo phases, and selected a threshold in grain size of2 mm, i.e. large grains were ferrite and small grains weremartensite. This method is supported by the fact that

4 Rotation axes for all boundaries in EBSD map of martensitic steel: major parts of high angle boundaries created by

transformation have misorientation in range 50–63u

5 a orientation map of retained austenite and b corresponding pole figure of austenite: all points may have allowed

orientation relationship if prior austenite grain boundary is present as indicated by black line; pole figure of marten-

site must also be checked to ensure that relationship follows Kurdjumov–Sachs or Nishiyama–Wasserman relation-

ship between austenite and martensite (colour in online version only)



many new boundaries are created during the displacivephase transformation from austenite to martensite andthe result is a good description of the structure althoughthere is some overlap between the two phases. It is alsoclear from the grain boundary distribution of the twophases (see Fig. 7) that this is a reasonable approach.The distribution for the high angle boundaries of theferrite more or less follows the Mackenzie distribution,as a randomly textured structure would, whereas thedistribution of boundaries in the martensite is typical forthat of a fast phase transformation, i.e. only low angleboundaries and boundaries .50u are present. It is likelythat this method may not be as successful in other dualphase materials, e.g. if the ferrite grains are smaller.

The laboratory at EDAX/TSL used a grain averagevalue of the ‘image quality’, a quality measure of thediffraction property of the analysed Kikuchi pattern.The diffraction pattern quality is lower for themartensite than for the ferrite but this is also the caseat grain boundaries. This property ‘image quality’should be equivalent to the ‘band contrast’ in theChannel 5 suite or the ‘pattern quality’ in the Oxford/Link system. The ‘image quality’ was averaged over eachgrain and a threshold was selected, grains above thisthreshold value, with good diffraction, was defined asferrite, and the grains with an average poor diffractionwas considered martensitic. The extra processing of thedata to evaluate the average ‘image quality’ value for

each grain, and not only for the single pixels, results in avery good separation of the two phases. A threshold ingrain size of 1.5u was used for the calculation of thegrain average ‘image quality’ but when the grain sizes ofthe different phases were determined, the standard value5u was used to separate grains, and that definition gavethe result in Fig. 8. This method gives a very goodseparation of ferrite and martensite and agrees well withthe understanding of the microstructure although somesmall areas inside the ferrite grains are identified asmartensite. Another successful attempt to separatemartensite, ferrite from bainite using a combination ofthe ‘image quality’ and the ‘confidence index’ with thesame EBSD system has been reported by Watershootet al.9

A different approach was made by the laboratory atHKL technology; they used the band slope of theKikuchi pattern, which was the slope of the intensitychange between the background of the pattern and theband, and could also be described as the sharpness of theband edges. With this measure, well separated peaks forthe two phases were obtained without extra processing(see Fig. 9) and this could be used to differentiatebetween martensite and ferrite. An additional cut off in

6 Austenite (in colour) in ferritic matrix (in grey) in TRIP

steel (colour in online version only)

7 a orientation map of dual phase steel with grains identified as ferrite in blue and martensite grains in colours and

grain boundaries distributions of b ferrite and c martensite grains: phases were separated by threshold in grain size,

i.e. small grains (,2 mm) are martensite and large grains are ferrite; Mackenzie distribution is indicated behind

observed

8 Separation of ferrite and martensite: according to results

from laboratory at EDAX/TSL, ferrite is light and marten-

site is dark; phase separation is based on ‘image quality’

value, averaged over each grain; ‘image quality’ property

is poorer for martensite than for ferrite



grain size was also used to enhance the separation. Thismethod describes the structure very well. The disadvan-tage of this method is that the Kikuchi band edges of thepatterns must be detected and in phases with poordiffraction, e.g. martensite, this setting may reduce thedetection rate compared with detecting Kikuchi bandcentre only.

Analysing bainite in steelsBainite is also a phase that transforms from austeniteduring fast cooling. Many of the requested microstruc-tural data are similar to those discussed under thesection on martensite, e.g. former austenite grain size,packet, lath sizes and grain boundary misorientations.This is usually not a problem for EBSD analysis sincethe diffraction is much better in bainite compared withmartensite.

Bainite is a eutectoid phase comprising of ferrite andcementite platelets where the carbide precipitates in acooperative process during the phase transformation.10

It is therefore impossible to separate bainite from ferritemerely by crystallography. To complicate things further,the structure of bainite depends also on the alloyingcomposition and on the transformation temperature.Bainite is conventionally separated in two types: upperand lower bainite, depending on its transformationtemperature resulting in differences in carbon partition-ing. Owing to the normally high cooling rates involved,there is usually more dislocations in bainite comparedwith ferrite, but not to the same extent as in martensite.As a result, it is difficult to differentiate between bainiteand ferrite, but the same approaches as described formartensite should be tried. To separate bainite fromWidmanstätten ferrite can also be difficult, particularlyif the content of carbon is low in the bainite.11 Asuccessful attempt to separate bainite from polygonalferrite and martensite in a sheet steel, based on a grainaverage of the band slope, as described before, is shownin Fig. 10. No attempt has been made to analyse if this isupper or lower bainite. The microstructure consists ofpolygonal ferrite, retained austenite, martensite andbainite. In this case ferrite grains were defined as grainswith a bcc structure and an average band slope value ofmore than 130 (on a scale from 0–255), bainite with aband slope between 90 and 130 and martensite grains

9 Description of a band slope method and b band slope

map to separate martensite from ferrite and c distribu-

tion of band slope in dual phase steel: this ferrite–

martensite separation method is suggested by HKL

technology

10 a band slope map and b phase map of steel with fer-

rite, retained austenite, martensite and bainite: auste-

nite was separated by crystal symmetry whereas

body centred phases were separated based on grain

average value of band slope; ferrite in yellow, bainite

in blue, martensite in green and austenite in red (col-

our in online version only)



had mean band slope values ,90. Separation of bainitefrom martensite in complex microstructures is normallydone by analysing the directions of the carbideprecipitates with high resolution microscopy. Carbidesin martensite precipitates from a solid solution and thereare three possible crystallographic planes on which theplate shaped carbides form, whereas only one set ofplanes can be active for the cooperative bainitetransformation. To separate martensite from bainitebased on the directions of the carbides is hardly anapplication for EBSD since the carbides in commercialsteels often are too small.

Misindexing (pseudosymmetry) in ferrite,martensite and bainiteIt is usual that the misorientation distributions for thethree phases have erroneous peaks at 30 and/or 60u (seeFig. 11). These errors will appear in the maps as singlepixels surrounded by a 60 or 30u misorientation or as‘checker board pattern’ inside grains (see Fig. 12). Thisis a result of using too few Kikuchi bands for theindexing procedure and/or band centre instead of bandedges. A high resolution in the Hough space may alsoreduce the problem with pseudo symmetric solutions.Other possibilities are to move the phosphor screen

closer to the sample and increase the minimum numberof bands for indexing. Changing the accelerating voltagecan also be tried; a decrease will widen the Kikuchibands but they will be more diffuse. The rotation axis is[111] for this pseudo symmetric relationship in bcc. Sincethe misorientation distribution for martensitic andbainitic material always have a true peak at 60u, owingto the phase transformation, errors may be hidden, butif the peak at 30u is present, it is clear that there areerrors in the data set.

Analysing plastic strain, qualitatively andquantitativelyResidual strain is important to evaluate in variousstructures, both deformation mechanisms and possiblerisk of failure. To study where the largest deformationsare located, the best principle is to use pixel to pixelmisorientations. The software then evaluates the mis-orientation of a pixel to its neighbours and colours thepixel according to the size of the misorientation. Highangle grain boundaries are neglected. An example fromsuch an orientation map of a compacted iron powderring is shown in Fig. 13. The measurements were madeto see if the strains were larger on the inner part of thering than on the outside and the example in Fig. 13 istaken from the inner part. Very little difference in strainwas found at different positions; the distributions ofmisorientation at different positions were very similar. Itis clear from the map that most of the deformation islocated at the surface of each powder particle since thecolour is green in these areas, whereas the interiors ofthe particles are less strained and are therefore coloured

11 Misorientation distribution for bainitic sample with

misindexed data: arrow indicates erroneous peak at

30u and possible error at 60u

12 Orientation map with ‘checker board pattern’, i.e. mis-

indexed data points owing to pseudosymmetry

13 a local strain map and b distribution of misorienta-

tions of pressed iron powder sample: highest misor-

ientations are found next to surface of particles and

interior of particles are less strained, i.e. they have

smaller local misorientation; orientation map is from

HKL technology (colour in online version)



blue, see also the legend in Fig 13. It is also obvious thatthe distribution of misorientations, see the legend inFig. 13, is highly dependent on the distance betweenadjacent measurements. A smaller step size will inevi-tably give a higher proportion of lower misorientationsthan a larger step size over the same distance (e.g. agrain) at the same strain. To compare data fromdifferent maps it is therefore extremely important thatthe same step sizes are being used and also the settingsinfluencing the angular accuracy. The effect of varioussettings on resolution has been discussed, e.g. byHumphreys et al.12 and will not be dealt with here.

The local misorientation data are correlated, i.e. onlyadjacent measurements are being compared, whereasuncorrelated data can be used to evaluate averagestrains. Average intra grain misorientation, i.e. uncorre-lated data from one grain, are less sensitive to the stepsize and are strongly related to the rotation of the crystaland the dislocation content. The average intra grainmisorientation is calculated from the average orientationof the grain and then all pixels within the graincompared with that average orientation which givesthe average misorientation inside that grain. Theprinciple is similar to the method described to separateferrite from martensite by TSL/EDAX in that averagevalues over single grains are used.

If all average misorientations from all grains in a mapare averaged, this value can be correlated and calibratedwith known strains, e.g. from tensile tests of the samematerial, as was reported by Angeliu13 and Sutliffe.14 Anexample of an average intra grain misorientation map ofa stainless steel strained to a true strain of 0.12 is shownin Fig. 14. The strain–misorientation correlation curve

for stainless steel and copper,13,14 together with theresults on a stainless steel made at KIMAB, is presentedin Fig. 15. Note that the presented relationship is notvalid for all materials, or if the deformation state shouldbe altered, but it is likely that a similar relationship canbe found.

At these levels of strain, there is a linear relationshipbetween the measured average misorientation and theapplied true strain. Again, it is important to set a lowerthreshold to what is accepted as a grain to have reliabledata that are not skewed by single pixels. Each grain inthe map has an even colour since it is the averagemisorientation of the grain that is shown in the map. Nolocal area of high misorientation can thus be shown in amap like this. Note that the same type of relationship isfound by different researchers, with different micro-scopes and in two different materials. This method isvery useful to evaluate strain close to welds or atsurfaces after e.g. cutting or grinding but this techniqueapplies to average values and should be used with carewhen the analysed area is reduced to a few grains.

ConclusionsAn EBSD round robin test has been performed onindustrial samples by laboratories at universities, insti-tutes, EBSD suppliers and in industrial laboratories.Some of the problems appearing for steel, that weredealt with during the test, have been discussed in thepresent paper. The purpose of the round robin exercisewas to test the current status of the technique, can itmeasure the parameters that are important to theperformance of commercial material?

The industrial partners asked for various measure-ments of grain size in martensitic and bainitic structuresthat were difficult to reveal by other metallographictechniques. The test showed that analysing martensite insteel might be difficult owing to poor diffraction butreliable data could be obtained with EBSD, even on lowtemperature tempered martensite, particularly if FEGSEMs and digital cameras were used. Prior austenitegrain size is difficult to analyse in martensitic steels sincethe old grain boundaries are partly destroyed during thedisplacive phase transformation. Packet size suffersfrom the same problem but lath size is easily measured.

14 a average intra grain misorientation map and b distri-

bution of average misorientations of stainless steel

strained to 0.12

15 Relationship between average intra grain misorienta-

tion and applied true strain for stainless steel and

copper



The industry wants measurements of retained auste-nite in martensitic structures and the experience of theauthors is that EBSD seldom gives the same high levelsas e.g. X-rays. This is probably due to the austenitetransforming to deformation induced martensite duringthe sample preparation or to relaxation of stressescaused by the introduction of a free surface and istherefore a problem that may be difficult to overcome.

Strain and the local distribution of strain after variousprocessing steps are also important information anddifficult to analyse with other techniques. Both corre-lated and uncorrelated EBSD data can be helpful in thisanalysis.

Much time is spent separating phases in multiphasesteels. If EBSD could separate martensite and bainitefrom ferrite, this could be a very valuable application.Several useful methods of separating martensite and/orbainite from ferrite are presented but all methods haveto be tuned depending on the specific sample studied.

All participants have put in a lot of effort into theround robin test and the conclusions are as follows.

1. All the samples had complex structures andprovided real challenges for the application of EBSD,or any other, metallographic technique. Most of thedifficulties that were observed could be overcome, asshown in the present paper.

2. Unique data from crystallographic structure, localorientation and misorientations have been used toanalyse phases, crystallographic texture, grain size andgrain size distributions and strain. This cannot beachieved by any other technique alone.

3. The results are very promising and many impor-tant parameters could be extracted also from the mostdifficult samples.

4. Most of these samples are difficult to analyse and itis an advantage to use high resolution FEG SEMs forthis analysis.

5. The use of digital cameras has also shown to beuseful, particularly binning, allowing fast acquisitionand/or the use of smaller apertures to obtain higherspatial resolution.

Acknowledgements

The work presented in the present paper has mainlybeen performed within the round robin project financedpartly by Nordisk Innovations Center and by SwedishSteel, Ruukki, Ovako Steel, Sandvik MaterialsTechnology, Höganäs AB, Otokumpu Stainless,Outokumpu Copper and Hydro Aluminium. Theparticipation of the laboratories at TSL/EDAX,Netherlands; HKL technology, Denmark; OxfordInstruments, England; NTNU, Norway; SSABTunnplåt, Sweden; SAPA, Sweden and OuluUniversity, Finland is hereby greatly acknowledged.Some of the results are from other projects andperformed at the Corrosion and Metals ResearchInstitute, KIMAB. Thanks to Professor Hutchinsonfor fruitful discussions and guidance during the projectand preparation of the present paper.

References1. J. A. Venables and C. J. Harland: Philos. Mag., 1973, 27, 1193–

1200.

2. D. J. Dingley, M. Longden, J. Weinbren and J. Alderman: Scan.

Electron Microsc., 1987, 1, 451–456.

3. N. Seljakow, G. Kurdjumow and Z. Goodtzow: Z. Physik, 1927,

45, 384.

4. C. Zener: Trans. AIME, 1946, 167, 550.

5. M. R. Suresh, I. Samajdar, A. Ingle, N. B. Ballal, P. K. Rao and

P. P. Sinha: Ironmaking Steelmaking, 2003, 30, 379–384.

6. C. Cayron, L. Briottet and P. H. Jouneau: Proc. Channel Users

Meet., Ribe, Denmark, June 2004, HKL Technology, 19–24.

7. A. F. Gourges, H. M. Flower and T. C. Lindley: Mater. Sci.

Technol., 2000, 16, 26.

8. D. W. Suh, J. H. Kang, K. H. Oh and H. C. Lee: Scr. Mater., 2002,

46, 375.

9. T. Waterschoot, L. Kestens, B. C. De Cooman: Metall. Mater.

Trans. A, 2002, 33A, 1091–1102.

10. G. R. Purdy and M. Hillert: Acta Metall., 1984, 32, 823–828.

11. G. Thewlis: Mater. Sci. Technol., 2004, 20, 143–160.

12. F. J. Humphreys and I. Brough: J. Microsc., 1999, 195, (1), 6–9.

13. T. M. Angeliu, P. L. Andresen, E. Hall, J. A. Sutliff and S. Sitzman:

Proc. Corrosion 2000 Conf., Houston, TX, March 2000, NACE

International, Paper 186.

14. J. A. Sutliff: Microsc. Microanal., 1999, 5, (Suppl. 2), 236.



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