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Wear 250 (2001) 492501

Effect of carbide fraction and matrix microstructure on thewear of cast iron balls tested in a laboratory ball mill

E. Albertin a,b,, A. Sinatora c

a Foundry Laboratory, Technological Research Institute IPT, So Paulo, Brazilb Instituto de Pesquisas Tecnolgicas I PT, Cidade Universitria, 5508- 901 So Paulo, Brazil

c Department of Mechanical Engineering, Polytechnic School, University of So Paulo, So Paulo, Brazil

Abstract

The effect of carbide volume fraction from 13 to 41% on the wear resistance of high chromium cast irons was evaluated by means ofball mill testing. Martensitic, pearlitic and austenitic matrices were evaluated.

The 50-mm diameter balls were tested simultaneously in a 40 cm diameter ball mill. Hematite, phosphate rock and quartz sand were

wet ground. The tests were conducted for 200 h.

Quartz sand caused the highest wear rates, ranging from 6.5 to 8.6 m/h for the martensitic balls, while the wear rates observed for the

phosphate rock ranged from 1.4 to 2.9m/h.

Increasing the carbide volume fraction resulted in decreased wear rates for the softer abrasives. The almost complete protection of the

matrix by carbides in eutectic microstructures caused the eutectic alloy to present the best performance against hematite or phosphate

rock. The opposite effect was observed for the quartz sand. The quartz abrasive rapidly wears out the matrix, continuously exposing and

breaking carbide branches. A martensitic steel presented the best performance against the quartz abrasive.

With phosphate rock, the wear rate of 30% carbide cast irons increased from 1.46 to 2.84 and to 6.39 m/h as the matrix changed,

respectively, from martensitic to austenitic and to pearlitic. Wear profiles of worn balls showed that non-martensitic balls presented deep

subsurface carbide cracking, due to matrix deformation. Similar behavior was observed in the tests with the other abrasives.

In pin-on-disc tests, austenitic samples performed better than the martensitic ones. This result shows that pin tests in the presence of

retained austenite can be misleading. 2001 Elsevier Science B.V. All rights reserved.

Keywords: White cast iron; Ball mill test; Wear mechanisms

1. Introduction

Wear is one of the most important cost factors during min-

eral processing [16]. In the cement industry grinding opera-

tions, high chromium cast iron became the standard material

for balls, replacing almost completely cast or forged steels,

since the performance of these alloyed cast irons in this ap-

plication is outstandingly superior [7,8]. On the other hand,in wet mineral grinding and, generally, in the most severe

wear applications, the differences in wear performance tend

to diminish. Consequently, a detailed technical and econom-

ical analysis is required to select the proper grinding media

material and production processes.

Many laboratory studies have been performed with high

chromium white cast irons in view of their great potential

as wear resistant materials in mineral processing and other

applications. However, since the laboratory wear conditions

Tel.: +55-11-3767-4245; fax: +55-11-3767-4037.

E-mail address: [email protected] (E. Albertin).

differ greatly from the industrial grinding applications, the

transfer of laboratory results to field service is a difficult

task. In practice, few laboratory tests results can be expected

to have wide acceptance among industrial professionals.

On the other hand, field studies of grinding media are

time consuming and expensive. Moreover, they exhibit

severe methodological shortcomings since, in production

conditions, it is almost impossible to keep constant all theoperating variables involved, which is essential to obtain

valid performance comparisons.

A fair compromise may be attained using laboratory ball

mill tests. In these tests, mills with diameters up to 1 m are

used [912], and the operation can be designed to repro-

duce, at least partially, the mineral/ball interactions that oc-

cur in a production mill, avoiding the main shortcomings

of both standard laboratory tests and field tests. If the tests

are properly conducted, information about the nature and

intensity of wear mechanisms drawn from such tests can ef-

fectively contribute to the development of grinding media

materials.

0043-1648/01/$ see front matter 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 0 4 3 -1 6 4 8 (0 1 ) 0 0 6 6 4 - 0


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E. Albertin, A. Sinatora / Wear 250 (2001) 492501 493

Compared with field tests, the advantages of laboratory

ball mill tests are the much shorter time required to perform

the tests and the possibility of introducing and controlling

operation and materials variables. A typical testing program

will vary the type and coarseness of abrasives, evaluating

their effect against a particular group of wear resisting ma-

terial or, else, for a fixed abrasive and operating conditions,the performance of a range of materials can be measured.

High chromium cast irons constitute a family of materials

that can present a wide range of characteristics depending

on chemical composition and heat treatment. These basic

variables can be manipulated to produce different carbide

volume fractions and compositions as well as radically

dissimilar metallic matrices. Additionally, more subtle vari-

ations can be introduced, using alloying elements and vary-

ing cooling rate during solidification. As mentioned, many

studies using standard laboratory tests, such as pin-on-disc

or rubber-wheel tests, have been conducted scrutinizing the

effects of most of these possible variations, but data origi-

nated from ball mills are scarce and poor. Most frequently,

published results refer to comparisons between roughly

dissimilar materials, such as steel versus high chromium

cast iron versus Ni-hard. Though useful for marketing

purposes, these data do not provide information necessary

to the continuous improvement of this important material,

and for the understanding of wear mechanisms.

During the present investigation a laboratory ball mill test

procedure has been developed, that permits, at the same time,

to perform practical studies, aimed to rank materials for

specific applications, and provide information concerning

the wear mechanisms present during ball mill operation.

The present paper discusses the mechanisms that deter-mine the wear rates in different systems, during tests in a

laboratory ball mill. A set of wear systems was obtained

varying the carbide content and the type of matrix of the

cast irons, and using two ores and silica sand as abrasives.

2. Experimental procedure

2.1. Test pieces

Cast, balls, 50 mm in diameter, were used as test pieces.

Five chemical compositions of high chromium cast irons

were selected to produce microstructures with carbide

Table 1

Chemical compositions, % carbides, hardness and densities of the alloys

Alloy Chemical composition Alloy characterization

Cr (%) C (%) Si (%) Mn (%) % Carbides Density (g/cm3) Hardness HRC

Steel 8.0 0.80 1.10 1.30 7.64 6265

A 12.2 1.65 0.37 1.33 13.1 7.61 5759

B 15.6 2.21 0.47 1.22 20.1 7.55 6163

C 18.0 2.96 0.96 1.50 28.9 7.48 6466

D 20.8 3.34 0.38 1.12 38.3 7.50 65

E 25.1 3.54 0.46 1.20 40.8 7.47 6263

fractions from 13 to 40%, which covers the range of com-

mercially produced alloys. A special composition matrix

steel was included to yield a microstructure with 0% car-

bides, with a metallic matrix similar to that of a typical

high chromium cast iron.

The chemical compositions, the carbide fractions and the

densities of the alloys are presented in Table 1. The carbidepercentages were determined by means of systematic point

count, using a 25-point grid along 60 fields.

A martensitic matrix was obtained for all compositions

by means of a heat treatment as follows, 700C for 1h,

air-cooling to room temperature; 950C for 5 h, forced-air

cooling to room temperature; tempering at 250C for 1h;

immersion in liquid nitrogen; tempering at 250C for 1h.

This heat treatment guaranteed a tempered martensite matrix

free of retained austenite (X-ray diffraction values lower

than 1%). Table 1 shows the hardness of these materials,

measured on the surface of the balls, after the run-in.

The effect of the type of matrix was studied in the cast

iron with 30% carbide fraction. A pearlitic matrix was ob-

tained treating the as-cast cast iron at 700C for 1h. An

austenitic matrix (80% austenite with martensite adjacent to

the carbides) was obtained in the as-cast condition, using Ni

and Mn as alloying elements. The hardness of the pearlitic

cast iron ranged from 44 to 46 HRC. The hardness of the

austenitic cast iron was 47 HRC before the wear test, in-

creasing to the range from 56 to 59, after the run-in.

2.2. Abrasives

The abrasives used were iron ore, phosphate rock and

quartz sand. The ores were sampled from the productionand supplied by mining companies. The iron ore-referred as

hematite in this paper-constitutes the feeding of the sec-

ond stage grinding for the production of pellets. Its compo-

sition is essentially hematite, with some silica and clay as

contaminants (amounting to 2.6%). The typical hardness of

this mineral ranges from 400 to 600 Hv.

The phosphate rock corresponds to the feeding of the

second stage grinding and has apatite as the main mineral.

The typical hardness of this mineral is around 300 Hv.

The quartz sand was a typical fine foundry sand, classified

as AFS 90/100. Its constitution was essentially quartz, with


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494 E. Albertin, A. Sinatora / Wear 250 (2001) 492501

Fig. 1. Wear rates evolution during run-in.

2.3. Wear tests

The tests were conducted in a Denver laboratory ball mill

having a 40 cm diameter 40 cm length grinding chamber,

lined with smooth steel plates. At the beginning of the tests

12 balls of each alloy were loaded. Wet milling was per-

formed for up to 200 h, at 53 rpm, which corresponded to

70% of the critical velocity. The balls and slurry occupied

40% of the chamber volume. The abrasive was fed at 70 kg/h

(50 kg/h for the phosphate rock), as a 60% solid slurry. A

classification system returned the ground product coarser

than 325 mesh (65 mesh for the phosphate rock) to regrind-

ing. The milling was interrupted periodically and the ballsremoved to measure weight losses.

A run-in period of 3050 h was conducted before each

test, using coarse silica sand as abrasive. The run-in re-

moved a layer of up to 200m, which concentrated the

heterogeneities coming from the casting and heat-treating

processes. Fig. 1 shows the evolution of the wear rates for

some of the tested alloys. The wear rates fall steeply from

the initial relatively high values. After grinding for about

Table 2

Weight losses of martensitic alloys in ball mill test-effect of % carbides and abrasives

Carbides

(%)

Accumulated weight loss per balla (g)

Hematite Phosphate Quartz

21.2c 65.7c 111.0c 144.9c 182.4c 170.9c 206.9c

Steel 0 2.15 6.61 11.28 14.52 18.8 0.28b 13.3 0.33 39.3 0.32

Alloy A 13.1 2.35 7.08 11.95 15.33 19.6 0.16 14.6 0.19 42.6 0.45

Alloy B 20.1 1.98 6.10 10.37 13.46 17.3 0.30 10.7 0.26 43.4 0.86

Alloy C 28.9 1.89 5.76 9.60 12.52 16.0 0.41 7.3 0.31 45.6 0.53

Alloy D 38.3 1.62 4.84 8.14 10.62 13.6 0.27 7.2 0.20 42.1 0.59

Alloy E 40.8 2.03 6.25 10.59 13.93 17.9 0.60 10.3 1.07 50.6 1.31

a Average of 10 balls.b Confidence interval.c Time (h).

Fig. 2. Effect of the type of abrasive on the wear rates of 30% carbide

martensitic balls.

30 h, the wear rates of the different materials tended to

stabilize at fairly constant values.

Pin tests were performed using 6.2 mm diameter pins ex-tracted from previously tested balls. To provoke conditions

very apart in terms of test severity the pins were tested ei-

ther against 80 mesh SiC, at a load of 19.65 N or against

400 mesh alumina, at a load of 9.85 N.

3. Results

3.1. Ball mill tests

3.1.1. Effect of carbide percentage and abrasive type

For each abrasive, the average accumulated weight losses

for each alloy were measured after grinding about 1 dayand then every 2 days. Table 2 presents the complete results

relative to the hematite series and the final accumulated

weight loss for the other abrasives. Figs. 2 and 4 summarize

these results.

3.1.2. Effect of the metallic matrix

The results of ball mill tests comparing the effects of

martensite, austenite and pearlite against the three abrasives


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Table 3

Effect of matrix type on the accumulated wear in ball mill test

Abrasive (grinding time) Accumulated weight loss per balla (g)

30% carbides high chromium cast irons

Martensite Austenite Pearlite

Phosphate rock (171 h) 7.28 24.29 31.46

Hematite (182 h) 15.97 21.23 34.62Quartz (207 h) 45.60 48.87 64.16

a Average of 10 balls.

Fig. 3. Effect of the type of matrix on the wear rates against hematite.

are presented in Table 3. The evolution of the wear of

pearlitic and martensitic balls during hematite grinding is

presented in Fig. 3.

3.2. Pin tests

The results of pin tests are presented using a dimension-

less parameter W, given by

W=P

AL(1)

where P is weight loss (g), the specific weight of the

material (g/cm3),A apparent contact area (cm2),L length of

the wear path (cm).

Pin test results concerning the effects of carbide percent-

age and of matrix type are presented in Table 4.

The laboratory ball mill test proved very effective to com-

pare the alloys, establishing performance rankings. In addi-tion, the test established the severity ranking of the abrasives.

Table 4

Results of pin tests with SiC and Alumina-effect of % carbides and matrix type

Abrasive Wear, W 106

Martensitic alloys Austenitic 30% carbide Pearlitic 30% carbide

Steel A B C D E Alloy Alloy

SiC, 80a 9.41 9.43 9.48 9.30 9.25 9.52 8.65 11.45

Al2O3, 400a 3.63 2.27 2.19 1.76 1.54 1.65 1.64 2.19

a Confidence interval.

The test program was very satisfactory concerning the

reproducibility of the results. For each group of balls, the

confidence interval of the average weight loss was kept under

6% (the only exception is discussed below) and the measured

wear rates for a given material/abrasive pair did not show

important variations along the various testing periods.

According to many authors, the rate of weight loss of amill ball at a given moment is proportional to its surface area

at that moment [5,9,1316]. This model results in a linear

reduction of the diameter with the milling time, according

to the equation

= o kt (2)

where ois the initial ball diameter,kthe constant (the slope

of the straight line) and t the grinding time.

Using this equation, the performance of different materi-

als can be expressed by the parameter k, since the slope of

the straight line corresponds to the wear rate of the balls. It

should be noted that kis independent of the initial ball di-ameter, permitting to compare results from different studies.

A lowkvalue corresponds to a good wear resistance of the

tested material.

The validity of this model holds as long as the wear is

caused solely by surface phenomena, such as abrasion and

corrosion. Results relative to industrial mills presented by

Norman and Loeb [9], Vermeulen [3], Nordquist and Moeller

[16], Barros et al. [5], among others, fit to the model. In

some industrial practices, however, the operating conditions

impose high kinetic energy to the interactions between balls,

mill lining and abrasives, so that the resulting wear rates are

partially proportional to the ball volume or mass. Some of

these conditions are high mill velocity, high mill diametersand low solid percent in the slurry.

To present the results in the form of Eq. (2), the measured

weight losses were converted to volume losses and then to

equivalent diameter reductions. For this, the densities pre-

sented in Table 1 were used. Figs. 2 and 3 show examples

of the straight lines obtained. Ther2 values were very close

to one in all cases.

Tables 5 and 6 present the calculated k values corre-

sponding to the wear observed, in, respectively, carbide

percentage and matrix variation series. Other calculated pa-

rameter are also presented in these tables-wear resistance,

calculated as 1/k, and wear resistance factor (WRF) cal-culated as the ratio of the wear resistance of a particular


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Table 5

Wear rates in the ball mill test-effect of abrasives and carbides percentages

Abrasive Alloy Carbides % Wear rate, k(m/h) Wear resistance (h/ m) WRFa

Phosphate rock Steel 0 2.63 380 55

A 13.1 2.89 346 51

B 20.1 2.13 469 69

C 28.9 1.46 685 100D 38.3 1.43 699 102

E 40.8 2.06 485 71

Hematite Steel 0 3.44 291 87

A 13.1 3.61 277 83

B 20.1 3.21 312 93

C 28.9 2.99 334 100

D 38.3 2.53 395 118

E 40.8 3.37 297 89

Quartz Steel 0 6.52 153 119

A 13.1 7.12 140 109

B 20.1 7.44 134 104

C 28.9 7.77 129 100

D 38.3 7.15 140 109

E 40.8 8.62 116 90a WRF: wear resistance factor, 30% carbide martensitic alloy as 100% reference.

Table 6

Wear rates in the ball mill test-effect of abrasives and matrix type

Abrasive Matrix 30% carbides cast iron Wear rate, k(m/h) Wear resistance, 1/ k(h/m) WRFa

Phosphate Rock Martensite 1.46 685 100

Austenite 2.84 352 51

Pearlite 6.39 152 23

Hematite Martensite 2.99 334 100



Quartz Martensite 7.77 129 100



Mart + 8% Pearlite 9.4 106 83

a Martensitic alloy as 100% reference.

Fig. 4. Effect of carbide percentage in the wear rates. Ball mill tests

grinding hematite, phosphate rock or quartz.

Fig. 5. Effect of the metallic matrix on the relative wear resistance against

three abrasives (100 attributed to martensitic matrix). Ball mill tests.


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material and that of a reference-in this case the martensitic

cast iron with 30% carbides.

Fig. 4 presents the wear rates of the martensitic alloys

as a function of the carbide percentage in the tests using

hematite, phosphate rock or quartz sand as abrasive.

The relative performances of cast irons with different ma-

trices are compared in Fig. 5. In the graph, the value 100was attributed to the wear resistance of the martensitic alloy

against each of the abrasives.

4. Discussion

4.1. Abrasives versus carbides

Fig. 4 shows that while the increase in the carbide con-

tent enhanced the wear resistance of the cast irons against

hematite and phosphate rock, the opposite effect was ob-

served when the abrasive was quartz.

To explain these results, the wear micromechanisms and

the severity of the test in each case must be considered.

Though the carbides are too hard to be cut by quartz, the in-

teractions between quartz grains and carbides are severe

enough to promote the breaking of carbide branches in the

prevailing conditions during ball milling. This effect is made

possible by the fact that the quartz grains easily cut away

the tempered martensite matrix, since the abrasive hardness

is much superior to that of the matrix. Once the matrix is re-

moved, carbide branches are exposed and subsequently bro-

ken by interactions with grains or any other element of the

system. In single scratch experiments, a quartz grain scratch-

ing a cast iron polished surface causes deep grooves in thematrix, while practically no effect is seen in the main por-

tion of the carbide particles. However, a detailed observa-

tion shows that microspalling occurs at the points where the

quartz particle impacts the carbides. Fig. 6 shows cracking

of a carbide particle as it arrests a quartz grain.

The results concerning the eutectic alloy (alloy D) present

a deviation from the general trend in the test with quartz,

as shown in Fig. 4. While in this test condition the general

behavior is an increase of the wear rate as the carbide con-

tent increases, it appears that the specific microstructure of

the eutectic alloy-shown in Fig. 7 puts some difficulty to

the wear process, by making the cutting of the matrix dif-

ficult. However, the mechanism of carbide breaking and re-

moval persists and the result is a wear rate higher than for

a martensitic steel. This behavior corroborates the proposed

mechanism.

For the softer abrasives hematite and phosphate rock, the

effect of the carbides as barriers against wear predominated.

It can be noted, from Fig. 4, that the behavior of the two

systems is very similar.

The above results show that valid qualitative evalua-

tions of ball materials wear resistance can be obtained by

means of tests using ores within a certain range of abrasive

characteristics, as is the case of hematite and phosphate

Fig. 6. Quartz particle scratch in a polished surface of white cast iron,

cracking of a carbide branch as it arrests a quartz particle.

rock. On the other hand, the use of an abrasive with too se-

vere characteristics can be misleading. It is clear that quartz

can not be recommended as a standard abrasive for the mod-

eling of industrial systems where it is not the main abrasive.

Results from Zum-Gahr and Eldis [17], shown in Fig. 8,

illustrate how the concept of severity can explain the differ-

ent effects caused by an important microstructural variable,

Fig. 7. Microstructure of eutectic cast iron, with 38% carbides, 1000.


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Fig. 8. Wear rate vs. % carbides. Pin-test results in different severity

conditions, from [17].

as the carbide content. While increasing the carbide content

reduces the wear caused by flint, a relatively soft abrasive,

increasingly higher wear rates are produced by 80 mesh

SiC, as more carbide is added to the microstructure. It is

clear that 80 mesh particles of a very hard abrasive impose

a highly severe testing condition, which is necessary to es-

tablish the mechanism of carbide microcracking. The result

for 220 mesh SiC shows that when the abrasive grain size

is decreased, the severity is lessened, and then the carbides

can promote an increase in wear resistance.

The severity of the test is determined not only by the

type and by the size of the abrasive, but also by the testing

system itself. Essentially the same mineral that caused a

severe condition in the present investigation-quartz, would

act as a mild abrasive in Zum-Gahrs testing conditions.

As a consequence of the above discussion, it is proposed

that the severity of a testing arrangement should be analyzed

to establish whether it is appropriate to evaluate the wear of

material in a particular application. The testing system and

the practical application must present the same correlation

between imposed conditions (such as type, hardness and

size of abrasives, as well as kind and intensity of the loads)

and wear behavior, specially concerning the occurrence of

mechanism changes. The results of the present investigationdo not recommend quartz to be used as abrasive in ball mill

tests, if the wear resistant material contains carbides or other

hard and brittle particles.

4.2. Abrasives versus matrix

Comparing materials with a fixed amount of carbides

(30%), the determining factor for the results obtained in the

ball mill tests was the ratio Ha/Hm, that is, hardness of the

abrasive divided by hardness of the matrix. As shown in

Fig. 9, rapid wear started for Ha/Hm higher than unity. That

Fig. 9. Effect of the ratio between abrasive and matrix hardness, Ha/Hm ,

on the wear resistance.

was the case for pearlite against any of the abrasives, but

also for martensite against quartz. In terms of mechanisms,

it can be said that a low wear resistance is obtained when-

ever the abrasive is able to easily cut the metallic matrix.

Comparison of the results presented in Fig. 5 shows that the

test using phosphate rock amplifies the performance differ-

ences among the various matrices. This occurs because the

low hardness of the abrasive makes it difficult to cut the

martensite, but it is enough for cutting the pearlite. The op-

posite effect is caused by quartz. Since quartz can cut all the

tested matrices, the results tend to gather.

Contrary to the results in the ball mill, the austenitic matrix

gave the best performance in the pin tests, a usual result inthe literature. A more detailed analysis of testing conditions

shows, however, that this kind of result is limited to pin tests

with extremely hard abrasives, such as SiC and alumina.

Gundlach and Parks [18] showed that using garnet, a less

hard abrasive, martensite performed better than austenite.

In laboratory jaw-crusher tests, martensite again showed the

best wear resistance [19].

The behavior of these cast irons in different testing con-

ditions can be explained considering two main factors, the

presence of brittle microconstituents and the differences in

the mechanical loading.

Fig. 10 shows metallographic sections perpendicular to

worn ball surfaces. In the case of the martensitic cast iron,

carbides under the ball surface do not crack. Apparently,

in spite of their fragile behavior in usual conditions, the

carbides near the surface can be plastically deformed and

bent, with the martensitic matrix acting as an anvil for the

carbides to support the imposed loads.

Austenitic and pearlitic matrices are too soft to give sup-

port to the carbides. In the case of these materials, cracked

carbides are observed under the surface until depths of about

20m. The wear process continually exposes these carbides

at the surface, where they are rapidly removed, resulting in

high wear rates.


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Fig. 10. Transverse sections of tested balls: (a) martensitic matrix: deformed carbides, without cracking; (b) austenitic matrix: subsurface carbide cracking.

It can be concluded that the mechanical efforts acting on

the balls can cause the cracking of carbides that are not sup-

ported by a strong matrix. The same behavior was observed

by Sare and Arnold [20], in laboratory jaw-crusher tests.

Considering the importance of the above conclusions,additional observations were made to check whether the

carbide cracking could have been produced by the sample

preparation. It was verified that the cracks were present ex-

clusively in worn pearlitic or austenitic balls. Additionally,

examination of worn balls surfaces polished without cutting

showed high densities of carbide cracking in the case of

pearlitic or austenitic balls, again a feature not shown by

martensitic balls.

The good performance of austenite in some pin test con-

ditions can be attributed to the fact that the interactions in

this case are more restricted to the surface of the test piece.

In this condition, the consumption of energy for the plasticdeformation and cold working of the austenite makes the

microcutting mechanism less effective, and the deep micro-

cracking of carbides is not promoted.

4.3. Hypereutectic alloy

The hypereutectic alloy, with 41% carbides, presented

the worse performance against all the abrasives, when com-

pared to the cast irons with carbide contents from 20 to

38%. The presence of coarse carbides, typical of the so-

lidification of hypereutectic cast irons, makes it easy for

the microcracking mechanism to start, leading to high wear

rates. Fig. 11 shows the remains of a large carbide, with

evidences that a portion has been cracked and removed.

It was observed that these materials presented the largest

standard deviations in the weight loss measurements, what

is, probably, a consequence of a discontinuous process of

coarse carbide cracking and removal.

In the maximum severity testing condition, represented

by the ball mill test with quartz, the wear rate of the hyper-

eutectic alloy follows the general trend, as shown in Fig. 4.

This shows that, except for the eutectic alloy, easy microc-

racking of the carbides is an important step in the prevail-

ing wear mechanism in the test with quartz. For the other

Fig. 11. Morphology of primary carbide in hypereutectic alloy. Evidence

of carbide cracking.

abrasives, this mechanism becomes relevant only for the hy-

pereutectic alloy, leading to a sudden increase in the wear

rate, as seen in Fig. 4.

The least severe of all testing conditions was the pin test

with fine alumina. In this system, even the presence of the

coarse primary carbides does not favor the microcracking

as the determining mechanism for the wear. Therefore, the

hypereutectic alloy shows a good wear resistance in this

particular testing condition.

In the pin test using coarse SiC, a severe wear condition

was attained, resulting in wear rates much higher than those

observed in the test with alumina. In this test, the hyper-eutectic alloy presented a much worse performance when

compared with those with 2038% carbides, similarly to the

observed behavior in the ball mill tests with iron and phos-

phate ores. Again, the reason for this is the promotion of

the microcracking mechanism due to presence of the coarse

carbides.

4.4. Practical applicability

To analyze the possible practical application of the testing

procedures and results described in this work, the present


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Fig. 12. Ball wear in a 3 m-diameter industrial mill.

results were compared to those obtained in an industrial ball

mill. The results of wear tests in a 3 m-diameter mill, carried

out in the mine that provided the phosphate rock for this

testing program, are summarized in Fig. 12 [5].

Comparing the performance of balls supplied by several

manufacturers, a range of wear rates from 2.5 to 7.3m/h

was obtained. To compare these results with those obtained

in the laboratory ball mill, a conversion factor-the square

root of the mill diameter-was used, based in the modeling

presented by Hukki [21]. Using this factor, the results of the

present work for martensitic balls would range from 3.8 to

7.8m/h. Since the laboratory and the field results fall prac-

tically in the same range, it can be concluded that the pro-

posed testing procedures can be used for the development ofmaterials for grinding media. Moreover, since the test pro-

vides quantitative results in terms of diameter reduction rate,

the in-service life of the grinding media can be estimated.

On the other hand, the results using quartz as abrasive

show the difficulty for establishing standard testing proce-

dures. Since the effects of increasing the carbide content of

the alloys were so completely opposed when using quartz

or the ores, it becomes clear that quartz should not be used

as standard abrasive for tests intended to evaluate and de-

velop materials for grinding media. The use of the same

ore of the intended practical application as the abrasive in

the laboratory ball mill test is obviously the best choice. If

only a qualitative ranking is required, iron ore, an abundant

abrasive, could be used to evaluate grinding media for many

other ores. Attention should be given to the fact the more

severe conditions (for example, a harder or coarser abrasive)

tend to compress the performance of the different materials,

while mild conditions tend to separate the results.

For martensitic cast irons, the general trends showed in

pin tests using SiC as abrasive were similar to those obtained

in the ball mill. The test using alumina did not show clearly

the adverse effect of coarse primary carbides.

The main drawback of pin test is the tendency to high-

light only the positive effects of austenitic matrices. This is

contrary to the practical field observation and to the results

of the ball mill tests of the present work. Results concerning

martensitic cast irons can be biased because of the presence

of non-controlled amounts of retained austenite.

5. Conclusions

1. The effect of the amount of carbides on the wear resis-

tance of high chromium cast irons depends on the test

severity. Severe conditions, as imposed by quartz in the

ball mill test, lead to increasing wear rates as the carbide

percentage increases. This occurs by a mechanism com-

bining rapid removal of the metallic matrix followed by

microcracking of the carbides. In less severe conditions,

as in the case of grinding of hematite or phosphate rock,

the carbides are effective to protect the metallic matrix

against the microcutting mechanism, and the wear rates

decrease as the carbide percentage increases, up to theeutectic composition.

2. Quartz sand is not recommended as standard abrasive in

ball mill wear tests.

3. The ratio of the abrasive hardness by the matrix hardness

was a determining factor for the wear rates, for 30%

carbide cast irons with different matrices.

4. Low resistance matrices, such as pearlite and austenite,

result in subsurface carbide cracking and low wear resis-

tance in the ball mill test. As a consequence, the superior

performance of austenitic cast irons in pin tests does not

reproduce in the ball mill test.

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