characterization of the carbides and the marten site phase in

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Characterization of the carbides and the martensite phase in

powder-metallurgy high-speed steel

Matja Godeca,, Barbara etina Batia, Djordje Mandrinoa, Ale Nagodea,Vojteh Leskoveka, Sreo D. kapinb, Monika Jenkoa

aInstitute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, SloveniabJozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

A R T I C L E D A T A A B S T R A C T

Article history:

Received 16 July 2009

Received in revised form

28 January 2010

Accepted 1 February 2010

A microstructural characterization of the powder-metallurgy high-speed-steel S390

Microclean was performed based on an elemental distribution of the carbide phase as

well as crystallographic analyses. The results showed that there were two types of carbides

present: vanadium-rich carbides, which were not chemically homogeneous and exhibited a

tungsten-enriched or tungsten-depleted central area; and chemically homogeneous

tungsten-rich M6C-type carbides. Despite the possibility of chemical inhomogenities, the

crystallographic orientation of each of the carbides was shown to be uniform. Using electron

backscatter diffraction the vanadium-rich carbides were determined to be either cubic VC or

hexagonal V6C5, while the tungsten-rich carbides were M6C. The electron backscatter

diffraction results were also verified using X-ray diffraction. Several electron backscatter

diffraction pattern maps were acquired in order to define the fraction of each carbide phaseas well as the amount of martensite phase. The fraction of martensite was estimated using

band-contrast images, while the fraction of carbides was calculated using the

crystallographic data.

2010 Elsevier Inc. All rights reserved.

Keywords:

Tool steel

Carbides

EBSD

XRD

AES

EDS

1. Introduction

Powder metallurgyis an importantalternative route for tool-steel

production. Its main advantage is the possibility to obtain a

refined steel that is almost free of inclusions and has a uniform

microstructure, which then leads to a steel with isotropic

performance [1,2]. The high-speed steel produced by powdermetallurgy contains primary carbides that are rich in vanadium,

tungsten and molybdenum. These carbides play an important

role in the mechanical properties of the steel, including the

hardness, the wear resistance and the temperature resistance.

It is known that in S390 Microclean, as well as in steels with

similar compositions, two carbide types, i.e., tungsten-rich M6C

and vanadium-rich MC, are formed [3]. However, with regard to

the vanadium-rich carbide phase, it is not clear whether the

carbides that are present are really VC. Q.L. Yong et al. [4]reported

that it is usual for most of the vanadium carbides in this type of

steeltobeV8C7or V6C5. Laterexperiments [5]usingTEM identified

the vanadium-rich carbides as V6C5 in PM HSS Vanadis 4 (V4).

The main goal of this study was to identify the carbide phases

in powder-metallurgy S390 Microclean by using the electron

backscatter diffraction (EBSD) technique, which was originallydeveloped for microtexture analysis. However, progress in

instrumentation and the development of software now mean

that it is possible for this technique to be used for phase analyses

[6]. Although one of the first applications of the EBSD technique

for the identification of carbides in steel was performed by V.

Randle and G. Laird in 1993 [7], publications concerning the

characterization of carbides using EBSD are still rare. However,

there is some related work concerning carbide analyses in

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 ) 4 5 2 4 5 8

Corresponding author. Tel.: +386 14701900; fax: +386 14701939.E-mail address: [email protected] (M. Godec).

1044-5803/$ see front matter 2010 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2010.02.003

a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

w w w . e l s e v i e r . c o m / l o c a t e / m a t c h a r
mailto:[email protected]://dx.doi.org/10.1016/j.matchar.2010.02.003http://dx.doi.org/10.1016/j.matchar.2010.02.003mailto:[email protected]


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different steels [810]. Besides carbide identification this tech-

nique wasalso used for martensite determinationbased on EBSD

pattern quality images (band contrast). A similar approach was

successfully applied by S. Zaefferer et al. [11]. In contrast to our

investigation where the martensite was separated from the

carbides to determine the fractions of each phase, S. Zaefferer

used this method to distinguish between bainite and ferrite. The

EBSD resultsin this studywere confirmedbythemoreestablishedX-ray diffraction (XRD) method. Since the carbides were not

homogenous, chemical characterization in the form of energy-

dispersive spectroscopy (EDS) and Auger electron spectroscopy

(AES) line profiles were performed.

2. Experimental

The material used was Bhler S390 Microclean with the

following chemical composition: C 1.6%, Cr 5%, Mo 2%, W 10.5%,

V 5%, Co 8%, and Fe balance. A plate specimen (20mm9 mm)

was cut and machined from a bar and subsequently heat treated

in a horizontal vacuum furnace with uniform, high-pressure gasquenching using N2 at a pressure of 5 bar. The specimen was

preheated in three steps: first to 650 C, then to 850 C and finally

to 1050 C. After this last preheat the specimen was heated

(25 C/min) to the austenitizing temperature of 1130 C, soaked

for 2 min, and then gas quenched to 80 C. The specimen was

then triple tempered at 540 C for 2 h.

The specimen was metallographically prepared using a

standard procedure, followed by colloidal silica oxide polishing

for 3 min and then cleaning in an ultrasonic bath. The specimen

wasanalyzedby a FE-SEMJEOLJSM 6500Ffield-emissionscanning

electron microscope using energy-dispersive X-ray spectroscopy

(EDS), an INCA X-SGHT LN2 type detector, INCA ENERGY 450

software, and an HKL Nordlys II EBSD camera using Channel5

software. The Auger electron spectroscopy (AES) was performed

in a VG Microlab 310F SEM/AES. The microstructures were

revealed by nital (HNO3 and alcohol). The plate specimen was

analyzed by X-ray diffraction (XRD) using Cu K radiation in the

angular 2 range from 30 to 70 with a step size of 0.02 and a

collection time of 10 s (D4 Endeavor, Bruker AXS, Karlsruhe,

Germany).

For the EDS analyses a 12-kV accelerating voltage and a probe

current of 0.7 nA were used. The parameters chosen represent a

good compromise between the size of the analyzing volume and

the overvoltage needed to detect the chemical elements that are

present. The EBSD was performed at a 20-kV accelerating voltage

anda 2.7-nA probe current. TheAES line profile,which examined

the same carbide particle as was analyzed using the EDS and

EBSD, was measured in the VG Microlab 310F. In this case a 10-kV

accelerating voltage was used, and prior to the acquisition of the

line profilethe specimen'ssurface wassputtercleaned using 3-kV

Ar+ to eliminate any surface contamination resulting from

exposure to the atmosphere.

The image analyses of the backscattered electron micro-

graphs were performed over 10 randomly chosen fields of view

using AnalySIS PRO 3.1 software. Each image of 2000 magni-

fication (an area of 2730 m2) contains about 530 darker carbide

particles and about 200 brighter carbide particles. The EBSD

mapping is a lengthier procedure and so only five fields of view,

with the same size as the backscattered electron micrographs,

were used. The obtained crystallographic data were analyzed

using the Channel5 software.

3. Results

The final microstructure of the investigated steel consisted of a

martensite matrix and a carbide phase. The carbides were evenly

distributedwithinthe matrixand were close toa regularspherical

shape with a diameter of less than 1 m. Fig. 1(a) shows the

microstructure of a nital-etched specimen obtained with the

secondary-electron detector, whereas Fig. 1(b) was recorded with

the backscatter detector using a polished specimen. Two carbide

types could be seen in the BE-mode image: a very bright-colored

carbide containing higher-atomic-number elements and a grey

carbidecontaininglower-atomic-numberelements.Usingimage-

analysis software the volume fraction of each phase was

estimated to be 12.70.8% and 4.20.3% for the grey and the

brightphases,respectively. Thematrix consistedof hardenedand

tempered martensite without proeutectoid carbides precipitated

on the prior austenite grain boundaries. No retained austenite

was observed in the matrix, and this preliminary evaluation was

later confirmed by XRD.

A higher-magnification BE image reveals the details of the

carbide morphology (Fig. 2). The brightcarbides are homogenous,

Fig. 1 Microstructure of the specimen: (a) SE image, (b) BE

image.

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whereas the grey carbides have a chemically inhomogeneous

microstructure,as isclear fromthe contrast in theimage. TheEDS

spot analyses and maps indicated that there were two types of

carbides,with chromium andmolybdenum being present in both.

The elemental distributions of vanadium, iron and tungsten are

shown in Fig. 3, while the EDS spot analysis is presented in Fig. 4.

In this image, only the main peaks are labelled, and there is a

significant overlap of the vanadium and chromium peaks. Themain difference between the two carbide phases relates to their

composition: the phase that is brighter in the BE image is rich in

tungsten and iron, while the darker one is rich in vanadium and

tungsten.

The EDS line analyses showed the chemical uniformity of the

tungsten-rich carbides. Two neighboringvanadium-richcarbides

one witha white patternin the centerand the other witha white

area forming a star pattern (along the line in Fig. 2) were chosen

and the EDS line analyses were performed across these two

carbides. On the basis of the 12-kV accelerating voltage and the

0.2-mspot distances in the EDS analyses a significant overlapof

the analyzed volumes was expected. This means that an exact

composition could not be accurately determined for the spot of

the analysis; however, a difference in the concentration profile

along theline across thecarbide, which corresponds to thedarker

and lighter areas in the carbide itself, could be identified (Fig. 5).

Significant changes in thevanadium andtungsten contents were

observed, which proves that the analyzed carbides were chem-

ically inhomogeneous.

In order toverify theresults an AESline profilewas performed;

this technique is surface sensitive and a significantly smaller

analysis volume is required. The results show a very similar

chemical distribution in the vanadium-rich carbidephase (Fig. 6).

However, the changes in the vanadium and tungsten contents

along the line arenot as pronounced as in the case of the EDS line

profile, which is due to the higher signal-to-noise ratio.

The chemical inhomogeneity of the vanadium-rich carbide

was confirmed by the EDS and AES analyses; however, it is not

clear whether the chemical inhomogeneity also leads to crystal-

lographic differences in a single carbide. Therefore, EBSD was

performed on the two carbide types (Fig. 7). The EBSD pattern of

the tungsten-rich carbide was best matched by the cubic Fe3W3C

pattern. The candidates for the vanadium-rich carbide phase

were the cubic VC and the hexagonal V6C5, because of their

similar V:C ratios. Despite having different crystal symmetries,

they give very similar EBSD patterns. Nevertheless, the crystal

orientation within a single carbide particle seems to be uniform,regardlessof thechemicaldifferences in theVC phases. TheEBSD

patterns from points 1, 2 and 3 in Fig. 2 are practically the same,

i.e., no rotation of thecrystal lattice wasobserved; therefore,it can

be concluded that the crystal structure is uniform.

XRD was performed in an attempt to distinguish between the

VCandtheV6C5. Unfortunately, however,theXRDmeasurements

did not provide a straightforward answer. In spite of the fact that

VC has a cubic symmetry and V6C5 has a hexagonal symmetry,

both phases have their main peaks at the same positions. The

tungsten-rich carbides Fe3W3C have peaks that are shifted to

higher 2 values or to lowerd values (Fig. 8). Considering only the

positions of the peaks is not enough. However, taking into

account the peak intensities, the carbides are closer to cubic VC.

The main three peaks of the measured vanadium-rich carbides

had the normalized intensity 920/999/310, whereas the VC and

V6C5 standards had 991/999/449 and 999/872/784, respectively.

The lattice parameters determined from the XRD data were

1.1030.002 nm and0.4180.001 nm forFe3W3C and VC, respec-

tively. The corresponding reference values [12] are 1.108 nm and

0.416 nm.

Thesize,distribution, type andfraction of thecarbide phase to

a large extent determine the properties of powder-metallurgy

high-speed steels. In order to obtain the carbide fraction of each

carbidetype EBSD phase-mapanalyses were performed.Basedon

previous results (EDS, EBSD and XRD) the VC and Fe3W3C phases

were chosen for the EBSD mapping (Fig. 9(a)). The fraction of VC

was 12.40.5%; the fraction of Fe3W3C was 3.70.2%; and the

balance belongs to martensite. These fractions were determined

from the crystallographic data obtained from the EBSD measure-

ments using the Channel5 software.The band-slope map [13] in Fig. 9(b) confirms that the

martensite pattern is of poorer quality (they are darker in the

image), whereas the carbides have a higher band-slope value

(they are brighter in the image). The band-contrast (pattern

quality) grey image level clearly separates the carbide phase

from the martensite phase, particularly because of the diffuse

martensite patterns.This also means that it is harder to obtaina

good-quality martensite pattern andtherefore it is impossibleto

crystallographically determine the fraction of martensite. Theband-contrast imagewith the rainbow-color threshold(Fig.9(c))

was used to separate the martensite phase from the carbide

phase. Fig. 9(d), the rainbow-color histogram, exhibits two

peaks. The peak at the lower value belongs to the martensite

phase, while the broader peak at higher values belongs to the

two carbide phases. Using such a calculation the fraction of

martensite is 83 1% and the balance is the two carbide phases.

4. Discussion

Specimen preparation is of crucial importance for EBSD mapping

as well as for BE imaging, especially for the determination of an

Fig. 2 BE image of the carbide phases. Note that the lighter

carbides appear to be homogeneous, while the darker ones

exhibit differences in coloration. The line corresponds to

the EDS and AES line analyses and the points mark the

spots of the EBSD analyses. EBSD patterns from the darker

vanadium-rich phase (spots 1, 2, 3) are shown in the image.

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individual phase fraction. In the ideal situation the specimens

should have as little topography as possible. However, a problem

arises when the material is composed of phases that have

different hardnesses. Since the carbides are harder than the

matrix, longer polishing, ion etching or another method of

material removal, leads to a more pronounced topography. The

best results were obtained by using shorter polishing times with

colloidal silica oxide and by rotating the specimen and the

polishing cloth in the same direction. To preserve the specimen

surface and to remove the residual dirt a special acetone-soluble

Fig. 3 (a) SE image and the corresponding X-ray elemental distributional images: (b) V K1, (c) Fe K1, (d) W M1. In (a), the

points from which EDS spectra were acquired are marked.

Fig. 4 EDS spectra from the points marked in Fig. 3. The front spectrum is Spectrum 1, representing the W-rich region and the

back spectrum is Spectrum 2, representing the V-rich region. The main peaks in both spectra are labeled.



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polymer foil was used, which was applied to the surface using a

drop of acetone and removed just before inserting the specimen

into the vacuum chamber of the SEM.The spatial resolutions of the AES and EDS techniques are

different. While AES is a surface-sensitive technique and the

Auger electrons exiting the specimen come from the top few

layers of the surface, with EDS it has to be taken into account

that the X-rays generated in a certain volume depend on the

accelerating voltage and the density of the material [14].

Comparing the results from the EDS and AES analyses (Figs. 5

and 6) it is clear that there is a better correspondence of the

EDS line profile with the BE image (Fig. 2). It is believed that

this is due to two reasons: firstly, the X-rays are produced at a

similar depth to the backscattered electrons and, secondly, the

signal-to-noise ratio was much better in the case of the EDS

analysis, so it was possible to observe smaller changes in the

compositions. Smaller fluctuations in the composition were

observed in the case of the AES. Another problem also lies in

the quantitative analysis being not as accurate. However, the

AES analysis is a better measure of the surface composition,

where the compositional differences are smaller [15]. The

compositional differences in the vanadium-rich carbides arise

from the production process and from their physical

Fig. 5 EDS line profile along the line shown in Fig. 2.

Fig. 6 AES line profile along the line shown in Fig. 2.

Fig. 7 Representative EBSD patterns acquired from the

carbide phases: (a) is determined to be Fe3W3C and (b) isdetermined to be VC.

Fig. 8 X-ray pattern. X-ray peak positions corresponding to

Fe3W3C, VC and V6C5 and -Fe are marked.



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properties: the free formation enthalpy, the solubility and the

diffusivity of carbide-forming elements.

In the XRD spectruma large, almost, linear background signal

as well as a lot of noise can be observed. This is the well known

effect of fluorescence [16]. The noise could also be caused by

amorphous material; however, this is not the case here. Another

featurethatcan be observed in the XRD pattern is the broadening

of the Fe peaks. Since the iron is in the martensite phase, it was

assumed that this phenomenon isassociated with a non-uniform

lattice distortion.The EBSD Kikuchipattern of themartensite also

appears as diffuse, which can be attributed to the same cause.

Certain problems were encountered in the analysis of the

vanadium-rich carbides. The Fe3W3C analysis was relatively

straightforward, since there were no alternative phases that

could match the EBSD patterns and the composition. However,

it is worth mentioning that in this case Fe3W3C can also mean

that other carbide-forming elements are present to form M6C

carbides.It was much harder to distinguish between the VC and

the V6C5. Both phases have their main XRD peaks in the same

position and their EBSD diffraction patterns were virtually the

same. In principle, the FCC and HCP structures have the same

packing ratio and a similar structure [17]. Since both XRD and

EBSDare diffractiontechniques, similar phenomenaand similar

problems can be expected. However, one of the advantages of

XRDis that thepeakpositionscan be accuratelydetermined and

thelattice parameterscan be calculated, whereas with EBSD it is

possible to investigate an individual carbide. Using electrolytic

carbide extraction would probably have given better results for

the XRD analysis, as there would be no Fe peaks present, and it

would also be possible to identify the less-intense carbide XRD

peaks [18,19]; however, in thiscaseXRD was used to supportthe

EBSD measurements and to provide information about the

carbide type.

Since VC and V6C5 have virtually the same EBSD patterns

(the mean angular deviation differs by only 0.004, which is

negligible) it is not possible to distinguish betweenthem based

only on the EBSD method. Thus, it was necessary to resort to

XRD. Due to the method of the PM steel's preparation, which

includes gas atomization, powder encapsulation, hot isostatic

Fig. 9 EBSD maps. (a) Band-contrast image, (b) phase image, (c) rainbow color image, (d) band-contrast histogram.

Table 1 Comparison of carbide and martensite fractionsusing different analytical approaches.

Phases BE image EBSD phase map Band contrast

Martensite 83.1 1 Balance 83 4

VC 12.7 0.8 12.4 0.5 Balance

Fe3W3C 4.2 0.3 3.7 0.2 Balance



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pressing, the carbides have no texture. Therefore, it was

possible to use the peak-intensity ratios for the determination

of the carbide types. In this case it was clear that VC was the

more likely candidate.

It is very important to characterize all the carbide phases

present in the material because the mechanical properties are

largely determined by their shape, size, distribution and their

volume fraction. To determine the amount of carbides andmartensite in the specimen, three independent methods were

used. In the analysis of the BE images additional problems

arise from the nature of backscattered electrons, which are

generated at a certain depth, rather than on the surface itself.

This means that more carbides could be counted. In the EBSD

mapping the main source of measurement errors is the local

topography of the specimen and the inaccuracy in the pattern

solving. However, the latter can be improved by using some

level of noise reduction. The third method was used to

separate the martensite from the carbides using EBSD pattern

band contrast. In this case the measurement uncertainty was

relatively high because the two separate peaks overlap

significantly. The results are presented in Table 1, where the

volume fraction of each phase obtained in different ways is

compared.

5. Conclusions

During isostatic pressing of the powder-metallurgy alloy, the

eutectic carbide phase spherodised and formed two cubic

carbide phases: Fe3W3C and VC. Auger electron spectroscopy

and electron-backscatter diffraction microchemical analyses

showed that theM6C carbidewas chemicallyuniform, while the

VC seemed to be non-uniform. The backscatter-electronimages

indicated a dendrite morphology of the carbide particles, which

most probably occurred during the spherodisation. Crystallo-

graphically, both carbides were uniform and monocrystalline.

Electron-backscatter diffraction and X-ray diffraction were

applied to confirm the carbide types. Due to the very similar

electron-backscatter diffraction patterns and the same X-ray

diffractionpeaks forthe VC and the V6C5 carbides, it washard to

distinguish between them. However, the carbides had no

texture and therefore the XRD peak intensity indicated a higher

probability of them being theVC carbide. Threedifferentwaysto

obtain the amount of each phase were demonstrated, with the

most precise result coming from the electron-backscatter-

diffraction phase map.

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