Sliding Friction and Wear Properties of 40CrNiMo Steel after Laser Hardening against GCr15 Steel under Oil Lubrication

Abstract

40CrNiMo steel, which is a commonly used material for sprag clutch wedges, is widely used in practice, and the surface wear will seriously affect the performance of the product. In this study, the surface of 40CrNiMo steel was strengthened at two scanning speeds by laser hardening. After laser hardening, the surface hardness can reach the use requirement of the clutch wedge. By changing the speed and temperature, the friction and wear testing machine was used to study the wear behavior of 40CrNiMo steel after laser hardening against GCr15 steel under oil lubrication, the friction coefficient, wear amount and wear morphology under different conditions were analyzed. The results show that, at the normal temperature, with the increase in the sliding speed, the friction coefficient showed a gradually decreasing trend, the width of the wear surface of the steel increased, and the wear amount increased. The increase in temperature led to an increase in the friction coefficient and the fluctuation, wear width and wear amount of the hardened layer. At the temperature of 20 °C, abrasive wear was the main form of wear, and the furrow formed special channels for the lubricating oil to help reduce the friction coefficient under oil lubrication. At high temperatures, the depth of the furrow further increased, and the degree of adhesive wear and oxidative wear gradually increased. The research provides a reference for the application of material in the sprag clutch.

1. Introduction

Wedge wear is an important factor affecting the performance of a sprag clutch [1]. The use of laser hardening technology has broad prospects for the reinforcement of a clutch wedge. Under oil lubricated conditions, the clutch wedge is in close contact with the inner and outer ring, and sliding wear becomes an inevitable phenomenon. 40CrNiMo steel is a kind of multi-purpose steel [2], and its performance is a hot research issue [3].

Chen et al. [4] found that multiple laser peening had a significant effect on the microhardness, residual stress and microstructure of 40CrNiMo steel. After three repeats of laser peening, the surface microhardness, compressive residual stress, strength and ductility were significantly improved. With an increase in the number of impacts, the crater depth and surface roughness increased. Wang et al. [5] conducted isothermal compression tests of as-cast 40CrNiMo steel at different temperatures. The true stress–strain curve exhibited a typical low deformation temperature and high strain rate. As the deformation temperature increased or the strain rate decreased, the true stress–strain curve gradually transformed into a dynamic recrystallization type.

Liu et al. [6] used austenite tempering above and below the martensitic transformation temperature for 40CrNiMo steel and obtained bainite and martensite multiphase with different volume fractions organization. The effects of preexisting martensite on the subsequent bainite microstructure transformation and mechanical properties were also studied. In the specimens austenitized below Ms, the significant improvement in the mechanical properties was mainly attributed to the formation of pre-existing martensite, thus, leading to an effective reduction in the size of the bainite plate and the size of the martensite/austenite composition.

Li et al. [7] analyzed the low temperature laser peening (CLP) effects on the high temperature wear properties and microstructure response of 40CrNiMo steel. The high-temperature wear characteristics and microhardness in the depth direction were measured, and the worn surface morphology was observed using scanning electron microscopy. The microstructure was observed by transmission electron microscopy. The experimental results showed that CLP can effectively improve the microhardness, and CLP generated a large number of twins, a higher density of dislocation structure and ultrafine grains, which significantly reduced the wear mass loss, thereby, improving the high temperature wear resistance.

Some scholars have also studied the wear of metal tribo-pair. Ferreira et al. [8] performed laser cladding on ultra-low carbon maraging steel 18Ni300 and H13 steel, dry sliding wear test using a pin-on-disk device, showed that H13 steel has a specific wear rate two orders of magnitude lower than that of 18Ni300 steel, the failure mode of 18Ni300 steel was mainly wear, while H13 steel was mainly a fatigue problem. Klug et al. [9] studied the sliding wear resistance of AISI M2, AISI M3:2 and AISI M35 steels under cryogenic treatment under dry sliding conditions. This depended on the HSS steel grade, as well as the contact conditions and wear pattern, and cryogenic treatment improved the dynamic wear and galling resistance.

Jia et al. [10] performed shot peening with different parameters on 4Cr9Si2 martensitic engine valve steel at high temperatures and experimented with a pin-on-disk wear tester. The results showed that shot peening refined the top surface grains and generated a large number of dislocations. At the same time, the compressive residual stress and microhardness of the treated surface increased, respectively. At high temperature, the hardness increase provided resistance to plastic deformation and improved the performance of the oxide film.

Liu et al. [11] used a fiber-based laser system to fabricate Ni60A coatings on 20CrNiMo steel and found that the coatings exhibited lower average friction coefficients and higher wear resistance compared to the substrate under high load and high temperature conditions. Pan et al. [12] analyzed the influence of the geometrical parameters of the surface microstructure on the wear of the steel ball and established a mathematical model including the geometrical parameters of the microstructure to calculate the surface wear of the microstructure, which can be used to predict the wear of the steel ball and the ball deployment wheel life as well as to aid design.

Meng et al. [13] used ultrasonic surface rolling technology to fabricate reinforced surface and line array textured surface on AISI 1045 substrate, and conducted a ball–disk reciprocating friction test. The results showed that textured surfaces with good quality and precision could be fabricated by ultrasonic rolling. The microhardness and residual compressive stress of the ultrasonically rolled surface will increase. Textured surfaces play an active role in improving tribological properties.

Gao et al. [14] found that sliding wear of nano-layered structure in martensitic steel induced grain coarsening through TEM and TKD, which is dominant without involving thermal effects. The nano stack structure did not show strong texture before or after roughening. Continuous grain growth slip occurs even when the stress is reduced during lubrication. The high density in the nanolayered structure promotes grain boundary movement and promotes grain coarsening. A variety of strengthening methods have been adopted for the study of the wear characteristics of steel, and a variety of experiences have been accumulated.

D. Jack et al. [15] studied a fan made of corten steel and found that the reason for the serious wear in the middle of the fan was the erosion of the oil fume caused by water contamination. Jamari, J et al. [16] researched the effect of surface texturing as dimples on the wear evolution of total hip arthroplasty. The results showed that the addition of dimples reduced the contact pressure and wear. Ammarullah. M et al. [17] analyzed the Tresca stress of metal-on-metal bearings with CoCrMo, SS 316L andTi6Al4V using a finite element model under the physiological conditions of the human hip joint during normal walking. The results show Ti6Al4V-on-Ti6Al4V had the best performance to reduce Tresca stress.

Under lubrication conditions, Cheng et al. [18] found that the friction pair of PTFE composites reinforced by glass fiber and chrome-plated nickel-based stainless-steel plate (18Cr2Ni4WA) has better wear performance in dry conditions than in oil lubrication. Under oil lubrication, the 90° texture direction of the steel plate relative to the motion direction was the texture direction with the best tribological performance. Shao et al. [19] prepared Cu10Al-MoS2 coatings on Q235 steel by laser cladding and conducted friction and wear tests on a ball–disk wear tester under oil lubrication.

The results showed that the hardness of the Cu10Al-Ti-MoS2 coating increased with the mass fraction of Ti, while the friction coefficient and wear rate demonstrated the opposite. Hua et al. [20] used an SPI fiber laser for laser texturing to produce a spherical convex texture on the surface of GCr15 steel. Friction and wear tests were performed to simulate the actual situation of sliding friction under full oil lubrication. Experiments showed that under full oil lubrication, the friction coefficient (FC) of raised texture specimens is positively correlated with texture density, and laser texture could greatly improve the surface wear resistance and friction reduction effect of sliding friction pairs.

Cheng et al. [21] studied the wear behavior of peak-aged Cu-15Ni-8Sn alloys prepared by powder metallurgy under oil lubrication. The results showed that the friction coefficient and wear rates of Cu-15Ni-8Sn alloy in the range of a normal load of 50–700 N and sliding speed of 0.05–2.58 m/s were both less than 0.14 and 2.8 × 10⁻⁶ mm³/mm and developed similar Stribeck curves and wear diagrams to describe the oil lubrication mechanism and wear behavior, where oil was squeezed into microcracks under severe wear conditions.

Laser surface treatment has also been used by some scholars. D. Janicki et al. [22] introduced research on the processing of titanium grade 1 with iron-nickel using diode lasers of different powers. The treated material has the characteristics of good adhesion, no pores, uniformity and no cracks. Research confirmed that the reasons for the increase in surface microhardness were the formation of new images and solid solutions. Unlike conventional quenching, as a fast and convenient surface strengthening method, laser hardening can improve the surface properties through rapid heating and rapid cooling [23].

Research on the friction and wear properties of laser-hardened steel by changing the friction conditions is rare, and the performance of laser-hardened steel under high temperature oil lubrication is not fully understood. There was no obvious conclusion regarding whether the friction and wear properties of laser-hardened steel and conventional hardened steel were consistent. In this paper, the 40CrNiMo steel that met the surface hardness requirement for clutch wedge after laser hardening was chosen, and the friction test of the sliding friction between 40CrNiMo steel and GCr15 steel under oil lubrication was performed.

The influence of different conditions on the friction behavior of 40CrNiMo steel was studied by changing the sliding speed and temperature, and the wear mechanism and lubrication mechanism of 40CrNiMo steel were summarized and analyzed under oil lubrication. This is expected to provide a basis for the use of the material in a clutch wedge.

2. Experimental Part

2.1. Specimen Preparation

In the experiment, the annealed 40CrNiMo steel was taken as the research object, and the GCr15 steel ball was used as the counter-material. The materials were provided by Daye Special Steel Co. Ltd., Huangshi, China. From the product qualification certificate, the chemical compositions of the tested materials were known. The main chemical components are listed in

Table 1. Element content (wt.%).

	C	Si	Mn	Cr	Ni	Mo	Fe
40CrNiMo	0.39	0.23	0.68	0.77	1.33	0.18	Balance
GCr15	0.97	0.25	0.4	1.46	0.07	0.01	Balance

.

The radius of GCr15 steel ball specimen was 6.25 mm, and the surface roughness was 0.32 μm. The initial hardness of 40CrNiMo steel was about 320 HV0.5, and the specimen was cut to a size of 50 × 40 × 8 mm by a DK7720 wire cutting machine, and was finished with a M7120 surface grinder to a roughness of 0.32 μm. The ball was heat-treated as follows: oil quenching at 1150 °C + tempering at 565 °C for 2 h, and the hardness was 63HRC. The specimens were sanded and polished, then sonicated with absolute ethanol clean for 5 min to remove tiny particles and contaminants on the surface, and the Contour GT-K 3D profilometer (Bruker, Karlsruhe Germany) was finally used to measure the surface roughness of the specimen to ensure Ra ≤ 1 μm.

The 40CrNiMo steel was conducted a laser quenching experiment using a ZKSX-3000-D01 fiber laser (Jiangsu ZhongkeSixiang Laser Technology Co. Ltd., Zhenjiang, China). The scanning speeds of the laser were 700 and 800 mm/min, the laser power was 2010 W, and the laser spot size was 1 × 14 mm. The sample was placed on a pad to prevent the equipment from being affected by the elevated temperature at the bottom of the sample. The laser spot moved from the middle position of the sample long side to the middle position of the other long side. During the experiment, the sample was in contact with the air.

After treatment, the surface Vickers hardness of the specimens after laser scanning speeds of 700 and 800 were 753 ± 7 HV0.5 and 797 ± 8 HV0.5, respectively, namely 62.2 ± 0.4 HRC and 63.8 ± 0.5 HRC. Generally speaking, the hardness requirement of the clutch wedge surface was between 61–66 HRC, and it can be seen that the surface hardness requirement of the clutch wedge was reached.

In order to obtain the characteristics of the laser hardened layer, a specimen of the hardening layer at a speed of 700 mm/min was chosen. First, a disc with a diameter of Φ3 mm and a thickness of 0.4 mm was cut using a DK7720 computer numerical control (CNC, Changde Machinery Co. Ltd., Taizhou, China) wire electrical discharge machining machine, and sandpaper with 600#, 1000# and 2000# was used to grind and polish both sides of the samples to about 40 μm. The specimen was pitted with the GATAN656 pit meter, and then the specimen was thinned to a thin enough area with GATAN695 ion thinner, and the specimen was observed with the JEM-F200 or JEM-2100 transmission electron microscope (JEOL, Tokyo, Japan).

2.2. Experiment Method

Considering the wedge of the clutch had continuous sliding friction in the overrunning state, the sliding friction was studied. Based on the sprag clutch working condition, the wear testing and the conditions was chosen.

The brief systematic figure to illustrate the workflow of experimental testing is shown in Figure 1.

The ball–disk reciprocating friction test was performed by using a CFI-1 friction and wear testing machine. The GCr15 steel ball was fixed and kept still, and the 40CrNiMo steel specimen after laser hardening was fixed in the lower plate. During the experiment, the specimens were immersed in lubricating oil to achieve a lubricated state. The Great Wall 4011 synthetic aviation lubricant was used in the experiment. The lubricating oil was a new type of low-viscosity lubricating oil refined from synthetic oil as base oil, and a variety of high-efficiency additives were added in the lubricating oil.

The kinematic viscosity of the lubricating oil is shown in

Table 2. Kinematic viscosity of the lubricating oil.

Project (mm/s2)	Temperature (°C)	Value
Kinematic viscosity	200	1.5
	110	4.11
	20	18.24

.

The flash point of the oil was 235 °C. The neutralization value of the oil was 0.26 mg KOH/g. The load was applied by the supporting spring, the friction force measurement processing system continuously recorded the change of the friction coefficient, and the friction environment was heated, and the temperature was controlled by the connecting temperature controller.

Two types of experiments were designed to study the effects of movement speed and temperature on the friction behavior of 40CrNiMo steel under oil lubrication. In the first category, the fixed load was 20 N, and the sliding speed was determined as a variable. For the 40CrNiMo steel after laser hardening at 700 and 800 mm/min, in order to study the effect of movement speed change on friction and wear, 300 mm/min, 400 mm/min and 500 mm/min were taken as the relative movement speeds, and the experiment time was 30 min.

For the second type, the fixed relative movement speed was 500 mm/min, in order to investigate the effect of temperature change, three temperatures of 20, 110, and 200 °C were chosen in the experiment. The temperature was regulated through the temperature controller. The specific test conditions are shown in

Table 3. Sliding speed effect experiment.

Load (N)	Laser Scanning Speed (mm/min)	Sliding Speed (mm/min)	Time (min)	Temperature (°C)
20	700	300	30	20 ± 1
		400
		500
	800	300
		400
		500

and

Table 4. Temperature effect experiment.

Load (N)	Laser Scanning Speed (mm/min)	Sliding Speed (mm/min)	Time (min)	Temperature (°C)
20	700	500	30	20 ± 1
				100 ± 2
				200 ± 2
	800			20 ± 1
				100 ± 2
				200 ± 2

. The choice of relative velocities was determined according to the typical relative velocities between the wedge and inner ring in the overrun state in previous studies.

The choice of temperature was determined according to the working temperature range of temperature resistant sprag clutch. The temperature range was 20–200 °C in our studies. Experiments were repeated three times per condition. The ambient temperature was controlled to 20 °C by Gree KFR-72LW air conditioning. The ambient humidity was controlled to 55% using a Yangtze CS10E dehumidifier.

A Bruker Contour GT-K 3D Optical Microscope was used to analyze the 3D profile of the wear scar after the friction experiment. Then, the wear condition after wear was determined, and the wear rate was calculated using Formula (1).

K = V/t

(1)

where V is the wear volume and t is the friction time of the GCr15 steel ball on the 40CrNiMo steel specimen.

The surface morphology of each sample was observed after the test using JEOL JSM-IT500 scanning electron microscope.

3. Results and Discussion

3.1. Organizational Observation

Through transmission electron microscope observation, the microscopic state before and after laser hardening can be seen, as shown in Figure 2.

The morphology before laser hardening can be seen from Figure 2a. There was cementite with higher carbon content on the ferrite. The cementite started to grow from the interface. There was no obvious lath-like material before laser hardening.

From Figure 2b, it can be seen that the lath martensites were generally arranged in parallel bundles with different sizes of martensite after laser hardening. Both lath martensite with a width of about 71 nm and lath martensite with a width of 433 nm can be generated. The martensite bundles growths were not very uniform. This was because the martensite formation order was different, and thus the space that martensite can grow was also different. It can be seen that dislocation was a common phenomenon. Deformation also occurred; however, no cracks appear, and thus the martensitic bundles had some plasticity.

It can be inferred that, after laser hardening, the original structure underwent phase transformation, and the internal stress was high.

3.2. Friction Coefficient

(1)

Friction coefficient at different movement speeds

Under dry sliding, the friction coefficient of the matrix material before laser hardening is shown in Figure 3.

As shown in Figure 3, the friction coefficient of the matrix material first increased significantly, and then the curve flattened with some fluctuation within a certain range. As the sliding speed increased, the value of the friction coefficient decreased.

Figure 4 shows the change of the typical friction coefficient between GCr15 steel and 40CrNiMo steel with laser scanning speeds of 700 and 800 mm/min. We found that, in the experimental range, when the load remained unchanged, with the increase in the sliding speed, the friction coefficient showed a decreasing trend; however, the increasing trend gradually became smaller. Compared with the friction coefficient of the matrix under the same sliding speed under dry sliding, the friction coefficient under oil lubrication decreased significantly.

Yang et al. [24] studied the grinding situation between laser-hardened steel with a surface hardness of 640.3–706.08HV and GCr13 steel. The test load was 15 N, the rotating speed of the turntable was 50 r/min, and the wear time was 20 min. The dry friction coefficient was between 0.4 and 0.6. The surface hardness of the material was slightly lower than that of the material after laser hardening in this experiment, and the experimental conditions were also different; however, the friction coefficients were far from the friction coefficient under oil lubrication conditions. Thus, it can be inferred that the dry friction coefficient was significantly greater than that under oil lubrication at room temperature.

In fact, the contact area of the friction pair surface was not the nominal contact area of the two contact surfaces but should be the sum of the micro-area of the contact spots formed by the contact of some surface profile peaks. When the contact area was plastically deformed by pressure, adhesion occurred on the contact surface, and the node was formed. When relative sliding occurred between the contact surfaces, these nodes were sheared apart. The more nodes, the greater the shear force required and the greater the coefficient of friction.

Under the oil lubrication, the overall surface clearance of the friction pair was very small, there were lubricating oil films between some contact surfaces, and there was mutual contact of the microconvex peaks of the remaining contact surface. As a result, the boundary film with low shear strength formed can effectively reduce the contact area of micro-convex peaks on the friction surface under oil lubrication, resulting in fewer nodes. Compared to dry sliding, the friction coefficient can be greatly reduced under oil lubrication. In addition, during the whole process, due to the flattening effect of the microscopic elastohydrodynamic local pressure on the roughness peak, the actual roughness was greatly reduced, which was an important reason for the decrease in the friction coefficient under the oil lubrication.

From the fluctuation of the friction coefficient, the fluctuation range of the friction coefficient was small under the oil lubrication, which was mainly because the lubricating oil film generated between the contact surfaces of the friction pair during the friction process. In addition, the lubricating oil can quickly take the heat generated in the friction process away from the contact area, so that the friction surface was always kept at a relatively low temperature, and there was no serious plastic deformation and melting of the wear surface similar to the dry sliding friction process; therefore, the friction coefficient fluctuated less.

(2)

Friction coefficient at different temperatures

Figure 5 shows the typical friction coefficient of 40CrNiMo steel Harden after laser scanning speeds of 700 and 800 mm/min. The sliding speed was 500 mm/min, and the temperatures were 20, 110 and 200 °C.

As shown in Figure 6, with the increase in temperature, the friction coefficient increased significantly. This was because at high temperature the volume of the lubricating oil increased, the distance between the molecules increased, and the attraction between the molecules decreased, and thus the lubricating oil became thinner, the viscosity decreased, and the lubricating effect of the lubricating oil reduced. In addition, the hardness and strength of the material decreased, the plasticity increased, the deformation resistance would decrease. However, when the temperature rose, the stable sliding period but not the stable wear period still existed.

There was a larger fluctuation in the friction coefficient with increasing temperature. When the temperature reached 200 °C, the friction coefficient no longer fluctuated at a lower value, the fluctuation range was very obvious, and even the friction coefficient exceeded the maximum value of the running-in period. This was because the temperature rose, on the one hand, thermal expansion increased the microscopic free volume of the object. The thermal motion capability of each motion unit was improved, and the activity space of each motion unit was increased. On the other hand, the lubricating effect of lubricating oil was significantly reduced.

In order to analyze the change of the friction coefficient better, two additional wear experiments were performed on the laser-hardened specimens, and the change trend of the friction coefficient obtained was basically the same. At room temperature, under oil lubrication, after reaching the stable period, the fluctuation of the friction coefficient was obviously small, and the wear was relatively stable. The friction coefficient during the wear period is shown in Figure 6.

Table 5. Friction coefficient under lubricated conditions.

Specimen Code	Sliding Speed (mm/min)	Laser Scan Speed (mm/min)	Friction Coefficient
1	300	700	0.215 ± 0.014
2	400	700	0.189 ± 0.012
3	500	700	0.161 ± 0.009
4	300	800	0.207 ± 0.013
5	400	800	0.189 ± 0.010
6	500	800	0.160 ± 0.008

shows the different friction coefficients under oil lubrication at room temperature.

From Table 5, the surface hardness of the 40CrNiMo steel after laser hardening with a scanning speed of 800 mm/min was slightly larger than that of the material after laser hardening with a scanning speed of 700 mm/min laser, the friction coefficient was also different but not greatly.

The friction coefficient decreased with the increase in the sliding speed. This was mainly because at the lower sliding speed, the friction pair squeezed less lubricating oil into the friction surface, and the bearing capacity of the liquid extrusion film would be weaker, the formation ability of the liquid film was poor, and there was a large proportion of solid–solid contact between the two solid surfaces.

As wear progressed, the surface of the contacting object would appear to a certain extent, and the surface was uneven, it is difficult to form a complete and continuous oil film. Therefore, the lubricating effect was poor. At high sliding speed, the action of the liquid extrusion film and the depth of the furrow would be deeper, thus, forming a better lubricating oil flow path, and then the better oil lubricating film was formed.

For laser-hardened materials with different scanning speeds, the decrease in friction coefficient would increase with the increase in the sliding speed. This may be because more furrows would be formed with the increase in the sliding speed, the better lubricating oil flow path was formed, which can make the lubrication effect better. Thereby, the coefficient of friction was reduced even more [25].

At a sliding speed of 500 mm/min, the friction coefficient values at different temperatures are shown in

Table 6. Friction coefficient at different temperatures.

Specimen Code	Temperature (°C)	Sliding Speed (mm/min)	Laser Scan Speed (mm/min)	Friction Coefficient
1	20	500	800	0.160 ± 0.008
2	110	500	800	0.241 ± 0.015
3	200	500	800	0.261 ± 0.019
4	20	500	700	0.161 ± 0.009
5	110	500	700	0.248 ± 0.016
6	200	500	700	0.299 ± 0.020

.

From

Table 7. Wear rate under lubricated conditions.

Specimen Code	Temperature (°C)	Sliding Speed (mm/min)	Laser Scan Speed (mm/min)	Wear Rate (10−3 mm/min)
1	20	300	700	0.1457 ± 0.015
2	20	400	700	0.1678 ± 0.017
3	20	500	700	0.1845 ± 0.018
4	110	500	700	0.4161 ± 0.029
5	200	500	700	0.5952 ± 0.04
6	20	300	800	0.1339 ± 0.014
7	20	400	800	0.1623 ± 0.016
8	20	500	800	0.1785 ± 0.017
9	110	500	800	0.3871 ± 0.026
10	200	500	800	0.5425 ± 0.038

, it can be seen that the material has been laser hardening with a scanning speed of 800 mm/min had a smaller friction coefficient and better resistance to fluctuations because, relatively speaking, the surface hardness of the material treated at this scanning speed was higher. With the increase in temperature, the increase in friction coefficient was larger; however, the increase would gradually decrease. This was mainly because the increase in thermal motion capacity would decrease to a certain extent within the experimental range.

3.3. Wear Amount

In order to know the wear width and wear amount, the wear scars of 40CrNiMo steel were observed with a three-dimensional profiler [26]. The wear scar width and wear amount of 40CrNiMo steel at room temperature and high temperature are shown in Figure 7.

With the sliding speed increased, the wear width of the laser-hardened surface would increase significantly. This was because with the increase in the sliding speed, the sliding distance would increase, the wear amount of the steel ball would also increase, and the bottom of the steel ball would gradually ground. The wear amount of the steel ball would increase, and the bottom of the steel ball would gradually ground. Under the action of vertical force, the contact width between the surface of the laser-strengthened 40CrNiMo steel and the steel ball would be larger, thus, resulting in an increase in the wear surface width.

3.4. Wear Morphology

(1)

3D morphology analysis of friction at different speeds at room temperature

The typical wear morphologies s at sliding speeds of 300, 400, and 500 mm/min without lighting effects enabled are shown in Figure 8.

From Figure 9, it can be seen that, with the increase in the speed, the wear pattern of the 40CrNiMo steel specimen hardened by the scanning speed of 700 mm/min or 800 mm/min changed. When the sliding speed was 300 mm/min, the wear depth was shallow, and wear in different areas was not exactly the same, and there was no obvious furrow along the sliding direction on the surface, which has a certain polishing effect. When the sliding speed increased to 400 mm/min, the furrows were gradually obvious; however, the furrows were shallower.

When the sliding speed was increased to 500 mm/min, the number of furrows increased, the furrows became more obvious, and the maximum wear depth also increased significantly. It can be seen that under the condition of oil lubrication, due to the high hardness of the material, under the combined action of normal load and tangential force, local ploughing and micro-cutting would appear, which were typical abrasive wear. Under the influence of lubricating oil, the increase in the sliding speed would bring more particles into between the two contact surfaces and gather together, deeper furrows were prone to appear.

(2)

3D morphology analysis of friction at different temperatures

Figure 9 showed the typical wear scars of the hardened layer without lighting effects enabled after laser hardening at a scanning speed of 700 and 800 mm/min, at 20 °C, 110 °C and 200 °C, at the sliding speed 500 mm/min.

As can be seen in Figure 10, at the same sliding speed, with the increase in temperature, the wear scars of the hardened layer under different scanning speeds have obvious differences.

(3)

SEM analysis of wear scars of the hardened layer with a scanning speed of 700 mm/min

The surface morphology of the hardened layer with a scanning speed of 700 mm/min at a normal force of 20 N and sliding speed of 300 mm/min, 400 mm/min and 500 mm/min respectively under 20 °C are shown in Figure 10.

As the equipment was different, the wear surface morphology was different from the one observed from the 3D Optical Microscope. Some surface features cannot be displayed when the 3D Optical Microscope companion software was used to analyze images without lighting effects enabled.

The shallow furrow and particle can be seen from Figure 10a, and thus abrasive wear occurred. Some areas appeared smooth, not enough for furrows, confirming there were a certain polishing effect again. More small particles and deeper furrow can be seen from Figure 10b, it can be inferred that abrasive wear has increased. Flake and deeper furrow can be seen from Figure 10c, and it can be inferred that the abrasive wear further increased.

The surface morphology of the hardened layer with a scanning speed of 700 mm/min at a normal force of 20 N and sliding speed of 500 mm/min, respectively, under 110 °C and 200 °C are shown in Figure 11.

From Figure 11, it can be seen that the furrows at the bottom of the friction surface of the hardened layer were more obvious and the depth was further deepened as the temperature increased. At the same time, due to the increase in temperature, there were obvious wear debits, thereby, indicating that the adhesive wear was intensifying.

In addition, the increase in temperature caused the hardness of the material to decrease, and the oxidation reaction would also increase, resulting in oxidative wear, and the generated oxides adhere to the wear surface under the action of lubricating oil. Since oxides were less hard than laser-hardened steel, they also promote abrasive wear during friction, resulting in deeper furrows.

3.5. Wear Rate

After laser hardening at scanning speeds of 700 and 800 mm/min, the wear rates of the hardened layer under different conditions are shown in Figure 12.

The wear rate values are shown in Table 7. From Table 7, it can be seen that with the increase in speed and temperature, the wear rate showed an increasing trend. The details as follows:

(1) After laser hardening with a scanning speed of 700 mm/min, under oil lubrication, the increments of the average wear rate of the hardened layer at the sliding speed of 500 mm/min relative to the sliding speed of 400 and the sliding speed of 400 mm/min relative to the sliding speed of 300 mm/min were 0.0167 and 0.0221 (10⁻³ mm/min), respectively. After laser hardening with a scanning speed of 800 mm/min, the average wear rate of the hardened layer at the sliding speed of 500 mm/min compared with the sliding speed of 400 mm/min and the sliding speed of 400 mm/min compared with the sliding speed of 300 mm/min.

The increments are 0.0162, 0.0284 (10⁻³ mm/min) respectively. At high sliding speed compared with low sliding speed, the sliding distance would increase, resulting in an increase in the amount of wear, and there would be more wear products in the lubricating oil and aggregated, resulting in deeper furrows.

It can be seen that the performance of the specimens hardened with a speed of 700 and 800 mm/min showed the same change law, that is, as the movement speed increased, the increase in the wear rate would change from more to less. This was because the friction was accompanied by the flow of lubricating oil. When the relative movement speed between the contact surfaces was slow, the flow of the lubricating oil was relatively slow, and thus that there may be more hard particles in the oil film between the contacting objects, which would promote a larger increase in wear. After reaching a certain the sliding speed, the speed continued to increase, and the oil flow accelerated to take away more wear products, which formed a relatively pure oil film between the friction objects, thereby the wear increment reduced.

(2) The increase in temperature had obvious effect on the increase in wear rate. Within the experimental range, the wear rates of the specimens hardened at 700 and 800 mm/min showed the same variation law. The wear rate at 110 °C was more than twice that at 20 °C, and the wear rate at 200 °C was about 140% of that at 110 °C. This was because the lubricating oil would become thinner and the viscosity would decrease after the temperature rose, which would change the characteristics of the lubricating oil film, the bearing capacity of the lubricating film would decrease, and the thickness of the oil film would be lower. Moreover, the increase in oil temperature would also promote oxidative wear and adhesive wear.

In particular, the research has certain limitations, and the contents of the article may be only applicable within the experimental parameters.

4. Conclusions

In the paper, the sliding friction and wear properties of 40CrNiMo steel after laser hardening against GCr15 steel under oil lubrication was investigated, and the following main conclusions were obtained.

In the hardened layer, there were different sizes of martensite, and dislocation was a common phenomenon. Deformation also occurred; however, no cracks appeared, and thus the martensitic bundles have some plasticity. Under oil lubrication, with the increase in the sliding speed, the friction coefficient of the friction pair showed a trend of gradually decreasing, and the wear width and wear amount of the hardened layer showed an increasing trend. The increase in temperature lead to the increase in friction coefficient and fluctuation, wear width and wear amount of the hardened layer. We explored the wear mechanism using scanning electron microscopy. It can be judged that abrasive wear was the main wear mechanism at a temperature of 20 °C under oil lubrication. As the temperature rose, the furrow depth would be further deepened, and adhesive wear and oxidative wear would further occur.