Method for determining the creep stage of aluminum-added ethylene cracking furnace tubes at a service temperature of 950℃

Abstract

This invention belongs to the field of ethylene cracking furnace tube testing technology, specifically relating to a method for determining the creep stage of an aluminum-added ethylene cracking furnace tube at a service temperature of 950°C. This invention, through microstructural observation and analysis of the cross-section of the aluminum-added ethylene cracking furnace tube, determines the influencing factors A (total area fraction of austenite grain boundaries and intragranular precipitates), B (total area fraction of austenite intragranular precipitates), C (area fraction of Ni3Al precipitates), and M (blocky M at austenite grain boundary edges). 23 The area fraction influence factor D of C6 precipitates was used to design the influence functions of the first and second stages of creep in the aluminum-added ethylene cracking furnace tube at 950℃, which can quickly determine the creep stage of the aluminum-added ethylene cracking furnace tube at 950℃. The determination method of this invention can provide a basis for evaluating the service status of the aluminum-added ethylene cracking furnace tube and determining the replacement time of the furnace tube, and provide a reference for petrochemical ethylene plant managers to formulate maintenance strategies and ensure the safe and stable operation of the plant.

Description

Technical Field

[0001] This invention belongs to the field of ethylene cracking furnace technology, and in particular, it is a method for judging the creep stage of aluminum-added ethylene cracking furnace tubes at a service temperature of 950°C. Background Technology

[0002] The ethylene cracking furnace is a core piece of equipment in a petrochemical ethylene plant, and the furnace tubes are its key components. They are typically centrifugally cast from heat-resistant alloy materials and include inlet and outlet tubes, with the inlet tubes operating at a temperature of 950℃. During long-term operation, high-temperature creep is one of the main creep damage mechanisms of the furnace tubes.

[0003] Creep damage in heat-resistant alloys is generally divided into three stages: creep stage 1, creep stage 2, and creep stage 3. Creep stage 1, also known as decelerating creep stage, is characterized by a gradual decrease in the alloy strain rate with increasing creep time. This stage typically accounts for 5% to 10% of the total creep time. Creep stage 2, or steady-state creep stage, is characterized by a relatively long duration of creep, with the alloy strain rate not changing significantly with increasing creep time. This stage typically accounts for 80% to 90% of the total creep time. Creep stage 3, or accelerated creep stage, is characterized by a rapid increase in the alloy strain rate with increasing creep time until creep fracture occurs. This stage typically has a shorter duration, generally accounting for 5% to 10% of the total creep time.

[0004] For ethylene cracking furnace tubes, once creep damage enters the third stage, it is often accompanied by the formation of creep voids and creep cracks, indicating that the furnace tube material has entered the end of its creep life. To ensure the long-term safe operation of ethylene plants, timely identification of the creep damage status of furnace tubes in service, determining whether the furnace tubes are in the first or second stage of creep, and providing technical reference for determining the timing of furnace tube maintenance and replacement, thus ensuring the safe and stable operation of furnace tubes during the maintenance cycle, has become an urgent problem to be solved in the ethylene industry.

[0005] The traditional method to determine the state of creep damage in furnace tube materials is to conduct high-temperature creep tests under certain temperature and stress conditions and judge based on the creep time-strain rate curve. However, this method has two drawbacks: 1) High-temperature creep tests are conducted below the high-temperature yield strength of the furnace tube material, so the test cycle is generally long, which is difficult to meet the requirement of short maintenance cycle of ethylene plant; 2) Creep tests are destructive sampling tests, and the amount of material required for sample processing is large, which is highly destructive to the furnace tube body. Summary of the Invention

[0006] One objective of this invention is to provide a method for determining the creep stage of an aluminum-added ethylene pyrolysis furnace tube at a service temperature of 950°C. This invention utilizes quantitative microstructural characteristic parameter analysis of the aluminum-added ethylene pyrolysis furnace tube at 950°C. The designed influence function can quickly determine the creep stage of the aluminum-added ethylene pyrolysis furnace tube material, overcoming the drawbacks of traditional high-temperature creep tests being time-consuming and highly destructive.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a method for determining the creep stage of an aluminum-added ethylene cracking furnace tube at a service temperature of 950°C, comprising the following steps:

[0008] S1. Microstructural observation and analysis were performed on the cross-section of the aluminum-added ethylene cracking furnace tube at a depth of 1 / 4 to 3 / 4 of the wall thickness:

[0009] The area S1 of the austenite grain boundaries and total intragranulation of the furnace tube material within the field of view S0 is measured. The area fraction A of the area S1 of the austenite grain boundaries and total intragranulation of the furnace tube material within the field of view S0 is calculated as A = S1 / S0 × 100%, unit 1.

[0010] The area S2 of the austenite precipitates within the furnace tube material in the field of view S0 is measured. The area fraction B of the austenite precipitates S2 relative to the total area S1 of the austenite grain boundaries and grains is calculated as B = S2 / S1 × 100%, unit 1.

[0011] The area of ​​the Ni3Al phase at the austenite grain boundary in the furnace tube material within the field of view S0 is measured as S3. The area fraction of the Ni3Al phase at the austenite grain boundary as a percentage of the total area of ​​the austenite grain boundary and intragranular precipitates S1 is calculated as C = S3 / S1 × 100%, unit 1.

[0012] The measured field area S0 shows the blocky M-shaped austenitic grain boundary edge of the furnace tube material. 23 C6 area S4, calculate the blocky M at the austenite grain boundary edge. 23 The area fraction of C6's area S4 relative to the total area S1 of austenite grain boundaries and intragranular precipitates is D = S4 / S1 × 100%, unit 1;

[0013] S2. Calculate the influence function of the aluminum-added ethylene cracking furnace tube on the creep stage at 950℃ using area fractions A, B, C, and D:

[0014] f(A,B,C,D)=[(aA+bB+cC+dD) / 100]×100%

[0015] Where a is 8, b is 800, c is 500, and d is 60;

[0016] S3. Delineate the creep stage of the aluminum-added ethylene cracking furnace tube at 950℃:

[0017] If f(A, B, C, D) ≤ 10%, it is determined that the creep state of the 27Cr44Ni5W3Al + micro-alloy furnace tube is the first stage of creep;

[0018] If 10% < f(A, B, C, D) ≤ 30%, it is determined that the creep state of the 27Cr44Ni5W3Al + micro-alloy furnace tube is the initial stage of the second stage of creep;

[0019] If 30% < f(A, B, C, D) ≤ 70%, it is determined that the creep state of the 27Cr44Ni5W3Al + micro-alloy furnace tube is the middle stage of the second stage of creep;

[0020] If 70% < f(A, B, C, D) ≤ 90%, it is determined that the creep state of the 27Cr44Ni5W3Al + micro-alloy furnace tube is the final stage of the second stage of creep.

[0021] As a further improvement to the method for judging the creep stage of the aluminized ethylene cracking furnace tube at a service temperature of 950°C:

[0022] Preferably, a GX53 type Olympus metallurgical microscope is used for microstructure observation, with a magnification of 1000 times, and not less than 20 fields of view are randomly selected for each specimen for microstructure observation.

[0023] Preferably, Image Pro Plus 6.0 software is used to analyze the microstructure of the microstructure photos, and the measurement data is the arithmetic mean of the measurement data of all fields of view.

[0024] Preferably, when using Image Pro Plus 6.0 software to analyze the microstructure photos, when selecting the area, range is selected as 10, thresh is selected as 3, smooth is selected as 1, and speed is selected as 2.

[0025] Preferably, the total precipitates in the austenite grain boundaries and grains of the furnace tube material include 23 M6C type carbides, M7C3 type carbides and Ni3Al phases.

[0026] Preferably, the aluminized ethylene cracking furnace tube is a 27Cr44Ni5W3Al + micro-alloy furnace tube.

[0027] The beneficial effects of the present invention compared with the prior art are as follows:

[0028] 1) Under normal operating conditions, the service temperature of the furnace tube wall (27Cr44Ni5W3Al + micro-alloy) at the inlet of the radiant section of the aluminized ethylene cracking furnace is generally 950°C. During service, under the action of high temperature and internal pressure, the furnace tube undergoes creep damage. As the service time prolongs, the degree of creep damage intensifies.

[0029] As service time increases, granular secondary carbides gradually precipitate within the austenite grains and eventually aggregate and grow at the austenite grain boundaries. Therefore, the number of austenite grain boundaries and intragranular carbides in the furnace tube material increases with creep. Simultaneously, the fine strip-shaped M7C3 type carbides within the austenite grain boundaries gradually transform into blocky M... 23 C6-type carbides transform and grow at the edges of austenite grain boundary precipitates. Therefore, as creep damage intensifies, the total number of austenite grain boundary precipitates increases, and blocky M-shaped precipitates appear at the edges of the austenite grain boundary precipitates. 23 The proportion of C6-type carbides also increases. Furthermore, under creep conditions at 950℃, granular or blocky Ni3Al precipitates gradually at the austenite grain boundaries and within the grains of the 27Cr44Ni5W3Al+ microalloyed material, and the amount increases with the extension of creep time. Typical microstructure images of grain boundaries and intragranular precipitates in the aluminum-added ethylene cracking furnace tube are shown below. Figure 1 .

[0030] 2) This invention determines the influence factors A (total area fraction of austenite grain boundaries and intragranular precipitates), B (total area fraction of austenite intragranular precipitates), C (area fraction of Ni3Al precipitates), and M (blocky precipitates at austenite grain boundary edges). 23 The area fraction influence factor D of C6 precipitates was used; an influence function was designed for the creep stage of the aluminum-added ethylene cracking furnace tube at 950℃, which can quickly determine the first and second creep stages of the aluminum-added ethylene cracking furnace tube. Creep time and strain rate curves were tested on the same aluminum-added ethylene cracking furnace tube before and after high-temperature service (typical creep time-strain rate curves at 950℃ and 35MPa, and schematic diagrams of different stages are shown in [reference needed]). Figure 2 The method of this invention calculates the creep stage of the furnace tube, confirming that it can quickly and accurately assess the creep damage state of the furnace tube in service, providing a basis for furnace tube replacement time and a reference for petrochemical ethylene plant managers to formulate maintenance strategies and ensure the safe and stable operation of the plant. Attached Figure Description

[0031] Figure 1 These are typical microstructure photographs of grain boundaries and intragranular precipitates in the aluminum-added ethylene cracking furnace tube of this invention;

[0032] Figure 2 This is a typical creep time-strain rate curve and schematic diagram of different stages of the aluminum-added ethylene cracking furnace tube under the conditions of 950℃ and 35MPa in this invention.

[0033] Figure 3 This is a photograph of the microstructure of the No. 1 aluminum-added ethylene cracking furnace tube in service in Embodiment 1 of the present invention;

[0034] Figure 4This is the creep time-strain rate curve of the unused No. 1 aluminum-added ethylene cracking furnace tube under conditions of 950°C and 32MPa in Example 1 of the present invention;

[0035] Figure 5 This is a photograph of the microstructure of the No. 2 aluminum-added ethylene cracking furnace tube in service in Embodiment 2 of the present invention;

[0036] Figure 6 This is the creep time-strain rate curve of the unused No. 2 aluminum-added ethylene cracking furnace tube under conditions of 950°C and 40MPa in Example 2 of the present invention.

[0037] Figure 7 This is a photograph of the microstructure of the No. 3 aluminum-added ethylene cracking furnace tube in service in Embodiment 3 of the present invention;

[0038] Figure 8 This is the creep time-strain rate curve of the unused No. 3 aluminum ethylene cracking furnace tube under conditions of 950℃ and 45MPa in Example 3 of the present invention. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0040] Example 1

[0041] This embodiment provides a method for determining the creep stage of an aluminum-added ethylene cracking furnace tube at a service temperature of 950°C, specifically including the following steps:

[0042] S1. A 27Cr44Ni5W3Al+ microalloyed furnace tube (designated as the No. 1 aluminum-added ethylene cracking furnace tube) at the inlet of an ethylene cracking furnace operating at 950℃ was selected. Microstructure was observed at half the wall thickness of the tube's cross-section using a GX53 Olympus metallographic microscope at 1000x magnification. Typical microstructure photographs are shown below. Figure 3 ;

[0043] S2. The microscopic tissue images were analyzed using Image Pro Plus 6.0 software. When selecting the region, the range was set to 10, the thresh setting to 3, the smooth setting to 1, and the speed setting to 2.

[0044] The area S1 of the austenite grain boundaries and total intragranulation of the furnace tube material within the field of view S0 is measured. The area fraction A of the area S1 of the austenite grain boundaries and total intragranulation of the furnace tube material within the field of view S0 is calculated as A = S1 / S0 × 100%, unit 1.

[0045] The area S2 of the austenite precipitates within the furnace tube material in the field of view S0 is measured. The area fraction B of the austenite precipitates S2 relative to the total area S1 of the austenite grain boundaries and grains is calculated as B = S2 / S1 × 100%, unit 1.

[0046] The area of ​​the Ni3Al phase at the austenite grain boundary in the furnace tube material within the field of view S0 is measured as S3. The area fraction of the Ni3Al phase at the austenite grain boundary as a percentage of the total area of ​​the austenite grain boundary and intragranular precipitates S1 is calculated as C = S3 / S1 × 100%, unit 1.

[0047] The measured field area S0 shows the blocky M-shaped austenitic grain boundary edge of the furnace tube material. 23 Calculate the area S4 of C6, and the blocky M at the austenite grain boundary edge. 23 The area fraction of C6's area S4 relative to the total area S1 of austenite grain boundaries and intragranular precipitates is D = S4 / S1 × 100%, unit 1;

[0048] Switch to the microstructure observation interface, and then randomly select 19 fields of view. Calculate the average area fraction A of the total precipitates at austenite grain boundaries and within grains of the No. 1 aluminum ethylene cracking furnace tube material in 20 tests. The average area fraction B of the precipitates within austenite grains is 0.5%, the average area fraction C of the Ni3Al phase at the austenite grain boundaries of the furnace tube material is 0.3%, and the area fraction M of the blocky austenite grain boundary edges is... 23 The average area fraction D of C6 is 4.2%;

[0049] S3. Based on the percentage of time at different stages of creep, define 0%t as the time before creep begins and 100%t as the time at the end of the third stage of creep; calculate the influence function of the creep stage on the No. 1 aluminum-added ethylene cracking furnace tube under 950℃ conditions:

[0050] f(A,B,C,D)=[(aA+bB+cC+dD) / 100]×100%

[0051] = [(8×0.41+800×0.005+500×0.003+60×0.0042) / 100]×100%

[0052] =9%;

[0053] S4, then f(A,B,C,D)≤10%, the creep state of the No.1 aluminum-added ethylene cracking furnace tube is determined to be the first stage of creep.

[0054] Verification test

[0055] A furnace tube of the same material, specification, and batch as the in-service No. 1 aluminum ethylene cracking furnace tube, but not yet in service, was selected for high-temperature creep testing at 950℃ and 32MPa. The creep time-strain rate curves are shown in [reference needed]. Figure 4 The total creep test time was t0, with the first, second, and third creep stages lasting t1, t2, and t3, respectively, and t0 = t1 + t2 + t3. The total creep rupture time was 285 hours, with the first, second, and third creep stages lasting 30 hours, 228 hours, and 27 hours, respectively.

[0056] A high-temperature creep test was conducted on the No. 1 aluminum ethylene cracking furnace tube under conditions of 950℃ and 32MPa. The total creep fracture time was 259h.

[0057] Compared to new furnace tubes not yet in service, the No. 1 aluminum-ethylene cracking furnace tube, after a period of service at 950℃ and 32MPa, experienced a creep life loss of 26 hours, approximately 9% of the total creep life of the new furnace tubes not yet in service. This indicates that the No. 1 aluminum-ethylene cracking furnace tube is in the first stage of creep. This assessment is consistent with the calculated results of the creep degree influence function.

[0058] Example 2

[0059] This embodiment provides a method for determining the creep stage of an aluminum-added ethylene cracking furnace tube at a service temperature of 950°C, specifically including the following steps:

[0060] S1. A 27Cr44Ni5W3Al+ microalloyed furnace tube (designated as the No. 2 aluminum-added ethylene cracking furnace tube) at the inlet of an ethylene cracking furnace operating at 950℃ was selected. Microstructure was observed at 3 / 5 of the wall thickness of the tube's cross-section using a GX53 Olympus metallographic microscope at 1000x magnification. Typical microstructure photographs are shown below. Figure 5 ;

[0061] S2. The microscopic tissue images were analyzed using Image Pro Plus 6.0 software. When selecting the region, the range was set to 10, the thresh setting to 3, the smooth setting to 1, and the speed setting to 2.

[0062] The area S1 of the austenite grain boundaries and total intragranulation of the furnace tube material within the field of view S0 is measured. The area fraction A of the area S1 of the austenite grain boundaries and total intragranulation of the furnace tube material within the field of view S0 is calculated as A = S1 / S0 × 100%, unit 1.

[0063] The area S2 of the austenite precipitates within the furnace tube material in the field of view S0 is measured. The area fraction B of the austenite precipitates S2 relative to the total area S1 of the austenite grain boundaries and grains is calculated as B = S2 / S1 × 100%, unit 1.

[0064] The area of ​​the Ni3Al phase at the austenite grain boundary in the furnace tube material within the field of view S0 is measured as S3. The area fraction of the Ni3Al phase at the austenite grain boundary as a percentage of the total area of ​​the austenite grain boundary and intragranular precipitates S1 is calculated as C = S3 / S1 × 100%, unit 1.

[0065] The measured field area S0 shows the blocky M-shaped austenitic grain boundary edge of the furnace tube material. 23 Calculate the area S4 of C6, and the blocky M at the austenite grain boundary edge. 23 The area fraction of C6's area S4 relative to the total area S1 of austenite grain boundaries and intragranular precipitates is D = S4 / S1 × 100%, unit 1;

[0066] Switch to the microstructure observation interface, and randomly select 32 fields of view. Calculate the average area fraction A of the total precipitates at austenite grain boundaries and within grains of the No. 2 aluminum ethylene cracking furnace tube material in 33 tests. The average area fraction B of the precipitates within austenite grains is 44%, the average area fraction C of the Ni3Al phase at austenite grain boundaries in the furnace tube material is 5.1%, and the average area fraction M of the blocky austenite grain boundary edges is [missing information]. 23 The average area fraction D of C6 is 5.6%;

[0067] S3. Based on the percentage of time at different stages of creep, define 0%t as the time before creep begins and 100%t as the time at the end of the third stage of creep; calculate the influence function of the creep stage on the No. 2 aluminum-added ethylene cracking furnace tube under 950℃ conditions:

[0068] f(A,B,C,D)=[(aA+bB+cC+dD) / 100]×100%

[0069] = [(8×0.44+800×0.041+500×0.051+60×0.056) / 100]×100%

[0070] =65%;

[0071] S4. If f(A,B,C,D) is between 30% and 70%, the creep state of the No. 2 aluminum-added ethylene cracking furnace tube is determined to be the middle stage of the second creep stage.

[0072] Verification test

[0073] A furnace tube of the same material, specification, and batch as the in-service No. 2 aluminum ethylene cracking furnace tube, but not yet in service, was selected for high-temperature creep testing at 950℃ and 40MPa. The creep time-strain rate curves are shown in [reference needed]. Figure 6The total creep test time was t0, with the first, second, and third creep stages lasting t1, t2, and t3, respectively, and t0 = t1 + t2 + t3. The total creep rupture time was 153 hours, with the first, second, and third creep stages lasting 11 hours, 129 hours, and 13 hours, respectively.

[0074] High-temperature creep tests were conducted on the No. 2 aluminum ethylene cracking furnace tube under conditions of 950℃ and 40MPa. The total creep fracture time was 259h.

[0075] Compared to new furnace tubes not yet in service, the No. 2 aluminum-ethylene cracking furnace tube, after a period of service at 950℃ and 40MPa, experienced a creep life loss of 97 hours, approximately 63% of the total creep life of the new furnace tubes not yet in service. This indicates that the No. 2 aluminum-ethylene cracking furnace tube is in the middle of the second stage of creep. This assessment is consistent with the calculated results of the creep degree influence function.

[0076] Example 3

[0077] This embodiment provides a method for determining the creep stage of an aluminum-added ethylene cracking furnace tube at a service temperature of 950°C, specifically including the following steps:

[0078] S1. A 27Cr44Ni5W3Al+ microalloyed furnace tube (referred to as the No. 3 aluminum-added ethylene cracking furnace tube) at the inlet of an ethylene cracking furnace operating at 950℃ was selected. Microstructure was observed at half the wall thickness of the tube's cross-section using a GX53 Olympus metallographic microscope at 1000x magnification. Typical microstructure photographs are shown below. Figure 7 ;

[0079] S2. The microscopic tissue images were analyzed using Image Pro Plus 6.0 software. When selecting the region, the range was set to 10, the thresh setting to 3, the smooth setting to 1, and the speed setting to 2.

[0080] The area S1 of the austenite grain boundaries and total intragranulation of the furnace tube material within the field of view S0 is measured. The area fraction A of the area S1 of the austenite grain boundaries and total intragranulation of the furnace tube material within the field of view S0 is calculated as A = S1 / S0 × 100%, unit 1.

[0081] The area S2 of the austenite precipitates within the furnace tube material in the field of view S0 is measured. The area fraction B of the austenite precipitates S2 relative to the total area S1 of the austenite grain boundaries and grains is calculated as B = S2 / S1 × 100%, unit 1.

[0082] The area of ​​the Ni3Al phase at the austenite grain boundary in the furnace tube material within the field of view S0 is measured as S3. The area fraction of the Ni3Al phase at the austenite grain boundary as a percentage of the total area of ​​the austenite grain boundary and intragranular precipitates S1 is calculated as C = S3 / S1 × 100%, unit 1.

[0083] The measured field area S0 shows the blocky M-shaped austenitic grain boundary edge of the furnace tube material. 23 Calculate the area S4 of C6, and the blocky M at the austenite grain boundary edge. 23 The area fraction of C6's area S4 relative to the total area S1 of austenite grain boundaries and intragranular precipitates is D = S4 / S1 × 100%, unit 1;

[0084] Switch to the microstructure observation interface, and then randomly select 25 fields of view. Calculate the average area fraction A of the total austenite grain boundaries and intragranular precipitates in the No. 3 aluminum ethylene cracking furnace tube material during 26 tests. The average area fraction B of the austenite intragranular precipitates is 46%, the average area fraction C of the Ni3Al phase at the austenite grain boundaries is 5.5%, and the average area fraction M of the blocky austenite grain boundary edges is 6.4%. 23 The average area fraction D of C6 is 7.2%;

[0085] S3. Based on the percentage of time at different stages of creep, define 0%t as the time before creep begins and 100%t as the time at the end of the third stage of creep; calculate the influence function of the creep stage on the No. 3 aluminum-added ethylene cracking furnace tube under 950℃ conditions:

[0086] f(A,B,C,D)=[(aA+bB+cC+dD) / 100]×100%

[0087] = [(8×0.46+800×0.055+500×0.064+60×0.072) / 100]×100%

[0088] =84%;

[0089] S4. If f(A,B,C,D) is between 70% and 90%, then the creep state of the No. 3 aluminum-added ethylene cracking furnace tube is determined to be the end of the second stage of creep.

[0090] Verification test

[0091] Furnace tubes of the same material, specification, and batch as those in service at the No. 3 aluminum ethylene cracking furnace, but not yet in service, were selected for high-temperature creep tests at 950℃ and 45MPa. The creep time-strain rate curves are shown in [reference needed]. Figure 8The total creep test time was t0, with the first, second, and third creep stages lasting t1, t2, and t3, respectively, and t0 = t1 + t2 + t3. The total creep rupture time was 71 hours, with the first, second, and third creep stages lasting 7 hours, 58 hours, and 6 hours, respectively.

[0092] High-temperature creep tests were conducted on the tubes of the No. 3 aluminum ethylene cracking furnace under conditions of 950℃ and 45MPa. The total creep fracture time was 13 hours.

[0093] Compared to new furnace tubes not yet in service, the creep life loss of the No. 3 aluminum-ethylene cracking furnace tube after a period of service at 950℃ and 45MPa was 58 hours, accounting for approximately 82% of the total creep life of the new furnace tubes not yet in service. This indicates that the No. 3 aluminum-ethylene cracking furnace tube is in the late stage of the second creep stage. This assessment is consistent with the calculation results of the creep degree influence function.

[0094] Those skilled in the art should understand that the above descriptions are merely several specific embodiments of the present invention, and not all embodiments. It should be noted that many modifications and improvements can be made by those skilled in the art, and all modifications or improvements not exceeding the scope of the claims should be considered within the protection scope of the present invention.

Claims

1. A method for determining the creep stage of an aluminum-added ethylene cracking furnace tube at a service temperature of 950°C, characterized in that, It includes the following steps: S1. Conduct microscopic structure observation and analysis at the 1 / 4 - 3 / 4 wall thickness part of the cross-section of the aluminum-added ethylene cracking furnace tube: The area S1 of the austenite grain boundaries and total intragranular precipitates of the furnace tube material within the field of view S0 was measured. The area fraction A of the austenite grain boundaries and total intragranular precipitates within the field of view S0 was calculated as A = S1 / S0 × 100%, unit 1; the austenite grain boundaries and total intragranular precipitates of the furnace tube material include M 23 C6 type carbides, M7C3 type carbides and Ni3Al phase; Measure the area S2 of the precipitates within the austenite grains of the furnace tube material within the field of view area S0, and calculate the area fraction B = S2 / S1×100% (unit: 1) of the area S2 of the precipitates within the austenite grains to the total area S1 of the precipitates at the austenite grain boundaries and within the grains; Measure the area S3 of the Ni3Al phase at the austenite grain boundaries of the furnace tube material within the field of view area S0, and calculate the area fraction C = S3 / S1×100% (unit: 1) of the area S3 of the Ni3Al phase at the austenite grain boundaries to the total area S1 of the precipitates at the austenite grain boundaries and within the grains; The measured field area S0 shows the blocky M-shaped austenitic grain boundary edge of the furnace tube material. 23 C6 area S4, calculate the blocky M at the austenite grain boundary edge. 23 The area fraction of C6's area S4 relative to the total area S1 of austenite grain boundaries and intragranular precipitates is D = S4 / S1 × 100%, unit 1; S2. Calculate the influence function of the aluminum-added ethylene cracking furnace tube at the creep stage under the condition of 950°C from the area fractions A, B, C, and D: f(A, B, C, D)=[(aA + bB + cC + dD) / 100]×100%; where a is 8, b is 800, c is 500, and d is 60; S3. The aluminum-added ethylene cracking furnace tube is a 27Cr44Ni5W3Al + micro-alloy furnace tube. Divide the creep stage of the aluminum-added ethylene cracking furnace tube under the condition of 950°C: If f(A, B, C, D)≤10%, determine that the creep state of the 27Cr44Ni5W3Al + micro-alloy furnace tube is the first creep stage; If 10% < f(A, B, C, D)≤30%, determine that the creep state of the 27Cr44Ni5W3Al + micro-alloy furnace tube is the initial stage of the second creep stage; If 30% < f(A, B, C, D)≤70%, determine that the creep state of the 27Cr44Ni5W3Al + micro-alloy furnace tube is the middle stage of the second creep stage; If 70% < f(A, B, C, D)≤90%, determine that the creep state of the 27Cr44Ni5W3Al + micro-alloy furnace tube is the final stage of the second creep stage.

2. The method for determining the creep stage of an aluminum-added ethylene cracking furnace tube at a service temperature of 950°C according to claim 1, characterized in that, Use a GX53 type Olympus metallurgical microscope for microscopic structure observation, with a magnification of 1000 times. Randomly select no less than 20 fields of view for each specimen for microscopic structure observation.

3. The method for determining the creep stage of an aluminum-added ethylene cracking furnace tube at a service temperature of 950°C according to claim 1 or 2, characterized in that, Use Image Pro Plus 6.0 software to conduct microscopic structure analysis on the microscopic structure photos, and the measured data is the arithmetic mean of the measured data of all fields of view.

4. The method for determining the creep stage of an aluminum-added ethylene cracking furnace tube at a service temperature of 950°C according to claim 3, characterized in that, Use Image Pro Plus 6.0 software to analyze the microscopic structure photos. When selecting the area, range is selected as 10, thresh is selected as 3, smooth is selected as 1, and speed is selected as 2.

Images