The wear of tungsten carbide-cobalt cemented carbides in a coal ash conditioner

The wear of tungsten carbide-cobalt cemented carbides in a coal ash conditioner

Wear, 375 153 (1992) 375-385 The wear of tungsten carbide-cobalt in a coal ash conditioner* cemented carbides A. Cuddon Engineering Investigati...

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Wear,

375

153 (1992) 375-385

The wear of tungsten carbide-cobalt in a coal ash conditioner*

cemented

carbides

A. Cuddon Engineering

Investigations, ESKOM,

Rosherville, Johannesburg

(South Africa)

C. Allen. Deparhent

of Materials Engineering,

University of Cape Town, Rondebosch

(South Africa)

(Received July 3, 1991)

Abstract An in-situ investigation has been carried out to assess the wear resistance of 13 grades of commercial tungsten carbidexobalt cemented carbides subjected to the action of flowing wet coal ash in a conditioner. All of the cemented carbides were found to have a superior wear resistance to a proprietary wear-resistant medium carbon steel currently being employed as mixer blades in ash conditioners. Cemented carbides containing fine grains and low binder contents exhibited the best wear resistance. The wear of cemented carbides has been shown to occur through the synergistic action of corrosion and abrasion mechanisms leading to the preferential loss of the cobalt binder phase followed by the removal of poorly supported whole tungsten grains. Explanations for the large variation in the wear resistance found between the different cemented carbide grades have been advanced.

1. Introduction

Power stations in South Africa generate electricity through the combustion of fine blended coal of low calorific value (approximately 16-22 MJ kg-‘). The resultant large quantities of fly ash are subsequently blended with effluent water in an ash conditioner in order to facilitate transportation on conveyor belts and to minimise dust emissions. Au ash conditioner is shown schematically in Fig. 1. It consists of two counter-rotating shafts, each fitted with 25 mixing blades set at an angle of 18.5” to the shaft axis, which move the fly ash through the containment bin. The blades are normally manufactured from medium carbon steel which has been roller quenched and’ tempered and have a maximum peripheral speed of 1.6 m s-l. The throughput of the hot ash-water mixtures averages 150 t h-i. The moisture content of the mixture can be varied between 8 and 25 vol.%. The effluent water used has a variable pH and contains sulphate, chloride and nitrate ions. The ash itself is extremely abrasive, being composed largely of alumina silicate particles with an average hardness of around 1200 I-IV. The aggressive nature of this mixture leads to rapid wear of the steel mixer blades and lifetimes as low as three months have been reported. *Paper presented at the International April 7-11, 1991.

0043-1648/92/$5.00

Conference on Wear of Materials, Orlando, PL, USA,

0 1992 - Elsevier Sequoia. All rights reserved

Fig. 1. Schematic and blades.

diagram of an ash conditioner

showing counter-rotating

worm mixing shafts

The resultant cost in terms of downtime for replacement together with the cost of manpower and materials was considered to be unacceptable and consequently attention has been given to blade material selection based on the testing and evaluation of materials in-situ. Previously reported results have demonstrated that the use of ceramic cappings on steel mixer blades can result in considerably improved lifetimes and financial savings [l]. This present work is an attempt to assess the wear resistance of other hard materials in similar circumstances, namely cemented carbides, and to correlate the wear resistance with microstmctural parameters.

2. Experimental

programme

2.1. Materials The wear resistance of 13 grades of tungsten carbidecobalt cemented carbides was evaluated in an ash conditioner. The grain sizes of the tungsten carbide ranged from 1.5 to 8 pm with cobalt binder contents between 11.3 and 31.5 vol.%. Microstructural characterisation and mechanical properties of the various grades of cemented carbides are shown in Table 1 and were provided by the manufacturer. A cemented carbide, Grade S20, containing a high cobalt content of 31.5 vol.% with a grain size of 2 pm was chosen as a reference standard in all the trials since it had already been demonstrated that the wear resistance of steel is too low for this purpose. The ash conditioner blades are normally manufactured from 0.45% plain carbon steel which has been roller quenched and tempered to a hardness of 190 HV30. 2.2. Abrasion testing The most meaningful method of collecting data through which materials can be ranked for wear resistance is to perform wear testing in-siru. Initial observations of worn mixer blades showed that wear occurs predominantly at the blade tip. As the blades move across an encrusted layer on the bin wall, additional ash is compacted between the blade tip and the wall giving rise to three-body abrasive wear of the

377 TABLE

1

Properties

of WC-Co

Grade

Vol.% co

S6 T6 GG K10 SlO GlO JlO H7 H8 H9 H12 G15 s20

alloys Hardness HV 30”

11.3 11.3 11.3 11.9 17.5 17.5 17.5 13.4 14.6 16.2 20.6 25.0 31.5

Trans. rupt. strengthb (MPa)

1550 1450 1350 1575 1335 1230 1120 1310 1275 1205 1130 1100 1030

1520 1590 1660 1250 1640 2000 2260 1690 1860 1930 2070 1920 2050

*Supplied by manufacturer. bBased on 4 point load. Steady state wear rate for medium

Fig. 2. Shows presence Fig. 3. Section millimetres.

Steady state wear rate

Average grain size (pm)

lOe-3 mm3 (h-*1

Variance

2 3 4 1.5 2 4 8 4 4 4 4 4 2

2.387 2.362 2.889 2.654 3.705 19.286 57.197 4.260 9.177 25.251 132.206 166.283 122.500

3.7 3.4 2.9 1.0 4.1 3.6 6.9 3.2 3.0 4.2 4.6 5.8 11.5

carbon steel: 551 X 10e3 mm3 h-’

(*%)

f 11%.

of wear tracks on tip of a hard faced mixer blade.

of conditioner

blade

showing

test pieces

and angle bracket.

Dimensions

in

metal blades. Figure 2 shows the typical ploughing and grooving at the steel blade tip associated with this process. Conversely the blade faces are largely protected by adhering ash particles and experience little wear. Consequently testing was carried out in an ash conditioner using the tip of one blade as a test site. The test samples consisted of triangular pieces soldered to threaded stainless steel studs which in turn were secured to stainless steel angle brackets by means of a nut. The angle brackets were attached to a removable top section of a conditioner blade which facilitated rapid exchange of test sections. Each bracket accommodated four

378

test pieces and each blade 11 angle brackets. The total arrangement of the test samples is shown in Fig. 3. The leading row of samples consisted of reference specimens and the back row of test specimens. Each test specimen was allocated a reference specimen which was offset on the blade such that both specimens experienced a similar abrasive wear situation. This arrangement is shown in Fig. 4. Prior to testing, the surfaces of the samples were polished down to a finish of 1 pm, using diamond paste, to remove any surface oxidation that could have occurred during soldering and to ensure a standard surface finish. After cleaning in acetone, the samples were weighed on a Mettler balance to an accuracy of 0.1 mg. The mass loss was monitored at regular intervals during the testing period of approximately 200 h by removing the specimens, cleaning in acetone, drying and weighing. The mass loss was converted to volume loss and the wear rate calculated with respect to time (Table 1). The relative wear resistance of each cemented carbide grade was calculated using the reciprocal of the steady-state wear rate for the reference standard, Grade S20; i.e. relative wear resistance equals the steady-state wear rate of S20 divided by the steady-state wear rate of the test grade. In this way the relative wear resistance of the different grades could be compared. 2.3. Structural examination Worn specimens were examined using both optical and scanning electron microscopy (SEM) on a Cambridge S200 stereoscan.

3. Results It was noticeable that the volume loss of material from the cemented carbides varied as a function of both constitution and position on the test blade periphery. Figure 5 illustrates this variation in volume loss with time for the series of reference samples as a function of position during testing. It is believed that the gradual formation of wear tracks across the blade edge is a result of the uneven accumulation of ash encrustations on the containment bin walls. Once a wear profile is developed, after approximately 100 h of service, it appears to maintain its shape. Consequently, the steady-state wear rate for the reference samples becomes similar with a variation in results of approximately 11.6% which is considered to be satisfactory for in-situ testing, particularly in view of the fluctuations in the amounts of ash processed with time together with variations in the water composition and moisture content of the mix (Fig. 6). Cumulative volume loss of the test samples also assumed a similar behavioural pattern as exhibited by the reference samples. This was not surprising since each test grade sample experiences movement against similar encrusted ash particles as their

Fig. 4. Test blade showing positioning of test pieces to account for orientation of wear tracks.

379

0’

1

2

3

4

Fig. 5. Variation

o

I 2

3

I, A

5

5

6

7

8 9 10 11 12 13 14 15 16 17 18 19 20 LO~TIONOFREFEREHCE SITE

in wear intensity

as a function

of position

/

/

I

6

( 7

8

/

1

9

10

I

/

11 12 13

I,

I

I

I

14 15 16 17 18 19 20

Fig. 6. Steady-state wear rates for the reference along the blade periphery.

21

across the test blade periphery.

_O 21

pieces Grade

S20, as a function

of position

relative reference sample. Table 1 shows that the normalised variation in the results of testing for all the test grade samples is less than approximately 7%. These results are based on three different test runs. The table also shows that the steady-state wear rates for the various grades of cemented carbides vary with an overall difference in performance of approximately 70 times. The relative wear resistance of the various test grades are illustrated more clearly in Fig. 7. Only two of the test grades, namely H12 and G15, had an inferior wear resistance to the standard reference grade S20. It should also be noted that the least wear-resistant carbide G15, had a wear resistance. three and a half times better than the roller quenched and tempered medium carbon steel normally used for-mixer blades in the ash conditioner. All of the other grades of cemented carbides exhibited superior wear resistance to S20 with two of the grades, T6 and S6, having a wear resistance approximately 47 times better than S20. Generally the best wear resistance. was shown by cemented carbides having a small grain size coupled with a low cobalt binder content, i.e. grades 56, T6, Gd, KlO.

380

I,,,

T6

S6

Fig. 7. Relative s20.

KlC

G6

I,/

SlO

Hi’

wear resistance

Fig. 8. Steady-state

H8 GlO U*TTERI*I

,,I

HQ

.I10

S20

of the test materials

wear rates for cemented

carbides

H12

GE

normalised

against the reference

as a function

material

of cobalt binder.

However, a variation in the grain size between 1.5 and 4 pm for a cemented carbide containing a binder content between 11% and 12% does not appear to influence greatly the wear resistance. Inspection of Table 1 shows that the measured steadystate wear rate for cemented carbides exhibiting such a constitution, grades S6, T6, G6 and KlO, are similar ( f 20%). As the cobalt binder content is increased beyond 12% the steady-state wear rates for two families of cemented carbides containing grain sizes of 2 and 4 m respectively, results in a rapid increase in wear rate regardless of the initial grain size (Fig. 8). For example, increasing the binder content from 11.3% to 25% for a cemented carbide with an average gram size of 4 pm results in an increase in the wear rate from 2.9 to 166X 10T3 mm3 mm-’ pm. This increase in wear rate with increase in cobalt binder content is greater for cemented carbides with larger grain sizes. Inspection of Fig. 9 reveals that as the hardness of the cemented carbide decreases from around 1600 HV the wear rate remains approximately constant until a hardness of 1300 HV is reached when the wear rate increases rapidly with a continued decrease

381

1

800

maa UITERIAL

1200 w*mNess

1400 w

1600

30

Fig. 9. Variation of abrasive wear rate with cemented carbide hardness.

f

2 z

z

loo05

1

0

0.3

1

L

0.6

0.Q

MEAN FREE

t

1.2

1.5

1.8

PATW IN MICRONS

Fig. 10. The effect of mean free path between carbide grains on steady-state wear rates.

It is interesting to note that all of the cemented carbides showing a hardness less than 1300 WV have a cobait binder content in excess of 14.6%. The measured wear rates for the various test grades also depend on the mean free path between the carbide grains (Fig. 10). As the mean free path increases from approximately 0.2 to 1.2 pm there is a corresponding increase in wear rate. Iiowever, the initial increase in wear rate with increase in mean free path (between 0.2 and 0.5 w) is only smail. Above a mean free path of 1.2 q the wear rate appears to decrease. The mean free path is detc~ned largely by the grain size and the binder content of the cemented carbide. Increases in either or both of these factors promote an increase in mean free path 121. in hardness.

4, Discussion The results of this work in which a variety of tungsten carbide-cobalt cemented carbides were subjected to wear testing in an ash conditioner show clearly that all these materials outperform a medium carbon roller quenched and tempered steel

382 which is currently

utilised for mixer blades (Table 1). It has also been established that the relative wear resistance of these cemented carbides varies widely by a factor of approximately 70 times. Clearly in order to interpret such wear data an understanding of the mechanisms which lead to material removal during service is a prerequisite. In the present instance it is considered that the loss of material is due to a number of interrelated factors. Examination of the worn surfaces using SEM revealed that preferential binder removal occurred for all the grades of cemented carbides (Fig. 11). This removal of the cobalt binder phase leaves the grains of tungsten carbide poorly supported and they are consequently easily dislodged from the surface through physical contact with moving or encrusted ash in the conditioner. This preferential removal of the binder occurs through two mechanisms primarily, namely corrosion and abrasion, which act synergistically as the cemented carbides move relative to wet abrasive coal ash. Cobalt is anodic to tungsten carbide and galvanic corrosion will take place when cemented carbides are subjected to a suitable electrolytic environment such as wet coal ash with the consequent loss of the cobalt binder phase [3]. The rate of corrosive attack is dependent on many factors such as the pH and electrolyte temperature and constitution. A low pH value leads to increased attack as does a rise in temperature. In the present instance the variation in the pH is known to be large with values as low as 1.7 having been recorded during the progress of this work. The temperature of the ash-water mixture is approximately 70 “C. The effluent water contains variable and high concentrations of salts including sulphates and chlorides which exacerbate corrosive attack of cemented carbides in acidic solutions. Thus, it is to be expected that corrosion of the cobalt binder will occur in practice and that the rate of corrosive attack will vary owing to the continuously changing environment. The influence of corrosion on the overall rate of wear will also be affected by the constitution of the cemented carbide. Although the ratio of the anodic to cathodic areas increases with an increase in cobalt binder content, the corrosion rate is unlikely to change significantly [3]. Consequently the mass loss owing to corrosion of the cobalt binder is also unlikely to vary to any great extent. Since the loss of tungsten carbide is dependent on binder removal, any increase in binder content will effectively reduce the rate of wear through the influence of corrosion alone.

Fig. 11. Showing the preferential removal of the cobalt binder. Note loss of discrete grain. Fig. 12. Showing development of pit or cavity on surface of carbide due to loss of many grains.

383

However, the loss of the binder phase is also assisted by the abrasive nature of the ash particles. As the particles penetrate between the grains of tungsten carbide leading to the loss of cobalt through abrasive wear. The rate at which cobalt can be removed through abrasion is intimately linked to the area of cobalt exposed to attack and the ease with which abrasive particles can penetrate between the carbide grains. It is to be expected that as the mean free path between the tungsten carbide grains increases, with an increase in grain size and cobalt binder content, then the mass loss owing to abrasion will increase [4]. Conversely, as the contiguity of the tungsten carbide grain structure improves the abrasive wear resistance should improve. Reduced free path between grains also results in increased plastic constraint at grain-binder interfaces, thereby strengthening the material structure, which is central in providing good wear resistance to tungsten carbide-cobalt cemented carbides [5]. At a critical mean free path linked to abrasive shape and size, penetration of abrasive between the grains will become so reduced that abrasive wear of the cobalt matrix should become approximately constant. In the present work this critical point appears to be at a mean free path of 0.6 pm, which corresponds to the lower limit of the measured abrasive particle size. The relative effects of abrasion and corrosion of cobalt on the overall wear rate are difficult to quantify because of the ever-changing process variables and is beyond the scope of the present work. There is little evidence that abrasive wear of the tungsten carbide grains occurs to any significant effect. This is not surprising in view of the large difference in hardness between the carbide (1600 HV) and abrasive (1200 I-IV). Once the ratio of material to abrasive hardness exceeds 0.8 abrasive wear rates fall significantly [6]. Whilst there appears to be a transition in the wear resistance of these cemented carbides at a material hardness ratio of one, such a transition cannot be compared with those reported elsewhere concerning the abrasive wear of homogeneous metallic materials [7]. The transition in wear rate for metallic materials is linked to a change in the mechanism of material removal from abrasive ploughing to abrasive cutting as the hardness of the material to abrasive hardness increases. In this instance the materials are inhomogeneous and more than one mechanism is responsible for material removal. Significant loss of tungsten carbide only occurs when the shearing forces that arise between two surfaces in intimate contact and moving relative to one another are sufficiently high to initiate intergranular fracture in the cemented carbide. Tungsten carbide grains which are poorly supported, owing to the loss of the binder phase through corrosion and abrasion, are susceptible to such forces and are torn from the surface (Fig. 12). Finer-grained cemented carbides are more able to resist such shearing forces since there is greater mechanical interlocking between the grains. Once individual grains are removed, enhanced removal of adjacent grains occurs because they are now less rigidly supported, leading to the eventual development of pits on the surface of the cemented carbide (Fig. 12). There was also evidence that the relative movement of grains, particularly in cemented carbides with large grain sizes, manifested itself in transgranular cracking of the carbide grain which ultimately assisted in the loss of tungsten carbide from the matrix (Fig. 13). It was also noticeable that in cemented carbides with high cobalt binder content considerable surface deformation and shearing had occurred during testing. Eventually this work-hardened layer is no longer able to sustain further deformation with the result that surface and subsurface cracking is initiated followed by the fracture and loss of discreet volumes of inhomogeneous material (Fig. 14). Such smearing will also lead to an overall lowering in the amount of material lost through corrosion of the

384

Fig. 13. Relative

displacement

Fig. 14. Shows smearing

of carbide

grains resulting

in transgranular

fracture.

of cobalt binder. Cracking in the work-hardened layer leads to loss of

volumes of inhomogeneous

material.

cobalt binder since it reduces the exposed tungsten carbide cathodic areas which are necessaq to sustain an electrochemical reaction. Thus it is considered that the wear resistance of cemented carbides exposed to moving wet coal ash mixtures is dependent on a number of factors. At low binder contents and small grain size the wear resistance is high and is controlled by the corrosion rate of the binder phase together with the inherent mechanical strength of the interlocked carbide grains resisting pull out. As the mean free path between the carbide grains enlarges with the increase in grain size and binder content, abrasive wear plays an increasingly dominant role, increasing the rate of loss of the binder phase and ultimately the loss of tungsten carbide grains. At high binder contents the effects of corrosion on the overall wear rate is low and the wear resistance is controlled by the work-hardening capacity of the cobalt matrix resisting abrasion. Whilst all the materials tested show an improvement in wear resistance when compared with steel, it should be recognised that any material replacement should be cost effective and readily available. The unit cost for locally producing a completely assembled carbide capped blade is approximately R800. This compares with a cost of approximately R60 for a steel blade. Thus, it could be argued that any carbide-capped blade which has a wear resistance at least 1.5 times better than steel would show a cost advantage. Since the large majority of cemented carbide grades exhibited wear resistances greatly in excess of this figure, three grades were in excess of 200 times, then the use of such composite blades will not only result in improved lifetimes but ‘have distinct cost advantages. It is anticipated that these lifetime improvements and cost advantages could be further enhanced by the use of nickel binders to promote better corrosion resistance.

5. Conclusions

(1) The wear resistance of 13 grades of tungsten carbide-cobalt cemented carbides has been shown to be superior to a medium carbon roller quenched and tempered steel when subjected to wear in a wet coal ash conditioner.

385

(2) The wear resistance of the cemented carbides varies widely and is related to the carbide grain size and cobalt binder content. Fine grains and low binder content promote better wear resistance. (3) The mechanism of wear has been found to be a ~mbination of corrosion and abrasion acting synergistically which preferentially removes the cobalt matrix, followed by loss of poorly supported tungsten carbide grains through physical interaction with the coal ash. (4) Substantial cost savings can be achieved by the use of cemented carbide capped mixer blades in coal ash conditions.

The authors acknowledge the assistance of the Engineering Investigations of ESKOM and the Lethabo Power Station in this work. The technical assistance of Mrs. H. Bohm, Mrs. S. Bet& Mr. B. Greeves and Mrs. A. C!. Ball in the preparation of the manuscript is gratefully acknowledged.

References A. Cuddon and C. Allen: The abrasive wear of ash conditioning plant in the semi-dry conveying of ash in a large power station, Anti Wear 88, Institute of Metals, London, Sept. 1988, pp. 37.1-37.8.. H. E. Exner and J. Gurland, Powder +ietalL, 13 (25) (1970) 13-31. W. J. TomIinson and C. R. Lime& Anodic polarisation and corrosion of cemented carbides with cobalt and nickel binders, J. Mater. Sci., 23 (1988) 914-918. M. K. Keshavan and N. Tee, Abrasion and erosion of WC-Cc Alloys, Met. Powder Rep., 42 (1987) 866-869. A. BaII and A. W. Patterson, Proc. 11th inf. Plansee Seminar, Ret&e, Aushia, Vol. 2, 1985, pp, 377-391. R. C. D. Richardson, The wear of metals by relatively soft abrasives, Wear, 11 (1968) 245-275. J. Larsen Basse, The effect of composition, microstructure and service conditions on the wear of cemented carbides, J. Met. (1983) 3542.