A protective material for a boiler tube, a preparation method thereof, and a method for protecting a boiler tube

Abstract

The present application belongs to the technical field of surface engineering, and provides a protective material for boiler pipes, a preparation method thereof, and a protective method for boiler pipes. The protective material for boiler pipes comprises the following components in mass fraction: Al: 6-20 wt.%; Cr: 18-24 wt.%; Mo: 5-7 wt.%; Co: 1-5 wt.%; Si: 0.5-1 wt.%; Zr: 1-3 wt.%; and Ni: the balance. The protective coating obtained by laser cladding of the protective material for boiler pipes is not easy to fall off from the base body, has good heat conduction performance and strong thermal shock resistance, can be used in working conditions of high-temperature chlorine corrosion or high-temperature chlorine corrosion-erosion synergistic effect, and can also be applied to corrosion protection in working conditions of high-temperature coexistence of alkali metal chlorides and sulfates; and can be applied to, but is not limited to, biomass boilers, waste incineration boilers and biomass circulating fluidized bed boilers.

Description

Technical Field

[0001] This invention relates to the field of surface engineering technology, and in particular to a protective material for boiler tubes, its preparation method, and a method for protecting boiler tubes. Background Technology

[0002] The use of waste and biofuels for power generation can play a crucial role in future energy security and reducing carbon dioxide emissions, and is currently showing a growing trend worldwide. However, these biofuels typically contain high levels of alkali metal chlorides (such as potassium chloride and sodium chloride), resulting in highly corrosive flue gas. Deposits formed on heated surfaces such as superheater tubes are often rich in alkali metal chlorides. The presence of these deposits significantly enhances high-temperature corrosion of boiler tubes, exacerbates the risk of boiler tube failure, and can even cause unplanned unit shutdowns. Therefore, high-temperature corrosion of metals and alloys is a serious problem for the safe and stable operation of boiler units. Maintaining lower steam temperatures can generally reduce the corrosion rate of boiler superheaters. However, this leads to poorer power generation efficiency. Higher temperatures often result in more severe corrosion. Therefore, preventing high-temperature corrosion failure to maintain boiler unit operation under high temperature and pressure has become a top priority.

[0003] Furthermore, flue gas often contains a large amount of corrosive gases and ash particles. With increasing unit operating parameters, due to the influence of high-temperature fluids, stress, and impurities, critical components such as boiler tubes, turbine blades, and valves often experience severe high-temperature corrosion accompanied by mechanical damage such as wear and high-temperature erosion. This seriously jeopardizes the safe operation of the boiler unit. For example, circulating fluidized bed boilers utilizing renewable energy sources such as biomass have significant potential because the temperature of the fluidized bed can be easily controlled to achieve stable biomass combustion. However, due to the impact of bed material particles (such as silica sand) in a harsh chlorine-containing atmosphere, the tube walls can rapidly thin or even burst under the synergistic effect of high-temperature corrosion and erosion wear, severely affecting the normal operation of the power plant boiler. Therefore, corrosion and wear are among the main causes of boiler tube bursts.

[0004] Preparing a protective coating on the surface of heated surfaces such as boiler tubes is an effective method. Laser cladding technology can deposit a high-quality, crack-free, and pore-free cladding layer that is metallurgically bonded to the substrate on the boiler tube surface. This effectively prevents the coating from peeling off or creating channels for corrosive media, thus avoiding loss of protective effect. Material selection and composition design are among the most critical factors determining the performance of the laser cladding layer. This complex service environment (thermal, chemical, and mechanical interactions) makes it extremely difficult for most current materials to meet all the required protective conditions. For example, common high-temperature alloys are designed based on a comprehensive consideration of strength, processing, and weldability; even if they have good corrosion resistance, their wear resistance is often unsatisfactory. While ceramic-metal matrix composites are chosen for cladding, they offer high high-temperature wear resistance. However, this material system is prone to developing through-cracks during laser cladding, making it unsuitable for corrosion protection. Furthermore, when ash accumulates on the heated surface of boiler tubes, molten salt corrosion often occurs due to the more complex components involved in the corrosion reaction and localized overheating. This makes the interface between the ceramic particle reinforcement phase and the metal matrix in the cladding layer susceptible to preferential corrosion.

[0005] Therefore, there is an urgent need to develop new materials suitable for laser cladding technology that can simultaneously provide high-temperature corrosion resistance and wear resistance. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a protective material for boiler tubes, a method for preparing the same, and a method for protecting boiler tubes. The protective material for boiler tubes provided by this invention has high-temperature corrosion resistance and wear resistance.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0008] This invention provides a protective material for boiler tubes, comprising the following components by mass fraction:

[0009] Al: 6–20 wt.%; Cr: 18–24 wt.%; Mo: 5–7 wt.%; Co: 1–5 wt.%; Si: 0.5–1 wt.%; Zr: 1–3 wt.%; Ni: balance.

[0010] The present invention also provides a method for preparing the protective material for boiler tubes described in the above technical solution, comprising the following steps:

[0011] Weigh the raw materials;

[0012] The raw material Ni is first melted, then Cr, Mo, Co, Si and Zr are added for a second melt, and then Al is added for a third melt to obtain an alloy melt;

[0013] The alloy molten liquid is atomized to obtain the protective material for boiler tubes.

[0014] The present invention also provides a method for protecting boiler tubes, comprising the following steps:

[0015] Prepare a protective coating on boiler tubes;

[0016] The protective coating is obtained by laser cladding of the boiler tube protective material described in the above technical solution or the boiler tube protective material prepared by the preparation method described in the above technical solution.

[0017] Preferably, a primer layer is provided between the boiler tube and the protective coating; the primer layer is obtained by laser cladding of a primer material; the primer material comprises the following components by mass fraction:

[0018] Al: 4–6 wt.%; Cr: 18–24 wt.%; Mo: 5–7 wt.%; Co: 1–5 wt.%; Si: 0.5–1 wt.%; Zr: 1–3 wt.%; Ni: balance.

[0019] Preferably, the thickness of the base layer is 300–500 μm.

[0020] Preferably, the laser cladding parameters for the underlayer include: the powder replenishing gas is high-purity nitrogen, the protective gas is high-purity argon, the laser is a fiber laser, the rated output power of the laser is 3-8kW, the laser beam focal spot diameter is 1-3mm, the relative motion speed between the laser spot and the workpiece is 5-25cm / s, and the axial step distance is 0.5-2.5mm.

[0021] Preferably, the protective coating includes a first protective coating or a second protective coating;

[0022] The raw materials for preparing the first protective coating include the following components in mass fractions: Al: 6–20 wt.%; Cr: 18–24 wt.%; Mo: 5–7 wt.%; Co: 1–5 wt.%; Si: 0.5–1 wt.%; Zr: 1–3 wt.%; Ni: balance;

[0023] The laser cladding parameters for the first type of protective coating include: the powder replenishing gas is high-purity nitrogen, the protective gas is high-purity argon, the laser is a fiber laser, the rated output power of the laser is 3-8kW, the laser beam focal spot diameter is 1-3mm, the relative motion speed between the laser spot and the workpiece is 5-25cm / s, and the axial step distance is 0.5-2.5mm.

[0024] The thickness of the first type of protective coating is 400–650 μm.

[0025] Preferably, the second protective coating comprises a first gradient protective coating and a second gradient protective coating stacked sequentially; the first gradient protective coating is in contact with the underlayer.

[0026] The raw materials for preparing the first gradient protective coating include the following components in mass fractions: Al: 8-12 wt.%; Cr: 18-24 wt.%; Mo: 5-7 wt.%; Co: 1-5 wt.%; Si: 0.5-1 wt.%; Zr: 1-3 wt.%; Ni: balance;

[0027] The raw materials for preparing the second gradient protective coating include the following components in mass fraction: Al: 15-20 wt.%; Cr: 18-24 wt.%; Mo: 5-7 wt.%; Co: 1-5 wt.%; Si: 0.5-1 wt.%; Zr: 1-3 wt.%; Ni: balance.

[0028] Preferably, the thickness of the first gradient protective coating and the second gradient protective coating is independently 400 to 600 μm.

[0029] Preferably, the laser cladding parameters of the first gradient protective coating include: the powder replenishing gas is high-purity nitrogen, the protective gas is high-purity argon, the laser is a fiber laser, the rated output power of the laser is 3 to 8 kW, the laser beam focal spot diameter is 1 to 3 mm, the relative motion speed between the laser spot and the workpiece is 5 to 25 cm / s, and the axial step distance is 0.5 to 2.5 mm.

[0030] The laser cladding parameters for the second gradient protective coating include: the powder replenishing gas is high-purity nitrogen, the protective gas is high-purity argon, the laser is a fiber-coupled output semiconductor laser or a fiber laser, the rated output power of the laser is 3 to 8 kW, the laser beam focal spot diameter is 1 to 3 mm, the relative motion speed between the laser spot and the workpiece is 5 to 25 cm / s, and the axial step distance is 0.5 to 2.5 mm.

[0031] This invention provides a protective material for boiler tubes, comprising the following components by mass fraction: Al: 6-20 wt.%; Cr: 18-24 wt.%; Mo: 5-7 wt.%; Co: 1-5 wt.%; Si: 0.5-1 wt.%; Zr: 1-3 wt.%; Ni: balance. The protective coating obtained by laser cladding of this invention is metallurgically bonded to the substrate, making it difficult to peel off. It exhibits good thermal conductivity and strong thermal shock resistance, and can be used in conditions of high-temperature chlorine corrosion or synergistic effects of high-temperature chlorine corrosion and erosion wear. It can also be applied to corrosion protection in conditions where alkali metal chlorides and sulfates coexist at high temperatures. It can be applied to, but is not limited to, biomass boilers, waste incineration boilers, and biomass circulating fluidized bed boilers.

[0032] The present invention also provides a method for protecting boiler tubes, comprising the following steps: preparing a protective coating on the boiler tube; wherein the protective coating is obtained by laser cladding of the boiler tube protective material described in the above technical solution or the boiler tube protective material prepared by the preparation method described in the above technical solution.

[0033] Furthermore, an underlayer is provided between the boiler tube and the protective coating. The underlayer isolates the Fe in the boiler tube substrate, preventing the dilution of Fe from affecting the performance of the protective coating. This allows the protective coating to better serve in environments with high-temperature corrosion, almost no erosion wear, or only a certain degree of wear.

[0034] Furthermore, the second protective coating comprises a first gradient protective coating and a second gradient protective coating stacked sequentially; the first gradient protective coating is in contact with the underlayer. This invention sets the protective coating as a multi-layered protective coating with a gradient change in aluminum content, which ensures that the inner layers of the protective coating are crack-free, prevents corrosion of the substrate, and enables the final protective coating to operate in high-temperature corrosive environments with severe high-temperature wear. Attached Figure Description

[0035] Figure 1 The images show the surface morphology of the protective coating obtained in Example 1 after 100 thermal shock tests. The left image shows the coating without the flaw detector after the thermal shock test, and the right image shows the coating with the flaw detector after the thermal shock test.

[0036] Figure 2 The cross-sectional distribution curves of the main elements in the protective coating obtained in Example 1 are shown.

[0037] Figure 3 The graph shows the weight loss per unit area of ​​the protective coating and Inconel 625 alloy laser cladding layer obtained in Example 1 after being exposed to a NaCl-KCl mixed salt at 700°C for 144 hours.

[0038] Figure 4 The images show the cross-sectional morphology of the protective coating and the Inconel 625 alloy laser cladding layer obtained in Example 1 after being exposed to a NaCl-KCl mixed salt at 700°C for 144 hours. The left image shows the Inconel 625 alloy laser cladding layer, and the right image shows the protective coating obtained in Example 1.

[0039] Figure 5 The cross-sectional distribution curves of the main elements in the protective coating obtained in Example 2 are shown.

[0040] Figure 6 The image shows a comparison of the mass loss of the protective coating obtained in Example 2 and the Inconel 625 alloy laser cladding layer after a high-temperature erosion test at 900°C for 8 minutes. Detailed Implementation

[0041] This invention provides a protective material for boiler tubes, comprising the following components by mass fraction:

[0042] Al: 6–20 wt.%; Cr: 18–24 wt.%; Mo: 5–7 wt.%; Co: 1–5 wt.%; Si: 0.5–1 wt.%; Zr: 1–3 wt.%; Ni: balance.

[0043] The boiler tube protective material provided by this invention comprises Al: 6-20 wt.%, preferably 8-18 wt.%, more preferably 10-15 wt.%. In this invention, an appropriate content of Al can simultaneously improve the high-temperature wear resistance and corrosion resistance of the protective coating obtained by laser cladding of the boiler tube protective material. First, Al is a basic constituent element for forming the γ'-Ni3Al phase. Adding Al to the protective coating not only provides solid solution strengthening but also forms Ni3Al with Ni for precipitation strengthening. Second, an Al2O3 oxide layer can be formed on the surface of the protective coating. Compared to the rapid growth rate of Fe or Ni oxides, resulting in a thick and easily peelable oxide layer, the Al2O3 oxide layer forms slowly, is thin, has good adhesion, is not easily peeled off, has high high-temperature hardness, and provides stronger protection to the substrate in high-temperature erosion and wear environments. Furthermore, Cl... - The diffusion activation energy in the Al2O3 oxide layer is approximately twice that in Cr2O3. In high-temperature corrosive environments containing chlorides, it is much more difficult to break down the Al2O3 oxide layer than Cr2O3, thus significantly improving the high-temperature chloride corrosion resistance of the protective coating.

[0044] The protective material for boiler tubes provided by this invention comprises Cr: 18-24 wt.%, preferably 19-23 wt.%, more preferably 20-22 wt.%, and even more preferably 21 wt.%. In this invention, the addition of Cr can cause lattice distortion in the protective coating formed by laser cladding, generating elastic stress field strengthening and playing a solid solution strengthening role. However, the Cr content should not be too high, otherwise it will lead to increased brittleness of the protective coating and easy cracking. This invention controls the Cr content between 18-24 wt%, ensuring that Cr atoms in the solid solution can also generate short-range ordered strengthening. In high-temperature environments, a continuous and dense protective Cr2O3 oxide layer can also be formed on the surface of the protective coating, thereby delaying the outward diffusion of metal elements and the inward diffusion of corrosive media such as O, Cl, and S, thus effectively improving the high-temperature corrosion resistance of the protective coating.

[0045] The protective material for boiler tubes provided by this invention comprises Mo: 5-7 wt.%, preferably 5.5-6.5 wt.%, and more preferably 6 wt.%. In this invention, Mo can reduce the high-temperature diffusion rate of elements such as Al and Cr in the protective coating obtained by laser cladding of the boiler tube protective material, strengthen the atomic bonding force in the solid solution, and improve the corrosion resistance, strength, and high-temperature processing performance of the protective coating. Furthermore, Mo can further accelerate the transformation from θ-Al₂O₃ to α-Al₂O₃, thereby obtaining a slowly growing dense oxide layer. The coupling effect of Mo and Cr also makes the oxide / metal interface flatter and more complete. However, the Mo content should not be too high. First, excessively high Mo content easily forms volatile refractory metal oxides, affecting the integrity of the oxide layer; second, due to the characteristics of laser cladding technology, excessively high Mo content easily causes severe segregation of the protective coating.

[0046] The protective material for boiler tubes provided by this invention comprises Co: 1-5 wt.%, preferably 2-4 wt.%, more preferably 3 wt.%. In this invention, Co can reduce the stacking fault energy of the protective coating obtained by laser cladding of the boiler tube protective material, making dislocation movement difficult, thereby increasing the strength of the material and causing solid solution strengthening. Studies have also shown that Co sulfides are relatively stable and can effectively inhibit internal sulfidation. This invention, through the interaction of Co and Cr, can improve the high-temperature corrosion performance of the protective coating in the presence of both alkali metal chlorides and sulfates. Furthermore, Co promotes the formation of protective oxides such as CoAl2O4, resulting in a denser corrosion layer and fewer pit defects. It can further inhibit the outward diffusion of metal atoms, slowing down the corrosion rate. However, Co is expensive and should not be added in excessive amounts. For protective coatings, if only high-temperature chlorine corrosion exists in the corrosive environment, the addition of Co can be minimized.

[0047] The boiler tube protective material provided by this invention comprises Si: 0.5–1 wt.%, preferably 0.6–0.9 wt.%, more preferably 0.7–0.8 wt.%. In this invention, Si can reduce the oxide content in the protective coating obtained by laser cladding of the boiler tube protective material. Furthermore, Si can alter the growth mechanism of the oxide layer, improve its adhesion, and significantly enhance its resistance to peeling. A small amount of Si can promote the formation of a protective, continuous Al2O3 oxide layer without disrupting the stability of the γ′ phase. A small amount of Si can also promote the formation of a dense Cr2O3 oxide layer on the surface, thereby improving the protective coating's resistance to oxidation, high-temperature corrosion, and high-temperature erosion wear.

[0048] The boiler tube protective material provided by this invention comprises: Zr: 1-3 wt.%, preferably 1.5-2.5 wt.%, more preferably 2 wt.%. In this invention, in a high-temperature corrosive environment, Zr segregates at the grain boundaries of the protective oxide layer (Al2O3, Cr2O3). Since the diffusion path of Zr ions is similar to, but not exactly the same as, that of Al ions, the "blocking effect" can significantly inhibit the continued outward diffusion of Al ions. In addition, Zr ions can also effectively delay the peeling of the protective oxide layer by inhibiting the growth of interfacial porosity and increasing the interfacial strength between the oxide layer and the protective coating obtained by laser cladding of the boiler tube protective material. In a high-temperature corrosive environment, Zr can rapidly diffuse to the surface of the cladding layer, thereby timely capturing S that directly enters the protective coating or is released by sulfide oxidation. This can effectively prevent the internal sulfidation-oxidation cycle reaction of the protective coating, further improving the high-temperature corrosion resistance of the protective coating when sulfate or chloride and sulfate are present simultaneously. However, excessive Zr content leads to the formation of ZrO2 particles at oxide grain boundaries, which in turn accelerates the transport of O and S, promoting internal oxidation and sulfidation. Furthermore, the growth of ZrO2 introduces significant localized internal stress into the oxide layer (Cr2O3, Al2O3), ultimately causing cracking and peeling. Therefore, the amount of Zr added should not be too high. Appropriate Zr content not only enhances the interfacial adhesion of the protective oxide layer but also gives it higher resistance to high-temperature corrosion and a lower growth rate, which is crucial for improving the durability of the protective coating in high-temperature corrosive environments.

[0049] The protective material for boiler tubes provided by this invention comprises Ni: balance. In this invention, Ni exhibits a face-centered cubic structure and shows no isomerization transformation from room temperature to high temperature. Furthermore, the third electron shell of Ni atoms is essentially filled, allowing it to dissolve more alloying elements for alloying while still maintaining the stability of the γ-austenite phase. This provides multiple ways to improve the high-temperature corrosion and wear resistance of the protective coating formed after laser cladding. In addition, Ni itself can prevent the inward diffusion of corrosive media by accumulating beneath the oxide layer, contributing to the excellent corrosion resistance of the protective coating formed after laser cladding.

[0050] The present invention also provides a method for preparing the protective material for boiler tubes described in the above technical solution, comprising the following steps:

[0051] Weigh the raw materials; melt the raw material Ni for the first time, then add Cr, Mo, Co, Si and Zr for the second time, then add Al for the third time to obtain the alloy melt;

[0052] The alloy molten liquid is atomized to obtain the protective material for boiler tubes.

[0053] In this invention, raw materials are weighed; Ni is first melted, then Cr, Mo, Co, Si and Zr are added for a second melt, and then Al is added for a third melt to obtain an alloy melt.

[0054] In this invention, the mass fraction of the raw materials weighed is preferably determined according to the component mass fraction of the boiler tube protective material described in the above technical solution. In this invention, the purity of the raw materials is preferably ≥99.9%.

[0055] In this invention, the first melting, second melting, and third melting are preferably carried out in a vacuum medium-frequency induction furnace, preferably model YC-2016028#. This invention does not specifically limit the temperature and time of the first melting, second melting, and third melting, as long as the raw materials can be melted.

[0056] After the third melting, the present invention preferably further includes a heat preservation treatment; the temperature of the heat preservation treatment is preferably 1400-1500℃, and the time is preferably 10-30min.

[0057] After obtaining the alloy melt, the present invention atomizes the alloy melt to obtain the protective material for boiler tubes.

[0058] In this invention, the atomizing medium is preferably nitrogen. In this invention, the atomization is preferably carried out in an atomization rapid condensation device. In this invention, the molten alloy is preferably placed in the crucible of the atomization rapid condensation device. In this invention, during the atomization process, the flow rate of the molten alloy is preferably 6–10 kg / min.

[0059] After atomization, the present invention preferably further includes sieving, wherein the aperture of the sieve is preferably 140 to 325 mesh.

[0060] In this invention, the particle size of the protective material for boiler tubes is preferably 53-105 μm.

[0061] The present invention also provides a method for protecting boiler tubes, comprising the following steps:

[0062] Prepare a protective coating on boiler tubes;

[0063] The protective coating is obtained by laser cladding of the boiler tube protective material described in the above technical solution or the boiler tube protective material prepared by the preparation method described in the above technical solution.

[0064] Before preparing the protective coating, the present invention preferably further includes rust removal treatment of the boiler tubes. The present invention does not specifically limit the operation of the rust removal treatment; any rust removal operation well known to those skilled in the art can be used.

[0065] In this invention, a primer layer is preferably provided between the boiler tube and the protective coating. In this invention, the thickness of the primer layer is preferably 300–500 μm. In this invention, the primer layer is preferably obtained by laser cladding of a primer material. In this invention, the primer material preferably comprises the following components in parts by weight: Al: 4–6 wt.%; Cr: 18–24 wt.%; Mo: 5–7 wt.%; Co: 1–5 wt.%; Si: 0.5–1 wt.%; Zr: 1–3 wt.%; Ni: balance.

[0066] In this invention, the laser cladding parameters for the bottom layer include: the powder replenishing gas is preferably high-purity nitrogen, the protective gas is preferably high-purity argon, the laser is preferably a fiber laser, the rated output power of the laser is preferably 3-8kW, the laser beam focal spot diameter is preferably 1-3mm, the relative movement speed between the laser spot and the workpiece is preferably 5-25cm / s, and the axial step distance is preferably 0.5-2.5mm.

[0067] In this invention, the protective coating preferably includes a first protective coating or a second protective coating.

[0068] In this invention, the raw materials for preparing the first protective coating preferably include the following components in mass fraction: Al: 6-20 wt.%; Cr: 18-24 wt.%; Mo: 5-7 wt.%; Co: 1-5 wt.%; Si: 0.5-1 wt.%; Zr: 1-3 wt.%; Ni: balance.

[0069] In this invention, the laser cladding parameters of the first protective coating include: the powder replenishing gas is preferably high-purity nitrogen, the protective gas is preferably high-purity argon, the laser is preferably a fiber laser, the rated output power of the laser is preferably 3 to 8 kW, the laser beam focal spot diameter is preferably 1 to 3 mm, the relative motion speed between the laser spot and the workpiece is preferably 5 to 25 cm / s, and the axial step distance is preferably 0.5 to 2.5 mm.

[0070] In this invention, the thickness of the first protective coating is preferably 400-650 μm.

[0071] In this invention, the first protective coating can be used for high-temperature corrosion protection and general high-temperature wear protection.

[0072] In this invention, the second protective coating preferably includes a first gradient protective coating and a second gradient protective coating stacked sequentially; the first gradient protective coating preferably contacts the underlayer.

[0073] In this invention, the thickness of the first gradient protective coating and the second gradient protective coating is preferably 400-600 μm independently.

[0074] In this invention, the raw material for preparing the first gradient protective coating, the boiler tube protective material, preferably contains the following mass fractions: aluminum 8-12 wt.%; Cr 18-24 wt.%; Mo 5-7 wt.%; Co 1-5 wt.%; Si 0.5-1 wt.%; Zr 1-3 wt.%; Ni: balance. In this invention, the laser cladding parameters for the first gradient protective coating include: the powder-filling gas is preferably high-purity nitrogen, the protective gas is preferably high-purity argon, the laser is preferably a fiber laser with a rated output power of 3-8 kW, a laser beam focal spot diameter of 1-3 mm, a relative motion speed between the laser spot and the workpiece of 5-25 cm / s, and an axial step distance of 0.5-2.5 mm.

[0075] In this invention, the raw material for preparing the second gradient protective coating, the boiler tube protective material, preferably contains the following mass fractions: aluminum 15-20 wt.%; Cr 18-24 wt.%; Mo 5-7 wt.%; Co 1-5 wt.%; Si 0.5-1 wt.%; Zr 1-3 wt.%; Ni: balance. In this invention, the laser cladding parameters for the second gradient protective coating include: the powder-filling gas is preferably high-purity nitrogen, the protective gas is preferably high-purity argon, the laser is preferably a fiber laser with a rated output power of 3-8 kW, a laser beam focal spot diameter of 1-3 mm, a relative motion speed between the laser spot and the workpiece of 5-25 cm / s, and an axial step distance of 0.5-2.5 mm.

[0076] In this invention, the second protective coating can be applied to environments with high-temperature corrosion and severe high-temperature wear.

[0077] In this invention, when performing laser cladding, for tubular samples, it is preferable that the laser head is fixed and the tubular sample performs a rotation-stepping composite motion; for plate samples, it is preferable that the steel plate is fixed and the laser head performs a linear reciprocating-stepping composite motion.

[0078] The following detailed description, in conjunction with embodiments, of the protective material for boiler tubes provided by the present invention, its preparation method, and the protection method for boiler tubes, should not be construed as limiting the scope of protection of the present invention.

[0079] Example 1: Preparation of a high-temperature corrosion-resistant laser cladding protective coating on the heated surface of a superheater tube.

[0080] I. Powder Composition and Preparation

[0081] 1. Alloy powder composition:

[0082] The raw materials for preparing the base layer include the following components by mass fraction: Al: 5 wt.%; Cr: 22 wt.%; Mo: 5 wt.%; Co: 2 wt.%; Si: 0.5 wt.%; Zr: 1.25 wt.%; Ni: balance.

[0083] The raw materials for preparing the first protective coating (high temperature corrosion resistant laser cladding protective coating) include the following components by mass fraction: Al: 10 wt.%; Cr: 20 wt.%; Mo: 7 wt.%; Co: 1 wt.%; Si: 0.75 wt.%; Zr: 1.5 wt.%; Ni: balance.

[0084] 2. Preparation of raw materials for the primer and the first protective coating:

[0085] Weigh Ni, Cr, Al, Mo, Co, Si, and Zr according to the mass fractions in step 1. First, add metallic Ni and heat it in a vacuum medium-frequency induction furnace to melt it. After Ni has completely melted, add Cr, Mo, Co, Zr, and Si. After Cr, Mo, Co, Zr, Ni, and Si have completely melted, add Al to melt it. Hold the molten alloy at 1400–1500 °C for 20 minutes to obtain the alloy melt.

[0086] The prepared alloy melt was poured into a crucible of an atomizing rapid condensation device, and the device was used to atomize and powder the alloy to obtain Ni-Cr-Al-Mo-Co-Si-Zr alloy powder, which is the raw material for preparing the protective coating, with a particle size of 53-105 μm. The atomizing medium was nitrogen gas, and the flow rate of the alloy melt was 6 kg / min.

[0087] II. Preparation of a base layer and a high-temperature corrosion-resistant laser cladding protective coating on the heat-receiving surface of the superheater tubes.

[0088] The superheater tubes are made of TP347H stainless steel, with an outer diameter of 30mm and a length of 6m.

[0089] 1. Preparation of the base layer

[0090] (1) Use an electric grinding wheel to remove rust from the outer surface of the superheater tubes.

[0091] (2) The raw materials for the underlayer are loaded into the powder storage container of the pneumatic synchronous powder feeding system. High-purity nitrogen is used as the powder replenishing gas, and high-purity argon is used as the protective gas. The underlayer is prepared by using a method of laser head stationary, superheater tube moving in a spiral motion relative to laser head, and overlapping cladding.

[0092] (3) A fiber laser with a rated power of 5kW was selected for laser cladding. The parameters were: laser output power of 4kW, laser beam focal spot diameter of 2mm, relative motion speed between laser spot and workpiece of 15cm / s, axial step distance of 1.2mm, resulting in an underlayer with an average thickness of about 400μm.

[0093] 2. Preparation of high-temperature corrosion-resistant laser cladding protective coating

[0094] (1) The Ni-Cr-Al-Mo-Co-Si-Zr alloy powder, the raw material for the preparation of the first protective coating, was loaded into the powder storage container of the pneumatic synchronous powder feeding system. The protective coating was prepared on the surface of the underlayer using the same method. A fiber laser with a rated power of 3.3kW was selected for laser cladding. The parameters were: laser output power of 4kW, laser beam focal spot diameter of 2mm, relative motion speed between laser spot and workpiece of 10cm / s, and axial step distance of 1.2mm. A high-temperature corrosion-resistant laser cladding protective coating with an average thickness of about 600μm was obtained.

[0095] Example 2: Preparation of a high-temperature corrosion-resistant and wear-resistant gradient laser cladding protective coating on the heated surface of a water-cooled wall tube.

[0096] I. Powder Composition and Preparation

[0097] 1. Alloy powder composition

[0098] The raw materials for preparing the base layer include the following components by mass fraction: Al: 5 wt.%; Cr: 18 wt.%; Mo: 5 wt.%; Co: 2 wt.%; Si: 0.5 wt.%; Zr: 1.25 wt.%; Ni: balance.

[0099] The raw materials for preparing the first-gradient protective coating include the following components by mass fraction: Al: 10 wt.%; Cr: 18 wt.%; Mo: 5 wt.%; Co: 2 wt.%; Si: 0.5 wt.%; Zr: 1.2 wt.%; Ni: balance.

[0100] The raw materials for preparing the second-gradient protective coating include the following components by mass fraction: Al: 20 wt.%; Cr: 20 wt.%; Mo: 6 wt.%; Co: 2 wt.%; Si: 0.5 wt.%; Zr: 1.25 wt.%; Ni: balance.

[0101] The preparation of raw materials for the base coat, the first gradient protective coating, and the second gradient protective coating is the same as in Example 1.

[0102] 2. Prepare an underlayer, a first gradient protective coating, and a second gradient protective coating sequentially on the heated surface of the water-cooled wall tube.

[0103] The water-cooled wall tube is made of 15CrMo steel, with an outer diameter of 60mm and a length of 8m.

[0104] (1) Use an electric grinder to remove rust from the outer surface of the water-cooled wall tube.

[0105] (2) The raw materials for the underlayer preparation are loaded into the powder storage container of the pneumatic synchronous powder feeding system. High-purity nitrogen is used as the powder replenishing gas, and high-purity argon is used as the protective gas. The first gradient protective coating is prepared by using a method of lap cladding with the laser head stationary and the water-cooled wall tube moving in a spiral motion relative to the laser head. A fiber laser with a rated power of 5kW is used for laser cladding with the following parameters: laser output power of 5kW, laser beam focal spot diameter of 2mm, relative motion speed between the laser spot and the workpiece of 15cm / s, and axial step distance of 1.6mm, resulting in an underlayer with an average thickness of about 400μm.

[0106] (3) The raw materials for the preparation of the first gradient protective coating are loaded into the powder storage container of the pneumatic synchronous powder feeding system. The first gradient protective coating is prepared on the surface of the prepared base layer using the same method. The main parameters are: the laser output power is 5kW, the laser beam focal spot diameter is 2mm, the relative motion speed between the laser spot and the workpiece is 10cm / s, and the axial step distance is 1.6mm. A second protective coating with an average thickness of about 400μm is obtained.

[0107] (4) The raw materials for the preparation of the second gradient protective coating are loaded into the powder storage container of the pneumatic synchronous powder feeding system. The second gradient protective coating is prepared on the surface of the first protective coating using the same method. The main parameters are: the laser output power is 5kW, the laser beam focal spot diameter is 2mm, the relative motion speed between the laser spot and the workpiece is 8cm / s, and the axial step distance is 1.5mm. A third gradient protective coating with an average thickness of about 400μm is obtained.

[0108] Performance testing

[0109] The superheater tube heating surface with a base coat and protective coating obtained in Example 1 was subjected to a thermal shock test. The specific procedure was as follows: the superheater tube heating surface with the base coat and protective coating was heated to 750°C and held at that temperature for 10 minutes, then rapidly quenched in water at 20–25°C, and this process was repeated 100 times. The results are as follows... Figure 1 As shown. Figure 1 The images show the surface morphology of the protective coating obtained in Example 1 after 100 thermal shock tests. The left image shows the coating without the flaw detector after the thermal shock test, and the right image shows the coating with the flaw detector after the thermal shock test. Figure 1 It can be seen that, after flaw detection, no cracks were found in the protective coating after 100 thermal shock tests, demonstrating excellent protective properties.

[0110] Figure 2 This is a cross-sectional distribution curve of the main elements within the protective coating obtained in Example 1. From... Figure 2 It can be seen that: Fe from the substrate decreases sharply after entering the protective coating and can be ignored at the surface; Cr and Mo are evenly distributed in the protective coating; Al content increases sharply after 500 μm from the substrate and meets the required Al content range near the surface.

[0111] Figure 3 The graph shows the weight loss per unit area of ​​the protective coating and Inconel 625 alloy laser cladding layer obtained in Example 1 after exposure to a NaCl-KCl mixed salt at 700°C for 144 hours. (From...) Figure 3 It can be seen that in the high-temperature corrosive environment of chloride deposited salts, the corrosion resistance of the protective coating of the present invention can reach about 15 times that of the commonly used boiler tube corrosion protection material (Inconel 625).

[0112] Figure 4 The images show the cross-sectional morphology of the protective coating obtained in Example 1 and the Inconel 625 alloy laser cladding layer after exposure to a NaCl-KCl mixed salt at 700°C for 144 hours. The left image shows the Inconel 625 alloy laser cladding layer, and the right image shows the protective coating obtained in Example 1. Figure 4 It can be seen that the thickness of the corrosion products on the surface of the Inconel 625 alloy laser cladding layer can reach approximately 110 μm, while the thickness of the corrosion products on the surface of the protective coating is only 2 μm.

[0113] Figure 5 This is a cross-sectional distribution curve of the main elements within the protective coating obtained in Example 2. From... Figure 5 It can be seen that the Fe content from the matrix decreases sharply after entering the protective coating, and can be ignored at the surface. The Al content increases in a gradient within the protective coating.

[0114] Figure 6 This is a comparison of the mass loss of the protective coating obtained in Example 2 and the Inconel 625 alloy laser cladding layer after a high-temperature erosion test at 900°C for 8 minutes. From... Figure 6 It can be seen that in a high-temperature erosion and wear environment, the high-temperature erosion and wear resistance of the protective coating can reach about 2.8 times that of the corrosion protection material commonly used for boiler tubes (Inconel 625).

[0115] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for protecting boiler tubes, characterized in that, Includes the following steps: Prepare a protective coating on boiler tubes; The protective coating is obtained by laser cladding of boiler tube protective material; The protective material for boiler tubes comprises the following components by mass fraction: Al: 6~20 wt.%; Cr: 18~24 wt.%; Mo: 5~7 wt.%; Co: 1~5 wt.%; Si: 0.5~1 wt.%; Zr: 1~3 wt.%; Ni: Balance; An underlayer is provided between the boiler tube and the protective coating; the underlayer is obtained by laser cladding of a base material; the base material comprises the following components by mass fraction: Al: 4~6 wt.%; Cr: 18~24 wt.%; Mo: 5~7 wt.%; Co: 1~5 wt.%; Si: 0.5~1 wt.%; Zr: 1~3 wt.%; Ni: Balance; The thickness of the base layer is 300~500μm.

2. The protection method according to claim 1, characterized in that, The preparation method of the protective material for boiler tubes includes the following steps: Weigh the raw materials; The raw material Ni is first melted, then Cr, Mo, Co, Si and Zr are added for a second melt, and then Al is added for a third melt to obtain an alloy melt; The alloy molten liquid is atomized to obtain the protective material for boiler tubes.

3. The protection method according to claim 1, characterized in that, The laser cladding parameters for the underlayer include: The powder replenishing gas is high-purity nitrogen, the protective gas is high-purity argon, the laser is a fiber laser, the rated output power of the laser is 3~8kW, the laser beam focal spot diameter is 1~3mm, the relative motion speed between the laser spot and the workpiece is 5~25cm / s, and the axial step distance is 0.5~2.5mm.

4. The protection method according to claim 1, characterized in that, The protective coating includes either a first protective coating or a second protective coating; The raw materials for preparing the first protective coating include the following components in mass fractions: Al: 6~20 wt.%; Cr: 18~24 wt.%; Mo: 5~7 wt.%; Co: 1~5 wt.%; Si: 0.5~1 wt.%; Zr: 1~3wt.%; Ni: Balance; The laser cladding parameters for the first type of protective coating include: the powder replenishing gas is high-purity nitrogen, the protective gas is high-purity argon, the laser is a fiber laser, the rated output power of the laser is 3~8kW, the laser beam focal spot diameter is 1~3mm, the relative motion speed between the laser spot and the workpiece is 5~25cm / s, and the axial step distance is 0.5~2.5mm. The thickness of the first type of protective coating is 400~650μm.

5. The protection method according to claim 4, characterized in that, The second type of protective coating includes a first gradient protective coating and a second gradient protective coating stacked sequentially; the first gradient protective coating is in contact with the underlayer. The raw materials for preparing the first gradient protective coating include the following components by mass fraction: Al: 8~12 wt.%; Cr: 18~24 wt.%; Mo: 5~7 wt.%; Co: 1~5 wt.%; Si: 0.5~1 wt.%; Zr: 1~3 wt.%; Ni: balance; The raw materials for preparing the second gradient protective coating include the following components in mass fraction: Al: 15~20wt.%; Cr: 18~24wt.%; Mo: 5~7wt.%; Co: 1~5wt.%; Si: 0.5~1wt.%; Zr: 1~3wt.%; Ni: balance.

6. The protection method according to claim 5, characterized in that, The thicknesses of the first gradient protective coating and the second gradient protective coating are independently 400~600μm.

7. The protection method according to claim 5 or 6, characterized in that, The laser cladding parameters for the first gradient protective coating include: the powder replenishing gas is high-purity nitrogen, the protective gas is high-purity argon, the laser is a fiber laser, the rated output power of the laser is 3~8kW, the laser beam focal spot diameter is 1~3mm, the relative motion speed between the laser spot and the workpiece is 5~25cm / s, and the axial step distance is 0.5~2.5mm. The laser cladding parameters for the second gradient protective coating include: the powder replenishing gas is high-purity nitrogen, the protective gas is high-purity argon, the laser is a fiber-coupled output semiconductor laser or a fiber laser, the rated output power of the laser is 3~8kW, the laser beam focal spot diameter is 1~3mm, the relative motion speed between the laser spot and the workpiece is 5~25cm / s, and the axial step distance is 0.5~2.5mm.

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