A pyrolysis furnace tube with a double-layer aluminum-rich composite corundum structure oxide film on its surface and a manufacturing method thereof

Abstract

The present invention belongs to the field of manufacturing cracking furnace tubes, and particularly relates to a cracking furnace tube with a double-layer aluminum-rich composite corundum structure oxide film on its surface and a manufacturing method thereof. A double-layer oxide film is formed on the surface of the cracking furnace tube, with the inner layer being a continuous and dense Al2O3 oxide layer and the outer layer being an aluminum-rich corundum structure M2O3 oxide layer. The content of Al element in the outer layer oxide film is relatively high, and the atomic ratio Al / (Cr + Ni + Fe + Si + Mn) > 1. In the present invention, when centrifugally casting the furnace tube, Al, Si, Mn, C and trace elements Hf, Y, Nb and Ta are directly added to the Fe-Ni-Cr alloy, and a two-step pre-oxidation heat treatment method with low oxygen pressure is adopted. After treatment, the deposition of coke on the inner wall of the cracking furnace tube can be greatly reduced, and the coking inhibition rate is up to more than 97% at most.

Description

Technical Field

[0001] The present invention belongs to the field of manufacturing cracking furnace tubes, and particularly relates to a cracking furnace tube with a double-layer aluminum-rich composite corundum structure oxide film on its surface and a manufacturing method thereof. Background Art

[0002] As one of the chemical products with the largest production in the world, the production of ethylene has become an important symbol to measure the development level of petrochemical industry in a country. The main production methods include the following three methods: thermal cracking, CTO / MTO, and light hydrocarbon cracking. Among them, the most important one is through the high-temperature cracking of petroleum hydrocarbons, and the cracking temperature is between 600°C and 1000°C, and its production capacity increases year by year. In the steam cracking process flow, in addition to the generation of various small molecule hydrocarbon products, a small amount of coke is generated at the same time, and the process of coke deposition is called coking. Coking will lead to a reduction in process efficiency and corrosion, erosion and degradation of the furnace tubes, and frequent decoking will shorten the service life of the equipment and cause huge economic losses.

[0003] Fe-Ni-Cr alloy is widely used as a high-temperature structural material for hydrocarbon high-temperature cracking furnace materials. However, Fe and Ni particles in the alloy will migrate at high temperatures, accumulate on the inner surface of the furnace tube and act as catalytic active centers, accelerating the deposition of coke on the cracking furnace tube. Therefore, it is crucial to improve the coke inhibition performance of Fe-Ni-Cr alloy. The coke inhibition methods being used or studied mainly include: using coke inhibitors, optimizing the ethylene production process, improving the furnace tube material, and pre-surface treatment of the furnace tube. Among them, the most economical, simple and effective method is the pre-treatment of the furnace tube surface.

[0004] A large number of studies have shown that using coke inhibitors will pollute the environment. Using surface coating technology to inhibit coking, in the high-temperature and high-carbon cracking environment, the coating cannot play a long-term role in anti-coking and carburization resistance. Improving the furnace tube material and selectively pre-oxidizing the inner surface of the cracking furnace tube is a method of inhibiting catalytic coking with low cost and simple operation. Its principle is to generate an oxide protective film on the surface of the furnace tube Fe-Ni-Cr alloy under a certain oxidation atmosphere to have the ability of anti-coking and anti-carburization. Among them, the cations in the oxide film all come from the furnace tube matrix, and the adhesion to the matrix is high.

[0005] Currently, Fe-Ni-Cr-based stainless steels are commonly used to generate Cr2O3 films to inhibit the catalytic effects of Fe and Ni on coking. However, when the temperature in the cracking furnace reaches above 1050 °C, Cr can react with C and form stable carbides. This will lead to the depletion of Cr on the alloy surface, inhibit the formation of the external Cr2O3 oxide film, and Cr2O3 is prone to spalling under thermal cycle shock. Due to spalling and incomplete coverage of the Cr2O3 oxide on the alloy surface, the Ni- and Fe-containing matrix can come into contact with the gas flow, promoting catalytic coking and resulting in poor persistence of anti-coking performance. In addition to the traditional Cr2O3 oxide film having the ability to inhibit coking, MnCr2O4 spinel also has high anti-coking and anti-carburization properties. However, the formed MnCr2O4 spinel has a loose structure and is prone to spalling after high-temperature cyclic use, with a short service life.

[0006] The ANK400 alloy furnace tube surface pretreatment technology developed by NOVA Company in Canada. The mass fraction of Cr in this heat-resistant chromium-nickel alloy is above 20%, and the mass fraction of Mn is above 1%. Through high-temperature pretreatment at 1000 °C in a low oxygen partial pressure atmosphere composed of H2 and H2O, Cr and Mn in the alloy are enriched on the surface to form a manganese-chromium spinel protective film. This furnace tube has strong coking inhibition performance, and the surface catalytic coking generation rate is significantly reduced. Jia Jingsheng et al. developed a technology for gas-coated cracking furnace tubes. When this technology was applied to a naphtha cracking evaluation device, it was found that the coking amount in the cracking process could be reduced by 51.4%. After multiple cracking and decoking cycles, the coating did not show obvious peeling, and the coking inhibition rate remained at about 50%. Wang Guoqing et al. developed a pretreatment technology for cracking furnace tubes. This technology uses gases such as CH4, H2O, and H2 to pretreat the alloy furnace tubes in a low oxygen partial pressure atmosphere. Finally, the obtained oxide film coverage rate is 90%, and the thickness is 2.6 μm. Through SEM and EDS elemental analysis, it was found that the oxide film formed on the surface of the treated furnace tube is relatively dense and has a strong bonding force with the matrix. Combining with the laboratory naphtha steam cracking coking evaluation device, it was found that the coking inhibition rate of the pretreated furnace tube reached 64%. Summary of the Invention

[0007] In order to further reduce the coking rate of cracking furnace tubes during the ethylene production process and improve the high-temperature creep performance of the furnace tubes, the present invention provides a cracking furnace tube with a double-layer aluminum-rich composite corundum structure oxide film on its surface and a manufacturing method thereof.

[0008] One of the present inventions is a cracking furnace tube with a double-layer aluminum-rich composite corundum structure oxide film on its surface.

[0009] The cracking furnace tube of the present invention with a double-layer aluminum-rich composite corundum structure oxide film on its surface includes a substrate and an oxide film combined with the substrate. The oxide film is a double-layer oxide film. Among them, the inner oxide film close to the substrate is a continuous and dense Al2O3 oxide layer, and the outer oxide film far from the substrate is an aluminum-rich corundum structure M2O3 oxide layer, where M includes elements such as Al, Cr, Ni, Fe, Si, and Mn, and the atomic ratio Al / (Cr + Ni + Fe + Si + Mn) > 1. As the ratio of Al / (Cr + Ni + Fe + Si + Mn) becomes larger, the technical effect of suppressing coking is better.

[0010] Preferably, in terms of weight percentage, the cracking furnace tube includes: Ni: 40 - 50%, Cr: 20 - 40%, Al: 1 - 5%, Si: 1 - 1.5%, Mn: 0.2 - 1.5%, C: 0.3 - 0.6%, trace elements 0.5 - 4%, and the balance is Fe; where the trace elements include Y, Hf, Nb, and Ta.

[0011] Preferably, in terms of weight percentage, the chemical composition of the outer aluminum-rich corundum structure M2O3 oxide layer includes: Al: 25 - 40%, Cr: 3 - 10%, Fe: 2 - 10%, Ni: 0 - 15%, Mn: 0 - 0.5%, Si: 0 - 1.2%, O: 30 - 50%, and the balance of other added elements 0 - 5%.

[0012] Preferably, the thickness of the double-layer oxide film is 2 - 5 μm.

[0013] In the cracking furnace tube of the present invention, the content of Ni element exceeds that of traditional HK-40, HP-40, and HP-45, and it is easy to form an FCC structure, which makes the diffusion coefficients of Al2O3 and Al in Ni become faster, promoting the external oxidation. At the same time, due to the increase in Ni content, the Fe content will relatively decrease, reducing the formation of Fe, Cr, and Mn oxides on the surface, and further promoting the formation of the Al2O3 protective film. The Nb element is beneficial to the formation of Al2O3 and can also improve the high-temperature creep strength. The Ta element inhibits the coarsening of the internal Al2O3 oxide. The addition of Ta induces the formation of TaO2, filling the voids in the external oxide layer, improving the density and stability of the protective film. The Ta element can also make the furnace tube have a certain creep recovery property.

[0014] The second aspect of the present invention is a manufacturing method of a cracking furnace tube with a double-layer aluminum-rich composite corundum structure oxide film on its surface, including the following steps:

[0015] (1) Directly add Al, Si, Mn, C, and trace elements Y, Hf, Nb, and Ta to the Fe-Ni-Cr alloy during the centrifugal casting process to directly make the furnace tube;

[0016] (2) Heat the furnace tubes prepared in step (1) to 600 - 900 °C in a low oxygen pressure oxidation atmosphere and keep the temperature constant for 6 - 20 h;

[0017] (3) Heat the furnace tubes treated in step (2) to 1000 - 1100 °C in a low oxygen pressure oxidation atmosphere and keep the temperature constant for 3 - 16 h.

[0018] Preferably, the oxidation atmosphere is a mixture of H2O and 4% H2 + Ar gas or a mixture of H2O and 3% CH4 + Ar gas, where the H2O accounts for 0.01% - 4% of the volume of the oxidation atmosphere gas. During the process of low oxygen pressure selective water oxidation, the water content in the mixed atmosphere is adjusted by regulating the water flow rate of the micro - pump at 0.01 ml / h - 0.6 ml / h to control the volume of H2O in the entire oxidation atmosphere gas, so as to control the oxygen partial pressure below 10 -17 atm.

[0019] The cracking furnace tubes of the present invention are heat - treated in a low oxygen pressure oxidation atmosphere. The heat treatment is divided into low - temperature heat treatment and high - temperature heat treatment. After two - step pre - oxidation treatment, the Al2O3 formed in situ on the alloy surface is transformed from a γ - Al2O3 oxide film to an α - Al2O3, and the structure of the oxide film is more stable.

[0020] The beneficial effects of the present invention are as follows:

[0021] 1. The alloy of the present invention adds Al element, forming a continuous and single Al2O3 structure in the inner layer and a stable corundum structure in the outer layer. As the furnace tubes are used in a high - temperature environment, the inner - layer Al2O3 oxide film continuously thickens and fills the Al2O3 in the surface solid solution, realizing self - growth and extending the service life of the furnace tubes.

[0022] 2. Compared with traditional HK - 40, HP - 40, and HP - 45 alloys, the Ni content in the present invention is higher, having more excellent high - temperature performance. And by adding Y, Hf, Nb, and Ta elements, oxide plugs can be formed between the matrix and the oxide layer, forming a pinning phenomenon, improving the adhesion and denseness of the oxide film. The carbides formed by Nb and Ta improve the high - temperature creep resistance of the furnace tubes. The present invention effectively improves the coking inhibition and high - temperature creep resistance of the furnace tubes.

[0023] 3. The present invention adopts a low oxygen pressure two - step pre - oxidation heat treatment method. First, low - temperature heat treatment is carried out to generate a Cr2O3 oxide film on the surface of the furnace tube alloy, reducing the oxygen pressure between the oxide layer and the matrix, promoting the internal oxidation of Al2O3, and also promoting the enrichment of Al element on the alloy surface to form an Al - rich phase. Then, high - temperature heat treatment is carried out to make the Al - rich phase generated at low temperature form a corundum structure rich in Al2O3 in the outer layer and a continuous and single Al2O3 structure in the inner layer on the surface layer of the furnace tube. Moreover, the heat treatment time of the present invention is short and the energy consumption is low, saving the production cost. Description of the Drawings

[0024] Figure 1 FIG. is the SEM image of the surface of the sample subjected to low oxygen pressure two-step pre-oxidation heat treatment in Example 1;

[0025] Figure 2 FIG. is the SEM image of the surface of the sample subjected to low oxygen pressure two-step pre-oxidation heat treatment in Example 2;

[0026] Figure 3 FIG. is the SEM images of the surface (a) and cross-section (b) of the sample subjected to low oxygen pressure two-step pre-oxidation heat treatment in Example 3;

[0027] Figure 4 FIG. is the SEM image of the surface of the sample subjected to low oxygen pressure two-step pre-oxidation heat treatment in Example 4;

[0028] Figure 5 FIG. is the SEM image of the surface of the sample subjected to low oxygen pressure one-step high-temperature pre-oxidation heat treatment in Comparative Example 1;

[0029] Figure 6 FIG. is the SEM image of the surface of the sample subjected to low oxygen pressure two-step pre-oxidation heat treatment in Comparative Example 2;

[0030] Figure 7 FIG. is the SEM image of the surface of the original alloy sample in Example 3;

[0031] Figure 8 FIG. is the curve graph of the relationship between creep strain and time of Example 1 and Example 3;

[0032] Figure 9 FIG. is the curve graph of the relationship between creep strain rate and time of Example 1 and Example 3. Detailed Embodiments

[0033] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings. The described embodiments are only a part of the embodiments of the present application, and these embodiments are only used to explain the present invention and do not limit the scope of the present invention. Example 1

[0034] By weight percentage, the composition of the cracking furnace tube includes: 45% Ni, 25% Cr, 4% Al, 1% Mn, 1% Si, 0.4% C, and the balance is Fe. The furnace tube is made by centrifugal casting, and after processing, the surface is bright and free of scale.

[0035] The furnace tube is subjected to low oxygen pressure two-step pre-oxidation heat treatment. The furnace tube is placed in a low oxygen partial pressure environment generated by a mixture of H2O and 4% H2 + Ar gas, and the flow rate of the H2 + Ar mixed gas is 30 ml / h.

[0036] The specific conditions for heat treatment are as follows:

[0037] Low-temperature heat treatment: The oxidation temperature is 600 °C, the oxidation time is 10 h, the oxygen pressure is 10 -28 atm, and the volume percentage of H2O is 0.0375%.

[0038] High-temperature heat treatment: The oxidation temperature is 1000 °C, the oxidation time is 7 h, the oxygen pressure is 10 -17 atm, and the volume percentage of H2O is 0.1018%.

[0039] After cooling, it was analyzed by scanning electron microscopy and energy dispersive spectrometer, and the results are shown in Table 1.

[0040] Table 1 Composition element distribution (wt.%) on the inner surface of the furnace tube before and after treatment in Example 1

[0041] O Al Si Cr Mn Fe Ni Others Thickness Before treatment 0.66 4.74 1.39 24.89 1.36 21.53 44.84 0.59 / After treatment 32.48 32.16 0.45 9.79 0.48 9.26 14.93 0.45 3.5μm

[0042] As can be seen from Table 1, there are many elements on the surface before treatment that are likely to cause catalytic coking. After the two-step pre-oxidation heat treatment with low oxygen pressure, the composition of the surface layer oxide on the alloy surface has changed greatly, and its surface morphology is as Figure 1 shown. The process of composition and microstructure evolution is a solid solution of Cr2O3—(Cr, Al)2O3—Al2O3 and other oxides, and more alumina-rich corundum structures appear on the surface. The high-temperature heat treatment makes the Al2O3 phase formed by the low-temperature heat treatment gradually transform into an alumina layer with a high Al content. The Al2O3 content on the alloy surface has exceeded Cr2O3, and the atomic percentages of some elements in the significantly film-forming part of the surface layer oxide are shown in Table 7.

[0043] Using industrial naphtha as the cracking raw material (the naphtha distillation range is 80 - 100), a coking evaluation test was carried out on the above-mentioned treated cracking furnace tube. The cracking furnace tube specimen was suspended in the cracking device. After washing the gas, the cracking furnace was heated up. When the furnace temperature reached 750 °C, deionized water was started to be introduced at a flow rate of 60 ml / h; after the temperature reached 850 °C, naphtha was introduced at a flow rate of 180 ml / h, and the coking experiment was timed. The cracking time was 3 h.

[0044] The experimental results show that the coking amount of the furnace tube in Example 1 is reduced by 86.48% compared with that of the original alloy furnace tube. Example 2

[0045] By weight percentage, the composition of the cracking furnace tube includes: 45% Ni, 25% Cr, 4% Al, 1% Mn, 1% Si, 0.4% C, 1.5% Nb, 0.1% Hf, 0.1% Y, and the balance is Fe. The furnace tube made by centrifugal casting has a bright surface and no scale after processing.

[0046] The furnace tubes are subjected to two-step pre-oxidation heat treatment under low oxygen pressure. The furnace tubes are placed in a low oxygen partial pressure environment generated by a gas mixture of H2O and 3% CH4 + Ar. The oxygen pressure generated by this atmosphere has the same effect as that of a gas mixture of H2O and 4% H2 + Ar. The flow rate of the CH4 + Ar gas mixture is 30 ml / h.

[0047] The specific conditions of the heat treatment are as follows:

[0048] Low-temperature heat treatment: The oxidation temperature is 600 °C, the oxidation time is 10 h, the oxygen pressure is 10 -28 atm, and the volume percentage of H2O is about 0.4%.

[0049] High-temperature heat treatment: The oxidation temperature is 1000 °C, the oxidation time is 7 h, the oxygen pressure is 10 -17 atm, and the volume percentage of H2O is about 3.7%

[0050] After cooling, it is analyzed by scanning electron microscopy and energy dispersive spectrometer, and the results are shown in Table 2.

[0051] Table 2 Composition element distribution (wt.%) on the inner surface of the furnace tube before and after treatment in Example 2

[0052] O Al Si Cr Mn Fe Ni Nb, Hf and Y Others Thickness Before treatment 0.96 4.19 1.09 25.54 1.08 19.33 45.39 1.96 0.46 / After treatment 35.47 31.18 0.73 9.75 0.40 7.82 13.23 0.83 0.59 3μm

[0053] It can be seen from Table 2 that in Example 2, trace elements Nb, Hf, and Y are added. After two-step pre-oxidation heat treatment under low oxygen pressure, a corundum structure protective film rich in aluminum is generated on the alloy surface, which plays a good role in inhibiting coking. Its surface morphology is as Figure 2 shown. Compared with Example 1, the addition of Nb, Hf, and Y elements further promotes the formation of surface Al2O3. The atomic percentages of elements in the significantly film-forming part of the surface oxide are shown in Table 7.

[0054] Using the same cracking raw materials and cracking test conditions as in Example 1, a coking evaluation test is carried out on the furnace tubes of Example 2. The experimental results show that the coking amount of the furnace tubes in Example 2 is reduced by 88.44% compared with that of the original alloy furnace tubes. Example 3

[0055] By weight percentage, the composition of the cracking furnace tube includes: 45% Ni, 25% Cr, 4% Al, 1% Mn, 1% Si, 0.4% C, 1.5% Nb, 1.5% Ta, 0.1% Hf, 0.1% Y, and the balance is Fe. The furnace tube made by centrifugal casting has a bright surface and no oxide scale after processing.

[0056] Using the same two-step pre-oxidation heat treatment conditions under low oxygen pressure as in Example 2.

[0057] After cooling, it is analyzed by scanning electron microscopy and energy dispersive spectrometer, and the results are shown in Table 3.

[0058] Table 3 Composition Element Distribution on the Inner Surface of the Furnace Tube before and after Treatment in Example 3 (wt.%)

[0059] O Al Si Cr Mn Fe Ni Nb, Ta, Hf and Y Others Thickness Before treatment 0.84 4.61 1.03 23.32 0.55 20.44 44.86 3.92 0.43 / After treatment 37.43 32.48 0.44 8.15 0.37 6.11 10.07 4.59 0.36 3μm

[0060] Figure 3 SEM images of the surface (a) and cross-section (b) of the sample after treatment in Example 3. It can be seen that in Example 3, trace elements Nb, Ta, Hf, and Y were added. After two-step pre-oxidation heat treatment under low oxygen pressure, a large amount of Al2O3 and aluminum-rich corundum structure appeared on the alloy surface, which had extended from a local area to most of the area compared with Example 1. The atomic percentages of elements in the significantly film-forming part of the surface oxide are shown in Table 7.

[0061] Using the same cracking raw materials and cracking test conditions as in Example 1, a coking evaluation test was carried out on the furnace tube of Example 3. The experimental results show that the coking amount of the furnace tube in Example 3 was reduced by 92.73% compared with that of the original alloy furnace tube. Example 4

[0062] Compared with Example 3, the heat treatment conditions were changed, specifically:[[]]

[0063] Low-temperature heat treatment: oxidation temperature was 600 °C, oxidation time was 8 h, oxygen pressure was 10 -28 atm, and the volume percentage of H2O was about 0.4%.

[0064] High-temperature heat treatment: oxidation temperature was 1000 °C, oxidation time was 16 h, oxygen pressure was 10 -17 atm, and the volume percentage of H2O was about 3.7%.

[0065] Other operations were the same as in Example 3.

[0066] After cooling, it was analyzed by scanning electron microscopy and energy dispersive spectrometer, and the results are shown in Table 4.

[0067] Table 4 Composition Element Distribution on the Inner Surface of the Furnace Tube before and after Treatment in Example 4 (wt.%)

[0068] O Al Si Cr Mn Fe Ni Nb, Ta, Hf and Y Others Thickness Before treatment 1.27 4.14 1.48 24.52 0.44 19.48 44.25 3.91 0.51 / After treatment 45.17 37.72 1.09 5.34 0.12 2.29 4.86 3.14 0.27 3.5μm

[0069] As can be seen from Table 4, by extending the high-temperature heat treatment time, Al2O3 on the alloy surface far exceeded other oxides. Its surface morphology is as Figure 4 shown. It can be seen that most of the area on the alloy surface was occupied by Al2O3, and the oxides of Cr, Ni, Si, and Fe were very few. The atomic percentages of elements in the significantly film-forming part of the surface oxide are shown in Table 7.

[0070] Using the same pyrolysis feedstock and pyrolysis test conditions as in Example 1, a coking evaluation test was carried out on the furnace tubes of Example 4. The experimental results show that the coking amount of the furnace tubes of Example 4 was reduced by 97.87% compared with that of the original alloy furnace tubes. Comparative Example 1

[0071] Compared with Example 1, low-oxygen-pressure one-step high-temperature pre-oxidation heat treatment was adopted. The specific heat treatment conditions were as follows:

[0072] The oxidation temperature was 1000 °C, the oxidation time was 10 h, the oxygen pressure was 10 -18 atm, and the volume percentage of H2O was 0.0761%. Other operations were the same as in Example 1.

[0073] After cooling, it was analyzed by scanning electron microscopy and energy dispersive spectrometer, and the results are shown in Table 5.

[0074] Table 5 Composition element distribution on the inner surface of the furnace tube before and after treatment in Comparative Example 1 (wt.%)

[0075] O Al Si Cr Mn Fe Ni Others Thickness Before treatment 1.31 4.19 1.34 24.46 1.22 22.07 44.92 0.49 / After treatment 30.13 0.25 0.22 47.04 15.27 2.85 3.93 0.31 4μm

[0076] Figure 5 Figure 5 is the SEM image of the surface of the sample of Comparative Example 1 with low-oxygen-pressure one-step high-temperature pre-oxidation heat treatment. It can be seen that the surface of the furnace tube alloy has a composite oxide film. The outer layer completely covers the composite oxide film of Cr2O3 and MnCr2O4 spinel, and the density is relatively low. The inner layer is a continuous and dense Al2O3 layer. Compared with Example 1, the Al element in the surface oxide film tends to 0, so Al / (Cr + Ni + Si + Fe + Mn) is not of reference significance, and the composite oxide film of Cr2O3 and MnCr2O4 spinel on the surface layer is unstable in the high-temperature working environment.

[0077] Using the same pyrolysis feedstock and pyrolysis test conditions as in Example 1, a coking evaluation test was carried out on the furnace tubes of Comparative Example 1. The experimental results show that the coking amount of the furnace tubes of Comparative Example 1 was reduced by 82.63% compared with that of the original alloy furnace tubes. Comparative Example 2

[0078] 4% Al by weight was added during the melting of HP-40 alloy, and then the same low-oxygen-pressure two-step pre-oxidation heat treatment method as in Example 2 was adopted.

[0079] After cooling, it was analyzed by scanning electron microscopy and energy dispersive spectrometer, and the results are shown in Table 6.

[0080] Table 6 Composition element distribution on the inner surface of the furnace tube before and after treatment in Comparative Example 2 (wt.%)

[0081] O Al Si Cr Mn Fe Ni Others Thickness Before treatment 0.69 3.96 0.79 24.12 1.21 33.39 35.36 0.48 / After treatment 39.53 7.45 1.78 28.54 0.31 14.52 7.47 0.40 3μm

[0082] Figure 6SEM image of the surface of the sample in Comparative Example 2 that underwent low-oxygen-pressure two-step pre-oxidation heat treatment. It can be seen that most of the Fe and Ni on its surface are covered by oxides, and the main components of these oxides are Cr2O3 and (Al, Cr)2O3. The atomic percentages of the elements in the significantly film-forming part of the surface oxide are shown in Table 7. Compared with Example 1, the content of Al2O3 on its surface is less because the Ni content in the furnace tube alloy is lower. When the Ni content increases, it is easier to form an FCC structure, and the diffusion coefficients of Al2O3 and Al in Ni become faster, promoting the formation of external oxidation Al2O3.

[0083] Using the same cracking raw materials and cracking test conditions as in Example 1, a coking evaluation test was carried out on the furnace tube of Comparative Example 2. The experimental results show that the coking amount of the furnace tube in Comparative Example 2 was reduced by 84.13% compared with that of the original alloy furnace tube.

[0084] Table 7 Element distribution (at.%) in the significantly film-forming part of the surface oxide by overall atomic percentage

[0085] O Al Si Cr Mn Fe Ni Al / (Cr + Ni + Si + Fe + Mn) Example 1 49.15 31.26 0.44 4.96 0.26 4.34 7.38 1.8 Example 2 48.94 33.23 0.47 6.72 0.31 3.98 4.75 2.05 Example 3 56.06 30.88 0.50 4.93 0.16 3.52 1.96 2.79 Example 4 60.28 32.77 0.86 2.24 0.04 0.91 1.81 5.60 Comparative Example 2 55.37 10.58 1.54 17.62 0.27 6.19 5.75 ＜1

[0086] Test Example

[0087] Examples 1 and 3 were selected for high-temperature creep tests. Using CTM304-A1 equipped with a low-stress sensor, the specific test conditions were: heating temperature 1000°C, stress 60 MPa. The results show that the fracture time of Example 1 was 2462.73 s, and the fracture time of Example 3 was 3378.74 s.

[0088] Compared with Example 1, the addition of Ta and Nb elements in Example 3 caused the formation of carbide (Nb,Ta)C in the original alloy, significantly improving the high-temperature creep resistance of the furnace tube. The surface of the original alloy sample in Example 3 is shown in Figure 7 .

[0089] Figure 8 It is a curve graph showing the relationship between creep strain and time. The creep fracture life of Example 3 is significantly better than that of Example 1 because the addition of Ta and Nb elements enhanced the high-temperature creep resistance of the furnace tube.

[0090] Figure 9 It is a curve graph showing the relationship between creep strain rate and time. It was found that under the same applied stress, the minimum creep strain rate of Example 3 was also less than that of Example 1, with better creep resistance and a longer creep life.

Claims

1. A cracking furnace tube with a double-layer aluminum-rich composite corundum structure oxide film on its surface, the cracking furnace tube comprising a substrate and an oxide film bonded to the substrate, characterized in that, The oxide film mentioned above is a double-layer oxide film. The inner oxide film close to the substrate is a continuous and dense Al2O3 oxide layer, and the outer oxide film far from the substrate is a corundum-rich structure M2O3 oxide layer, where M includes elements such as Al, Cr, Ni, Fe, Si, and Mn, and the atomic ratio Al / (Cr + Ni + Fe + Si + Mn) > 1; by weight percentage, the cracking furnace tube includes: Ni: 40 - 50%, Cr: 20 - 40%, Al: 1 - 5%, Si: 1 - 1.5%, Mn: 0.2 - 1.5%, C: 0.3 - 0.6%, trace elements 0.5 - 4%, and the balance is Fe; among them, the trace elements include Y, Hf, Nb, and Ta.

2. The cracking furnace tube with an oxide film having a double-layer aluminum-rich composite corundum structure on the surface according to claim 1, characterized in that, By weight percentage, the chemical composition of the corundum-rich structure M2O3 oxide layer includes: Al: 25 - 40%, Cr: 3 - 10%, Fe: 2 - 10%, Ni: 0 - 15%, Mn: 0 - 0.5%, Si: 0 - 1.2%, O: 30 - 50%, and the remaining additive elements 0 - 5%.

3. The cracking furnace tube with a double-layer aluminum-rich composite corundum structure oxide film on the surface according to claim 1, characterized in that, The thickness of the double-layer oxide film is 2 - 5 μm.

4. A manufacturing method of a cracking furnace tube with a double-layer aluminum-rich composite corundum structure oxide film on the surface according to any one of claims 1 to 3, characterized in that, It includes the following steps: (1) Add Al, Si, Mn, C, and trace elements Y, Hf, Nb, and Ta directly during the centrifugal casting process of the Fe-Ni-Cr alloy to directly make the furnace tube; (2) Heat the furnace tube made in step (1) to 600 - 900 °C in a low-oxygen-pressure oxidation atmosphere and keep it at a constant temperature for 6 - 20 h; (3) Heat the furnace tube treated in step (2) to 1000 - 1100 °C in a low-oxygen-pressure oxidation atmosphere and keep it at a constant temperature for 3 - 16 h.

5. The manufacturing method of the cracking furnace tube according to claim 4, characterized in that, The oxidizing atmosphere is a gas mixture of H2O and 4% H2 + Ar or a gas mixture of H2O and 3% CH4 + Ar, where the H2O accounts for 0.01% to 4% of the volume of the oxidizing atmosphere gas; the oxygen partial pressure ≤ 10 -17 atm.

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