MICROSTRUCTURE OF BORONIZED PM Cr-V COLD WORK...

Materials Engineering, Vol. 17, 2010, No. 4

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MICROSTRUCTURE OF BORONIZED PM Cr-V COLD WORK LEDEBURITIC TOOL STEEL

Peter Jurči1, Mária Hudáková

2

Received 4th

December 2010; accepted in revised form 12th

December 2010

Abstract

The PM Vanadis 6 cold work tool steel has been boronized at various processing parameters, austenitized,

quenched and tempered to a core hardness of 700 HV. Microstructure, phase constitution and microhardness of

boronized layers were investigated. It was found that the boronized layers are of two-phase FeB/Fe2B

constitution, with an addition of small portion of CrB. Below the boronized layer, intermediate region with

elevated carbides ratio was developed. Boronized layers contain also carbides, as a result of the fact that they

did not undergo dissolution in the austenite during processing. The hardness of compound layers ranged between

1850 and 2100 HV 0.1 for the FeB-region and between 1750 and 1850 HV 0.1 for the Fe2B-region, respectively.

Keywords: boronizing, Vanadis 6 steel, heat treatment, microstructure, microhardness, phase constitution.

1. Introduction

Chromium and chromium-vanadium

ledeburitic steels are, due to their high wear

resistance and hardness, nowadays used in many

industrial operations like metal cutting, wood

working, fine blanking, drawing etc. In these

operations, the material must meet various

requirements. It has to withstand the compressive

stresses, abrasive and/or adhesive wear, but also

chipping and total tool collapse. To meet these

demands, the materials must have an optimal

chemistry, as well as the phase constitution.

In addition, a proper heat treatment must be done

before the use of the material. This brings an

appropriate combination of toughness and

strength to the material.

Boronizing is a thermochemical process

used for the formation of borides on the steel

surfaces. As a product of treatment, thin, very

hard wear resistant and corrosion resistant

compound layers are formed. The transition area,

in contrast, is significantly softer and often

contains carbides, as a result of carbon

redistribution towards to the substrate during

the boronizing. Depending on the nature of

the substrate material and processing conditions,

single phase (Fe2B) or double phase (FeB+Fe2B)

layers can be formed. If the material contains

a sufficiently high amount of chromium, also

the chromium based or complex (Fe, Cr)-borides

are formed [1]. But, as found by Dybkov [2],

the chromium content tends to decrease in boride

layer compared to the substrate. The Fe2B single

phase layer has normally a hardness between

1400 and 1700 HV and fracture toughness

around 5-6 MPa.m1/2

[3-6]. For the double-phase

layer, the hardness is increased because the FeB

has a hardness over 2000 HV. Moreover,

the hardness further slightly increases for higher

alloyed materials, for instance ledeburitic tool

steels [1,7,8]. The fracture toughness of the FeB-

phase is two-four times lower than that of

the Fe2B-phase [9].

1 P. Jurči, Doc. Dr. Ing. – Department of Materials Engineering, Faculty of Mechanical Engineering,

Czech Technical University in Prague, Karlovo nám 13, 121 35 Prague, Czech Republic, 2 M. Hudáková, Doc. Ing. PhD. – Faculty of Materials and Technology, STU, J. Bottu 52, 917 24 Trnava,

Slovak Republic * Corresponding author, e-mail address: [email protected]


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Owing to the diffusion of boron into the

steel substrate, boronized layers exhibit a good

adhesion. The adhesion onto the substrate is

further enhanced by the morphology of the

interface between the substrate and the

compound layer. The morphology of the

interface depends on many factors. First of all, it

is very important whether the steel contains

alloying elements forming substitutional solid

solutions or not. In plain carbon steels,

the borides exhibit mostly typical sawtooth

interface between compound layer and substrate

[10]. However, higher carbon content often leads

to alteration of the interface morphology. Kulka

[11], for instance, reported that in pre-carburised

low carbon steel, the tips of the teething on the

interface became weak. Also for other high

carbon cold work tool steels, similar phenomena

were observed [6,9]. For alloyed steels,

Carbucichio and Palumbarini [12] established

that the elements like Mo, Cr and V inhibit

the growth kinetics of the compound layer.

According their observations, substitutional

atoms of these elements concentrate at the tips

of boride columns and reduce the boron flux in

this region. As a result, the interface becomes

an irregular form.

The thickness of boronized layers is

commonly up to 0.4 mm. The thickest layers are

formed on the surface of low carbon steels.

Increasing carbon content tends to decrease of

the layer thickness [11]. Also, according to

Campos [10], chromium inhibits the layer

growth. In the case of boronizing of high

chromium steels, in addition, the FeB-layer tends

to form more easily and, as indicated for instance

by Li [5], Dybkov [2] and Oliveira et al. [13], the

it can make up to 50% of the total compound

layer thickness in some cases.

The aim of this paper is to describe the

structure and phase constitution of boronized

layers formed on the surface of widely used and

very popular Cr-V cold work steel Vanadis 6.

2. Experimental

The experimental material was the cold

work ledeburitic tool steel Vanadis 6 with actual

chemical composition of 2.1 wt. % C, 1.0 wt. %

Si, 0.4 wt. % Mn, 6.8 wt. % Cr, 1.5 wt. % Mo,

5.4 wt. % V and Fe as balance. The steel has

been manufactured by P/M (HIP of rapidly

solidified particles) and soft annealed to

a hardness of 284 HV 10.

Round shaped plate specimens of 20 mm

in diameter and 5 mm in thickness were fine

ground to a surface roughness of Ra = 0.1 ÷ 0.2

µm. Specimens were cleaned, degreased and

boronized using the Durborid® powder mixture

in hermetically sealed containers at a temperature

of 1030 oC for 45, 75 and 150 min. After

the boronizing, the containers with specimens

were furnace cooled down slowly to a room

temperature, and then the specimens were

removed and subjected to standard vacuum heat

treatment. This procedure consisted of

austenitizing at 1000 oC for 30 min., nitrogen gas

quenching (pressure of 6 bar) and double

tempering, each tempering cycle at 550 oC for

2 hours. After each tempering cycle, the samples

were cooled down slowly to a room temperature.

Resulting core hardness of the steel was 60 HRC

(700 HV).

The light and scanning electron

microscopy after a deep etching were used for

the microstructural evaluation. For the EDS

mapping and point chemical analysis, the EDS-

detector was used whereas the acceleration

voltage of the SEM was lowered to 1 kV.

For the EDS-analysis of boronized layer

and carbides below that, twenty measurements

were made and the mean values and standard

deviations were calculated. Microhardness of

boronized layer, transient region and core

material was measured with a Hanemann

indenter placed in a Zeiss Neophot 21 light

microscope, at a load of 100 g (HV 0.1). At least

ten measurements have been made to obtain the

mean value and other statistical data, according

to methods elaborated in [14]. X-ray patterns of


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the boride layers were recorded using a Phillips

PW 1710 device with Fe-momochromatic

radiation. Data were recorded in the range 27 –

120o of the two-theta angle.

3. Results, discussion

Light micrograph, Fig. 1, shows the micro-

structure of the substrate steel after quenching

and tempering to a hardness of 700 HV. The

material consists of the martensitic matrix and

fine (size of several microns) globular carbide

particles. Previous experimental investigations

revealed that the carbides are of two types –

vanadium rich MC-phase and chromium rich

M7C3-phase [15].

Fig. 1. Microstructure of the substrate material

All the boronized compound layers

consisted of two characteristic regions, Fig. 2.

On the free side, there is the FeB-phase region.

Below that, the region formed with the Fe2B

phase can be observed. The interface between the

two compounds exhibits clearly shown sawtooth

morphology in all the cases. Also the interface

between the Fe2B region and the intermediate

region has a sawtooth morphology, but,

the teething becomes slightly smoother.

The intermediate region contains a lot of

carbides, more than the equilibrium ratio typical

for the Vanadis 6 steel after a given heat

treatment.

The thickness of compound layer is

relatively low and reaches only up to 60 µm for

the given processing parameters. This can be

attributed to high carbon and alloying elements

content in the material, which hinders the boron

diffusion into the core material. With prolonging

processing time, the thickness of boronized layer

increases only slightly, Table 1. These results

correspond well to the observations of various

authors [10-12], where the negative effect

of either carbon or chromium content on the

layer growth rate was found.

Similarly, with increased processing time,

the relative ratio of FeB-phase region increases

up to over 50% of the total layer thickness. Also

this observation is in a good agreement with the

established data for the high alloyed ledeburitic

tool steels and chromium steels [2,5,13] and

confirms that chromium, as a substitutional

element, strongly inhibits as the layer growth in

general so the boron diffusion through the

compound layer.

Boronized layers produced at all of

the processing parameters contain a great number

of pores, mainly in the FeB-phase region, Fig. 3.

In the Fe2B-region, substantially lower number

of pores was observed. But, there is also a great

number of fine particles in both regions. Their

nature will be discussed later. All the boronized

layers contain microcracks oriented parallel to

the surface and located close the FeB/Fe2B

interface. This is a common effect of the stress

discontinuity at the interface (the FeB-region

Tab. 1

Parameters of developed boronized layers

Boronizing Thickness [µm]

FeB ratio [-] Compound layer FeB Fe2B

1030 oC/45 min. 49.8 ± 1.7 20.5 ± 0.9 29.3 ± 2 0.41

1030 oC/75 min. 53.4 ± 3.6 26 ± 4.1 27.4 ± 1.8 0.48

1030 oC/150 min. 59 ± 4.4 31.5 ± 2.8 27.5 ± 3.3 0.53


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C 30 µm

30 µm A B 30 µm

Fig. 2. Boronized layers on the Vanadis 6

steel formed at 1030 oC for: A - 45 min.,

B - 75 min., C - 150 min.

C 7 µm

B 7 µm 7 µm A

Fig. 3. Detail micrographs of boronized layers

on the Vanadis 6 steel formed at 1030 oC for:

A - 45 min., B - 75 min., C - 150 min.


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contains tensile stresses while the Fe2B-region

compressive stresses) and extremely low fracture

toughness of both compounds [3,5,6,9].

Concerning the nature of the carbides

incorporated in the boronized layers it is believed

that they are not a product of the boronizing

itself but their existence results from the previous

history of the material. Earlier investigations [16]

revealed that the MC carbides remain practically

unaffected by the heat treatment up to 1200 oC.

The M7C3-carbides, on the other hand, undergo

dissolution in the austenite. But, the dissolution

is only in a limited extent at the boronizing

temperature used for current experiments.

Therefore, both types of the carbides remain in

the structure also during the boronizing and

subsequent processing carried out at lower

temperature, Fig. 4. The MC-carbides are

apparently shown in all the EDS-maps and look

bright on the vanadium map, dark on iron map

and in an intermediate brightness on chromium

map (they normally contain some portion of

chromium). The M7C3 particles are coarser and

they are visible mainly on the chromium map as

clearly bright formations.

Fig. 4. EDS maps of the boronized layer

developed formed at 1030 oC for 150 min.

A – image, B – carbon, C – iron,

D – chromium, E – vanadium

A 5 µm B

C D

E


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Figure 5 shows the results of the EDS

mapping of the intermediate region of boronized

layer, formed at 1030 oC for 45 min. Firstly, it is

apparently shown that considerable alloying

elements redistribution took place in this region

during processing. Iron is accumulated mainly in

the solid solution. Less iron is contained in the

compound boride layer and, of course, also in the

carbides, Fig. 5b. This phenomenon is given

from the nature of the carbides because it is

known that the MC phase can contain only up to

10% of iron [17-20] and the M7C3-carbide only

up to the ratio Cr:Fe of approx. 1:1. Also

chromium assists in the processes in the

boronized layer and its vicinity. The cause is that

the chromium rich M7C3-carbides dissolve in

a significant extent in the austenite during

the processing. This makes it possible to form

new carbides beneath the compound layer, due to

the carbon redistribution from the compound

layer towards the substrate. These carbides are

visible as on the SEM micrograph so in the EDS-

mapping of chromium, Fig. 5a,c. On the other

hand, vanadium does not form new carbides

below the compound layer. The nature is, as

expected, that vanadium rich MC-carbides are

very stable and they do not undergo the

dissolution in the austenite during the processing.

As such, they remain unaffected and their

distribution corresponds well to the “as-

received” state of the steel, Fig. 5d.

X-ray diffraction, Table 2, revealed that

besides the major phases FeB and Fe2B, also a

certain ratio of CrB has been formed during the

processing. This observation seems to be logical.

On the one side, boronizing may lead to slight

decrease of the chromium concentration in the

layer, compared to the nominal content in the

alloy, as found previously by Dybkov for pure

Fe-10%Cr system [2]. On the other hand, for the

ledeburitic steel Vanadis 6, the chromium

required for the CrB formation must be firstly

Fig. 5. EDS maps of the intermediate region of boronized layer formed at 1030 oC for 45 min.

A – image, B – iron, C – chromium, D – vanadium

A 5 µm

M7C3

MC alfa

Fe2B

„new“

carbide

B

C D


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gained due to the dissolution of M7C3- carbides

in the austenite. Therefore, one can believe that

the flux of chromium towards the core material is

slightly hindered and the formation of CrB-

particles is enabled.

Table 3 presents the results of point

chemical analysis of both compound regions.

It is shown that chromium content in both FeB

and Fe2B has a slight trend to increase with

the increasing processing time. This fact could be

attributed to better dissolution of M7C3-carbides

and more saturated solid solution with

chromium. The vanadium content shows

practically constant tendency because the MC

carbides are dissolved only in negligible extent

in the austenite [16] and the M7C3-phase does not

contain high enough vanadium. However,

the assumption on the probability of the

formation of the CrB-boride particles is anyway

supported by the observation of high chromium

content in both boride regions.

Hardness measurements of boronized

layers are summarized in Table 4. For short-time

treatment, the hardness of FeB-region ranges

around 1850 HV 0.1. Prolonging of the

processing time to 150 min. leads to a significant

hardness increase, over 2114 HV 0.1. Also the

Fe2B-regions formed at 45 and 75 min. do not

differ in hardness. But, prolonging of the

processing time leads again to the hardness

increase in this region, to almost 1850 HV 0.1.

The intermediate region has a hardness between

776 and 802 HV 0.1, e.g. by approx 70-100 HV

0.1 higher than the core material. This is

attributed to the formation of “new carbides”

here. Nevertheless, it is not clear what caused a

hardness increase in the compound layer formed

for 150 min. One can say that the principal

explanation could be an enhanced volume

fraction of the chromium borides, Table 2. But it

is very difficult to determine their volume

fraction in the layers exactly, based on only

X-ray measurements. Anyway, the explanation

of the hardness increase will require further

investigations.

Tab. 2

Results of X-ray diffraction measurements of boronized layers on the Vanadis 6 steel

Boronizing Phase constitution

FeB Fe2B CrB

1030 oC/45 min. intermediate strong tracks

1030 oC/75 min. strong strong tracks

1030 oC/150 min. strong strong intermediate

Tab. 3

Results of EDS measurements of compound layer and carbides in the intermediate region

Boronizing FeB Fe2B

Fe Cr V Fe Cr V

1030 oC/45 min. 63 ± 4.07 5.45 ± 1.23 3.17 ± 1.04 77 ± 9.2 7.92 ± 1.12 3.01 ± 0.38

1030 oC/75 min. 60 ± 3.11 5.53 ± 0.47 3.09 ± 1.77 70.33 ± 3.73 7.32 ± 0.65 2.92 ± 0.9

1030 oC/150 min. 60 ± 4.76 5.56 ± 0.98 3.03 ± 1.4 72 ± 1.87 8.81 ± 0.55 3.13 ± 0.21

Tab. 4

Summary of hardness measurements of boronized layers

Boronizing Microhardness HV 0.1

FeB Fe2B intermediate region

1030 oC/45 min. 1852 ± 72 1756 ± 27 799 ± 9

1030 oC/75 min. 1857 ± 54 1759 ± 43 776 ± 17

1030 oC/150 min. 2114 ± 71 1847 ± 39 802 ± 12


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4. Conclusions

1) Short – time boronizing of the Vanadis 6

steel produced thin compound layers having

up to 60 µm in thickness. Small thickness of

the layers can be attributed to the hindering

effect of carbon and alloying elements with

respect to the layer growth.

2) The layers are double-phase, consisting of

FeB and Fe2B-regions, with small addition of

CrB. Moreover, the compound layers contain

small carbides, resulting from the

metallurgical nature of the material and its

thermal history.

3) The interface between the compound layers

and intermediate region showed a typical

sawtooth morphology.

4) Below the compound layers, there were

intermediate regions detected. They are

enriched with carbon, which provided a

formation of a lot of carbides in this area.

These carbides contain some portion of iron

and chromium.

5) Boronized layers have very high hardness.

For the FeB-region it was found to be 1850

HV 0.1 for short-time processing and more

than 2100 HV 0.1 when the boronizing took

150 min. For the Fe2B-region, there was a

hardness of 1750 HV 0.1 found for short

time processing and it increased to 1850 HV

0.1 for the processing for 150 min.

Intermediate layers have also slightly

increased hardness compared to the

substrate, due to the “carbide excess” formed

there during the treatment.

References

[1] Kusý, M. et al.: In: Proc. of 21st Int. Conf. on

Heat Treatment, 28-30 November, Jihlava,

Czech Republic, ATZK Prague, 2006, 289.

[2] Dybkov, V.I., Lengauer, W., Barmak, K.: J.

Alloys Compd. 398 (2005) 113.

[3] Campos, I. et al.: Appl. Surf. Sci., 254 (2008)

2967.

[4] Sen, U., S. Sen, S.: Mater. Charact. 50 (2003)

261.

[5] Li, Ch., Shen, B., Yang, Ch.: Surf. Coat.

Techn. 202 (2008) 5882.

[6] Ozbek, I., Bindal, C.: Surf. Coat. Techn. 154

(2002) 14.

[7] Hudáková, M. et al.: Materials and

Technology 41 (2007) 81.

[8] Hudáková, M. et al.: In: Proc. of Int. Conf.

“Coatings and layers 2007“, October 28–30,

Rožnov pod Radhoštěm, Czech Republic,

LISS Rožnov pod Radhoštěm, CD-ROM.

[9] Sen, S. et al.: Surf. Coat. Techn. 135 (2001)

173.

[10] Campos, I. et al.: Appl. Surf. Sci., 252 (2006)

8662.

[11] Kulka, M., Pertek, A.: Appl. Surf. Sci., 214

(2003) 161.

[12] Carbucicchio, M., Palombarini, G.: j. Mater.

Sci. Lett. 6 (1987 1147.

[13] Oliveira, C. K. N. et al.: Vacuum 84 (2010)

792.

[14] Kučerová, M.: Application of method of

planed experiments in the quality

management, (Uplatnenie metódy plánovania

experimentov v manažérstve kvality),

Trnava, AlumniPress, 2010.

[15] Jurči, P. et al.: Materials and Technology, 38

(2004) p. 13.

[16] Jurči, P.: Structural changes in Cr-V

ledeburitic steel during austenitizing,

accepted paper for the publication in

Materials Engineering.

[17] Fredriksson, H., Hillert, M., Nica, M.: Scand.

J. Metall. 8 (1979) 115.

[18] Versaci, R. A.: J. Mater. Sci. Letters, 7

(1988) 273.

[19] Piotrkowski, R.,Versaci, R. A.: J. Mater. Sci.

Lett. 11 (1992) 390.

[20] Stiller, K., Svensson, L. E., Howell, P. R.:

Acta Metall. 32 (1984) 1457.

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