- Email: [email protected]
Effective parameters for erosion caused by water droplet impingement and applications to surface treatment technology
Wear 263 (2007) 386–394
Effective parameters for erosion caused by water droplet impingement and applications to surface treatment technology Y.I. Oka ∗ , S. Mihara, H. Miyata Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan Received 29 August 2006; received in revised form 23 November 2006; accepted 24 November 2006 Available online 19 March 2007
Abstract Component materials in the steam environment of energy generation systems have severe problems of erosion caused by the impingement of water droplets. An evaluation of the resistance of such materials to erosion is very important in the maintenance of power plant systems and to prolong the life span of the components. A water-jet peening system, which is normally used for surface treatment technology, is based on the phenomenon of erosion damage. Parameters associated with water droplet impingement which dominate erosion rates are less controllable when a water-jet system is used. In this study, the use of a water-jet apparatus for erosion tests was conducted on an aluminum alloy to determine the effective parameters for erosion damage which are dependent on water pressure and the stand off distance between the nozzle and specimen. Parameters such as impact velocity, impact frequency and diameter of water droplets were examined by observations of craters formed on a thin aluminum film. Erosion damage was characterized with respect to incubation period and damage depth rate. Optimum surface treatment conditions were evaluated for an incubation period and damage depth rate after the incubation period. The incubation period and damage depth rate were greatly affected by the velocity and impact frequency of water droplets and the surface treatment technology should be sustained by the optimum impingement conditions which are controlled by the water pressure and stand off distance of the nozzle in the water-jet apparatus. © 2007 Elsevier B.V. All rights reserved. Keywords: Water-jet; Water droplet impingement; Incubation period; Damage depth rate; Impingement conditions of droplets; Surface treatment
1. Introduction The environments associated with liquid impact erosion or water droplet impingement on component materials are frequently seen in wet steam turbine systems of energy generation plants. Parameters that affect erosion damage need to be understood in order to estimate or predict erosion damage caused by water droplet impingement in energy generation systems and to select suitable materials for use as components which are highly erosion resistant. The characteristics of damage to materials and parameters related to water droplet impingement have been extensively reviewed, and a number of fundamental studies on liquid impact or water droplets have been reported [1–4]. A few studies have recently reported on the damage characteristics of turbine blades [5,6], using a rotating arm apparatus [7]. It is reasonable to evaluate erosion damage to materials or to select materials, as the droplet velocity can be readily controlled by the
∗
Corresponding author. Tel.: +81 824 24 7845; fax: +81 824 24 5494. E-mail address: [email protected] (Y.I. Oka).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.11.022
revolution of the rotor. On the other hand, water-jet peening has recently emerged as an alternate surface treatment technology to the sand blast or shot peening methods [8–11]. However, it has not adequately been compared to other impingement parameters except for impingement velocity in a water-jet apparatus. The testing time on the abscissa in erosion damage curves is prone to change as a function of jet pressure, type of nozzle and, importantly, the impact frequency of the impinging droplets. Predicting the life span of candidate materials used in actual plant components such as turbine blades is therefore impossible if precise parameters that are dependent on operating conditions are not clearly understood. Applications of the water-jet to surface treatment technology are also inadequate for the same reason. The impingement conditions for water droplets in actual plants should be investigated to relate experimental erosion data to actual damage in order to accurately predict erosion damage. The aim of this study was to characterize the effective parameters related to damage characteristics caused by water droplet impingement in a water-jet apparatus, in an attempt to predict or estimate erosion damage to materials that are used in actual plant components, and to determine the optimum conditions
Y.I. Oka et al. / Wear 263 (2007) 386–394
Fig. 1. Schematic diagrams of water-jet testing apparatus and measurement rigs.
for applications of water-jet impingement in surface treatment technology. Erosion tests were conducted on an aluminum alloy under different conditions of water pressure and stand off distance with a commercial orifice nozzle. Craters formed on a thin aluminum film were observed, in order to visualize the traces of the impinging droplets. Impingement parameters such as velocity, impact frequency and diameter of water droplets were investigated under some of the same conditions as were used in erosion tests. Effective parameters related to erosion damage by water-jet impingement and the best use of the water-jet for surface treatment technology are discussed based on the erosion test results and the determination of impingement conditions.
387
were measured with an electronic balance and a surface profilometer, respectively. In addition, simulated surface treatment tests were conducted at various scanning speeds at pressures of 30 and 70 MPa. The damaged surfaces were observed by both an optical microscope and scanning electron microscopy (SEM). The distributions of water droplets were obtained as craters formed on an aluminum foil with a thickness of 12 m mounted on the specimen. The reason for using the aluminum foil was to distinguish the size and impact number (impact frequency) per unit area and time of the water droplets from the water-jet owing to the softness of the aluminum foil. A slit board, 1 mm in width located at a distance of 10 mm under the nozzle was quickly moved into place for short duration of 0.02 ms in front of the water-jet. Craters that were formed were observed with an optical microscope after the short duration tests. The impact frequency and size of the water droplets were measured based on observations of the craters on the aluminum foil at the central part on the nozzle axis at various water pressures and SOD. A droplet collector with a duct of 0.5 mm in inner diameter was installed on the stage in order to obtain the localized mass flow rate in the central part of the water-jet at various stand off distances. The mass flow rate (g mm−2 s−1 ) was measured from the mass of water for durations from 1 to 10 s. A force detection rod unit with a diameter of 1 mm was installed to measure the force generated on the central part of the water-jet, as shown in Fig. 1. The strain on the stainless steel plate was measured and converted to output voltage. The voltage and force was calibrated using the same detection unit and a universal testing machine.
2. Experimental procedures 3. Results Schematic diagrams of a water-jet apparatus and the measurement rigs are shown in Fig. 1. Pressurized tap water was supplied from a plunger pump and was injected in an orifice nozzle with diameter of about 0.4 mm (exactly 0.425 mm). The water-jet was allowed to impinge on the aluminum alloy Al 5083 (Mg: 4.5, Mn: 0.7) at normal angle. The reason for using an aluminum alloy was that it was a soft material (Hv = 71) and considerable damage would be expected within a relatively short duration under low impingement conditions. The water pressure was controlled at 10, 30, 50 and 70 MPa. The flow rate and average velocity at the outlet of the nozzle are summarized in Table 1. The average velocity was calculated by the flow rate divided by the crosssection area of the nozzle. The stand off distance (SOD) between the nozzle and specimen was regulated from 30 to 500 mm. Mass loss of the whole specimen (30 mm × 40 mm × 5 mm) and the maximum damage depth in the central part on the nozzle axis
Fig. 2 shows examples of plots of mass loss versus testing time at a SOD of 200 mm at various water pressures for Al 5083. The mass loss is not linear and changes gradually with testing time. An incubation period, during which the mass loss
Table 1 Flow rate and average flow velocity, and damage characteristics at a stand off distance of 200 mm Pressure (MPa)
Fr (cm3 s−1 )
Vav (m s−1 )
10 30 50 70
12.7 21.7 29.0 35.3
89 153 204 248
Ip (s) 300 0.71 0.25 0.15
Rd (mm s−1 ) 0.00042 0.073 0.216 0.378
Fr: flow rate; Vav : average velocity; Ip : incubation period; Rd : damage depth rate.
Fig. 2. Mass loss vs. testing time curves at a stand off distance of 200 mm at various water pressures.
388
Y.I. Oka et al. / Wear 263 (2007) 386–394
Fig. 3. Damage depth vs. testing time curves at a stand off distance of 200 mm at various water pressures.
of the specimen cannot be measured, can be clearly found at a pressure of 30 MPa but not at higher pressures because of the shorter incubation period. The incubation period at a pressure of 10 MPa was 300 s (see Table 1 and Fig. 13). As a result, a mass loss curve could not be obtained for this short scale of testing time. The mass loss increased with water pressure and was typical for water droplet impingement. Fig. 3 shows damage depth versus testing time curves under the same conditions as in Fig. 2. The damage depth was the maximum damage depth, as measured in the central part of a damaged area on the nozzle axis. The curves for damage depth were similar to the mass loss curves, but the increase in damage depth in an early stage of testing time was relatively steeper than that for mass loss (Fig. 2). The damage depth curves can be approximated by two linear curves, as shown in Fig. 3. An incubation period was not clearly recognized for extended testing time. Damage depth curves for the initial testing time in Fig. 3 are shown in Fig. 4. The incubation periods for various water pressures are clearly indicated on the testing time axis. The incubation period decreases with an increase in water pressure. On the contrary the damage depth rate, which means the slope of the damage depth curve, increases rapidly with an increase in water pressure. The incubation periods and initial damage depth rates obtained from Fig. 4 are summarized in Table 1. Fig. 5 shows SEM observations at a testing time of 3 s, a SOD of 200 mm and a pressure of 30 MPa. The damaged area was a spot within a diameter of about 0.6–0.7 mm. Plastic deformation in the center was observed appearing as hills and valleys and relatively flat indentations, and rolled up grains around the crater were observed as configurations caused by the small amount of water droplet impingement. The crater size spread with increasing testing time. Fig. 6 shows the effect of SOD on damage depth rate which shows the slope for damage depth versus initial testing time curves after the incubation period in Fig. 4. The maximum dam-
Fig. 4. Damage depth vs. testing time curves in short duration at a stand off distance of 200 mm at various water pressures.
age depth rates were commonly observed for a SOD from 120 to 200 mm at all water pressures. The damage depth rates decreased for either a shorter or longer stand off distance. The damage area increased with the increase in SOD, although the damage depth rate decreased at a SOD of over 200 mm. The shape of the distribution of damage depth rate is similar among the three water pressures.
Fig. 5. SEM observations of an eroded crater at a stand off distance of 200 mm, pressure of 30 MPa and testing time of 3 s (a) whole crater, (b) the rim of the crater.
Y.I. Oka et al. / Wear 263 (2007) 386–394
389
Fig. 6. Effect of stand off distance on damage depth rate at various water pressures.
Fig. 7 shows examples of optical microscope observations of aluminum foil surfaces after the impingement of a small amount of water-jet at various stand off distances at a pressure of 10 MPa. The cross sign indicates the center of the water-jet. Fig. 7(a) was not frequently observed but the large crater in the center appeared to be caused by a water column in the water-jet. Irregularly shaped craters were observed around the large crater. The number of craters seemed to be largest, but their size was smaller at a stand off distance of 365 mm (Fig. 7(b)). Based on these photographs the number (impact frequency) and average diameter of water droplets were measured at various stand off distances at water pressures of 10 and 30 MPa. These observations were impossible at pressures of over 30 MPa probably because of the strong impingement of water droplets. Figs. 8 and 9 show the distributions of impact frequency and average diameter of water droplets at the center of the damage area against SOD at water pressures of 10 and 30 MPa, from visual inspections of the aluminum foils (Fig. 7). The maximum impact frequencies of water droplets were 107 number mm−2 s−1 at a pressure of 30 MPa, 5 × 105 number mm−2 s−1 at a pressure of 10 MPa in the stand off distance from 200 to 300, and the minimum diameters of water droplets were about 50 m at a pressure of 30 MPa, 150 m at a pressure of 10 MPa. The broken lines in Figs. 8 and 9 were estimated from the similar curves at the two pressures according to the equations in the figures. Where Freq is the impact frequency, D is the diameter of the water droplet and P is the water pressure. Fig. 10 shows the relations between SOD, and impingement force and mass flow rate in the central part of the water-jet at pressures of 10 and 30 MPa. The value for impingement force was constant at a SOD of less than 100 mm and decreased gradually with the increase in SOD. The behaviour of the mass flow rate was similar to that for the impingement force, but the slope of the linear curves at a SOD of over 100 mm was different between the impingement force and the mass flow rate. On the other hand, the impingement force was clearly different between pressures
Fig. 7. Optical microscope observations of aluminum foils at a pressure of 10 MPa, at stand off distances of (a) 100 mm, (b) 365 mm, (c) 500 mm.
of 10 and 30 MPa within a SOD of 100 mm, but the mass flow rate was only slightly different between the two pressures. Fig. 11 shows examples of simulated surface treatments for the aluminum alloy at various specimen scanning speeds and surface profiles. Two types of surface roughness, in the incubation period and in the early region of the damage depth curve, were formed at pressures of 30 and 70 MPa. The stand off distance was 200 mm at which the maximum damage depth rate was obtained, as shown in Fig. 6. In order to achieve a similar
390
Y.I. Oka et al. / Wear 263 (2007) 386–394
Fig. 8. Distributions of the impact frequency of water droplets at various pressures.
surface coarseness, the scanning speed was varied with different water pressures. The band width at a pressure of 70 MPa was narrower than that at a pressure of 30 MPa in the case of deeper grooving, however, the scanning speed used was very fast at a pressure of 70 MPa. 4. Discussion 4.1. Evaluation of erosion damage Erosion damage is characterized by mass loss, damage depth and these rates, and the incubation period. The mass loss curve gradually changes with testing time in water-jet impingement, as shown in Fig. 2. In other words, the mass loss rate is not constant and its behaviour is well known [12]. This phenomenon
Fig. 9. Distributions of the diameter of water droplets at various pressures.
Fig. 10. Relations between stand off distance, and impingement force and mass flow rate in the central part of the water-jet.
is considered to be caused by the repetition of plastic deformation and removal of the material tip. The decrease in mass loss rate after a long duration is due to the shock absorption of a water film formed on the bottom of the crater. The mass loss of the specimen arises from overall crater damage but impingement conditions are possibly different in the respective points of the crater, as described in Section 4.2. The mass loss, then, means the sum of the amount of damage at different areas and changes (curvilinear) with testing time. It is therefore difficult to evaluate erosion damage based on the mass loss curve with a variable damage rate. The maximum damage rate in the mass loss curve is a useful indicator of erosion damage caused by water droplet impingement, but does not necessarily encompass
Fig. 11. Appearances of simulated surface treatments and measurements of cross sectional surface roughness at a stand off distance of 200 mm, at various pressures and scanning speeds.
Y.I. Oka et al. / Wear 263 (2007) 386–394
all of the damage behaviour and is useful only for comparing erosion resistance among various candidate materials. On the other hand, damage depth means the local volume of damage per small area and is indicated by a linear curve (constant damage depth rate) after the incubation period (see Fig. 4), as expressed by Eq. (1). Ed = Rd (t − Ip )
(1)
where Ed is the damage depth (mm), Rd is the damage depth rate (mm s−1 ), t is the testing time (s) and Ip is the incubation period (s). Damage depth behaviour at long testing time can be ignored, taking into account the actual life span or the degradation in running efficiency regarding the incubation period (initiation point of damage depth) or the allowable damage depth of the component materials. These parameters of Rd and Ip are obtained from the spot condition at the central part of the damaged area so that the relations between damage depth and impingement parameters can be obtained more easily and more reliably. Both Rd and Ip are then concluded to be good indicators in the damage depth curve. From Figs. 2 and 3, the behaviour of damage depth as local point damage is different from that of mass loss as overall damage. The straight line in the damage depth curve is the direct result of the constant rate of local damage transmitted by the energy of water droplet impingement. The fact that the damage area is extended as the stand off distance is increased implied that the impingement conditions are largely changed by the water pressure and the stand off distance. The water pressure dominates the impingement velocity directly and the stand off distance dominate impingement parameters of impact frequency, diameter and velocity of water droplets as mentioned in Section 4.2. A maximum damage depth rate depending on the stand off distance clearly exists (Fig. 6) and is considered to be induced by the optimum impingement conditions.
391
act cyclic stresses. The impact frequency of the water droplets is too low to cause erosion damage at the short stand off distance used. The increase in the impact frequency of water droplets directly induces a proportional increase in damage depth rate and a decrease in incubation period in Eq. (1). The optimum impingement condition for inducing a maximum damage rate is a SOD of 120–200 mm. This stand off distance at maximum damage rate did not coincide with the maximum impact frequency and the minimum diameter of the water droplet. It is therefore very likely that the third parameter, i.e., droplet velocity is a very important factor in damage to a material by the impingement of water droplets. We also attempted to obtain the velocity of the water-jet. It is well known that the change in momentum is equal to the impulse in Eq. (2). mv = Ft
(2)
where m is the mass of the water-jet, v is the change in velocity, F is the force and t is the time. The impingement force F is constant if the water-jet impinges on a wall (specimen) at a constant velocity. The change in velocity is equal to the initial velocity of the water-jet. The velocity of the water-jet is defined by the force divided by the mass flow rate Q as rewritten in Eq. (3). v=
F F = m/t Q
(3)
The velocity of the water-jet at the central axis at an arbitrary stand off distance can be derived from the experimentally obtained impingement force (N) and mass flow rate (g mm−2 s−1 ) in Fig. 10, taking into account the area of the rod with a diameter of 1 mm, as shown in Fig. 12. The velocities at pressures of 10 and 30 MPa were found to be 93 and 161 m s−1 , respectively, nearly the same values as the average velocity obtained from the mass flow rate of the nozzle (see
4.2. Impingement conditions in water-jet The parameters that control water droplet impingement are impact frequency, diameter and droplet velocity and are affected by water pressure, SOD and the shape or type of a nozzle used. Effects of a nozzle type on impingement parameters were not the focus of this study, and a commercial orifice nozzle of 0.4 mm in diameter was used exclusively. However, it is generally difficult to visualize droplets in a water-jet. In this study damage caused by water-jet impingement was confirmed to be due to the impingement of a droplet, since craters were observed on the aluminum foils within a very short duration (0.02 ms) using a slit slider (see Fig. 7). It is thought that the diameter of droplets is nearly the same as that of the craters measured in this study because droplets can easily deform the thin aluminum foil and the mounted glue beneath the foil. The very short stand off distance less than 30 mm caused negligible material removal, as shown in Fig. 6 and probably generate not water droplets but, rather, a water column, in spite of the small amount of deceleration in water-jet velocity near the outlet of the nozzle. The continuous water column only presses the specimen but does not
Fig. 12. Relations between stand off distance and droplet velocity in the central part of the water-jet.
392
Y.I. Oka et al. / Wear 263 (2007) 386–394
Table 1). From Fig. 12, the droplet velocity gradually decreases from a SOD of 200 mm and is as inversely proportional to the stand off distance (slope of −1 in a logarithmic scale). Moreover, the shape of the curves was the same between pressures of 10 and 30 MPa, indicating that the ratio of droplet velocity between the two pressures √ at given stand off distances v1 /v2 is constant and is about 3 (P1 = 10, P2 = 30 MPa) according to the Bernoulli theory (Eq. (4)). v = 2P/ρ, v1 /v2 = P1 /P2 (4) where P is the pressure of the water-jet and ρ is the density of water. The velocity √ of the water-jet at a pressure of 70 MPa was estimated to be 7 times of the velocity at 10 MPa as shown by the broken line in Fig. 12. This suggests that the substantial decrease in damage depth rate at further stand off distances is primarily caused by a decrease in water-jet velocity. 4.3. Effects of impingement velocity on damage characteristics The effects of impingement velocity on damage characteristics can be easily evaluated from the test results obtained using the water-jet apparatus, as the impingement velocity distributions against SOD are obtained and estimated at various pressures as shown in Fig. 12. Both the incubation period and the damage depth rate are important factors in characterizing erosion damage by water droplet impingement. The relation between impingement velocity and incubation period is shown in Fig. 13. Several types of data were extracted from Fig. 6. The relation was excellent under the wide range of impingement conditions of velocity, impact frequency and diameter of water droplets. The shock pressure Ps by the impingement of a droplet is well known from a basic equation of Eq. (5) [1]. Ps = ρCv
(5)
where ρ is the water density, C is the speed of sound in water, which are physical properties of water and constants. As a result, the shock pressure is proportional to impingement velocity. The diameter of a water droplet is not necessarily a radical parameter for shock pressure. The ordinate in Fig. 13 can be then regarded as an applied stress to a material and can be additionally described as shock pressure with ρ = 1000 kg m−3 , C = 1500 m s−1 . The abscissa in Fig. 13 is similar to the cycle number of water droplets per unit area (mm2 ), if the approximate impact frequency of water droplets is assumed from a few data points at low and high velocities as shown in Fig. 13. It is therefore considered that the curve in Fig. 13 resembles an S–N curve for fatigue. A value of 60 MPa is nearly the fatigue limit of the aluminum alloy used in this study [13]. Thiruvengadam et al. discussed the relation between shock pressure and fatigue limits and the existence of a threshold velocity [14], but the remarkable threshold velocity was not found in this study. It is possible that corrosion fatigue could occur from the tap water in which its corrosiveness cannot be ignored and that a fatigue limit cannot be reached at about 60 MPa or 109 numbers of impact frequency of water droplets. The impingement velocity versus incubation period or number of cycles curve is worth using for predicting the life span of component materials, if candidate materials are tested in this apparatus. It is very important to discuss the dependence of velocity on the amount of erosion damage caused by water droplet impingement. For example, the amount of erosion damage caused by solid particle impact is generally related to the square of the particle impact velocity with the deviation in velocity exponent [15]. This indicates that the amount of erosion damage is basically proportional to the impact energy. On the other hand, the exponent of the impingement velocity dependence of damage caused by liquid impact is said to be 5–7, which was experimentally obtained [14]. It is, however, probable that the higher exponent includes the effects of other impingement parameters such as the impact frequency of droplets in terms of individually uncontrollable impingement parameters. The amount of erosion damage per unit mass of droplets E (mm3 kg−1 ) can be modified from the damage depth rate Rd (mm s−1 ), impact frequency Freq (Number mm−2 s−1 ), diameter D (mm) and density ρ (kg mm−3 ) of droplet as shown in Eq. (6). E=
Fig. 13. Relation between incubation period and droplet velocity extracted from various test data.
Rd Freq ρ(πD3 /6)
(6)
where πD3 /6 is the volume of a droplet (mm3 ), then ρ(πD3 /6) is the mass of the droplet. The denominator means the mass of impacted droplets per unit area and time. Rd is the same as the volume loss per unit area and time. Fig. 14 shows that erosion damage versus the impingement velocity of droplets curve is approximated with the two linear curves on a logarithmic scale. The exponent took a value of about 6 in a region of low velocity less than 100 m s−1 . It can be found that the intensity of water droplets against material strength rapidly decreases with a decrease in droplet velocity but in the high velocity region. However, the exponent is about 2 in a region of higher velocity and erosion damage is directly related to the energy of the
Y.I. Oka et al. / Wear 263 (2007) 386–394
393
distance and choosing as high pressures (high droplet velocity) as possible, that is, a very short incubation period and very high damage depth rate are recommended. The performance on operating time and area was directed toward finding optimum impingement conditions for droplet frequency and a high energy of the water-jet. The knowledge from this study, which outlines implicit impingement parameters and optimum conditions for the use of a water-jet, will be very useful for surface treatment technology. 5. Conclusions
Fig. 14. Relation between droplet velocity and modified erosion damage extracted from various test data.
impinging droplets. This behaviour is the same as in erosion by solid particle impact. The transmission efficiency from impingement energy into material removal was approximately one tenth that for erosion damage to the same material by the impact of solid SiO2 particles, at an impact velocity of 104 m s−1 [16]. The lower transmission efficiency obtained by the impinging of water droplets was considered to be due to easier deformation of a water droplet during impingement than to tip destruction of a solid particle. The velocity dependence of erosion damage (mm3 kg−1 ) in water droplet impingement can be useful for estimating damage to industrial materials. 4.4. Applications to surface treatment technology It was found from Fig. 11 that the two ratios of scanning speeds between pressures of 30 and 70 MPa were from about 5 to 6, respectively, and that these were the same values as the comparisons of incubation periods and the damage depth rate between the two pressures, as shown in Fig. 4. Damage depth versus testing time curves can be therefore utilized to control the scanning speed of a surface treatment. From the clarification of the impingement conditions of water droplets as mentioned in the foregoing sections, the optimum operations of the water-jet can be introduced in application to surface treatment technology. Surface treatment technology by water-jet impingement is a technology which can cause the maximum damage to material surfaces by the impingement of water droplets, as damage to a material by a water-jet was found to be clearly caused by the impingement of water droplets related to the parameters of impact frequency, diameter and velocity as mentioned in Section 4.2. The type and size of the nozzle is definitely a major factor in damage characteristics for the incubation period and damage depth rate. The optimum conditions for water droplets with respect to SOD were recognized and erosion damage is directly related to impingement velocity or energy. Surface treatment technology should be applied at an appropriate stand off
Erosion tests using a water-jet apparatus were conducted on an aluminum alloy under conditions of different water pressures from 10 to 70 MPa and stand off distances from 30 to 500 mm with a fixed orifice nozzle of 0.4 mm in diameter. The impingement conditions which include impact frequency, the diameter and velocity of water droplets in the water-jet apparatus, were confirmed by measurements of craters on the aluminum foils, impingement force and mass flow rate in the central part of the water-jet. The conclusions are: (1) Erosion damage was dependent on both water pressure and stand off distance. The erosion damage was caused by the impingement of water droplets in the water-jet and the impingement conditions associated with the velocity and impact frequency of the water droplet were enhanced by a higher water pressure and an optimum stand off distance. The incubation period and damage depth rate were good indicators of damage characteristics in damage depth curves. (2) It was confirmed that the damage produced by the impingement of the water-jet was directly caused by the impingement of water droplets, from the fact that craters were formed on the aluminum foils. The velocity and impact frequency of the water droplets increased with the relative ratio of water pressure exponents of 0.5 and 3, respectively. The diameter of the water droplets decreased with the water pressure exponent of 0.5. (3) The relation between incubation period and droplet velocity was obtained, and was similar to an S–N curve for fatigue. However, a fatigue limit did not exist at about 60 MPa, which is the fatigue limit of the aluminum alloy used in this study, as shock pressure or 109 numbers of the impacted water droplet. This result suggested the possibility of a corrosion fatigue by a tap water. The impingement velocity versus the incubation period curve can be useful for predicting the life span of component materials, as the incubation period is representative of the number of cycles of water droplets. (4) The relation between the impingement velocity and erosion damage per unit mass of water droplets indicated a high exponent number of about 6 in the region of low velocity because the water droplet intensity against the material strength rapidly decreases with the decrease in droplet velocity. However, the velocity exponent of 2 was recognized in the high velocity region. This indicates that the erosion damage is basically related to the energy of imping-
394
Y.I. Oka et al. / Wear 263 (2007) 386–394
ing droplets in the high velocity region. This was the same as erosion by solid particle impact. The transmission efficiency from impingement energy into material removal was approximately one tenth that for erosion damage to the same material by the impact of SiO2 solid particle at a velocity of 104 m s−1 . This was considered to be due to the ease of deformation of the water droplet itself than due to the tip destruction of a solid particle. (5) The scanning speed at different pressures in a simulated surface treatment reflected the test results for the different incubation period and damage depth rate. The damage depth versus testing time curves and the impingement conditions of the water droplet assisted in the application of this approach to the surface treatment technology.
[5]
[6]
[7] [8] [9]
[10] [11]
Acknowledgement [12]
The authors wish to express their gratitude to Mr. R. Kaneda and T. Tsuda, Hiroshima University.
[13]
References
[14]
[1] G.S. Springer, Erosion by Liquid Impact, John Wiley & Sons, New York, 1976. [2] J.E. Field, ELSI conference: invited lecture liquid impact: theory, experiment, applications, Wear 233–235 (1999) 1–12. [3] W.F. Adler, Waterdrop impact modeling, Wear 186–187 (1995) 341–351. [4] T. Mabrouki, K. Raissi, A. Cornier, Numerical simulation and experimental study of the interaction between a pure high-velocity waterjet and tar-
[15]
[16]
gets: contribution to investigate the decoating process, Wear 239 (2000) 260–273. B. Staniˇsa, Z. Schauperl, K. Grilec, Erosion behaviour of turbine rotor blades installed in the Krsko nuclear power plant, Wear 254 (2003) 735–741. J.A. Krzyzanowski, A.E. Kowalski, A.L. Shubenko, Some aspects of erosion prediction of steam turbine blading, J. Eng. Gas Turbines Power Trans. ASME 116 (1994) 442–451. J. Dehouve, P. Nardin, M. Zeghmati, Erosion study of final stage blading of low pressure steam turbines, Appl. Surf. Sci. 144–145 (1999) 238–243. A.W. Momber, Y.C. Wong, E. Budidharma, R. Tjo, Surface profiling of low-carbon steel with supersonic waterjets, Wear 249 (2002) 853–859. M. Ramulu, K. Kunaporn, M. Jenkins, M. Hashish, J. Hopkins, Fatigue performance of high-pressure waterjet-peened, aluminum alloy, Trans. ASME 124 (2002) 118–123. S. Kunaporn, M. Ramulu, M. Hashish, Mathematical modeling of ultrahigh-pressure waterjet peening, Trans. ASME 127 (2005) 186–191. H.Z. Kuyumcu, L. Rolf, Application of high-pressure waterjets for comminution, Int. J. Miner. Process 74S (2004) S191–S198. M.M. Vijay, W.H. Brierley, A study of erosion by high-pressure cavitating and noncavitating waterjets, Erosion: Prevention and Useful Applications, ASTM STP 664, W.F. Adler, ASTM, 1979, pp. 512–529. J. Soc. of Mats. Sci., Japan, Kinzoku no Hirou (in Japanese), Maruzen Co. Ltd., 1964, p. 114. A. Thiruvengadam, S.L. Rudy, M. Gunasekaran, Experimental and analytical investigations on liquid impact erosion, ASTM STP 474 (1970) 249–287. Y.I. Oka, K. Okamura, T. Yoshida, Practical estimation of erosion damage caused by solid particle impact. Part 1. Effects of impact parameters on a predictive equation of erosion damage, Wear 259 (2005) 95–101. Y.I. Oka, T. Yoshida, Practical estimation of erosion damage caused by solid particle impact. Part 2. Mechanical properties of material directly associated with erosion damage, Wear 259 (2005) 102–109.
Effective parameters for erosion caused by water droplet impingement and applications to surface treatment technology Y.I. Oka ∗ , S. Mihara, H. Miyata Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan Received 29 August 2006; received in revised form 23 November 2006; accepted 24 November 2006 Available online 19 March 2007
Abstract Component materials in the steam environment of energy generation systems have severe problems of erosion caused by the impingement of water droplets. An evaluation of the resistance of such materials to erosion is very important in the maintenance of power plant systems and to prolong the life span of the components. A water-jet peening system, which is normally used for surface treatment technology, is based on the phenomenon of erosion damage. Parameters associated with water droplet impingement which dominate erosion rates are less controllable when a water-jet system is used. In this study, the use of a water-jet apparatus for erosion tests was conducted on an aluminum alloy to determine the effective parameters for erosion damage which are dependent on water pressure and the stand off distance between the nozzle and specimen. Parameters such as impact velocity, impact frequency and diameter of water droplets were examined by observations of craters formed on a thin aluminum film. Erosion damage was characterized with respect to incubation period and damage depth rate. Optimum surface treatment conditions were evaluated for an incubation period and damage depth rate after the incubation period. The incubation period and damage depth rate were greatly affected by the velocity and impact frequency of water droplets and the surface treatment technology should be sustained by the optimum impingement conditions which are controlled by the water pressure and stand off distance of the nozzle in the water-jet apparatus. © 2007 Elsevier B.V. All rights reserved. Keywords: Water-jet; Water droplet impingement; Incubation period; Damage depth rate; Impingement conditions of droplets; Surface treatment
1. Introduction The environments associated with liquid impact erosion or water droplet impingement on component materials are frequently seen in wet steam turbine systems of energy generation plants. Parameters that affect erosion damage need to be understood in order to estimate or predict erosion damage caused by water droplet impingement in energy generation systems and to select suitable materials for use as components which are highly erosion resistant. The characteristics of damage to materials and parameters related to water droplet impingement have been extensively reviewed, and a number of fundamental studies on liquid impact or water droplets have been reported [1–4]. A few studies have recently reported on the damage characteristics of turbine blades [5,6], using a rotating arm apparatus [7]. It is reasonable to evaluate erosion damage to materials or to select materials, as the droplet velocity can be readily controlled by the
∗
Corresponding author. Tel.: +81 824 24 7845; fax: +81 824 24 5494. E-mail address: [email protected] (Y.I. Oka).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.11.022
revolution of the rotor. On the other hand, water-jet peening has recently emerged as an alternate surface treatment technology to the sand blast or shot peening methods [8–11]. However, it has not adequately been compared to other impingement parameters except for impingement velocity in a water-jet apparatus. The testing time on the abscissa in erosion damage curves is prone to change as a function of jet pressure, type of nozzle and, importantly, the impact frequency of the impinging droplets. Predicting the life span of candidate materials used in actual plant components such as turbine blades is therefore impossible if precise parameters that are dependent on operating conditions are not clearly understood. Applications of the water-jet to surface treatment technology are also inadequate for the same reason. The impingement conditions for water droplets in actual plants should be investigated to relate experimental erosion data to actual damage in order to accurately predict erosion damage. The aim of this study was to characterize the effective parameters related to damage characteristics caused by water droplet impingement in a water-jet apparatus, in an attempt to predict or estimate erosion damage to materials that are used in actual plant components, and to determine the optimum conditions
Y.I. Oka et al. / Wear 263 (2007) 386–394
Fig. 1. Schematic diagrams of water-jet testing apparatus and measurement rigs.
for applications of water-jet impingement in surface treatment technology. Erosion tests were conducted on an aluminum alloy under different conditions of water pressure and stand off distance with a commercial orifice nozzle. Craters formed on a thin aluminum film were observed, in order to visualize the traces of the impinging droplets. Impingement parameters such as velocity, impact frequency and diameter of water droplets were investigated under some of the same conditions as were used in erosion tests. Effective parameters related to erosion damage by water-jet impingement and the best use of the water-jet for surface treatment technology are discussed based on the erosion test results and the determination of impingement conditions.
387
were measured with an electronic balance and a surface profilometer, respectively. In addition, simulated surface treatment tests were conducted at various scanning speeds at pressures of 30 and 70 MPa. The damaged surfaces were observed by both an optical microscope and scanning electron microscopy (SEM). The distributions of water droplets were obtained as craters formed on an aluminum foil with a thickness of 12 m mounted on the specimen. The reason for using the aluminum foil was to distinguish the size and impact number (impact frequency) per unit area and time of the water droplets from the water-jet owing to the softness of the aluminum foil. A slit board, 1 mm in width located at a distance of 10 mm under the nozzle was quickly moved into place for short duration of 0.02 ms in front of the water-jet. Craters that were formed were observed with an optical microscope after the short duration tests. The impact frequency and size of the water droplets were measured based on observations of the craters on the aluminum foil at the central part on the nozzle axis at various water pressures and SOD. A droplet collector with a duct of 0.5 mm in inner diameter was installed on the stage in order to obtain the localized mass flow rate in the central part of the water-jet at various stand off distances. The mass flow rate (g mm−2 s−1 ) was measured from the mass of water for durations from 1 to 10 s. A force detection rod unit with a diameter of 1 mm was installed to measure the force generated on the central part of the water-jet, as shown in Fig. 1. The strain on the stainless steel plate was measured and converted to output voltage. The voltage and force was calibrated using the same detection unit and a universal testing machine.
2. Experimental procedures 3. Results Schematic diagrams of a water-jet apparatus and the measurement rigs are shown in Fig. 1. Pressurized tap water was supplied from a plunger pump and was injected in an orifice nozzle with diameter of about 0.4 mm (exactly 0.425 mm). The water-jet was allowed to impinge on the aluminum alloy Al 5083 (Mg: 4.5, Mn: 0.7) at normal angle. The reason for using an aluminum alloy was that it was a soft material (Hv = 71) and considerable damage would be expected within a relatively short duration under low impingement conditions. The water pressure was controlled at 10, 30, 50 and 70 MPa. The flow rate and average velocity at the outlet of the nozzle are summarized in Table 1. The average velocity was calculated by the flow rate divided by the crosssection area of the nozzle. The stand off distance (SOD) between the nozzle and specimen was regulated from 30 to 500 mm. Mass loss of the whole specimen (30 mm × 40 mm × 5 mm) and the maximum damage depth in the central part on the nozzle axis
Fig. 2 shows examples of plots of mass loss versus testing time at a SOD of 200 mm at various water pressures for Al 5083. The mass loss is not linear and changes gradually with testing time. An incubation period, during which the mass loss
Table 1 Flow rate and average flow velocity, and damage characteristics at a stand off distance of 200 mm Pressure (MPa)
Fr (cm3 s−1 )
Vav (m s−1 )
10 30 50 70
12.7 21.7 29.0 35.3
89 153 204 248
Ip (s) 300 0.71 0.25 0.15
Rd (mm s−1 ) 0.00042 0.073 0.216 0.378
Fr: flow rate; Vav : average velocity; Ip : incubation period; Rd : damage depth rate.
Fig. 2. Mass loss vs. testing time curves at a stand off distance of 200 mm at various water pressures.
388
Y.I. Oka et al. / Wear 263 (2007) 386–394
Fig. 3. Damage depth vs. testing time curves at a stand off distance of 200 mm at various water pressures.
of the specimen cannot be measured, can be clearly found at a pressure of 30 MPa but not at higher pressures because of the shorter incubation period. The incubation period at a pressure of 10 MPa was 300 s (see Table 1 and Fig. 13). As a result, a mass loss curve could not be obtained for this short scale of testing time. The mass loss increased with water pressure and was typical for water droplet impingement. Fig. 3 shows damage depth versus testing time curves under the same conditions as in Fig. 2. The damage depth was the maximum damage depth, as measured in the central part of a damaged area on the nozzle axis. The curves for damage depth were similar to the mass loss curves, but the increase in damage depth in an early stage of testing time was relatively steeper than that for mass loss (Fig. 2). The damage depth curves can be approximated by two linear curves, as shown in Fig. 3. An incubation period was not clearly recognized for extended testing time. Damage depth curves for the initial testing time in Fig. 3 are shown in Fig. 4. The incubation periods for various water pressures are clearly indicated on the testing time axis. The incubation period decreases with an increase in water pressure. On the contrary the damage depth rate, which means the slope of the damage depth curve, increases rapidly with an increase in water pressure. The incubation periods and initial damage depth rates obtained from Fig. 4 are summarized in Table 1. Fig. 5 shows SEM observations at a testing time of 3 s, a SOD of 200 mm and a pressure of 30 MPa. The damaged area was a spot within a diameter of about 0.6–0.7 mm. Plastic deformation in the center was observed appearing as hills and valleys and relatively flat indentations, and rolled up grains around the crater were observed as configurations caused by the small amount of water droplet impingement. The crater size spread with increasing testing time. Fig. 6 shows the effect of SOD on damage depth rate which shows the slope for damage depth versus initial testing time curves after the incubation period in Fig. 4. The maximum dam-
Fig. 4. Damage depth vs. testing time curves in short duration at a stand off distance of 200 mm at various water pressures.
age depth rates were commonly observed for a SOD from 120 to 200 mm at all water pressures. The damage depth rates decreased for either a shorter or longer stand off distance. The damage area increased with the increase in SOD, although the damage depth rate decreased at a SOD of over 200 mm. The shape of the distribution of damage depth rate is similar among the three water pressures.
Fig. 5. SEM observations of an eroded crater at a stand off distance of 200 mm, pressure of 30 MPa and testing time of 3 s (a) whole crater, (b) the rim of the crater.
Y.I. Oka et al. / Wear 263 (2007) 386–394
389
Fig. 6. Effect of stand off distance on damage depth rate at various water pressures.
Fig. 7 shows examples of optical microscope observations of aluminum foil surfaces after the impingement of a small amount of water-jet at various stand off distances at a pressure of 10 MPa. The cross sign indicates the center of the water-jet. Fig. 7(a) was not frequently observed but the large crater in the center appeared to be caused by a water column in the water-jet. Irregularly shaped craters were observed around the large crater. The number of craters seemed to be largest, but their size was smaller at a stand off distance of 365 mm (Fig. 7(b)). Based on these photographs the number (impact frequency) and average diameter of water droplets were measured at various stand off distances at water pressures of 10 and 30 MPa. These observations were impossible at pressures of over 30 MPa probably because of the strong impingement of water droplets. Figs. 8 and 9 show the distributions of impact frequency and average diameter of water droplets at the center of the damage area against SOD at water pressures of 10 and 30 MPa, from visual inspections of the aluminum foils (Fig. 7). The maximum impact frequencies of water droplets were 107 number mm−2 s−1 at a pressure of 30 MPa, 5 × 105 number mm−2 s−1 at a pressure of 10 MPa in the stand off distance from 200 to 300, and the minimum diameters of water droplets were about 50 m at a pressure of 30 MPa, 150 m at a pressure of 10 MPa. The broken lines in Figs. 8 and 9 were estimated from the similar curves at the two pressures according to the equations in the figures. Where Freq is the impact frequency, D is the diameter of the water droplet and P is the water pressure. Fig. 10 shows the relations between SOD, and impingement force and mass flow rate in the central part of the water-jet at pressures of 10 and 30 MPa. The value for impingement force was constant at a SOD of less than 100 mm and decreased gradually with the increase in SOD. The behaviour of the mass flow rate was similar to that for the impingement force, but the slope of the linear curves at a SOD of over 100 mm was different between the impingement force and the mass flow rate. On the other hand, the impingement force was clearly different between pressures
Fig. 7. Optical microscope observations of aluminum foils at a pressure of 10 MPa, at stand off distances of (a) 100 mm, (b) 365 mm, (c) 500 mm.
of 10 and 30 MPa within a SOD of 100 mm, but the mass flow rate was only slightly different between the two pressures. Fig. 11 shows examples of simulated surface treatments for the aluminum alloy at various specimen scanning speeds and surface profiles. Two types of surface roughness, in the incubation period and in the early region of the damage depth curve, were formed at pressures of 30 and 70 MPa. The stand off distance was 200 mm at which the maximum damage depth rate was obtained, as shown in Fig. 6. In order to achieve a similar
390
Y.I. Oka et al. / Wear 263 (2007) 386–394
Fig. 8. Distributions of the impact frequency of water droplets at various pressures.
surface coarseness, the scanning speed was varied with different water pressures. The band width at a pressure of 70 MPa was narrower than that at a pressure of 30 MPa in the case of deeper grooving, however, the scanning speed used was very fast at a pressure of 70 MPa. 4. Discussion 4.1. Evaluation of erosion damage Erosion damage is characterized by mass loss, damage depth and these rates, and the incubation period. The mass loss curve gradually changes with testing time in water-jet impingement, as shown in Fig. 2. In other words, the mass loss rate is not constant and its behaviour is well known [12]. This phenomenon
Fig. 9. Distributions of the diameter of water droplets at various pressures.
Fig. 10. Relations between stand off distance, and impingement force and mass flow rate in the central part of the water-jet.
is considered to be caused by the repetition of plastic deformation and removal of the material tip. The decrease in mass loss rate after a long duration is due to the shock absorption of a water film formed on the bottom of the crater. The mass loss of the specimen arises from overall crater damage but impingement conditions are possibly different in the respective points of the crater, as described in Section 4.2. The mass loss, then, means the sum of the amount of damage at different areas and changes (curvilinear) with testing time. It is therefore difficult to evaluate erosion damage based on the mass loss curve with a variable damage rate. The maximum damage rate in the mass loss curve is a useful indicator of erosion damage caused by water droplet impingement, but does not necessarily encompass
Fig. 11. Appearances of simulated surface treatments and measurements of cross sectional surface roughness at a stand off distance of 200 mm, at various pressures and scanning speeds.
Y.I. Oka et al. / Wear 263 (2007) 386–394
all of the damage behaviour and is useful only for comparing erosion resistance among various candidate materials. On the other hand, damage depth means the local volume of damage per small area and is indicated by a linear curve (constant damage depth rate) after the incubation period (see Fig. 4), as expressed by Eq. (1). Ed = Rd (t − Ip )
(1)
where Ed is the damage depth (mm), Rd is the damage depth rate (mm s−1 ), t is the testing time (s) and Ip is the incubation period (s). Damage depth behaviour at long testing time can be ignored, taking into account the actual life span or the degradation in running efficiency regarding the incubation period (initiation point of damage depth) or the allowable damage depth of the component materials. These parameters of Rd and Ip are obtained from the spot condition at the central part of the damaged area so that the relations between damage depth and impingement parameters can be obtained more easily and more reliably. Both Rd and Ip are then concluded to be good indicators in the damage depth curve. From Figs. 2 and 3, the behaviour of damage depth as local point damage is different from that of mass loss as overall damage. The straight line in the damage depth curve is the direct result of the constant rate of local damage transmitted by the energy of water droplet impingement. The fact that the damage area is extended as the stand off distance is increased implied that the impingement conditions are largely changed by the water pressure and the stand off distance. The water pressure dominates the impingement velocity directly and the stand off distance dominate impingement parameters of impact frequency, diameter and velocity of water droplets as mentioned in Section 4.2. A maximum damage depth rate depending on the stand off distance clearly exists (Fig. 6) and is considered to be induced by the optimum impingement conditions.
391
act cyclic stresses. The impact frequency of the water droplets is too low to cause erosion damage at the short stand off distance used. The increase in the impact frequency of water droplets directly induces a proportional increase in damage depth rate and a decrease in incubation period in Eq. (1). The optimum impingement condition for inducing a maximum damage rate is a SOD of 120–200 mm. This stand off distance at maximum damage rate did not coincide with the maximum impact frequency and the minimum diameter of the water droplet. It is therefore very likely that the third parameter, i.e., droplet velocity is a very important factor in damage to a material by the impingement of water droplets. We also attempted to obtain the velocity of the water-jet. It is well known that the change in momentum is equal to the impulse in Eq. (2). mv = Ft
(2)
where m is the mass of the water-jet, v is the change in velocity, F is the force and t is the time. The impingement force F is constant if the water-jet impinges on a wall (specimen) at a constant velocity. The change in velocity is equal to the initial velocity of the water-jet. The velocity of the water-jet is defined by the force divided by the mass flow rate Q as rewritten in Eq. (3). v=
F F = m/t Q
(3)
The velocity of the water-jet at the central axis at an arbitrary stand off distance can be derived from the experimentally obtained impingement force (N) and mass flow rate (g mm−2 s−1 ) in Fig. 10, taking into account the area of the rod with a diameter of 1 mm, as shown in Fig. 12. The velocities at pressures of 10 and 30 MPa were found to be 93 and 161 m s−1 , respectively, nearly the same values as the average velocity obtained from the mass flow rate of the nozzle (see
4.2. Impingement conditions in water-jet The parameters that control water droplet impingement are impact frequency, diameter and droplet velocity and are affected by water pressure, SOD and the shape or type of a nozzle used. Effects of a nozzle type on impingement parameters were not the focus of this study, and a commercial orifice nozzle of 0.4 mm in diameter was used exclusively. However, it is generally difficult to visualize droplets in a water-jet. In this study damage caused by water-jet impingement was confirmed to be due to the impingement of a droplet, since craters were observed on the aluminum foils within a very short duration (0.02 ms) using a slit slider (see Fig. 7). It is thought that the diameter of droplets is nearly the same as that of the craters measured in this study because droplets can easily deform the thin aluminum foil and the mounted glue beneath the foil. The very short stand off distance less than 30 mm caused negligible material removal, as shown in Fig. 6 and probably generate not water droplets but, rather, a water column, in spite of the small amount of deceleration in water-jet velocity near the outlet of the nozzle. The continuous water column only presses the specimen but does not
Fig. 12. Relations between stand off distance and droplet velocity in the central part of the water-jet.
392
Y.I. Oka et al. / Wear 263 (2007) 386–394
Table 1). From Fig. 12, the droplet velocity gradually decreases from a SOD of 200 mm and is as inversely proportional to the stand off distance (slope of −1 in a logarithmic scale). Moreover, the shape of the curves was the same between pressures of 10 and 30 MPa, indicating that the ratio of droplet velocity between the two pressures √ at given stand off distances v1 /v2 is constant and is about 3 (P1 = 10, P2 = 30 MPa) according to the Bernoulli theory (Eq. (4)). v = 2P/ρ, v1 /v2 = P1 /P2 (4) where P is the pressure of the water-jet and ρ is the density of water. The velocity √ of the water-jet at a pressure of 70 MPa was estimated to be 7 times of the velocity at 10 MPa as shown by the broken line in Fig. 12. This suggests that the substantial decrease in damage depth rate at further stand off distances is primarily caused by a decrease in water-jet velocity. 4.3. Effects of impingement velocity on damage characteristics The effects of impingement velocity on damage characteristics can be easily evaluated from the test results obtained using the water-jet apparatus, as the impingement velocity distributions against SOD are obtained and estimated at various pressures as shown in Fig. 12. Both the incubation period and the damage depth rate are important factors in characterizing erosion damage by water droplet impingement. The relation between impingement velocity and incubation period is shown in Fig. 13. Several types of data were extracted from Fig. 6. The relation was excellent under the wide range of impingement conditions of velocity, impact frequency and diameter of water droplets. The shock pressure Ps by the impingement of a droplet is well known from a basic equation of Eq. (5) [1]. Ps = ρCv
(5)
where ρ is the water density, C is the speed of sound in water, which are physical properties of water and constants. As a result, the shock pressure is proportional to impingement velocity. The diameter of a water droplet is not necessarily a radical parameter for shock pressure. The ordinate in Fig. 13 can be then regarded as an applied stress to a material and can be additionally described as shock pressure with ρ = 1000 kg m−3 , C = 1500 m s−1 . The abscissa in Fig. 13 is similar to the cycle number of water droplets per unit area (mm2 ), if the approximate impact frequency of water droplets is assumed from a few data points at low and high velocities as shown in Fig. 13. It is therefore considered that the curve in Fig. 13 resembles an S–N curve for fatigue. A value of 60 MPa is nearly the fatigue limit of the aluminum alloy used in this study [13]. Thiruvengadam et al. discussed the relation between shock pressure and fatigue limits and the existence of a threshold velocity [14], but the remarkable threshold velocity was not found in this study. It is possible that corrosion fatigue could occur from the tap water in which its corrosiveness cannot be ignored and that a fatigue limit cannot be reached at about 60 MPa or 109 numbers of impact frequency of water droplets. The impingement velocity versus incubation period or number of cycles curve is worth using for predicting the life span of component materials, if candidate materials are tested in this apparatus. It is very important to discuss the dependence of velocity on the amount of erosion damage caused by water droplet impingement. For example, the amount of erosion damage caused by solid particle impact is generally related to the square of the particle impact velocity with the deviation in velocity exponent [15]. This indicates that the amount of erosion damage is basically proportional to the impact energy. On the other hand, the exponent of the impingement velocity dependence of damage caused by liquid impact is said to be 5–7, which was experimentally obtained [14]. It is, however, probable that the higher exponent includes the effects of other impingement parameters such as the impact frequency of droplets in terms of individually uncontrollable impingement parameters. The amount of erosion damage per unit mass of droplets E (mm3 kg−1 ) can be modified from the damage depth rate Rd (mm s−1 ), impact frequency Freq (Number mm−2 s−1 ), diameter D (mm) and density ρ (kg mm−3 ) of droplet as shown in Eq. (6). E=
Fig. 13. Relation between incubation period and droplet velocity extracted from various test data.
Rd Freq ρ(πD3 /6)
(6)
where πD3 /6 is the volume of a droplet (mm3 ), then ρ(πD3 /6) is the mass of the droplet. The denominator means the mass of impacted droplets per unit area and time. Rd is the same as the volume loss per unit area and time. Fig. 14 shows that erosion damage versus the impingement velocity of droplets curve is approximated with the two linear curves on a logarithmic scale. The exponent took a value of about 6 in a region of low velocity less than 100 m s−1 . It can be found that the intensity of water droplets against material strength rapidly decreases with a decrease in droplet velocity but in the high velocity region. However, the exponent is about 2 in a region of higher velocity and erosion damage is directly related to the energy of the
Y.I. Oka et al. / Wear 263 (2007) 386–394
393
distance and choosing as high pressures (high droplet velocity) as possible, that is, a very short incubation period and very high damage depth rate are recommended. The performance on operating time and area was directed toward finding optimum impingement conditions for droplet frequency and a high energy of the water-jet. The knowledge from this study, which outlines implicit impingement parameters and optimum conditions for the use of a water-jet, will be very useful for surface treatment technology. 5. Conclusions
Fig. 14. Relation between droplet velocity and modified erosion damage extracted from various test data.
impinging droplets. This behaviour is the same as in erosion by solid particle impact. The transmission efficiency from impingement energy into material removal was approximately one tenth that for erosion damage to the same material by the impact of solid SiO2 particles, at an impact velocity of 104 m s−1 [16]. The lower transmission efficiency obtained by the impinging of water droplets was considered to be due to easier deformation of a water droplet during impingement than to tip destruction of a solid particle. The velocity dependence of erosion damage (mm3 kg−1 ) in water droplet impingement can be useful for estimating damage to industrial materials. 4.4. Applications to surface treatment technology It was found from Fig. 11 that the two ratios of scanning speeds between pressures of 30 and 70 MPa were from about 5 to 6, respectively, and that these were the same values as the comparisons of incubation periods and the damage depth rate between the two pressures, as shown in Fig. 4. Damage depth versus testing time curves can be therefore utilized to control the scanning speed of a surface treatment. From the clarification of the impingement conditions of water droplets as mentioned in the foregoing sections, the optimum operations of the water-jet can be introduced in application to surface treatment technology. Surface treatment technology by water-jet impingement is a technology which can cause the maximum damage to material surfaces by the impingement of water droplets, as damage to a material by a water-jet was found to be clearly caused by the impingement of water droplets related to the parameters of impact frequency, diameter and velocity as mentioned in Section 4.2. The type and size of the nozzle is definitely a major factor in damage characteristics for the incubation period and damage depth rate. The optimum conditions for water droplets with respect to SOD were recognized and erosion damage is directly related to impingement velocity or energy. Surface treatment technology should be applied at an appropriate stand off
Erosion tests using a water-jet apparatus were conducted on an aluminum alloy under conditions of different water pressures from 10 to 70 MPa and stand off distances from 30 to 500 mm with a fixed orifice nozzle of 0.4 mm in diameter. The impingement conditions which include impact frequency, the diameter and velocity of water droplets in the water-jet apparatus, were confirmed by measurements of craters on the aluminum foils, impingement force and mass flow rate in the central part of the water-jet. The conclusions are: (1) Erosion damage was dependent on both water pressure and stand off distance. The erosion damage was caused by the impingement of water droplets in the water-jet and the impingement conditions associated with the velocity and impact frequency of the water droplet were enhanced by a higher water pressure and an optimum stand off distance. The incubation period and damage depth rate were good indicators of damage characteristics in damage depth curves. (2) It was confirmed that the damage produced by the impingement of the water-jet was directly caused by the impingement of water droplets, from the fact that craters were formed on the aluminum foils. The velocity and impact frequency of the water droplets increased with the relative ratio of water pressure exponents of 0.5 and 3, respectively. The diameter of the water droplets decreased with the water pressure exponent of 0.5. (3) The relation between incubation period and droplet velocity was obtained, and was similar to an S–N curve for fatigue. However, a fatigue limit did not exist at about 60 MPa, which is the fatigue limit of the aluminum alloy used in this study, as shock pressure or 109 numbers of the impacted water droplet. This result suggested the possibility of a corrosion fatigue by a tap water. The impingement velocity versus the incubation period curve can be useful for predicting the life span of component materials, as the incubation period is representative of the number of cycles of water droplets. (4) The relation between the impingement velocity and erosion damage per unit mass of water droplets indicated a high exponent number of about 6 in the region of low velocity because the water droplet intensity against the material strength rapidly decreases with the decrease in droplet velocity. However, the velocity exponent of 2 was recognized in the high velocity region. This indicates that the erosion damage is basically related to the energy of imping-
394
Y.I. Oka et al. / Wear 263 (2007) 386–394
ing droplets in the high velocity region. This was the same as erosion by solid particle impact. The transmission efficiency from impingement energy into material removal was approximately one tenth that for erosion damage to the same material by the impact of SiO2 solid particle at a velocity of 104 m s−1 . This was considered to be due to the ease of deformation of the water droplet itself than due to the tip destruction of a solid particle. (5) The scanning speed at different pressures in a simulated surface treatment reflected the test results for the different incubation period and damage depth rate. The damage depth versus testing time curves and the impingement conditions of the water droplet assisted in the application of this approach to the surface treatment technology.
[5]
[6]
[7] [8] [9]
[10] [11]
Acknowledgement [12]
The authors wish to express their gratitude to Mr. R. Kaneda and T. Tsuda, Hiroshima University.
[13]
References
[14]
[1] G.S. Springer, Erosion by Liquid Impact, John Wiley & Sons, New York, 1976. [2] J.E. Field, ELSI conference: invited lecture liquid impact: theory, experiment, applications, Wear 233–235 (1999) 1–12. [3] W.F. Adler, Waterdrop impact modeling, Wear 186–187 (1995) 341–351. [4] T. Mabrouki, K. Raissi, A. Cornier, Numerical simulation and experimental study of the interaction between a pure high-velocity waterjet and tar-
[15]
[16]
gets: contribution to investigate the decoating process, Wear 239 (2000) 260–273. B. Staniˇsa, Z. Schauperl, K. Grilec, Erosion behaviour of turbine rotor blades installed in the Krsko nuclear power plant, Wear 254 (2003) 735–741. J.A. Krzyzanowski, A.E. Kowalski, A.L. Shubenko, Some aspects of erosion prediction of steam turbine blading, J. Eng. Gas Turbines Power Trans. ASME 116 (1994) 442–451. J. Dehouve, P. Nardin, M. Zeghmati, Erosion study of final stage blading of low pressure steam turbines, Appl. Surf. Sci. 144–145 (1999) 238–243. A.W. Momber, Y.C. Wong, E. Budidharma, R. Tjo, Surface profiling of low-carbon steel with supersonic waterjets, Wear 249 (2002) 853–859. M. Ramulu, K. Kunaporn, M. Jenkins, M. Hashish, J. Hopkins, Fatigue performance of high-pressure waterjet-peened, aluminum alloy, Trans. ASME 124 (2002) 118–123. S. Kunaporn, M. Ramulu, M. Hashish, Mathematical modeling of ultrahigh-pressure waterjet peening, Trans. ASME 127 (2005) 186–191. H.Z. Kuyumcu, L. Rolf, Application of high-pressure waterjets for comminution, Int. J. Miner. Process 74S (2004) S191–S198. M.M. Vijay, W.H. Brierley, A study of erosion by high-pressure cavitating and noncavitating waterjets, Erosion: Prevention and Useful Applications, ASTM STP 664, W.F. Adler, ASTM, 1979, pp. 512–529. J. Soc. of Mats. Sci., Japan, Kinzoku no Hirou (in Japanese), Maruzen Co. Ltd., 1964, p. 114. A. Thiruvengadam, S.L. Rudy, M. Gunasekaran, Experimental and analytical investigations on liquid impact erosion, ASTM STP 474 (1970) 249–287. Y.I. Oka, K. Okamura, T. Yoshida, Practical estimation of erosion damage caused by solid particle impact. Part 1. Effects of impact parameters on a predictive equation of erosion damage, Wear 259 (2005) 95–101. Y.I. Oka, T. Yoshida, Practical estimation of erosion damage caused by solid particle impact. Part 2. Mechanical properties of material directly associated with erosion damage, Wear 259 (2005) 102–109.