Wear, 84 (1983) 297 - 312 297

A STUDY OF CAVITATION EROSION OF CAST IRON

T. OKADA and Y. IWAI

Department of Mechanical Engineering, Fukui University, Bunkyo 3-9-1, Fukui (Japan)

A. YAMAMOTO

Matsushita Electric Industrial Co. Ltd., Osaka (Japan)

(Received May 14, 1982; in revised form July 30,1982)

Summary

Cavitation erosion tests were carried out in ion-exchanged water using four cast irons (gray or spheroidal graphite) together with carbon steels with the same matrix as the cast irons. The erosion mechanism of the cast irons is explained through the growth of eroded cracks which depends on both the notch action of graphite and the notch sensitivity of the matrix. The erosion resistances of the cast irons and of the carbon steels were compared. A linear relation was obtained from the logarithmic curve between the reciprocal of the maximum mass loss rate and H,*/E& (where Hv is the Vickers micro- hardness, E is the elastic modulus and & is the fatigue strength reduction factor for cast iron). It is thus concluded that the cavitation erosion resis- tance of cast iron is less by a factor of (l//3,)‘*’ than that of carbon steel of the same hardness as the cast iron matrix.

1. Introduction

Cavitation damage of cast iron is initiated from graphite, so the erosion resistance is influenced greatly by the graphite size, shape and distribution and by the matrix strength [ 11. Thus it was reported [2] that spheroidal graphite cast iron is better than gray cast iron and pearlitic cast iron is superior to ferritic cast iron in erosion resistance. Endo [3] showed that much erosion damage in cast irons arises from the free graphite, so the erosion resistance is dependent on the size and shape of the graphite even if the matrix is hardened by heat treatment. If it is assumed that the erosion rate of cast iron is higher because the free graphite acts as a surface notch in the formation of erosion particles, it is necessary to observe the relationship between the shape and size of graphite and the sensitivity of the matrix to a notch just as in the study of metal fatigue.

In the present work, cavitation erosion tests were carried out using various specimens, i.e. gray and spheroidal graphite cast iron with a matrix

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of pearfite or ferrite. The erosion mechanism is explained through the behaviour of erosion cracks which initiate and propagate depending on both the graphite shape and the matrix. The erosion resistances of various cast irons are also compared with carbon steels for more general use.

2. Test materials and experimental procedure

2.1. Test matertats Test materials were ferritic gray cast iron or pearlitic gray cast iron (FClO

or FCZO respectively; ASTM A48) and ferritic or pearlitic spheroidal graphite cast iron (FCD40 or FCD70; ASTM A536). For comparison, 0.15% C steel (S15G; AISI-SAE 1015), 0.54% C steel (S55C; AISI-SAE 1055) and tool steel (Japanese designation SK4) were also tested, The chemical composi- tions and heat treatments used are listed in Tables 1 and 2. FClO is obtained by ferritizing FC20. The carbon contents of the various cast irons are similar. Magnesium is added to obtain the spheroidal graphite of FCD40 and FCD70.

The mechanical properties were measured in the Fukui University laboratory, and they are summarized in Table 3. The Brine11 hardness & is

TABLE 1

Chemical compositions (wt.%] of the test materials

Material c Si Mn P S &

FClO, FC20 3.27 1.71 0.34 0.12 0.063 FCD40 3.47 2.71 0.31 0.033 0.013 0.033 FCD70 3.25 2.68 0.24 0.030 0.010 0.035 s15c 0.15 0.25 0.47 0.015 0.013 S55C 0.54 0.27 0.82 0.016 0.017 SK4 1.11 0.34 0.98 0.014 0.009

TABLE 2

Heat treatments of the test materials

Material Details of the following heat treatments

Before machining After machining.

Annealing Normalizing annealing

FC20 - - I FClO FCD70 FCD40 SK4 S15C

s55c

940 “C for 120 min -

- 940 “C for 60 min 540 “C for 30 min

940 “C for 120 min - J - 940 “C! for 60 min 910 “C for 60 min - 600 “C for 30 min

850 “C for 60 min -

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TABLE 3

Mechanical properties of the test materials

Material aB@'fPa) E (GPa) Hv HB

FClO 110 71.5 152 91 FC20 201 97.0 350 172 FCD40 414 167 201 126 FCD70 860 167 385 278 s15c 420 204 150 101 s55c 706 204 240 190 SK4 1117 204 370 331

uw WW

49.0 58.8

137 235 211

390

&

4.3 6.7 1.5 1.7

the measure of the macrohardness and the Vickers hardness Hv is the mea- sure of the microhardness of the matrixes. u, is the fatigue limit, which is measured in a rotating-bending fatigue test using unnotched specimens in air (cantilever-type test at a stress cycle frequency of 1200 rev min-l). & is a fatigue strength reduction factor for cast iron, i.e. [4]

P, = u,(carbon steel with the same structure as the matrix of cast iron)

uW (cast iron)

The test materials were cast in bars of 30 mm diameter and heat treated to obtain the desired microstructure. Test pieces were then machined to the size shown in Fig. 1. The test surface was buff finished. The specimens were then vacuum annealed to eliminate cold-work effects.

MlOX1.25

Fig. 1. Shape and dimensions of specimen (all dimensions are given in millimetres; thread diameter, 10 mm; thread pitch, 1.25 mm).

Figure 2 shows the microstructures of four cast irons. The length of the flake graphite, or the diameter of the spheroidal graphite, and the number of graphite particles per unit area are listed in Table 4. The size and number of graphite particles are similar regardless of the matrix structure.

2.2. Experimental procedure The cavitation erosion tests were carried out using the magnetostrictive

vibratory facility described previously [ 51. The test specimen was attached to the free end of the horn and vibrated in water at 14.5 kHz. The double amplitude was maintained at 40 pm by controlling the output current of the

(a)

(c) Cd)

Fig. 2. Microstructures of test materials: (a) FClO; (b) FC20; (c) FCD40; (d) FCD70.

TABLE 4

Size and number of graphite particles

Type of cast iron Test Number of Size parameters of the graphite particles a material graphite

particles Average size Standard deviation

(mm-*) (Pm) (Pm)

Gray cast iron FClO 371 57.4 32.5 FC20 376 59.8 39.3

Spheroidal graphite FCD40 200 27.0 11.6 cast iron FCD70 220 21.5 13.2

‘For flake graphite, the particle length was measured; for spheroidal graphite, the particle diameter was measured.

oscillator, which is almost proportional to the amplitude. Ion-exchanged water was used as the test liquid to minimize corrosive effects. The test vessel (a glass beaker with a capacity of 500 cm3) was surrounded by circu-

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lating water (the temperature was held constant by an electronic cooling bath) and the test water was maintained at 25 f 1 “C. The end plane of the test piece was immersed to a depth of about 3 - 4 mm. The mass loss was mea- sured with a precision balance (sensitivity, 0.01 mg). Eroded surfaces and erosion particles were observed by scanning electron microscopy (SEM). In addition, the cracks in the cross sections of the test pieces were observed at test termination.

3. Experimental results and discussion

3.1. Cavitation erosion of pearlitic cast iron In general, pearlitic cast iron is more often used in practical applications

than ferritic cast iron so we discuss here mostly the erosion of pearlitic cast iron. Figure 3 shows mass loss rate curves for FC20 and FCD70 together

0 120 240

Exposure time min

Fig. 3. Mass loss rate curves of pearlitic cast irons.

with those of S55C carbon steel and SK4 tool steel for comparison. The erosion processes for cast iron are similar to those for plain carbon steels [6] , namely there exists an “initial period”, where a comparatively large mass loss occurs, followed by an “incubation period” where there is little further loss. There is a subsequent “transition period” with an increasing mass loss rate and then a “stable period” with an approximately constant maximum rate. Finally, there is a “deceleration period”, during which the erosion rate decreases gradually. However, the mass loss rate for cast iron increases strongly during the initial period, and also during the incubation period. These phenomena, as will be discussed in detail, are caused by surface graphite.

The mass loss rates in the stable period for FC20 and FCD70 are greater by factors of about 10 and about 2 than the mass loss rate for SK4, whose hardness is the same as that of the matrix of the cast irons. Although the matrix of FC20 is harder than that of S55C, the mass loss rate is larger for FC20 than for S55C. These results may be due to the notch action of

Fig. 4. Eroded surfaces in the initial period (after 5 min): (a) FC20; (b) FCD70.

Fig. 5. Eroded surfaces in the incubation period:, (a) FC20 (after 10 min); (b) FCD70 (after 20 min).

graphite in the matrix. Notch action of graphite is greater for gray cast iron than for spheroidal graphite cast iron.

To study the behaviour of graphite during the erosion process, speci- men surfaces were observed by SEM. Figure 4 shows an eroded surface which had been exposed to cavitation for 5 min. This is during the initial period. Graphite, and any other compounds on the surface, is easily ejected. However, the matrix has not yet changed so the larger mass loss rates in the initial period may be due to the removal of graphite particles. Figure 5 shows the eroded surfaces of FC20 cavitated for 10 min and of FCD70 cavitated for 20 min. These are from the incubation period. The matrix material surrounding the graphite is pounded into small fragments, which are then ejected. Thus a larger mass loss rate occurs in the incubation period for cast iron. The matrix slips along the flake graphite boundaries for the FC20. This means that considerably longer cracks are developed inside the matrix. Figure 6 shows the eroded surfaces of FC20 cavitated for 30 min and of

Fig. 6. Eroded surfaces in the transition period: (a) FC20 (after 30 min); (b) FCD70

(a)

(cl

Fig. 7. Series of photomicrographs taken at the same test surface location for FC20 during the stable period: (a) after 160 min; (b) after 170 min; (c) after 180 min.

(a)

(cl

Fig. 8. Series of photomicrographs taken at the same test surface location for FCD70 during the stable period: (a) after 360 min; (b) after 375 min; (c) after 390 min.

FCD70 cavitated for 100 min. Large particles are removed along the flake graphite boundaries for FC20. For FCD70, the matrix surrounding the crater formed by the ejection of spheroidal graphite is eroded, and accord- ingly the craters are further extended.

Figure 7 is a series of photomicrographs taken at the same test surface location for FC20 during the stable period. Large craters are formed on the spongy surface. It is considered that larger particles are removed in succes- sion, resulting in large local craters, and simultaneously spongy surfaces are fragmented into small pieces and then ejected into the liquid. Figure 8 shows a similar series of photomicrographs for FCD70 in the stable period. Many cracks surround the crater formed by spheroidal graphite removal Thus small debris particles are ejected from the crater walls.

Figure 9 shows roughness profiles of the eroded surface during the stable period. These correspond to a cumulative mass loss of about 70 mg. The mass loss rate is larger for S55C than for SK4, but both steels are eroded almost uniformly over the whole surface. By contrast, FC20 suffers rela- tively deep eroded pits (up to about 200 pm in depth). The FCD70 surface

305

(b)

(d)

Fig. 9. Roughness profiles of eroded surfaces obtained in the stable period (the profiles correspond to mass losses of about 70 mg): (a) FC20 (test duration, 180 min; mass loss, 72 mg); (b) FCDPO (test duration, 720 min; mass loss, 72 mg); (c) SK4 (test duration, 1440 min; mass loss, 73 mg); (d) S55C (test duration, 120 min; mass loss, 67 mg).

is eroded approximately uniformly but is much rougher than both SK4 and S55C. These results indicate that craters are caused by graphite removal. Cast iron forms therefore a particularly rough surface because of the presence of flake or spheroidal graphite in the matrix.

Figures 10(a) and 10(c) show the cross sections of specimens in the stable period. The cracks observed are of two types. Type I cracks are short cracks in the matrix, unrelated to the graphite. Type II cracks are cracks which propagate from the bottom of a graphite particle into the matrix, or through several such graphite particles. Typical cracks of both types are shown schematically in Figs. 10(b) and 10(d).

The average crack length and the number of cracks per unit length of cross section are summarized in Table 5. For FC20 the average type II crack length is about five times longer than the type I cracks (the longest cracks of type II are up to about 100 pm in length). For FCD70, most of the cracks of

(a)

Eroded matrix

Type Ii Type II

(b)

(c) Cd)

Fig. 10. Cross sections of test pieces in the stable period and corresponding schematic models of the cracks: (a) cross section of FC20; (b) schematic model corresponding to (a); (c) cross section of FCD70; (d) schematic model corresponding to (c).

TABLE 5

Average crack length and number of cracks per unit length of cross section in the stable period

Type of cast iron Test material Average lengths Numbers of cracks

(Pm) (cm-l)

Type 1 Type II Type I Type II Total

Pearlite FC20 5.5 29.0 229 247 476 FCD70 3.6 4.5 420 373 793 SK4 6.7 - 529 - 529

Ferrite FClO 7.5 21.7 702 160 862 FCD40 9.5 12.8 632 123 755 s15c 9.8 - 651 - 651

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both types are less than 10 pm in length, but the total number of cracks is about 1.5 times that for FC20 and SK4.

For comparison, erosion particles ejected into liquid were observed by SEM. Figure 11 shows cumulative distributions of particle size and mass in the stable period. The particle mass is calculated by assuming that the particles are spheres. Distributions for FCD70 and SK4 are similar. However, for FC20 there are more larger particles so that the mass frequency is greater for the larger size.

From these results, we thus conclude that type II cracks propagate more deeply for FC20 and larger erosion particles are ejected. This results in locally deeper erosion craters and a substantial mass loss rate increase. For FCD70 many cracks originate at the boundary between graphite and matrix but do not propagate deeply because of a lower notch action. Thus smaller erosion particles fall quickly from the graphite boundary, resulting in an increased graphite void size and erosion rate.

3.2. Cavitation erosion of ferritic cast iron Figure 12 shows mass loss rate curves for FClO and FCD40 and for

S15C, which has the same structure as the matrix of both cast irons. The mass loss rates in the stable period are larger for ferritic than for pearlitic cast irons as shown in Fig. 3. The mass loss rate ratios of cast irons to steel are 2.5:0.8:1 (FClO:FCD40:S15C) for ferritic cast iron and 10:2:1 (FC20: FCD70:SK4) for pearlitic cast iron. The mass loss rate ratio is thus least for ferritic cast irons because the graphite notch action is less for a ferritic than a pearlitic matrix.

O.Ol I 0 0 60 120 0

SIX of part1cles pm Exposure time mln

Fig. 11. Cumulative frequency distributions for size and mass of erosion particles in the stable period.

Fig. 12. Mass loss rate curves of ferritic cast irons.

(a)

(b) Br

Fig. 13. Roughness profiles of eroded surfaces obtained in the stable period (the profiles correspond to mass losses of about 70 mg): (a) FClO (test duration, 40 min; mass loss, 68 mg); (b) FCD40 (test duration, 90 min; mass loss, 66 mg); (c) S15C (test duration, 90 min; mass loss, 64 mg).

Figure 13 shows the surface roughness of FClO, FCD40 and S15C during the stable period. The cumulative mass loss is about 70 mg for all materials. By comparing Figs. 9 and 13 it can be seen that S15C is eroded relatively uniformly over the entire surface, and it does not differ much from S55C or SK4. However, FClO shows many deep eroded craters and a substantial increase in roughness compared with FC20. Similarly, the rough- ness of FCD40 is significantly greater than that of FCD70.

By comparing cracks in the ferritic and pearlitic matrix (Table 5) it can be seen that the average crack length of both type I and type II cracks for FCD40 is about three times that for FCD70, but for FClO the type II crack length is shorter than for FC20. These results show that erosion cracks are longer in a ferritic than in a pearlitic matrix for both carbon steel and spheroidal graphite cast iron, but the trend is reversed for gray cast iron. These tendencies are opposite to those for the mass loss rate and the eroded surface roughness (Figs. 12 and 13).

Figure 14 shows the distributions of cumulative frequencies for the size and mass of the erosion particles. The mass distributions show little dif- ference between ferritic cast irons (FClO and FCD40) and steel (S15C). Thus the erosion particle size (the largest particle is less than about 40 pm in size for the three materials) is much smaller for FClO than for FC20 but is larger for FCD40 than for FCD70. The following conclusion can be drawn from these

Y

2 10

E

: FCIO FCD40 S15C

1

Frequency q q q 0.1

0 50 100

Sme of partIcks pm

Fig. 14. Cumulative frequency distributions for size and mass of erosion particles in the stable period.

results. For ferritic gray cast iron (FClO), although cracks propagate more slowly because of the lower notch sensitivity of the matrix, erosion particles are ejected quickly before the cracks propagate deeply because of the low matrix strength. It may thus be considered that local surface erosion becomes greater and the mass loss rate is increased substantially for ferritic gray cast iron in comparison with pearlitic gray cast iron. For ferritic sphe- roidal graphite cast iron (FCD40), the number of cracks occurring around the spheroidal graphite is only about one-third of that for the pearlitic cast iron (Table 5) because of the low notch action and lower notch sensitivity of the matrix. However, the cracks propagate further in proportion to the re- duction in matrix strength, resulting in increased size of erosion particles, and hence increased erosion rate.

3.3. Erosion resistance of cast iron Maximum mass loss rates in the stable period are listed in Table 6 for all

materials together with the improvement ratio for erosion resistance when the matrix is strengthened by transforming ferrite into pearlite. The improve- ment ratio is greatest for carbon steel but least for gray cast iron because of the severe notch action of graphite. It has been concluded (e.g. in ref. 6) that cavitation erosion is caused by local fatigue fractures from repeated bubble collapse pressures on the surface. Generally, the fatigue strength of carbon steel increases in proportion to one-half of the tensile strength for smooth specimens. However, the degree of strength increase for notched specimens becomes smaller according to the sharpness of the notch [ 71. Therefore, by comparing the lower improvement ratios of cast irons with the fatigue

310

TABLE 6

Improvement ratio of erosion resistance when the matrix is strengthened by transforming ferrite into pearlite

Material Test material designations and Improvement ratio of corresponding mass loss rates erosion resistance (mg hP’) for the following (ferrite in to pearlite) material structures

Ferrite Pearlite

Gray cast iron FClO; 159.6 FC20; 30.5 5.2 Spheroidal graphite cast iron FCD40; 56.7 FCD70; 6.0 9.5 Steel S15C; 67.3 SK4; 3.1 21.7

strength increase of notched specimens for tensile strength, the erosion resistance is found to exhibit tendencies similar to fatigue.

Until now, it has often been found (e.g. in refs. 8 - 12) that the erosion resistance shows a better correlation with oB2/2E or HB2/E, which means the strain energy to fracture, rather than with a single mechanical property such as tensile strength or hardness. However, when the erosion resistance (e.g. the reciprocal of maximum mass loss rate in the stable period) is plotted against HB2/E, cast iron shows a lower resistance than those of carbon steels or various other metals [12]. This may be due to the treatment of cast iron as macroscopically homogeneous. It is thus necessary to study further the erosion resistance of cast iron with the consideration that the matrix strength for repeated stressing is reduced because of the notch action by the graphite.

The reciprocals of the maximum mass loss rate for the cast irons and steels are plotted (Fig. 15) as functions of Hv2/E& instead of Hv2/E, where /3, is the fatigue strength reduction factor for cast iron. A single linear

1

Fig. 15. Relation between erosion resistance and HV2/E&.

311

relation was then obtained on a logarithmic graph for the cast irons as well as for the steels. The slope (calculated by the least-squares method) of the line was 1.7 (equivalent to about 60”). The linear correlation coefficient was 0.99. It is thus concluded that the cavitation erosion resistance of cast iron is smaller by a factor of (l/p,) 1.7 than that of carbon steel of the same hardness as the cast iron matrix.

4. Conclusions

Cavitation erosion tests were performed using gray cast irons, spheroidal graphite cast irons and carbon steels with the same matrix as the cast irons. The following conclusions are drawn.

(1) Although cavitation erosion of cast irons exhibits erosion processes similar to carbon steel with increasing exposure duration, erosion rates in the initial and/or incubation periods are larger for cast irons than for carbon steels because of the effects of graphite.

(2) For gray cast irons, long erosion cracks occur because of the severe notch action at the graphite interfaces and locally deep erosion craters are formed, resulting in an increased erosion rate. For spheroidal graphite cast irons, many erosion cracks originate along the boundary between graphite and matrix, and accordingly the craters are extended from the spheroidal graphite holes. The erosion rate is thus increased. As a result, erosion damage is higher for gray cast iron than for spheroidal graphite cast iron.

(3) Erosion resistance is higher for pearlitic than for ferritic cast irons. The improvement ratios, however, are smaller than those assumed from the increased strength of the matrix.

(4) A linear relation was obtained on a logarithmic plot between the reciprocal of the maximum mass loss rate and Hv2/E&, where & is the fatigue strength reduction factor for cast iron. Thus the cavitation erosion resistance of cast iron is (l//3,)1*7 times smaller than that of carbon steel with the same matrix hardness as the cast iron.

Acknowledgments

This work was performed in Japan as part of a joint research pro- gramme between Fukui University, Japan, and the Cavitation and Multiphase Flow Laboratory, University of Michigan, MI, U.S.A. The authors would like to extend their thanks to Professor F. G. Hammitt, University of Michigan.

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