Effect of ultrasonic surface rolling on the friction and wear properties of 8620H carburized steel at elevated temperatures

The surface strengthening treatment of 8620H carburized steel was carried out by ultrasonic surface rolling (USR) process, and the samples with and without the treatment were subjected to friction and wear experiments at 25°C, 100°C, and 150°C. The experimental results showed that the samples with USR treatment had higher surface hardness, lower surface roughness, deeper hardened layer depth, and higher surface residual compressive stress than the samples without USR treatment. Meanwhile, the wear degree of the samples deteriorated as the temperature increased, but the anti-wear performance of the samples with USR treatment increased by 52.9%, 29.8%, and 14.9% compared with the samples without USR treatment at 25°C, 100°C, and 150°C, respectively, indicating that the USR treatment still had a better strengthening effect at high temperatures.


Introduction AISI 8620H
steel is an alloy structural steel renowned for its high hardness and strength, making it a widely used material in the manufacture of gears and bearings.Furthermore, it proves useful in manufacturing mechanical parts for challenging environments such as those with high temperatures and pressures.This steel exhibits good oxidation resistance at elevated temperatures and can endure a certain amount of thermal fatigue and creep [1] .However, improving the service life of parts under elevated temperatures is very important to reduce costs and improve efficiency, as the service life of parts under elevated temperatures is generally reduced.Ultrasonic surface rolling is a process that uses an ultrasonic tool to apply constant pressure to the surface of a part and vibrate at an ultra-high frequency.This process can increase the hardness of the parts' surface, reduce the surface roughness, and form a deeper hardened layer and higher residual stress on the parts' surface to achieve the effect of improving the wear resistance.Currently, numerous reports examine the friction and wear performance of steel with surface strengthening treatment.For instance, LI Y et al. [2] studied the effect of laser peening on the wear resistance of 20CrNiMo steel, and the results indicated that the wear resistance of the samples improves with an increasing laser impact overlap rate.Zhang Y L et al. [3] have investigated the influence of shot peening (SP) on the friction and wear properties of 17Cr2Ni2MoVNb steel, which showed that the SPtreated samples have better wear resistance.Hu X F et al [4] conducted a study on the influence of discrete surface laser quenching on the friction and wear performance of 25CrNi2MoV steel and found that the use of a discrete surface laser quenching process led to a substantial decrease in the wear volume of the samples by approximately 93.2%.As for the study of the wear resistance of steel at different temperatures, this also has a lot of research such as He Q [5] et al. and Emami M [6] et al. studied the wear resistance of 9Cr18Mo steel and 31CrMoV9 nitride steel at various high temperatures, respectively.However, there is limited research on the investigation of friction and wear characteristics of steel at varying temperatures after USR treatment.Therefore, in this paper, we initially strengthened 8620H carburized steel samples utilizing ultrasonic surface rolling.Subsequently, we performed friction and wear experiments on the above samples with and without the USR treatment to investigate their friction and wear performance at 25°C, 100°C, and 150°C.

Preparation of the sample
The chemical composition of AISI 8620H steel is shown in Table 1 [7] .The material selected consisted of a 50 mm diameter bar.All samples were dis-shaped, measuring φ45 mm × 6 mm, cutting from this bar.The samples received a carburizing heat treatment, and the process is depicted in Figure 1.First, place the samples in a carburizing furnace set at 940°C for 3.5 hours, then cool to 850°C and keep for 0.5 hours.Thereafter, quenched in oil and maintained at 160°C for 2.5 hours before air-cooling and tempering.After the heat treatment process, the samples underwent uniform grinding on a surface grinder to eliminate the decarburization layer on the surface.Following that, an ultrasonic cleaner was employed to complete the pretreatment of the samples.Subsequently, the samples were cleaned and blown dry, resulting in a surface hardness of approximately 62 HRC after the completion of the pretreatment process.The ultrasonic surface rolling apparatus is utilized to conduct USR processing on the sample, and its processing principle is illustrated in Figure 2. The device's front end features a 10 mm diameter silicon nitride ceramic rolling ball with a hardness of approximately 95 HRC.An air compressor and an ultrasonic generator are employed to ensure a constant static pressure is exerted on the rolling ball, which carries out high-frequency vibration on the sample's surface whilst maintaining a constant pressure.The high-frequency vibrations cause grain refinement and plastic deformation of the sample's surface material.In USR processing, the sample is secured onto the machine tool and co-rotated, while the ultrasonic rolling device applies perpendicular pressure onto the sample's surface.The rolling device then moves reciprocally along the radial direction of the sample, permitting a specific range of multipass USR processing on its surface.During USR processing, heat can easily accumulate on the surface of contact.To prevent the ball and sample from reaching high temperatures, lubricating oil can continuously flow out from around the ball.This process helps avoid high temperatures and provides lubrication and cooling for the rolling ball and sample.The optimized processing parameters for the USR, derived from laboratory experimentation, are presented in Table 2 below.

Friction and wear tests
The test at elevated temperatures of 8620H carburized steel was performed using the MMU-5G friction and wear tester.The counter-abrasion form was applied using the pin and disc contact mode, and the custom-made GCr15 ball-end pin was used as the counter-abrasion sample.The pin is 18 mm long, with a head diameter of 3 mm, and heat-treated to achieve a hardness of 60-62 HRC.The ball-end region was polished and ground to a surface roughness of Ra 0.8 μm.At the start of the experiment, the testing machine's hydraulic spindle exerts a vertical upward load on the lower fixture.It then moves the disc sample upward, bringing the two samples in close contact.Subsequently, it applies a force to achieve and maintain the pre-determined load between the sample and the pin.Last, the heating furnace is activated to elevate the temperature.Once the heating furnace has reached its set temperature and remained stable for at least 5 minutes, the upper fixture's spindle will engage, causing the pin to rotate on the sample's surface.This enables the performance of friction and wear tests under high-temperature work conditions.The friction wear tester records real-time data on various test parameters including loading load, spindle speed, friction coefficient, test temperature, and test duration.The device automatically stops the test when the predetermined time limit is reached.Table 3 displays the friction wear test parameters applied in this study.2, but compared with the effect of USR treatment in [8], there are still more unclosed scratches on the surface of this experiment, which does not achieve the effect of complete smoothing.This is due to the high hardness of the surface of the 8620H carburized sample, which reduces the ability of the USR processing to close the abrasive marks on the sample surface [9] .a) Untreated sample b) USR-treated sample  4 displays the surface hardness, surface roughness, and surface residual compressive stress of the samples with and without USR treatment.As depicted in the table, the sample's hardness augmented from 760 HV0.2 to 834 HV0.2, accompanied by a decline in its surface roughness from 0.798 to 0.406 and an elevation in its residual compressive stress from -207 MPa to -621 MPa.These results demonstrate that USR treatment can significantly enhance the surface properties of 8620H carburized steel, and thus increase the samples' resistance to wear and fatigue [10] .The cross-sectional organization of samples with and without USR treatment is shown in Figure 5, the organization of untreated sample mainly consists of slaty martensite and residual austenite, and the density distribution of the organization is relatively uniform in the cross-sectional, whereas the organization of the USR-treated sample produces a significant change in the range of 24 μm from the surface, in which the distance between martensite is significantly reduced.In this range, the martensite morphology becomes finer and shorter than that of the untreated sample, and the volume of residual austenite is significantly smaller or even disappears, indicating that USR treatment under this parameter can produce obvious organization refinement of the surface organization of the 8620H carburized steel, and enhance its surface hardness, thus improving its wear resistance.

Frictional performance
Figure 6 shows the sliding friction coefficients of the samples with and without USR treatment at different temperatures.As shown in Figures 6(a), 6(b), 6(d), and 6(e), the friction coefficients of the samples at 25°C and 100°C increased slowly at the beginning of the experiment and then remained stable, indicating that the initial break-in phase lasted for a long time at these two temperatures.This is because the surface of the samples has a higher hardness at relatively low temperatures.At the beginning of contact between the two samples, the contact mode between the ball joint pin and the surface of the sample is the point contact mode.At this time, the friction force is small.As wear progresses, the contact form of the sample gradually changes to the surface contact mode, and the friction force also gradually increases, resulting in a longer time in the initial break-in phase.While at 150°C, as shown in Figure 6(c) and (f), the break-in phase is shorter and the range of fluctuations in the friction coefficient is significantly smaller compared to the samples at 25°C and 100°C, which demonstrates that the wear process of the samples is smoother under high-temperature conditions.This is because the sample reaches a low-temperature tempering temperature at 150°C, causing a gradual decrease in its hardness.At the same axial load, the surface material is also more susceptible to abrasion, resulting in a reduced friction force and less sudden changes.In addition, comparing Figures 6(a 6(e), 6(f), it can be seen that the fluctuation range of the coefficient of friction of the samples with USR treatment is smaller than that of the samples without USR treatment at these three temperatures, which suggests that the USR treatment can make the wear process of the samples smoother because the USR processing has greatly reduced the roughness of the sample surface, which in turn suppressed the possibility of the friction to be subjected to sudden changes.Figure 7 shows the average friction coefficients in the steady-state of the samples with and without USR treatment at various temperatures, and the results demonstrate that the average friction coefficients in the steady-state of the samples with and without USR treatment at 25°C, 100°C, and 150°C, are 0.521, 0.498, 0.450 and 0.496, 0.454, 0.438 respectively.This shows that the friction coefficients of the samples at various temperatures were reduced after USR treatment, indicating that the USR treatment can significantly enhance the friction performance of the samples at various temperatures.Moreover, as the temperature increases, the average coefficient of friction in a steady state decreases gradually.This implies that under high-temperature conditions, the friction force produced by the samples is lesser than that under low temperatures.Such a decrease may be attributed to the reduction in surface hardness of the samples when exposed to high temperatures or the development of an adhesive layer on the sample surface.

Wear performance
The 3D morphology of sliding wear of samples with and without USR treatment at various temperatures is shown in Figure 8, from which the wear area and depth of wear on the surfaces of the samples increase gradually with the increase in temperature, which indicates that the samples experience a higher degree of wear as the temperature increases.This is because as the temperature increases, the surface hardness of the sample diminishes, which reduces the specimen's ability to resist wear.In addition, the ability of the sample to resist crack propagation diminishes as the temperature increases, which may also contribute to the greater degree of wear on the sample.Comparing Figures 8 (a1), 8(b1), 8(c1) and 8 (d1), 8(e1), 8(f1) show the corresponding wear contours of the samples with and without USR treatment at three temperatures, respectively, and it can be seen from the contour plots that the average depth of the wear pits of the samples with USR treatment is smaller than that of the samples without USR treatment at all three temperatures.Calculate the area enclosed by the contour curve and the straight line with the depth of the abrasion mark of 0 and integrate in the circumferential direction to find out the wear volume of the sample under various working conditions.As shown in Figure 9, the wear volumes of samples with and without USR treatment are 0.129 mm 3 , 0.219 mm 3 , 0.355 mm 3, and 0.274 mm 3 , 0.312 mm 3 , 0.417 mm 3 at 25°C, 100°C, and 150°C, respectively.The wear volumes of the samples at 25°C, 100°C, and 150°C after USR treatment are reduced by 52.9%, 29.8%, and 14.9%, indicating that the USR treatment can significantly enhance the wear performance of the samples at various temperatures, but its enhancement effect decreases with the rise of temperature.

Analysis of wear mechanisms
The wear morphology of the samples without USR treatment at various temperatures is shown in Figures 10(a), 10(b), and 10(c), from which the wear surfaces of the samples at the three temperatures have a lot of adhesive layers and ploughings, which indicates that the samples have obvious adhesive wear and abrasive wear mechanisms at the three temperatures.The EDS elemental analyses of the adhesive layer of the samples at these three temperatures are shown in Figure 10(a1), 10(b1), and 10(c1), respectively, which show that the adhesive layer contains a high concentration of oxygen, indicating that there is a significant oxidative wear phenomenon for the untreated samples at all three temperatures.This is because the wear surface of the sample is in contact with the air and thus oxidation occurs.However, comparing Figures 10(a), 10(b), and 10(c), the area of spalling pits on the sample surface expands with the increase of temperature, and at 150°C, the surface of the sample has a long strip of spalling cracks and the phenomenon of delamination occurs, indicating that the phenomenon of fatigue wear occurring in the untreated sample is more pronounced with the increase of temperature.This is because as the temperature increases, the surface hardness of the sample decreases and the residual compressive stress on the surface is slowly released, so the ability of the sample to resist crack extension also decreases and the spalling pit gradually becomes larger.
Figures 10(d), 10(e), and 10(f) show the wear morphology of the samples with USR treatment at various temperatures.As shown in the figure, the wear surfaces of the samples have obvious adhesive layers and ploughings at all three temperatures, which indicates that the samples with USR treatment still have the mechanisms of adhesive wear and abrasive wear at all three temperatures, but compared with the samples without USR treatment, the area of adhesive layers on the surfaces and the amount of the plowings are smaller at all three temperatures.This is because the USR treatment improves the hardness of the sample surface, which produces less hard abrasive debris during the experiment.Therefore, less hard abrasive debris adheres to the sample surface, and it is also more difficult for the hard abrasive debris to produce significant plowing of the sample surface.Figures 10(d1), 10(e1), and 10(f1) show the EDS elemental analysis of the adhesive layer in Figures 10(d), 10(e), and 10(f), respectively, from which all the adhesive layers have a certain content of oxygen elements, but compared to the samples without treatment, the oxygen content overall decreased, which indicates that the USR treatment has improved the samples' resistance to oxidative wear.
In addition, the figures show the presence of small spalling pits on the surface of samples with USR treatment at 25°C and 100°C.At 25°C, powdering was observed around the spalling pit on the sample surface, whereas at 100°C, the entire spalling pit was heavily powdered and a large number of powdered particles were observed around the spalling pit.At 150°C, the number and area of the spalling pits on the surface increase, and cracks appear around them.This highlights the presence of fatigue wear phenomenon on the USR samples at all three temperatures, with the degree of fatigue wear increasing as the temperature rises.This is because even though the samples with USR treatment exhibit an increase in surface hardness and residual compressive stress, the efficacy of this strengthening effect declines as the temperature rises.Consequently, under high-temperature working conditions, the surface of the samples with USR treatment still exhibited large spalling pits.However, comparing Figures 10(a

Conclusions
The USR treatment was employed to strengthen the surface of 8620H carburized steel.To evaluate the effectiveness of this treatment, high-temperature abrasive wear experiments were conducted on samples with and without USR treatment at temperatures of 25°C, 100°C, and 150°C.The findings are presented below.
(1) The USR treatment can enhance the hardness and residual compressive stress of the sample, decrease roughness, and refine grain organization, ultimately bolstering sample wear resistance.
(2) The wear volume of the samples rose as the temperature increased.However, the wear volume of the samples with USR treatment decreased by 52.9%, 29.8%, and 14.9% at temperatures of 25°C, 100°C, and 150°C, respectively compared to the samples without USR treatment.
(3) The degree of adhesive wear, abrasive wear, oxidative wear, and fatigue wear of the samples with and without USR treatment increased as temperature rose, but the degree of occurrence of the above wear mechanisms in the samples with USR treatment was reduced as compared to that in the samples without USR treatment.

Figure 2 .
Figure 2. Schematic diagram of USR processing.Table2.Parameters of USR processing.

Figure 3 Figure 3 .
Figure 3. Experimental sample and schematic diagram of the tester.

Figure 4
displays the surface SEM morphology of the samples with and without USR treatment.It is clear that the surface of the sample without USR treatment has more noticeable and deep scratches, while after USR treatment, most of the scratches on the surface of the sample disappeared, and only a few deep scratches remain on the sample surface, which indicates that the sample surface can achieve a relatively flat effect under the USR parameters shown in Table

Figure 4 .
Figure 4. Surface morphology of samples.Table4displays the surface hardness, surface roughness, and surface residual compressive stress of the samples with and without USR treatment.As depicted in the table, the sample's hardness augmented from 760 HV0.2 to 834 HV0.2, accompanied by a decline in its surface roughness from 0.798 to 0.406 and an elevation in its residual compressive stress from -207 MPa to -621 MPa.These results demonstrate that USR treatment can significantly enhance the surface properties of 8620H carburized steel, and thus increase the samples' resistance to wear and fatigue[10] .Table4.Surface properties of different samples.

Figure 5 .
Figure 5. Cross-sectional organization of samples with and without USR treatment.

Figure 6 .
Figure 6.Sliding friction coefficients of samples at various temperatures.

Figure 7 .
Figure 7. Average friction coefficients in steady-state samples at different temperatures.
Figures 8(a), 8(b), and 8(c) with Figures 8(d), 8(e), and 8(f), the wear degree of samples with USR treatment at various temperatures is reduced compared to untreated samples, which is most obvious at ambient conditions.

Figure 8 .
Figure 8. 3D morphology of sliding wear of samples at different temperatures.

Figure 9 .
Figure 9. Wear the volume of samples at different temperatures.

Figure 10 .
Figure 10.Wear patterns of samples with and without USR treatment at various temperatures.

Table 2 .
Parameters of USR processing.
4.1 Ultrasonic surface rolling strengthening properties.

Table 4 .
Surface properties of different samples.