Anti-coking furnace tube with good high-temperature stability oxide layer and preparation method thereof

Abstract

The present application belongs to the field of preparation of cracking furnace tube, and particularly relates to an anti-coking furnace tube with an oxidation layer with good high-temperature stability and a preparation method thereof. The present application generates a complete and dense, single and uniformly distributed Al2O3 oxidation layer on the surface of the furnace tube by optimizing the alloy composition and adopting a two-step low-temperature and high-temperature pre-oxidation process, i.e. after low-temperature heat treatment of the furnace tube in a low-oxygen partial pressure atmosphere, the furnace tube is heated to high temperature at a constant heating rate for heat treatment, and the oxidation layer can exhibit excellent high-temperature stability and adhesion during high-temperature cyclic oxidation at 1000 DEG C for 460 h.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of cracking furnace tube preparation, and particularly relates to a coking-resistant furnace tube with an oxidation layer of good high-temperature stability and a preparation method thereof. BACKGROUND

[0002] Currently, Fe-Ni-Cr centrifugal casting alloy (HK, HP) is commonly used for cracking furnace tubes, and has good oxidation resistance, corrosion resistance and high-temperature stability. Fe, Ni and other elements are necessary to ensure the temperature and mechanical properties of the alloy. However, in the process of steam thermal cracking of petroleum hydrocarbons, Fe, Ni and other transition elements on the surface of the furnace tube are prone to electron pairing with electron-rich hydrocarbons, catalyzing the deposition of coke on the inner wall of the furnace tube, thereby deteriorating the performance and service life of the material. Although Cr can form a continuous and dense Cr2O3 oxidation layer on the surface of the furnace tube to isolate the catalytic effect of Fe and Ni on coking, at high temperatures, Cr can react with C and form stable carbides, which will lead to the depletion of Cr on the surface of the alloy and inhibit the formation of the external Cr2O3 oxidation film; at the same time, Cr2O3 has poor stability in a high-temperature water vapor environment, and is prone to peeling under the action of thermal cycle impact; peeling and incomplete Cr2O3 coverage can make the Ni-containing and Fe-containing matrix come into contact with the gas flow, promoting catalytic coking.

[0003] Compared with the Cr2O3 oxidation film, the Al2O3 oxidation film has better thermal stability, excellent mechanical properties and adhesion, and better resistance to thermal cycle impact, and can more effectively inhibit the deposition of coke on the surface of the furnace tube. Therefore, an austenitic heat-resistant stainless steel (AFA) capable of spontaneously forming an Al2O3 protective film on the surface of the alloy is considered as an alternative material for traditional cracking furnace alloys. Under low oxygen pressure conditions, the alloy will undergo selective oxidation, and among various oxides, Al2O3 exhibits strong bonding force between Al and O and the lowest Gibbs free energy, making it more thermodynamically favorable to form. However, the formation of a complete Al2O3 oxidation film requires Al content to exceed a certain critical value, but Al and Cr are alpha-Fe forming elements. When the Al and Cr contents are high, the matrix will change from single gamma-Fe to gamma+alpha dual-phase structure, resulting in a decrease in high-temperature creep resistance of the alloy. To ensure single austenite, the austenite stability can be increased by increasing the Ni content. Currently, high-Ni austenitic alloys with an aluminum oxide protective film and nickel-based high-temperature alloys have high creep strength and excellent oxidation resistance, but the production cost is high and they cannot be widely used, and research on Fe-based aluminum oxide forming austenitic stainless steel is less.

[0004] The patent CN116445182A proves that the Al2O3 oxide layer can well inhibit the formation of coke, but the oxide layer of the anti-coking furnace tube prepared by the method peels off and changes obviously and cracks appear in the high-temperature cyclic oxidation process. Therefore, the application of the anti-coking furnace tube with a good high-temperature stability oxide layer and the preparation method thereof have important significance for effectively improving the stability and service life of the oxide layer on the surface of the furnace tube in the high-temperature complex working environment. SUMMARY

[0005] In order to solve the above technical problems, the application provides an anti-coking furnace tube with a good high-temperature stability oxide layer and a preparation method thereof. A single Al2O3 outer oxide layer is obtained by two-step low-oxygen partial pressure pre-oxidation treatment of the iron-based AFA stainless steel, the oxide layer is complete and dense, and the thickness is 1-3 microns. The alloy composition is optimized on the basis of the process to increase the high-temperature stability of the alloy oxide layer. The alloy high-temperature oxidation layer of the application can exhibit excellent high-temperature stability and adhesion during the high-temperature cyclic oxidation process of 1000 DEG C for 460 hours.

[0006] In order to achieve the above-mentioned purpose, the specific schemes of the application are as follows:

[0007] The application provides a preparation method of an anti-coking furnace tube with a good high-temperature stability oxide layer. Raw materials are prepared according to the chemical components required by the anti-coking furnace tube, and after melting, the furnace tube is prepared by centrifugal casting. Then, the furnace tube is subjected to low-temperature heat treatment in a low-oxygen partial pressure atmosphere, and then subjected to heat treatment at a constant heating rate to high temperature. After cooling, a single and uniformly distributed Al2O3 oxide layer is obtained on the surface of the anti-coking furnace tube.

[0008] Preferably, the low-temperature heat treatment temperature is 750-900 DEG C, and the time is 5-10 hours; the high-temperature heat treatment temperature is 1000-1200 DEG C, and the time is 10-20 hours; the heating rate is 5 DEG C / min; and the oxygen partial pressure is 1.8*10 -26 atm-1.2*10 -18 atm.

[0009] Preferably, the low-oxygen partial pressure atmosphere is a mixed atmosphere composed of reducing gas, water vapor and inert gas; the reducing gas is a mixture of 4% H2 and 0.2% CH4; the water vapor content is 0.1%-0.8% of the total volume; and the inert gas is Ar.

[0010] Preferably, the chemical composition of the anti-coking furnace tube is, in percentage by weight, Ni: 20-30%, Cr: 18-25%, Al: 4-5%, Si: 0.8-1.2%, Nb: 0.5-2%, C: 0.2-0.5%, Mn: 0.3-0.8%, Y: 0.1-0.2%, and the rest is iron.

[0011] The anti-coking furnace tube with the oxidation layer of good high-temperature stability prepared by the method has an Al2O3 oxidation layer with a thickness of 1-3 microns.

[0012] The present application has the following advantages:

[0013] 1. The present application can form a complete and dense Al2O3 oxidation layer with anti-coking and high-temperature oxidation resistance on the surface of the iron-based AFA stainless steel through the optimized low-oxygen partial pressure two-step pre-oxidation process of low temperature + high temperature. The water content and oxygen partial pressure in the alloy oxidation atmosphere can be regulated by setting the micro pump flow rate of the oxidation process parameters, so that the thickness and morphology of the oxidation layer can be flexibly adjusted, and therefore the present application has strong flexibility and practicality in the preparation process.

[0014] 2. The addition of appropriate amount of Si in the alloy can improve the diffusion coefficient and activity of Al, increase the generation driving force of Al2O3, and accelerate the formation of B2-NiAl phase, which is an Al storage phase and can continuously provide the required Al element for the formation of Al2O3 oxidation layer during the alloy oxidation process, thereby promoting the formation of the surface Al2O3 oxidation layer and effectively reducing the requirements of the oxidation process.

[0015] 3. The present application optimizes and changes the addition amount of Mn and Si to prevent excessive Mn element from forming coarse CrMn 1.5 O4 in the high-temperature water vapor environment, which can destroy the continuity of the Al2O3 oxidation film and reduce its thermal stability. Si is a strong ferrite-forming element, which can reduce the stability of austenite, and high content of Si can reduce the welding performance and creep performance of the alloy. Therefore, in the present application, the Si content in the furnace tube is reduced to improve the high-temperature austenite structure stability of the furnace tube while maintaining the promoting effect of Si on the oxidation layer.

[0016] 4. The anti-coking furnace tube with the oxidation layer of good high-temperature stability and the preparation method thereof provided by the present application can be applied to iron-based austenitic high-temperature alloy, and can effectively reduce the cost compared with nickel-based high-temperature alloy and high-nickel austenitic alloy.

[0017] 5、Y element can increase the grain boundary energy of AFA alloy, thus enhancing the atomic diffusion rate at the grain boundary of AFA alloy, reducing the oxidation activation energy, accelerating the formation of protective oxide layer, thus avoiding more oxidation. At the same time, Y can improve the problem of oxide skin peeling during the cyclic oxidation process of the alloy. The alloy prepared by adding an appropriate amount of Y element in the alloy and combining with the two-step pre-oxidation process can exhibit excellent high-temperature stability and adhesion during the high-temperature cyclic oxidation process of 1000℃, 460h, significantly improving the service life of the alloy oxide layer and the furnace tube. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 SEM images of the surface (left) and cross-section (right) of the alloy oxide layer before high-temperature cyclic oxidation of Example 1;

[0019] Figure 2 SEM images of the surface (left) and cross-section (right) of the alloy oxide layer after high-temperature cyclic oxidation of Example 1;

[0020] Figure 3 SEM images of the surface of the alloy oxide layer before (left) and after (right) high-temperature cyclic oxidation of Example 2;

[0021] Figure 4 SEM images of the surface of the alloy oxide layer before (left) and after (right) high-temperature cyclic oxidation of Comparative Example 1;

[0022] Figure 5 SEM images of the surface of the alloy oxide layer before (left) and after (right) high-temperature cyclic oxidation of Comparative Example 2;

[0023] Figure 6 SEM images of the surface of the alloy oxide layer before (left) and after (right) high-temperature cyclic oxidation of Comparative Example 3;

[0024] Figure 7 SEM images of the surface of the alloy oxide layer before (left) and after (right) high-temperature cyclic oxidation of Comparative Example 4;

[0025] Figure 8 SEM images of the surface of the alloy oxide layer before (left) and after (right) high-temperature cyclic oxidation of Comparative Example 5;

[0026] Figure 9 SEM images of the surface of the alloy oxide layer before (left) and after (right) high-temperature cyclic oxidation of Comparative Example 6;

[0027] Figure 10 SEM images of the surface of the alloy oxide layer before (left) and after (right) high-temperature cyclic oxidation of Comparative Example 7;

[0028] Figure 11For Examples 1-2, the plots of spallation weight (left) and total weight gain (right) of the oxide layer of the furnace tube sample of Comparative Example 1-7 as a function of time during high-temperature cyclic oxidation at 1000°C for 460h. DETAILED DESCRIPTION

[0029] The application is further described below by combining specific examples. The examples described below are only used to explain the application, and do not limit the protection scope of the application.

[0030] Example 1

[0031] The chemical composition of the furnace tube alloy is 4% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb and 0.15% Y by weight percentage, and Fe is the balance.

[0032] The method for preparing the anti-coking furnace tube with an oxidation layer having good high-temperature stability comprises the following specific steps:

[0033] S1, the stainless steel raw material is proportioned by weight percentage and placed in a non-consumable magnetic control arc furnace for smelting. Before smelting starts, the furnace chamber is repeatedly pumped to vacuum by a vacuum pump, and then pure argon gas with a purity of 99.99% is flushed to fully absorb the oxygen and other impurity gases in the furnace chamber, which is repeated 5 times. Then the alloy is smelted, the smelting current is 136A, and the alloy is smelted on both sides for 2 times, 6 minutes each time, so that the alloy is fully fused to prevent composition segregation. The vacuum degree of smelting is ≤5Pa. The smelted alloy is placed in a centrifugal casting machine, and a furnace tube is made by centrifugal casting. The furnace tube sample made is placed in a water oxidation furnace, vacuum is drawn, and pure argon gas with a purity of 99.99% is flushed repeatedly 3 times. Then the surface of the furnace tube is machined to make the surface bright and remove the oxide skin.

[0034] S2, the furnace tube treated in step S1 is subjected to low-temperature pre-oxidation treatment, the oxidation temperature is 850°C, the time is 6h, the oxidation atmosphere is 4% H4+0.2% CH4+Ar mixed gas, the flow rate of the mixed gas is 30l / h, the water content is 0.2%, and the oxygen partial pressure is 1.03×10 -24 atm.

[0035] S3, the furnace tube treated in step S2 is subjected to high-temperature pre-oxidation treatment, the furnace is continued to be heated at a heating rate of 5°C / min to 1100°C, and high-temperature oxidation is carried out at 1100°C for 12h, the oxidation atmosphere is 4H4+0.2% CH4+Ar mixed gas, the flow rate of the mixed gas is 30l / h, the water content is 0.2%, and the oxygen partial pressure is 1.21×10 -22 atm. After cooling, the furnace tube sample is taken out and weighed. At this time, the oxide layer on the surface of the furnace tube is a dense, single and uniformly distributed Al2O3 oxidation layer with a thickness of about 2μm.

[0036] S4, the furnace tube sample after step S3 is placed in a corundum crucible and weighed, and then placed in an annealing furnace for cyclic oxidation. The crucible and sample are taken out every certain period of time, and after cooling, weighed separately. After the weighing is completed, the sample is placed back in the annealing furnace for high-temperature oxidation, repeated several times, and the specific parameters are as follows:

[0037] Cyclic oxidation temperature: 1000°C

[0038] Cyclic oxidation time: 2h, 4h, 6h, 12h, 18h, 28h, 50h, 100h, 240h

[0039] The total cyclic oxidation time is 460h

[0040] The surface and cross-sectional morphology and composition of the oxidation layer before and after high-temperature cyclic oxidation are observed and analyzed using a scanning electron microscope (SEM) and an energy-dispersive spectrometer (EDS). The composition of the oxidation layer after high-temperature cyclic oxidation is shown in Table 1, and the surface and cross-sectional morphology of the oxidation layer before and after high-temperature cyclic oxidation is shown in Figure 1 、 Figure 2 .

[0041] According to Table 1 and Figure 1 、 Figure 2 , it can be seen that after the oxidation layer generated in Example 1 is subjected to high-temperature cyclic oxidation at 1000°C for 460h, the alloy surface is still covered with a complete and dense Al2O3 oxidation layer, and there is no peeling or change in oxidation products. The cross-section of the sample after high-temperature cyclic oxidation is observed, and it is found that the Al2O3 oxidation layer is continuous and uniform, with a thickness of about 1.8μm.

[0042] Example 2

[0043] Compared with Example 1, the amount of rare earth element Y added to the furnace tube alloy is increased in Example 2. The chemical composition of the furnace tube alloy, by weight percentage, is: 4% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb, and 0.2% Y, with Fe being the balance. The two-step low-oxygen partial pressure pre-oxidation treatment of steps S1-S4 is carried out as in Example 1. The surface morphology and composition of the oxidation layer after high-temperature cyclic oxidation are observed and analyzed using a scanning electron microscope and an energy-dispersive spectrometer, and the composition is shown in Table 1, and the surface morphology of the oxidation layer before and after cyclic oxidation is shown in Figure 3 .

[0044] As can be seen from Table 1 and Figure 3 , the surface morphology and element content of the oxidation layer generated in Example 2 after high-temperature cyclic oxidation at 1000°C for 460h are similar to those of Example 1, and there is no peeling or change in oxidation products.

[0045] Comparative Example 1

[0046] Compared to Example 1, rare earth element Y was not added, serving as a control sample to investigate the effect of Y on the high-temperature stability of the oxide layer. The chemical composition of the furnace tube alloy, by weight percentage, was: 4% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb, with Fe as the balance. The two-step low-oxygen partial pressure pre-oxidation treatment (steps S1-S4) was performed as in Example 1. The surface morphology and composition of the oxide layer after high-temperature cyclic oxidation were observed and analyzed using scanning electron microscopy and energy dispersive spectroscopy. The composition is shown in Table 1, and the surface morphology of the oxide layer before and after cyclic oxidation is shown in Table 1. Figure 4 As shown.

[0047] As shown in Table 1, after high-temperature cyclic oxidation at 1000℃ for 460 hours, the oxide layer generated in Comparative Example 1 showed a significant decrease in Al content and an increase in Fe and Cr content compared to Example 1. Combined with... Figure 4 As can be seen, the surface of the furnace tube in Comparative Example 1 is covered with a black Al2O3 oxide film and gray MCr2O3 spinel (M represents Fe and Mn). The bright white area in the figure is the area where the oxide layer has peeled off. It can be seen that without the addition of rare earth element Y, the oxide layer in Comparative Example 1 has limited resistance to high-temperature cyclic oxidation, and the protective oxide layer has peeled off and transformed.

[0048] Comparative Example 2

[0049] Compared to Example 1, the amount of rare earth element Y added to the furnace tube alloy was changed to investigate the effect of Y content on the high-temperature stability of the oxide layer as a control sample. The chemical composition of the furnace tube alloy, by weight percentage, was: 4% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb, and 0.1% Y, with Fe as the balance. The two-step low-oxygen partial pressure pre-oxidation treatment (steps S1-S4) was performed as in Example 1. The surface morphology and composition of the oxide layer after high-temperature cyclic oxidation were observed and analyzed using scanning electron microscopy and energy dispersive spectroscopy. The composition is shown in Table 1, and the surface morphology of the oxide layer before and after cyclic oxidation is shown in Table 1. Figure 5 As shown.

[0050] From Table 1 and Figure 5 It can be seen that after the oxide layer generated in Comparative Example 2 is subjected to high-temperature cyclic oxidation at 1000℃ for 460h, the surface of the furnace tube is covered by a complete Al2O3 oxide layer, and a small amount of granular and blocky (Al,Cr)2O3 corundum oxides are evenly distributed on the surface of the Al2O3 oxide layer.

[0051] Comparative Example 3

[0052] The water content in the low oxygen partial pressure atmosphere gas is 1.5% by volume percentage, and the rest of the process is the same as that of Example 1, which is used as a comparison sample to explore the influence of the water content in the oxidation atmosphere on the high-temperature stability of the oxidation layer.

[0053] The chemical composition of the furnace tube alloy is 4% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb, and 0.15% Y by weight percentage, and Fe is the balance. The specific steps are as follows:

[0054] S1, the stainless steel raw material is proportioned by weight percentage and put into a non-consumable magnetic control arc furnace for smelting. Before smelting starts, the furnace chamber is repeatedly pumped by a vacuum pump, and then pure argon gas with a purity of 99.99% is flushed to fully absorb the oxygen and other impurity gases in the furnace chamber, which is repeated 5 times. Then the alloy is smelted, the smelting current is 136A, and the alloy is smelted on both sides for 2 times, 6 minutes each time, so that the alloy is fully fused to prevent composition segregation. The smelting vacuum degree is ≤5Pa. The smelted alloy is put into a centrifugal casting machine, and a furnace tube is made by centrifugal casting. The furnace tube sample made is put into a water oxidation furnace, vacuumized, and flushed with pure argon gas with a purity of 99.99% for 3 times. Then the surface of the furnace tube is machined to make the surface bright and remove the oxide skin.

[0055] S2, the furnace tube treated in step S1 is subjected to low-temperature pre-oxidation treatment, the oxidation temperature is 850℃, the time is 6h, the oxidation atmosphere is 4%H4+0.2%CH4+Ar mixed gas, the mixed gas flow rate is 30l / h, the water content is 1.5%, and the oxygen partial pressure is 1.38×10 -22 atm.

[0056] S3, the furnace tube treated in step S2 is subjected to high-temperature pre-oxidation treatment, the furnace is continued to be heated at a heating rate of 5℃ / min to 1100℃, and high-temperature oxidation is carried out at 1100℃ for 12h, the oxidation atmosphere is 4H4+0.2%CH4+Ar mixed gas, the mixed gas flow rate is 30l / h, the water content is 1.5%, and the oxygen partial pressure is 1.01×10 -19 atm. After cooling, the furnace tube sample is taken out and weighed.

[0057] S4, the furnace tube sample treated in step S3 is subjected to high-temperature cyclic oxidation treatment.

[0058] The surface morphology and composition of the oxidation layer after high-temperature cyclic oxidation are observed and analyzed by scanning electron microscope and energy spectrometer. The surface morphology of the oxidation layer before and after cyclic oxidation is shown in Figure 6 .

[0059] From Table 1, it can be seen that the oxide layer generated in Comparative Example 3 has a significantly reduced Al content and increased Fe and Cr contents after high-temperature cyclic oxidation at 1000℃ for 460h, as compared with Example 1. In combination with the fact that the oxide layer is white and has a peeling area, it can be concluded that the (Al,Cr)2O3oxide layer obtained after pre-oxidation is transformed into a FeCr2O3spinel oxide layer during high-temperature cyclic oxidation. Therefore, when the H2O content in the oxidation atmosphere is high, the alloy cannot generate a single Al2O3oxide layer after pre-oxidation, and the transformation and peeling of the oxide layer will occur during high-temperature cyclic oxidation. Figure 6 It can be seen that, during high-temperature cyclic oxidation, the (Al,Cr)2O3oxide layer obtained after pre-oxidation is transformed into a FeCr2O3spinel oxide layer, and a white and bright peeling area of the oxide layer appears. Therefore, when the H2O content in the oxidation atmosphere is high, the alloy cannot generate a single Al2O3oxide layer after pre-oxidation, and the transformation and peeling of the oxide layer will occur during high-temperature cyclic oxidation.

[0060] Comparative Example 4

[0061] In comparison with Example 1, the oxidation process is changed to investigate the influence of the oxidation process on the high-temperature stability of the oxide layer.

[0062] The chemical composition of the furnace tube alloy is 4% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb and 0.15% Y by weight, and Fe is the balance. The specific steps are as follows:

[0063] S1, the stainless steel raw material is proportioned according to the weight percentage and put into a non-consumable magnetic control arc furnace for smelting. Before smelting starts, the furnace chamber is repeatedly pumped to vacuum, and then pure argon gas with a purity of 99.99% is flushed to fully absorb the oxygen and other impurity gases in the furnace chamber, which is repeated for 5 times. Then the alloy is smelted, the smelting current is 136A, and the alloy is smelted on both sides for 2 times, 6 minutes each time, so that the alloy is fully fused to prevent composition segregation. The vacuum degree of smelting is ≤5Pa. The smelted alloy is put into a centrifugal casting machine, and a furnace tube is made by centrifugal casting. The furnace tube sample made is put into a water oxidation furnace, vacuumized and flushed with pure argon gas with a purity of 99.99% for 3 times. Then the surface of the furnace tube is machined to make it bright and remove the oxide skin.

[0064] S2, the furnace tube treated in step S1 is subjected to low-temperature pre-oxidation treatment. The oxidation temperature is 650℃, the time is 10h, the oxidation atmosphere is 4%H4+0.2%CH4+Ar mixed gas, the flow rate of the mixed gas is 30l / h, the water content is 0.2%, and the oxygen partial pressure is 1.58x10 -26 atm.

[0065] S3, the furnace tube treated in step S2 is subjected to high-temperature pre-oxidation treatment. The furnace tube is directly transferred to a furnace at 1050℃ for high-temperature pre-oxidation treatment, the time is 10h, the oxidation atmosphere is 4H4+0.2%CH4+Ar mixed gas, the flow rate of the mixed gas is 30l / h, the water content is 0.2%, and the oxygen partial pressure is 1.87x10 -22atm. After cooling, the furnace tube sample was taken out and weighed.

[0066] S4, the furnace tube sample after step S3 was treated by high temperature cyclic oxidation treatment.

[0067] The surface morphology and composition of the oxide layer after high temperature cyclic oxidation were observed and analyzed by scanning electron microscope and energy spectrometer, and the surface morphology of the oxide layer before and after cyclic oxidation was shown in Figure 7

[0068] From Table 1 and Figure 7 It can be seen that the oxide layer generated in Comparative Example 4 was covered by Al2O3 oxide layer and gray (Al, M)2O3 oxide layer after high temperature cyclic oxidation at 1000℃ for 460h, and the peeling phenomenon of (Al, M)2O3 oxide layer occurred during the high temperature cyclic oxidation process, compared with Example 1.

[0069] Comparative Example 5

[0070] Compared with the examples, the composition of the furnace tube was changed to explore the effect of Al element on the high temperature stability of the oxide layer. The chemical composition of the furnace tube alloy was 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb and 0.15% Y, and Fe was the balance. The two-step low oxygen partial pressure pre-oxidation treatment of steps S1-S4 was carried out as the same as Example 1. The surface morphology and composition of the oxide layer after high temperature cyclic oxidation were observed and analyzed by scanning electron microscope and energy spectrometer, and the surface morphology of the oxide layer before and after cyclic oxidation was shown in Figure 8

[0071] From Table 1, it can be seen that the oxide layer generated in Comparative Example 5 was Fe2O3 after high temperature cyclic oxidation at 1000℃ for 460h. Combined with Figure 8 It can be seen that the surface of the furnace tube in Comparative Example 5 was covered by loose and porous Fe2O3 oxide layer, because when Al element was not added in the furnace tube, the protective oxide layer of the furnace tube sample after low oxygen partial pressure pre-oxidation was single Cr2O3, due to the high temperature instability of Cr2O3, therefore the oxide layer generated in Comparative Example 5 was all volatilized and peeled off during the high temperature cyclic oxidation process, exposing the matrix alloy, and in the subsequent high temperature oxidation process, the furnace tube continued to oxidize to generate loose and porous Fe2O3 oxide layer, which would seriously affect the service life and high temperature creep properties of the furnace tube.

[0072] Comparative Example 6

[0073] ​​Comparative Example 5 changes the addition amount of Al element in the furnace tube alloy as a comparative sample to explore the influence of Al element content on the high-temperature stability of the oxidation layer. The chemical composition of the furnace tube alloy is 2.5% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb, and 0.15% Y by weight percentage, and Fe is the balance. The two-step low-oxygen partial pressure pre-oxidation treatment of steps S1-S4 is carried out as in Example 1. The surface morphology and composition of the oxidation layer after high-temperature cyclic oxidation are observed and analyzed by a scanning electron microscope and an energy spectrometer, and the surface morphology of the oxidation layer before and after cyclic oxidation is shown in Figure 9 .

[0074] As can be seen from Table 1, the oxidation layer generated in Comparative Example 6 has a significantly reduced Al content and increased Fe and Cr content after 460h of high-temperature cyclic oxidation at 1000℃ compared with Example 1. Combined with Figure 9 It can be seen that the surface of the furnace tube in Comparative Example 6 is covered with a gray blocky Cr2O3 oxidation layer and a granular FeCr2O3 spinel, and the white bright area in the figure is the peeling area of the oxidation layer. It can be seen that when the Al content is low, the oxidation layer in Comparative Example 6 has poor high-temperature cyclic oxidation resistance, and peeling and transformation of the protective oxidation layer occur.

[0075] Comparative Example 7

[0076] The anti-coking furnace tube is prepared according to the existing patent "CN116445182A: Anti-coking furnace tube with gradient composite oxidation layer and manufacturing method thereof". Compared with the example, the chemical composition of the furnace tube alloy of the method is 4% Al, 20% Cr, 25% Ni, 0.3% C, 1% Mn, 1.5% Si, 1% Nb, and 0.2% Y, and Fe is the balance. After step S1, steps S2 and S3 are different from Example 1. The oxidation atmosphere is 3% CH4+Ar mixed gas, the low-temperature pre-oxidation treatment temperature is 650℃, the time is 10h, and then it is directly transferred to a furnace at 1050℃ for high-temperature pre-oxidation treatment, the time is 10h, and the volume percentage of H2O in the low-oxygen partial pressure gas is 1.5%. The sample after two-step low-oxygen partial pressure pre-oxidation treatment is subjected to high-temperature cyclic oxidation treatment of step S4. The surface morphology and composition of the oxidation layer after high-temperature cyclic oxidation are observed and analyzed by a scanning electron microscope and an energy spectrometer, and the surface morphology of the oxidation layer before and after cyclic oxidation is shown in Figure 10 .

[0077] The furnace tube sample generates a composite oxidation layer with an (Al, M)2O3 oxidation layer on the outer surface and an Al2O3 oxidation layer on the inner surface after low-oxygen partial pressure pre-oxidation. As can be seen from Table 1 and Figure 10It can be seen that the surface morphology and the element contents of the oxide layer generated in Comparative Example 7 after high temperature cyclic oxidation at 1000°C for 460h are similar to those of Comparative Example 1. The surface of the furnace tube in Comparative Example 7 is covered by black Al203oxide film and gray MCr203spinel (M is Fe and Mn). It can be seen that the (Al, M)2O3oxide layer on the surface of the alloy at this time has obviously peeled off and transformed during high temperature cyclic oxidation, and the Al203oxide layer has cracking zones in the figure. It can be seen that the high temperature cyclic oxidation resistance of the protective oxide layer in Comparative Example 7 is lower than that of the examples.

[0078] Table 1: Element mass percentage (wt%) of the oxide layer on the surface of the furnace tube in Examples 1-2 and Comparative Examples 1-7

[0079] O Al Si Cr Mn Fe Ni Nb Y Example 1 61.40 35.50 0.03 2.15 0.08 0.49 0.14 0.21 0.00 Example 2 63.04 35.41 0.04 0.96 0.05 0.31 0.14 0.00 0.04 Comparative Example 1 55.32 19.49 0.43 10.88 5.30 5.95 2.30 0.33 — Comparative Example 2 61.31 34.67 0.11 2.91 0.02 0.67 0.31 0.02 -0.02 Comparative Example 3 62.37 1.37 0.06 18.32 0.95 16.25 0.62 0.06 0.00 Comparative Example 4 61.63 18.00 0.41 15.56 0.11 2.56 1.48 0.24 0.01 Comparative Example 5 57.51 — 0.01 5.56 0.07 32.75 4.24 -0.14 0.00 Comparative Example 6 61.31 0.02 -0.02 27.37 0.27 7.84 3.34 -0.09 -0.03 Comparative Example 7 54.15 21.55 1.00 12.70 0.79 6.56 2.23 1.00 0.02

[0080] Figure 11 The curve of the peeling weight (left) and the total weight gain (right) of the furnace tube samples in Examples 1-2 and Comparative Examples 1-7 with time during high temperature cyclic oxidation at 1000°C for 460h. From the curve of the peeling weight (left), it can be seen that after high temperature cyclic oxidation, the peeling weight of Examples 1-2 and Comparative Examples 1-7 is 0.2mg / cm Figure 11 2 , 0.29mg / cm 2 , 12.84mg / cm 2 , 2.86mg / cm 2 , 18.8mg / cm 2 , 7.91mg / cm 2 , 23.78mg / cm 2 , 19.6mg / cm 2 , 9.4mg / cm 2 , respectively. Among them, the peeling weight of Examples 1 and 2 is the least and has no obvious difference, and the peeling weight of Comparative Examples 5 and 6 is the highest. From the curve of the total weight gain (right), it can be seen that after high temperature cyclic oxidation, the total weight gain of Examples 1-2 and Comparative Examples 1-7 is 0.63mg / cm Figure 11 2 , 1.11mg / cm 2 , 10.13mg / cm 2 , 5.4mg / cm 2 , 11.24mg / cm 2 , 6.97mg / cm 2 , 30.88mg / cm 2 , 18.03mg / cm 2 , 8.84mg / cm 2 ​​The total weight gain of Example 1 is the least, and the total weight gain of Comparative Example 5 and Comparative Example 6 is the highest. Thus, when the added content of the rare earth element Y reaches 0.15%, the oxidation layer obtained by the two-step low-oxygen partial pressure pre-oxidation process of the present application has the best anti-peeling and anti-oxidation performance in the high-temperature oxidation process. Therefore, the preparation method of the anti-coking furnace tube with the oxidation layer having good high-temperature stability provided by the present application can obtain the Al2O3 oxidation layer with excellent high-temperature stability and adhesion performance by optimizing the alloy composition and adopting the low-temperature and high-temperature two-step pre-oxidation process in the present application.

Claims

1. A method for preparing an anti-coke oven tube with a good high-temperature stable oxide layer, characterized in that, Raw materials are prepared according to the required chemical composition for anti-coking furnace tubes. After melting, the furnace tubes are centrifugally cast. The tubes are then subjected to low-temperature heat treatment in a low-oxygen partial pressure atmosphere, followed by high-temperature heat treatment at a constant heating rate. After cooling, a single and uniformly distributed Al2O3 oxide layer is obtained on the surface of the anti-coking furnace tube. The low-temperature heat treatment temperature is 750~900℃, and the time is 5~10h; the high-temperature heat treatment temperature is 1000~1200℃, and the time is 10~20h; the heating rate is 5℃ / min; and the oxygen partial pressure is 1.8×10⁻⁶. -26 atm~1.2×10 -18 atm; the low oxygen partial pressure atmosphere is a mixed atmosphere composed of reducing gas, water vapor and inert gas; the reducing gas is a mixture of two gases: 4% H2 + 0.2% CH4; the water vapor content accounts for 0.1%~0.8% of the total volume; the inert gas is Ar; by weight percentage, the chemical composition of the anti-coking furnace tube is Ni: 20~30%, Cr: 18~25%, Al: 4~5%, Si: 0.8~1.2%, Nb: 0.5~2%, C: 0.2~0.5%, Mn: 0.3~0.8%, Y: 0.15~0.2%, with the remainder being iron.

2. The anti-coke oven tube with a good high-temperature stable oxide layer prepared by the preparation method according to claim 1, characterized in that, The oxide layer of the anti-coking furnace tube is an Al2O3 oxide layer with a thickness of 1~3µm.

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