CN117926168A - Anti-coking furnace tube with oxidation layer with good high-temperature stability and preparation method thereof - Google Patents
Anti-coking furnace tube with oxidation layer with good high-temperature stability and preparation method thereof Download PDFInfo
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- 238000007254 oxidation reaction Methods 0.000 title claims abstract description 141
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- 238000002360 preparation method Methods 0.000 title claims abstract description 14
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- 239000002994 raw material Substances 0.000 claims description 8
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- 125000004122 cyclic group Chemical group 0.000 abstract description 54
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- 229910002064 alloy oxide Inorganic materials 0.000 description 11
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- 229910001566 austenite Inorganic materials 0.000 description 5
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Abstract
The invention belongs to the field of cracking furnace tube preparation, and particularly relates to an anti-coking furnace tube with an oxidation layer with good high-temperature stability and a preparation method thereof. According to the invention, through optimizing alloy components and adopting a low-temperature and high-temperature two-step preoxidation process, after the furnace tube is subjected to low-temperature heat treatment under a low-oxygen partial pressure atmosphere, the temperature is increased to high temperature at a constant temperature increasing rate for heat treatment, so that a complete, compact, single and uniformly distributed Al 2O3 oxide layer is generated on the surface of the furnace tube, and the oxide layer can show excellent high-temperature stability and adhesiveness in the high-temperature cyclic oxidation process at 1000 ℃ for 460 hours.
Description
Technical Field
The invention belongs to the field of cracking furnace tube preparation, and particularly relates to an anti-coking furnace tube with an oxidation layer with good high-temperature stability and a preparation method thereof.
Background
At present, 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. The Fe, ni and other elements are necessary for ensuring the temperature and mechanical properties of the alloy, but in the steam thermal cracking process of petroleum hydrocarbon, fe, ni and other transition elements on the surface of a furnace tube are extremely easy to carry out electronic pairing with electron-rich hydrocarbon, and the deposition of coke on the inner wall of the furnace tube is catalyzed, so that the performance and the service life of the material are deteriorated. While Cr element can form a continuous, dense Cr 2O3 oxide layer on the furnace tube surface to isolate Fe and Ni from the catalytic action of coking, at high temperatures Cr can react with C and form stable carbides, which will lead to Cr depletion at the alloy surface and inhibit the formation of an external Cr 2O3 oxide film; meanwhile, cr 2O3 has poor stability in a high-temperature vapor environment, and volatile CrO 3 or CrO 2(OH)2 can be generated and is easy to peel off under the action of thermal cycle impact; spalling and incomplete Cr 2O3 coverage can bring Ni-and Fe-containing substrates into contact with the gas stream, promoting catalytic coking.
Compared with a Cr 2O3 oxide film, the Al 2O3 oxide film has better thermal stability, excellent mechanical property and adhesion, better thermal cycle shock resistance and more effective inhibition of coke deposition on the surface of the furnace tube. Therefore, an austenitic heat-resistant stainless steel (AFA) capable of spontaneously forming a protective film of Al 2O3 on the alloy surface is considered as an alternative material to conventional cracker alloys. Under low oxygen conditions, the alloy undergoes selective oxidation, and among the various oxides, al 2O3 exhibits a strong binding force between Al and O and a minimum Gibbs free energy, making its formation thermodynamically more favorable. However, it is necessary that the Al content exceeds a certain critical value to form a complete Al 2O3 oxide film, but Al and Cr are α -Fe forming elements. When the Al and Cr contents are high, the matrix is converted from single gamma-Fe into gamma+alpha dual-phase structure, resulting in reduced high temperature creep resistance of the alloy. To ensure single austenite, the austenite stability can be improved by increasing the Ni content. The existing high-Ni austenitic alloy and nickel-based superalloy with aluminum oxide as a protective film have high creep strength and excellent oxidation resistance, but the production cost is high and cannot be widely used, and meanwhile, the research on Fe-based aluminum oxide forming austenitic stainless steel is less.
Chinese patent CN116445182A proves that the Al 2O3 oxide layer can play a good role in inhibiting the generation of coke, but the oxide layer of the anti-coking furnace tube prepared by the method has obvious spalling and conversion in the high-temperature cyclic oxidation process, and a cracking zone appears. Therefore, the anti-coking furnace tube with the oxidation layer with good high-temperature stability and the preparation method thereof have important significance for effectively improving the stability and the service life of the oxidation layer on the surface of the furnace tube in a high-temperature complex working environment.
Disclosure of Invention
In order to solve the technical problems, the invention provides an anti-coking furnace tube with an oxidation layer with good high-temperature stability and a preparation method thereof. The single Al 2O3 external oxide layer is obtained by carrying out two-step low-oxygen partial pressure pre-oxidation treatment on the iron-based AFA stainless steel, the oxide layer is complete and compact, the thickness is 1-3 mu m, and the high-temperature stability of the alloy oxide layer is increased by optimizing alloy components on the basis of the process. The alloy high-temperature resistant oxidation layer provided by the invention can show excellent high-temperature stability and adhesiveness in a high-temperature cyclic oxidation process at 1000 ℃ for 460 hours.
In order to achieve the above object, the present invention is specifically as follows:
The invention provides a preparation method of an anti-coking furnace tube with an oxidation layer with good high temperature stability, which comprises the steps of preparing raw materials according to chemical components required by the anti-coking furnace tube, smelting, centrifugally casting to prepare the furnace tube, carrying out low-temperature heat treatment on the furnace tube under a low oxygen partial pressure atmosphere, then raising the temperature to high temperature at a constant temperature raising rate for heat treatment, and cooling to obtain a single and uniformly distributed Al 2O3 oxidation layer on the surface of the anti-coking furnace tube.
Preferably, the low-temperature heat treatment temperature is 750-900 ℃ and the time is 5-10 h; the high-temperature heat treatment temperature is 1000-1200 ℃ and the time is 10-20 h; the heating rate is 5 ℃/min; the oxygen partial pressure is 1.8X10 -26atm～1.2×10-18 atm.
Preferably, the atmosphere with low oxygen partial pressure is a mixed atmosphere composed of reducing gas, water vapor and inert gas; the reducing gas is two mixed gases of 4%H 2+0.2％CH4; the water vapor content accounts for 0.1-0.8% of the total volume; the inert gas is Ar.
Preferably, the chemical composition of the anti-coking furnace tube is Ni: 20-30%, cr: 18-25%, al: 4-5% of Si:0.8 to 1.2 percent of Nb: 0.5-2%, C:0.2 to 0.5 percent of Mn:0.3 to 0.8 percent, Y:0.1 to 0.2 percent and the balance of iron.
The anti-coking furnace tube with the oxidation layer with good high temperature stability prepared by the method has the oxidation layer of Al 2O3 and the thickness of 1-3 mu m.
The beneficial effects of the invention are as follows:
1. The invention can form a complete and compact oxidation layer of Al 2O3 resisting coking and high-temperature oxidation on the surface of the iron-based AFA stainless steel by optimizing a low-temperature and high-temperature low-oxygen partial pressure two-step preoxidation process. Meanwhile, the water content and the oxygen partial pressure in the alloy oxidizing atmosphere can be regulated and controlled by setting the flow rate of a micro pump according to the oxidation process parameters, so that the thickness and the morphology of the oxide layer can be flexibly regulated, and the preparation process has strong flexibility and practicability.
2. By adding a proper amount of Si into the alloy, the diffusion coefficient and activity of Al can be improved, and the generation driving force of Al 2O3 is increased; meanwhile, the formation of a B2-NiAl phase is quickened, the B2-NiAl phase is an Al storage phase, required Al elements can be continuously provided for the formation of an Al 2O3 oxide layer in the alloy oxidation process, the formation of the surface Al 2O3 oxide layer is further promoted, and the requirements of an oxidation process can be effectively reduced.
3. According to the invention, the addition amount of Mn and Si is optimized and changed, so that excessive Mn element is prevented from forming coarse CrMn 1.5O4 to damage the continuity of the Al 2O3 oxide film in a high-temperature steam environment, and the thermal stability of the Al 2O3 oxide film is reduced. Si is a ferrite element which is strongly formed, so that the stability of austenite is reduced, and meanwhile, the welding performance and creep property of the alloy are reduced when the content is high, so that the Si content in the furnace tube is reduced to improve the high-temperature austenite structure stability of the furnace tube while the promotion effect of Si on an oxide layer is maintained.
4. The anti-coking furnace tube with the oxidation layer with good high-temperature stability and the preparation method thereof provided by the invention can be applied to iron-based austenitic superalloy, and can effectively reduce cost compared with nickel-based superalloy and high-nickel austenitic alloy.
5. The Y element can improve the grain boundary energy of the AFA alloy, so that the atomic diffusion rate at the grain boundary of the AFA alloy is enhanced, the oxidation activation energy is reduced, and a protective oxide layer is formed in an accelerating way, so that more oxidation is avoided. Meanwhile, the problem of oxide scale falling in the cyclic oxidation process of the alloy can be solved by Y. According to the invention, a proper amount of Y element is added into the alloy, and the alloy high-temperature oxidation resistant layer generated by combining the two-step preoxidation process can show excellent high-temperature stability and adhesiveness in the high-temperature cyclic oxidation process at 1000 ℃ for 460 hours, so that the service lives of the alloy oxidation layer and a furnace tube are obviously prolonged.
Description of the drawings:
FIG. 1 is a SEM image of the surface (left) and cross-section (right) of an alloy oxide layer prior to high temperature cyclic oxidation of example 1;
FIG. 2 is a SEM image of the surface (left) and cross-section (right) of an alloy oxide layer after high temperature cyclic oxidation of example 1;
FIG. 3 is a surface SEM image of the alloy oxide layer before (left) and after (right) high temperature cyclic oxidation of example 2;
FIG. 4 is a surface SEM image of the alloy oxide layer before (left) and after (right) high temperature cyclic oxidation of comparative example 1;
FIG. 5 is a surface SEM image of the alloy oxide layer before (left) and after (right) high temperature cyclic oxidation of comparative example 2;
FIG. 6 is a surface SEM image of the alloy oxide layer before (left) and after (right) high temperature cyclic oxidation of comparative example 3;
FIG. 7 is a surface SEM image of the alloy oxide layer before (left) and after (right) high temperature cyclic oxidation of comparative example 4;
FIG. 8 is a surface SEM image of the alloy oxide layer before (left) and after (right) high temperature cyclic oxidation of comparative example 5;
FIG. 9 is a surface SEM image of the alloy oxide layer before (left) and after (right) high temperature cyclic oxidation of comparative example 6;
FIG. 10 is a surface SEM image of the alloy oxide layer before (left) and after (right) high temperature cyclic oxidation of comparative example 7;
FIG. 11 is a graph of the spalling weight of the oxide layer (left) and the total weight gain of the coupon (right) over time during a 1000 ℃ 460h high temperature cyclical oxidation of the coupon samples of examples 1-2, comparative examples 1-7.
Detailed Description
The invention is further described below by means of a combination of specific examples. The examples described below are only for the purpose of illustrating the invention and do not limit the scope of protection of the invention.
Example 1
The furnace tube alloy comprises the following chemical components in percentage by weight: 4% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb and 0.15% Y, the balance being Fe.
The preparation method of the anti-coking furnace tube with the oxidation layer with good high temperature stability comprises the following specific steps:
S1, proportioning stainless steel raw materials according to weight percentage, putting the stainless steel raw materials into a non-consumable magnetic control electric arc furnace for smelting, repeatedly vacuumizing a hearth by using a vacuum pump before smelting, then flushing argon with the purity of 99.99%, fully absorbing oxygen and other impurity gases in the hearth, and repeating for 5 times; and then smelting the alloy, wherein the smelting current is 136A, the front side and the back side of the alloy are respectively smelted for 2 times, and each time is 6 minutes, so that the alloy is fully fused to prevent component segregation, and the smelting vacuum degree is less than or equal to 5Pa. Putting the smelted alloy into a centrifugal casting machine, manufacturing a furnace tube by adopting centrifugal casting, putting a manufactured furnace tube sample into a water oxidation furnace, vacuumizing, flushing with argon with the purity of 99.99%, repeating for 3 times, and then machining the surface of the furnace tube to make the surface bright and remove oxide scales.
S2, performing low-temperature pre-oxidation treatment on the furnace tube treated in the step S1, wherein the oxidation temperature is 850 ℃, the time is 6 hours, the oxidation atmosphere is 4%H 4+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 1.03X10 -24 atm.
S3, carrying out high-temperature pre-oxidation treatment on the furnace tube treated in the step S2, continuously heating the furnace to 1100 ℃ at a heating rate of 5 ℃/min, and carrying out high-temperature oxidation at 1100 ℃ for 12 hours, wherein the oxidation atmosphere is a mixed gas of 4H 4+0.2％CH4 and Ar, the flow rate of the mixed gas is 30l/H, the water content is 0.2%, and the oxygen partial pressure is 1.21 multiplied by 10 -22 atm. After cooling, the furnace tube sample was taken out and weighed. At this time, the oxide layer on the surface of the furnace tube is a dense, single and uniformly distributed Al 2O3 oxide layer, and the thickness is about 2 μm.
S4, placing the furnace tube sample processed in the step S3 into a corundum crucible, weighing, then placing the corundum crucible into an annealing furnace for cyclic oxidation, taking out the crucible and the sample at intervals, respectively weighing after cooling, and placing the sample into the annealing furnace again for high-temperature oxidation after the weighing is finished, wherein the specific parameters are as follows:
Cyclic oxidation temperature: 1000 DEG C
Cyclic oxidation time: 2h, 4h, 6h, 12h, 18h, 28h, 50h, 100h, 240h
Total duration of cyclic oxidation: 460h
The surface, cross-sectional morphology and composition of the oxide layer before and after the high-temperature cyclic oxidation were observed and analyzed by using a Scanning Electron Microscope (SEM) and an energy spectrometer (EDS), the composition of the oxide layer after the high-temperature cyclic oxidation is shown in table 1, and the surface, cross-sectional morphology of the oxide layer before and after the high-temperature cyclic oxidation are shown in fig. 1 and 2.
As can be seen from table 1, fig. 1 and fig. 2, the oxide layer generated in example 1 is still a complete and dense Al 2O3 oxide layer on the surface of the alloy after being subjected to high-temperature cyclic oxidation at 1000 ℃ for 460 hours, and no flaking and conversion of oxidation products occur; the section of the sample after high-temperature cyclic oxidation was observed, and it was found that the Al 2O3 oxide layer was continuous and uniform, and the thickness was about 1.8. Mu.m.
Example 2
In example 2, the addition amount of the rare earth element Y in the furnace tube alloy was increased as compared with example 1. The furnace tube alloy comprises the following chemical components in percentage by weight: 4% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb and 0.2% Y, the balance being Fe. The two-step low oxygen partial pressure pre-oxidation treatments of steps S1-S4 were performed in the same manner as in example 1. The surface morphology and composition of the oxidized layer after high temperature cyclic oxidation were observed and analyzed by a scanning electron microscope and an energy spectrometer, the composition of which is shown in table 1, and the surface morphology of the oxidized layer before and after cyclic oxidation is shown in fig. 3.
As can be seen from Table 1 and FIG. 3, the oxide layer formed in example 2 was subjected to high-temperature cycle oxidation at 1000℃for 460 hours, and the surface morphology and the content of each element of the oxide layer were similar to those of example 1, and no flaking and conversion of oxidation products occurred.
Comparative example 1
In comparison with example 1, the effect of the element Y on the high-temperature stability of the oxide layer was investigated as a comparative sample without adding the rare earth element Y. The furnace tube alloy comprises the following chemical components in percentage by weight: 4% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb, fe as the balance. The two-step low oxygen partial pressure pre-oxidation treatments of steps S1-S4 were performed in the same manner as in example 1. The surface morphology and composition of the oxidized layer after high temperature cyclic oxidation were observed and analyzed by a scanning electron microscope and an energy spectrometer, the composition of which is shown in table 1, and the surface morphology of the oxidized layer before and after cyclic oxidation is shown in fig. 4.
As is clear from table 1, the oxide layer produced in comparative example 1 was subjected to high-temperature cycle oxidation at 1000 ℃ for 460 hours, and then the Al content was significantly reduced and the Fe and Cr contents were increased as compared with example 1. As can be seen from fig. 4, the furnace tube surface in comparative example 1 is covered with black Al 2O3 oxide film and gray MCr 2O3 spinel (M is Fe and Mn), and the white and bright area in the figure is the peeling zone of the oxide layer, so that the oxide layer in comparative example 1 has limited resistance to high-temperature cyclic oxidation without adding rare earth element Y, and peeling and transformation of the protective oxide layer occur.
Comparative example 2
The effect of the content of the rare earth element Y on the high temperature stability of the oxide layer was investigated as a comparative sample by changing the addition amount of the rare earth element Y in the furnace tube alloy as compared with example 1. The furnace tube alloy comprises the following chemical components in percentage by weight: 4% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb and 0.1% Y, the balance being Fe. The two-step low oxygen partial pressure pre-oxidation treatments of steps S1-S4 were performed in the same manner as in example 1. The surface morphology and composition of the oxidized layer after high temperature cyclic oxidation were observed and analyzed by a scanning electron microscope and an energy spectrometer, the composition of which is shown in table 1, and the surface morphology of the oxidized layer before and after cyclic oxidation is shown in fig. 5.
As is clear from Table 1 and FIG. 5, the oxide layer formed in comparative example 2 was subjected to high temperature cycle oxidation at 1000℃for 460 hours, the furnace tube surface was covered with the complete Al 2O3 oxide layer, and a small amount of granular and massive (Al, cr) 2O3 corundum oxide was uniformly distributed on the Al 2O3 oxide layer surface.
Comparative example 3
In contrast to example 1, the water content was 1.5% by volume of the low oxygen partial pressure atmosphere gas, and the rest of the process was the same, so that the effect of the water content in the oxidizing atmosphere on the high temperature stability of the oxide layer was investigated as a comparative sample.
The furnace tube alloy comprises the following chemical components in percentage by weight: 4% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb and 0.15% Y, the balance being Fe. The method comprises the following specific steps:
S1, proportioning stainless steel raw materials according to weight percentage, putting the stainless steel raw materials into a non-consumable magnetic control electric arc furnace for smelting, repeatedly vacuumizing a hearth by using a vacuum pump before smelting, then flushing argon with the purity of 99.99%, fully absorbing oxygen and other impurity gases in the hearth, and repeating for 5 times; and then smelting the alloy, wherein the smelting current is 136A, the front side and the back side of the alloy are respectively smelted for 2 times, and each time is 6 minutes, so that the alloy is fully fused to prevent component segregation, and the smelting vacuum degree is less than or equal to 5Pa. Putting the smelted alloy into a centrifugal casting machine, manufacturing a furnace tube by adopting centrifugal casting, putting a manufactured furnace tube sample into a water oxidation furnace, vacuumizing, flushing with argon with the purity of 99.99%, repeating for 3 times, and then machining the surface of the furnace tube to make the surface bright and remove oxide scales.
S2, performing low-temperature pre-oxidation treatment on the furnace tube treated in the step S1, wherein the oxidation temperature is 850 ℃, the time is 6 hours, the oxidation atmosphere is 4%H 4+0.2％CH4 +Ar mixed gas, the flow rate of the mixed gas is 30l/h, the water content is 1.5%, and the oxygen partial pressure is 1.38x -22 atm.
S3, carrying out high-temperature pre-oxidation treatment on the furnace tube treated in the step S2, continuously heating the furnace to 1100 ℃ at a heating rate of 5 ℃/min, and carrying out high-temperature oxidation at 1100 ℃ for 12 hours, wherein the oxidation atmosphere is a mixed gas of 4H 4+0.2％CH4 and Ar, the flow rate of the mixed gas is 30l/H, the water content is 1.5%, and the oxygen partial pressure is 1.01x10 -19 atm. After cooling, the furnace tube sample was taken out and weighed.
S4, performing high-temperature cyclic oxidation treatment on the furnace tube sample processed in the step S3.
The surface morphology and composition of the oxidized 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 oxidized layer before and after cyclic oxidation is shown in fig. 6.
As is clear from Table 1, the oxide layer produced in comparative example 3 was subjected to high-temperature cycle oxidation at 1000℃for 460 hours, and the Al content was significantly reduced and the Fe and Cr contents were increased as compared with example 1. As can be seen in connection with fig. 6, the (Al, cr) 2O3 oxide layer obtained after the pre-oxidation was converted into FeCr 2O3 spinel oxide layer during the high temperature cyclic oxidation, and white and bright oxide layer spalling regions appeared. From this, it is clear that when the content of H 2 O in the oxidizing atmosphere is high, the alloy cannot form a single Al 2O3 oxide layer after pre-oxidation, and transformation and exfoliation of the oxide layer occur during high-temperature cyclic oxidation.
Comparative example 4
The effect of the oxidation process on the high temperature stability of the oxide layer was investigated as a comparison with example 1, by changing the oxidation process.
The furnace tube alloy comprises the following chemical components in percentage by weight: 4% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb and 0.15% Y, the balance being Fe. The method comprises the following specific steps:
S1, proportioning stainless steel raw materials according to weight percentage, putting the stainless steel raw materials into a non-consumable magnetic control electric arc furnace for smelting, repeatedly vacuumizing a hearth by using a vacuum pump before smelting, then flushing argon with the purity of 99.99%, fully absorbing oxygen and other impurity gases in the hearth, and repeating for 5 times; and then smelting the alloy, wherein the smelting current is 136A, the front side and the back side of the alloy are respectively smelted for 2 times, and each time is 6 minutes, so that the alloy is fully fused to prevent component segregation, and the smelting vacuum degree is less than or equal to 5Pa. Putting the smelted alloy into a centrifugal casting machine, manufacturing a furnace tube by adopting centrifugal casting, putting a manufactured furnace tube sample into a water oxidation furnace, vacuumizing, flushing with argon with the purity of 99.99%, repeating for 3 times, and then machining the surface of the furnace tube to make the surface bright and remove oxide scales.
S2, performing low-temperature pre-oxidation treatment on the furnace tube treated in the step S1, wherein the oxidation temperature is 650 ℃, the time is 10 hours, the oxidation atmosphere is 4%H 4+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 1.58 multiplied by 10 -26 atm.
S3, performing high-temperature pre-oxidation treatment on the furnace tube processed in the step S2, directly transferring the furnace tube into a furnace with the temperature of 1050 ℃, performing high-temperature pre-oxidation treatment for 10 hours, wherein the oxidation atmosphere is a mixed gas of 4H 4+0.2％CH4 and Ar, the flow rate of the mixed gas is 30l/H, the water content is 0.2%, and the oxygen partial pressure is 1.87 multiplied by 10 -22 atm. After cooling, the furnace tube sample was taken out and weighed.
S4, performing high-temperature cyclic oxidation treatment on the furnace tube sample processed in the step S3.
The surface morphology and composition of the oxidized 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 oxidized layer before and after cyclic oxidation is shown in figure 7.
As is clear from table 1 and fig. 7, the oxide layer formed in comparative example 4 was subjected to high-temperature cyclic oxidation at 1000 ℃ for 460 hours, and compared with example 1, the furnace tube surface was covered with Al 2O3 oxide layer and gray (Al, M) 2O3 oxide layer, and the peeling phenomenon of (Al, M) 2O3 oxide layer occurred during the high-temperature cyclic oxidation.
Comparative example 5
Compared with the examples, the composition of the furnace tube was changed, and the effect of Al element on the high temperature stability of the oxide layer was investigated as a comparative sample. The furnace tube alloy comprises the following chemical components in percentage by weight: 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb and 0.15% Y, the balance being Fe. The two-step low oxygen partial pressure pre-oxidation treatments of steps S1-S4 were performed in the same manner as in example 1. The surface morphology and composition of the oxidized 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 oxidized layer before and after cyclic oxidation is shown in fig. 8.
As is clear from Table 1, the oxide layer formed in comparative example 5 was oxidized at 1000℃for 460 hours, and the oxidized product on the alloy surface was Fe 2O3. As can be seen from fig. 8, the surface of the furnace tube in comparative example 5 is covered by the loose porous Fe 2O3 oxide layer, because when no Al element is added into the furnace tube, the protective oxide layer of the furnace tube sample after the low oxygen partial pressure pre-oxidation is single Cr 2O3, and because of the high temperature instability of Cr 2O3, the oxide layer generated in comparative example 5 is completely volatilized and peeled off during the high temperature cyclic oxidation process, exposing the base alloy, and in the subsequent high temperature oxidation process, the furnace tube continuously undergoes oxidation reaction to generate the loose porous Fe 2O3 oxide layer, which can seriously affect the service life and high temperature creep performance of the furnace tube.
Comparative example 6
Compared with the examples, comparative example 5 was examined as a comparative sample for the effect of the content of Al element on the high temperature stability of the oxide layer by changing the addition amount of Al element in the furnace tube alloy. The furnace tube alloy comprises the following chemical components in percentage by weight: 2.5% Al, 20% Cr, 25% Ni, 0.3% C, 0.5% Mn, 1% Si, 1% Nb and 0.15% Y, the balance being Fe. The two-step low oxygen partial pressure pre-oxidation treatments of steps S1-S4 were performed in the same manner as in example 1. The surface morphology and composition of the oxidized 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 oxidized layer before and after cyclic oxidation is shown in fig. 9.
As is clear from Table 1, the oxide layer produced in comparative example 6 was subjected to high-temperature cycle oxidation at 1000℃for 460 hours, and the Al content was significantly reduced and the Fe and Cr contents were increased as compared with example 1. As can be seen from fig. 9, the furnace tube surface in comparative example 6 is covered with a gray lump of Cr 2O3 oxide layer and granular FeCr 2O3 spinel, and the white and bright area in the figure is the peeling zone of the oxide layer, so that it is known that when the Al content is low, the oxide layer in comparative example 6 has poor resistance to high-temperature cyclic oxidation, and peeling and transformation of the protective oxide layer occur.
Comparative example 7
According to the prior patent "CN116445182A: compared with the embodiment, the anti-coking furnace tube with the gradient composite oxide layer and the manufacturing method thereof are characterized in that the chemical components of the furnace tube alloy are as follows: 4% Al, 20% Cr, 25% Ni, 0.3% C, 1% Mn, 1.5% Si, 1% Nb and 0.2% Y, the balance being Fe. After step S1 was performed, unlike in example 1, in steps S2 and S3, the oxidizing atmosphere was 3% of a mixed gas of CH 4 +Ar, the low-temperature pre-oxidation treatment temperature was 650℃for 10 hours, and then, the mixture was directly transferred to a 1050℃furnace for high-temperature pre-oxidation treatment for 10 hours, and H 2 O was 1.5% by volume of the low-oxygen partial pressure atmosphere gas. And (3) performing high-temperature cyclic oxidation treatment in the step S4 on the sample subjected to the low-oxygen partial pressure pre-oxidation treatment by the two-step method. The surface morphology and composition of the oxidized 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 oxidized layer before and after cyclic oxidation is shown in fig. 10.
After the furnace tube sample is subjected to low oxygen partial pressure pre-oxidation, a composite oxide layer with the outer surface being (Al, M) 2O3 oxide layer and the inner surface being Al 2O3 oxide layer is generated. As can be seen from Table 1 and FIG. 10, the oxide layer formed in comparative example 7 was subjected to high temperature cycle oxidation at 1000℃for 460 hours, and the surface morphology and the element content of the oxide layer were similar to those of comparative example 1, and the furnace tube surface in comparative example 7 was covered with a black Al 2O3 oxide film and gray MCr 2O3 spinel (M is Fe and Mn). It is clear that the (Al, M) 2O3 oxide layer on the surface of the alloy is obviously peeled off and transformed in the high-temperature cyclic oxidation process, and meanwhile, the Al 2O3 oxide layer in the figure shows a cracking zone, so that the high-temperature cyclic oxidation resistance of the alloy protection oxide layer in comparative example 7 is lower than that in the embodiment.
Table 1: examples 1-2 and comparative examples 1-7 were conducted on the furnace tube surface oxide layer in terms of elemental mass percent (wt%)
| 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 |
FIG. 11 is a graph of the spalling weight of the oxide layer (left) and the total weight gain of the coupon (right) over time during a 1000 ℃ 460h high temperature cyclical oxidation of the coupon samples of examples 1-2, comparative examples 1-7. As can be seen from the graph of the peel weights (left) of FIG. 11, the peel weights of examples 1-2, comparative examples 1-7 were 0.2mg/cm2、0.29mg/cm2、12.84mg/cm2、2.86mg/cm2、18.8mg/cm2、7.91mg/cm2、23.78mg/cm2、19.6mg/cm2、9.4mg/cm2., respectively, after high temperature cyclic oxidation, with the peel weights of example 1 and example 2 being the least and no significant difference, and the peel weights of comparative examples 5 and comparative example 6 being the highest. As can be seen from the graph of fig. 11 for total weight gain (right), after high temperature cyclic oxidation, examples 1-2, comparative examples 1-7, respectively, were 0.63mg/cm2、1.11mg/cm2、10.13mg/cm2、5.4mg/cm2、11.24mg/cm2、6.97mg/cm2、30.88mg/cm2、18.03mg/cm2、8.84mg/cm2. where the total weight gain for example 1 was the least and the total weight gain for comparative examples 5 and 6 was the highest. Therefore, when the addition content of the rare earth element Y reaches 0.15%, the oxide layer obtained by the two-step low-oxygen partial pressure pre-oxidation process has optimal stripping resistance and oxidation resistance in the high-temperature oxidation process. Therefore, the preparation method of the anti-coking furnace tube with the oxidation layer with good high-temperature stability can obtain the Al 2O3 oxidation layer with excellent high-temperature stability and adhesion performance by optimizing alloy components and adopting the low-temperature and high-temperature two-step pre-oxidation process.
Claims (3)
1. The preparation method of the anti-coking furnace tube with the oxidation layer with good high temperature stability is characterized in that raw materials are prepared according to chemical components required by the anti-coking furnace tube, smelting is carried out, centrifugal casting is carried out to prepare the furnace tube, the furnace tube is subjected to low-temperature heat treatment under the atmosphere of low oxygen partial pressure, then is heated to high temperature at a constant heating rate for heat treatment, and a single and uniformly distributed Al 2O3 oxidation layer is obtained on the surface of the anti-coking furnace tube after cooling; the low-temperature heat treatment temperature is 750-900 ℃ and the time is 5-10 h; the high-temperature heat treatment temperature is 1000-1200 ℃ and the time is 10-20 h; the heating rate is 5 ℃/min; the oxygen partial pressure is 1.8X10 -26atm～1.2×10-18 atm; the atmosphere with low oxygen partial pressure is a mixed atmosphere composed of reducing gas, water vapor and inert gas; the reducing gas is two mixed gases of 4%H 2+0.2％CH4; the water vapor content accounts for 0.1-0.8% of the total volume; the inert gas is Ar.
2. The method for preparing an anti-coking furnace tube with an oxidation layer with good high temperature stability according to claim 1, wherein the chemical composition of the anti-coking furnace tube is as follows in percentage by weight: 20-30%, cr: 18-25%, al: 4-5% of Si:0.8 to 1.2 percent of Nb: 0.5-2%, C:0.2 to 0.5 percent of Mn:0.3 to 0.8 percent, Y:0.1 to 0.2 percent and the balance of iron.
3. The anti-coking furnace tube with the oxidation layer with good high-temperature stability prepared by the method according to any one of claims 1 to 2, wherein the oxidation layer of the anti-coking furnace tube is an Al 2O3 oxidation layer, and the thickness is 1-3 μm.
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