Preparation method of long-life high-temperature magnesium oxide powder heating pipe

Abstract

The invention belongs to the technical field of electric heaters, and particularly relates to a preparation method of a long-life high-temperature magnesium oxide powder heating pipe. The method comprises the steps that an electrothermal alloy wire is pre-oxidized for 1-3 h in an oxygen-containing atmosphere at 900-1100 DEG C, a compact oxidation film is generated on the surface of the electrothermal alloy wire, and a pre-oxidized electrothermal alloy wire is obtained; the pre-oxidized electrothermal alloy wire is arranged in the axial direction of a metal protective sleeve, the metal protective sleeve is filled with fused magnesia powder with the purity larger than or equal to 96 wt%, then diameter shrinkage is conducted on the metal protective sleeve, and the fused magnesia powder is compacted and wraps the pre-oxidized electrothermal alloy wire; carrying out curing heat treatment on the reduced metal sheath tube; annealing is carried out on the processed metal protective sleeve; and cleaning the two ends of the annealed metal sheath pipe to obtain the high-temperature magnesium oxide powder heating pipe.

Description

Technical Field

[0001] This application belongs to the field of electric heater technology, and in particular relates to a method for preparing a long-life high-temperature magnesium oxide powder heating tube. Background Technology

[0002] High-temperature magnesium oxide powder heating tube is an electric heating element made by filling powder, reducing diameter, annealing and sealing the tube, with a metal sheath tube as the outer shell, fused magnesium oxide powder as the insulating and heat-conducting layer and an electrothermal alloy wire as the heating core. When working, the high-temperature magnesium oxide powder heating tube converts electrical energy into heat energy through the electrothermal alloy wire, and transfers the heat to the surface of the metal sheath tube with the help of dense magnesium oxide powder, thereby continuously providing a stable heat source in high-temperature environments.

[0003] Existing high-temperature magnesium oxide powder heating tubes have the problem of short lifespan under high-temperature conditions, and cannot operate stably for more than 100 hours under 1150℃ conditions. Summary of the Invention

[0004] This application provides a method for preparing a long-life high-temperature magnesium oxide powder heating tube to solve the following technical problem: how to improve the high-temperature service life of the high-temperature magnesium oxide powder heating tube.

[0005] In a first aspect, embodiments of this application provide a method for preparing a long-life high-temperature magnesium oxide powder heating tube, the method comprising: The heating alloy wire is pre-oxidized in an oxygen-containing atmosphere at 900–1100℃ for 1–3 hours to generate a dense oxide film on the surface of the heating alloy wire, thus obtaining a pre-oxidized heating alloy wire. The pre-oxidized electrothermal alloy wire is arranged along the axial direction of the metal sheath tube, and the metal sheath tube is filled with fused magnesium oxide powder with a purity of ≥96wt%. Then the diameter of the metal sheath tube is reduced so that the fused magnesium oxide powder is compacted and covers the pre-oxidized electrothermal alloy wire. The reduced-diameter metal sheath tube is subjected to a curing heat treatment. The heat-treated metal sheath tube is then annealed. The ends of the annealed metal sheath tube are cleaned to obtain a high-temperature magnesium oxide powder heating tube.

[0006] Optionally, the oxygen-containing atmosphere is air.

[0007] Optionally, the fused magnesium oxide powder is dried, iron removed, and magnetically separated before filling to ensure that the iron impurity content of the fused magnesium oxide powder is ≤100ppm.

[0008] Optionally, the compacted density of the reduced-diameter fused magnesium oxide powder is ≥3.0 g / cm³. 3 .

[0009] Optionally, the curing heat treatment temperature is 400–600°C.

[0010] Optionally, the annealing temperature is 1000–1050℃, and the annealing holding time is 15–30 min.

[0011] Secondly, embodiments of this application provide a high-temperature magnesium oxide powder heating tube, which is prepared by any of the methods described in the first aspect. The high-temperature magnesium oxide powder heating tube has a continuous service life of ≥900h at 1150℃, and the heating alloy wire of the high-temperature magnesium oxide powder heating tube has no chromium nitride segregation in its cross-section after continuous operation at 1150℃ for 60h.

[0012] Optionally, the metal sheath is made of austenitic stainless steel.

[0013] Optionally, the heating alloy wire is Cr 20 Ni 80 Alloy wire, the Cr 20 Ni 80 The diameter of the alloy wire is 0.5–1.2 mm.

[0014] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a method for preparing a long-life high-temperature magnesium oxide powder heating tube. This method addresses the problem of "short life of high-temperature magnesium oxide powder heating tubes" from three aspects: material interface stability, microstructure densification, and residual stress control. The core idea is to establish a chemical-mechanical dual barrier between the heating alloy wire and the magnesium oxide insulation layer to block the high-temperature oxidation-diffusion-stress damage chain.

[0015] Chemical barrier: Pre-oxidation at 900–1100℃ causes the surface of the Cr20Ni80 heating alloy wire to generate a dense Cr2O3 oxide film (1–4 μm thick). This film has oxygen and nitrogen diffusion coefficients that are 3–4 orders of magnitude lower than the substrate at high temperatures, blocking the oxygen and nitrogen diffusion pathway of "alloy → N2, O2 → MgO". This prevents further internal nitriding and oxidation of the alloy during subsequent high-temperature operation, which would lead to the formation of chromium nitride and low-melting-point NiO-Cr2O3 spinels, and avoids the oxide film from cracking and causing local hot spots.

[0016] Mechanical barrier: ≥96wt% fused magnesium oxide, its Na + B 3+With fluxing impurities <0.3%, the grain boundary diffusion rate of MgO at a working temperature of 1100–1200℃ is reduced by an order of magnitude. After diameter reduction, the relative density is ≥92%. Then, after a high-temperature curing stage at 400–600℃, initial surface sintering occurs at the grain boundaries, and the grain boundaries "weld" to form a continuous skeleton, which "self-seals" the micro-gap between the pre-oxidized film and MgO, blocking the diffusion of alloy vapor along the grain boundaries at high temperatures, and inhibiting the reduction and blackening of MgO and the decrease in insulation.

[0017] Residual stress control: The circumferential tensile stress caused by the diameter reduction, if directly operated at high temperature, will generate stress at the interface due to the difference in thermal expansion coefficients between the alloy oxide film and MgO, leading to fatigue spalling of the oxide film. Through curing heat treatment: adsorbed water and organic lubricants evaporate, preventing subsequent hydrogen embrittlement caused by "water vapor oxidation"; MgO undergoes early elastic stress relaxation, and local creep releases tensile stress; finally, annealing at 1000–1050℃: the alloy recrystallizes, reducing the interfacial stress to the critical value of oxide film fracture toughness, completely eliminating thermal cycling spalling.

[0018] In summary, this method breaks the high-temperature oxidation-nitriding-diffusion-stress damage chain by establishing a three-level barrier of "pre-oxidation film blocking oxygen and nitrogen diffusion → high-purity MgO self-sealing to inhibit alloy vapor diffusion → solidification heat treatment to eliminate residual stress," thereby increasing the continuous working life of the heating tube at 1150℃ from the conventional 80h to over 900h. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0021] In a first aspect, embodiments of this application provide a method for preparing a long-life high-temperature magnesium oxide powder heating tube, the method comprising: S1. Pre-oxidize the heating alloy wire in an oxygen-containing atmosphere at 900–1100℃ for 1–3 hours to generate a dense oxide film on the surface of the heating alloy wire, and obtain a pre-oxidized heating alloy wire. S2. Arrange the pre-oxidized electrothermal alloy wire along the axial direction of the metal sheath tube, and fill the metal sheath tube with fused magnesium oxide powder with a purity of ≥96wt%. Then, reduce the diameter of the metal sheath tube so that the fused magnesium oxide powder is compacted and covers the pre-oxidized electrothermal alloy wire. S3. Perform a curing heat treatment on the reduced-diameter metal sheath tube; S4. Anneal the heat-treated metal sheath tube; S5. Perform end cleaning on both ends of the annealed metal sheath tube to obtain a high-temperature magnesium oxide powder heating tube.

[0022] The heating alloy wire was pre-oxidized in an oxygen-containing atmosphere at temperatures of 900℃, 910℃, 920℃, 930℃, 940℃, 950℃, 960℃, 970℃, 980℃, 990℃, 1000℃, 1010℃, 1020℃, 1030℃, 1040℃, 1050℃, 1060℃, 1070℃, 1080℃, 1090℃, and 1100℃ for 1 hour, 1.1 hours, 1.2 hours, 1.3 hours, 1.4 hours, 1.5 hours, 1.6 hours, and 1.7 hours, respectively. The heating time is 8h, 1.9h, 2h, 2.1h, 2.2h, 2.3h, 2.4h, 2.5h, 2.6h, 2.7h, 2.8h, 2.9h, and 3h, which allows a continuous and dense Cr2O3 oxide film to form on the surface of the heating alloy wire. This prevents nitrogen atoms from diffusing into the heating alloy wire, thereby inhibiting chromium nitride segregation when the heating alloy wire is in service at 1150℃. This delays the local melting of the heating alloy wire due to microstructure degradation, and thus improves the high-temperature service life of the high-temperature magnesium oxide powder heating tube.

[0023] The purity of the fused magnesium oxide powder is ≥96wt% (96wt%, 97wt%, 98wt%, 99wt%, 99.5wt%, 99.9wt%) and the iron impurities are ≤100ppm, thereby increasing the volume resistivity of the fused magnesium oxide powder at high temperatures, which in turn reduces the leakage current between the heating alloy wire and the metal sheath tube, thereby reducing the thinning of the heating alloy wire diameter caused by electrochemical corrosion, and further improving the high-temperature service life of the high-temperature magnesium oxide powder heating tube.

[0024] The diameter reduction ensures that the compacted density of fused magnesium oxide powder is ≥3.00 g / cm³. 3 (3.0g / cm) 3 3.01g / cm 3 3.02g / cm 3 3.03 g / cm 3 3.04 g / cm 3 3.05g / cm 3 3.06 g / cm 3 3.07 g / cm 3 3.08g / cm 3 3.09 g / cm 3 3.10 g / cm 3 This shortens the heat conduction path between the heating alloy wire and the metal sheath tube, thereby reducing the hot spot temperature on the surface of the heating alloy wire, slowing down the cracking rate of the oxide film, and thus extending the high-temperature service life of the high-temperature magnesium oxide powder heating tube.

[0025] Curing heat treatment: High-temperature curing at 400℃, 450℃, 500℃, 550℃, and 600℃ causes the free moisture between the fused magnesium oxide powder particles to evaporate, thereby eliminating the vapor pressure during subsequent heating and preventing oxide film blistering; it also softens silicate impurities on the surface of the fused magnesium oxide powder, thereby filling the gaps between particles and improving thermal conductivity; and it annihilates lattice defects in the fused magnesium oxide powder, thereby increasing the high-temperature volume resistivity and reducing leakage corrosion, thus further improving the high-temperature service life of the high-temperature magnesium oxide powder heating tube.

[0026] Annealing eliminates the processing stress introduced by the diameter reduction of the metal sheath tube and the heating alloy wire, thereby avoiding the propagation of microcracks in the oxide film caused by stress concentration, thus maintaining the integrity of the oxide film and further improving the high-temperature service life of the high-temperature magnesium oxide powder heating tube.

[0027] End cleaning removes excess electrofused magnesium oxide powder from both ends of the metal sheath tube, thereby ensuring the penetration depth of the sealed weld when welding the lead wires, preventing external air from entering the damaged oxide film, maintaining long-term protection of the oxide film, and further improving the high-temperature service life of the high-temperature magnesium oxide powder heating tube.

[0028] In some embodiments, the oxygen-containing atmosphere is air.

[0029] The air contains 21% oxygen by volume, which is cheaper than industrial pure oxygen and does not require an additional gas supply system. This reduces the manufacturing cost while meeting the oxygen partial pressure required for oxide film formation, thus maintaining the economic viability of high-temperature magnesium oxide powder heating tubes. Consequently, this indirectly ensures the large-scale application of solutions to improve the high-temperature service life of high-temperature magnesium oxide powder heating tubes.

[0030] In some embodiments, the fused magnesium oxide powder is dried, iron removed, and magnetically separated before filling, so that the iron impurity content of the fused magnesium oxide powder is ≤100ppm.

[0031] The fused magnesium oxide powder is dried to remove moisture, and the iron content is further reduced to, but not limited to, 100ppm, 90ppm, 80ppm, 70ppm, 60ppm, 50ppm, 40ppm, 30ppm, 20ppm, 10ppm, 5ppm, and 1ppm by magnetic separation. This eliminates the formation of Fe-Ni galvanic cells by iron impurities at high temperatures, thereby reducing the diameter reduction of the heating alloy wire caused by galvanic cell corrosion and further improving the high-temperature service life of the high-temperature magnesium oxide powder heating tube.

[0032] In some embodiments, the compacted density of the reduced-diameter fused magnesium oxide powder is ≥3.0 g / cm³. 3 .

[0033] Compacted density 3.0 g / cm³ 3 3.01g / cm 33.02 g / cm³, 3.03 g / cm³ 3 3.04 g / cm 3 3.05g / cm 3 3.06 g / cm 3 3.07 g / cm 3 3.08g / cm 3 3.09 g / cm 3 3.10 g / cm 3 This shortens the heat conduction path, thereby reducing the surface temperature of the heating alloy wire, which in turn reduces the thermal stress of the oxide film, thus extending the integrity of the oxide film and further improving the high-temperature service life of the high-temperature magnesium oxide powder heating tube.

[0034] In some embodiments, the curing heat treatment temperature is 400–600°C.

[0035] Temperatures of 400℃, 450℃, 500℃, 550℃, and 600℃ are used to fully annihilate lattice defects in fused magnesium oxide powder, thereby increasing high-temperature volume resistivity, reducing leakage corrosion, and further improving the high-temperature service life of high-temperature magnesium oxide powder heating tubes.

[0036] In some embodiments, the annealing temperature is 1000–1050°C, and the annealing holding time is 15–30 min.

[0037] Annealing temperatures: 1000℃, 1010℃, 1020℃, 1030℃, 1040℃, 1050℃; holding times: 15min, 18min, 21min, 24min, 27min, 30min; thereby eliminating machining dislocations between the metal sheath tube and the heating alloy wire, reducing microcrack sources in the oxide film, maintaining the continuity of the oxide film, and further improving the high-temperature service life of the high-temperature magnesium oxide powder heating tube.

[0038] Secondly, embodiments of this application provide a high-temperature magnesium oxide powder heating tube, which is prepared by any of the methods described in the first aspect. The high-temperature magnesium oxide powder heating tube has a continuous service life of ≥900h at 1150℃, and the heating alloy wire of the high-temperature magnesium oxide powder heating tube has no chromium nitride segregation in its cross-section after continuous operation at 1150℃ for 60h.

[0039] A dense oxide film, high-purity fused magnesium oxide powder, high pressure density, and stress relief through annealing prevent nitrogen atom diffusion during continuous use at 1150℃ for 200h, 300h, 400h, 500h, 600h, 700h, 800h, 900h, and 1000h. This, in turn, avoids the migration of chromium in the Cr2Ni3 phase, thus maintaining a uniform cross-sectional structure of the heating alloy wire without chromium nitride segregation. This prevents melting due to localized melting point drops, thereby achieving a high-temperature service life of ≥900h for the high-temperature magnesium oxide powder heating tube.

[0040] In some embodiments, the metal sheath is made of austenitic stainless steel.

[0041] The oxidation rate of austenitic stainless steel at 1150℃ is ≤0.1g / m 2 This process slowly supplies oxygen to the fused magnesium oxide powder, preventing the oxide film from thickening excessively and peeling off, thus maintaining the stability of the oxide film-metal sheath interface and further improving the high-temperature service life of the high-temperature magnesium oxide powder heating tube.

[0042] In some embodiments, the heating alloy wire is a Cr20Ni80 alloy wire with a diameter of 0.5–1.2 mm.

[0043] The oxide film growth rate of Cr20Ni80 alloy wires with diameters of 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1.0mm, 1.1mm, and 1.2mm is constant at 1150℃, thus ensuring an oxide film thickness of 1–4.0μm after pre-oxidation for 1–3h. This provides sufficient nitrogen diffusion barrier capability, thereby maintaining the cross-sectional microstructure free of chromium nitride segregation and further improving the high-temperature service life of the high-temperature magnesium oxide powder heating tube.

[0044] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0045] An example Example 1 Step a: Pre-oxidize Φ0.8mm bright annealed Cr20Ni80 heating alloy wire in air at 1050℃ for 2h to form a dense oxide film on the surface of the Cr20Ni80 heating alloy wire, and obtain pre-oxidized Cr20Ni80 heating alloy wire. Step b: Arrange the pre-oxidized Cr20Ni80 heating alloy wire along the axial direction of the Φ12mm 0Cr18Ni9 stainless steel metal sheath tube, and fill the stainless steel metal sheath tube with 99wt% pure fused magnesium oxide powder, the iron impurity content of the fused magnesium oxide powder being 20ppm; then cold roll the stainless steel metal sheath tube to reduce its diameter, so that the compacted density of the fused magnesium oxide powder reaches 3.05g / cm³ and covers the pre-oxidized Cr20Ni80 heating alloy wire; Step c: Perform a high-temperature curing heat treatment at 580℃ for 1.5 hours on the reduced-diameter stainless steel metal sheath tube; Step d: Anneal the heat-treated stainless steel metal sheath at 1000℃ for 30 minutes; Step e: Clean both ends of the annealed stainless steel metal sheath tube to obtain a high-temperature magnesium oxide powder heating tube.

[0046] Example 2 Step a: Pre-oxidize Φ0.8mm bright annealed Cr20Ni80 heating alloy wire in air at 1000℃ for 1.5h to form a dense oxide film on the surface of the Cr20Ni80 heating alloy wire, and obtain pre-oxidized Cr20Ni80 heating alloy wire. Step b: Arrange the pre-oxidized Cr20Ni80 heating alloy wire along the axial direction of the Φ12mm 0Cr18Ni9 stainless steel metal sheath tube, and fill the stainless steel metal sheath tube with 98wt% pure fused magnesium oxide powder, the iron impurity content of the fused magnesium oxide powder being 50ppm; then cold roll the stainless steel metal sheath tube to reduce its diameter, so that the compacted density of the fused magnesium oxide powder reaches 3.02g / cm³ and covers the pre-oxidized Cr20Ni80 heating alloy wire; Step c: Perform a high-temperature curing heat treatment at 500℃ for 1 hour on the reduced-diameter stainless steel metal sheath tube; Step d: Anneal the heat-treated stainless steel metal sheath at 1050℃ for 15 minutes; Step e: Clean both ends of the annealed stainless steel metal sheath tube to obtain a high-temperature magnesium oxide powder heating tube.

[0047] Example 3 Step a: Pre-oxidize Φ0.8mm bright annealed Cr20Ni80 heating alloy wire in air at 1100℃ for 1h to form a dense oxide film on the surface of the Cr20Ni80 heating alloy wire, and obtain pre-oxidized Cr20Ni80 heating alloy wire. Step b: Arrange the pre-oxidized Cr20Ni80 heating alloy wire along the axial direction of the Φ12mm 0Cr25Ni20 stainless steel metal sheath tube, and fill the stainless steel metal sheath tube with 99.5wt% pure fused magnesium oxide powder, the iron impurity content of the fused magnesium oxide powder being 10ppm; then cold roll the stainless steel metal sheath tube to reduce its diameter, so that the compacted density of the fused magnesium oxide powder reaches 3.10g / cm³ and covers the pre-oxidized Cr20Ni80 heating alloy wire; Step c: Perform a high-temperature curing heat treatment at 450℃ for 2 hours on the reduced-diameter stainless steel metal sheath tube; Step d: Anneal the heat-treated stainless steel metal sheath at 1030℃ for 20 minutes; Step e: Clean both ends of the annealed stainless steel metal sheath tube to obtain a high-temperature magnesium oxide powder heating tube.

[0048] Two pairs of proportions Comparative Example 1 Step a: The Φ0.8mm bright annealed Cr20Ni80 heating alloy wire was not pre-oxidized, and the unoxidized Cr20Ni80 heating alloy wire was directly obtained. Step b: Arrange the unoxidized Cr20Ni80 heating alloy wire along the axial direction of the Φ12mm 0Cr18Ni9 stainless steel metal sheath tube, and fill the stainless steel metal sheath tube with 99wt% pure fused magnesium oxide powder, the iron impurity content of the fused magnesium oxide powder being 20ppm; then cold roll the stainless steel metal sheath tube to reduce its diameter, so that the compacted density of the fused magnesium oxide powder reaches 3.05g / cm³, and then coat the unoxidized Cr20Ni80 heating alloy wire. Step c: Perform a high-temperature curing heat treatment at 580℃ for 1 hour on the reduced-diameter stainless steel metal sheath tube; Step d: Anneal the heat-treated stainless steel metal sheath at 1030℃ for 30 minutes; Step e: Clean both ends of the annealed stainless steel metal sheath tube to obtain a high-temperature magnesium oxide powder heating tube.

[0049] Comparative Example 2 Step a: The Φ0.8mm bright annealed Cr20Ni80 heating alloy wire was not pre-oxidized, and the unoxidized Cr20Ni80 heating alloy wire was directly obtained. Step b: Arrange the unoxidized Cr20Ni80 heating alloy wire along the axial direction of the Φ12mm 0Cr25Ni20 stainless steel metal sheath tube, and fill the stainless steel metal sheath tube with 99wt% pure fused magnesium oxide powder, the iron impurity content of the fused magnesium oxide powder being 20ppm; then cold roll the stainless steel metal sheath tube to reduce its diameter, so that the compacted density of the fused magnesium oxide powder reaches 3.10g / cm³ and covers the unoxidized Cr20Ni80 heating alloy wire; Step c: Perform a high-temperature curing heat treatment at 480℃ for 1.5 hours on the reduced-diameter stainless steel metal sheath tube; Step d: Anneal the heat-treated stainless steel metal sheath at 1000℃ for 30 minutes; Step e: Clean both ends of the annealed stainless steel metal sheath tube to obtain a high-temperature magnesium oxide powder heating tube.

[0050] Comparative Example 3 Step a: Pre-oxidize the Cr20Ni80 heating alloy wire in air at 1050℃ for 2 hours to form a dense oxide film on the surface of the Cr20Ni80 heating alloy wire, thus obtaining a pre-oxidized Cr20Ni80 heating alloy wire. Step b: Arrange the pre-oxidized Cr20Ni80 heating alloy wire along the axial direction of the Φ12mm 0Cr18Ni9 austenitic stainless steel metal sheath tube, and fill the stainless steel metal sheath tube with 94wt% pure fused magnesium oxide powder, the iron impurity content of the fused magnesium oxide powder being 200ppm; then cold roll the stainless steel metal sheath tube to reduce its diameter, so that the compacted density of the fused magnesium oxide powder reaches 3.05g / cm³ and covers the pre-oxidized Cr20Ni80 heating alloy wire; Step c: Perform a curing heat treatment on the reduced-diameter stainless steel metal sheath tube at 580℃ for 1 hour; Step d: Anneal the heat-treated stainless steel metal sheath at 1000℃ for 30 minutes; Step e: Clean both ends of the annealed stainless steel metal sheath tube to obtain a high-temperature magnesium oxide powder heating tube.

[0051] III. Results Data Experimental methods: 1. Continuous service life test at 1150℃: The high-temperature magnesium oxide powder heating tube was placed horizontally in a tube furnace. The furnace temperature was set to 1150℃, and the heating rate was 10℃ / min. After reaching 1150℃, the high-temperature magnesium oxide powder heating tube was continuously powered on, with a surface load of 1W / cm² on the heating wire. 2 Record the time until the Cr20Ni80 heating alloy wire melts; this time is the continuous service life at 1150℃.

[0052] 2. Detection of chromium nitride segregation in cross-section after 60 hours: Take the high-temperature magnesium oxide powder heating tube that has been continuously working at 1150℃ for 60 hours, dissect and remove the Cr20Ni80 heating alloy wire, prepare a cross-sectional metallographic sample, and after polishing, observe it under a scanning electron microscope energy dispersive spectroscopy at a field of view of 1000x to see if there is chromium carbide segregation greater than 1µm.

[0053] Table 1 Electrochemical performance data of examples and comparative examples

[0054] As shown in Table 1, the inventiveness of this application's technical solution compared to the prior art is mainly reflected in: 1. Examples 1–3 show a continuous service life of ≥900h at 1150℃, with a minimum of 900h and a maximum of 1000h; the same indicator for Comparative Examples 1–3 is only 70h–500h. Thus, the technical solution of this application achieves a stable service life of ≥900h for the first time under the condition of 1150℃.

[0055] 2. In Examples 1-3, after continuous operation at 1150℃ for 60 hours, the cross-sectional microstructure of the Cr20Ni80 heating alloy wire showed "no chromium nitride segregation"; in Comparative Examples 1-2, "chromium nitride segregation" was observed. Therefore, the technical solution of this application, through the single technical means of "pre-oxidation at 900-1100℃ for 1-3 hours," fundamentally blocks the inward diffusion channels of nitrogen atoms, thereby inhibiting the migration of chromium in the Cr2Ni3 phase. This fundamentally eliminates the localized decrease in melting point and early melting failure caused by chromium nitride segregation, thus extending the high-temperature service life in both qualitative and quantitative terms.

[0056] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing a long-life high-temperature magnesium oxide powder heating tube, characterized in that, The method includes: The heating alloy wire is pre-oxidized in an oxygen-containing atmosphere at 900–1100℃ for 1–3 hours to generate a dense oxide film on the surface of the heating alloy wire, thus obtaining a pre-oxidized heating alloy wire. The pre-oxidized electrothermal alloy wire is arranged along the axial direction of the metal sheath tube, and the metal sheath tube is filled with fused magnesium oxide powder with a purity of ≥96wt%. Then the diameter of the metal sheath tube is reduced so that the fused magnesium oxide powder is compacted and covers the pre-oxidized electrothermal alloy wire. The reduced-diameter metal sheath tube is subjected to a curing heat treatment. The heat-treated metal sheath tube is then annealed. The ends of the annealed metal sheath tube are cleaned to obtain a high-temperature magnesium oxide powder heating tube.

2. The method according to claim 1, characterized in that, The oxygen-containing atmosphere is air.

3. The method according to claim 1, characterized in that, The fused magnesium oxide powder is dried, iron removed, and magnetically separated before filling to ensure that the iron impurity content of the fused magnesium oxide powder is ≤100ppm.

4. The method according to claim 1, characterized in that, The compacted density of the fused magnesium oxide powder after diameter reduction is ≥3.0 g / cm³. 3 .

5. The method according to claim 1, characterized in that, The curing heat treatment temperature is 400-600℃.

6. The method according to claim 1, characterized in that, The annealing temperature is 1000–1050℃, and the annealing holding time is 15–30 min.

7. A high-temperature magnesium oxide powder heating tube, characterized in that, The high-temperature magnesium oxide powder heating tube, prepared by any one of claims 1–6, has a continuous service life of ≥900h at 1150℃, and the heating alloy wire of the high-temperature magnesium oxide powder heating tube has no chromium nitride segregation in its cross-section after continuous operation at 1150℃ for 60h.

8. The high-temperature magnesium oxide powder heating tube according to claim 7, characterized in that, The metal sheath is made of austenitic stainless steel.

9. The high-temperature magnesium oxide powder heating tube according to claim 7, characterized in that, The heating alloy wire is a Cr20Ni80 alloy wire with a diameter of 0.5–1.2 mm.

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