A gradient nickel-based alloy coating resistant to thermal fatigue cracking, a preparation method and applications thereof

Abstract

This invention discloses a gradient nickel-based alloy coating resistant to thermal fatigue cracking, its preparation method, and its application, relating to the field of thermal fatigue resistant pipeline technology. The chemical composition of the underlayer cladding metal powder, by mass percentage, is Ni 40–65 wt.%, Fe 10–30 wt.%, Cr 5–10 wt.%, Mo 10–15 wt.%, and W 10–15 wt.; the chemical composition of the surface layer cladding metal powder, by mass percentage, is Ni 50–70 wt.%, Fe 2–10 wt.%, Cr 15–25 wt.%, Mo 2–10 wt.%, and W 2–10 wt. The underlayer and surface layer are distributed in a double-layer gradient. The average linear expansion coefficient of the underlayer and the 12Cr1MoVg pearlitic heat-resistant steel substrate differs by less than 6% within the range of 500℃ at room temperature. The surface layer exhibits excellent resistance to thermal fatigue cracking and high-temperature oxidation. The combination of these two layers forms a gradient coating to solve the technical problem of surface cracking due to thermal fatigue in existing desuperheater downstream pipelines.

Description

Technical Field

[0001] This invention relates to the field of thermal fatigue resistant pipeline technology, and in particular to a gradient nickel-based alloy coating, preparation method and application for thermal fatigue cracking resistance. Background Technology

[0002] In high-temperature and high-pressure environments such as thermal power generation and petrochemical plants, the reliability and lifespan of welded seams in pressure-bearing pipelines directly affect the safe operation of the equipment. Downstream pipelines of boiler reheaters or desuperheaters are particularly prone to thermal fatigue failure. During service, this area is subjected to cyclical alternating thermal stresses caused by frequent start-ups and shutdowns, load fluctuations, and the injection of desuperheating water.

[0003] Common boiler reheaters or spray desuperheaters use 12Cr1MoVG alloy tubes, which are low-alloy pearlitic heat-resistant steels. The "12" indicates a carbon content of 0.12%, and the steel contains chromium, molybdenum, and vanadium. "G" stands for high pressure and boiler. It possesses excellent high-temperature strength and creep resistance, making it suitable for engineering applications in high-temperature and high-pressure environments.

[0004] The existing 12Cr1MoVG alloy pipes have inherent defects in their weld seams, such as abrupt changes in microstructure, residual stress, and geometric discontinuities, making them weak points for crack initiation and propagation. After tens of thousands of hours of service, on-site non-destructive testing revealed that dense, network-like thermal fatigue cracks frequently appeared on the inner surface of the pipe downstream of the desuperheater, while circumferential, straight-line cracks often appeared on the surface of the weld seams at pipe bends and other locations. Once these cracks initiate, they can easily propagate rapidly along grain boundaries under continuous cyclic heat loads and internal pressure stress, leading to a reduction in the effective load-bearing wall thickness of the pipe, causing through-thickness leaks or even pipe rupture accidents, forcing unplanned shutdowns of the unit and resulting in significant economic losses.

[0005] To address the problem of thermal fatigue cracking of the inner wall of heat-resistant steel pipes, existing technologies mainly employ the following measures:

[0006] Firstly, a thermal fatigue-resistant sleeve is installed on the inner wall of the downstream pipe of the desuperheater. This sleeve is mainly used to prevent the droplets of cooling water sprayed from the desuperheater from directly contacting the inner wall of the pipe, inhibiting frequent hot and cold alternations on the inner wall, and reducing the level of thermal fatigue stress. However, this sleeve often needs to be installed after machining inside the pipe, which makes the process design relatively complex. Moreover, the length of the sleeve is limited by the space of the pipe diameter. Beyond a certain length, the inner wall of the downstream pipe still faces relatively serious thermal fatigue damage.

[0007] Secondly, using base materials and welding materials with better resistance to thermal fatigue, such as martensitic stainless steel or nickel-based alloys instead of low-alloy steel, is an option. However, this approach involves high material costs, and welding dissimilar steels presents complex technical challenges such as interface carbon migration and mismatched physical properties.

[0008] Third, by optimizing welding processes and performing post-weld heat treatment, residual stress in the weld can be reduced, weld microstructure improved, and resistance to thermal fatigue cracking enhanced. However, these measures cannot fundamentally change the thermal stress state of the weld surface under service conditions and have no protective effect on non-welded areas of the pipeline inner wall.

[0009] Therefore, how to design low-cost coating protection materials and preparation processes without introducing new failure risks, and effectively improve the thermal fatigue crack resistance of the inner wall surface of the heat-resistant steel pipe downstream of the desuperheater, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0010] To address the aforementioned needs, this invention proposes a gradient nickel-based alloy coating, its preparation method, and its application to resist thermal fatigue cracking. The aim is to improve the fatigue resistance of the inner wall of heat-resistant steel pipes by combining the high toughness of the coating with the gradient coefficient of linear expansion, thereby solving the existing technical problem of reducing surface cracking caused by thermal fatigue in downstream pipes of desuperheaters. This provides a novel technical solution for extending the service life and strengthening high-temperature and high-pressure pipelines.

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] A gradient nickel-based alloy coating resistant to thermal fatigue cracking, its preparation method and application.

[0013] This coating system is designed for 12Cr1MoVg pearlitic heat-resistant steel substrates and is especially suitable for high thermal shock areas downstream of boiler reheaters or spray desuperheaters. A composite protective coating with gradient thermal expansion coefficient matching and high thermal fatigue resistance is prepared in situ on the inner wall of heat-resistant steel pipes through laser cladding process.

[0014] (1) Coating structure design

[0015] The protective coating has a double-layer gradient structure, consisting of an underlayer that is directly fused onto the surface of the 12Cr1MoVG steel substrate and a surface layer that is fused onto the underlayer from the inside out.

[0016] The gradual transition in composition and properties between the two layers effectively mitigates abrupt changes in the coefficient of thermal expansion between the coating and the substrate, reduces interfacial stress, and prevents coating peeling. The continuous transition in composition between the layers forms a metallurgical bonding interface, avoiding the physical interface delamination problem common in traditional coatings.

[0017] The thickness of a single coating layer is 0.4–1.2 mm, and the total thickness is 0.8–2.4 mm.

[0018] (2) Coating composition design

[0019] The alloy material of the coating is made of pure metal powder with a purity of ≥99.9% by weight, and the particle size range of the pure metal powder is 250 to 500 mesh.

[0020] The chemical composition of the cladding metal powder for the underlayer, by mass percentage, is: Ni 40–65 wt.%, Fe 10–30 wt.%, Cr 5–10 wt.%, Mo 10–15 wt.%, W 10–15 wt.%.

[0021] Effect: Through compositional design, the base coat maintains an average linear expansion coefficient of 12.5 × 10⁻⁶ within the range of room temperature to 500°C. -6 At approximately / ℃, compared to 11.8×10⁻⁶ of the 12Cr1MoVg matrix. -6 / ℃ matching, thereby significantly reducing interfacial thermal stress during thermal cycling.

[0022] The chemical composition of the cladding metal powder in the surface layer is as follows (by mass percentage): Ni 50–70 wt.%, Fe 2–10 wt.%, Cr 15–25 wt.%, Mo 2–10 wt.%, W 2–10 wt.%.

[0023] Effects: Increasing the Ni content in the surface layer composition design improves the surface toughness of the coating, enhancing its ability to prevent cracking when subjected to alternating hot and cold shocks from desuperheated water. At the same time, increasing the Cr content allows for the formation of a dense oxide film in a high-pressure steam environment, extending the long-term service life of the coating.

[0024] (3) Laser cladding mixed powder preparation method

[0025] Weigh out the powders of each element according to the design ratio, mix them using mechanical ball milling for ≥120 minutes to ensure uniform distribution of each component; place the mixed powder in a 150℃ constant temperature drying oven for more than 30 minutes to remove adsorbed moisture and prevent pores from being generated during laser cladding.

[0026] Laser cladding process:

[0027] The synchronous powder feeding method is used for cladding. First, the bottom layer is clad on the 12Cr1MoVg substrate, and then the surface layer is laser clad after cooling to below 100℃.

[0028] The laser heat source uses a fiber-coupled semiconductor laser with a beam wavelength of 1060nm and a laser spot size of 20 mm × 1.0 mm; the overlap rate is 15%–30%, and the powder feeding speed is 15–40 g / min to ensure that the coating is formed smoothly without any unfused areas; the protective gas is high-purity argon with a gas flow rate of 20–40 L / min to prevent oxidation of the molten pool; the laser power is 2.0–6.0 kW; and the laser scanning speed is 120–600 mm / min.

[0029] The present invention has the following beneficial effects:

[0030] This invention employs a double-layer gradient nickel-based alloy coating. The underlayer, by controlling the content of Mo and W elements, ensures that the thermal expansion coefficients of the coating and the substrate are close, resulting in an average linear expansion coefficient difference of less than 6% within the temperature range of 0–500℃. This significantly inhibits the initiation of thermal fatigue cracks at the interface, reduces interfacial thermal stress during thermal cycling, and suppresses the initiation of thermal fatigue cracks at the coating / substrate interface. The appropriate amount of Fe in the underlayer avoids drastic concentration gradient changes between the Ni-based alloy coating and the 12Cr1MoVg substrate, inhibiting the formation of brittle phases at the interface. The high Ni content in the surface layer gives the coating higher plastic deformation capacity under alternating thermal stress, delaying crack propagation rate. Furthermore, the appropriate Cr and Mo content in the surface layer can form a dense Cr2O3 protective film in a high-temperature steam environment, extending the effective protective life of the coating.

[0031] Compared to existing methods and the installation of inner wall sleeves, this invention has the advantages of in-situ laser cladding, no limitation on pipe diameter, and full-area protection; compared to replacing martensitic stainless steel or nickel-based alloy base materials, it has gradient coating, avoids dissimilar steel joint problems, and has the advantage of controllable cost; compared to optimizing the welding process, it has full inner wall coverage, including weld and non-weld areas, and has a wider range of applicability.

[0032] This invention achieves thermal expansion matching and suppresses the initiation of interfacial cracks by applying a bottom layer, delays crack propagation by a high Ni surface layer, and extends high-temperature service life by forming an oxide film with a high Cr / Mo ratio. These three factors work synergistically to significantly improve the thermal fatigue crack resistance of downstream pipelines of reheaters / desuperheaters. Attached Figure Description

[0033] Figure 1 The following are metallographic images of the surface layer, the underlayer, the interface region, and the 12Cr1MoVg base material in Embodiment 1 of the present invention.

[0034] Figure 2 This is a surface crack inspection image of the simulated thermal shock specimen in Example 1 of the present invention;

[0035] Figure 3 This is a graph showing the oxidation weight gain of the cladding surface layer and the 12Cr1MoVg substrate in Example 1 of the present invention.

[0036] Figure 4 This is a comparison diagram of the surface oxidation morphology of the cladding surface layer and the 12Cr1MoVg substrate in Example 1 of the present invention. Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0038] Example 1

[0039] This embodiment develops a gradient nickel-based alloy coating resistant to thermal fatigue cracking, its preparation method, and its application. The applicable substrate material for steam pipelines is 12Cr1MoVg steel pipelines.

[0040] Including the following:

[0041] 1. The coating structure adopts a double-layer gradient structure with a total thickness of approximately 1.0 mm. The base layer is 0.4 mm thick, and the surface layer is 0.6 mm thick.

[0042] 2. Coating composition design: The base coat is composed of Ni 50wt.%, Fe 22wt.%, Cr 5wt.%, Mo 13wt.%, and W 10wt.% by mass percentage. The surface layer is composed of Ni 65wt.%, Fe 5wt.%, Cr 20wt.%, Mo 5wt.%, and W 5wt.% by mass percentage.

[0043] 3. A composite powder for cladding was prepared using pure metal alloy powders. The Ni, Fe, and Cr metal powders used for the underlayer had a particle size of 300 mesh and a purity of 99.7 wt.%, while the Mo and W metal powders had a particle size of 400 mesh and a purity of 99.8 wt.%. The above metal powders were mixed in the specified proportions and then ball-milled using a planetary ball mill. A stainless steel grinding jar and cemented carbide (WC) grinding balls were used, with a ball-to-powder ratio of 8:1, a mill speed of 200 r / min, and a milling time of 120 min.

[0044] The surface layer uses Ni, Fe, and Cr metal powders with a particle size of 250 mesh and a purity of 99.7 wt.%, and Mo and W metal powders with a particle size of 300 mesh and a purity of 99.8 wt.%. These metal powders were mixed in a specific ratio and then ball-milled using a planetary ball mill. A stainless steel grinding jar and cemented carbide (WC) grinding balls were used, with a ball-to-powder ratio of 8:1. The ball mill speed was 200 r / min, and the milling time was 120 min. The milled powder was then dried in a 150℃ drying oven for 60 min to remove moisture.

[0045] 4. The laser cladding substrate is 10mm thick 12Cr1MoVg pearlitic heat-resistant steel. Before cladding, mechanical grinding is used to remove the oxide layer and oil stains on the surface.

[0046] 5. The laser cladding process is set as follows: a fiber-coupled semiconductor laser is used, with a laser wavelength of 1060nm and a laser beam size of 20mm×1.0mm. The process parameters used for the underlayer cladding are as follows: laser power 5.0 kW, laser scanning speed 240mm / min, powder feeding speed 20 g / min, overlap rate 25%, and argon flow rate 25 L / min.

[0047] 6. After the base layer cladding is completed, the temperature of the cladding layer is detected using an infrared thermometer. During the subsequent surface layer cladding, the interlayer temperature is controlled to be below 100℃. The process parameters used for surface layer laser cladding are as follows: laser power 3.5kW, laser scanning speed 240mm / min, powder feeding speed 30 g / min, overlap rate 20%, argon flow rate: 25 L / min. After cladding, the surface is allowed to cool naturally to room temperature.

[0048] 7. Inspection of the microstructure and properties of the cladding layer. A comparative inspection of the microstructure and properties of the surface layer and the 12Cr1MoVg substrate was conducted. The metallographic structures of the cladding surface layer, the cladding underlayer, the 12Cr1MoVg interface, and the 12Cr1MoVg base material are shown in the appendix. Figure 1 The surface layer of the cladding is dominated by uniform equiaxed crystals, with a fine and defect-free structure. The underlayer and interface zone of the cladding are dominated by columnar and cellular crystals, without porosity or crack defects. The 12Cr1MoVg base material has a mixed structure of pearlite and ferrite.

[0049] The coefficient of linear expansion for the cladding underlayer was measured, and the average coefficient of linear expansion was 12.1 × 10⁻⁶ within the temperature range of room temperature to 500°C. -6 / ℃. The average coefficient of linear expansion of the 12Cr1MoVg heat-resistant steel base material is 11.8 × 10⁻⁶ °C in the range of room temperature to 500 °C. -6 / ℃, the difference is less than 3%.

[0050] The thermal fatigue cracking resistance of the cladding surface layer and the 12Cr1MoVg base material was compared and tested using a simulated thermal shock method. 3cm × 3cm samples were cut, held at 600℃ for 10 min in a furnace, and then rapidly immersed in 20℃ cooling water for quenching. After 100 cycles, the surface crack rate was examined using dye penetrant testing. (See attached image.) Figure 2 To simulate the surface crack inspection results of the sample, no peeling or cracking was found in the cladding surface layer, while a network of fatigue cracks appeared on the surface of the 12Cr1MoVg base material, with a crack density reaching 18.5 mm / cm². 2 This indicates that the cladding surface layer has a significant ability to resist fatigue cracking.

[0051] To compare the high-temperature oxidation resistance of the cladding surface layer and the 12Cr1MoVg base material, samples with dimensions of 10mm × 10mm × 0.5mm were cut and subjected to a high-temperature oxidation test at 600℃. The oxidation time was 100h, and the weight gain of the samples was tested.

[0052] Appendix Figure 3 For comparison of the oxidation weight gain curves of the samples. (Attached) Figure 4 The results show a comparison of the surface oxidation morphology of the samples under scanning electron microscopy. The results indicate that the 12Cr1MoVg parent material showed a weight gain of 22.4 mg / cm³ after 100 h of oxidation. 2 The oxidation weight gain of the cladding surface layer was 3.6 mg / cm³. 2 It is only 16.1% of the heat-resistant steel matrix, and its high-temperature oxidation resistance, measured by oxidation weight gain, is about 6.2 times that of the parent material.

[0053] Example 2

[0054] This embodiment differs from Embodiment 1 in process parameters and powder formulation, and is primarily suitable for the inner wall protection of steam pipes with large thicknesses and higher temperatures. The percentages of Mo and W elements in the underlayer are increased, further reducing the coefficient of linear expansion, while the percentage of Fe element is increased, resulting in a smoother transition of elemental distribution at the interface between the underlayer and 12Cr1MoVg steel. The laser cladding process corresponding to the underlayer increases heat input, leading to more uniform fusion of the underlayer powder. The percentage of Cr element in the surface layer is increased, further enhancing resistance to high-temperature oxidation.

[0055] 1. The coating structure adopts a double-layer gradient structure with a total thickness of approximately 1.8 mm. The base layer is 0.6 mm thick, and the surface layer is 1.2 mm thick.

[0056] 2. Coating composition design: The base coat consists of Ni 43wt.%, Fe 25wt.%, Cr 5wt.%, Mo 15wt.%, and W 12wt.% by mass percentage. The surface layer consists of Ni 62wt.%, Fe 3wt.%, Cr 25wt.%, Mo 5wt.%, and W 5wt.% by mass percentage.

[0057] 3. A composite powder for cladding is prepared using pure metal alloy powders. The metal powder used for the underlayer has a Ni, Fe, and Cr particle size of 300 mesh and a purity of 99.7 wt.%, and a Mo and W particle size of 400 mesh and a purity of 99.8 wt.%. These metal powders are mixed in the specified proportions and then ball-milled using a planetary ball mill. A stainless steel grinding jar and cemented carbide (WC) grinding balls are used, with a ball-to-powder ratio of 10:1, a mill speed of 200 r / min, and a milling time of 200 min. The metal powder used for the surface layer has a Ni, Fe, and Cr particle size of 250 mesh and a purity of 99.7 wt.%, and a Mo and W particle size of 300 mesh and a purity of 99.8 wt.%. These metal powders are also mixed in the specified proportions and then ball-milled using a planetary ball mill. A stainless steel ball mill jar and cemented carbide WC grinding balls were used, with a ball-to-material ratio of 10:1. The ball mill speed was 200 r / min, and the milling time was 200 min. The mixed powder after ball milling was dried in a drying oven at 150℃ for 60 min to remove moisture.

[0058] 4. The laser cladding substrate is 30mm thick 12Cr1MoVg pearlitic heat-resistant steel. Before cladding, mechanical grinding is used to remove the oxide layer and oil stains on the surface.

[0059] 5. The laser cladding process is set as follows: a fiber-coupled semiconductor laser is used, with a laser wavelength of 1060nm and a laser beam size of 20mm×1.0mm. The process parameters used for the underlayer cladding are as follows: laser power 6.0kW, laser scanning speed 180mm / min, powder feeding speed 25g / min, overlap rate 30%, and argon flow rate 25L / min.

[0060] 6. After the base layer cladding is completed, an infrared thermometer is used to monitor the temperature of the cladding layer. During subsequent surface layer cladding, the interlayer temperature is controlled to be below 100℃. The process parameters used for surface layer laser cladding are as follows: laser power 4.0kW, laser scanning speed 200mm / min, powder feeding speed 30g / min, overlap rate 30%, and argon flow rate 25L / min. After cladding, the surface is allowed to cool naturally to room temperature.

[0061] 7. Inspection of the microstructure and properties of the cladding layer. A comparative inspection of the microstructure and properties of the surface layer and the 12Cr1MoVg substrate was conducted. The microstructure of the cladding surface layer consisted of uniform, fine equiaxed crystals with a grain size finer than that of Example 1, exhibiting a dense and defect-free structure. The cladding underlayer and interface region were dominated by columnar and cellular crystals. Due to the increased Fe element ratio, the element concentration gradient at the interface decreased, resulting in a smoother microstructure transition without defects such as pores or cracks.

[0062] The resistance to thermal fatigue cracking of the cladding surface layer and the 12Cr1MoVg base material was compared using a simulated thermal shock method. The material was heated to 650℃ for 10 min in a furnace, then rapidly immersed in 20℃ cooling water for quenching. After 100 cycles, the surface crack rate was examined using dye penetrant testing. The results showed that no peeling or cracking was observed in the cladding surface layer, while the 12Cr1MoVg base material exhibited a dense network of fatigue cracks with a crack density reaching 26.4 mm / cm². 2 Compared to Example 1, this example maintains excellent resistance to thermal fatigue cracking under higher temperatures and more severe cycling conditions.

[0063] To compare the high-temperature oxidation resistance of the cladding surface layer and the 12Cr1MoVg base material, a high-temperature oxidation test was conducted at 600℃ for 100 hours, and the weight gain of the samples was measured. The results showed that after 100 hours of oxidation, the weight gain of the 12Cr1MoVg base material reached 28.6 mg / cm³. 2 The oxidation weight gain of the cladding surface layer was 3.1 mg / cm³. 2 The Cr content is only 10.8% of that of the heat-resistant steel substrate, and the high-temperature oxidation resistance, measured by oxidation weight gain, is approximately 9.2 times that of the base material. Compared to Example 1, the surface layer Cr content is increased to 25 wt.%, resulting in a denser and more stable Cr2O3 oxide film formed at higher temperatures, significantly improving the high-temperature oxidation resistance.

[0064] This embodiment achieves a gradient nickel-based alloy coating suitable for thicker, higher-temperature conditions (650°C) by optimizing the composition design and process parameters, which can meet the protection requirements of supercritical and above-parameter steam pipelines.

[0065] Comparative Example 1

[0066] This comparative example is used to verify the crucial role of the undercoat layer in alleviating thermal stress and improving resistance to thermal fatigue cracking in gradient coating structures. This comparative example uses a single-layer protective coating without an undercoat layer transition; it directly clads a coating with the same surface layer composition as in Example 1. The substrate material is also 12Cr1MoVg heat-resistant steel pipe.

[0067] 1. The coating structure adopts a single-layer coating with a total thickness of approximately 1.0 mm.

[0068] 2. The coating composition is designed to be the same as that of the surface layer in Example 1, with the following metal element mass percentages: Ni 65wt.%, Fe 5wt.%, Cr 20wt.%, Mo 5wt.%, W 5wt.%.

[0069] 3. A composite powder for cladding was prepared using pure metal alloy powder. The particle size, purity, and mixing process of the metal powder used were the same as those used in the preparation of the surface layer powder in Example 1.

[0070] 4. The laser cladding substrate is 10mm thick 12Cr1MoVg pearlitic heat-resistant steel. Before cladding, the surface oxide layer and oil stains are removed by mechanical grinding.

[0071] 5. The laser cladding process was set as follows: a fiber-coupled semiconductor laser with a wavelength of 1060 nm and a beam size of 20 mm × 1.0 mm was used. Single-layer cladding was performed directly, and the process parameters were the same as those for the surface layer cladding in Example 1. After cladding, the surface was allowed to cool naturally to room temperature.

[0072] 6. Cladding Layer Microstructure and Performance Inspection. Metallographic observation of the cladding coating revealed that it was predominantly composed of equiaxed crystals, similar to the surface layer in Example 1, with a fine and dense structure free of obvious pores or cracks. At the interface between the coating and the 12Cr1MoVg substrate, due to the absence of a transition underlayer, a significant abrupt change in microstructure was observed in the interfacial bonding zone. The substrate side consisted of pearlite and ferrite, while the coating side consisted of nickel-based solid solution, with significant differences in lattice constants and thermophysical properties between the two.

[0073] The resistance to thermal fatigue cracking was tested using the same simulated thermal shock method as in Example 1. The crack density of the coating reached 8.5 mm / cm². This is a significant contrast to the gradient coating in Example 1, which showed no cracking or peeling under the same conditions.

[0074] Comparative Example 2

[0075] This comparative example is used to compare and verify the key roles of Cr and Mo elements in the surface layer on the coating's resistance to high-temperature oxidation and thermal fatigue cracking. A two-layer gradient coating was prepared in this comparative example. The composition and process of the underlayer were the same as in Example 1, but Cr and Mo elements were not added to the surface layer. The proportions of other components were adjusted to be supplemented with Ni, Fe, and W. All other process parameters remained consistent with Example 1.

[0076] 1. The coating structure adopts a double-layer gradient structure with a total thickness of approximately 1.0 mm. The base layer is 0.4 mm thick, and the surface layer is 0.6 mm thick.

[0077] 2. The undercoat composition design is the same as in Example 1. The surface layer does not contain Cr and Mo elements, but consists only of Ni, Fe, and W, with the following composition: Ni 80 wt.%, Fe 15 wt.%, W 5 wt.%.

[0078] 3. Powder Preparation: The base layer powder was identical to that used in Example 1. The surface layer powder was prepared in the following ratio: Ni 80 wt.%, Fe 15 wt.%, W 5 wt.%. The Ni and Fe metal powders had a particle size of 250 mesh, and the W metal powder had a particle size of 300 mesh, with a purity of ≥99.7 wt.%. The same ball milling process was used to mix the powders. The ball-to-powder ratio was 8:1, the milling speed was 200 r / min, and the time was 120 min. After mixing, the powders were dried in a 150℃ drying oven for 60 min.

[0079] 4. The laser cladding substrate is the same as in Example 1, 10mm thick 12Cr1MoVg pearlitic heat-resistant steel. Before cladding, the oxide layer and oil stains are removed by mechanical grinding.

[0080] 5. The laser cladding process is the same as in Example 1. After cladding, the laser cladding is allowed to cool naturally to room temperature.

[0081] 6. Inspection of the microstructure and properties of the cladding layer. Metallographic observation of the cladding coating was performed. The microstructure of the underlayer and interface was similar to that of Example 1, consisting of columnar and cellular crystals without pores or cracks. The surface layer microstructure was still dominated by equiaxed crystals, but due to the absence of Cr and Mo, the grains were coarsened, and the solid solution strengthening effect was weakened.

[0082] The resistance to thermal fatigue cracking was tested using the same simulated thermal shock method as in Example 1. The results showed that after 100 cycles, obvious tortoise-shell-like cracks appeared on the surface layer, with a crack density reaching 7.2 mm / mm². 2 Compared to Example 1 (no cracks), the resistance to thermal fatigue cracking is significantly reduced. This is due to the increased coefficient of linear expansion caused by the lack of Cr and Mo, the weakened solid solution strengthening effect, and the insufficient ability of the surface layer to resist thermal cyclic stress.

[0083] Comparison of high-temperature oxidation resistance. A high-temperature oxidation test was conducted at 600℃ for 100 hours, and the weight gain after oxidation was measured. After 100 hours of oxidation, the surface layer of this comparative example showed an oxidation weight gain of 18.6 mg / cm³. 2 The oxidation weight gain of the base material was 22.4 mg / cm³. 2 The oxidation weight gain of the cladding layer was 83% of that of the base material, only slightly better than the substrate. In contrast, the oxidation weight gain of the surface layer containing 22% Cr and 5% Mo in Example 1 was only 3.6 mg / cm³. 2The comparison shows that, due to the absence of Cr and Mo, a dense Cr2O3 or Mo-rich oxide film cannot be formed, resulting in a sharp decline in the coating's resistance to high-temperature oxidation, almost completely eliminating its protective effect. The table below shows the performance parameters of Example 1, Example 2, Comparative Example 1, and Comparative Example 2.

[0084]

[0085] Comparative Example 1 shows that if a single surface layer is used instead of an underlayer, the coating’s resistance to fatigue cracking will be significantly reduced due to the differences in thermophysical properties between the cladding layer and the base material, as well as the abrupt changes in elemental and microstructure at the interface.

[0086] Comparative Example 2 demonstrates that the surface layer nickel-based alloy must contain appropriate amounts of Cr and Mo to simultaneously achieve good resistance to thermal fatigue cracking and excellent high-temperature oxidation resistance. The Ni-Fe-W system alone cannot meet the protection requirements for the inner walls of high-temperature steam pipes.

[0087] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A gradient nickel-based alloy coating resistant to thermal fatigue cracking, comprising a 12Cr1MoVg pearlitic heat-resistant steel substrate; Its features are, include: The underlayer is clad onto a 12Cr1MoVg pearlitic heat-resistant steel substrate. The chemical composition of the cladding metal powder for the underlayer is, by mass percentage: Ni 40–65 wt.%, Fe 10–30 wt.%, Cr 5–10 wt.%, Mo 10–15 wt.%, and W 10–15 wt.%. The surface layer is clad onto the underlayer. The chemical composition of the cladding metal powder in the surface layer is as follows (by mass percentage): Ni 50–70 wt.%, Fe 2–10 wt.%, Cr 15–25 wt.%, Mo 2–10 wt.%, W 2–10 wt.%. The underlayer and surface layer are distributed in a double-layer gradient. The average linear expansion coefficient of the underlayer and the 12Cr1MoVg pearlitic heat-resistant steel substrate differs by less than 6% within the range of 500℃ at room temperature.

2. The gradient nickel-based alloy coating resistant to thermal fatigue cracking according to claim 1, characterized in that, The thickness of a single layer of the base layer or surface layer is 0.4–1.2 mm, and the total thickness is 0.8–2.4 mm.

3. A method for preparing a gradient nickel-based alloy coating resistant to thermal fatigue cracking, characterized in that: Includes the following steps: Step 1, Raw material preparation: Prepare composite powder for cladding using pure metal alloy powder; A. The metal powder used for the base coat has a particle size of 300 mesh for Ni, Fe, and Cr, with a purity of 99.7 wt.%; and a particle size of 400 mesh for Mo and W, with a purity of 99.8 wt.%. After mixing the above-mentioned metal powders according to the mass ratio for the underlayer as described in claim 1, Ball milling ensures uniform distribution of all components; B. The surface layer uses Ni, Fe, and Cr metal powders with a particle size of 250 mesh and a purity of 99.7 wt.%; Mo and W metal powders with a particle size of 300 mesh and a purity of 99.8 wt.%. After mixing the above-mentioned metal powders according to the surface layer ratio described in claim 1, the mixture is ball-milled to ensure that each component is evenly distributed. Step 2, Material selection: The laser cladding substrate is 12Cr1MoVg pearlitic heat-resistant steel. Before cladding, mechanical grinding is used to remove the oxide layer and oil stains on the surface. Step 3, Laser Cladding: Cladding is performed using a synchronous powder feeding method; First, laser cladding is performed on the bottom layer: the laser heat source uses a fiber-coupled semiconductor laser to ensure that the coating is formed smoothly and without any unfused areas; protective gas is supplied synchronously; laser power is 2.0 to 6.0 kW; laser scanning speed is 120 to 600 mm / min; After the undercoat cladding is completed, the temperature of the cladding layer is monitored until the surface temperature of the undercoat layer drops below 100°C. Then, surface layer laser cladding is performed: the laser heat source adopts fiber-coupled semiconductor laser to ensure that the coating is formed smoothly and without unfused areas; the laser power is 3.0 to 4.0 kW and the laser scanning speed is 200 to 240 mm / min; After the cladding is completed, allow it to cool naturally to room temperature; Step 4: Inspect the microstructure and properties of the cladding layer. Qualified products complete the preparation of a gradient nickel-based alloy coating resistant to thermal fatigue cracking.

4. The method for preparing a gradient nickel-based alloy coating resistant to thermal fatigue cracking according to claim 3, characterized in that: In step 1, a planetary ball mill is used for ball milling and mixing. The grinding balls are stainless steel grinding jars and cemented carbide WC grinding balls. The ball-to-material ratio is (8-10):

1. The ball mill speed is 200 r / min and the ball milling time is ≥120 minutes.

5. The method for preparing a gradient nickel-based alloy coating resistant to thermal fatigue cracking according to claim 3, characterized in that: After the bottom layer of metal powder from step 1 is placed in the ball mold, the mixed powder is placed in a constant temperature drying oven at 150°C for more than 30 minutes to remove moisture. After the surface layer metal powder in step 1 is ball-milled, the mixed powder is dried in a 150°C drying oven for 60 min to remove moisture and prevent pores from forming during the laser cladding process.

6. The method for preparing a gradient nickel-based alloy coating resistant to thermal fatigue cracking according to claim 3, characterized in that: The laser cladding process parameters for the underlayer in step 3 are as follows: beam wavelength 1060nm, laser spot size 20 mm * 1.0 mm; overlap rate 15%–30%, powder feeding speed 15–40 g / min. High-purity argon is used as the protective gas, with a flow rate of 20–40 L / min, to prevent oxidation of the molten pool.

7. The method for preparing a gradient nickel-based alloy coating resistant to thermal fatigue cracking according to claim 3, characterized in that: In step 3, the laser cladding beam wavelength of the surface layer is 1060nm, the laser spot size is 20 mm*1.0mm, the overlap rate is 15% to 30%, the powder feeding speed is 15 to 40g / min, and the protective gas is high-purity argon with a gas flow rate of 20 to 40 L / min to prevent oxidation of the molten pool.

8. The method for preparing a gradient nickel-based alloy coating resistant to thermal fatigue cracking according to claim 3, characterized in that: The high-purity argon gas has a purity exceeding 99.99%.

9. The application of a gradient nickel-based alloy coating resistant to thermal fatigue cracking, characterized in that, Used in high-temperature steam pipelines.

10. The application of the gradient nickel-based alloy coating resistant to thermal fatigue cracking according to claim 9, characterized in that, It is used in boiler reheaters or spray desuperheaters.

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