Method for evaluating service safety of Super304H steel pipe with abnormally thick and large crystal grains

Abstract

The invention relates to the technical field of metal material service safety evaluation, in particular to a method for evaluating the service safety of a Super304H steel pipe with abnormally coarse grains, which comprises the following steps: firstly, accurately simulating the structure evolution behavior of the Super304H steel pipe in the long-term service process of a coarse grain area through a solid solution and high-temperature aging coupling treatment method of a pre-stretching sample; a P function value in each aging state is calculated through a Parson-Miller parametric method, namely a P function, then a hardness value in each aging state is calculated, data fitting is carried out on hardness value descending section data and the P function value, and a correlation model between the hardness parameter and the coarse grain region structure aging degree (P function) is constructed. A service remaining service life evaluation model is established by combining the condition that the hardness of the Super304H steel is at a lower limit value (150HV), accurate evaluation of the service safety of the outer-wall coarse-grain Super304H steel pipe is realized, technical support is provided for scientific operation management and safety evaluation of an ultra-supercritical unit, and the method is expected to play a key role in the field of operation and maintenance of the ultra-supercritical unit and has a wide application prospect. And efficient and safe development of the power industry is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of service safety assessment of metal materials, and particularly to a method for assessing the service safety of Super304H steel pipes with abnormally coarse grains. Background Technique

[0002] With the improvement of steam parameters of ultra-supercritical units, the high-temperature heating surface tubes of boilers face a more severe service environment. Super304H steel is widely used in the high-temperature superheater and reheater tubes of ultra-supercritical boilers due to its excellent steam oxidation resistance and creep properties; conventional fine-grained Super304H heat-resistant steel is generally prepared by the double-solution process of electric arc furnace or converter steelmaking plus secondary refining, solution softening at a high temperature above 1230°C, rapid cooling treatment and cold rolling deformation, and then solution treatment again at no less than 1150°C followed by rapid cooling, with an average grain size of 9-10 grades; however, in actual operation, it is found that the surface grains of some service pipe sections become abnormally coarsened after 30,000 hours of operation, with a grain size of 2 grades, seriously deviating from the requirement in the GB / T 5310 standard that "the grain size of finished steel pipes of 10Cr18Ni9NbCu3BN (similar steel to Super304H) is 6-10 grades"; research shows that the formation of the coarse grain zone is due to the uneven strain distribution on the outer wall of the steel pipe in the as-supplied state. During long-term service in a high-temperature and high-pressure environment, stress-induced grain boundary migration occurs, and low-distortion grains gradually engulf high-distortion grains, resulting in continuous grain growth; compared with the fine grain zone, the microstructure aging in the coarse grain zone is more serious, significantly reducing the room temperature and high-temperature mechanical properties of the steel pipe, becoming the weak link of the whole pipe, and greatly increasing the risk of tube expansion or even bursting.

[0003] Although it is clear that the existence of the coarse-grained zone has a significant adverse effect on the continued service of Super304H steel pipes, given the certain universality of the coarse-grained phenomenon, it is obviously unrealistic to adopt a large-scale pipe replacement strategy to avoid the risks brought by it. Not only is the cost high, but the implementation difficulty is also huge. In the existing technology, Chen Guohong et al. proposed a comparative study on the damage and mechanical property attenuation of service-aged and aged Super304H heat-resistant steel structures in Electric Power Construction (Vol. 34, No. 9, September 2013, pp. 105-111). The study shows that based on the same P function, the hardness of the aged Super304H steel pipe is the same as that of the inner side of the service-aged Super304H steel pipe, and the two have similar degrees of structural damage. However, this study did not point out the relationship between the coarse-grained zone on the outer wall of the steel pipe and the P function, nor did it study the remaining service life of the Super304H steel pipe with abnormally large grains, and it is impossible to evaluate its service safety; and the existing relevant standards only focus on the evaluation of fine-grained structures and do not cover the coarse-grained situation. Therefore, it is of great significance to develop an accurate and convenient method to evaluate the service safety of Super304H steel pipes with abnormally large grains by means of daily metal supervision, so as to ensure the safe service of Super304H heat-resistant steel pipes and then ensure the stable and reliable operation of the unit. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for evaluating the service safety of Super304H steel pipes with abnormally large grains. It can accurately simulate the tissue evolution behavior during the long-term service process of the coarse-grained zone of Super304H steel pipes through the solution-aging coupling treatment method of pre-stretched specimens. The present invention constructs an association model between the hardness parameter and the degree of tissue aging in the coarse-grained zone, and realizes the accurate evaluation of the service safety of Super304H steel pipes with coarse grains on the outer wall.

[0005] To achieve the above purpose, the present invention provides the following technical solution: A method for evaluating the service safety of Super304H steel pipes with abnormally large grains, the method comprising the following steps:

[0006] Step S1: Sampling

[0007] Use wire cutting to cut room-temperature tensile specimens on the as-supplied Super304H steel pipes, and the number of specimens is not less than 20.

[0008] Step S2: Tensile

[0009] Perform room-temperature tensile on the specimens, and the pre-deformation amount is ΔL.

[0010] Step S3: Solution treatment

[0011] An initial coarse-grained specimen with the same coarse-grained size as the outer wall of the in-service steel pipe was prepared by solution treatment.

[0012] Step S4: Aging treatment

[0013] Based on the Larson-Miller parameter method, the specimens were subjected to high-temperature treatment with different aging times, and the P function values at different holding times (different aging states) were calculated; the formula of the Larson-Miller parameter method is:

[0014] P = T(C + lgt) (1)

[0015] In the formula: P is the Larson-Miller parameter, which is a function related to stress, T is the aging temperature, K; t is the holding time, h; C is a constant, taking 15.

[0016] Step S5: Hardness test

[0017] Using an HVS-50 type Vickers hardness tester, the Vickers hardness of the specimens in each aging state was measured.

[0018] Step S6: Construct a linear relationship model between hardness and the P function

[0019] Based on the Vickers hardness and P function values of the specimens in each aging state, the data in the hardness decline section and the P function values were linearly fitted using a mathematical fitting tool, and the functional relationship between hardness and P was obtained as:

[0020] HV = -0.1P + 2000 (2)

[0021] In the formula, HV is the Vickers hardness; P is a function related to stress.

[0022] Step S7: Construct an evaluation model for the remaining service life

[0023] For the in-service Super304H heat-resistant steel pipe with abnormally grown austenite grains, its remaining service life is:

[0024] t r = t 理论 - t 实际 (3)

[0025] In the formula, t r is the remaining service life, h; t 理论 is the theoretical service time when the hardness of Super304H steel is at the lower limit value (150 HV), h; t 实际 is the actual service time, h;

[0026] From equations (1), (2) and (3), the evaluation model for the remaining service life in the case of abnormally large grains (grain size level 2) can be derived:

[0027]

[0028] In the formula, t r is the remaining service life, in h; T is the aging temperature, in K; HV is the Vickers hardness.

[0029] Preferably, in the step S2, the value of the pre-deformation amount ΔL is 5%.

[0030] Preferably, in the step S3, the specific operation method of the solution treatment is:

[0031] Set the resistance furnace temperature to 1170 °C. When the temperature rises to 1170 °C, put the pre-strained specimen into the furnace chamber of the resistance furnace. After the temperature rises to 1170 °C, keep the specimen for 20 min, and finally take out the specimen and quickly water-cool it.

[0032] Preferably, in the step S4, the specific operation method of the aging treatment is:

[0033] Set the resistance furnace temperature to 700 °C. When the temperature rises to 700 °C, put the solution-treated specimen into the furnace chamber of the resistance furnace. After the temperature rises to 700 °C, treat the specimen for different holding times, and finally take out the specimen and air-cool it.

[0034] Preferably, in the step S5, the loading load of the HVS-50 type Vickers hardness tester is 5 kgf, and the pressure holding time is 10 s; measure 5 values for each specimen, and take the average value as the Vickers hardness value of the specimen.

[0035] Preferably, the value range of the holding time is 0 - 5000 h.

[0036] Preferably, the holding time increases in a gradient manner, and the holding time difference between adjacent specimens is 100 - 1000 h.

[0037] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0038] 1. Through the solution-aging coupling treatment method of pre-stretching the specimen, the present invention accurately simulates the microstructure evolution behavior during the long-term service of the coarse-grained region of the Super304H steel pipe.

[0039] 2. By constructing a correlation model between the hardness parameter and the degree of microstructure aging in the coarse-grained region, the present invention establishes an evaluation model for the remaining service life, realizes the accurate evaluation of the service safety of the Super304H steel pipe with a coarse-grained outer wall, and provides technical support for the scientific operation management and safety evaluation of ultra-supercritical units.

[0040] 3. The present invention establishes an evaluation model for the remaining service life of steel pipes through simple and feasible means, and the calculation of the model is relatively simple and fast. It can be used as a daily metal supervision means to evaluate the service safety of Super304H steel pipes with abnormally coarse grains. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a flowchart of the overall working method of Embodiment 1 of the present invention;

[0042] Figure 2 It is a fitting linear relationship diagram between the hardness of each aging coarse-grained Super304H steel specimen and the P function;

[0043] Figure 3 It is a metallographic photograph of the coarse-grained area on the outer wall of a Super304H steel pipe that has served at 650°C for 45,000 h;

[0044] Figure 4 It is a metallographic photograph of a Super304H steel specimen after 5% pre-strain + solution treatment at 1170°C for 20 min;

[0045] Figure 5 It is a metallographic photograph of a Super304H steel specimen after 5% pre-strain + solution treatment at 1170°C for 20 min + aging treatment at 700°C for 4400 h;

[0046] Figure 6 It is a SEM photograph of the coarse-grained area on the outer wall of a Super304H steel pipe after serving at 650°C for 45,000 h;

[0047] Figure 7 It is a SEM photograph of a Super304H steel specimen after 5% pre-strain + solution treatment at 1170°C for 20 min + aging treatment at 700°C for 4400 h;

[0048] Figure 8 It is a metallographic photograph of a Super304H steel specimen after 5% pre-strain + solution treatment at 1000°C for 20 min;

[0049] Figure 9 It is a metallographic photograph of a Super304H steel specimen after 3% pre-strain + solution treatment at 1200°C for 20 min;

[0050] Figure 10 It is a metallographic photograph of a Super304H steel specimen after 9% pre-strain + solution treatment at 1170°C for 20 min;

[0051] Figure 11 It is a SEM photograph of a Super304H steel specimen after 5% pre-strain + solution treatment at 1170°C for 20 min + aging treatment at 750°C for 540 h. Detailed implementation manners

[0052] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0053] As an index characterizing the comprehensive mechanical properties of materials, hardness is highly sensitive to microstructural changes, which are mainly reflected in: the weakening of solid solution strengthening due to the precipitation of solute atoms, softening caused by dislocation recovery and second-phase coarsening, and the reduction of grain boundary effects caused by grain growth. Research shows that under the condition of constant stress σ, the hardness HV of heat-resistant steel during service or aging has the following relationship with the P function:

[0054] P = A(σ)HV + B(σ) (5)

[0055] In the formula, P is the Larson-Miller parameter, which is a function related to stress; A(σ) and B(σ) are both functions of the working stress σ; HV is the Vickers hardness; among them, the formula of the P function is:

[0056] P = T(C + lgt) (1)

[0057] In the formula, P is the Larson-Miller parameter, which is a function related to stress, T is the aging temperature, K; t is the holding time, h; C is a constant, taking 15;

[0058] A(σ) = A1 + A2lgσ + A3(lgσ) 2 (6)

[0059] In the formula, A i (i = 1, 2, 3) are material constants, and σ is the stress;

[0060] B(σ) = B1 + B2lgσ + B3(lgσ) 2 (7)

[0061] In the formula, B i (i = 1, 2, 3) are material constants, and σ is the stress;

[0062] This indicates that when the working stress σ is constant, there is a certain linear relationship between the attenuation of material hardness and the P function. This linear relationship provides a theoretical basis for calculating the remaining service life of heat-resistant steel pipes. By extrapolating the P function value at a specific hardness, the service safety of heat-resistant steel pipes can be effectively evaluated.

[0063] The present invention provides a method for evaluating the service safety of Super304H steel pipes with abnormally large grains. First, tensile specimens are cut from the as-supplied Super304H steel pipes. Through small-strain tensile deformation combined with short-time solution treatment, initial coarse-grained specimens with the same coarse grain size as the outer wall of the service-state steel pipes are prepared. Subsequently, based on the Larson-Miller parameter method, the specimens are subjected to high-temperature accelerated aging treatment to induce the precipitation of second-phase particles from the grain interior and grain boundaries, simulating the service aging process of the steel pipes. By measuring the hardness of the specimens with different aging durations, the relationship between hardness and the P function is established, and the remaining service life of the Super304H steel pipes with abnormally large grains is deduced using the lower limit of the hardness of the standard material, so as to evaluate the service safety of the heat-resistant steel pipes.

[0064] Example 1

[0065] A method for evaluating the service safety of Super304H steel pipes with abnormally large grains. For the method flow chart, please refer to Figure 1 , which specifically includes the following steps:

[0066] Step S1: Sampling

[0067] Wire cutting is used to cut room-temperature tensile specimens from the as-supplied Super304H steel pipes. The specimen size refers to GB / T228.1-2021 (ISO 6892-1:2019, MOD) "Tensile testing of metallic materials - Part 1: Method of test at room temperature". The number of specimens is not less than 20. In this example, the number of specimens is not limited.

[0068] Step S2: Tensile

[0069] The specimens are placed on a microcomputer-controlled electronic universal testing machine. The control program is selected as displacement control, and then room-temperature tensile is carried out with a pre-deformation of 5%.

[0070] Step S3: Solution treatment

[0071] Initial coarse-grained specimens with the same coarse grain size as the outer wall of the service-state steel pipes are prepared through solution treatment. The specific operation method is as follows: The temperature of the resistance furnace is set to 1170°C. When the temperature rises to 1170°C, the pre-strained specimens are put into the furnace chamber of the resistance furnace. After the temperature rises to 1170°C, the specimens are kept warm for 20 minutes, and finally the specimens are taken out and rapidly water-cooled.

[0072] Step S4: Aging treatment

[0073] Based on the Larson-Miller parameter method, the specimens are subjected to high-temperature treatment with different aging times, and the P function values under different holding times (different aging states) are calculated. The formula of the Larson-Miller parameter method is:

[0074] P = T(C + lgt) (1)

[0075] In the formula: P is the Larson-Miller parameter, which is a function related to stress, T is the aging temperature, K; t is the holding time, h; C is a constant, taking 15.

[0076] By aging treatment, the second-phase particles are induced to precipitate from the intragranular and grain boundary, simulating the service aging process of the steel pipe. The specific operation method of aging treatment is as follows: the temperature of the resistance furnace is set at 700 °C. When the temperature rises to 700 °C, the solution-treated sample is put into the furnace chamber of the resistance furnace. After the temperature rises to 700 °C, the samples are treated for different holding times. Finally, the samples are taken out and air-cooled. The range of the holding time is 0 - 5000 h, and it increases in gradient. The difference in holding time between adjacent samples is 100 - 1000 h. In this embodiment, the specific values and differences of the holding time are not limited.

[0077] Step S5: Hardness test

[0078] According to GB / T 4340.1 - 2009 (ISO 6507 - 1:2005, MOD) "Metallic materials - Vickers hardness test - Part 1: Test method", using an HVS - 50 Vickers hardness tester, measure the hardness values of the coarse-grained Super304H steel samples in each aging state; the loading load of the HVS - 50 Vickers hardness tester is 5 kgf, and the holding time of the pressure is 10 s. Five values are measured for each sample, and the average value is taken as the Vickers hardness of the sample.

[0079] Step S6: Construct a linear relationship model between hardness and the P function

[0080] By performing hardness tests on the coarse-grained Super304H samples with different aging durations, it is found that the hardness evolution shows significant regularity: in the initial stage, the hardness rises steadily. When aging for 3400 h, it can be obtained from formula (1) that P = 18031 at this time. Due to the combined action of the excessive precipitation and coarsening of the M23C6 phase and the significant widening of the austenite grain boundary, the hardness begins to decrease. Use a mathematical fitting tool to perform linear fitting on the data in the hardness decrease section (P > 18031). Please refer to Figure 2 As shown, it is the linear relationship diagram between the hardness decrease section and the P function, and the functional relationship between hardness and P is obtained as:

[0081] HV = -0.1P + 2000 (2)

[0082] In the formula, HV is the Vickers hardness; P is a function related to stress.

[0083] Step S7: Construct an evaluation model for the remaining service life

[0084] For the in-service Super304H heat-resistant steel pipe with abnormal austenite grain growth, its remaining service life is:

[0085] t r =t 理论 -t 实际 (3)

[0086] In the formula, t r is the remaining service life, h; t 理论 is the theoretical service time when the hardness of Super304H steel is at the lower limit value (150 HV), h; t 实际 is the actual service time, h.

[0087] It can be seen from Equation (2) that when the hardness is 150 HV, then P = 18500.

[0088] From Equations (1), (2) and (3), the evaluation model of the remaining service life under the condition of abnormally large grain size (grain size level 2) can be deduced:

[0089]

[0090] In the formula, t r is the remaining service life, h; T is the aging temperature, K; HV is the Vickers hardness.

[0091] Example 2

[0092] Super304H steel pipes in a domestic power plant served at 650 °C for 45,000 hours. According to Equation (1), P = 18140 was calculated. Conventional inspection found that abnormal grain growth occurred in the outer wall area, the grain size reached level 2 - 4, and the thickness of the coarse grain zone was 2 mm. The metallographic photo of the outer wall coarse grain zone of the service - state steel pipe is as Figure 3 shown; after detection, the Vickers hardness of the coarse grain zone was 185 HV. According to the evaluation model of the remaining service life, that is, Equation (4), the remaining service life of this steel pipe was approximately 64347 hours. The accurate simulation of the service state of the coarse - grained Super304H steel pipe is the key to the reliability of this model. According to the experimental process of the present invention, please refer to Figure 4 , which is the metallographic photo of the Super304H steel sample after 5% pre - deformation + solution treatment at 1170 °C for 20 min. Its austenite grain size is equivalent to that of the coarse grain zone in the service state, and there are few precipitates in the grains and at the grain boundaries. And for the Super304H steel sample aged at 700 °C for 4400 h (P = 18140), the metallographic observation was carried out, and the obtained metallographic photo is as Figure 5 shown. It was found that its austenite grain size and morphology are highly similar to those of the coarse grain zone of the service - state steel pipe served at 650 °C for 45,000 hours (see Figure 3 ). Please refer to Figure 6 and Figure 7 , which are the SEM photos of the coarse grain zone of the service - state steel pipe and the SEM photos of the aged sample respectively. ComparingFigure 7 And Figure 6 It can be seen that for the coarse-grained specimens prepared by the method of the present invention, the morphology, size and dispersion degree of the precipitation phases at the austenite grain boundaries and within the grains are highly consistent with those in the coarse-grained region of the in-service steel pipe. The Vickers hardness test result shows 186 HV, which is close to the hardness value of the coarse-grained region of the in-service steel pipe, indicating that the mechanical properties are also relatively close to the actual in-service state.

[0093] Example 3

[0094] A domestic power plant's Super304H steel pipe served at 650 °C for 78,000 hours (P = 18360), and an abnormal coarse-grained region was found in the outer wall area with a grain size of grade 2. After testing, the Vickers hardness of the coarse-grained region was 167 HV. According to the hardness-P function relationship formula (2) fitted by the present invention, it was calculated that the Vickers hardness of the aged coarse-grained region corresponding to this state was 164 HV. The fitting result was close to the actual value, and the error was within a reasonable range, indicating that the fitting result had a high accuracy. At this time, substituting it into the formula for constructing the remaining service life assessment model to calculate, the remaining service life of this steel pipe was about 39,972 hours.

[0095] Comparative Example 1

[0096] The difference from the method of Example 1 is that in step S3, the temperature of the resistance furnace was set to 1000 °C. When the temperature rose to 1000 °C, the pre-stretched specimen was put into the furnace chamber, kept at 1000 °C for 20 min, then the specimen was taken out and water-cooled to obtain the initial coarse-grained specimen. The results are shown in Figure 8 , because the solution temperature was relatively low, the austenite grains could not grow sufficiently, resulting in the grain size being significantly smaller than that in the coarse-grained region of the in-service state.

[0097] Comparative Example 2

[0098] The difference from the method of Example 1 is that in step S2, the deformation amount was 3%, and in step S3, the temperature of the resistance furnace was set to 1200 °C. When the temperature rose to 1200 °C, the pre-stretched specimen was put into the furnace chamber, kept at 1200 °C for 20 min, then the specimen was taken out and water-cooled to obtain the initial coarse-grained specimen. The results are shown in Figure 9 , it can be seen that due to the relatively small deformation amount, the austenite grain size was smaller than that in the coarse-grained region of the in-service state.

[0099] Comparative Example 3

[0100] The difference from the method of Example 1 is that in step S2, the deformation amount was 9%; the results are shown in Figure 10 , because the deformation amount was relatively large (9%), higher than the critical deformation amount, the obtained grain size was smaller than that in the coarse-grained region of the in-service state.

[0101] Comparative Example 4

[0102] It is different from the method of Example 1 in that in step S4, the aging temperature is 750°C. Please refer to Figure 11 , which is the SEM photograph of the Super304H steel sample after aging treatment at 750°C for 540 h; according to the P function calculation formula (1), it can be known that serving at 650°C for 45,000 hours (P = 18137) should have a similar aging state to aging at 750°C for 540 h (P = 18137); by comparing Figure 11 and Figure 6 , it can be seen that for the Super304H coarse-grained sample after aging at 750°C for 540 h, the data sizes of its precipitation phases are very different from those in the actual service state. It can be seen that when accelerating aging simulation according to the P function principle, if the aging temperature is too high, the simulation accuracy will drop significantly. For the Super304H steel pipe with a service temperature of 650°C, choosing 700°C for accelerating aging has a good simulation effect.

[0103] The above detailed comparison fully shows that the Super304H steel coarse-grained sample obtained by the present invention has a very high similarity to the coarse-grained area of the Super304H steel pipe in the service state in terms of both the microstructure and the mechanical properties of the material, and can accurately simulate the complex aging state of the coarse-grained area of the Super304H steel pipe in the service state, laying a reliable foundation for the safety assessment of the steel pipe in service.

[0104] In summary, the present invention accurately simulates the microstructure evolution behavior during the long-term service of the coarse-grained area of the Super304H steel pipe through the solution-aging coupling treatment method of the pre-stretched sample. An association model between the hardness parameter and the aging degree of the coarse-grained area microstructure is constructed and a service remaining service life assessment model is established, realizing the accurate assessment of the service safety of the Super304H steel pipe with a coarse-grained outer wall, providing technical support for the scientific operation management and safety assessment of ultra-supercritical units. The present invention has been verified by rigorous theoretical derivation, fine experimental design and actual case, is practical and feasible, and is expected to play a key role in the operation and maintenance field of ultra-supercritical units, helping the efficient and safe development of the power industry.

[0105] It should be noted that, in this document, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising an..." does not exclude the presence of additional identical elements in the process, method, article or device comprising said element.

[0106] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for evaluating the service safety of Super304H steel pipes with abnormally large grains, characterized in that: The method includes the following steps: Step S1: Sampling Wire cutting is used to cut room temperature tensile specimens on the as-supplied Super304H steel pipes, and the number of specimens is not less than 20; Step S2: Tensile The specimens are subjected to room temperature tensile, and the pre-deformation is ΔL; Step S3: Solution treatment Initial coarse grain specimens with the same outer wall coarse grain size as that of the in-service steel pipes are prepared through solution treatment; Step S4: Aging treatment Based on the Larson-Miller parameter method, the specimens are subjected to high temperature treatment with different aging, and the P function values under different holding times (different aging states) are calculated; the formula of the Larson-Miller parameter method is: P = T(C + lgt) (1) In the formula: P is the Larson-Miller parameter, which is a function related to stress, T is the aging temperature, K; t is the holding time, h; C is a constant, taking 15; Step S5: Hardness test Using an HVS-50 type Vickers hardness tester, the Vickers hardness of the specimens in each aging state is measured; Step S6: Construct a linear relationship model between hardness and P function Based on the Vickers hardness and P function values of the specimens in each aging state, using a mathematical fitting tool to linearly fit the data in the hardness drop section and the P function values, the functional relationship between hardness and P is obtained as: HV = -0.1P + 2000 (2) In the formula, HV is the Vickers hardness; P is a function related to stress; Step S7: Construct an evaluation model for the remaining service life For the in-service Super304H heat-resistant steel pipes with abnormal austenite grain growth, the remaining service life is: t r = t 理论 -t 实际 (3) where t r is the remaining service life, in h; t 理论 is the theoretical service time when the hardness of Super304H steel is at the lower limit value (150 HV), in h; t 实际 is the actual service time, in h; From formulas (1), (2) and (3), the evaluation model for the remaining service life in the case of abnormally coarse grains (grain size level 2) can be deduced: where t r is the remaining service life, h; T is the aging temperature, K; HV is the Vickers hardness.

2. A method for evaluating the service safety of a Super304H steel pipe with abnormally coarse grains according to claim 1, characterized in that: In the said step S2, the value of the pre-deformation ΔL is 5%.

3. A method for evaluating the service safety of a Super304H steel pipe with abnormally large grains according to claim 1, characterized in that: In the said step S3, the specific operation method of the solution treatment is: The temperature of the resistance furnace is set to 1170 °C. When the temperature rises to 1170 °C, the pre-strained specimens are placed into the furnace chamber of the resistance furnace. After the temperature rises to 1170 °C, the specimens are held for 20 min, and finally the specimens are taken out and rapidly water-cooled.

4. A method for evaluating the service safety of a Super304H steel pipe with abnormally coarse grains according to claim 1, characterized in that: In the said step S4, the specific operation method of the aging treatment is: The temperature of the resistance furnace is set to 700 °C. When the temperature rises to 700 °C, the specimens after solution treatment are placed into the furnace chamber of the resistance furnace. After the temperature rises to 700 °C, the specimens are treated with different holding times, and finally the specimens are taken out and air-cooled.

5. A method for evaluating the service safety of a Super304H steel pipe with abnormally coarse grains according to claim 1, characterized in that: In the said step S5, the loading load of the HVS-50 type Vickers hardness tester is 5 kgf, and the pressure holding time is 10 s; 5 values are measured for each specimen, and the average value is taken as the Vickers hardness value of the specimen.

6. A method for evaluating the service safety of a Super304H steel pipe with abnormally large grains according to claims 1 and 4, characterized in that: The value range of the said holding time is 0 - 5000 h.

7. A method for evaluating the service safety of a Super304H steel pipe with abnormally large grains according to claim 4, characterized in that: The holding time increases in gradient, and the difference in holding time between adjacent specimens is 100 - 1000 h.

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