ELSEVIER Wear 198 (1996) 150-155

WEAR

Progress of erosive wear in spheroidal graphite cast iron

K. Shimizu ~'*, T. Noguchi b, T. Kamada b, H. Takasaki b •Oita National College of Technology 1666 Maki, Oita 870-01, Japan

b Faculty of Engineering, Hokkaido University, North 13 West 8, Klta.~, Sapporo 060, Japan

Received 5 October 1995; accepted 1 February 1996

Abstract

To clarify the erosion mechanism of spheroidal graphite cast iron, the authors carried out erosion tests and observed the continuous structural change in the vertical section near the surface. The erosion of spheroidal graphite cast iron progresses as [ollows: (l) the spheroidal graphites on the surface layer deform gradually; (2) lips grow in the impact direction; (3) they expand and drop off. Although the erosion progresses with the repetition of lip-growth and dropping off, the cycle differs depending on the materials. The wear growth rate, which serves as the standard cycle of lip-growth and drop off, decreases as the peaflite ratio increases and corresponds well to the erosion rate. We also confirmed that our continuous observation method is practical and useful for elucidating the erosion mechanism.

Keywords: Erosive wear; Spheroidal graphite cast iron

1, Introduction

The phenomenon of material surfaces being damaged and removed by particle-impact is called "Erosion" and it is a serious problem in transportation by pneumatic conveyance [ 1-14]. The authors conducted a series of tests of erosive wear of ductile iron, studying erosive wear characteristics and wear mechanisms [ 1-3]. Observations of the structure of cross sections has made it clear that erosive wear forms lips by plastic deformation in the neighborhood of the eroded surface, and that these lips are repeatedly damaged, grow and chop off [1-3]. However, the process of lip formation and growth has not been established.

This study performed tests on the erosion of ductile irons with different structures and hardnesses. To observe the changes in the eroded sections, the test pieces were combined after being cut in half, and the formation and process of the erosion was made clear. The rate of removal of the material from the surface was directly calculated, and the relationship between the hardness of the structure and the removed, eroded volume was considered.

2. Materials and methods

The test pieces were ferritic ductile iron (FDI) and ductile iron with pearlite ratios of 30% (PDI 30), 50% (PDI 50)

* Corresponding author.

0043-16481961515,00 © I996 Elsevier Science S.A. All rights reserved Pll S 0043-1648 ( 9 6 ) 06951-7

and 70% (PD170). The chemical compositions and mechan- ical properties are as in Table 1, and the structure is shown in Fig. 1.

The test piece was 50x50x 10 mm and the surface was finished by grinding. The authors used a blast machine in the test, and the particles were steel grits with mean a diameter of 660 i~m and Vickers hardness number (HV) 420 (load 0.01 N). The grit was changed after each test. The impact velocity (V) was about 145 m s- ~, and the particle mass was about 37 g s- ' , which was found in pneumatic conveying. The nozzle diameter was 6 nun, the distance between the nozzle and samples was 50 ram. The impact angle was set at nine values with 100 intervals, and each test lasted 7200 s [1-3], At set times, the eroded volume was obtained by calculating the eroded mass with an electronic microweight (sensitivity 0.01 rag). After the test, the eroded surface and the metal structure of the cross section were observed macro- and microscopically [ 1-3].

3. Results

In discussing the erosion resistance of materials of different densities, it is not appropriate to simply compare the weights of the eroded material [ 1-8]. However, using average den- sities, the erosion rate can be compared. Fig. 2 shows the relationship between the impact angle and the erosion rate at 7200 s for each test piece. Both FDI and PDI have the max.

K Shimizu et aLI Wear 198 (1996) 150-1.55

Table I Chemical composition (mass%) and mechanical pcoperties of specimens

151

c si Mn P S Ca Mg ea $ HB p

FDI 3.75 2.08 0.34 0,0It 0,007 0.04 0.03"] 400 25.8 140 7.00 PDi (30) 3,74 2.16 0.30 0.020 0.008 0,27 0.035 520 18.8 187 7.00 PDI (50) 3.83 2.30 0.40 0.017 0.007 0.42 0,041 650 12,8 217 7,00 PDI (70) 3.65 2,34 0.48 0,020 0.008 0,75 0,043 750 7.2 255 7.00

~'B, tensile s~ngth (MPa); ~, elongation(%); HB, Brin~ll hardness (HB !0/3000), p, ~asity (gcm- 3).

Fig, I, Microstructur¢ of specimens.

h . , , . d

~l,,,m

FDI - t -pot

.~ 20 7200s J-A- ~1

,P

.2 m lo n

5

l _ _

°o 80 6o 90 Impact anise (deg,)

Fig. 2. Erosion rate vs. impact angle for FDI and PDI.

imum erosion rate at the 600 impact angle. The minimum erosion rate is for PDI (70), one-third of FDI. The erosion rate decreased with increases in the hardness and the amount of pearlit¢ in the matrix, but did not correspond with Brinell hardness (HB). The HB of PDI (30) and PDI (50) arc about 20% different, while the erosion rate of PDI (50) is about half that of PDI (30). On the other hand, although PDI (50) and PDI (70) have the same difference of HB (about 20%), the erosion rates do not differ much. The authors measured surface hardness with the Rockwell hardness test and this is shown in Fig. 3, arranged according to erosion rate and impact angle. The eroded surface became harder than the initial hardness, caused by the particle impacts. The hardness after the test did not depend on impact angle. The initial hardness of PDI (50) and PDI (70) scarcely changed after the test, as shown by the erosion rates in Fig. 2. Hence, the

x10.4~, . , . , ] • Pl~{30) I. I • Pol(~)

"~ ::,o F o I A PDIFO)

-~ r o , ;15~ %o " , .~Io[ o o o , ~ +°[

o~.,c , .: , _&, • 70 80 90 100 110

Hm'dne~ (HRBI

Fig. 3. Erosion rate vs. hardness (HRB) for FDI ami PDI.

hardness of the eroded surface has much to do with hardness of the material.

4, Observation of the eroded surface

To consider the above results in relation to changes in the eroded surface, the authors observed cross sections of the eroded surface. Fig. 4 shows cross sectionsoferodcd surfaces with impact angles :200 and 60 o blasted for 7200 s. Here there- fore are lips and structural deformation in the direction of depth.

The specimens form lips by plastic deformation in ~he area of impact. Table 2 shows the maximum length of lips of each specimen, In all specimens the lips caused by erosion rate of 60 ° are longer than those produced at 20 ° . With the impact at

152 /t', Shimizu et al. / Wear 198 (1996) 150-155

V=145m/s

Fig. 4. Microstructure of the eroded surfaces of FDI and PDI at the 20 ° and ~o impact angles.

I J 50~m

Table 2 Lengths of eroding surface structures (p,m)

20 + 60 °

FDI 150 175 PDI (30) 100 175 PD! (50) "/5 175 PDI ('/0) 50 150

20 °, the lip of FDI is about 150 txm, and longer than that of PDI. As the pearlite in the matrix increases, the 20 ° lip becomes shorter, and with PDI (70) it is 50 ltm. At 60 °, all specimens show lips of similar lengths, 150-175 ltm. As the length of the lip does not reach more than about 175 itm, it may be concluded that the differences in the material cause differences in erosion rates because the time of formation, growth, and dropping off of the lip is different for materials.

Test specimen (half)

$ p a ~ . . . $pacer

Set bolt

Fig. 5. Sketch oftl~ specimen holder.

5. Continuous observation oft.he erosion process

To determine the erosion process of the specimens, the authors observed the effect of deformation, growth and drop- ping off of lips on the structure near the eroded surface in the depth direction. Specimens were cut in half along the center line, fixed at the base with jigs, as shown in Fig. 5, and the surface was observed with a microscope after a set time of blasting. In this test, the grit jet did not erode the edge of

K. $himiz~ et al. 1 Wear 198 (1996) 150-155

V=145m/s a=60* ~z~

153

1001.tin Fig. 6. Miemseopic continuous observation of eroding surfaces of FD! and PDI (50) at the 60 ° imraet angle.

sample at all. Fig. 6 is the result of observations of the cross The figure clearly shows that, initially, the spheroidal graphite sections of FDI and PDI (50) when the erosion rate is max- on the eroded surface deforms gradually, then extends in the imum (at 60 °) and the blasting time is 60, 300, 600 and 900 s, direction of blasting, and finally drops out.

154 K. Shimizu et aLI Wear 198 (1996) 150.-155

500

40O

g

i °

100

V=145r~s (z. 60*

,1-<>-~o, ! I + Pint.) i . , 0

~ o ~ 600 8oo lOOO Total hieing ~11e (see,)

Fig. 7. Wear depth vs. total blasting time for FDI and PDI.

between the removed depth and time variation. The removed depth increases linearly with time for every specimen. The linear gradient may be regarded as the growth rate in the direction of eroded depth. This may be defined as the "wear penetration rate" (l~m s- ~).

The wear penetration rate of FDI is about 0.47 izm s-~, PDI (30) about 0.26 ~m, PDI (50) about 0.2 Fro, and PDI (70) about 0.17 ~m. The wear penetration rate becomes smaller as pearlite volume increases and shows a correspon- dence with the above-mentioned erosion rate.

Fig. 8 shows the relation between wear penetration rate and erosion rate. The relationship shows good iinearity, which indicates that the erosion rate can forecast wear pene- nation rate. Therefore, the values of wear penetration rates are useful in estimating the life of pipes subject to erosion.

V=145m/s a-6o ° 7200se0. x104 ~ f . . . . . . . . .

em O.

151"

10 POl('/O} t~

0 0.1 0.2 0.3 0.4 0,5 Wear pom~ttonmte ( ~m/s~

Fig. 8. Erosion rate vs. wear penetration rate for FDI and PDI.

The structure and spheroidal graphite on the surface of the FDI specimen begins to deform in the direction of impact after 60 s of blasting. After 300 s, the surface becomes rougher and the spheroidal graphite deforms to a depth of 100 I~m, with visible lips. The lips are more than 50 I~m long, and the intervals and lengths are not constant. Near the sur- face, after 600 s, the deformed spheroidal graphite forms lips of 150-175 l~m with the ferrite phase near the spheroidal graphite, and there are craters made by the broken off lips. At 900 s the erosion is deeper and further extended.

With PDI (50), like FDI, at 60 s the structure and sphe- roidal graphite has begun to deform. At 300 s, the graphite in the surface down to about 100 i~m is deformed ellipticatly and about 50 I~m lips are formed near the surface. The rough- ness on the surface is smaller than that on FDI. At 600 s, spheroidal graphite, deformed elliptieally at 300 s, is pressed down and has been flattened by the impact of particles. The deformed spheroidal graphite grew with the ferrite phase and pearlite phase around it and produced lips of about 100- 175 I~m. At 900 s, erosion developed further in the depth direction. These phenomena are similar in both PDI (30) and PDI (70).

The spheroidal graphite behavior in the area of variation

is shown in Fig. 6. From the variation of the surface, the removed depth can be obtained; Fig. 7 shows the relationship

6. Conclusion

To make the erosion mechanism of spheroidal graphite ductile iron clear, erosion tests directly observing cross sec- tions near the surface wore conducted. The results are as follows.

In the erosion mechanism of PDI the spheroidal graphite at the surface deforms gradually, lips are produced in the direction of the blast, and they extend, and finally drop off. This process of growth, extension, and dropping off is repeated, but life spans are different depending on the material.

The growth and dropping offthe lips becomes an indication of the wear penetration rate, and corresponds to the erosion rate as it becomes smaller with increases in pearlite.

The above explains the mechanism of spheroidal graphite ductile iron erosion by formation, growth and dropping off of the lips. The material life is different depending on the volume of pearlitc. The authors have shown that the erosion rate and wear penetration rates of materials are different. This investigation confirms that the continuous method of obser- vation is of great use in the study of erosion mechanisms.

Acknowledgements

The authors wish to express their gratitude to Hitachi Met- als, Co. Ltd and Dr Masato Goka (Hitachi Metals, Co, Ltd) for supplying specimens and suggestions in performing the experiments.

References

[ I ] K. Shimizu and T. Noguchi, Wear. 176 (1994), 255-260. [2] K. $himizu and T. Noguchi, Trans. Am. Foandrymen's See.. 93.078

(1993), 225-229. [3] K. Shimizu andT, Noguchi, Trans, Am, Foundrlmen'sSoc., 94.-063

(1994) 285-289. [4] I, Finne. Wear, 3 (1960) 87-103.

K. Shimizu el aL /Wear 198 (1996) 150-155 155

[5] I.G.A. einer, Wear, 6 (1963) 5-21. [6] J.G.A. Bitter, Wear, 6 (1963) 169-190. [7] J.H. Neilson and A. Gilchdst, Wear, I1 (1968) ! i !. [8] K.V. Pool, CK.H. Dharan and I. Rnne, Wear, 107 (1986) 1-12. [9] T.H. Tsiang, ASIM SlP, 1003 (1989) 55-74.

[ 10] A.W. Ruffand L.K. [yes, Wear, 35 (1975) 195-199. [ 11 ] S. Okazak~ and K. Hasegawa, JSMEa, 56.-527 (1990) 1668-1671. [ 12] R.C Tucker, Wear f~lures, in Metals Handbook, VoL I I, 1986, pp.

145-162. [13] R.C. Tucker, Wear testing, in Metals Handbook, VoL II, 1986, pp.

602--607. [ 14] I. Finne and Y.H. KabiL Wear, 8 (1965) 60-69.

Biographies

K. Shimizu: graduated from Hokkaido University in 1986. He was a design and maintenance engineer at Nippon Steel Co. from 1986 to 1988. He then joined Oita National College of Technology as an assistant in the Mechanical Engineering department, and then assistant professor in Computer and Control Engineering. He has authored 15 papers on wear

(mainly erosion) and engineering drawings, He is a member of the Japan Society of Mechanical Engineers, Japan Society of Powder Technology, Japan Foundrymen's Society, and the Japan Society for Design and Draftings in Graphic Science.

T. Noguchi: gr~uated from Hokkaido University in 1968. He then became an assistant professor in the Mechanical Engineering department at the same university, and a profes- sor in 1989. He has authored 50 papers on strength of mate- rials (mainly fatigue) and casting. He is a member of the Japan Society of Mechanical Engineers, and the Japan Foun- drymen's Society.

T. Kamada: is a student of mechanical engineering at Hok- kaido University, He is a member of the Japan Society of Mechanical Engineers,

H. Takasaki: is a student of mechanical engineering at Hok- kaido University. He is a member of the Japan Society of Mechanical Engineers.