Bachelor Thesis

Stockholm, Sweden 2011

Equilibrium studies in the Co-Cr-C system Solubility of Cobalt in M23C6 and M3C2

KARIN EKSTRÖM SHANAR KORDBACHE

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2011

Karin Ekström

Shanar Kordbache

Bachelor Thesis MMK 2011:x CMATD3

Equilibrium Studies in the Co-Cr-C System

Karin Ekström

Shanar kordbache

Approved

2011-05-05

Examiner

Anders Eliasson

Supervisor

Malin Selleby

Commissioner

Department of Materials Science and

Engineering

Supervisor at Sandvik

Susanne Norgren

Abstract

This project concerns cemented carbides and the carbides that are formed when adding

chromium. Cemented carbides are composites, often consisting of tungsten-carbide particles

embedded in a cobalt-rich matrix, and are because of their extreme hardness used in for

example cutting tools and drills. Chromium is sometimes added when making cemented

carbides in order to lower the melting point, reduce grain growth and/or increase corrosion

resistance. When adding chromium there is a risk of forming unwanted carbides such as

M23C6, M7C3 and M3C2. It is therefore of great interest to know the stability of these carbides.

The purpose of this work was to investigate the solubility of Co in M23C6 and M3C2 by

equilibrium studies.

The aim was to produce samples equilibrated in the three-phase regions between liquid‐M23C6‐M7C3 and M3C2‐M7C3‐graphite, to study the solubility of Co in M23C6 and M3C2 respectively.

Initial studies were performed at Sandvik Mining and Construction (SMC) to determine the

compositions of the samples to be produced and temperatures for the heat treatments.

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The alloys were heat‐treated at 1450⁰C for three days and were thereafter investigated with LOM and XRD. The work was carried out in collaboration with Sandvik Mining and

Construction (SMC).

It was difficult to analyze the results with XRD since the intensity peaks in the diffractograms

are close or overlapping for the M23C6 and M7C3 carbides. The solubility of Co in M23C6 could not be investigated accurately.

The M3C2-M7C3‐graphite sample did not reach equilibrium in the three days of heat treatment.

The conclusions that can be drawn from this project are that further work, using longer

annealing times, has to be done in order to get more knowledge about the Co solubility in

M23C6 and M7C3 carbides.

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Acknowledgments

We would like to thank our supervisors Susanne Norgren at Sandvik and Associated Professor

Malin Selleby at Department of Materials Science and Engineering, KTH.

Special thanks to Andreas Markström (Thermo-Calc Software AB) and Bartek Kaplan

(Sandvik Tooling) who helped us with the experimental part and guided us through our

project.

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Nomenclature

Designations

Symbol Description

C Celsius

K Kelvin

M Co, Cr

Abbreviations

Co Cobalt

Cr Chromium

C Carbon

LOM Light Optical Microscopy

XRD X-ray Diffraction

SEM Scanning electron microscope

EDS Electronic Data Systems

SMC Sandvik Mining and Construction

WC Tungsten carbide

PEG Polyethylene glycol

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ABSTRACT ................................................................................................................ 2

ACKNOWLEDGMENTS ............................................................................................. 5

NOMENCLATURE ..................................................................................................... 7

1 INTRODUCTION ..................................................................................................... 9

1.1 Aim ...................................................................................................................... 10

1.2 Literature survey ............................................................................................... 11

1.3 Background ........................................................................................................ 13 1.3.1 Manufacturing Cemented Carbides ..................................................................................... 13 1.3.2 Milling ...................................................................................................................................... 13 1.3.3 Pressing .................................................................................................................................. 13 1.3.4 Sintering .................................................................................................................................. 14

1.3.5 Characterization methods .............................................................................. 14 1.3.5.1 Light Optical Microscope (LOM) ................................................................................... 14 1.3.5.2 XRD .................................................................................................................................. 14

2 EXPERIMENTAL PROCEDURE ........................................................................... 15

2.1 Solubility of cobalt in M23C6 .............................................................................. 15 2.1.1 Sample preparation ................................................................................................................ 15 2.1.2 Sample analysis ..................................................................................................................... 16 2.1.3 Results .................................................................................................................................... 17

2.1.3.1 LOM .................................................................................................................................. 17 2.1.3.2 XRD .................................................................................................................................. 18

2.1.4 Discussion .............................................................................................................................. 18

2.2 Solubility of cobalt in M3C2 .............................................................................. 19 2.2.1 Sample preparation ................................................................................................................ 19 2.2.2 Results .................................................................................................................................... 19 2.2.3 Discussion .............................................................................................................................. 19

3 CONCLUSIONS AND FUTURE WORK ................................................................ 20

4 REFERENCES ...................................................................................................... 21

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1 Introduction

Cemented carbides are composite materials, composed of hard particles of a refractory

carbide ceramic such as tungsten carbide (WC) or titanium carbide (TiC), embedded in a

metal matrix such as cobalt or nickel. The first cemented carbide was a WC-Co-alloy. The

hard, brittle tungsten particles were cemented with a soft, ductile cobalt-rich binder.

Cemented carbides are manufactured from powdered raw materials by liquid phase sintering.

Cemented carbides are used for their hardness, wear resistance and temperature stability. The

main applications for cemented carbides are cutting or drilling tools and other wear parts,

where the material is expected to have high hardness as well as some toughness.

The hard carbide particles provide the hardness needed for cutting or drilling but, being

extremely brittle, are not in themselves capable of withstanding the stresses during such

operations. The toughness comes from the inclusion of carbide particles in the ductile metal

matrix, which isolates the particles from one another and prevents particle to particle crack

propagation. Both particulate and matrix phases are to some extent refractory, to withstand the

high temperatures generated by the cutting action on hard materialsi.

Chromium is often added to cemented carbides to inhibit grain growth during sintering and to

increase the corrosion resistance along with mechanical strength of the binder by solution

hardeningii. However, when adding Cr there is a risk of forming unwanted carbides such as

M23C6, M3C2 and M7C3. It is therefore of great interest to know the stability of these carbides.

In previous workiii,iv

, the solubility of cobalt in M7C3 has been studied and it was found that

the solubility of Co in the M7C3 carbide was much higher than the values assessed by

thermodynamic calculations.

In this project equilibrium studies was made to investigate the solubility of Co in the M23C6 and M3C2 carbides at temperatures higher than previous studies. In figure 1, the compositions of the samples produced in this work are presented. In order to find the Co solubility in M23C6

and M3C2, samples with compositions between the points in the phase diagram should be

produced.

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Figure 1- The ternary system C-Co-Cr, calculated at 1723 K, with the desired compositions for the samples in this

work iv

.

1.1 Aim

The aim of this work was to increase the knowledge on the ternary Co-Cr-C system. The aim

was to produce samples in equilibrium between liquid- M23C6- M7C3 for the investigation of

the solubility of cobalt in M23C6.

For the study of the solubility of cobalt in M3C2 the compositions of the samples were

targeted to be within the three phase equilibrium between M3C2-M7C3-graphite.

Initial studies were performed at Sandvik Mining and Construction (SMC) to determine the

sample compositions and temperatures for the heat treatment. The alloys were to be heat

treated and thereafter investigated with LOM and XRD.

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1.2 Literature survey

To gain the information needed to do this project a literature survey was made. The studied

articles concern the Co-Cr-C system.

A Thermodynamic Evaluation of the Co-Cr and the Co-Cr-C Systems

Alexandra Kusoffsky and Bo Jansson

CALPHAD vol. 21, number 3, (1997), p 321-333

The thermodynamic properties of the systems Co-Cr and Co-Cr-C was analyzed with the

CALPHAD technique using a computerized optimization procedure. The Gibbs Energy of the

stable phases was evaluated applying the compound energy model. A good agreement with a

set of selected consistent experimental phase diagram and thermo chemical data was obtained.

Investigation of (Co, Cr)7C3-fcc-Graphite Equilibrium in the

Temperature Interval 1373 to 1473K

Thérèse Sterneland, Andreas Markström, Susanne Norgren, Ragnhild E. Aune and

Seshadri Seetharaman, Metallurgical and Materials Transactions A, vol. 37, (2006,)

number 10, p. 3023-3028

The purpose of this article was to investigate the solubility of cobalt in M7C3 as well as the

solubility of Cr in the Co rich fcc-phase, since earlier studies have indicated higher solubility

than the ones calculated. Heat treatments of appropriate mixtures of Cr7C3 and Co were

performed at 1373, 1423 and 1473 K. The samples were then quenched in liquid nitrogen and

examined by SEM and XRD.

From the results, the compositional regions of the three-phase triangle M7C3+FCC+graphite

were determined. The results showed that the Co solubility in the Cr7C3 within the

experimental temperature interval was higher than previous investigations performed at

higher temperatures.

Experimental and Thermodynamic Evaluation of the Co-Cr-C system

Andreas Markström, Susanne Norgren, Karin Frisk, Bo Jansson, Thérèse Sterneland

Int.J.Mat.res 97(2006)9

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In this study the solubility of cobalt in M7C3 in equilibrium with liquid was investigated and a

thermodynamic description of the system was made. Heat treatments were performed at 1523

and 1723 K. The samples were analyzed with EDS, WDS and SEM and based on the

experimental results the Co-Cr-C system was analyzed with the CALPHAD technique.

The results of this study showed a higher solubility of cobalt in M7C3 than previous

investigations.

Equilibrium study of chromium containing cemented carbides

Solubility of chromium in tungsten carbide and η-phase

Bonnie Lindahl, Master of Science Thesis, Stockholm, Sweden, 2010

In this study the W-C-Co-Cr quaternary system at low carbon contents was studied. The aim

was to find the four phase equilibrium between WC, Co-binder phase; η-phase (M6C) and a

fourth unknown phase, and study the solubility of chromium in WC and establish the fourth

phase. The first step was choosing a number of alloys which compositions were calculated

with thermodynamic calculations and then analyzed by LOM, SEM, EDS, WDS and XRD.

The result showed that no four phase equilibrium could be found. The solubility of chromium

was proved to be much higher in the η-phase than previous investigations indicated.

Thermodynamic Modeling of Carbides in Multicomponent Systems

Andreas Markström , Licentiate thesis 2009, ISRN KTH/MSE- -09/04- -

SE+THERM/AVH SE-100 44 Stockholm

ISBN 978-91-7415-237-1

This thesis was about thermodynamic modeling of carbides in multicomponent systems with

focus on systems interesting for cemented carbide production.

A re-assessment of the Co-W-C system was presented and new experimental results on the

maximum solubility of Co in the M7C3 were defined along with a new thermodynamic

description of the Co-Cr-C system which accurately describes the temperature interval 1373-

1723 K.

Also, experimental work on the C-Co-Ti-V-W-Zr system was done in order to determine the

extension of the miscibility gaps in TiC-ZrC and VC-ZrC into the (TiC or VC)-ZrC-WC

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system. The new experimental information was used to assess the thermodynamic description

for the TiC-ZrC system.

1.3 Background

Cemented carbides consist of a group of very hard and wear resistant materials that are used

in cutting tools, drilling parts etc. The high hardness comes from a high fraction of carbides,

typically 90 % or higher. The carbide particles, which have sizes from 10 µm to 100 nm, are

bonded together by a metallic aggregate, typically cobalt.

Cemented carbides are fabricated by powder metallurgy. Casting a melt with the right

composition would require very high temperatures in order to get a homogenised melt and the

solidification process would not give the required microstructurev.

1.3.1 Manufacturing Cemented Carbides

There are different ways to fabricate powders. One method is Comminution of Solid

Materials. In this process the chosen material is crushed and separated from other materials

and then mixed with carbon. The mix is then sent into a furnace. The new material from the

furnace is crushed again and the wanted material is sorted out.

Another process to produce powder of metal is using electrolysis, which is really expensive.

In the Atomization Process the metal is first melted and then separated into small droplets.

The droplets will then rapidly be cooled down with either water or gas (argon or nitrogen)vi

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1.3.2 Milling

When the desired raw materials have been chosen, the first stage is milling, in order to get a

powder with a homogenous composition and to reduce the particles to the desired size. The

simplest ways of milling are ball milling and wet milling. Ball milling consists of a cylindrical

jar filled with solid cemented carbide milling bodies, which are added together with the

powder. When the cylinder rotates, the milling bodies will fall down and crush the powdered

materialvii

. Ethanol is added during the milling in order to protect the powder from oxidation

together with polyethylene glycol (PEG), which acts as a binder when the powder is later

compacted providing sufficient strength for the compacted body. Before the powder can be

used it will be spray dried with warm nitrogen which gives spherical granules and prevents

further agglomerationviii

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1.3.3 Pressing

The powder, with particles with sizes around 1 µm or less, is hard to work with because it will

form agglomerates. To facilitate the procedure a controlled agglomeration will be performed

which gives spherical granules. Each granule is glued together by an additive. The powder

will then be easier to handle and can be pressed into a mold. If the powder does not keep the

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shape after the pressing the procedure has to be redone which costs money and gives a poor

result v.

1.3.4 Sintering

Sintering of a compacted powder body most often results in a dense and pore-free material,

depending on the sintering conditions. The first step is to remove the PEG-additive from the

compacted body, so-called debinding. The PEG-additive is burnt off at low temperatures, and

the actual sintering is performed at 1350-1500 C. At that temperature the cobalt melts. During sintering the porosity is eliminated through different diffusion processes which lead to a linear

shrinkage of about 18%v. The driving force in sintering is the reduction of surface energy

when the granules are agglomerated. There are two ways of sintering. One of them is called

Solid Phase Sintering, where particles are bound together through necking at elevated

temperatures below the melting temperature of the component with the lowest melting point.

Liquid Phase Sintering implies that part of the material is melted. In the case of cemented

carbides the Co-rich binder phase melts. Because part of the material has melted, a more

extensive densification is possible. When sintering cemented carbides, both solid phase

sintering and liquid phase sintering are used vii

.

1.3.5 Characterization methods

1.3.5.1 Light Optical Microscope (LOM)

LOM is a type of microscope which uses visible light and a system of lenses to magnify

images. The image from a light optical microscope can be captured by normal light-sensitive

cameras to generate a micrograph. LOM is the easiest characterization method and is used to

investigate different microstructures. A LOM is demonstrated in figure 2.

Figure 2 - image of a light optical microscope.

1.3.5.2 XRD

X-ray diffractometry (XRD) based on the elastic scattering of X-rays from electron clouds of

the individual atoms in the system. By observing the scattered intensity of an X-ray beam

hitting a sample as a function of incident and scattered angle, polarization, and wavelength or

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energy a material can be characterized based on thickness, present phases and their

crystallographic structure, chemical composition and physical properties (for example

residual stresses).

2 Experimental procedure

2.1 Solubility of cobalt in M23C6

2.1.1 Sample preparation

According to observations from previous studies, 1410 C is the minimum heat treatment temperature needed to be able to study the equilibrium in liquid+M23C6+M7C3. The result

from earlier experiment showed that this alloy contains the solid phase (fcc) instead of M23C6.

In previous work at 1500⁰C and 3 hours heat treatment, equilibrium has been reached in liquid+M23C6+M7C3, but the proportion of melt to solid material has been high.

On the basis of these results, the samples in this project were heat treated at 1450⁰C for 3 days. The compositions of the samples were determined by previous studies and are

presented together with the sintering times in table 1.

The samples were prepared by weighing Cr3C2, Co and Cr powders to get the right

composition. The raw materials were mixed by vibration in order to get a homogenous

powder.

Table 1- Shows the compositions and sintering times and temperatures for the liq. +M23C6+M7C3

samples.

The powder was not milled in this project, since it could easily have been contaminated in the

milling process.

The powder was pressed into pellets in a PTC (Powder Testing Center), which is a small scale

press for laboratory experiments. No additives were added since there was no demand on the

samples keeping their shape during sintering.

The heat treatments were performed in a STA 409 CD furnace at 1450⁰C for three days with stationary argon. With flowing argon the samples could have oxidized even with the very low

oxygen level in the argon. The samples were then cooled by filling the furnace with cold gas.

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The cooling rate was set on 50 degrees per minute. The cooling procedure is demonstrated in

table 2.

Table 2- Cooling rate after sintering

The sintered samples were hot-mounted in Bakelite to make them easier to analyse with

LOM. The samples were then ground with abrasive paper and polished with a diamond slurry

in order to get smooth samples with as few scratches as possible.

Before analysing the samples, they were etched with Murakami’s etching reagent, which

consists of 10g potassium ferricyanide, 10g sodium hydroxide and 100 ml water.

2.1.2 Sample analysis

The prepared samples, for detemening the solubility of chromium in M23C6, were analyzed

with LOM and XRD.

LOM was used to investigate the microstructure in the samples.

The XRD analysis was carried out to investigate which phases that had formed after the heat

treatments. The XRD model that was used was a PanAnalytical X'Pert Pro Multipurpose

Diffractometer.

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2.1.3 Results

2.1.3.1 LOM

In figure 3-8 the microstructure, gained from LOM, of the liq+ M23C6 +M7C3 samples are

presented. The black phase could be M23C6, the dark gray M7C3 and the liquid has solidified

in a eutectic reaction of fcc (light gray), M7C3 and M23C6. According to the LOM images

Sample AM400 is the only sample in the equilibrium with the melt.

AM400

Figure 3-AM400-etching line Figure 4-AM400

AM401

Figure 5-AM401 Figure 6-AM401- etching line

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AM402

Figure 7-AM402 Figure 8-AM402 etching line

2.1.3.2 XRD

The samples were difficult to analyze in the XRD. However it seemed that most samples

contained large amounts of M7C3 but since the intensity peaks for M23C6 are close to or

overlap those for M7C3 it is difficult to distinguish them from each other. From looking at the

XRD results it looked like some Cr-rich oxide had formed and that some of the original raw

material (Cr3C2) remained in the samples.

2.1.4 Discussion

It was hard to analyze the results from the experiments since the XRD results were so limited.

To get better results that were easier to interpret, the samples should probably have been

crushed and then mixed with a standard material. A well-defined standard material would be a

good reference when comparing the results with structures in the database.

The LOM images were hard to analyze without accurate XRD. However when looking at

figure 3-8 it is reasonable to assume that the AM400 sample is the only one that surely

reached equilibrium with liquid.

The sample with the highest Co content should contain the most liquid. This means that

sample AM402 should have the highest amount of liquid phase (see table 1). Therefore it is

possible that samples AM400 and AM402 were mixed up during the sample preparations or

the heat treatments.

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2.2 Solubility of cobalt in M3C2

2.2.1 Sample preparation

The heat treatments of these samples have been too short to reach equilibrium between

M3C2+M7C3+graphite (3 hours at 1500⁰C). In this work longer heat treatments were performed. The samples would probably need a heat treatment at least for a week but in this

work the samples were heat treated together with the liq+M23C6+M7C3 samples, for three days

at 1450⁰C. The compositions of the samples were determined by previous studies and are presented together with the sintering times in table 2.

The samples were prepared (mixed, pressed and sintered) in the same way as the

liquid+M23C6+M7C3 samples.

Table 3; Shows the compositions and sintering times for the M3C2+M7C3+graphite samples

2.2.2 Results

Only one of the samples was sintered, sample10AM500, because of problems with the

furnace. However it was clear that the 10AM500 sample (M3C2+M7C3+graphite) had not

reached equilibrium. The sample was impossible to polish and prepare for further analysis due

to incomplete sintering.

2.2.3 Discussion

It was clear that the M3C2+M7C3+graphite sample that was sintered did not reach equilibrium.

To reach equilibrium in the samples the time of the heat treatment would probably have to be

doubled.

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3 Conclusions and future work

The conclusions made from this work are first and foremost that the liquid+M23C6+M7C3 samples should be further analyzed in order to investigate the solubility of Co in the M7C3

carbide. In future work the samples should be analyzed with SEM to get clearer images than

the LOM images and new XRD tests should be made.

For the solubility of Co in the M3C2 carbide it is clear that three days heat treatment is not

enough to reach equilibrium between M3C2+M7C3+graphite. In future work the heat

treatments should probably be about a week.

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4 References

i William D. Callister, Jr, Materials Science and Engineering An Introduction, 7th edition, USA, 2007

ii A. Kusoffsky and B. Jansson, A Thermodynamic Evaluation of the Co-Cr and the Co-Cr-C Systems, Calphad,

vol. 21, (1997), p. 321-333 iii

T. Sterneland, A. Markström, S. Norgren, R. Aune and S. Seetharaman, Investigation of (Cr;Co)7C3-fcc-

Graphite Equilibrium in the Temperature Interval 1373 to 1473K, Metallurgical and Materials Transactions A,

vol. 37,(2006) number 10, p. 3023-3028 iv A. Markström, S. Norgren, K. Frisk, B. Jansson and T. Sterneland, Experimental and thermodynamic

evaluation of the Co-Cr-C system, Int.J.Mat, vol 97, (2006), number 9, p. 1243-1250 v M. Hillert, J. Ågren and A. Borgenstam, Mikro och nanostrukturer i materialdesign, Institutionen för

Materialvetenskap, Kungliga Tekniska Högskolan, 2005 vi Randall M. German, Powder Metallurgy & Particulate Materials Processing, 2005

vii Bonnie Lindalh, Master of science thesis, 2010

viii Björn Uhrenius, Pulvermetallurgi, Stockholm, 2000

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