Effect of multiple factors on sediment erosion characteristics of hydro turbine materials

Hydraulic turbines on sediment-laden rivers suffer from cavitation-silt erosion. This study conducted cavitation-silt erosion tests on three materials (06Cr16Ni5Mo, Q355B, 06Cr20Ni11) with different sediment concentrations and impact velocities using a rotating disk facility, and used a scanning electron microscope (SEM) to observe the cavitation-silt erosion characteristics of the materials. The results show that the cumulative weight loss of all materials continuously increases with the sediment concentration and impact velocity rises. The weight loss of 06Cr16Ni5Mo is the lowest, and 06Cr20Ni11 is the highest. As the impact velocity rises, the cumulative weight loss increases exponentially. As the sediment concentration rises, there are obvious scour marks and fatigue spalling on the material surface. As the impact velocity rises, the size and depth of micro-cutting marks and cavitation pits increases. Due to the lower surface hardness of 06Cr20Ni11, the deep platelet pits appear on the material surface.


Introduction
The sediment erosion damage of hydraulic machinery has always been a challenging issue that affects the safe operation of hydropower stations and pumping stations.The combined effects of sediment erosion and cavitation erosion can lead to material damage, causing failure of the overflow parts and significantly reducing equipment lifespan.In severe cases, this result in strong vibrations, noise, and load fluctuations, directly impacting the safe and stable operation of hydraulic machinery [1][2].The Jinghe River is one of the famous rivers with high sediment concentrations, high hardness, and small particle sizes in northwest China, which has damaged the hydraulic turbines seriously.The Dongzhuang Power Station is a key project on Jinghe River planned by the government.In the flood season of 2022, the maximum sediment concentration exceeds 1100 kg/m 3 .Sediment erosion will cause fatal damage to the overflow parts of the hydro turbine, which must be solved in the Dongzhuang Power Station Project.
Li Ming [3] conducted hydraulic erosion tests on a rotating disk with various coatings using sediment from the Yellow River.The study revealed that the volume erosion rate increased with higher sediment concentrations and provided recommendations for material selection for impeller and runner components.Through using a rotating disk plane turbulence test device, Liu Juan [4][5][6] found that median particle size and sediment concentration significantly influenced material anti-erosion performance.The depth of erosion increased with larger median particle size and sediment concentration.By evaluating the sediment erosion capacity and testing the anti-erosion characteristics of alternative materials, an estimation of the hydraulic turbine erosion caused by sediment was made, providing technical support for pump material selection and the formulation of protective measures for power station projects.
In actual engineering projects, hydraulic machinery overflow parts are not only suffered from sand particle wear but also cavitation erosion.Some scholars have tested the combined effect of sediment erosion and cavitation erosion through the creation of cavitation holes on rotating disks and the addition of cavitation columns.Liu Juan [7] conducted an cavitation-silt erosion test on HVAF coatings of hydraulic machinery parts using a rotating disk device.Lu Jinling [8][9] conducted cavitation-silt erosion tests on sediment with varying particle sizes at low concentrations, using a rotating disk jet device.The study obtained the trend of material cumulative weight loss over time under different sediment particle sizes, finding that weight loss increased with larger particle sizes.Pang Youxia [10][11] investigated the cavitation erosion resistance performance of pump impeller materials of HT200 using a rotating disk device.
Currently, most research on sediment erosion focuses on operating conditions involving large particle sizes and low sediment concentrations.Therefore, this study employs a rotating disk jet device to conduct cavitation-silt erosion tests on three commonly used materials in hydraulic machinery: ZG06Cr16Ni5Mo, Q355B, and 06Cr20Ni11.The tests are performed under varying sediment concentrations and impact velocities.The cumulative weight loss curve and microstructure morphology are studied and analyzed.This research significantly contributes to the understanding of cavitation-silt erosion characteristics in hydraulic machinery, providing valuable guidance for the protection of hydraulic machinery against cavitation-silt erosion.

Test equipment
The cavitation-silt erosion characteristics of hydraulic machinery materials were tested by a rotating disk facility, which is capable of operating at a high sediment concentration condition.The cavitationsilt erosion test system consists of a cleaning tank, a mixing tank, a diaphragm pump, a surge tank, a rotating disk chamber, a compressor, a cooling system, and a control cabinet.As shown in Figure 1.During the test, water and sediment in a certain proportion were mixed in the mixing tank.The sediment water was circulated in the system through the diaphragm pump.The surge tank could maintain stable impact velocity, and then the sediment water was sprayed on the surface of the specimens through the nozzle in the rotating disk chamber.Meanwhile, the cooling system maintained the test system at constant temperature about 25℃.The cavitation-silt erosion test system was equipped with a bypass pipe, which could clean the test system after cavitation-silt erosion tests, thus reducing the risk of sediment accumulation and pipe blocking in the test system during the high concentrations of cavitationsilt erosion.The flow chart of the test system is shown in Figure 2. The flow velocity was adjusted by adjusting the rotational speed of the rotating disk and the flow velocity of the nozzle, as shown in Figure 3(a), and a rotating disk was used to fix the specimens, as shown in Figure 3(b).In order to make the results of cavitation-silt erosion tests more consistent with the actual erosion results of the real machine, the following materials were intended to be used: (1) Quartz sand with a median grain size of 16 um was selected for the tests, and its characteristics were similar to those of the Jinghe River sediment.The microstructure of sediment grains is shown in Figure 4. (2) The metal specimens were tested with the materials of the flow parts of the Dongzhuang Hydraulic Project: 06Cr16Ni5Mo, Q355B and 06Cr20Ni11, with the surface roughness of 0.16 um.The three material mechanical properties are shown in Table 2.
Table 2. Mechanical properties of three materials.

Test method and parameter settings
There are four kinds of variable concentration and velocities in the cavitation-silt erosion tests.The 12 hours continuous test was carried out at each sediment concentration and velocity.Before and after the cavitation-silt erosion tests, the specimens were cleaned and dried with ethanol, respectively, and weighed by LICHEN-2204 electronic balance with an accuracy of 10 -4 g.The surface microstructure of the specimens were observed and analyzed using Zeiss EVO10 scanning electron microscope.Specific test parameters are shown in Table 3 below.

Material surface microstructure and mechanical properties
Figure 5 illustrates the initial surface microstructure of 06Cr16Ni5Mo, Q355B, and 06Cr20Ni11.From the figure, it can be seen that due to polishing reason, there are obvious regular and directional polishing marks in the surface of the three materials of specimens.Additionally, it should be noted that Q355B is prone to oxidation and corrosion, and a few oxidation pits formed by oxidation can be seen on the surface microstructure.

Cumulative weight loss on cavitation-silt erosion on materials
Figure 6 illustrates the cumulative weight loss of three materials during cavitation-silt erosion tests conducted for 12 hours at various sediment concentrations, with an impact angle of 30° and an impact velocity of 32 m/s.It is evident that the cumulative weight loss of all materials progressively increases as the sediment concentration rises.Below a sediment concentration of 34.48 kg/m 3 , the weight loss of all materials demonstrates a rapid increase.However, beyond this sediment concentration of 34.48 kg/m 3 , the weight loss rate slows down with sediment concentration rises.These observations indicate that, at an impact velocity of 32 m/s and an impact angle of 30°, the increased sediment concentration leads to a higher collision frequency between sand particles and the material surface, resulting in rapid weight loss for all materials.Once the sediment concentration surpasses 34.48 kg/m 3 , continuous impacts strengthen the material surface, initiating a stable period.The enhanced resistance to deformation gradually diminishes the cavitation-silt erosion effect, resulting in a reduced rate of weight loss.At a sediment concentration of 8.76 kg/m 3 , the cumulative weight loss of the three materials is relatively close.However, above this concentration, the cumulative weight loss of the three materials significantly increases, with the weight loss of 06Cr16Ni5Mo is the lowest, and 06Cr20Ni11 is the highest.At a sediment concentration of 57.04 kg/m 3 , the weight loss of 06Cr16Ni5Mo is 159.65 mg, which is 10.72 times higher than that at a sediment concentration of 8.76 kg/m 3 .The weight loss of Q355B is 183.15 mg, which is 6.78 times higher than at a sediment concentration of 8.76 kg/m 3 .Furthermore, the weight loss of 06Cr20Ni11 is 211.18 mg, which is 11.12 times higher than at a sediment concentration of 8.76 kg/m 3 .As the sediment concentration rises, the difference in weight loss among the three materials becomes more pronounced.At a sediment concentration of 57.04 kg/m 3 , the weight loss of Q355B exceeds that of 06Cr16Ni5Mo by 14.72%, and the weight loss of 06Cr20Ni11 exceeds that of 06Cr16Ni5Mo by 32.28%.These findings suggest that variation in mechanical properties among the three materials leads to differences in surface hardness, with 06Cr16Ni5Mo having the highest surface hardness and 06Cr20Ni11 the lowest.Consequently, as the sediment concentration increases, there is a notable distinction in weight loss among the materials.Figure 6.Cumulative weight loss of three materials with different concentrations.Figure 7 illustrates the cumulative weight loss of three materials during cavitation-silt erosion tests conducted for 12 hours at various impact velocities.The tests were performed with an impact angle of 30° and a sediment concentration of 34.48 kg/m 3 .As depicted in the figure, the cumulative weight loss of all three materials exhibits an exponential increase with the rise in impact velocity.This indicates that, at an impact angle of 30° and a sediment concentration of 34.48 kg/m 3 , the weight loss significantly increases as the impact velocity and the kinetic energy of sand particles and the cavitation action intensify.
Furthermore, as the impact velocity increases, the cumulative weight loss for 06Cr16Ni5Mo is measured at 1.43 mg, 28.68 mg, 143.19 mg, and 476.38 mg, respectively, while for 06Cr20Ni11, the values are 1.12 mg, 46.03 mg, 202.36 mg, and 681.08 mg, respectively.When the impact velocity is below 24 m/s, the cumulative weight loss of all materials is relatively close, with a difference in weight loss of less than 20 mg.However, when the impact velocity exceeds 24 m/s, the weight loss differences between materials increases, with a maximum weight loss difference of 205 mg.The weight loss of 06Cr16Ni5Mo remains the lowest as the impact velocity rises, while 06Cr20Ni11 experiences the highest weight loss.This indicates that, due to differences in mechanical properties, 06Cr16Ni5Mo has the highest surface hardness, whereas 06Cr20Ni11 has the lowest surface hardness among all the materials.Therefore, as the impact velocity increases, 06Cr16Ni5Mo demonstrates the lowest weight loss and exhibits superior anti-erosion performance.
Moreover, when the sediment concentration rises from 34.48 kg/m 3 to 57.04 kg/m 3 under an impact velocity of 32 m/s and an impact angle of 30°, the weight loss of 06Cr16Ni5Mo increases by 16.46 mg.Additionally, an increase in impact velocity from 32 m/s to 40 m/s while maintaining the sediment concentration at 34.48 kg/m 3 and the impact angle at 30° results in a weight loss increase of 333.19 mg for 06Cr16Ni5Mo.Comparing the effects of changing sediment concentration and impact velocity, it becomes evident that material damage caused by variations in impact velocity is more severe.This suggests that the impact kinetic energy of sand particles and the cavitation action exert a significantly greater influence as the impact velocity rises, resulting in substantial material damage and rapid weight loss.Overall, the anti-erosion performance of all materials under impact velocity and sediment concentration conditions can be ranked as follows: 06Cr16Ni5Mo > Q355B > 06Cr20Ni11.

Effects of sediment concentration on cavitation-silt erosion behaviour
Figure 8 illustrates the surface microstructure of 06Cr16Ni5Mo, Q355B, and 06Cr20Ni11 under cavitation-silt erosion tests for 12 hours with different sediment concentrations.Comparing the SEM images under two sediment concentration conditions: relatively low (18.86 kg/m 3 ) and high (57.04kg/m 3 ).
As shown in Figure 8(a) and (b), 06Cr16Ni5Mo exhibits obvious scour marks and micro-cutting at sediment concentration of 18.86 kg/m 3 , accompanied by a little of dispersed pinhole-shaped cavitation pits on the material surface.At sediment concentration of 57.04 kg/m 3 , more obvious scour marks can be observed, along with larger and denser micro-cutting and pits of material fatigue failure.This is because impact of sand particles with a certain amount of kinetic energy on the material surface, resulting in micro-cutting under shear stress.Additionally, the collapse of bubbles causes pinholeshaped cavitation pits under normal stress.As the sediment concentration rises and the kinetic energy of sand particles increases, the size of micro-cutting also increases.The repeated impact of sand particles at the position of micro-cutting leads to stress concentration, and then cause the fatigue spalling of the material.As shown in Figures 8(c) and (d), it can be observed that Q355B has a certain number of dispersed cavitation pits and micro-cutting on its surface at sediment concentration of 18.86 kg/m 3 .At sediment concentration of 57.04 kg/m 3 , the scour marks are more obvious, with the size and density of micro-cutting increases.The metal chips appear at the front end of the micro-cutting, and with multiple pits of material fatigue spalling.This is because Q355B is a ferritic alloy steel with relatively low surface hardness and weak resistance to cavitation, resulting in a certain number of dispersed cavitation pits on the surface of material.As the sediment concentration rises, the sand particles have greater kinetic energy and enhance collision with the material surface, resulting in the formation of micro-cutting and material fatigue spalling.As shown in Figures 8(e) and (f), 06Cr20Ni11 exhibits a little of pits of material fatigue spalling at sediment concentration of 18.86 kg/m 3 , forming deep platelet pits without a large number of micro-cutting found in the previous materials.At sediment concentration of 57.04 kg/m 3 , there are obvious scour marks, micro-cutting, and a little of platelet pits.This is because 06Cr20Ni11 is an austenitic stainless steel with good toughness but low surface hardness.Therefore, a significant amount of plastic deformation occurs and a few pits of material fatigue spalling appear on the material surface at sediment concentration of 18.86 kg/m 3 .At sediment concentration of 57.04 kg/m 3 , the  Figure 9 illustrates the surface microstructure of 06Cr16Ni5Mo, Q355B, and 06Cr20Ni11 under cavitation-silt erosion tests for 12 hours at different impact velocities.SEM images were compared under two impact velocity conditions: relatively low (16m/s) and high (32 m/s).
As shown in Figures 9(a) and (b), 06Cr16Ni5Mo exhibits shallow micro-cutting marks and pinholeshaped cavitation pits caused by cavitation at an impact velocity of 16 m/s.At an impact velocity of 32 m/s, more pronounced scour marks are observed, with larger and denser micro-cutting sizes than those at the low impact velocity.This is attributed to the increased impact kinetic energy of sand particles resulting from higher velocities under the same sediment concentration conditions.The increased tangential stress leads to the formation of larger micro-cutting marks on the material surface.
As shown in Figures 9(c) and (d), Q355B exhibits shallow micro-cutting marks and cavitation pits at an impact velocity of 16 m/s.However, at an impact velocity of 32 m/s, more evident scour marks, increased micro-cutting size, and intensified cavitation damage are observed.This can be attributed to the relatively low surface hardness and weak resistance to cavitation of Q355B, a ferritic alloy steel.As the impact velocity rises, the kinetic energy of sand particles and cavitation effects increase, resulting in the formation of larger and more micro-cutting marks and cavitation pits.
As shown in Figures 9(e) and (f), 06Cr20Ni11 exhibits larger and longer micro-cutting marks compared to the other materials at an impact velocity of 16 m/s.At an impact velocity of 32 m/s, deep platelet pits and micro-cutting marks, accompanied by a significant number of deep cavitation pits, are observed.The austenitic stainless steel 06Cr20Ni11 exhibits good toughness but low surface hardness, leading to extensive plastic deformation on its surface with increasing impact velocity.
It can be observed that, at a low impact velocity of 16 m/s, the primary failure modes for the three materials are shallow micro-cutting marks along with a certain number of cavitation pits.Notably, the surface of 06Cr20Ni11 sustains more severe damage compared to the other materials.Then, at a high impact velocity of 40 m/s, the primary failure modes for the three materials involve larger and denser micro-cutting marks accompanied by significant cavitation pits resulting from increased impact kinetic energy.

Conclusion
Three commonly used materials in hydraulic machinery (06Cr16Ni5Mo, Q355B, 06Cr20Ni11) were subjected to cavitation-silt erosion tests at different sediment concentrations and impact velocities using a rotating disk facility.After analyzing the cumulative weight loss data and microstructure morphology of cavitation-silt erosion tests, the following conclusions were obtained.
(1) When the sediment concentration is below 34.48 kg/m 3 , the cumulative weight loss of the three materials increases rapidly as the sediment concentration rises.When the sediment concentration exceeds 34.48 kg/m 3 , the rate of cumulative weight loss slows down.As the impact velocity increases, the cumulative weight loss of three materials exhibits an exponential increase with the rise in impact velocity.The anti-erosion performance of all materials under varied impact velocity and sediment concentration conditions can be ranked as follows: 06Cr16Ni5Mo > Q355B > 06Cr20Ni11.
(2) With an increase in sediment concentration from 34.48 kg/m 3 to 57.04 kg/m 3 , the weight loss of 06Cr16Ni5Mo increases by 16.46 mg.Similarly, with an increase in impact velocity from 32 m/s to 40 m/s, the weight loss of 06Cr16Ni5Mo increases by 333.19 mg.The material responds more sharply to the damage caused by the increase in impact velocity.
(3) As the sediment concentration increases, the main forms of cavitation-silt erosion for the three materials are micro-cutting and pinhole-shaped cavitation pits.Higher surface hardness can result in the formation of material fatigue spalling pits, whereas lower surface hardness leads to irregular squeezing pits.With the increase in impact velocity, the higher impact kinetic energy of sand particles leads to an enlargement of marks and cavitation pits, and the surface of the material is more severely damaged than sediment concentration changes.

Future works
The effects of multiple factors on the cavitation and sediment erosion characteristics and their synergic impact on materials will be studied, the mechanism of sediment erosion will be clarified, and a prediction model for hydro turbine sediment erosion will be established.

Figure 1 .
Figure 1.Physical diagram of the rotating disk facility.

Figure 2 .Figure 3 .
Figure 2. The flow chart of the cavitation-silt erosion test system.

Figure 4 .
Figure 4. Microstructure of sediment grains for tests.(2)The metal specimens were tested with the materials of the flow parts of the Dongzhuang Hydraulic Project: 06Cr16Ni5Mo, Q355B and 06Cr20Ni11, with the surface roughness of 0.16 um.The three material mechanical properties are shown in Table2.Table2.Mechanical properties of three materials.

Figure 5 .
Figure 5.Initial surface microstructure of Three Materials.

Figure 7 .
Figure 7. Cumulative weight loss of three materials with different impact velocities.

3 Figure 8 .
Figure 8. SEM images of three materials at sediment concentrations of 18.86 kg/m 3 and 57.04 kg/m 3 .micro-cuttingincreases.The metal chips appear at the front end of the micro-cutting, and with multiple pits of material fatigue spalling.This is because Q355B is a ferritic alloy steel with relatively low surface hardness and weak resistance to cavitation, resulting in a certain number of dispersed cavitation pits on the surface of material.As the sediment concentration rises, the sand particles have greater kinetic energy and enhance collision with the material surface, resulting in the formation of micro-cutting and material fatigue spalling.As shown in Figures8(e) and (f), 06Cr20Ni11 exhibits a little of pits of material fatigue spalling at sediment concentration of 18.86 kg/m 3 , forming deep platelet pits without a large number of micro-cutting found in the previous materials.At sediment concentration of 57.04 kg/m 3 , there are obvious scour marks, micro-cutting, and a little of platelet pits.This is because 06Cr20Ni11 is an austenitic stainless steel with good toughness but low surface hardness.Therefore, a significant amount of plastic deformation occurs and a few pits of material fatigue spalling appear on the material surface at sediment concentration of 18.86 kg/m 3 .At sediment concentration of 57.04 kg/m 3 , the

Figure 9 .
Figure 9. SEM images of three materials at impact velocities of 16 m/s and 32 m/s.

Cleaning tank Mixing tank Rotating disk chamber Diaphragm pump Liquid flow meter Compressor Surge tank Cooling circulation Motor Bypass pipe
The median grain size of the sediment of Jinghe River in the late stage of sediment retention is 16 um.The sediment concentration passed through the hydraulic turbine with different operating periods is shown in Table1.

Table 1 .
Average sediment concentration in different operating periods.