On the formation of impurity-containing phases in cemented carbides

On the formation of impurity-containing phases in cemented carbides

Refractory Metals & Hard Materials 10 (1991) 45-55 '.. k~. '?j On the Formation of Impurity-Containing Phases in Cemented Carbides Bj6rn Uhrenius*, H...

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Refractory Metals & Hard Materials 10 (1991) 45-55 '.. k~. '?j

On the Formation of Impurity-Containing Phases in Cemented Carbides Bj6rn Uhrenius*, Hrl6ne Brandrup-Wognsen AB Sandvik Coromant, Box 42056, S-126 12 Stockholm, Sweden

Ulla Gustavsson, Anders Nordgren, B6rje Lehtinen Swedish Institute for Metals Research, Stockholm, Sweden

& Helena Manninen Helsinki University of Technology, Espoo, Finland

Abstract: This work is a part of a COST programme undertaken to study the

influence of some impurities on the properties of cemented carbides. The precipitation of inclusions containing Ca, A1, Si, P, S and O was studied by combining calculated phase equilibria with sintering experiments based on two cemented carbide grades. Minor amounts of Ca, A1, P and Si were added to the raw materials used and by sintering in furnace atmospheres, having increased sulphur or oxygen potentials, the formation of oxides and sulphides was facilitated. A comprehensive investigation of as-sintered surfaces, polished cross sections and fracture surfaces was made, and the type and amount of impurity-containing phases were compared to the calculated diagrams. It was found that CaS is always formed whenever Ca and S are present. Excess amounts of either Ca or S were evaporated, and also Si to some extent. Aluminium is often found in oxides together with Ca and Si. Both A1 and P are present in the sintered material to the same extent as had been added to the raw materials. No phosphorus or very small amounts of the phosphorus added were found in the inclusions investigated. It is believed that P is evenly distributed throughout the material to a high degree.

materials o r in the f u r n a c e a t m o s p h e r e d u r i n g sintering. Inclusions o f brittle phases such as sulphides o r oxides m a y be d e t r i m e n t a l to the m e c h a n i c a l p r o p e r t i e s o f the finished p r o d u c t s a n d the objective o f the present w o r k was to find o u t u n d e r which c i r c u m s t a n c e s such phases precipitate a n d to w h a t e x t e n t they m i g h t influence the t e c h n o l o g i c a l p r o p e r t i e s o f c e m e n t e d carbides. A S w e d i s h - F i n n i s h project was p l a n n e d in 19841985 a n d was financially s u p p o r t e d b y the Swedish I r o n m a s t e r s A s s o c i a t i o n a n d the Swedish B o a r d for T e c h n i c a l D e v e l o p m e n t . This project was linked to the C O S T 503 p r o g r a m m e in 1985 a n d the results

1 INTRODUCTION T r a c e elements like calcium, a l u m i n i u m a n d silicon are always p r e s e n t in small a m o u n t s in the raw materials used for the p r o d u c t i o n o f c e m e n t e d carbides. T h e level o f these impurities is usually very low, o n l y 10-30 p p m , a l t h o u g h occasionally the c o n t e n t m i g h t be in the r a n g e 50-100 p p m . A l t h o u g h these elements readily dissolve in the b i n d e r metal, they o f t e n precipitate in the f o r m o f oxides a n d sulphides d u e to the high affinity o f these elements to o x y g e n a n d s u l p h u r present in the raw * Present address : The Swedish Institute for Metals Research, Drottning Kristinas v/ig 48, S-114 28 Stockholm, Sweden. 45

Refractory Metals & Hard Materials 0263~,368/91 $3.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain. 6-2

46

BjOrn Uhrenius et al.

were presented in a report o f the Swedish Ironmasters Association. 1 Raw materials, as for all projects within COST 503, were delivered by H. C. Starck AG, Berlin, in early 1986. A group with members from OY Airam, Kovametalli, AB Sandvik C o r o m a n t , the Swedish Institute for Metals Research and Helsinki University of Technology has been doing work in different areas of the project. The results of the characterization o f the raw materials used are presented in a previous report s and the present work is only concerned with the sintering practice used and phases precipitated containing the impurity elements added.

P as well as an undoped reference all having grain sizes suitable for the two grades. These carbides as well as Co powders, (Ta, Nb)C powders and soot were obtained from H. C. Starck. All raw materials were carefully characterized by chemical analysis and by the determination o f grain sizes and grain size distribution. Similar characterizations of the same materials were made in parallel COST projects at H. C. Starck and Metallwerk Plansee. The impurity content of the powder mixtures used (based on the analysis of the raw materials) is compared to the impurity content after sintering in Table 1.

2 RAW M A T E R I A L S

3 SINTERING TECHNIQUES

The present work describes the results obtained on two cemented carbide grades; one metal cutting grade for milling applications containing 93.6% W C (1-5/tm), 0.4 % (Ta, Nb)C and 6 % Co (referred to as grade I), and one grade for rock drilling purposes continuing 90 % W C (6/zm) and l0 % Co (referred to as grade II). The metal cutting grade was sintered and tested at Sandvik C o r o m a n t and the rock drilling grade at OY Airam, Kovametalli. These two grades were sintered on the basis of tungsten carbide powders, doped with about 200 ppm of one o f each o f the elements Ca, A1, Si or

Pressed inserts and some o f the rock drill bits were sintered in three different atmospheres in a Sandvik laboratory vacuum-sintering furnace to simulate atmospheres o f production furnaces containing increased oxygen or sulphur potentials as well as a reference run corresponding to normal sintering practice. The vacuum-sintering furnace used had an effective volume o f 40 litres. The pressed material was put on graphite trays. After dewaxing in a hydrogen atmosphere, heating to sintering temperature was done directly in a vacuum. During heating to the sintering temperature, some charges were exposed

Table 1. Results of the chemical analysis of sintered inserts

Sin tering variant

vacuum (reference)

vacuum + H ~S

vacuum + COs

Trace elemen t added

none Ca AI Si P none Ca A1 Si P none Ca AI Si P

Chemical analysis (ppm) Ca

Al

Si

< 40 60 < 40 < 40 < 40 < 40

20 40

< 40 < 40 < 40

110

< 40 < 40 < 40 < 40 80

< 40 < 20 < 40

190

< 20 20 40 30 200

50 50 20 < 20 180

40 30

P

20 20 20 20

70

< 40 < 40 60 < 40 90 < 40 < 40 < 40 < 40 70

< 40

130

< 20 20 < 20 < 20 120

< < < <

20 20 20 20

100

S

20 30

20 20 20 60 120

50 90 40 20 50

20 20 20

Content of dopant in ready to press powder (ppm)

-140 Ca 160 AI 110 Si 110 P -140 Ca 160 A1 110 Si 110 P -140 Ca 160 A1 110 Si 110 P

Formation of impurity-containing phases in cemented carbides to an atmosphere containing small additions of CO 2 or H2S, 0.10% by volume at a total pressure of 7 mbar. These additions were made to a gas where the major constituents were H 2, Ar and CO. The reactive gases (CO 2 and H2S ) were added in the temperature range 800-1200°C where the pore system was still open to enable oxygen and sulphur to react with the impurity elements added. After the reactive step, sintering was performed at 1400°C (rock drill bits) and at 1450°C (milling inserts). Specimens were then allowed to cool down at a slow rate. The vacuum furnace used was equipped with a mass spectrometry system allowing for the analysis of gaseous species with a molecular weight less than 100. The resolution was about 0-01% vol. The partial pressures of six gaseous species (CO2, CO, H 2, Ar, N 2 and H2S) were registered during the heating to sintering temperature. Due to reactions between the gases and the furnace lining or the furnace load, it was not always possible to keep the pressure at a constant level. It is, however, evident from the analysis of the sintered material that the amount of sulphur let into the furnace was enough to form sulphides in the sintered material. It was more difficult to maintain the increased level of carbon dioxide due to reactions between the carbon dioxide and the graphite materials inside the furnace. Due to the oxygen-rich inclusions formed on the sintered carbides it is believed that the load was exposed to an increased oxygen potential. After sintering the cemented carbides were characterized by chemical methods, metallographic inspection, SEM and microprobe analysis. 4 C A L C U L A T E D P H A S E EQUILIBRIA DESCRIBING T H E C o - C - C a - S i - A I - O - S S Y S T E M AT 1400°C Thermodynamic equilibria describing the precipitation of phases containing trace elements like Ca, A1, Si, S and O were presented in a paper at a Plansee Seminar? Similar calculations have also been made in the present work to increase the understanding of the experimental results obtained. The data describing the thermodynamic properties of the dilute cobalt liquid have been somewhat modified since the previous calculations. No solution phases between oxides or silicates were considered. All phases, except the cobalt melt, were treated as stoichiometric compounds although it is known that silicate melts are formed at 1400°C

47

or already at somewhat lower temperatures. By neglecting the possibility of forming such melts the tendency to precipitate these phases was underestimated. However, in view of all the other uncertainties involved, this simplification was not believed to have any major influence on the conclusions drawn. As all elements, except carbon and cobalt, are present in very low amounts ( < 200ppm), a simplified regular solution model was used to describe the properties of the liquid cobalt phase. All interactions between two or more of the ' trace' elements were neglected. Thus only interactions between cobalt a n d / o r carbon and these elements were considered. The calculations were limited to one temperature, 1400°C, which might present a compromise between the sintering temperatures used in the present case, 1400 and 1450°C. A number of calculations were thus made simulating the situation during the sintering of cemented carbides containing minor amounts of the elements Ca, A1, Si, O and S. All calculations were made by using a computer program for phase diagram calculations, POLY, 4 developed at the Institute of Technology in Stockholm. The following expression was used for the description of the excess energy of the liquid cobalt phase: c E=

x,(Xoo A,_co + xo A,_o) i

where i represents the elements in the sevencomponent system; xi stands for atom fraction of component i; and the parameters A describe the pairwise interaction between the elements in the liquid phase. The C o - W - C system has previously been described in thermodynamic terms by one of the present authors 5 and the description of the C o - W - C liquid used in that work was also adopted in the present work. The influence of tungsten, however, was neglected and only the parameters describing the C o - C interactions were used. A tabulation of all parameter values is given in Table 2. A review of thermodynamic data on dilute cobalt liquids by Sigworth and Elliott 6 gives information about the interaction between cobalt or carbon and AI, Si, O and S. This information is in most cases restricted to 1600°C. For the present purpose extrapolations to 1400°C were done by assuming regular solution behaviour. Experimental information on the properties of calcium in liquid cobalt was not found in the

Bj6rn Uhrenius e t al.

48

Table 2. P a r a m e t e r s describing the interaction between elements dissolved in liquid cobalt at 1400°C

Element (i) AI C Ca O (2~-O2) S(½S2) Si

Reference state liquid graphite liquid gas (1 atm) gas (1 atm) liquid

oGi Hq- oGj rer

Ai_c

(J/mole)

(J/mole)

0 68 730 0 -26870 -29850 0

literature but estimates could be made on the basis of information found for the Ni-Ca system s and on the assumption that cobalt behaves in much the same way as nickel. Data on cobalt liquids compiled by Schenck e t al. 9 were also used in order to find an estimate for the interaction coefficient between calcium and carbon. In a similar way the activity coefficient of calcium in dilute cobalt liquids could be estimated on the basis of work by Lange. 1° The resuits of these estimates are also presented in Table 2. A search in a thermodynamic text book 11 and in the SGTE-database xz available at the Institute of Technology in Stockholm gave information of approximately 25 different compounds formed between the elements Ca, A1, Si, S and O. The data chosen to represent the Gibbs energy of formation of these compounds are presented in a more detailed report describing this part of the COST programme. 1 Equilibria in a seven-component system are difficult to represent in an easy way and in order to facilitate calculations the following restrictions were introduced. Sintering of cemented carbides is often made at conditions close to graphite saturation and, even if a deficit of carbon is present, carbon activities (relative to graphite) will not be reduced by more than a factor of three or four. It was thus decided to put the carbon activity, a c, equal to unity throughout all calculations. Oxygen and sulphur are volatile at sintering temperatures and the chemical potential of these elements is thus strongly influenced by sintering furnace conditions. In general the amount of these two elements is probably not only determined by the amount present in the raw material but also influenced by residues in the furnace lining and by condensates on the furnace walls. It was therefore chosen to allow a variation of the chemical potential of these elements. By presenting these calculations in the form of predominance area diagrams and by putting the sulphur and oxygen potentials on the

A~_c

(J/mole)

- 82 500 - 51200 0 5100 0 - 104000

- 97 700 -- 148 600 -515000 14850 - 106000

Reference number 6 5 estimate 6 6 6, 7

axes, the degrees of freedom were obtained, as needed for these elements. A seven-component system offers a maximum of six degrees of freedom (keeping temperature and pressure constant and by using mole fractions rather than moles as composition variables). After the choice of carbon, sulphur and oxygen potentials, three more degrees of freedom remain to be defined in a single phase domain. The 'trace' elements, Ca, A1 and Si, might be evaporated to some extent during sintering but by choosing the amounts of these elements corresponding to the level obtained after sintering the three remaining degrees of freedom were defined. Calculations could thus be made representing a situation close to sintering conditions (see Figs 1-4). 5 E X A M I N A T I O N OF SINTERED CARBIDES

The results of the chemical analysis of carbides from two test runs with additions of H2S and CO 2 as well as from a reference batch without the addition o f ' r e a c t i v e ' gases are shown in Table 1. An increased level of sulphur in inserts sintered with

14 -18

[/II1

l +CaO / ~

16

l+CaS

-12 -8 log( pS 2/ MPa )

Fig. 1, Calculated p r e d o m i n a n c e area diagram for the C o 4 ~ C a - A I - S i ~ O - S system at 1400°C. C a r b o n activity, a c = 1. I m p u r i t y c o n t e n t : A1, 200 p p m ; Si, 10 p p m ; Ca, 10 ppm.

Formation of impurity-containing phases in cemented carbides

-14l+silicates+CaO_ ~,

~- -

-16l+ C a O ~

a'Al-°xides

z~ - t 8 -

l+CaS

-20-

-22-

liquid-Co -16

I

.112 I -8t log( pS 2/ MPa )

T

Fig, 2. Calculated predominance area diagram for the C o - C Ca-AI-Si-O-S system at 1400°C. Carbon activity, a c = 1. Impurity content: Ca, 200 ppm; Si, 10 ppm; AI, 10 ppm. -14-

I+silicates -16

~

_- --

,f

l+CaO ~ / ~ C a , A l - o x i d e s ¢q

-18-20-

I+CaS liquid-Co

-22A

I

-k

'

s

the addition of H~S is evident, especially for the calcium-doped material. An addition of CO 2 was less efficient, since a rapid reaction with the surrounding graphite only resulted in an increased amount of CO. This on the other hand indicates that the oxygen potential might be rather insensitive to minor additions of oxygen-rich species, provided the initial amount of carbon monoxide is high enough. The results indicate that calcium is evaporated during sintering if the sulphur level is insufficient to precipitate calcium as CaS. The amount of silicon added to the carbide is also reduced during sintering. However, both phosphorus and aluminium are present at the same level after sintering as before. In the following a code is used to describe the material which has been subject to a certain treatment. According to this 'code' A1/O describes the material sintered on the basis of a carbide doped with aluminium and which is heat-treated in an atmosphere somewhat enriched in CO s. In the same way Ca/S describes the Ca-doped material sintered in an atmosphere with an addition of H2S. The notation R / R describes the reference material. In order to detect any difference regarding the kind or amount of precipitates on the surfaces of the sintered carbides or in the core of the specimens, inspections were made on the following surfaces:

i

log( pS 2/ MPa ) Fig. 3. Calculated predominance area diagram for the C o - C Ca-A1-Si~O-S system at 1400°C. Carbon activity, a c = 1. Impurity content: Si, 200 ppm; Ca, l0 ppm; A1, 10 ppm.

-14-

l+silicates r

-16- l +silicates +CaO/// l+Ca,Al-oxtdfs

J

~-18-

l+CaS "~2o-

-22-

~-~ liquid-Co I' -16

t

I

] -12 -8 log( pS2/ MPa)

i

Fig. 4. Calculated predominance area diagram for the Co-C Ca-AI-Si~3-S system at 1400°C. Carbon activity, a c --1. Impurity content: A1, 200 ppm; Ca, 200 ppm; Si, 200 ppm.

49

(1) as-sintered surfaces (2) fracture surfaces from TRS-tests (3) polished cross sections

Grade I Grade II Milling Rock inserts drill bits yes yes yes yes

yes no

For the investigation of these surfaces a scanning electron microscope, equipped with an energy dispersive X-ray analyser, was used. Two different SEM-EDX instruments were used for the analysis of the milling inserts and the rock drill bits. With the EDX equipment used for the analysis of milling inserts it was possible to detect light elements including oxygen. X-ray distribution maps were used to describe the distribution of the different elements in the complex phases containing the trace elements. It should be emphasized that in some cases severe overlap between X-ray energies from different elements occurs. One example is the overlap between SiK and W M energies. A W-signal from the carbide phase is always detected and makes it difficult to see whether Si is present or not,

50

Bj~rn Uhrenius et al.

especially when the Si content is low. However, in large inclusions with high Si contents, the presence of Si could be accurately detected. The amount and the size distribution of the inclusions found in the A1/O and A1/R were determined using an automatic image analyser based on a S E M / E D X system (PASEM--Particle Analysing Scanning Electron Microscope). The system combines automatic image analysis with Xray analysis of the individual inclusions. PASEM gives a qualitative description of the composition of the inclusions. Light elements (e.g. oxygen) cannot be detected. This investigation was carried out on polished surfaces of milling inserts only (grade 1). Transverse rupture tests (TRS) were made for two reasons. One reason, and the most important reason in the present case, was to obtain fracture surfaces to study with respect to the presence of inclusions. The other reason was to detect any impact of the dopants on the mechanical strength of the sintered inserts. Three samples from each combination of dopant and atmosphere were tested. Only one of the sintering experiments made in the H2S-enriched atmosphere was successful with respect to the carbon content of the sintered material and with respect to the partial pressure of sulphur maintained, and in this experiment only the Cadoped variant was included. There are thus no reliable values of TRS tests from the other variants sintered in this atmosphere. The TRS values are shown in Table 3. The specimens were HIP-ed before they were ground to shape and lapped. The reason for HIP-ing was to get a fully dense material for mechanical testing as the pores obtained in all laboratory-milled material would otherwise act as starting points for fractures, and probably make the influence of the inclusions impossible to detect.

In most cases a very good correlation between the observations of inclusions and the TRS values was obtained. The lowest of the three values was obtained for a sample where the authors could observe a big inclusion at the initiation point and the highest value was reached for a sample where no inclusions were observed. An exception to this is the group of undoped specimens R / R of grade I, which did not reach the highest values, in spite of the fact that no inclusions were found in those.

5.1 The image analysis of sintered carbides Most of the comments below refer to investigations based on grade I. However, the results of grade II are similar but for the experiment with the H2Senriched atmosphere. This experiment seemed to have failed regarding the pick-up of sulphur during heating and the results indicate an increased oxygen potential. As a result these specimens contain rather coarse aluminium oxides, which caused abnormally low TRS values for this variant. No sulphides were detected. A slight indication of decreased TRS values also for the P-doped material was obtained for this grade. However, statistical calculations did not give any significant difference between doped and undoped material. 5.1.1 A l - d o p e d m a t e r i a l

Al-rich 'islands' or spots were found on both upper and lower surfaces of the sintered inserts of material A1/R and material A1/O (Figs 5 and 6). These inclusions were uniformly distributed over the surfaces. The sintering of AI/R resulted in smaller but more frequent islands on the lower surface (Fig. 5) as compared to specimen A1/O (Fig. 6). On the upper surface of the inserts a high number of small

Table 3. Results of three-point transverse rupture tests, (MN/m 2) Grade I

Grade H

Sintering atmosphere

Sintering atmosphere

Trace element added

' Vacuum' (reference)

+ CO 2

none A1 Si Ca P

2750 2 550 3000 2 500 2600

2800 2 600 2950 2750 2450

+ H2S

(2950) (2650) (2950) 2150 (2650

' Vacuum' (reference)

2400 2 500 2450 2150 2100

pure H 2

2700 2800 2700 2 750 2700

+ H2S

very low very low very low very low very low

The variation of the above values is approximately + 200 to ___350. Values within brackets are single values, others are the average of at least three measurements.

Formation of impurity-containing phases in cemented carbides

Fig. 5. Lower surface of an A I / R insert with Al-rich islands.

51

Fig. 7. Large, Al-rich islands ( ~ 20 pm on the lower surface of an A l / O insert.

F&~,~

Fig. 6. Lower surface of an AI/O insert with relatively large Al-rich islands.

spots were observed but with no significant differences between the two sintering atmospheres. Examples of the Al-rich islands with the corresponding element distribution maps are given in Figs 7 and 8. The sizes of the islands on the lower surface of A1/O (Fig. 7) are about 20 pm, which is approximately twice the size of the islands on the corresponding surface for material A1/R. The Xray distribution map in Fig. 8 shows an increased concentration of aluminium and oxygen along the periphery of the almost circular precipitates. This was not observed on any other surface. The sizes of the spots on the upper surface of the inserts were approximately 5 pm. The results from the image analysis (PASEM) of inclusions in specimen A1/O and A1/R are shown in Fig. 9. In both materials the inclusions are mainly Al-rich particles coarser than 10 pm. Light element X-ray analysis of similar particles on the fracture surfaces shows that these inclusions are oxides. A relatively small number of inclusions containing other elements such as (A1 + Ca, AI + Ca + S + Ti, A1 + P) was also detected. The amount of inclusions

~.



Fig. 8. Element distributions on the surface above. High aluminium (AI) and oxygen (O)-intensity along the periphery of the islands.

Al/O

0.005

Others : At*Ca At*Ca*S*Ti AI*P

~ o.~ z

0.003 i

AI/R ~'

0.002

0.001

Fig. 9. Area fraction of aluminium (Al)-containing defects larger than 10 pm in the materials A1/R and A1/O.

containing several elements is higher for A1/O specimens as compared to AI/R. The results indicate that, when the oxygen potential in the sintering atmosphere is increased, the amount of inclusions in the cemented carbide is increased too. The amount of calcium sulphide in material A1/O was also measured. The CaS particles are much more frequent but considerably smaller than the AIoxides. Only a small number of calcium sulphides

Bj6rn Uhrenius et al.

52

larger than 10/zm was observed. Therefore the influence of individual CaS inclusions on the fracture initiation process, e.g. during bend testing is small compared to the influence of the relatively large aluminium oxides. Based on microscopic observations of CaS in A1/R, the size distribution for CaS in A1/O is also assumed to be representative for A1/R. The CaS distribution in A I / R was only estimated on the basis of a comparison with A1/O in a light optical microscope. In many cases the large defects are pores where the Al-containing inclusions occupy parts of the pore volume, or where the inclusions cover the internal surface of the pore (Fig. 10). The smaller particles (Fig. 11) on the contrary probably consist of Al-containing oxides without any surrounding porosity. These different types of inclusions were also identified in the fracture surfaces of the TRS test specimens. It was always possible to find the area on the fracture surface where the fracture was initiated. In

Fig. 10. Aluminium oxides of different sizes in a polished cross section through an AI/R insert.

Fig. 11. A small aluminium oxide in A1/R.

many cases it was also possible to detect an individual inclusion at the site of fracture initiation. In Fig. 12 an example is given of a critical defect found in the Al-doped material. The figure shows a relatively large defect found approximately 0"5 m m from the tensile surface. The defect shown in Fig. 12 is assumed to correspond to the inclusion types observed on the polished cross sections.

5.1.2 Ca-doped material In this case it was much more difficult to detect the impurity elements on the sintered surfaces due to their low amount and to the very low contrast in the SEM. Special attention was paid to the lower surface of Ca/O-inserts. Small calcium sulphides were found (~< 5/zm). An example of the surface of the cemented carbide and the corresponding element distributions is shown in Figs 13 and 14. The CaS is probably present in the form of a thin layer on the central part of the image of the surface. Individual WC-grains in Fig. 13 can easily be distinguished although they are covered with a layer of CaS. The lower surfaces of C a / R and C a / S were also scanned. No inclusions of phases similar to those in Fig. 12 were detected. However, based on the rather limited SEM investigations it is difficult to compare the relative amounts of CaS on the three specimens. Occasionally small (2/tm) CaS spots were also observed on the upper surfaces of the Ca/O-inserts. Two different types of inclusions were found on the fracture surfaces: small CaS inclusions and large Ca- and Al-containing inclusions. The individual CaS particles found in the C a / R material are very small but on some fracture surfaces several CaS particles lying close to each other were observed. Fracture due to CaS inclusions is assumed to be caused by interaction between several CaS

Fig. 12. Fracture caused by a relatively large, aluminium oxide-containing defect. Al-rich areas are indicated by arrows.

Formation of impurity-containing phases in cemented carbides

Fig. 13. Lower surface of a Ca/O cutting insert. A thin layer of calcium sulphide covers the WC grains.

Fig. 14. Element distributions on the surface above. Relatively high intensities of Ca and S. particles forming larger defects. The individual calcium sulphides are too small ( < 5/zm) to initiate the fracture in these cemented carbides. A fracture surface from the C a / R - m a t e r i a l showed a relatively large inclusion (30/~m) containing both Ca and A1. Only small amounts of sulphur were detected in some areas of that inclusion. The inclusion is assumed to be mainly oxidic. A duplex (Ca-sulphide + Al-oxide) inclusion which was found on a fracture surface o f a C a / O specimen is shown in Figs 15 and 16. The size o f that inclusion is approximately 30/tm. The elemental distribution maps show that the aluminium oxide is surrounded by a sulphide phase

(CaS). In the C a / S specimens no observations of large Al-containing inclusions were made on the fracture surfaces. However, occasionally agglomerates of CaS were found.

5.1,3 Si-doped material Large agglomerates o f inclusions were found on one fracture surface. The phases present in that

53

Fig. 15. Fracture caused by a large (30/tin) duplex aluminium oxide-calcium sulphide inclusion in a Ca/O TRS test specimen.

Fig. 16. Element distribution in the inclusions above. The aluminium oxide phase is surrounded by calcium sulphides. inclusion were calcium sulphide and aluminium oxide. Silicon was detected in some regions together with Ca and S. The presence of Si was confirmed by careful manual point analysis using the S E M / E D X system. High intensities of the binder phase (Co) signals were also detected together with these inclusions.

5.1.4 P-doped material Even though large inclusions were observed on the fracture surfaces, no phosphorus was detected in these inclusions or on the surrounding fracture surface. Large aluminium oxide inclusions were found in fracture surfaces.

5.1.5 Reference material No inclusions were found on the fracture surfaces on the reference material (R/R). However, it should be emphasized that only a limited number of fracture surfaces were investigated for each material and it cannot therefore be concluded that large inclusions are not present in the reference material.

Bj6rn Uhrenius et al.

54

Table 4. Investigations of fracture surfaces of bend test specimens--summary of results (grade I)

Trace element added

Sintering atmosphere

A1

Reference CO s

Ca

Reference

P

CO s H2S Reference

Si

CO 2 CO s

Undoped

Reference

Critical defects Large, thin, plate-type of pore/inclusion Small, round inclusion Large, thin, plate-type of pore/inclusion Large inclusion Several small calcium sulphides Large, duplex, inclusion Small calcium sulphide Co-binder pool; no P detected Large inclusion Large, duplex, inclusion Co-binder pool

In Table 4 a compilation of the observed inclusions is presented, giving the combinations of elements obtained in the EDX analysis. 6 CONCLUSIONS A comparison between the analysis of the raw materials and sintered carbides shows that aluminium and phosphorus in general are remaining in the material after sintering whereas calcium and silicon to some extent are evaporated. Calcium is, however, strongly bound to sulphur and when sulphur is present, either added in the gas phase or in the raw material, an almost stoichiometric relation between calcium and sulphur is prevailing in the sintered material. The calculations of predominance area diagram show that calcium sulphide is precipitated already at very low sulphur potentials and a certain amount of sulphide will probably always be present in sintered carbides. Aluminium, on the other hand, is present in the form of oxides or silicates and investigations show that almost all specimens contain some aluminium oxides. The presence of these seems to be more serious than that of the sulphides as the former generally tend to be coarser and more likely to initiate crack formation. Pronounced aluminium oxide precipitates were observed on the surfaces on the A1/R and A1/O inserts. Calcium sulphide precipitates were detected

Frequency (No. particles/ No. of fr. surfaces)

Size (1.tm)

A1203

1/3

6O

AI~O3 AI203

2/3 1/3

10 40

Ca, A120a CaS

2/3 1/3

30-50 10

AI~Oa + CaS CaS --

1/3 1/6 1/3

30 <5

AI~Oa AI203 + Ca with Si

2/3 1/3

35 50

Composition

1/3

on the surface of the C a / O inserts and were generally much smaller than the aluminium oxide precipitates. No trace elements or inclusions were detected on the surface of the P-doped inserts. F r o m the compilation of the observed inclusions it can be seen that A1 is present in the oxidic inclusions in all variants investigated. Apparently aluminium is also a very important element in the formation of inclusions large enough to cause fracture during bend testing. It is also observed that the addition of Ca and Si is reflected in the composition of the inclusions in these materials. Duplex inclusions consisting of both sulphide and oxide phases were detected on the fracture surfaces of the Ca- and Si-doped materials. The sizes of the Al-containing oxide inclusions identified as fracture initiation sites were in the order of 30-60 pm. The calcium sulphides observed on the fracture surface of the Ca-doped material were considerably smaller. Agglomerates of CaS are, however, assumed to be of importance for the crack initiation. Calcium sulphides were also present in the Al-doped material, but were not found on the fracture surfaces of this material. 7 ACKNOWLEDGEMENT This work is the result of a joint programme between parties in Sweden and Finland. The authors would like to gratefully acknowledge the financial support of the Swedish Ironmasters

Formation of impurity-containing phases in cemented carbides

Association, the Swedish Board for Technical Development, the Swedish Institute for Metals Research, OY Airam Kovametalli, the Ministry of Trade in Finland and AB Sandvik Coromant, and express their sincere appreciation to all parties within the COST 503 involved in this project, especially we would like to mention H. C. Starck who put the raw materials at our disposal. References 1. Uhrenius, B., Brandrup-Wognsen, H., Almqvist, V., Gustavsson, U., Nordgren, A., Lehtinen, B. & Manninen, H., The influence of trace elements on the properties of cemented carbides. Swedish Ironmasters Association Report D620, Stockholm, 1988. 2. Ortner, H. M., Willhartitz, P. & Lux, B., The influence of trace elements on the properties of cemented carbides--a joint EuropeanoStudy within COST 5031, Austria, 1988. 3. Uhrenius, B., Akesson, L. & Mikus, M., High Temperatures--High Pressures, 18 (1986) 337-46.

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4. Sundman, B., Jansson, B. & Andersson, J.-O., Calphad, 9 (1985) 149-85. 5. Uhrenius, B., Calculated and experimental equilibria in the Co-Ti-W-C system. Internal report, AB Sandvik Coromant, Stockholm, 1985. 6. Sigworth, G. K. & Elliott, J. F., Canadian Metallurgical Quarterly, 15 (1976) 123-7. 7. Chart, T., High Temperatures--High Pressures, 5 (1973) 241-52. 8. Hultgren, R., Desai, P., Hawkins, D., Gleiser, M. & Kelley, K., Selected values of the thermodynamic properties of binary alloys. ASM, Metals Park, Ohio, 1973. 9. Schenck, H., Frohberg, M. G. & Steinmetz, E., Cobalt, 23 (1964) 88-93. 10. Lange, K. W., Archivfiir das Eisenhfittenwesen, 41 (1970) 1-6. 11. Kubaschewski, O. & Alcock, B., Metallurgical Thermochemistry. Pergamon Press, Oxford, 1979. 12. Sundman, B. & Jansson, B., The SGTE Database, Implemented on the Thermocalc System. Division of Physical Metallurgy, Royal Institute of Technology Stockholm, Sweden.