Gradiently overlapped martensitic stainless steel surface laser cladding coating and method for preparing same

Abstract

The present application relates to a kind of gradient lap joint martensitic stainless steel surface laser cladding coating and its preparation method, belong to laser cladding and remanufacturing technical field.Gradient lap joint martensitic stainless steel surface laser cladding coating preparation method includes using homogeneous coating raw material on the surface of martensitic stainless steel substrate after preheating laser cladding, sequentially preparing bottom layer, intermediate layer, surface layer, obtain homogeneous coating semi-finished product;The starting position of adjacent layer laser cladding melt channel of the bottom layer, intermediate layer, surface layer exists from the offset distance of the outer edge of substrate to the center of substrate;After heat preservation, cooling, finishing, preparation is completed to homogeneous coating semi-finished product.The present application can form three-dimensional staggered grid structure in homogeneous coating, effectively refine grain structure, improve coating interface bonding strength, anti-cavitation performance, reduce porosity, provide new direction for the high-quality repair of martensitic stainless steel overcurrent component.

Description

Technical Field

[0001] This invention belongs to the field of laser cladding and remanufacturing technology, specifically relating to a gradient-overlapping laser cladding coating on the surface of martensitic stainless steel and its preparation method. Background Technology

[0002] Martensitic stainless steel, due to its excellent comprehensive mechanical properties and good corrosion resistance, is widely used in the manufacture of industrial flow-through components. During long-term service, these components can develop and propagate cavitation damage, which can lead to component failure and threaten the safe operating life of the equipment. Laser cladding technology, with its advantages of a small heat-affected zone, controllable dilution rate, and metallurgical bonding with the substrate, has become an ideal method for surface repair and strengthening of martensitic stainless steel flow-through components.

[0003] In multi-pass laser cladding, the overlap ratio is one of the key process parameters affecting coating quality. The overlap ratio directly determines the degree of overlap between adjacent weld passes, thus affecting the interfacial bonding strength, porosity, surface smoothness, and residual stress distribution of the coating. In existing technologies, the overlap ratio is usually selected using a fixed value or an empirical range, such as a single overlap ratio of 40-50% commonly used in conventional laser cladding processes. Practice shows that using a single fixed overlap ratio makes it difficult to simultaneously meet the functional requirements of the coating in different thickness directions: a higher overlap ratio, while beneficial for eliminating incomplete fusion defects between weld passes and enhancing interfacial bonding, leads to an expansion of the repeated heating area, intensified heat accumulation effects, severe surface oxidation, and increased roughness; a lower overlap ratio, while improving surface quality, easily forms incomplete fusion zones at the weld pass boundaries, reducing interfacial bonding strength. Chinese invention patent application CN119040881A discloses a laser-clad nickel-based superalloy wear-resistant gradient coating and its preparation method. This method employs a high overlap rate of 50%–70% to prepare a heterogeneous composition gradient coating containing two transition layers and one functional layer, with each layer having the same overlap rate. Furthermore, conventional cladding processes use a single melt channel arrangement, with each melt channel stacked along the same direction. This easily leads to columnar crystals penetrating multiple cladding layers and undergoing continuous epitaxial growth, forming a coarse, directional solidification structure. This structure exhibits significant anisotropy, limiting its resistance to cavitation erosion.

[0004] To address the aforementioned issues, researchers have attempted to improve coating quality by optimizing parameters such as laser power and scanning speed, but with limited success. Currently, there is a lack of laser cladding technology that can simultaneously achieve high-strength interfacial bonding, uniform and refined microstructure, and high-quality surface formation within a homogeneous material system through the synergistic control of overlap rate and melt channel arrangement. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies that rely on a single fixed overlap rate to simultaneously ensure coating interface bonding strength and surface quality. This invention provides a gradient overlap laser cladding coating for martensitic stainless steel and its preparation method. By finely controlling the overlap rate layer by layer and staggering the overlap offset layer by layer, the growth of columnar crystals can be effectively controlled at the microstructure level, thereby obtaining a homogeneous coating with high bonding strength, good surface quality, and excellent cavitation resistance, achieving a synergistic improvement in coating bonding strength and surface quality.

[0006] The technical solution adopted in this invention is as follows: In a first aspect, the present invention provides a method for preparing a laser cladding coating on a gradient-overlapping martensitic stainless steel surface, comprising the following steps: S1. Prepare homogeneous coating materials; S2. Pre-treat the martensitic stainless steel substrate and preheat the pre-treated martensitic stainless steel substrate to obtain the substrate. S3. Using homogeneous coating materials, laser cladding is performed on the substrate surface to prepare a bottom layer, an intermediate layer, and a surface layer in sequence, resulting in a homogeneous coating semi-finished product; the laser cladding overlap rate of the bottom layer, intermediate layer, and surface layer decreases in gradient; the starting position of the laser cladding melt channel between adjacent layers of the bottom layer, intermediate layer, and surface layer has an offset distance from the outer edge of the substrate to the center of the substrate. S4. After heat preservation, the homogeneous coating semi-finished product is cooled and finely processed to complete the preparation of the homogeneous coating.

[0007] Furthermore, the homogeneous coating material mentioned in step S1 is a powder material with the same chemical composition as the martensitic stainless steel matrix; the type of martensitic stainless steel includes 0Cr. 13 Ni5Mo, 2Cr 13 1Cr 17 One of Ni2, wherein the particle size distribution of the homogeneous coating material is controlled within the range of 15-150 μm to ensure good powder flowability and stability of the cladding process.

[0008] By adopting the above technical solution and using homogeneous powder with the same composition as the substrate for laser cladding repair, it is possible to ensure that the coating and the substrate have good physical compatibility and corrosion resistance consistency, and avoid the interfacial stress problem caused by heterogeneous materials.

[0009] Furthermore, the pretreatment of the martensitic stainless steel substrate in step S2 includes grinding, cleaning and degreasing, and flaw detection. Grinding can remove fatigue layer and oxides (rust removal), acetone is used for cleaning and degreasing, and flaw detection is performed to ensure that there are no hidden cracks on the surface. The specific tools and reagents used for pretreatment can be implemented in accordance with conventional methods in the field, without too many restrictions.

[0010] Furthermore, the preheating conditions in step S2 are as follows: the pretreated martensitic stainless steel substrate is preheated at a temperature of 200-350℃ and held for 25-60 minutes to reduce the temperature gradient during the laser cladding process and reduce thermal stress.

[0011] Furthermore, in step S3, the laser cladding overlap rate of the bottom layer, the intermediate layer, and the surface layer decreases in a gradient, including: the laser cladding overlap rate of the bottom layer is 45-50%, the laser cladding overlap rate of the intermediate layer is 35-45%, and the laser cladding overlap rate of the surface layer is 25-35%.

[0012] Furthermore, in step S3, the number of laser cladding layers in the bottom layer is 1-3, and the thickness of a single layer in the bottom layer is 0.3-0.5mm; the number of laser cladding layers in the middle layer in step S3 is 1-3, and the thickness of a single layer in the middle layer is 0.3-0.5mm; the number of laser cladding layers in the surface layer in step S3 is 1-3, and the thickness of a single layer in the surface layer is 0.3-0.5mm.

[0013] Furthermore, the laser cladding process parameters for the bottom layer, intermediate layer, and surface layer described in step S3 shall meet the following conditions: laser power 800-1500W, scanning speed 6-12mm / s, spot diameter 1.5-1.7mm, and powder feeding amount 15-25g / min.

[0014] This invention employs a gradient overlap rate layering control strategy. The bottom layer uses a high overlap rate of 45-50% for cladding, ensuring a fully remelted zone between adjacent weld channels, thereby eliminating interface incomplete fusion defects and forming a strong metallurgical bond between the coating and the substrate. The middle layer uses a medium overlap rate of 35-45% for cladding, balancing heat input and microstructure uniformity, allowing the heat-affected zone to transition smoothly along the thickness direction, avoiding stress concentration, and effectively releasing internal stress within the coating. The surface layer uses a low overlap rate of 25-35% for cladding, reducing the number of repeated scans, decreasing heat accumulation effects, suppressing surface oxidation and alloy element burn-off, and reducing molten pool fluctuations, thereby obtaining a smooth and flat surface morphology.

[0015] Furthermore, the starting position of the laser cladding track between adjacent layers of the bottom layer, intermediate layer, and surface layer has an offset distance from the outer edge of the substrate to the center of the substrate, including: The offset distance is 1 / 3 to 1 / 2 of the overlap width, the overlap width is 0.7 to 1.5 mm, and the overlap width is the width of the overlapping area of ​​adjacent laser cladding channels; wherein, the overlap width, the channel width and the overlap rate have the following relationship: overlap width = channel width × overlap rate.

[0016] Furthermore, the offset distance between the starting positions of the laser cladding tracks of adjacent layers from the outer edge of the substrate to the center of the substrate means that: the starting position of the laser cladding track of the intermediate layer is offset from the starting position of the laser cladding track of the bottom layer from the outer edge of the substrate to the center of the substrate; the starting position of the laser cladding track of the surface layer is offset from the starting position of the laser cladding track of the intermediate layer from the outer edge of the substrate to the center of the substrate.

[0017] This invention employs an overlapping offset staggered strategy to form a three-dimensional staggered mesh structure within the coating. This structure blocks the continuous epitaxial growth of the underlying columnar crystals along the deposition direction, forcing the solidification front to re-nucleate at the staggered interface, resulting in significant grain refinement. Within this three-dimensional staggered mesh structure, the staggered melt channel interfaces form a three-dimensional spatial network reinforcement structure, which acts as a mechanical reinforcement similar to a steel mesh, improving the overall impact toughness and deformation resistance of the coating. Furthermore, the three-dimensional staggered mesh structure effectively blunts the microcrack tips generated during cavitation impact, altering the crack propagation path, increasing crack propagation resistance, and preventing cracks from extending into depth, thereby extending the service life of the coating.

[0018] Furthermore, the cooling after heat preservation in step S4 involves heat preservation at 200-300℃ for 1-3 hours followed by furnace cooling to eliminate residual thermal stress.

[0019] Furthermore, the finishing process in step S4 involves grinding and polishing the surface of the coating after it has cooled down following heat preservation, so that the surface roughness Ra ≤ 0.9 μm, which meets the surface quality requirements of the flow-through components and completes the preparation of the homogeneous coating.

[0020] Secondly, the present invention provides a gradient-overlapping laser cladding coating on the surface of martensitic stainless steel, which is prepared by the above-mentioned method for preparing a gradient-overlapping laser cladding coating on the surface of martensitic stainless steel. The laser cladding coating on the surface of martensitic stainless steel has the following characteristics: interfacial bonding strength ≥1220MPa; porosity ≤0.3%; average grain size ≤4.5μm; cavitation erosion weight loss rate ≤0.28mg / h, and its cavitation erosion resistance is more than 3.8 times that of the martensitic stainless steel substrate.

[0021] Compared with the prior art, the beneficial effects of the present invention are: This invention achieves significant technical results through a synergistic process innovation of "gradient overlap rate layered control and alternating overlap offset." Firstly, regarding interfacial bonding strength, the bottom layer of the homogeneous coating employs a high overlap rate of 45-50%, completely eliminating the inter-pass lack of fusion defects commonly found in traditional processes. This achieves a strong metallurgical bond between the coating and the substrate, with an interfacial bonding strength exceeding 1220 MPa, approximately 30% higher than conventional single overlap rate processes. Simultaneously, the intermediate layer uses a medium overlap rate of 35-45%, effectively balancing heat input and microstructure uniformity. The surface layer employs a low overlap rate of 25-35%, significantly reducing the heat accumulation effect caused by repeated heating, lowering the coating surface roughness to Ra≤0.9μm, and controlling the porosity to within 0.3%, thus achieving high-quality surface forming.

[0022] In terms of microstructure control, this invention constructs a three-dimensional interlaced grid structure within the coating by offsetting the starting position of the laser cladding channels between adjacent layers (bottom, intermediate, and surface layers) from the outer edge of the substrate to the center. This structure effectively blocks the continuous epitaxial growth of columnar crystals along the deposition direction, forcing the solidification front to re-nucleate at the interlaced interfaces, promoting grain refinement, and reducing the average grain size to below 4.5 μm. The three-dimensional interlaced grid structure also provides mechanical reinforcement; the interlaced channels form a spatial network reinforcement structure, improving the coating's strength and impact toughness.

[0023] Ultimately, the aforementioned technological innovations led to improved cavitation erosion resistance. Testing showed that the cavitation erosion weight loss rate of the laser-clad coating on the martensitic stainless steel surface prepared by this invention was reduced to below 0.28 mg / h, and its cavitation erosion resistance was more than 3.8 times that of the martensitic stainless steel substrate. While maintaining complete homogeneity with the martensitic stainless steel substrate, this invention, through refined design of the overlap ratio, achieved a synergistic improvement in coating bonding strength, microstructure uniformity, surface quality, and cavitation erosion resistance, providing an effective technical solution for high-quality repair of martensitic stainless steel components, and possessing promising industrial application prospects and widespread application value. Attached Figure Description

[0024] Figure 1 This is an overall schematic diagram of the gradient-overlapping laser cladding coating on the surface of martensitic stainless steel prepared in Example 1 of the present invention. Figure 2 This is a schematic diagram illustrating the preparation principle of the gradient overlapping laser cladding coating on the surface of martensitic stainless steel as shown in Embodiment 1 of the present invention. Figure 3 This is a schematic diagram of the metallographic morphology of the coating surface prepared in Example 1 of the present invention; Figure 4 This is a schematic diagram of the metallographic morphology of the coating surface prepared in Comparative Example 1 of the present invention; Reference numerals: Substrate 1, Bottom layer 2, Intermediate layer 3, Surface layer 4, Bottom layer overlap rate 501, Intermediate layer overlap rate 502, Surface layer overlap rate 503, First offset distance 601, Second offset distance 602. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the implementation schemes of this invention will be described in detail below with reference to specific embodiments. Unless otherwise defined, the technical and scientific terms used in this invention have the meanings commonly understood by those skilled in the art. Without departing from the concept of this invention, those skilled in the art can make various improvements and changes to the specific embodiments described in this specification, and all such improvements and changes fall within the scope of protection of this invention.

[0026] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0027] Example 1: This embodiment provides a method for preparing a laser cladding coating on a gradient-overlapping martensitic stainless steel surface, including the following steps: S1. Prepare homogeneous coating materials, including: 0Cr stainless steel with the same composition as the martensitic stainless steel matrix was prepared by vacuum atomization. 13 The Ni5Mo martensitic stainless steel powder has the following chemical composition by weight percentage: C 0.06%, Cr 13.5%, Ni 4.8%, Mo 1.2%, Si 0.3%, Mn 0.5%, with the balance being Fe and unavoidable impurities. The martensitic stainless steel powder is sieved to obtain spherical powder with a particle size distribution of 15-150 μm, a flowability ≤18s / 50g, and a loose density ≥4.2g / cm³. 3 .

[0028] S2. Pre-treating the martensitic stainless steel substrate and preheating the pre-treated martensitic stainless steel substrate to obtain a substrate, including: Select 0Cr 13 Ni5Mo martensitic stainless steel was used as the base material, with dimensions of 100mm×100mm×20mm. The surface of the martensitic stainless steel base to be clad was polished by using 240#, 400# and 600# sandpaper in sequence, then ultrasonically cleaned with acetone for 15 minutes to remove oil stains, rinsed with anhydrous ethanol and dried with cold air.

[0029] The pretreated martensitic stainless steel substrate is placed in an induction heating device and heated to 250°C at a heating rate of 10°C / min. The temperature is held for 25 minutes to ensure that the temperature of the martensitic stainless steel substrate is uniform and to reduce the temperature gradient in the subsequent cladding process, thus obtaining the substrate.

[0030] S3. Using homogeneous coating materials, laser cladding is performed on the substrate surface to sequentially prepare the bottom layer, intermediate layer, and surface layer, resulting in a homogeneous coating semi-finished product, including: A coaxial powder feeding laser cladding system was used for multi-pass lap cladding. The laser was a fiber laser with a wavelength of 1080nm, a spot diameter of 1.6mm, and a cladding width of approximately 3mm.

[0031] The bottom layer is prepared by laser cladding on the substrate surface: the bottom layer has an overlap rate of 45% (overlap width of 1.35mm), laser power of 1200W, scanning speed of 8mm / s, powder feed rate of 20g / min, and cladding of 2 layers with a single layer thickness of 0.4mm and a total thickness of 0.8mm; the high overlap rate of this layer ensures that a sufficient remelting zone is formed between adjacent melt channels.

[0032] An intermediate layer is prepared by laser cladding on the bottom surface: the starting position of the laser cladding channel of the intermediate layer is offset from the starting position of the laser cladding channel of the bottom layer by a distance from the outer edge of the substrate to the center of the substrate. Taking the starting position of the channel of the bottom layer as 0mm (reference zero point), the starting position of the channel of the intermediate layer is offset by 0.6mm towards the center of the substrate (approximately 44% of the overlap width of the bottom layer); the intermediate layer adopts an overlap rate of 40% (overlap width of 1.2mm), laser power of 1100W, scanning speed of 8mm / s, powder feed rate of 20g / min, cladding 2 layers, single layer thickness of 0.4mm, total thickness of 0.8mm; the medium overlap rate of this layer achieves the microstructure transition.

[0033] A surface layer is prepared by laser cladding on the intermediate layer: the starting position of the laser cladding track of the surface layer is offset by 0.4 mm from the outer edge of the substrate to the center of the substrate compared with the starting position of the laser cladding track of the intermediate layer (approximately 33% of the overlap width of the intermediate layer); the surface layer adopts an overlap rate of 30% (overlap width of 0.9 mm), laser power of 1000 W, scanning speed of 8 mm / s, powder feed rate of 20 g / min, cladding 1 layer, single layer thickness of 0.4 mm, and total thickness of 0.4 mm; the low overlap rate of this layer reduces heat accumulation and improves surface quality.

[0034] Argon gas was used as the protective gas throughout the laser cladding process, with a flow rate of 15 L / min, to ensure no oxidation and to obtain a homogeneous coating semi-finished product.

[0035] S4. After heat preservation, the homogeneous coating semi-finished product is cooled and finely processed to complete the preparation of the homogeneous coating, including: The homogeneous coating semi-finished product is immediately placed in a heat treatment furnace and cooled to 250°C at a cooling rate of 5°C / min. It is then held at that temperature for 2 hours for annealing and then cooled to room temperature in the furnace to eliminate residual thermal stress.

[0036] The finishing process involves grinding and polishing the surface of the coating after heat preservation and cooling to remove surface micro-protrusions, reduce surface roughness to Ra≤0.9μm, and achieve a final coating thickness of 2.0mm, thus completing the preparation of the homogeneous coating.

[0037] A schematic diagram of the gradient-overlapping laser cladding coating on the surface of martensitic stainless steel prepared in this embodiment is shown below. Figure 1 As shown, a gradient-overlapping martensitic stainless steel surface laser cladding coating is provided on the substrate 1, including a bottom layer 2, an intermediate layer 3, and a surface layer 4 arranged sequentially.

[0038] The schematic diagram illustrating the preparation principle of the laser cladding coating on the surface of the gradient-overlapping martensitic stainless steel prepared in this embodiment is shown below. Figure 2 As shown, the bottom layer 2 prepared according to the preparation method has 2 cladding layers, the middle layer 3 has 2 cladding layers, and the surface layer 4 has 1 cladding layer; the bottom layer overlap rate 501 of the bottom layer cladding of the bottom layer 2 is 45%, the middle layer overlap rate 502 of the middle layer cladding of the middle layer 3 is 40%, and the surface layer overlap rate 503 of the surface layer cladding of the surface layer 4 is 30%; there is a first offset distance 601 between the bottom layer 2 and the middle layer 3, and a second offset distance 602 between the middle layer 3 and the surface layer 4.

[0039] Example 2: The difference between this embodiment and Embodiment 1 is that: S3. Using homogeneous coating materials, laser cladding is performed on the substrate surface to sequentially prepare the bottom layer, intermediate layer, and surface layer, resulting in a homogeneous coating semi-finished product, including: A coaxial powder feeding laser cladding system was used for multi-pass lap cladding. The laser was a fiber laser with a wavelength of 1080nm, a spot diameter of 1.6mm, and a cladding width of approximately 3mm.

[0040] The bottom layer is prepared by laser cladding on the substrate surface: the bottom layer has an overlap rate of 50% (overlap width of 1.5mm), laser power of 1400W, scanning speed of 7mm / s, powder feed rate of 22g / min, and 3 layers are clad, with a single layer thickness of 0.3mm and a total thickness of 0.9mm; the high overlap rate of this layer ensures that a sufficient remelting zone is formed between adjacent melt channels.

[0041] An intermediate layer is prepared by laser cladding on the bottom surface: the starting position of the laser cladding channel of the intermediate layer is offset from the starting position of the laser cladding channel of the bottom layer by a distance from the outer edge of the substrate to the center of the substrate. Taking the starting position of the channel of the bottom layer as 0mm (reference zero point), the starting position of the channel of the intermediate layer is offset by 0.7mm towards the center of the substrate (approximately 46.7% of the overlap width of the bottom layer); the intermediate layer adopts an overlap rate of 45% (overlap width of 1.35mm), laser power of 1200W, scanning speed of 8mm / s, powder feed rate of 20g / min, cladding 2 layers, single layer thickness of 0.4mm, total thickness of 0.8mm; the medium overlap rate of this layer achieves the microstructure transition.

[0042] A surface layer is prepared by laser cladding on the surface of the intermediate layer: the starting position of the laser cladding track of the surface layer is offset by 0.5 mm from the outer edge of the substrate to the center of the substrate compared with the starting position of the laser cladding track of the intermediate layer (approximately 37% of the overlap width of the intermediate layer); the surface layer adopts an overlap rate of 35% (overlap width of 1.05 mm), laser power of 1000 W, scanning speed of 9 mm / s, powder feed of 18 g / min, cladding 1 layer, single layer thickness of 0.4 mm, and total thickness of 0.4 mm; the low overlap rate of this layer reduces heat accumulation and improves surface quality.

[0043] Argon gas was used as the protective gas throughout the laser cladding process, with a flow rate of 15 L / min, to ensure no oxidation and to obtain a homogeneous coating semi-finished product.

[0044] Example 3: The difference between this embodiment and Embodiment 1 is that: S3. Using homogeneous coating materials, laser cladding is performed on the substrate surface to sequentially prepare the bottom layer, intermediate layer, and surface layer, resulting in a homogeneous coating semi-finished product, including: A coaxial powder feeding laser cladding system was used for multi-pass lap cladding. The laser was a fiber laser with a wavelength of 1080nm, a spot diameter of 1.5mm, and a cladding width of approximately 2.8mm.

[0045] The bottom layer is prepared by laser cladding on the substrate surface: the bottom layer has an overlap rate of 50% (overlap width of 1.4mm), laser power of 1500W, scanning speed of 6mm / s, powder feed rate of 25g / min, and cladding of 2 layers with a single layer thickness of 0.4mm and a total thickness of 0.8mm; the high overlap rate of this layer ensures that a sufficient remelting zone is formed between adjacent melt channels.

[0046] An intermediate layer was prepared by laser cladding on the substrate surface. The starting position of the laser cladding channel in the intermediate layer was offset from the starting position of the laser cladding channel in the substrate from the outer edge to the center. Taking the starting position of the channel in the substrate as 0mm (zero point), the starting position of the channel in the intermediate layer was offset 0.7mm towards the center (50% of the overlap width of the substrate). The intermediate layer used an overlap rate of 35% (overlap width of 0.98mm), a laser power of 1200W, a scanning speed of 8mm / s, a powder feed rate of 20g / min, and cladding one layer with a single layer thickness of 0.4mm and a total thickness of 0.4mm. This moderate overlap rate achieved a smooth transition of the microstructure.

[0047] A surface layer is prepared by laser cladding on the surface of the intermediate layer: the starting position of the laser cladding track of the surface layer is offset by 0.45 mm from the outer edge of the substrate to the center of the substrate compared with the starting position of the laser cladding track of the intermediate layer (approximately 46% of the overlap width of the intermediate layer); the surface layer adopts an overlap rate of 25% (overlap width of 0.7 mm), laser power of 950 W, scanning speed of 11 mm / s, powder feed rate of 18 g / min, cladding one layer, single layer thickness of 0.4 mm, and total thickness of 0.4 mm; the low overlap rate of this layer reduces heat accumulation and improves surface quality.

[0048] Argon gas was used as the protective gas throughout the laser cladding process, with a flow rate of 15 L / min, to ensure no oxidation and to obtain a homogeneous coating semi-finished product.

[0049] Example 4: The difference between this embodiment and Embodiment 1 is that: In S1, 2Cr with the same composition as the martensitic stainless steel matrix was prepared by vacuum atomization. 13 Martensitic stainless steel powder.

[0050] In step S2, the pretreated martensitic stainless steel substrate is placed in an induction heating device and heated to 200°C at a heating rate of 10°C / min. The temperature is then held for 60 minutes to ensure that the temperature of the martensitic stainless steel substrate is uniform and to reduce the temperature gradient in the subsequent cladding process, thereby obtaining the substrate.

[0051] In S3, the spot diameter is 1.7 mm; the bottom layer is prepared by laser cladding on the substrate surface: laser power 800 W, scanning speed 6 mm / s, powder feed rate 15 g / min, cladding 1 layer, single layer thickness 0.5 mm.

[0052] An intermediate layer was prepared by laser cladding on the bottom surface: laser power 1100W, scanning speed 6mm / s, powder feed rate 20g / min, cladding 3 layers, and single layer thickness 0.3mm.

[0053] A surface layer was prepared by laser cladding on the intermediate layer: laser power 1100W, scanning speed 12mm / s, powder feed rate 20g / min, cladding 3 layers, and single layer thickness 0.3mm.

[0054] In S4, the homogeneous coating semi-finished product is immediately placed in a heat treatment furnace and cooled to 200°C at a cooling rate of 5°C / min. It is then held at that temperature for 3 hours for annealing and then cooled to room temperature with the furnace to eliminate residual thermal stress.

[0055] Example 5: The difference between this embodiment and Embodiment 1 is that: In S1, 1Cr with the same composition as the martensitic stainless steel matrix was prepared by vacuum atomization. 17 Ni2 martensitic stainless steel powder.

[0056] In step S2, the pretreated martensitic stainless steel substrate is placed in an induction heating device and heated to 350°C at a heating rate of 10°C / min. The temperature is then held for 30 minutes to ensure that the temperature of the martensitic stainless steel substrate is uniform and to reduce the temperature gradient in the subsequent cladding process, thereby obtaining the substrate.

[0057] In S3, an intermediate layer is prepared by laser cladding on the bottom surface: 1 layer is clad with a single layer thickness of 0.5 mm.

[0058] A surface layer is prepared by laser cladding on the intermediate layer: one layer is clad with a single layer thickness of 0.5 mm.

[0059] In S4, the homogeneous coating semi-finished product is immediately placed in a heat treatment furnace and cooled to 300°C at a cooling rate of 5°C / min. It is then held at that temperature for 1 hour for annealing and then cooled to room temperature with the furnace to eliminate residual thermal stress.

[0060] Comparative Example 1: This comparative example uses a traditional single-overlap ratio laser cladding process. The difference between the laser cladding coating preparation method on the surface of martensitic stainless steel and Example 1 is that: Step S3 involves multi-layer cladding using a coaxial powder-feeding laser cladding system. The entire cladding process employs a single, fixed overlap rate of 45%, a laser power of 1200W, a scanning speed of 8mm / s, a powder feed rate of 20g / min, and a spot diameter of 1.5mm. A total of 5 layers are clad, with a total thickness of 2.0mm. During the cladding process, the starting position of each cladding channel is the same, and there is no offset distance between the starting positions of adjacent cladding channels. Argon gas is used as the protective gas throughout the laser cladding process, with a flow rate of 15L / min.

[0061] Comparative Example 2: This comparative example is used to verify the advantages of the interlacing of overlapping offsets (i.e., the design advantage of the offset distance). The difference between the laser cladding coating preparation method on the surface of martensitic stainless steel and Example 1 is that there is no offset distance between the starting positions of the cladding channels between adjacent layers in step S3, and the starting positions of the channels in each layer are the same.

[0062] Performance testing: The coatings prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to systematic performance tests, and the test methods are as follows: (1) Interface bonding strength (tensile strength) test: The tensile method was adopted and the test was carried out on a universal testing machine in accordance with GB / T 8642-2002 standard. Five samples were tested in each group and the average value was taken.

[0063] (2) Microhardness test: Vickers hardness tester was used with a load of 200g and a holding time of 15s. A point was tested every 0.2mm along the coating depth direction, and the average value was taken.

[0064] (3) Porosity test: The porosity of the metallographic photographs of the coating section was statistically analyzed using the image analysis method. Ten fields of view were counted for each sample, and the average value was taken.

[0065] (4) Grain size test: Electron backscatter diffraction technology was used to statistically analyze the grain size of the coating cross section. The statistical area of ​​each sample was ≥5000μm. 2 .

[0066] (5) Cavitation resistance test: In accordance with GB / T 6383-2009 standard, the ultrasonic vibration cavitation test method was adopted. The test medium was deionized water, the temperature was 25±2℃, the vibration frequency was 20kHz, the amplitude was 60μm, and the sample immersion depth was 12mm.

[0067] Test Results and Analysis: (1) Data on interfacial bonding strength (tensile strength) and microhardness are shown in Table 1.

[0068]

[0069] Table 1 shows that Example 1, using the gradient overlap rate layering control of the present invention, achieved a tensile strength of 1250 MPa, while Comparative Example 1 (single overlap rate) achieved 1140 MPa. Furthermore, the hardness of Example 1 was significantly higher than that of Comparative Example 1, indicating that the interfacial bonding strength of Example 1 was superior to that of Comparative Example 1. This demonstrates that gradient overlap rate layering can effectively improve the interfacial bonding strength between the coating material and the substrate material. Compared to Comparative Example 2, Example 1 used the same gradient overlap rate layering control, the only difference being that Comparative Example 2 did not employ a staggered overlap offset method. The tensile strength of Comparative Example 2 decreased by 2.7%, and the hardness also decreased, proving that overlap offset also helps improve the interfacial bonding strength between the coating material and the substrate material. Moreover, the interfacial bonding strength and hardness of Examples 1-3 were all higher than those of Comparative Example 1 and Comparative Example 2.

[0070] (2) Data on porosity and average grain size are shown in Table 2.

[0071]

[0072] Both Example 1 and Comparative Example 2 employed the same gradient overlap rate layering control, differing only in whether a layer-by-layer staggered overlap offset strategy was used. The average grain sizes of Example 1 and Comparative Example 2 were 4.2 μm and 5.5 μm, respectively, with porosities of 0.25% and 0.35%, respectively, while the average grain size of Comparative Example 1 was 10.5 μm. Figure 3 , Figure 4 The metallographic morphology of the two studies clearly shows a significant difference in average grain size. Furthermore, the average grain size and porosity of Examples 1-3 are all smaller than those of Comparative Examples 1 and 2, demonstrating that gradient overlap and overlap offset have a significant effect on refining grains and reducing porosity.

[0073] (3) Data on cavitation resistance are shown in Table 3.

[0074]

[0075] The cavitation erosion resistance of Example 1 is 3.9 times that of the substrate, which is far superior to the comparative example, proving that the present invention can significantly improve the cavitation erosion resistance of the coating and verifying the necessity of the technical solution of the present invention.

[0076] In summary, this invention, through the synergistic process innovation of gradient overlap rate layer control and layer-by-layer overlap offset, achieves simultaneous improvement in coating interface bonding strength, microstructure uniformity, surface quality, and cavitation resistance while maintaining complete homogeneity with the substrate. The technical effect is significantly better than existing processes.

[0077] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or basic characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects.

[0078] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be appropriately combined to form other embodiments that can be understood by those skilled in the art. The above content is only for illustrating the technical concept of the present invention and should not be used to limit the scope of protection of the present invention. Any modifications made to the technical solutions based on the technical concept proposed in this invention fall within the scope of protection of this invention.

Claims

1. A method for preparing a gradient-overlapping laser cladding coating on the surface of martensitic stainless steel, characterized in that, Includes the following steps: The pretreated martensitic stainless steel matrix is ​​preheated to obtain the substrate. Laser cladding is performed on the substrate surface using homogeneous coating raw materials to prepare a bottom layer, an intermediate layer, and a surface layer in sequence, resulting in a homogeneous coating semi-finished product; the laser cladding overlap rate of the bottom layer, intermediate layer, and surface layer decreases in gradient; the starting position of the laser cladding melt channel between adjacent layers of the bottom layer, intermediate layer, and surface layer has an offset distance from the outer edge of the substrate to the center of the substrate. The homogeneous coating semi-finished product is kept at a certain temperature, then cooled and finely processed to complete the preparation of the homogeneous coating; The laser cladding overlap rate of the bottom layer, the middle layer, and the surface layer decreases in a gradient, including: the laser cladding overlap rate of the bottom layer is 45-50%, the laser cladding overlap rate of the middle layer is 35-45%, and the laser cladding overlap rate of the surface layer is 25-35%. The starting position of the laser cladding track between adjacent layers of the bottom layer, intermediate layer and surface layer has an offset distance from the outer edge of the substrate to the center of the substrate, including: the offset distance is 1 / 3 to 1 / 2 of the overlap width, the overlap width is 0.7 to 1.5 mm, and the overlap width is the width of the overlapping area of ​​adjacent laser cladding tracks.

2. The method for preparing a gradient-overlapping laser cladding coating on the surface of martensitic stainless steel according to claim 1, characterized in that, The homogeneous coating material is a powder material with the same chemical composition as the martensitic stainless steel matrix; the type of martensitic stainless steel includes 0Cr. 13 Ni5Mo, 2Cr 13 1Cr 17 One of Ni2, wherein the particle size distribution of the homogeneous coating raw material is controlled within the range of 15-150 μm.

3. The method for preparing a gradient-overlapping laser cladding coating on the surface of martensitic stainless steel according to claim 1, characterized in that, The preheating conditions are as follows: the pretreated martensitic stainless steel matrix is ​​preheated at a temperature of 200-350℃ and held for 25-60 minutes.

4. The method for preparing a gradient-overlapping laser cladding coating on the surface of martensitic stainless steel according to claim 1, characterized in that, The bottom layer has 1-3 laser cladding layers, and the thickness of a single bottom layer is 0.3-0.5 mm; the middle layer has 1-3 laser cladding layers, and the thickness of a single middle layer is 0.3-0.5 mm; the surface layer has 1-3 laser cladding layers, and the thickness of a single surface layer is 0.3-0.5 mm.

5. The method for preparing a gradient-overlapping laser cladding coating on the surface of martensitic stainless steel according to claim 1, characterized in that, The laser cladding process parameters for the bottom layer, intermediate layer, and surface layer meet the following conditions: laser power 800-1500W, scanning speed 6-12mm / s, spot diameter 1.5-1.7mm, and powder feeding rate 15-25g / min.

6. The method for preparing a gradient-overlapping laser cladding coating on the surface of martensitic stainless steel according to claim 1, characterized in that, The cooling after heat preservation refers to the process of heat preservation at 200-300℃ for 1-3 hours followed by cooling in the furnace for the homogeneous coating semi-finished product.

7. The method for preparing a gradient-overlapping laser cladding coating on the surface of martensitic stainless steel according to claim 1, characterized in that, The finishing process involves grinding and polishing the surface of the coating after it has cooled following heat preservation, so that the surface roughness Ra is ≤ 0.9 μm.

8. A laser cladding coating on a gradient-overlapping martensitic stainless steel surface, characterized in that, The laser cladding coating on the surface of martensitic stainless steel, prepared by the method of any one of claims 1-7, has the following characteristics: interfacial bonding strength ≥1220MPa; porosity ≤0.3%; average grain size ≤4.5μm; cavitation erosion weight loss rate ≤0.28mg / h.

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