Wear Properties of Metal Material in Dredging Equipment

In dredging engineering, the parts of dredging and conveying system which contact with dredging mixture often suffer from serious wear and short service life. In order to improve the durability of materials, new materials and generation processes are constantly proposed. It is always a difficult work to analyze the wear properties and mechanism of materials.In this paper, dry friction and abrasive wear tests were carried out on Cr15, Cr26, Fedur®40, medium manganese steel and XGG with Q235 carbon steel as the comparison material. The wear mechanism of materials was analyzed by scanning electron microscope (SEM) observation on the wear surface of materials. Under the test conditions, the dry friction wear rates of the five materials were 6.98×10−6 mm3/Nm,5.71×10−6 mm3/Nm,20.2×10−6 mm3/Nm,21.9×10−6 mm3/Nm and 5.39×10−6 mm3/Nm respectively.The wear rates of abrasive wear tests were 58.3×10−6 mm3/Nm,25.2×10−6 mm3/Nm,83.4×10−6 mm3/Nm,30.9×10−6 mm3/Nm and 32.1×10−6 mm3/Nm respectively. The results show that the metal surface is covered with coarse and deep cutting and chipping marks under abrasive wear, while the metal surface only has fine furrowing and a little fatigue spalling under dry friction wear. The wear rate of abrasive wear of five wear-resistant materials is much higher than that of dry friction wear. Cr26 and XGG showed better wear performance than other materials under the two test conditions.


Introduction
In dredging engineering, the equipment directly acting on the dredging medium, such as dredging system, conveying system, etc., due to the long-term impact, friction and erosion with sediment, gravel and even rock and other media, the tool teeth, flow parts often exist cracking, falling off, wear failure and short service time and other problems [1].Therefore, a large number of wear resistant materials are used in such equipment components, most of which are metal materials.With the deepening of research and technological innovation, new types and generation processes of wear resistant materials have been developed and proposed.The research on the wear properties and mechanisms of materials has always been a difficult task [2][3][4][5].
In this paper, the wear mechanism and wear properties of several metal wear-resistant materials for dredging equipment were studied and discussed under dry friction and abrasive wear conditions by sliding wear test, in order to provide reference suggestions for the evaluation and selection of material properties.

Test material
Several wear-resistant metal materials, such as high chromium cast iron [6], Fedur® plate, medium manganese steel [7] and XGG [8], which are commonly used in dredging tool teeth, mud pump flow parts and dredging pipes, were selected for the test.Among them, Fedur® plate is a kind of hot rolled double layer composite material, registered trademark of the Netherlands company GCC, its upper wear resistant material grain fine, hardness is not less than HRC62.XGG is a kind of hard particles such as chromium alloy distributed to the austenitic matrix surface through surfacing production process, formed wear-resistant composite steel plate [1].In this experiment, Q235 carbon steel was used as the comparison material, Q235, Cr15, Cr26, Fedur®40, medium manganese steel,XGG numbered 1-6 to carry out the test.

Sliding wear test
The sliding wear test referred to the standard GB/T 12444-2006, wear test equipment using M-2000 multifunctional friction and wear testing machine.The processing size and surface treatment requirements of relevant samples were carried out according to the standard.Figure 1 shows the processing size of the sample of the material to be tested, Figure 2 shows the processing size requirements of the ring friction pair of GCr15 bearing steel stipulated in the standard, and the contact mode of sliding wear is shown in Figure 3.The surface roughness of the test sample and bearing steel ring was controlled within 0.06~0.08μm,which met the test standard Ra≤0.4μm.Treatment of samples before and after the test: All samples, including the friction pair steel ring, were cleaned, dried and weighed before the wear test.All samples were weighed and placed in a drying apparatus for testing.After the wear test, all samples were cleaned, dried and weighed again, and the average weight loss of wear was taken.Wear test conditions: The test was divided into dry friction, quartz abrasive wear two conditions.The test abrasive selected quartz sand, particle size of 100-150 mesh, quartz sand flow control at 1.5-2.0kg/h.
Friction pair and operating conditions: Bearing steel GCr15 was selected as the steel ring material for friction pair, the diameter was 50mm, the speed was 200r/min, and the test load was 200N (20kg).The sliding friction and wear time was set at 2h, the test temperature was room temperature, and the relative humidity was 40-90.The test parameters are detailed in Table 1.
Table 1 Wear data processing: Archard wear model was used to calculate the wear rate [9].For different wear forms, the calculation formula of the wear rate is slightly deformed.The calculation formula of the volume wear rate of sliding friction is as follows: Where, Wv is the volume wear rate, mm 3 /Nm; Δm is wear weight loss, mg; ρ is the density of the test material, g/cm 3 ; N is applied load, N; L is wear stroke, m (L=3768m in this test).

Coefficients of sliding friction
Figure 4 shows the curves of friction coefficients over time, recorded in the dry friction test of six materials.As can be seen from the figures, the change law of the friction coefficients of the six materials is that the friction running stage increases rapidly, then decreases slightly, and then tends to be stable.The fluctuation of friction coefficients is mainly due to the change of surface roughness of material contact surface due to friction sliding, which accords with the change law of friction coefficient of tribology.

Wear losses and wear rates of sliding wear
Figure 6 shows the test results under dry friction.It can be seen that under the same wear condition, the wear weight losses and wear rates of samples 1, 4 and 5 are higher than that of samples 2, 3 and 6.

Dry friction and wear mechanism
S-3000N scanning electron microscope was used to observe the microscopic morphology of the wear surface of the materials.Figure 8-Figure 13 are the SEM images of samples 1-6, respectively.In order to present the wear morphology characteristics as far as possible, low-power and high-power images are given for each material.As can be seen from the figure, thin and shallow furrows and local fatigue spalling are distributed on the material surface.Because the metal surface treatment cannot achieve absolute smooth, there are a lot of micro concave and convex shapes on the surface.The sliding effect of micro convex metal bodies on each other may be the main cause of the formation of furrow marks.In addition, the friction sliding between the test material and the friction pair generated a large amount of heat energy, which maked the surface temperature of the sample rise rapidly.High temperature caused changes in the surface properties of the material, such as the reduction of the toughness of the local metal structure, combined with the continuous external force, resulting in fatigue spalling of the sample surface material.Therefore, the main mechanism of dry friction and wear is ploughing and fatigue spalling.
Carefully observed, the surface wear characteristics of materials 1, 4 and 5 are more obvious and more distributed than materials 2, 3 and 6.For example, the surface of material 1 and 4 show a wider range of fatigue spalling phenomena, while material 5 shows deeper and denser plough grooves, which is also consistent with the above test data.

Abrasion mechanism of quartz sand
Figure 14-Figure 19 respectively show the microstructure of abrasive wear surface of quartz sand of samples 1-6.The wear surfaces of all six materials are distributed with obvious coarse and deep cutting grooves and gouging pits, and the wear surfaces are relatively rough, with local fatigue peeling also present.In the form of abrasive wear, hard and sharp quartz sand abrasive is the main cause of material wear damage, quartz sand contains the main component SiO2, the average Mohs hardness of about 4.5, usually sharp shape, is a typical three-body abrasive, its damage to the wear surface greatly increased.
The abrasive wear mechanism is that at the moment when abrasive particles enter the friction surface, under the combined action of impact compressive stress and tangential friction force under the action of load force, impact chipping pits, particle cutting grooves, fatigue spalling and other phenomena occur on the material surface.The main influencing factors include material properties and abrasive properties.The harder and tougher the material surface is, the more reasonable the distribution of hard metallographic structure on the surface, the stronger the resistance to chipping damage, and the smaller the chipping pit is formed [10].The abrasive properties mainly refer to the shape, size and hardness of abrasive particles.
Under the condition of abrasive wear, the surface wear characteristics of the six materials are more intense and rough, which is also consistent with the test results of the wear rate is much higher than dry friction wear phenomenon.However, according to the SEM images, the surface morphology of the six materials is dominated by a large number of cutting marks in the case of abrasive wear, which also increases the difficulty in finding a basis consistent with the test results.

Conclusions
1.The wear rate of abrasive wear is much higher than that of dry friction wear, and the surface wear morphology observed by SEM also changes greatly.The metal surface is covered with coarse and deep cutting and chiseling marks under abrasive wear, while there are only fine furrowing and a little fatigue spalling under dry friction wear.
2. Under different wear forms, the wear performance and wear mechanism of materials are quite different.In the test evaluation of material durability, the test conditions should reflect the actual working conditions as far as possible.
3. The test results show that under two wear conditions, No. 3 Cr26 and No. 6 XGG both show low wear loss and wear rate, and their comprehensive wear resistance is better than other materials.Therefore, it is recommended that these two materials be applied to dredge pump overflow components and dredge ship pipelines, which can effectively improve their service life.

Figure 3 .
Figure 3.The contact mode of sliding wear.

Figure 1 .
Figure 1.Processing size of the test samples to be tested.

Figure 2 .
Figure 2. Processing size requirements of the ring friction pair.

Figure 4 .
Figure 4. Variation curves of dry friction coefficients.Figure 5 shows the comparison of the average friction coefficients of dry friction under 200N load.The average friction coefficients were taken after the data fluctuated smoothly.The friction coefficient of sample 1 is 0.25, sample 2 is 0.24, sample 3 is 0.24, sample 4 is 0.20, sample 5 is 0.26, and sample 6 is 0.25.Except sample 4 is slightly lower, the friction coefficients of other samples is basically the same.

Figure 5
Figure 4. Variation curves of dry friction coefficients.Figure 5 shows the comparison of the average friction coefficients of dry friction under 200N load.The average friction coefficients were taken after the data fluctuated smoothly.The friction coefficient of sample 1 is 0.25, sample 2 is 0.24, sample 3 is 0.24, sample 4 is 0.20, sample 5 is 0.26, and sample 6 is 0.25.Except sample 4 is slightly lower, the friction coefficients of other samples is basically the same.

Figure 5 .
Figure 5. Average friction coefficients of dry friction.
The wear weight losses of No.1, No.4 and No. 5 samples are 173.6 mg, 120.1 mg and 130.4 mg, respectively, and the wear rates are 29.2×10 - mm 3 /Nm, 20.2×10 -6 mm 3 /Nm and 21.9×10 -6 mm 3 /Nm, which are basically at the same wear resistance level.The wear weight losses of No. 2, No.3 and No.6 samples are 41.6 mg, 33.9 mg and 32.1 mg, respectively, and the wear rates are 6.98×10 -6 mm 3 /Nm, 5.71×10 -6 mm 3 /Nm and 5.39×10 -6 mm 3 /Nm, all of which are at the same wear resistance level.It can be seen that samples No. 2, No. 3 and No. 6 are more suitable for wear conditions without hard abrasive particles, and sample No. 6 has the lowest wear rate.

Figure 6 .
Figure 6.Weight losses and wear rates at 200N under dry friction.

Figure 7 6 mm 3 /
Figure 7 shows the test results in the form of abrasive wear of quartz sand.Under the same wear condition, the weight losses and wear rates of samples No. 3, 5 and 6 are significantly lower than that of samples No. 1, No. 2 and No. 4. The wear losses of No. 3, No.5 and No.6 samples are 149.5 mg, 183.6 mg and 190.9 mg, respectively, and the wear rates are 25.2×10 -6 mm 3 /Nm, 30.9×10 -6 mm 3 /Nm and 32.1×10 -6 mm 3 /Nm.The wear losses of No. 1, No.2 and No.4 samples are 209.3mg, 346.7 mg and 495.9 mg, respectively, and the wear rates are 35.21×10 - mm 3 /Nm, 58.3×10 -6 mm 3 /Nm and 83.4×10 - 6 mm 3 /Nm.It can be seen that No. 3, No. 5 and No. 6 samples are more suitable for wear conditions of hard abrasives.In addition, it can be seen from the above test results that under the two wear conditions, samples No. 3 and No. 6 both show low wear loss and wear rate, and their comprehensive wear resistance are better than other samples.

Figure 7 .
Figure 7. Weight losses and wear rates of 200N under the condition of quartz sand abrasive.

Figure 15 .
Figure 15.Worn surface morphology of quartz sand of sample 2.

Figure
Figure Worn surface morphology of quartz sand of sample 5(SEM).

.
Sliding wear test parameters.