High-temperature-wear-resistant nickel-based alloy and preparation method thereof

Abstract

According to the high-temperature-wear-resistant nickel-based alloy and the preparation method thereof, the components in the nickel-based alloy are optimized, the novel high-temperature-wear-resistant nickel-based alloy is prepared and has excellent tensile strength and fracture toughness in the temperature range of 700-800 DEG C, and the service life of the alloy is prolonged.

Description

Technical Field

[0001] The invention belongs to the technical field of nickel-based alloys, and in particular relates to a high-temperature wear-resistant nickel-based alloy and a preparation method thereof. Background Art

[0002] Nickel-based alloy refers to a type of high-temperature alloy with high strength and good comprehensive properties such as anti-oxidation and anti-corrosion at high temperatures of 600℃ to 1000℃. Nickel-based alloys are mainly used in mechanical industries such as automobiles, ships, and aerospace. For components such as turbine blades and combustion chambers in aerospace, they usually need to undergo special high-temperature treatment to obtain performance requirements that meet their service conditions. However, special high-temperature treatment often involves high temperatures, high loads, and repeated hot and cold alternations, which can cause wear and cracking of the alloy, affecting its service life. Therefore, higher requirements are placed on the high-temperature wear resistance of the alloy.

[0003] Therefore, there is an urgent need for a nickel-based alloy that is resistant to high temperature wear. Summary of the invention

[0004] The present invention provides a high temperature wear resistant nickel-based alloy and a preparation method thereof, which are used to solve the problems of easy wear and cracking of nickel-based alloys in the prior art when used in the aerospace field after special high temperature treatment.

[0005] In the first aspect, the present invention provides a high temperature wear resistant nickel-based alloy, comprising the following components in mass percentage: Ni: 48.0-50.0wt%, Cr: 18.0-20.0wt%, C: less than 0.07wt%, Si: less than 0.35wt%, Mn: 2.0-3.0wt%, Mo: 3-5wt%, Nb: 4-6wt%, N: 0.2-0.3wt%, Al: 0.8-1.0wt%, Ti: 1.0-1.2wt%, B: 0.5-0.8%, W: 2.0-3.0%, and the balance is Fe and unavoidable impurities.

[0006] Compared with the prior art, the invention is beneficial in that: the invention optimizes the components in the nickel-based alloy to prepare a new type of high-temperature wear-resistant nickel-based alloy, which has excellent tensile strength and fracture toughness in the temperature range of 700°C-800°C compared with the existing nickel-based alloy, and in particular, the high-temperature wear resistance is significantly improved; wherein the high-temperature tensile strength is ≥580MPa, the yield strength is ≥460MPa, the elongation after fracture is ≥34.6%, and the friction loss is ≤1.31mg; therefore, the alloy of the invention has high wear resistance when used in the temperature range of 700°C-800°C, thereby improving the service life of the alloy.

[0007] Furthermore, the mass percentage of C in the high temperature wear resistant nickel-based alloy is 0.04-0.07wt%.

[0008] Furthermore, the mass percentage of Si in the high temperature wear resistant nickel-based alloy is 0.30-0.35wt%.

[0009] Further, in terms of element mass percentage: W / B is 2.7 to 5.5; and / or,

[0010] The mass percentage of W and B in the high temperature wear resistant nickel-based alloy is 2.7-3.5wt%.

[0011] Furthermore, calculated by element mass percentage: (W+B) / N is 10-17.

[0012] Furthermore, the N source includes MnN and CrN; wherein the molar ratio of MnN to CrN is (1-1.2): (1.3-2.0).

[0013] Furthermore, the high temperature wear resistant nickel-based alloy has a high temperature tensile strength of ≥580 MPa at 700-800° C., a yield strength of ≥460 MPa, an elongation after fracture of ≥34.6%, and a friction loss of ≤1.31 mg.

[0014] In a second aspect, the present invention provides a method for preparing the high temperature wear resistant nickel-based alloy according to the first aspect, comprising the following steps:

[0015] (1) melting a raw material system including a nickel source, a chromium source, a silicon source, a molybdenum source, an aluminum source, a titanium source, a niobium source, a tungsten source, and a boron source to obtain a first molten steel;

[0016] (2) When the molten steel is heated to 1350-1400° C., MnN and CrN are added to the first molten steel to obtain a second molten steel;

[0017] (3) subjecting the second molten steel to a refining treatment and a cooling and standing treatment in sequence to obtain a third molten steel;

[0018] (4) Deoxidizing the third molten steel to obtain a high temperature wear resistant nickel-based alloy.

[0019] Furthermore, in step (2), MnN is first added to the first molten steel, and then CrN is added.

[0020] Further, the particle size of the raw material system is 100 to 120 meshes; and / or,

[0021] The refining treatment temperature is 1550-1600°C and the time is 3-5 minutes; and / or,

[0022] The temperature of the cooling and standing treatment is 1350-1370°C; and / or,

[0023] The deoxidation treatment uses silicon calcium barium deoxidizer, and the addition amount of the silicon calcium barium deoxidizer accounts for 0.12-0.15% of the mass of the third steel liquid. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a graph showing the relationship between the tensile strength and temperature of the alloys in various embodiments of the present invention and comparative examples;

[0025] Figure 2 is a graph showing the relationship between the yield strength and temperature of the alloys in various embodiments and comparative examples of the present invention;

[0026] Figure 3 is a relationship diagram between the elongation after fracture and the temperature of the alloys in various embodiments of the present invention and comparative examples;

[0027] Figure 4 is a relationship diagram between the wear amount and temperature of the alloy in each embodiment of the present invention and the comparative example;

[0028] Figure 5 is a SEM image of the alloy in Example 1 of the present invention;

[0029] Figure 6 This is the SEM image of the alloy after friction and wear in Example 1 of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] In the first aspect, the present invention provides a high temperature wear resistant nickel-based alloy, which includes the following components in mass percentage: Ni: 48.0-50.0wt%, Cr: 18.0-20.0wt%, C: less than 0.07wt%, Si: less than 0.35wt%, Mn: 2.0-3.0wt%, Mo: 3-5wt%, Nb: 4-6wt%, N: 0.2-0.3wt%, Al: 0.8-1.0wt%, Ti: 1.0-1.2wt%, B: 0.5-0.8%, W: 2.0-3.0%, and the balance is Fe and unavoidable impurities.

[0032] The high temperature wear resistant nickel-based alloy provided by the present invention improves the wear resistance of the alloy through a specific element ratio. It can be understood that the wear resistance of the alloy is not achieved by a single element, but by the contribution of alloy elements such as chromium, molybdenum, niobium, boron, and tungsten. The inventor found that chromium can combine with carbon and nitrogen elements in the alloy to form hard phases such as high hardness chromium carbonitride. In addition, boron and tungsten also have similar effects. These hard phases are like "skeletons" in the alloy, effectively resisting the erosion of external wear media and improving the wear resistance of the alloy. The addition of molybdenum and niobium can refine the grains of the alloy, improve the strength and toughness of the alloy, and this strengthening effect not only helps to resist external impact and wear, but also maintain the integrity of the alloy during the wear process and prevent the expansion of cracks. These elements cooperate with each other in the alloy to form a complex strengthening mechanism, which jointly improves the wear resistance of the alloy. At the same time, through reasonable composition design and casting process, the distribution and morphology of the elements of these alloys can be further optimized, thereby further improving the wear resistance and service life of the alloy.

[0033] Furthermore, the mass percentage of C in the high temperature wear resistant nickel-based alloy is 0.04-0.07wt%.

[0034] The addition of carbon can improve the high temperature strength and wear resistance of the alloy. By specifically limiting the C content, the weldability of the alloy can be further improved and its structural stability during long-term high temperature service can be improved.

[0035] Furthermore, the mass percentage of Si in the high temperature wear resistant nickel-based alloy is 0.30-0.35wt%.

[0036] Si can improve the hardness and wear resistance of austenite. Adding Si to the alloy can form a solid solution, thereby increasing the hardness of the alloy and making it more resistant to wear and deformation. In addition, Si can affect the organizational structure of the alloy and reduce the formation of defects and cracks. By adjusting the microstructure of the alloy, Si reduces the internal stress of the alloy and improves its fatigue resistance and wear resistance. At the same time, when Si is used in combination with other alloying elements such as Cr, Mo, etc., the wear resistance of the alloy can be further improved. For example, the combination of Si and Cr can significantly improve the hardenability and tempering stability of the alloy, thereby enhancing the wear resistance of the alloy.

[0037] In a specific embodiment, calculated by the mass percentage of the elements: W / B is 2.7-5.5, and the mass percentage of W and B in the high temperature wear resistant nickel-based alloy is 2.7-3.5wt%.

[0038] By further limiting the mass ratio of W element and B element, the hardness and wear resistance of the alloy can be further improved. When W element and B element exist in the above-defined ratio, the red hardness and thermal strength of steel can be further improved, and the wear resistance of steel can be significantly improved, and the hardenability of steel can be increased exponentially, thereby enhancing the hardness and wear resistance of the material. The inventor analyzed that limiting the mass ratio of W to B can be beneficial for the W element to react with C to generate hard phases such as WC and Fe3W3C, which can significantly improve the hardness and wear resistance of the cladding layer, further control the amount of B element added in steel, further improve the hardenability of steel, and then improve the hardness and wear resistance of the material.

[0039] Furthermore, calculated by element mass percentage: (W+B) / N is 10-17.

[0040] The effect of N forming nitrides with W and B elements on the wear resistance of the alloy is particularly obvious. By further limiting the mass ratio of W element to B element to N element, nitrides of BN and WN can be further formed, further strengthening the solid solution strengthening effect on the alloy. On the other hand, BN and WN form dispersed hard particles in the alloy, hindering dislocation movement, thereby improving the wear resistance and hardness of the alloy; and by determining the mass ratio of W element to B element to N element, pores can be further reduced and the high temperature performance of the alloy can be improved.

[0041] Specifically, the N source includes MnN and CrN; wherein the molar ratio of MnN to CrN is (1-1.2):(1.3-2.0).

[0042] The source of N needs to take into account the content of Mn and Cr. According to the element ratio of nitride, the content and ratio of Mn and Cr elements should be controlled, and the optimal ratio is obtained through a large number of experimental tests.

[0043] The N element is added in the form of MnN and CrN, which further dissolves the N element in the alloy and adjusts the content of Mn and Cr elements. The N and Mn elements can expand and stabilize the austenite zone, play the role of dissolving more high-temperature alloy elements, and also make up for the impact of Ni reduction; another purpose of N is to form BN with B, further improving the high-temperature wear resistance of the alloy.

[0044] Compared with the existing nickel-based alloys, the high-temperature wear-resistant nickel-based alloy provided by the present invention has excellent tensile strength and fracture toughness in the temperature range of 700°C-800°C, and in particular, the high-temperature wear resistance is significantly improved. The high-temperature tensile strength is ≥580MPa, the yield strength is ≥460MPa, the elongation after fracture is ≥34.6%, and the friction loss is ≤1.31mg. Therefore, the alloy of the present invention has high wear resistance when used in the temperature range of 700°C-800°C, which improves the service life of the alloy.

[0045] In a second aspect, the present invention provides a method for preparing the high temperature wear resistant nickel-based alloy of the first aspect, comprising the following steps:

[0046] (1) melting a raw material system including a nickel source, a chromium source, a silicon source, a molybdenum source, an aluminum source, a titanium source, a niobium source, a tungsten source, and a boron source to obtain a first molten steel;

[0047] (2) When the molten steel is heated to 1350-1400° C., MnN and CrN are added to the first molten steel to obtain a second molten steel;

[0048] (3) subjecting the second molten steel to a refining treatment and a cooling and standing treatment in sequence to obtain a third molten steel;

[0049] (4) Deoxidizing the third molten steel to obtain a high temperature wear resistant nickel-based alloy.

[0050] Furthermore, in step (2), MnN is first added to the first molten steel, and then CrN is added.

[0051] Specifically, 2.0-3.0 wt% of manganese is added first, and then 0.2-0.3 wt% of nitrogen is added.

[0052] Among them, the nickel source, chromium source, silicon source, molybdenum source, aluminum source, titanium source, niobium source, tungsten source and boron source are all single substance powders.

[0053] In a specific embodiment, the particle size of the raw material system is 100-120 mesh; the temperature of the refining treatment is 1550-1600°C, and the time is 3-5 minutes; the temperature of the cooling and standing treatment is 1350-1370°C; the deoxidation treatment uses a silicon calcium barium deoxidizer, and the addition amount of the silicon calcium barium deoxidizer accounts for 0.12-0.15% of the mass of the third molten steel.

[0054] The high temperature wear resistant nickel-based alloy provided by the present invention is described in detail below through specific embodiments.

[0055] Example 1

[0056] The raw materials of this embodiment include the following components in mass percentage: Ni: 50.0wt%, Cr: 18.0wt%, C: 0.07wt%, Si: 0.35wt%, Mn: 2.0wt%, Mo: 5wt%, Nb: 5wt%, N: 0.2wt%, Al: 0.9wt%, Ti: 1.2wt%, B: 0.5%, W: 2.3%, and the balance is Fe and unavoidable impurities.

[0057] The following steps are involved:

[0058] (1) Electrolytic nickel powder, chromium powder, silicon powder, molybdenum powder, aluminum powder, titanium powder, niobium powder, tungsten powder, boron powder, etc. are respectively charged into a medium frequency furnace, and heated and melted in a short time. After all the steel materials are melted, a first molten steel is obtained. When the temperature reaches 1350-1400° C., 2.5 wt % of manganese powder is added, and then 0.20 wt % of nitrogen (nitrogen element is added in the form of MnN and CrN with a molar ratio of 1:2) is added to obtain a second molten steel;

[0059] (2) After the second molten steel is completely melted, the temperature is raised for refining treatment, the refining temperature is 1550-1600° C., the refining time is 3-5 min, the heating is stopped, the temperature is lowered and the steel is allowed to stand, thereby obtaining a third molten steel;

[0060] (3) The temperature of the third molten steel is measured by a thermocouple. When the temperature is close to the pouring temperature, 0.12% of the mass of the molten steel in the amount of silicon, calcium and barium is added to the molten steel for deoxidation. After deoxidation, a high temperature wear-resistant nickel-based alloy is obtained.

[0061] Example 2

[0062] The difference between this embodiment and embodiment 1 is that the raw material system, calculated by mass percentage, includes: Ni: 49.0wt%, Cr: 18.0wt%, C: 0.07wt%, Si: 0.35wt%, Mn: 2.5wt%, Mo: 5wt%, Nb: 5wt%, N: 0.25wt%, Al: 0.9wt%, Ti: 1.2wt%, B: 0.6%, W: 2.6%, and the balance is Fe and unavoidable impurities.

[0063] Example 3

[0064] The difference between this embodiment and embodiment 1 is that the raw material system, calculated by mass percentage, includes: Ni: 48.0wt%, Cr: 18.0wt%, C: 0.07wt%, Si: 0.35wt%, Mn: 3.0wt%, Mo: 5wt%, Nb: 5wt%, N: 0.3wt%, Al: 0.9wt%, Ti: 1.2wt%, B: 0.8%, W: 3.0%, and the balance is Fe and unavoidable impurities.

[0065] Example 4

[0066] The difference between this embodiment and embodiment 1 is that, in terms of the raw material system, the amount of C added is 0.02wt%, and the remaining raw material system is Ni: 50.0wt%, Cr: 18.0wt%, Si: 0.35wt%, Mn: 3.0wt%, Mo: 5wt%, Nb: 5wt%, N: 0.2wt%, Al: 0.9wt%, Ti: 1.2wt%, B: 0.7%, W: 2.8%, and the balance is Fe and unavoidable impurities.

[0067] Example 5

[0068] The difference between this embodiment and embodiment 1 is that, in terms of the raw material system, the amount of Si added is 0.01wt%, and the remaining raw material systems are Ni: 49.0wt%, Cr: 18.0wt%, C: 0.07wt%, Mn: 2.5wt%, Mo: 5wt%, Nb: 5wt%, N: 0.25wt%, Al: 0.9wt%, Ti: 1.2wt%, B: 0.6%, W: 2.6%, and the balance is Fe and unavoidable impurities.

[0069] Example 6

[0070] The difference between this embodiment and embodiment 1 is that, in terms of the raw material system, the amount of W added is 3.5wt%, and the remaining raw material systems are Ni: 50.0wt%, Cr: 18.0wt%, C: 0.07wt%, Si: 0.35wt%, Mn: 2.0wt%, Mo: 5wt%, Nb: 5wt%, N: 0.2wt%, Al: 0.9wt%, Ti: 1.2wt%, B: 0.5%, and the balance is Fe and unavoidable impurities.

[0071] Example 7

[0072] The difference between this embodiment and embodiment 1 is that, in terms of the raw material system, the amount of N added is 0.35wt%, and the remaining raw material systems are Ni: 48.0wt%, Cr: 18.0wt%, C: 0.07wt%, Si: 0.35wt%, Mn: 3.0wt%, Mo: 5wt%, Nb: 5wt%, Al: 0.9wt%, Ti: 1.2wt%, B: 0.8%, W: 3.0%, and the balance is Fe and unavoidable impurities.

[0073] Example 8

[0074] The difference between this embodiment and embodiment 1 is that the molar ratio of MnN to CrN is 2:1.

[0075] Comparative Example 1

[0076] The difference between this comparative example and Example 1 is that the composition of each element in the nickel-based alloy includes the following components in weight parts: 0.01wt% C, 61.6wt% Ni, 0.55wt% Si, 8.7wt% Mo, 3.7wt% Nb, 22wt% Cr, 0.4wt% Ti, 0.1wt% Al, 1.4wt% Mn, and Fe as the balance.

[0077] Comparative Example 2

[0078] The difference between this comparative example and Example 1 is that the composition of each element in the 316L alloy includes the following components in weight parts: 12.0wt% Ni, 0.37wt% Si, 2.5wt% Mo, 17wt% Cr, 1.8wt% Mn, and Fe as the balance.

[0079] Test Example 1

[0080] The tensile strength, yield strength, elongation after fracture and wear amount of Examples 1-8 and Comparative Examples 1-2 were measured at high temperature conditions of 700°C-800°C. The tensile test conditions were as follows: the sample was heated to the set temperature at a heating rate of 10°C / s and then kept at that temperature for 10 minutes. The sample was stretched at a tensile speed of 0.1mm / min, and the test was terminated until all the samples broke. The high temperature wear test was performed using an MMG-5G friction and wear high temperature end surface tester. The friction disc was made of quenched and tempered 45# steel, cut into The disk has a positioning hole diameter of The center distance of the positioning holes is 35 mm. The test results are shown in Table 1 and Figure 1 to Figure 4 shown.

[0081] Table 1 Mechanical properties of different alloys

[0082]

[0083]

[0084] According to the performance measurement results in Table 1, the novel high temperature wear resistant nickel-based alloy of the present invention has excellent performance at a high temperature of 700°C-800°C, wherein the tensile strength is ≥580MPa, the yield strength is ≥460MPa, the elongation after fracture is ≥34.6%, and the friction loss is ≤1.31mg. Compared with the novel high temperature wear resistant nickel-based alloy of the present invention, when the content of tungsten and boron in the element composition increases, the high temperature wear resistance of the alloy is significantly improved.

[0085] Test Example 2

[0086] The alloy in Example 1 was subjected to SEM test and the Figure 5 ,Depend on Figure 5 It can be seen that the precipitated phases are mainly in the shape of grids and fishbones. The N element can increase the stability of supercooled austenite when it is dissolved in the alloy. As its content increases, the nitrogen content of the solid solution reaches a saturated state. The supersaturated nitrogen element will first combine with the boron element to form the BN phase. It will further form Cr(C,N) with Cr, which will improve the hardness and wear resistance of the alloy.

[0087] A friction and wear test was carried out on the alloy in Example 1. The sample was placed in an MMG-5G friction and wear high-temperature end face testing machine. First, click to start loading, then clear the load. When the friction pair contacts the pin disk, the load will suddenly increase. At this time, click to load. Subsequently, the machine will gradually load. When loaded to the load required by the experiment (150N), tighten the turntable, clear the friction torque, and click to start the turntable to begin the experiment. At this time, the load will fluctuate around 150N. After 10 minutes, click to save and record the data. After completing a set of experiments, click to unload and switch to the next set of test pieces. The surface of the friction and wear samples was characterized by SEM, and the results were obtained. Figure 6 .

[0088] from Figure 6 It can be seen from the figure that shallow furrows appear on the surface of the sample. These furrows have the same direction and are nearly parallel to each other. There are also some particles distributed in the furrows. The furrows are mainly generated in the friction experiment. When the hard phase in the friction pair material slides and rubs against the alloy surface, the compressive stress causes abrasive wear on the alloy surface. Figure 6 It can be seen from the observation that the furrows are shallow, indicating that the alloy has strong wear resistance under the lateral force of the friction pair.

[0089] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein by equivalents. If these modifications and variations fall within the scope of the claims of the present invention and their equivalent technologies, they should be considered to be within the protection scope of the present invention.

Claims

1. A high temperature wear resistant nickel-based alloy, characterized in that: The following components are included by mass percentage: Ni: 48.0~50.0wt%, Cr: 18.0~20.0wt%, C: less than 0.07wt%, Si: less than 0.35wt%, Mn: 2.0~3.0wt%, Mo: 3~5wt%, Nb: 4~6wt%, N: 0.2~0.3wt%, Al: 0.8~1.0wt%, Ti: 1.0~1.2wt%, B: 0.5~0.8%, W: 2.0~3.0%, and the balance is Fe and unavoidable impurities.

2. The high temperature wear resistant nickel-based alloy according to claim 1, characterized in that: The mass percentage of C in high temperature wear resistant nickel-based alloy is 0.04~0.07wt%.

3. The high temperature wear resistant nickel-based alloy according to claim 1, characterized in that: The mass percentage of Si in high temperature wear resistant nickel-based alloy is 0.30~0.35wt%.

4. The high temperature wear resistant nickel-based alloy according to claim 1, characterized in that: Calculated by element mass percentage: W / B is 2.7~5.5; and / or, The mass percentage of W and B in high temperature wear resistant nickel-based alloy is 2.7~3.5 wt%.

5. The high temperature wear resistant nickel-based alloy according to claim 1, characterized in that: Calculated by element mass percentage: (W+B) / N is 10~17.

6. The high temperature wear resistant nickel-based alloy according to claim 1, characterized in that: The N source includes MnN and CrN; wherein the molar ratio of MnN to CrN is (1-1.2): (1.3-2.0).

7. The high temperature wear resistant nickel-based alloy according to any one of claims 1 to 6, characterized in that: The high temperature wear resistant nickel-based alloy has a high temperature tensile strength of ≥580MPa at 700~800℃, a yield strength of ≥460MPa, an elongation after fracture of ≥34.6%, and a friction loss of ≤1.31mg.

8. A method for preparing a high temperature wear resistant nickel-based alloy according to any one of claims 1 to 7, characterized in that: The following steps are involved: (1) melting a raw material system including a nickel source, a chromium source, a silicon source, a molybdenum source, an aluminum source, a titanium source, a niobium source, a tungsten source, and a boron source to obtain a first molten steel; (2) When the molten steel is heated to 1350-1400° C., MnN and CrN are added to the first molten steel to obtain a second molten steel; (3) performing a refining treatment and a cooling and standing treatment on the second molten steel in sequence to obtain a third molten steel; (4) The third molten steel is deoxidized to obtain a high temperature wear resistant nickel-based alloy.

9. The method for preparing a high temperature wear resistant nickel-based alloy according to claim 8, characterized in that: In step (2), MnN is first added to the first molten steel, and then CrN is added.

10. The method for preparing a high temperature wear resistant nickel-based alloy according to claim 8 or 9, characterized in that: The particle size of the raw material system is 100-120 mesh; and / or, The refining treatment temperature is 1550~1600℃ and the time is 3~5min; and / or, The temperature of the cooling and standing treatment is 1350-1370°C; and / or, The deoxidation treatment uses silicon calcium barium deoxidizer, and the addition amount of silicon calcium barium deoxidizer accounts for 0.12~0.15% of the mass of the third steel liquid.

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