Effect of Solution and Aging Treatment on the Tribological Properties of K452 Alloy in a Wide Temperature Range

Highlights

What are the main findings?

•

Utilizing the high-temperature strength and high-temperature oxidation resistance of nickel-based superalloys, the K452 nickel-based superalloy is applied in the field of high-temperature friction and wear. It has been rarely studied in the literature.

•

Under normal conditions, K452 alloy undergoes a three-step heat treatment process, as follows: (1) solution treatment, (2) intermediate treatment, and (3) aging. This study showed that the K452 alloy can achieve optimal wear resistance after a two-step heat treatment in a wide temperature range from RT to 600 °C.

•

Further research in this article indicates that the wear resistance of K452 alloy has completely deteriorated at 800 °C.

What are the implications of the main findings?

•

The heat-treated K452 alloy exhibits excellent wear resistance for critical hot-end components in aero-engines up to 600 °C.

•

The deterioration of tribological properties at 800 °C due to oxidative wear provides a clear operational upper limit for the alloy.

•

The study guides the optimization of superalloy microstructures via precipitation strengthening for high-temperature tribological applications.

Abstract

This study focuses on China’s domestically developed K452 alloy. Using Si₃N₄ ceramic balls as the counterface material, the tribological properties of the K452 alloy were investigated after heat treatment over a wide temperature range (RT–800 °C), and the wear mechanisms were analyzed. The results show that the heat treatment process enhances the material hardness slightly by promoting the dissolution of the γ′-strengthening phase and the precipitation of the η phase. From RT to 600 °C, the wear rate of the K452 alloy remains at a relatively low level, on the order of 10⁻⁶ mm³·m⁻¹·N⁻¹. Compared with the as-cast condition, intermediate treatment exhibits a significant reduction in the wear rate. Compared with traditional processes, it reduces one step of heat treatment. This improvement is attributed to the precipitation of the uniformly fine η phase, along with the re-dissolution of the γ′-strengthening phase. When the testing temperature is raised to 800 °C, the tribological performance of the K452 alloy deteriorates significantly, with the wear rate increasing to the order of 10⁻⁵ mm³·m⁻¹·N⁻¹. Microstructural characterization confirms that the in situ formations of dense Cr₂O₃ and Al₂O₃ oxide films during friction are the primary mechanism for improved wear resistance from RT to 600 °C. But when the temperature rises to 800 °C, the dynamic equilibrium of the oxide layers is disrupted, leading to oxidative wear becoming the dominant mechanism.

1. Introduction

The K4 series of nickel-based superalloys was independently developed in China. It is specifically designed for high-temperature corrosive environments and complex stress conditions. This series is characterized by high chromium content and a composite strengthening mechanism, offering excellent high-temperature strength and hot corrosion resistance. It is widely used in hot-end components of aero-engines and gas turbines [1]. The K452 alloy consists of a γ matrix phase composed of Ni. The γ phase has an FCC structure, providing high-temperature stability. Elements such as Cr and Al form oxide films that resist high-temperature oxidation and corrosion. Al and Ti are key elements for the formation of the strengthening γ′ phase (Ni₃(Al, Ti)), which is primarily produced through coherent precipitation and plays a critical role in enhancing the overall performance of the alloy [2]. In addition, MC-types (e.g., TaC, NbC) and M₂₃C₆-type carbide strengthening phases collectively contribute to the alloy’s excellent high-temperature performances, enabling superior durability in complex environments. The alloy is used in the manufacturing of modern shipborne and land-based industrial heavy-duty gas turbines, and performs particularly well in first- to sixth-stage guide vanes with operating temperatures not exceeding 950 °C [3].

According to incomplete statistics, one-third of the world’s primary energy is consumed by friction and wear, and more than half of mechanical failures are attributed to friction and wear. In industrialized countries, annual losses caused by friction and wear account for 2% to 7% of the gross national product (GNP) [4,5,6,7]. Therefore, research on the tribological properties of materials and their industrial applications plays a crucial role in addressing issues related to resources, energy, and environmental pollution [8]. Wear-resistant materials, owing to their high hardness, low friction coefficient, and favorable physical and chemical stability, occupy an important position in applications involving high temperatures, high stresses, and complex environments, particularly in aerospace [9,10], energy [11,12], chemical engineering [13,14], iron and steel [15], and other fields.

As an important means of improving the performance of metallic materials [16], heat treatment can optimize the microstructures [17] and mechanical properties [18] of materials, and also plays a significant role in enhancing wear resistance, oxidation resistance, and corrosion resistance [19,20], and fatigue resistance [21]. Continuous innovations in heat treatment processes, along with coordinated optimization of structures and performance, enable superalloys to adapt to higher-temperature and more complex application environments, meeting the growing demands for material performance across various fields. This study focuses on the K452 alloy, subjecting it to the solution and aging treatment. Utilizing the high-temperature strength and high-temperature oxidation resistance of nickel-based superalloys, the K452 nickel-based superalloy is applied in the field of high-temperature friction and wear with a wide temperature range from RT to 800 °C. It has been rarely studied in the literature.

2. Materials and Methods

2.1. Experimental Materials

In this paper, the K452 alloy is in this experiment obtained by vacuum melting (Jinchuan Group Co., Ltd., Jinchang, China). The main composition of it is shown in

Table 1. The composition of K452 alloy (wt.%).

Cr	Co	Al	Ti	Nb	Mo	W	C	Ni
20.9	11.2	2.5	3.5	0.25	0.6	3.5	0.11	Bal.

.

2.2. Heat Treatment Regimes

The experimental materials were cut into cylindrical sheets with a size of φ20 mm × 3 mm, and the heat treatment regimens are shown in

Table 2. Heat treatment processes of K452 alloy.

Process No.	Heat Treatment Regime
1	As-cast
2	1170 °C × 4 h, WC
3	1170 °C × 4 h, AC + 1050 °C × 4 h, AC
4	1170 °C × 4 h, AC + 1050 °C × 4 h, AC + 850 °C × 16 h, AC

. The first group was as-cast samples; the second group was water-cooled (WC) after holding at 1170 °C for 4 h; the third group was air-cooled (AC) after heating to 1050 °C and holding for 4 h following the treatment of the second group; the fourth group was air-cooled after heating to 850 °C and holding for 16 h following the treatment of the third group. The heating equipment is the SX-G01863 energy-saving box-type resistance furnace (Tianjin Zhonghuan Furnace Corp., Tianjin, China).

2.3. Friction and Wear Experiments

The tribological behaviors of the K452 alloy in different test temperatures were studied using a high-temperature vacuum tribometer (GHT-1000E, Zhongke Kaihua Co., Ltd., Lanzhou, China). The friction test temperatures were set at room temperature (RT), 300 °C, 600 °C, and 800 °C, and the friction test environment was dry friction. The counterface material was φ6 mm Si₃N₄ ceramic balls, the friction time was 20 min, the applied load was 5 N, the friction radius was 4 mm, and the sliding speed was 0.19 m/s. The friction coefficient was automatically recorded by the tester, and the wear rate was calculated by Formula (1).

$$W={\displaystyle \frac{V}{PS}}$$

(1)

where W is the wear rate of the test sample (mm³·N⁻¹·m⁻¹), V is the wear volume of the test sample (mm³), P is the applied load (N), and S is the sliding distance (m). The wear volumes were measured by a non-contact 3D optical profiler (MicroXAM-800, KLA Instruments, Milpitas, CA, USA).

2.4. Characterizations

The hardness was tested by the HRS-150 Rockwell hardness tester (Buehler Ltd., Lake Bluff, IL, USA); the hardness of five points on the surface of the K452 alloy was measured, and the average value was taken. The microstructures of the K452 alloy were observed by an Axioscope 5 intelligent microscope (Zeiss Co., Ltd., Oberkochen, Germany). The worn surfaces after the tribological tests were thoroughly characterized using a scanning electron microscope (SEM, JSM-5600LV, Tokyo, Japan) in conjunction with an energy dispersive spectrometer (EDS) (Oxford In-struments, Oxford, UK). The composition of the wear scars of the Inconel K452 alloy was determined by using the X-ray diffraction technique (XRD, EMPYREAN, Panalytical, Almelo, The Netherlands). And the analysis involved the use of Cu Kα radiation, with a scanning speed of 8°/min and a scanning range of 30° to 80°.

3. Results and Discussion

3.1. Microstructural Analysis

Figure 1 shows the microstructures of the K452 alloy after different heat treatments. It can be seen from Figure 1a that the main structure of the as-cast K452 alloy sample consists of the matrix γ phase, γ′ phase, and carbides. In Figure 1b, the γ′ phase and carbides in the sample partially dissolve after solution treatment, resulting in a uniform structure. Figure 1c shows that the γ′ strengthening phase in the alloy redissolves and precipitates dispersively during the cooling process after intermediate treatment, and at the same time, fine black η phase precipitates, with a relatively uniform structure distribution. In the aged sample shown in Figure 1d, the quantities of γ′ phase and η phase increase significantly, and the structure coarsens to a great extent.

3.2. Mechanical Property Analysis

Figure 2 shows the hardness histogram of the K452 alloy under different heat treatment processes. The hardness of the as-cast sample is 33.4 HRC, and decreases slightly to 32.7 HRC after solution treatment because of the γ′ phase and carbides in the sample partially dissolving. The hardness of the sample after intermediate treatment increases slightly to 36.0 HRC, and then decreases slightly to 35.4 HRC after aging treatment. After intermediate treatment, the precipitation of γ′ phase and carbides and the refinement of the structure improve the hardness of the alloy. However, a large amount of structures coarsening and the aggregation of secondary phases lead to the deterioration of mechanical properties in the aging treatment, as shown in Figure 1d.

3.3. Tribological Properties

3.3.1. Room Temperature Friction and Wear

Figure 3 shows the relationship between friction coefficients and time variation in the K452 alloy under different heat treatment processes at RT. After the running stage, the friction coefficient of the as-cast sample stabilizes and fluctuates between 0.5 and 0.6. The friction coefficient of the solution treatment sample does not show an obvious running stage, presenting periodic fluctuations, and generally remains between 0.45 and 0.55. After a very short running stage, the intermediate treatment sample enters a stable stage, and the friction coefficient generally remains between approximately 0.4 and 0.5. Then it increases and stabilizes at around 0.55. After the running stage, the friction coefficient of the aged sample stabilizes at around 0.5. As a whole, the friction coefficient of the intermediate treatment sample is lower and more stable compared to the other three samples.

Figure 4 shows the wear rates of the K452 alloy under different heat treatment processes at RT. The as-cast sample has the highest wear rate, which is 1.08 × 10⁻⁵ mm³·m⁻¹·N⁻¹. After solution treatment, the sample’s wear rate decreases significantly to 2.93 × 10⁻⁶ mm³·m⁻¹·N⁻¹. The intermediate treatment sample has the lowest wear rate, which is 1.12 × 10⁻⁶ mm³·m⁻¹·N⁻¹, a decrease of one order of magnitude. After aging treatment, the wear rate of the sample increases slightly to 4.25 × 10⁻⁶ mm³·m⁻¹·N⁻¹. After heat treatment, the precipitation of γ′ phase and carbides and the refinement of the structure improve the strength and wear resistance of the alloy.

Figure 5 shows the wear scar morphologies of the as-cast and intermediate treatment K452 alloys at RT. The wear scar of the as-cast sample has a complex morphology and is relatively deep, as shown through local magnification of the wear scar surface (Figure 5b). It can be seen that a large number of particles peel off on the wear surface, resulting in the accumulation of residual particles. In contrast, the wear scar of the intermediate-treated K452 alloy is relatively shallow, and the wear surface is relatively smooth (Figure 5d), with fine particles on the surface, mainly showing the characteristics of abrasive wear. This is because after the intermediate treatment, the γ′ phase and carbides in the K452 alloy precipitate dispersively and distribute on the matrix, which greatly improves the strength and wear resistance of the alloy.

Figure 6 shows the XRD patterns of the wear scars of the K452 alloy at RT. The results show that the main components on the wear scar surface of the K452 alloy are the Ni-Cr matrix phase, Ni(Al/Ti) strengthening phase, and M₂₃C₆ carbides. The diffraction peaks of the as-cast sample are mainly composed of the γ phase (111) and γ′ phase (110), which form the highest peaks. After intermediate treatment, the intensities of the diffraction peaks composed of the γ phase (111) and γ′ phase (110), as well as the diffraction peaks composed of the γ phase (200) and M₂₃C₆ on the wear scar surface of the alloy, increase significantly, which indicates that a large number of γ′ phases and M₂₃C₆ phases are formed in the alloy. The Ni₃(Al/Ti) strengthening phase and carbides significantly hinder the movement of dislocations, thereby improving the strength and wear resistance of the material.

3.3.2. Friction and Wear at 300 °C

Figure 7 shows the relationship between friction coefficients and time variation in the K452 alloy under different heat treatment regimes at 300 °C. It can be observed that the friction coefficient changes in all samples are similar. After a short running stage, they stabilize and fluctuate within a certain range. The friction coefficients of the as-cast and aged samples are close, fluctuating around 0.5, while the friction coefficient of the solution treatment sample is slightly lower, fluctuating around 0.45. But the intermediate treatment sample has the lowest friction coefficient with the smallest fluctuation. After a running stage of approximately 5 min, the friction coefficient stabilizes at around 0.2. After 12 min, the fluctuation increases and shows a certain downward trend.

Figure 8 shows the wear rates of the K452 alloy after different heat treatments at a high temperature of 300 °C. As can be seen from the figure, the wear rate of the as-cast sample is 7.75 × 10⁻⁶ mm³·m⁻¹·N⁻¹. The wear rate of the solution treatment sample is close to that of the as-cast sample, which is 7.18 × 10⁻⁶ mm³·m⁻¹·N⁻¹. The intermediate treatment sample has the lowest wear rate, which is 7.2 × 10⁻⁷ mm³·m⁻¹·N⁻¹, a decrease of one order of magnitude compared with other samples. The aged sample has the highest wear rate, reaching 9.62 × 10⁻⁶ mm³·m⁻¹·N⁻¹. This indicates that in a friction environment at 300 °C, the uniformly precipitated γ′ phase and carbides after intermediate treatment play a positive role in improving the wear resistance of the K452 alloy.

Figure 9 shows the wear morphologies and surface element distribution maps of the as-cast and intermediate treatment K452 alloy samples under friction at 300 °C. It can be seen that the as-cast sample has obvious plow groove wear scars. After local magnification (Figure 9b), many lamellar peelings and large particles can be observed. The wear scar surface of the aged sample is smooth, and after local magnification (Figure 9d), the surface is also smooth with milder delamination, and there is a small amount of unpeeled cracked lamellae. From the surface element distribution map, it can be seen that O elements are concentrated, indicating the formation of oxides such as Cr₂O₃ and Al₂O₃. This protective oxide film reduces the direct contact between the alloy and the external environment, improving the wear resistance of the alloy. At the same time, the matrix elements Ni and Cr are evenly distributed, and the structure is uniformly refined. Various strengthening mechanisms enable the alloy to maintain solid structural uniformity and stability under friction conditions at 300 °C.

3.3.3. Friction and Wear at 600 °C

Figure 10 shows the relationship between friction coefficients and time variation in the K452 alloy under different heat treatment regimes at 600 °C. After an extremely short running stage, the friction coefficient of the as-cast sample generally fluctuates around 0.32. The friction coefficient of the solution treatment sample stabilizes at around 0.25. The friction coefficient of the intermediate treatment sample stabilizes at around 0.34. And the friction coefficient of the aged sample stabilizes at around 0.24. Compared with the tests at 300 °C and RT, the friction coefficient of each sample decreases at 600 °C, except for the intermediate treatment sample.

Figure 11 shows the wear rates of the K452 alloy samples subjected to different heat treatment regimes at a high temperature of 600 °C. As can be seen from the figure, the wear rate of the as-cast sample is 1.82 × 10⁻⁶ mm³·m⁻¹·N⁻¹. The solution treatment sample has the highest wear rate, which is approximately three times that of the as-cast sample, reaching 6.16 × 10⁻⁶ mm³·m⁻¹·N⁻¹. The intermediate treatment sample has the lowest wear rate, which is 3.6 × 10⁻⁷ mm³·m⁻¹·N⁻¹, reaching the order of 10⁻⁷. After aging treatment, the wear rate increases slightly to 1.27 × 10⁻⁶ mm³·m⁻¹·N⁻¹. This indicates that the K452 alloy can maintain a lower wear rate at 600 °C.

Figure 12 shows the wear morphologies of the K452 alloy at different heat treatment stages at 600 °C. Observing the wear scar morphology of the as-cast sample, slight plow grooves and scratch wear scars can be seen. After magnification (Figure 12b), it can be seen that the wear scar surface is flat with certain peeled particles and fine flake structures. The friction morphology of the alloy after intermediate treatment is relatively smooth overall. After magnification (Figure 12d), a layered structure composed of dense particles can be seen. It is speculated that, because the K452 alloy forms a complete and dense oxide layer during friction at 600 °C, the K452 alloy has high strength and wear resistance.

Figure 13 shows the XRD patterns of the wear scars of the as-cast and intermediate treatment K452 alloys at 600 °C. The results show that the main components on the wear scar surface of the K452 alloy are the Ni-Cr matrix phase, Ni₃(Al/Ti) strengthening phase, and M₂₃C₆ carbides. The background of the as-cast sample pattern has a significant influence. After intermediate treatment, the highest peak on the wear scar surface of the alloy is composed of the γ phase (200) and M₂₃C₆ carbide. The γ phase is the alloy matrix phase, and M₂₃C₆ is the key precipitation strengthening phase of the alloy. M₂₃C₆ carbide significantly hinders the movement of dislocations, thereby improving the strength and wear resistance of the material.

The above studies showed that the K452 alloy can achieve optimal wear resistance after a two-step heat treatment in a wide temperature range from RT to 600 °C. The first step is to heat to 1170 °C and hold for 4 h, then water cooling (WC). The second step is to heat again to 1050 °C and hold for 4 h, then air cooling (AC).

3.3.4. Friction and Wear at 800 °C

Figure 14 shows the relationship between friction coefficients and time variation in the K452 alloy under different heat treatment regimes at 800 °C. After a running stage, the friction coefficients of all samples stabilize between 0.6 and 0.7, showing a slow increasing trend. They all exhibit intense volatility. This phenomenon may be due to the complete destruction of the oxide protective film on the contact surface.

Figure 15 shows the wear rates of the K452 alloy after different heat treatment regimes at 800 °C. As can be seen from the figure, when the test temperature is 800 °C, the wear rates of all samples showed a significant increase compared to the wear rates in the temperature range from RT to 600 °C. Whether or not heat treatment is adopted, the wear rate of the K452 alloy increases to the order of 10⁻⁵ mm³·m⁻¹·N⁻¹. This indicates that the wear resistance of K452 alloy has significantly decreased at 800 °C.

Figure 16 shows the wear morphologies and element distribution maps of the K452 alloy at 800 °C. As shown in the figures, the wear scar surface of the as-cast alloy in the environment of 800 °C is flat. After magnification (Figure 16b), a certain layered structure and particles can be seen. There are some obvious cracks on the wear scar; this phenomenon is caused by the damage of oxides under pressure. The wear scar surface of the intermediate treatment alloy has obvious plow grooves. After magnification (Figure 16d), plow grooves and large area lamellar peeling can be seen. From the element distribution maps, it can be seen that a certain oxide film is formed, and the alloy matrix on the wear scar surface is exposed. The high temperature causes the continuous formation of an oxide film on the alloy surface during the wear process, but it is quickly worn and peeled off. This cycle is repeated, resulting in oxidative wear of the alloy under the condition of 800 °C.

4. Conclusions

(1)

The as- ast K452 alloy is mainly composed of γ phase, γ′ phase, and carbides, with the γ′ phase dispersively distributed in the matrix. After solution and aging treatment, the γ′ phase in the alloy precipitates dispersively, with a certain amount of black η phase distributed, the structures are uniform, and the grains are refined. After heat treatment, the hardness of the alloy increases slightly from 33.4 HRC (as-cast) to 36.0 HRC (intermediate treatment).

(2)

In a wide temperature range from RT to 600 °C, heat treatment significantly improved the friction and wear properties of the K452 alloy. After solution and intermediate treatment, the wear rate of the alloy decreases significantly to the order of 10⁻⁶~10⁻⁷ mm³·m⁻¹·N⁻¹. But after aging treatment, the wear resistance of the alloy decreases due to the aggregation and growth of secondary phases.

(3)

When the temperature of the friction environment rises to 800 °C, the wear rate of the alloy increases significantly, generally increasing to the order of 10⁻⁵ mm³·m⁻¹·N⁻¹. This indicates that the wear resistance of K452 alloy has significantly decreased at 800 °C.