An anti-coking furnace tube with a gradient composite oxide layer and a manufacturing method thereof

Abstract

The present invention belongs to the field of manufacturing cracking furnace tubes, and particularly relates to an anti-coking furnace tube with a gradient composite oxide layer and a manufacturing method thereof. The composition of the anti-coking furnace tube includes Fe, Ni, Cr, Si, Nb, Al, and C, as well as trace elements Mn and Y. After subjecting the anti-coking furnace tube to low-temperature and high-temperature heat treatments in a low oxygen pressure atmosphere, a gradient composite oxide layer is formed on the surface of the furnace tube. The inner surface is a continuous and dense Al2O3 oxide layer, and the outer surface is an (Al, M)2O3 oxide layer, where M is any one of Mn, Cr, or Si. This gradient composite oxide layer has good adhesion, high-temperature stability, and anti-coking performance.

Description

Technical Field

[0001] The present invention belongs to the field of manufacturing cracking furnace tubes, and particularly relates to an anti-coking furnace tube with a gradient composite oxide layer and a manufacturing method thereof. Background Art

[0002] Ethylene is one of the most produced chemical products in the world. Its production capacity is an important standard for measuring a country's ethylene competitiveness. The output directly affects the supply of ethylene and other derivatives, and is also an important symbol for measuring a country's petrochemical industry. At present, the production of ethylene is mainly formed by cracking petroleum hydrocarbons in a steam cracking furnace. During the thermal cracking process of hydrocarbons, carbon formed by the condensation of a large number of carbon atoms and containing a small amount of hydrogen is generated, and this carbon is called coke. The process of coke deposition on the cracking furnace tubes during the cracking process is called coking, which will lead to increased energy consumption, reduced process efficiency, and corrosion, erosion and degradation of the furnace tubes, resulting in huge economic losses.

[0003] Cracking furnace tubes are usually made of stainless steel or HK-40 and HP-40 alloys produced by centrifugal casting. Its main components are Fe, Cr, and Ni elements, which have good oxidation resistance, corrosion resistance and high-temperature stability. Elements such as Fe and Ni are necessary to ensure the temperature and mechanical properties of the alloy. However, during the steam thermal cracking process of petroleum hydrocarbons, transition elements such as Fe and Ni on the surface of the furnace tubes are extremely easy to pair electrons with electron-rich hydrocarbons, catalyzing the deposition of coke on the inner wall of the furnace tubes, thereby deteriorating the performance and service life of the materials. Currently, the main means of suppressing coking are adding coking inhibitors, coating the inner surface of the furnace tubes, and improving the furnace tube materials.

[0004] Adding coking inhibitors is a relatively mature and commonly used method at present. It can effectively inhibit the catalytic coking of metals by passivating the surface of the furnace tubes, and at the same time promote the reaction of surface coke and water vapor for subsequent coke removal. The diethyl thiodiphosphite studied by Nalco Company in the United States as a coking inhibitor can reduce the coking amount by 92%; Xi'an Hangping New Technology Company has developed a YHCS sulfiding agent that can reduce the coking amount by about 70%. However, most coking inhibitors contain toxic substances such as sulfur and phosphorus, which will inevitably cause environmental pollution and other problems, and at the same time increase equipment and maintenance costs.

[0005] The inner surface coating technology of furnace tubes is to coat a layer on the surface of furnace tubes to isolate the high-temperature gas flow of hydrocarbon cracking from contacting the Fe and Ni substrates, thereby effectively inhibiting the occurrence of catalytic coking. At the same time, it can also make up for the defects exposed when the oxide scale on the furnace tube surface exfoliates. The "CoatAlloy 1100" coating developed by Westaim SEP Company in Canada has high stability and anti-coking and anti-carburization capabilities; the "AlcroPlex" technology developed by Alon Company in the United States can improve the operation cycle of cracking furnaces and extend the service life of furnace tubes; the SiO2 / S coating developed by East China University of Science and Technology can reduce coke by 39%. However, the material of the coating is different from that of the furnace tube material. The surface coating obtained by coating technology usually has a weak bonding force with the substrate at high temperatures and is easy to exfoliate. At the same time, there may be a problem of blocking the furnace tube due to exfoliation.

[0006] By improving the furnace tube material, the generation of coke can be effectively inhibited, the cracking depth and production efficiency can be improved. At the same time, adding some alloying elements can also improve the high-temperature creep strength of the furnace tube and extend its service life. Currently, Fe-Ni-Cr-based stainless steel is commonly used to form a Cr2O3 oxide film to inhibit the catalytic effect of Fe and Ni on coking. As the working temperature of the cracking furnace tube further increases, the current Cr2O3 film is difficult to meet its requirements. At high temperatures, Cr can react with C and form stable carbides, which will lead to the depletion of Cr on the alloy surface and inhibit the formation of the external Cr2O3 oxide film; at high temperatures, due to the action of thermal cycle shock, Cr2O3 is easy to exfoliate; at the same time, exfoliation and incomplete Cr2O3 coverage can make the Ni- and Fe-containing substrates contact with the gas flow, promoting catalytic coking. Compared with the Cr2O3 oxide film, the Al2O3 oxide film has better thermal stability, excellent mechanical properties and adhesion, better anti-thermal cycle shock performance, and can more effectively inhibit the deposition of coke on the furnace tube surface.

[0007] Compared with ferritic stainless steel and duplex steel, austenitic stainless steel has better high-temperature strength, resistance to steam oxidation and corrosion resistance. Ni can expand the austenite phase region and improve high-temperature stability, while Al and Cr are ferrite-forming elements. When their content is relatively high in austenitic stainless steel, the alloy will transform into an austenite-ferrite duplex structure and promote the precipitation of σ phase, reducing the creep resistance of the alloy. Therefore, the composition design of the alloy should first reasonably control the contents of Al, Cr and Ni to ensure the stability of austenite.

[0008] Although a single oxide film can effectively inhibit the formation of coke on the furnace tube surface, under the long-term high-temperature thermal cycle shock, the bonding force between the surface oxide film and the substrate is limited. Once damaged, there is a risk of exposing the substrate and promoting catalytic coking. Summary of the Invention

[0009] To solve the above technical problems, the present invention provides an anti-coking furnace tube with a gradient composite oxide layer and a manufacturing method thereof. The composite oxide film on the surface of the anti-coking furnace tube manufactured by the present invention has good adhesion and high-temperature stability, can effectively alleviate the coke deposition generated on the inner wall of the furnace tube during the cracking of petroleum hydrocarbons, and has good antioxidant performance.

[0010] The specific solution of the present invention is as follows:

[0011] An anti-coking furnace tube with a gradient composite oxide layer, comprising a substrate and a composite oxide layer combined with the substrate. The composite oxide layer includes a continuous and dense Al2O3 oxide layer close to the substrate and an (Al, M)2O3 oxide layer far from the substrate; where M is any one of Mn, Cr, or Si.

[0012] Preferably, by weight percentage, the chemical composition of the anti-coking furnace tube is C: 0.1 - 0.5%, Cr: 15 - 30%, Ni: 20 - 35%, Al: 2 - 5%, Si: 0.3 - 2%, Nb: 0.5 - 2%, Y: 0.1 - 0.3%, and Mn: 0 - 1%, and the rest is iron.

[0013] Preferably, the thickness of the composite oxide layer is 2 - 6 μm; where the thickness of the Al2O3 oxide layer is 1 - 3 μm.

[0014] The manufacturing method of the anti-coking furnace tube with a gradient composite oxide layer is: preparing raw materials according to the chemical components required for the anti-coking furnace tube, melting, centrifugally casting to form a furnace tube, and then performing low-temperature and high-temperature heat treatment on the furnace tube in a low oxygen partial pressure atmosphere. The oxygen partial pressure is 3.2×10 -29 ~1.6×10 -14 atm.

[0015] Preferably, the low oxygen partial pressure atmosphere is a mixed atmosphere composed of a reducing gas, water vapor, and an inert gas; the reducing gas is one or a mixture of two gases of CH4 and H2; the oxygen pressure can be accurately controlled by adjusting the water flow rate with a micro-injection pump; the volume percentage of H2O in the low oxygen partial pressure atmosphere gas is 0.2% - 3%.

[0016] Preferably, the low-temperature heat treatment temperature is 650 - 800 °C, and the time is 5 - 10 h. At low temperatures, Al diffuses outward better than other elements, increasing the surface Al content and making it easier to form an Al2O3 oxide film on the surface. The Al2O3 oxide film formed at low temperatures is thinner and sparser. The high-temperature heat treatment temperature is 1000 - 1200 °C, and the time is 5 - 10 h. Since Al has a stronger affinity for oxygen, "selective oxidation" of alloying elements occurs under low oxygen pressure, preferentially forming Al2O3 and inhibiting the formation of other oxides. When the alloy treated at low temperature is further subjected to high-temperature oxidation, a stable corundum structure is formed on the surface, and γ-Al2O3 on the inner surface is transformed into more stable α-Al2O3.

[0017] The beneficial effects of the present invention are as follows:

[0018] 1. The furnace tube prepared by the present invention forms a composite oxide layer with an outer surface of (Al, M)2O3 oxide layer and an inner surface of Al2O3; the oxide layers are tightly connected and not easily peeled off. The Al2O3 oxide layer has good adhesion to the substrate and good high-temperature stability, and can effectively inhibit the deposition of coke on the surface of the furnace tube; at the same time, the addition of Si element fills the gap between the oxide layer and the substrate, and the Y element can reduce the oxidation activation energy, promote the external oxidation of Al in AFA stainless steel, and further improve the adhesion of the oxide film to the substrate.

[0019] 2. Compared with a single oxide layer, the gradient composite oxide layer has a greater thickness of the oxide layer and an additional protective effect. When the outer oxide film peels off or has defects, it will not expose the substrate, resulting in coking and carburization. At the same time, in the low oxygen pressure atmosphere of ethylene cracking, it can spontaneously repair the damaged outer oxide film, effectively reducing the process requirements and costs for generating the protective coating and increasing the service life of the furnace tube.

[0020] 3. The furnace tube of the present invention is made of an iron-based material, and the alloy structure is a single-phase austenite structure, which can effectively save production costs compared with nickel-based superalloys and high-nickel austenitic alloys. Description of the Drawings

[0021] Figure 1 SEM images of the surface (a) and cross-section (b) of the furnace tube after pre-oxidation heat treatment in Example 1;

[0022] Figure 2 SEM image of the surface of the furnace tube after pre-oxidation heat treatment in Comparative Example 1;

[0023] Figure 3 SEM images of the surface (a) and cross-section (b) of the furnace tube after pre-oxidation heat treatment in Comparative Example 2;

[0024] Figure 4 SEM image of the cross-section of the furnace tube after pre-oxidation heat treatment in Example 2;

[0025] Figure 5 SEM image of the furnace tube cross-section after pre-oxidation heat treatment in Example 3;

[0026] Figure 6 SEM image of the furnace tube cross-section after pre-oxidation heat treatment in Comparative Example 3;

[0027] Figure 7 SEM image of the furnace tube cross-section after pre-oxidation heat treatment in Example 4;

[0028] Figure 8 SEM image of the furnace tube cross-section after pre-oxidation heat treatment in Example 5;

[0029] Figure 9 SEM image of the furnace tube cross-section after pre-oxidation heat treatment in Example 6;

[0030] Figure 10 SEM image of the furnace tube cross-section after pre-oxidation heat treatment in Comparative Example 4;

[0031] Figure 11 Comparison chart of weight gain per unit area after 4 hours of reaction in a petroleum pyrolysis atmosphere at 850 °C for the 25Ni-20Cr type original furnace tube, Example 1, Example 5, Comparative Example 2, and Comparative Example 4 furnace tubes. Detailed implementation manners

[0032] The present invention will be further introduced below by combining specific embodiments. The embodiments described below are only used to explain the present invention and do not limit the protection scope of the present invention.

[0033] Example 1

[0034] By weight percentage, the chemical composition of the furnace tube alloy is: 4% Al, 20% Cr, 25% Ni, 0.3% C, 1% Mn, 1.5% Si, 1% Nb, and 0.2% Y, with Fe as the balance.

[0035] The manufacturing method of the anti-coking furnace tube with a gradient composite oxide layer is as follows:

[0036] S1, Put the alloy raw materials according to the above weight percentages into a vacuum melting furnace for melting. The melting current is 120 A. The front and back sides of the alloy are each melted 2 times, and the melting duration for each time is 4 min to avoid alloy composition segregation.

[0037] S2, The melted alloy is cast into a seamless furnace tube with a diameter of 8 mm and a wall thickness of 1.2 mm by centrifugal casting. The inner surface of the furnace tube is machined to make the surface bright and remove the oxide scale, and then ultrasonically oscillated in an ethanol solution for 5 min to remove grease.

[0038] S3. Perform low-temperature pre-oxidation treatment on the furnace tubes after the treatment in step S2. The oxidation temperature is 650 °C, the time is 10 h, the oxidation atmosphere is a 3% CH4 + Ar mixed gas, the flow rate of the mixed gas is 30 l / h, the volume percentage of H2O in the low oxygen partial pressure atmosphere gas is 1.5%, and the oxygen partial pressure is 1.0×10 -26 atm.

[0039] S4. Perform high-temperature pre-oxidation treatment on the furnace tubes after the treatment in step S3. The oxidation temperature is 1050 °C, the time is 10 h, the oxidation atmosphere is a 3% CH4 + Ar mixed gas, the flow rate of the mixed gas is 30 l / h, the volume percentage of H2O in the low oxygen partial pressure atmosphere gas is 1.5%, and the oxygen partial pressure is 1.0×10 -23 atm.

[0040] Observe and analyze the morphology and composition of the surface and cross-section of the furnace tubes before and after pre-oxidation heat treatment by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The composition is shown in Table 1. The SEM images of the surface and cross-section of the furnace tubes after pre-oxidation heat treatment are as Figure 1 shown.

[0041] Comparative Example 1

[0042] Compared with Example 1, the high-temperature pre-oxidation treatment in step S4 is not carried out, and the remaining operations are the same as those in Example 1. Observe and analyze the surface morphology and composition of the furnace tubes after pre-oxidation heat treatment by scanning electron microscopy and energy dispersive spectrometer. The composition is shown in Table 1. The SEM image of the surface of the furnace tubes after pre-oxidation treatment is as Figure 2 shown.

[0043] Comparative Example 2

[0044] Compared with Example 1, the low-temperature pre-oxidation treatment in step S3 is not carried out, and the remaining operations are the same as those in Example 1. Observe and analyze the morphology and composition of the surface and cross-section of the furnace tubes after pre-oxidation heat treatment by scanning electron microscopy and energy dispersive spectrometer. The composition is shown in Table 1. The SEM images of the surface and cross-section of the furnace tubes after pre-oxidation heat treatment are as Figure 3 shown.

[0045] Table 1 Mass percentages (wt%) of surface elements of the furnace tubes before and after pre-oxidation heat treatment in Example 1 and Comparative Examples 1-2

[0046] O Al Si Cr Mn Fe Ni Nb Y Before treatment 1.47 4.22 1.42 19.71 0.94 44.94 26.03 1.08 0.19 Example 1 48.15 27.55 1.00 10.70 0.79 8.56 2.23 1.00 0.02 Comparative Example 1 6.33 8.22 1.59 14.32 0.33 43.76 24.34 1.01 0.1 Comparative Example 2 37.74 17.90 0.92 17.57 2.70 16.90 5.53 0.71 0.03

[0047] As can be seen from Table 1, before the heat treatment, the main elements on the surface of the furnace tubes are Fe, Ni, and Cr. On the surface of the furnace tubes in Example 1, the contents of Fe and Ni are significantly reduced, the contents of Al and O are significantly increased, and the contents of the remaining elements have no obvious change. Combining Figure 1(a) It can be seen that the surface of the furnace tube in Example 1 is covered by a grayish-white Al2O3 oxide film and a grayish-black (Al, Cr)2O3 oxide film. The coverage area of the (Al, Cr)2O3 oxide layer is about 70%, mainly covering the surface layer and being relatively thin, while the inner part is a dense Al2O3 oxide film. Combining with Figure 1 (b) It can be seen that the inner surface of the furnace tube is a continuous and dense Al2O3 oxide film with a thickness of 2 - 3 μm, and the outer surface is a relatively thin (Al, Cr)2O3 oxide layer with a composite oxide layer thickness of about 3 μm.

[0048] On the surface of the furnace tube in Comparative Example 1, the Al content slightly increases, and the contents of other elements do not change significantly, and no obvious oxide film is formed. Combining with Figure 2 it can be known that a rich Al phase appears on the surface of the furnace tube in Comparative Example 1, and at the same time, the B2-NiAl phase is enriched at the grain boundaries. The B2-NiAl phase is an Al storage phase. When the furnace tube undergoes high-temperature oxidation, the B2-NiAl phase can continuously provide the necessary Al element for the formation of the Al2O3 oxide film on the surface, thereby ensuring that the furnace tube has excellent and stable oxidation resistance.

[0049] From Table 1 and Figure 3 (a) It can be seen that the outer layer of the surface of the furnace tube in Comparative Example 2 is mainly covered by (Fe, Cr)2O3 spinel and (Al, Cr)2O3 corundum oxide layers. A small amount of black Al2O3 oxide film can be found inside. Compared with Example 1, the surface oxide film coverage is incomplete, and the (Fe, Cr)2O3 spinel has limited anti-coking performance compared to the Al2O3 oxide film. Combining with Figure 3 (b) It can be seen that the outer surface of the furnace tube is a relatively thick (Al, Cr)2O3 corundum oxide layer with a thickness of about 2 μm, and the inner surface is a relatively complete but thin Al2O3 oxide film.

[0050] Example 2

[0051] Compared with Example 1, by weight percentage, the chemical composition of the anti-coking furnace tube is changed to: 4% Al, 20% Cr, 25% Ni, 0.3% C, 1% Mn, 1.5% Si, 1% Nb, 0.1% Y, and the balance is Fe. The rest of the operations are the same as those in Example 1.

[0052] The cross-sectional morphology and composition of the furnace tube before and after pre-oxidation heat treatment were observed and analyzed by a scanning electron microscope and an energy spectrometer. The composition is shown in Table 2, and the SEM cross-sectional diagram of the furnace tube after pre-oxidation heat treatment is as Figure 4 shown.

[0053] Example 3

[0054] Compared with Example 1, by weight percentage, the chemical composition of the anti-coking furnace tube was changed to: 4% Al, 20% Cr, 25% Ni, 0.3% C, 1% Mn, 1.5% Si, 1% Nb, 0.3% Y, with the balance being Fe. The remaining operations were the same as those in Example 1.

[0055] The cross-sectional morphology and composition of the furnace tube before and after pre-oxidation heat treatment were observed and analyzed by scanning electron microscopy and energy dispersive spectrometer. The composition is shown in Table 2, and the SEM diagram of the cross-section of the furnace tube after pre-oxidation heat treatment is as Figure 5 shown.

[0056] Comparative Example 3

[0057] Compared with Example 1, by weight percentage, the chemical composition of the anti-coking furnace tube was changed to: 4% Al, 20% Cr, 25% Ni, 0.3% C, 1% Mn, 1.5% Si, 1% Nb, with the balance being Fe. The remaining operations were the same as those in Example 1.

[0058] The cross-sectional morphology and composition of the furnace tube before and after pre-oxidation heat treatment were observed and analyzed by scanning electron microscopy and energy dispersive spectrometer. The composition is shown in Table 2, and the SEM diagram of the cross-section of the furnace tube after pre-oxidation heat treatment is as Figure 6 shown.

[0059] Table 2: Element mass percentages (wt%) on the surface of the furnace tube before and after pre-oxidation heat treatment in Examples 2-3 and Comparative Example 3

[0060]

[0061] It can be seen from Table 4 and Figure 4 that when the addition amount of Y is 0.1%, Al2O3 has been formed on the inner surface of the furnace tube, but the oxide layer is thinner compared with that in Example 1. When the addition amount of Y is 0.3%, its cross-sectional morphology is as Figure 5 shown, which is basically the same as that in Example 1. The Al2O3 oxide layer on the inner surface is uniform and dense, with a thickness of 2-3 μm. The outer surface is a relatively thin (Al, Cr)2O3 corundum oxide layer, and the thickness of the composite oxide layer is about 3 μm.

[0062] Y element was not added to the alloy in Comparative Example 3. It can be seen from Figure 6 that the oxidation products on the surface of the furnace tube are (Al, Cr)2O3 and Al2O3, with a thickness of 1.5-2 μm. Compared with Example 1, the composite oxide layer in Comparative Example 3 is thinner and irregular. Part of the (Al, Cr)2O3 oxide layer has not been completely transformed into Al2O3 on the inner surface, because the Y element can reduce the oxidation activation energy, thereby accelerating the formation of the protective oxide layer and promoting the external oxidation of Al in AFA stainless steel, increasing the adhesion of the oxide.

[0063] It can be seen from this that when the content of element Y is 0.1-0.3%, the formation of the Al2O3 oxide layer can be well promoted.

[0064] Example 4

[0065] Compared with Example 1, the composition of the anti-coking furnace tube was changed. During the centrifugal casting of the 25Ni-20Cr furnace tube alloy, 2.5% Al by weight was added, and the low-temperature pre-oxidation heat treatment in step S3 was not carried out. Other operations were the same as those in Example 1. The alloy composition of the 25Ni-20Cr furnace tube was: 20% Cr, 25% Ni, 0.3% C, 1% Mn, and the balance was Fe.

[0066] The cross-sectional morphology and composition of the furnace tube before and after the pre-oxidation heat treatment were observed and analyzed by a scanning electron microscope and an energy spectrometer. The composition is shown in Table 3, and the SEM diagram of the cross-section of the furnace tube after the pre-oxidation heat treatment is as Figure 7 shown.

[0067] Example 5

[0068] Compared with Example 1, the composition of the anti-coking furnace tube was changed. During the centrifugal casting of the 25Ni-20Cr furnace tube alloy, 4% Al by weight was added, and the low-temperature pre-oxidation heat treatment in step S3 was not carried out. Other operations were the same as those in Example 1. The alloy composition of the 25Ni-20Cr furnace tube was: 20% Cr, 25% Ni, 0.3% C, 1% Mn, and the balance was Fe.

[0069] The cross-sectional morphology and composition of the furnace tube before and after the pre-oxidation heat treatment were observed and analyzed by a scanning electron microscope and an energy spectrometer. The composition is shown in Table 3, and the SEM diagram of the cross-section of the furnace tube after the pre-oxidation heat treatment is as Figure 8 shown.

[0070] Example 6

[0071] Compared with Example 1, the composition of the anti-coking furnace tube was changed. During the centrifugal casting of the 25Ni-20Cr furnace tube alloy, 5% Al by weight was added, and the low-temperature pre-oxidation heat treatment in step S3 was not carried out. Other operations were the same as those in Example 1. The alloy composition of the 25Ni-20Cr furnace tube was: 20% Cr, 25% Ni, 0.3% C, 1% Mn, and the balance was Fe.

[0072] The cross-sectional morphology and composition of the furnace tube before and after the pre-oxidation heat treatment were observed and analyzed by a scanning electron microscope and an energy spectrometer. The composition is shown in Table 3, and the SEM diagram of the cross-section of the furnace tube after the pre-oxidation heat treatment is as Figure 9 shown.

[0073] Comparative Example 4

[0074] Compared with Example 1, for the centrifugally cast 25Ni-20Cr furnace tube, the low-temperature pre-oxidation heat treatment in step S3 is not carried out, and other operations are the same as those in Example 1. The alloy components of the 25Ni-20Cr furnace tube are: 20% Cr, 25% Ni, 0.3% C, 1% Mn, and the balance is Fe.

[0075] The cross-sectional morphology and composition of the furnace tube before and after the pre-oxidation heat treatment were observed and analyzed by a scanning electron microscope and an energy spectrometer. The composition is shown in Table 3, and the SEM diagram of the cross-section of the furnace tube after the pre-oxidation heat treatment is as Figure 10 shown.

[0076] Table 3: Element mass percentages (wt%) on the surface of the furnace tubes in Examples 4-6 and Comparative Example 4 before and after the pre-oxidation heat treatment

[0077]

[0078] From Table 3 and Figure 7 it can be seen that in Example 4, when 2.5 wt% Al is added to the 25Ni-20Cr furnace tube alloy, during the high-temperature pre-oxidation heat treatment, in addition to forming a complete Cr2O3 oxide film on the outer surface, there is also a dispersed but closely distributed Al2O3 oxidation zone on the inner surface of the alloy, and a complete Al2O3 oxide layer has not been formed in this oxidation zone.

[0079] From Figure 8 and Figure 9 it can be seen that when the Al content in the 25Ni-20Cr furnace tube alloy reaches 4 wt% and 5 wt%, a complete and dense Al2O3 oxide layer appears in the inner surface area of the furnace tube. This is because in the initial stage of high-temperature and low oxygen partial pressure pre-oxidation, due to the relatively high content and fast diffusion rate of Cr, the Cr element diffuses to the matrix surface and combines with oxygen preferentially to form a stable Cr2O3 oxide film. When an oxide layer is formed on the alloy surface, as the depth of oxygen atom diffusion into the matrix increases, the oxygen partial pressure will drop rapidly and cannot form a stable oxide with Cr. However, due to the stronger affinity of Al with oxygen, when the Al content reaches a certain level, selective oxidation of the Al element will occur, thereby forming a continuous and dense oxide layer on the inner surface of the matrix. This oxide layer can still effectively protect the matrix from coke deposition when the outer layer Cr2O3 oxide film fails due to spalling or transformation into carbides.

[0080] From Table 3 and Figure 10 it can be seen that in Comparative Example 4, the commonly used 25Ni-20Cr furnace tube alloy protects the matrix by forming a layer of Cr2O3 oxide film in a high-temperature pre-oxidation environment. The thickness of the oxide film is about 1.5 μm. However, in a high-temperature environment, the stability of Cr2O3 is poor, and it is easy to form carbides and spall, and long-term effective anti-coking and anti-oxidation properties cannot be achieved.

[0081] Test Example

[0082] The 25Ni-20Cr furnace tubes and the furnace tubes after pre-oxidation heat treatment in Example 1, Example 5, Comparative Example 2 and Comparative Example 4 were placed in a cracking atmosphere with industrial naphtha as the cracking raw material (naphtha distillation range: 80-100) for coking experiment testing. The specific steps are as follows: First, the furnace tube specimens after treatment in each example and comparative example were suspended in the cracking device. After vacuum pumping and gas washing, the cracking furnace was heated up at a heating rate of 10 °C / min. When the furnace temperature rose to 700 °C, deionized water was introduced, and the flow rate of deionized water was 60 ml / h; when the furnace temperature rose to 850 °C, naphtha was introduced, and the flow rate of naphtha was 180 ml / h. The introduced deionized water and naphtha would both pass through a vaporization furnace at 180 °C and turn into water vapor and naphtha gas and then enter the cracking furnace for cracking reaction. The cracking time was 4 h.

[0083] The furnace tube specimens before and after the coking experiment were weighed and measured, and the measurement results of the weight gain per unit area of each furnace tube were plotted as a scatter line graph, as Figure 11 shown. It can be seen that compared with the 25Ni-20Cr original furnace tube, the anti-coking furnace tube of Comparative Example 4 after high-temperature pre-oxidation heat treatment can reduce the coking amount by 41.76%. This is because the dense Cr2O3 oxide film formed on its surface can inhibit the formation of coke on its surface to a certain extent, but the anti-coking effect of a single Cr2O3 oxide film is limited. Compared with the 25Ni-20Cr original furnace tube, 4% Al element was added to the anti-coking furnace tube of Example 5. After high-temperature pre-oxidation heat treatment, the coking amount was reduced by 56.67%, indicating that the Al2O3 oxide film formed on the furnace tube surface has a better anti-coking effect than the Cr2O3 oxide film. After the anti-coking furnace tube of Comparative Example 2 was subjected to high-temperature pre-oxidation heat treatment, the (Al, Cr)2O3 corundum structure oxide layer on its surface has a more effective and stable anti-coking and anti-carburization effect than the Cr2O3 oxide film. Compared with the 25Ni-20Cr original furnace tube, the coking amount was reduced by 82.45%. When the anti-coking furnace tube of Example 1 was subjected to low-temperature and high-temperature pre-oxidation heat treatment and then carried out the coking experiment under the same conditions, its coking amount was further reduced. Compared with the 25Ni-20Cr original furnace tube, the coking amount was reduced by 92.49%.

Claims

1. An anti-coking furnace tube with a gradient composite oxide layer, characterized in that, It includes a substrate and a composite oxide layer combined with the substrate. The composite oxide layer includes: a continuous and dense Al2O3 oxide layer close to the substrate and an (Al, Cr)2O3 oxide layer far from the substrate; By weight percentage, the chemical composition of the anti-coking furnace tube is C: 0.1~0.5%, Cr: 15~30%, Ni: 20~35%, Al: 2~5%, Si: 0.3~2%, Nb: 0.5~2%, Y: 0.1~0.3% and Mn: 1%, and the rest is iron; The manufacturing method of the anti-coking furnace tube with a gradient composite oxide layer is as follows: Prepare raw materials according to the chemical components required for the anti-coking furnace tube. After melting, the furnace tube is made by centrifugal casting, and then the furnace tube is subjected to low-temperature and high-temperature heat treatments in an atmosphere with a low oxygen partial pressure; the oxygen partial pressure is 3.2×10 -29 ~1.6×10 -14 atm; the low-temperature heat treatment temperature is 650~800 °C, and the time is 5~10 h; the high-temperature heat treatment temperature is 1000~1200 °C, and the time is 5~10 h. The low oxygen partial pressure atmosphere is a mixed atmosphere composed of a reducing gas, water vapor, and an inert gas; the reducing gas is one or a mixture of two of CH4 and H2; the volume percentage of water vapor in the low oxygen partial pressure atmosphere gas is 0.2%~3%.

2. The anti-coking furnace tube with a gradient composite oxide layer according to claim 1, wherein The thickness of the composite oxide layer is 2~6 µm; among which, the thickness of the Al2O3 oxide layer is 1~3 µm.

3. A manufacturing method of an anti-coking furnace tube with a gradient composite oxide layer as described in any one of claims 1 to 2, characterized in that, Prepare raw materials according to the chemical components required for anti-coking furnace tubes. After melting, the furnace tubes are made by centrifugal casting, and then the furnace tubes are subjected to low-temperature and high-temperature heat treatments in an atmosphere with a low oxygen partial pressure; the oxygen partial pressure is 3.2×10 -29 ~1.6×10 -14 atm; the low-temperature heat treatment temperature is 650~800°C, and the time is 5~10h; the high-temperature heat treatment temperature is 1000~1200°C, and the time is 5~10h; The low oxygen partial pressure atmosphere is a mixed atmosphere composed of a reducing gas, water vapor and an inert gas; the reducing gas is one or a mixture of two of CH4 and H2; the volume percentage of the water vapor in the low oxygen partial pressure atmosphere gas is 0.2%~3%.

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