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Scanning electron microscopy observation on the incubation period of vibratory cavitation erosion
303
Wear, 142 (1991) 303314
Scanning electron microscopy observation on the incubation period of vibratory cavitation erosion S. M. Ahnted”, K. Hokkirigawa”, Y. Itob and R. Obab aF~~y of E~~~~, ~~u~an~
bInstitute
of F%uid Sciace,
~~~~
Unhm-sitgt~ Sendai 980
Y. Matsudaira Tokyo Metropolitan
Institute
sf Technology,
H$m, Tokyo 191 (Japan.
(Received February 12, 1990; revised July 23, 1990; accepted September 21, 1990)
Abstract The behavlour of the vibratory cavitation erosion on typical machinery material, SUS 304 stainless steel, is lnvestlgated using a scanning electron microscope and a profllometer. The tests are carried out under conditions of uniform cavitation nuclei size distribution. It is observed that the polishing lines act as weak points for the formation of pits. The diameter and the shape of these pits do not change with the test time (t<20 min). The rate of formation of pits with test time is h&h initially but decreases after t = 15 min. It is also observed that most of the test surface is 1ocalIy deformed plastically from the beginning of the vibration, resulting in characterkstically wavy surface undulations. As the time passes, these undulations do not change in shape and number but they do change in width as well as ln height. In addition, the direction of undulation greatly affects the roughness on the eroded surface.
1. introduction Owing to the rapid increase in their speed, hydraulic machinery and instruments are very often fatally damaged by cavitation erosion. A growing interest has therefore been focused on fInding a practical method to predict precisely the probable rate of the erosion or to minimize it by clarifying the erosion mechanism to allow new erosion-resistant materials to be developed. As is well known, there are several types of cavitation with specific aspects, behaviour and ability to cause the erosion f 1, 2 1, and they are sensitive to the cavitation nuclei in the test water [2]. However, according to the literature on vibratory erosion tests [3-14 1, to our knowledge there have been no rigorous erosion results with respect to cavitation aspects and the cavitation nuclei of their dolt factor. In a previous paper f 151, therefore, we reported the time dependence of the nuclei in typical vibratory erosion tests conforming to the ASTM Standard. We then carefully observed the erosion patterns and the surface roughness aspects with respect to the test time and found a correspondence between the developing stages of the Q Elsevier SequoWPrintedIn The NetherIands
304
erosion and the roughness [ 161. However, the important question of why such a relationship occurs has yet to be solved. In order to investigate the mechanism of the erosion, most investigators were interested in the initial stage of the erosion, i.e. the incubation period, where the impact damage was not superimposed. Even in these investigations, however, data on the dominant factor were scarcely shown. In this paper, therefore, typical vibratory erosion tests on a typical ductile material, SUS 304 stainless steel, conforming to the ASTM Standard [ 171 are carried out and the erosion aspects are systematically obsenred during the incubation period under a specified condition of uniform cavitation nuclei size distributions by means of scanning electron microscopy (SEM).
2. Literature on pit fo~ation It is known that the incubation period is of paramount importance in understanding the basic mechanism of the erosion, and pit formation is one of the characteristics of this period. In vibratory erosion tests, however, pit formation is still the subject of controversy among many investigators [ 18-241 partially because the test conditions are somewhat different from each other. According to the literature, Plesset and Devine [ 181 generated cavitation erosion on vibratory test pieces by means of a magnetostrictive oscillator and found a few visible indentations in a very soft material after only 10 cycles of exposure, which showed a high degree of work hardening on the test surface. In contrast, solids which had a high surface hardness and high yield strength showed no visual surface changes even after exposure for many thousand of cycles. In short, Plesset and Devine demonstrated the important role of the plastic deformation in this period. Karimi and AvelIan [20] mentioned in their tests on vibratory brass test pieces that the erosion consisted of an overall surface deformation without fracture, which was revealed by surface ~d~~o~ and the appearance of grain boundaries and twin boundaries during the incubation period. Very few small craters were detected there. These scarcely grew with time and the largest was 4 pm in diameter. However, they concluded that these types of pit were not characteristic of the vibratory erosion. Contrary to the above, Hansson and Mprrch [ 191 showed in their tests on stationary alu~um and stainless steel test pieces that all test pieces were visibly eroded from the onset of exposure and pits appeared as shallow indentations, which had a diameter of the order of 25 pm. On their vibratory carbon steel test pieces, Okada et al. [24] found an initial period before the incubation where a few square pits several microns in diameter were formed in a brittle fracture manner, while the mass removal was negligibly small. As the time passed, the surface was progressively work hardened, resulting in a reduction in such pit formation; then the period transfers into the incubation period. They also observed several slip lines within the ferrite structure in the incubation period.
305
3. Experimental procedure In order to stabilize the gas content and to release entrained gases, tap water (the test water) was allowed to stand in an open atmosphere for at least 24 h. The nuclei distributions of the test water are measured before and after the tests as shown in Fig. 1. It is clear from this figure that the nuclei distributions scarcely change before and after every test run. The SEM observations and surface roughness measurements require at least 2 days for every test run, so that the entire experiments require about 2 weeks. During such a long period, the nuclei distributions shown by t= 0 min in Fig. 1 vary somewhat for every test run at a different test time. Considering the unfavourable effects of the test conditions, i.e. the size of the test piece, the frequency and the amplitude of the vibrations, the present experiments are carried out with a standard vibratory apparatus and simple flat-surfaced test pieces of disc type, which precisely conform to ASTM Standard G328.5 [ 171. The vibratory frequency is 19.5 kHz, the peak-to-peak displacement amplitude is 50 pm, and the type 304 stainless steel test pieces are 15.9 mm in diameter and 11 mm high. The water temperature T,,. is 22 & 1 “C and is controlled by cooled water circulated around the beaker. The details of the apparatus and the test procedure have been described elsewhere [ 15, 161. To ensure sampling from the same population, all the test pieces are cut from a single block of type 304 stainless steel with uniform chemical composition. The mechanical properties and the chemical composition are shown in Table 1 conforming to the Japan Industrial Standard. In order to minimize the surface roughness effects, which are still not necessarily clear, all test pieces are very smoothly polished by grade 3000 emery paper. The resulting maximum height roughness R,, is 0.025 pm. Development of material erosion on the test surface is carefully observed with a scanning electron microscope and a commercial surface proillometer (SESC, Kosaka Ltd.). The rapidly changing cavitation aspects with time are photographed with a xenon flash lamp of 1 w exposure time. The cavitation nuclei are measured with a specially made Coulter counter [25], which
IO
20
Nuclei
40
60
diameter
10
20
40
d,
prr
E
Fig. 1. Change in nuclei size distribution with respect to test time t.
306 TABLE 1 Mechanical properties and composition of SUS 304 Density (kg m-“) Yield stress (MPa) Tensile strength (MPa) Elastic modulus (MPa) Hardness (Bhn) Elongation (%) Nominal chemical composition (%)
8027 517 758 193 x 103 240 60 C, 0.08 (max.); Mn, 2 (max.); Si, 1 (max.); P, 0.045 (max.); S, 0.03 (max.); Cr, 18-20; Ni, 8-12
monitored the free microbubbles (nuclei) and the dust particles separately. The air content rate a$~ is measured with a van Slyke apparatus [ 261, where (Y is the total air content and a$ is the saturated air content at the water temperature T,.
4. Results 4.1. Aspects of the erosion pits First, four typical positions on the test pieces are selected: two, C1 and Cz, near the centre and the others, El and Ez, near the edge (radial distance r= 6 mm). Then the aspects of the pit characteristics, i.e. the number, the sizes and the shapes, and the role played by these pits in forming a crack or in material removal, are systematically investigated by taking a series of scanning electron photographs at these positions. To clarify the shape of the pits and their distribution with time, scanning electron photographs are taken at the position C for various test times t = 0, 5, 10, 15 and 20 n-tin,where the time is measured from the beginning of the vibrations, as shown in Fig. 2. As is clearly seen on the photograph at t = 0 min without exposure, there are a number of horizontal parallel polishing lines (0.025 pm in height). A nmber of black spots are seen on the photograph at t = 5 min; these are the pictures of dust particles on the test piece (it should be noted that these spots do not appear on the other photographs), so that the lines and the spots are not related to the phenomenon. In order to show these pits more clearly, circles are drawn round them as shown in Fig. 2. The number of pits is ~~i~~y counted, where the observed areas are 0.025 mm2 and 0.034 mm2 at the centre and at the edge respectively. At a typical time t = 10 min, Fig. 3 shows a sampling area for counting pits at the centre C, and at the edge El. Circles are also drawn round the pits. The number of white lines developed across the polishing traces show the surface undulations due to the plastic deformations as will be discussed later.
Cd>
Ccl
Fig. 2, Scam&g electron p~ot~g~~~s of the pit fo~~~o~ min; (c) 15 mixq (d) 20 min.
(a)
for various t: (a) 0 min; (b) 5
@I
Fig. 3. Comparison of pit formation at the centre (a) and the edge (b) of the test piece at t=lO min.
308
As clearly seen in Fig. 2, the pits consist of the following two types: (i) pits develop along the polishing traces which act as favourite sites for the stress concentration, as shown in the lower part of the photograph; (ii) other pits are randomly distributed on the photographs. Pits of type (i) suggest the important fact that even very small roughness such as 0.025 pm or less may considerably a.fYectthe erosion. This subject will be discussed again later. Pits of type (ii), however, appear as shallow indentations, very irregular in shape and of the order of 2 pm in maximum diameter. Even with a considerable increase in the time t, their diameters and shapes do not change at all as shown in Pig. 2, as if they resulted from single cavitation blows. However, the number initially increases with time rapidly, and then the increasing rate of the number of pits becomes much lower (Table 2 and Pig. 4). Such an interesting tendency can be easily explained by the rapid progress of the work hardening of the test surface, even if the high impulsive cavitation pressure scarcely changes with time. It is also very clear from Pig. 4 that the number of pits near the edge is larger than that near the centre. This observation and other observations of the largest plastic deformations near the edge, which will be discussed later, illustrate why the erosion becomes shallower at the centre and deeper at the edge [Z?]. In short, such characteristic aspects of the pits and their stochastic distribution in both space and time suggest that they have resulted directly from local high cavitation pressures higher than the material strength as if powerful microjets have resulted from erosive bubbles. Such pits are expected to be very similar to those reported by Karimi and Avellan [ZO] and are quite different from the square pits observed by Okada et al. [24]. However, we do not observe very large pits, up to 25 Frn in diameter, as Hansson and Merch [ 191 observed from the onset of exposure. As is well known, most researchers have tried to estimate the depth of pits from the roughness measurements but, as will be shown later, the number of pits is small and the surface deformation is dominated by severe plastic deformations, especially ln the incubation period. So the depth cannot be estimated from such roughness measurements. 4.2. Plastic defcvrmatiims In the present vibratory tests according to the ASTM Standard, most of the test surface is locally deformed plastically, from the beginning of the vibrations, which results in wavy surface undulations and the appearance of the grain boundaries, slip bands and twin lines, as shown in Fig. 5 (e.@ several white lines with a left-hand slope appeared on the left side of Fig. 5(b) or with a right-hand slope on the right side). Such plastic deformations clearly show that a huge number of moderate cavitation pressures lower than the material strength are distributed over the test surface. With an increase in t, the undulations do not change in shape, length (where the plastic deformations potentially develop) and number, but they do change in width as well as height, as expected from the typical nature of plastic deformations. This corresponds well to the previous results (161 as the
5 10 15 20
Sapling point (t, min)
8 11 17 18
0.00734
G (A,=
mm”) 10 20 27 33
0.00680)
0.00801)
4 10 13 16
E, (A,=
C, (A,= 0.00783)
EP Gb= 8 18 20 22
2 4 6 8
0.00999)
C,(Ao=
-
-
C;!
Centre
Centre Else
Test piece no. 2
Test piece no. 1
Number of pits with increasing t
TABLE 2
10 20 31 37
0.01080)
E, (-Jo=
Edge
7 19 27 36
E, (A,= 0.00938) 596 1048 1478 1744
Centre
1040 2278 3067 3730
Edge
Number of pits np (mm-“)
Test tune
t
rn~n
Fig. 4. Numbers of pits near the centre and the edge plotted against t.
(a)
(cl
Cdl
Fig. 5. Development of plastic deformation near the centre for various t: (a) 0 min; (b) 5 min; (c) 15 min; (d) 20 min.
311
surface roughness steadily increases with increasing t in the incubation period (Fig. 6). The maximum length and width of plastic deformation lines are 40 pm and 10 pm respectively. Clearly, the plastic deformations have significant directional qualities. Such a locality of the deformations can be manifested by measuring the surface roughness in one direction and then in the perpendicular direction. It is clear in Fig. 7 that roughness in a specific direction, shown by the bold lines, is often quite different from that in the perpendicular direction, shown by the broken lines. The difference has to be a maximum when the direction of the undulation lines agrees with the specific direction or the perpendicular direction. Also, since the roughness is measured at only five points in the radial direction for each t, the data are somewhat scattered, but a marked difference between the corresponding points at the same radius r has to result from the difference between the directions of the undulation and the roughness measurement lines. Generally, the cavitation erosion distribution on the surface is not uniform, because of many parameters such as the distribution of bubbles and the radial flow in the vibratory test. This is clear in Fig. 7, since the distributions of the surface roughness, the average roughness R,, the r.m.s. roughness R r.m.s. and the maximum peak-to-valley height roughness R_ vary with the radius for various times t. To elucidate this, the scanning electron photographs
I 15
IO Test time
t
I 20 min
Fig. 6. The maximum roughness R_, R, plotted against t.
01 5.95
/ 3.55
I
I.15 Radial
,
0 Center distance
1.56
3.96 I
6.36
mm
the r.m.s. roughness R,.,.,, and the average roughness
Fig. 7. Surface roughness distributions on the test piece in a specific direction and perpendicular to it.
312
at the centre and the edge at two values of t are shown in Fig. 8. Clearly from this figure, plastic deformation at the edge is higher than at the centre. 6. Conclusions Vibratory erosion tests based on the ASTM Standard were carried out on a typical machine material, SUS 304 stainless steel, under a specified condition of uniform nuclei size distributions. The eroded surface for the initial stage of the incubation period is examined using SEM and by profilometry. The results obtained are summarized as follows. (1) The roughness, even if it is very small (R_ = 0.025 ,um), acts as weak points for pit formation along the polishing lines. (2) The diameters and shapes of the pits do not change with the test time as if they result from single cavitation blows. The number of these pits initially increases with increasing test time rapidly, and then the increasing rate of the number of pits becomes much lower. (3) From the beginning of the vibration, most of the eroded surface is plastically deformed and results in wavy surface undulations and appearance
(a)
(Cl
Cd>
Fig. 8. Comparison of the wavy pattern of plastic deformation between the centre and the edge: (a) centre, t = 15 min; (b} centre, t = 20 min; (c) edge, t 9 15 min; (d) edge, t = 20 min.
313
of the grain boundaries, slip bands and twin lines. As the time passes, these undulations do not change in shape and number but they do change in width as well as height. (4) The undulation directional qualities greatly affect the roughness on the eroded surface. It is also observed that the roughness and the deformations at one edge are higher than at the other edge and at the centre.
Acknowledgments This work was financially supported by the International Scientific Research Programme of the Japanese Ministry of Education under Grant 63044009. The authors wish to express their thanks to Mr. J. Higuchi for his help in preparing the manuscript.
References 1 C. A. Johnsson, Cavitation inception on head forms. Further tests, Proc. 12th In.!. Towing Tank CmJ, Rome, 1969, ITTC, 1969, p. 381. 2 R. Oba and Y. Ito, Cavitation aspects in a Venturi (Johnsson effects in a low turbulent flow), Rep. Inst. High Speed Mech., Tohoku Univ., 46 (358) (1983) 89. 3 W. J. Rheingans, Accelerated-cavitation research, fians. ASlkZ, 72 (5) (1950) 705. 4 S. L. Kerr, Determination of the relative resistance to cavitation erosion by the vibratory method, Trans. ASME, 59 (4) (1937) 373. 5 M. S. Plesset, Temperature effects in cavitation damage, J. Basic Eng., 94 (4) (1972) 559. 6 F. G. Hammitt and D. 0. Roger, Effects of pressure and temperature in vibratory cavitation damage test, J. Mech. Eng. Sci., 12 (6) (1970) 432. 7 A. Thiruvengadam and H. S. Preiser, On testing materials for cavitation damage test, J. Ship Res., 8 (3) (1964) 39. 8 H. G. Olson and F. G. Hammitt, High speed photographic studies of uItrasonicaIIyinduced cavitation, J. Acoust. Sot. Am., 45 (5) (1969) 1272. 9 F. Numachi,UItraschaIIweIIendurch Hohlsog beim magnetostriktivenSchwingungsversuche der Korrosion, Forsch. Ingenieurwes, 34 (4) (1968) 107. 10 K. Endo, T. Okada and M. Nakashima, Erosion of bearing alloys in oil iIlm, Bull_ JSME, II (45) (1968) 536. 11 T. Okada, Fundamental studies on cavitation erosion (Observation of the eroded surface by SEM under corrosive liquid), BuU. JSME, 24 (189) (1981) 461. 12 T. Okada, Corrosive liquid effects on cavitation erosion, J. Ship Res., 25 (4) (1981) 271. 13 B. Vyas and C. M. Preece, Stress produced in a solid by cavitation, J. Appl, Phys., 47 (12) (1976) 5133. 14 L. E. Robinson, B. A. Holmes and W. C. Leith, Progress report on standardization of the vibratory-cavitation test, Pruns. AS&fE, 80 (1) (1958) 103. 15 S. M. Ahmed, Y. Ito, J. Higuchi and R. Oba, A peculiar behavior of cavitation-nuclei-size distributionsin sample water under vibratory erosion tests, Z?-un.s. JSME, 55 (5 14B) (1989) 1495 (in Japanese). 16 S. M. Ahmed, K. Hokkirigawa, R. Oba and Y. Matsudaira,Developing stage of ultrasonically produced cavitation erosion and corresponding surface roughness, Jpn. Sot. Mech. Eng. Zti. J., 33 (1) (1990) 11. 17 ASTM Stand. G32-85, inAnnual Book of ASTMStandards, Part 03.02, ASTM, Philadelphia, PA, 1985, p. 187.
314 18 M. S. Plesset and R. E. Devine, Effect of exposure time on cavitation damage, J. Basic Eng., 88 (1966) 691. 19 I. Hansson and K. A. Merch, Comparison of the initial stage of vibration and flow cavitation erosion, Proc. 5th Int. Cm5 072Erosion by Solid and Impact, Cambridge, 1979, Cavendish Laboratory, Cambridge, 1979, paper 60. 20 A. Karimi and F. AveIlan, Comparison of erosion mechanisms in different types of cavitation, Wear, 113 (1986) 305. 21 B. Vyas and C. M. Preece, Cavitation-induced deformation of ahuninum, in Erosion, Wear and Interfmes with Ccn-rosiun, ASTM Spec. Tech. Publ. 567, 1974, p. 77. 22 B. Vyas and C. M. Preece, Cavitation erosion of face centered cubic metals, Metall. Trans., 8A (1977) 915. 23 E. H. R. Wade and C. M. Preece, Cavitation erosion of iron and steel, Metall. Trans., 9A (1977) 1299. 24 T. Okada, J. Iwamoto and K. Sano, Fundamental studies on cavitation erosion (observation of the eroded surface by scanning electron microscope), BUU. JSME, 20 (147) (1977) 1067. 25 R. Oba, K. T. Kim, H. Niitsuma, T. Ikohagi and R. Sato, Cavitation-nuclei-measurements by a newly made Coulter counter without adding salt in water, Rep. Inst. High Speed Mech., Tohoku Univ., 43 (340) (1981) 163. 26 F. Numachi, Profihnessungen on vier FIiigelproilIen bei HohIsog, Forsch. ZngenieuPwes, 11 (1946) 303. 27 T. McGuinness and A. Thiruvengadam, Cavitation erosion-corrosion modeling, in Erosion Wear and Interfaces with Con-osian, ASTM Spec. Tech. Publ. 567, 1974, p. 30.
Wear, 142 (1991) 303314
Scanning electron microscopy observation on the incubation period of vibratory cavitation erosion S. M. Ahnted”, K. Hokkirigawa”, Y. Itob and R. Obab aF~~y of E~~~~, ~~u~an~
bInstitute
of F%uid Sciace,
~~~~
Unhm-sitgt~ Sendai 980
Y. Matsudaira Tokyo Metropolitan
Institute
sf Technology,
H$m, Tokyo 191 (Japan.
(Received February 12, 1990; revised July 23, 1990; accepted September 21, 1990)
Abstract The behavlour of the vibratory cavitation erosion on typical machinery material, SUS 304 stainless steel, is lnvestlgated using a scanning electron microscope and a profllometer. The tests are carried out under conditions of uniform cavitation nuclei size distribution. It is observed that the polishing lines act as weak points for the formation of pits. The diameter and the shape of these pits do not change with the test time (t<20 min). The rate of formation of pits with test time is h&h initially but decreases after t = 15 min. It is also observed that most of the test surface is 1ocalIy deformed plastically from the beginning of the vibration, resulting in characterkstically wavy surface undulations. As the time passes, these undulations do not change in shape and number but they do change in width as well as ln height. In addition, the direction of undulation greatly affects the roughness on the eroded surface.
1. introduction Owing to the rapid increase in their speed, hydraulic machinery and instruments are very often fatally damaged by cavitation erosion. A growing interest has therefore been focused on fInding a practical method to predict precisely the probable rate of the erosion or to minimize it by clarifying the erosion mechanism to allow new erosion-resistant materials to be developed. As is well known, there are several types of cavitation with specific aspects, behaviour and ability to cause the erosion f 1, 2 1, and they are sensitive to the cavitation nuclei in the test water [2]. However, according to the literature on vibratory erosion tests [3-14 1, to our knowledge there have been no rigorous erosion results with respect to cavitation aspects and the cavitation nuclei of their dolt factor. In a previous paper f 151, therefore, we reported the time dependence of the nuclei in typical vibratory erosion tests conforming to the ASTM Standard. We then carefully observed the erosion patterns and the surface roughness aspects with respect to the test time and found a correspondence between the developing stages of the Q Elsevier SequoWPrintedIn The NetherIands
304
erosion and the roughness [ 161. However, the important question of why such a relationship occurs has yet to be solved. In order to investigate the mechanism of the erosion, most investigators were interested in the initial stage of the erosion, i.e. the incubation period, where the impact damage was not superimposed. Even in these investigations, however, data on the dominant factor were scarcely shown. In this paper, therefore, typical vibratory erosion tests on a typical ductile material, SUS 304 stainless steel, conforming to the ASTM Standard [ 171 are carried out and the erosion aspects are systematically obsenred during the incubation period under a specified condition of uniform cavitation nuclei size distributions by means of scanning electron microscopy (SEM).
2. Literature on pit fo~ation It is known that the incubation period is of paramount importance in understanding the basic mechanism of the erosion, and pit formation is one of the characteristics of this period. In vibratory erosion tests, however, pit formation is still the subject of controversy among many investigators [ 18-241 partially because the test conditions are somewhat different from each other. According to the literature, Plesset and Devine [ 181 generated cavitation erosion on vibratory test pieces by means of a magnetostrictive oscillator and found a few visible indentations in a very soft material after only 10 cycles of exposure, which showed a high degree of work hardening on the test surface. In contrast, solids which had a high surface hardness and high yield strength showed no visual surface changes even after exposure for many thousand of cycles. In short, Plesset and Devine demonstrated the important role of the plastic deformation in this period. Karimi and AvelIan [20] mentioned in their tests on vibratory brass test pieces that the erosion consisted of an overall surface deformation without fracture, which was revealed by surface ~d~~o~ and the appearance of grain boundaries and twin boundaries during the incubation period. Very few small craters were detected there. These scarcely grew with time and the largest was 4 pm in diameter. However, they concluded that these types of pit were not characteristic of the vibratory erosion. Contrary to the above, Hansson and Mprrch [ 191 showed in their tests on stationary alu~um and stainless steel test pieces that all test pieces were visibly eroded from the onset of exposure and pits appeared as shallow indentations, which had a diameter of the order of 25 pm. On their vibratory carbon steel test pieces, Okada et al. [24] found an initial period before the incubation where a few square pits several microns in diameter were formed in a brittle fracture manner, while the mass removal was negligibly small. As the time passed, the surface was progressively work hardened, resulting in a reduction in such pit formation; then the period transfers into the incubation period. They also observed several slip lines within the ferrite structure in the incubation period.
305
3. Experimental procedure In order to stabilize the gas content and to release entrained gases, tap water (the test water) was allowed to stand in an open atmosphere for at least 24 h. The nuclei distributions of the test water are measured before and after the tests as shown in Fig. 1. It is clear from this figure that the nuclei distributions scarcely change before and after every test run. The SEM observations and surface roughness measurements require at least 2 days for every test run, so that the entire experiments require about 2 weeks. During such a long period, the nuclei distributions shown by t= 0 min in Fig. 1 vary somewhat for every test run at a different test time. Considering the unfavourable effects of the test conditions, i.e. the size of the test piece, the frequency and the amplitude of the vibrations, the present experiments are carried out with a standard vibratory apparatus and simple flat-surfaced test pieces of disc type, which precisely conform to ASTM Standard G328.5 [ 171. The vibratory frequency is 19.5 kHz, the peak-to-peak displacement amplitude is 50 pm, and the type 304 stainless steel test pieces are 15.9 mm in diameter and 11 mm high. The water temperature T,,. is 22 & 1 “C and is controlled by cooled water circulated around the beaker. The details of the apparatus and the test procedure have been described elsewhere [ 15, 161. To ensure sampling from the same population, all the test pieces are cut from a single block of type 304 stainless steel with uniform chemical composition. The mechanical properties and the chemical composition are shown in Table 1 conforming to the Japan Industrial Standard. In order to minimize the surface roughness effects, which are still not necessarily clear, all test pieces are very smoothly polished by grade 3000 emery paper. The resulting maximum height roughness R,, is 0.025 pm. Development of material erosion on the test surface is carefully observed with a scanning electron microscope and a commercial surface proillometer (SESC, Kosaka Ltd.). The rapidly changing cavitation aspects with time are photographed with a xenon flash lamp of 1 w exposure time. The cavitation nuclei are measured with a specially made Coulter counter [25], which
IO
20
Nuclei
40
60
diameter
10
20
40
d,
prr
E
Fig. 1. Change in nuclei size distribution with respect to test time t.
306 TABLE 1 Mechanical properties and composition of SUS 304 Density (kg m-“) Yield stress (MPa) Tensile strength (MPa) Elastic modulus (MPa) Hardness (Bhn) Elongation (%) Nominal chemical composition (%)
8027 517 758 193 x 103 240 60 C, 0.08 (max.); Mn, 2 (max.); Si, 1 (max.); P, 0.045 (max.); S, 0.03 (max.); Cr, 18-20; Ni, 8-12
monitored the free microbubbles (nuclei) and the dust particles separately. The air content rate a$~ is measured with a van Slyke apparatus [ 261, where (Y is the total air content and a$ is the saturated air content at the water temperature T,.
4. Results 4.1. Aspects of the erosion pits First, four typical positions on the test pieces are selected: two, C1 and Cz, near the centre and the others, El and Ez, near the edge (radial distance r= 6 mm). Then the aspects of the pit characteristics, i.e. the number, the sizes and the shapes, and the role played by these pits in forming a crack or in material removal, are systematically investigated by taking a series of scanning electron photographs at these positions. To clarify the shape of the pits and their distribution with time, scanning electron photographs are taken at the position C for various test times t = 0, 5, 10, 15 and 20 n-tin,where the time is measured from the beginning of the vibrations, as shown in Fig. 2. As is clearly seen on the photograph at t = 0 min without exposure, there are a number of horizontal parallel polishing lines (0.025 pm in height). A nmber of black spots are seen on the photograph at t = 5 min; these are the pictures of dust particles on the test piece (it should be noted that these spots do not appear on the other photographs), so that the lines and the spots are not related to the phenomenon. In order to show these pits more clearly, circles are drawn round them as shown in Fig. 2. The number of pits is ~~i~~y counted, where the observed areas are 0.025 mm2 and 0.034 mm2 at the centre and at the edge respectively. At a typical time t = 10 min, Fig. 3 shows a sampling area for counting pits at the centre C, and at the edge El. Circles are also drawn round the pits. The number of white lines developed across the polishing traces show the surface undulations due to the plastic deformations as will be discussed later.
Cd>
Ccl
Fig. 2, Scam&g electron p~ot~g~~~s of the pit fo~~~o~ min; (c) 15 mixq (d) 20 min.
(a)
for various t: (a) 0 min; (b) 5
@I
Fig. 3. Comparison of pit formation at the centre (a) and the edge (b) of the test piece at t=lO min.
308
As clearly seen in Fig. 2, the pits consist of the following two types: (i) pits develop along the polishing traces which act as favourite sites for the stress concentration, as shown in the lower part of the photograph; (ii) other pits are randomly distributed on the photographs. Pits of type (i) suggest the important fact that even very small roughness such as 0.025 pm or less may considerably a.fYectthe erosion. This subject will be discussed again later. Pits of type (ii), however, appear as shallow indentations, very irregular in shape and of the order of 2 pm in maximum diameter. Even with a considerable increase in the time t, their diameters and shapes do not change at all as shown in Pig. 2, as if they resulted from single cavitation blows. However, the number initially increases with time rapidly, and then the increasing rate of the number of pits becomes much lower (Table 2 and Pig. 4). Such an interesting tendency can be easily explained by the rapid progress of the work hardening of the test surface, even if the high impulsive cavitation pressure scarcely changes with time. It is also very clear from Pig. 4 that the number of pits near the edge is larger than that near the centre. This observation and other observations of the largest plastic deformations near the edge, which will be discussed later, illustrate why the erosion becomes shallower at the centre and deeper at the edge [Z?]. In short, such characteristic aspects of the pits and their stochastic distribution in both space and time suggest that they have resulted directly from local high cavitation pressures higher than the material strength as if powerful microjets have resulted from erosive bubbles. Such pits are expected to be very similar to those reported by Karimi and Avellan [ZO] and are quite different from the square pits observed by Okada et al. [24]. However, we do not observe very large pits, up to 25 Frn in diameter, as Hansson and Merch [ 191 observed from the onset of exposure. As is well known, most researchers have tried to estimate the depth of pits from the roughness measurements but, as will be shown later, the number of pits is small and the surface deformation is dominated by severe plastic deformations, especially ln the incubation period. So the depth cannot be estimated from such roughness measurements. 4.2. Plastic defcvrmatiims In the present vibratory tests according to the ASTM Standard, most of the test surface is locally deformed plastically, from the beginning of the vibrations, which results in wavy surface undulations and the appearance of the grain boundaries, slip bands and twin lines, as shown in Fig. 5 (e.@ several white lines with a left-hand slope appeared on the left side of Fig. 5(b) or with a right-hand slope on the right side). Such plastic deformations clearly show that a huge number of moderate cavitation pressures lower than the material strength are distributed over the test surface. With an increase in t, the undulations do not change in shape, length (where the plastic deformations potentially develop) and number, but they do change in width as well as height, as expected from the typical nature of plastic deformations. This corresponds well to the previous results (161 as the
5 10 15 20
Sapling point (t, min)
8 11 17 18
0.00734
G (A,=
mm”) 10 20 27 33
0.00680)
0.00801)
4 10 13 16
E, (A,=
C, (A,= 0.00783)
EP Gb= 8 18 20 22
2 4 6 8
0.00999)
C,(Ao=
-
-
C;!
Centre
Centre Else
Test piece no. 2
Test piece no. 1
Number of pits with increasing t
TABLE 2
10 20 31 37
0.01080)
E, (-Jo=
Edge
7 19 27 36
E, (A,= 0.00938) 596 1048 1478 1744
Centre
1040 2278 3067 3730
Edge
Number of pits np (mm-“)
Test tune
t
rn~n
Fig. 4. Numbers of pits near the centre and the edge plotted against t.
(a)
(cl
Cdl
Fig. 5. Development of plastic deformation near the centre for various t: (a) 0 min; (b) 5 min; (c) 15 min; (d) 20 min.
311
surface roughness steadily increases with increasing t in the incubation period (Fig. 6). The maximum length and width of plastic deformation lines are 40 pm and 10 pm respectively. Clearly, the plastic deformations have significant directional qualities. Such a locality of the deformations can be manifested by measuring the surface roughness in one direction and then in the perpendicular direction. It is clear in Fig. 7 that roughness in a specific direction, shown by the bold lines, is often quite different from that in the perpendicular direction, shown by the broken lines. The difference has to be a maximum when the direction of the undulation lines agrees with the specific direction or the perpendicular direction. Also, since the roughness is measured at only five points in the radial direction for each t, the data are somewhat scattered, but a marked difference between the corresponding points at the same radius r has to result from the difference between the directions of the undulation and the roughness measurement lines. Generally, the cavitation erosion distribution on the surface is not uniform, because of many parameters such as the distribution of bubbles and the radial flow in the vibratory test. This is clear in Fig. 7, since the distributions of the surface roughness, the average roughness R,, the r.m.s. roughness R r.m.s. and the maximum peak-to-valley height roughness R_ vary with the radius for various times t. To elucidate this, the scanning electron photographs
I 15
IO Test time
t
I 20 min
Fig. 6. The maximum roughness R_, R, plotted against t.
01 5.95
/ 3.55
I
I.15 Radial
,
0 Center distance
1.56
3.96 I
6.36
mm
the r.m.s. roughness R,.,.,, and the average roughness
Fig. 7. Surface roughness distributions on the test piece in a specific direction and perpendicular to it.
312
at the centre and the edge at two values of t are shown in Fig. 8. Clearly from this figure, plastic deformation at the edge is higher than at the centre. 6. Conclusions Vibratory erosion tests based on the ASTM Standard were carried out on a typical machine material, SUS 304 stainless steel, under a specified condition of uniform nuclei size distributions. The eroded surface for the initial stage of the incubation period is examined using SEM and by profilometry. The results obtained are summarized as follows. (1) The roughness, even if it is very small (R_ = 0.025 ,um), acts as weak points for pit formation along the polishing lines. (2) The diameters and shapes of the pits do not change with the test time as if they result from single cavitation blows. The number of these pits initially increases with increasing test time rapidly, and then the increasing rate of the number of pits becomes much lower. (3) From the beginning of the vibration, most of the eroded surface is plastically deformed and results in wavy surface undulations and appearance
(a)
(Cl
Cd>
Fig. 8. Comparison of the wavy pattern of plastic deformation between the centre and the edge: (a) centre, t = 15 min; (b} centre, t = 20 min; (c) edge, t 9 15 min; (d) edge, t = 20 min.
313
of the grain boundaries, slip bands and twin lines. As the time passes, these undulations do not change in shape and number but they do change in width as well as height. (4) The undulation directional qualities greatly affect the roughness on the eroded surface. It is also observed that the roughness and the deformations at one edge are higher than at the other edge and at the centre.
Acknowledgments This work was financially supported by the International Scientific Research Programme of the Japanese Ministry of Education under Grant 63044009. The authors wish to express their thanks to Mr. J. Higuchi for his help in preparing the manuscript.
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314 18 M. S. Plesset and R. E. Devine, Effect of exposure time on cavitation damage, J. Basic Eng., 88 (1966) 691. 19 I. Hansson and K. A. Merch, Comparison of the initial stage of vibration and flow cavitation erosion, Proc. 5th Int. Cm5 072Erosion by Solid and Impact, Cambridge, 1979, Cavendish Laboratory, Cambridge, 1979, paper 60. 20 A. Karimi and F. AveIlan, Comparison of erosion mechanisms in different types of cavitation, Wear, 113 (1986) 305. 21 B. Vyas and C. M. Preece, Cavitation-induced deformation of ahuninum, in Erosion, Wear and Interfmes with Ccn-rosiun, ASTM Spec. Tech. Publ. 567, 1974, p. 77. 22 B. Vyas and C. M. Preece, Cavitation erosion of face centered cubic metals, Metall. Trans., 8A (1977) 915. 23 E. H. R. Wade and C. M. Preece, Cavitation erosion of iron and steel, Metall. Trans., 9A (1977) 1299. 24 T. Okada, J. Iwamoto and K. Sano, Fundamental studies on cavitation erosion (observation of the eroded surface by scanning electron microscope), BUU. JSME, 20 (147) (1977) 1067. 25 R. Oba, K. T. Kim, H. Niitsuma, T. Ikohagi and R. Sato, Cavitation-nuclei-measurements by a newly made Coulter counter without adding salt in water, Rep. Inst. High Speed Mech., Tohoku Univ., 43 (340) (1981) 163. 26 F. Numachi, Profihnessungen on vier FIiigelproilIen bei HohIsog, Forsch. ZngenieuPwes, 11 (1946) 303. 27 T. McGuinness and A. Thiruvengadam, Cavitation erosion-corrosion modeling, in Erosion Wear and Interfaces with Con-osian, ASTM Spec. Tech. Publ. 567, 1974, p. 30.