Method for evaluating creep damage degree of heat-resistant steel based on band contrast

Abstract

The invention discloses a heat-resistant steel creep damage degree evaluation method based on contrast, and as the deformation of heat-resistant steel always presents a monotonically increasing change rule in the creep process, the creep damage degree of the heat-resistant steel can be researched and judged through a deformation measurement method. The method is based on the crystallographic level, utilizes the quality reduction of the Kikuchi zone in the creep process of the heat-resistant steel, explores the internal correlation between the contrast ratio of the Kikuchi zone and the material strain, focuses on the parameter change of the pixel points at the grain boundary, and characterizes the creep degradation degree of the heat-resistant steel from the microscopic scale. Therefore, a more effective damage evaluation method is obtained, and the problem that the creep damage of the important pressure-bearing component of the ultra-supercritical unit is difficult to evaluate is solved.

Description

Technical Field

[0001] The present application belongs to the technical field of creep evaluation of heat-resistant steel of critical units, and specifically relates to a method for evaluating the degree of creep damage of heat-resistant steel based on contrast. Background Art

[0002] The development of ultra-supercritical units is an important path to improve the efficiency of thermal power generation, reduce coal consumption and emissions. As power plant units develop towards high parameters, large capacity, low energy consumption and low emissions, their operating temperature / pressure will be further increased, and key components of the units such as water-cooled walls, superheaters, reheaters and other heat-resistant steel materials will face a greater risk of failure. How to accurately characterize the degree of creep damage of the above materials has become one of the practical needs faced by the thermal power field.

[0003] Traditional physical and chemical monitoring technology for power station metals is difficult to characterize the early creep damage of heat-resistant steel materials. Conventional methods such as microhardness testing, strength testing, and impact toughness testing are only effective in the final stage of the life of heat-resistant steel. On the contrary, in the long early steady-state creep stage, the dislocation configuration in the microstructure of heat-resistant steel will not produce obvious degradation, and the mechanical properties of the material fluctuate little. Therefore, the aging degree of heat-resistant steel cannot be effectively characterized by hardness and strength alone.

[0004] On the other hand, conventional non-destructive testing techniques are also difficult to evaluate the early creep damage of heat-resistant steel materials. For example, the A-type pulse ultrasonic method is only sensitive to volume defects with a size close to half a wavelength (such as cracks, millimeter-level holes, etc.). In contrast, the size of defects generated in heat-resistant steel within the creep life range is mostly micron-sized, which makes the effect of conventional ultrasonic testing methods unsatisfactory. This is because micron-sized defects have little effect on the linear propagation of ultrasonic waves. Summary of the invention

[0005] The present application provides a method for evaluating the creep damage degree of heat-resistant steel based on contrast, so as to solve the problem of characterizing the creep degradation degree of heat-resistant steel from a microscopic scale, hoping to obtain a more effective damage evaluation method and break through the technical problem of the difficulty in evaluating creep damage of important pressure-bearing components of ultra-supercritical units.

[0006] In order to solve the above technical problems, a technical solution adopted in the present application is: a method for evaluating the creep damage degree of heat-resistant steel based on contrast, comprising:

[0007] S1. Based on austenitic heat-resistant steel material, the austenitic heat-resistant steel material is processed by wire cutting equipment to obtain a standard sample;

[0008] S2. Perform a digital image correlation test on the standard sample to obtain a speckle paint pattern on the surface of the standard sample;

[0009] S3. Perform an accelerated creep rupture test on a standard specimen under high temperature and high pressure conditions to obtain the creep rupture life of austenitic heat-resistant steel and the first functional relationship between the creep strain value and time;

[0010] S4. Conduct a creep interruption test under the same test conditions as step S3, with the time being set to 20%, 40%, 60%, and 80% of the rupture life in step S3, respectively, to obtain a group of creep specimens of austenitic heat-resistant steel with increasing creep damage degrees, providing sample materials for subsequent Kikuchi belt contrast quality test;

[0011] S5. Perform EBSD test on the creep sample to obtain the Kikuchi band parameters;

[0012] S6. performing data processing on the Kikuchi band parameters and performing data fitting using an exponential function fitting method to obtain a second functional relationship between the Kikuchi band parameters and the creep strain value;

[0013] S7. Based on the second functional relationship, an EBSD test is performed on the actual austenitic heat-resistant steel to obtain the creep strain value of the actual austenitic heat-resistant steel.

[0014] Further, the method of performing digital image correlation method testing in step S1 includes:

[0015] Clean the surface of the standard sample to ensure that there is no oil or dust; select one side of the standard sample, spray high temperature resistant white paint as background primer, and leave it to dry for 45 minutes;

[0016] On the white primer, a high temperature resistant black paint is then sprayed using a spray gun, so that the black paint adheres to the white primer in spots, generating a speckled paint pattern of random black spots;

[0017] During the creep experiment of the standard sample, the speckle paint image on the surface of the standard sample is photographed in real time to obtain the first functional relationship between the creep strain value and time.

[0018] Furthermore, in the creep test in step S3, the test temperature is greater than 600° C., and the test stress is less than 1 / 3 of the yield strength of the austenitic heat-resistant steel material.

[0019] Furthermore, the Kikuchi band parameter is the brightness contrast between the Kikuchi band and the background. Based on formula (1), the Kikuchi band parameter is obtained; wherein formula (1) is:

[0020]

[0021] Where BC represents the Kikuchi belt parameter; I band Indicates the brightness of the Kikuchi belt; I background Indicates the background brightness.

[0022] Furthermore, the method for data processing of the Kikuchi belt parameters includes:

[0023] The following three types of data processing are performed on the Kikuchi belt parameters:

[0024] S61. Sum and average all the pixels on a Kikuchi belt scan image, denoted as BC ave ;

[0025] S62. Randomly select some grains on a Kikuchi tape scanning image, sum and average the BC values ​​of some pixel points in the grain, and record it as

[0026] S63. Randomly select some grains on a Kikuchi tape scanning image, sum and average the pixel points at the grain boundary, and record it as

[0027] Further, the method of using exponential function fitting method to fit data includes:

[0028] The creep strain of austenitic heat-resistant steel is set as the horizontal axis. The parameter is the ordinate, and the parameter curve is drawn using Origin software to obtain the change law of the contrast of austenitic heat-resistant steel strip with the increase of its creep strain in actual conditions. The exponential function fitting method is used for data fitting to obtain the Kikuchi band parameters of austenitic heat-resistant steel The second functional relationship between and creep strain value.

[0029] The beneficial effects of the present application are as follows: the present application uses the crystallographic level to utilize the quality degradation of the Kikuchi band that occurs in the creep process of heat-resistant steel to explore the intrinsic relationship between the contrast of the Kikuchi band and the material strain, focusing on the parameter changes of the pixel points at the grain boundaries, and characterizing the degree of creep degradation of the heat-resistant steel from a microscopic scale, so as to obtain a more effective damage evaluation method and break through the problem of the difficulty in creep damage evaluation of important pressure-bearing components of ultra-supercritical units. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a flow chart of an embodiment of a method for evaluating creep damage degree of heat-resistant steel based on contrast according to the present application;

[0031] Figure 2 is a first functional relationship diagram between creep strain value and time in an embodiment of a heat-resistant steel creep damage degree evaluation method based on contrast according to the present application;

[0032] Figure 3 It is a scanned image of the Kikuchi belt in an embodiment of a method for evaluating creep damage degree of heat-resistant steel based on belt contrast of the present application;

[0033] Figure 4 yes Figure 3 The grain boundary pixel map in the yellow box in ;

[0034] Figure 5 The Kikuchi band parameter BC in the first embodiment of the method for evaluating the degree of creep damage of heat-resistant steel based on band contrast of the present application is ave The fitting curve diagram of

[0035] Figure 6 The Kikuchi belt parameters in one embodiment of the method for evaluating the degree of creep damage of heat-resistant steel based on belt contrast in this application are The fitting curve diagram of

[0036] Figure 7 The Kikuchi belt parameters in one embodiment of the method for evaluating the degree of creep damage of heat-resistant steel based on belt contrast in this application are The fitting curve diagram of

[0037] Figure 8 It is a Kikuchi scan of a steel pipe component with a service time of 38,000 hours in an embodiment of a method for evaluating creep damage degree of heat-resistant steel based on contrast in the present application;

[0038] Fig. 9 It is a Kikuchi scan of a steel pipe component with a service time of 75,000 hours in an embodiment of a method for evaluating creep damage degree of heat-resistant steel based on contrast in the present application;

[0039] Fig.10 This is a Kikuchi scan of a steel pipe component with a service time of 735,000 hours in an embodiment of a method for evaluating the degree of creep damage of heat-resistant steel based on contrast in the present application. DETAILED DESCRIPTION

[0040] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with specific embodiments.

[0041] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from the description. Therefore, the present invention is not limited to the specific embodiments of the following disclosure.

[0042] See also Figure 1 , Figure 1 1 is a flow chart of an embodiment of a method for evaluating creep damage degree of heat-resistant steel based on contrast ratio in the present application. The method comprises:

[0043] S1. Based on austenitic heat-resistant steel material, the austenitic heat-resistant steel material is processed by wire cutting equipment to obtain a standard sample.

[0044] Specifically, a typical austenitic heat-resistant steel for superheater and reheater pipes, Super304H steel, was selected as the research object, and the material was processed into a standard creep specimen using a wire cutting equipment.

[0045] S2. Perform a digital image correlation test on the standard sample to obtain a speckle paint pattern on the surface of the standard sample and obtain a first functional relationship between the creep strain value and time.

[0046] Specifically, in order to carry out the digital image correlation (DIC) test, first clean the surface of the sample to ensure that there is no oil or dust; secondly, spray high-temperature resistant white paint as a background primer on one side of the Super304H steel sample and let it dry for 45 minutes; on the white primer, use a spray gun to spray high-temperature resistant black paint, and note that the black paint adheres to the white primer in spots, generating random black spots.

[0047] Since the camera needs to capture the speckle image on the sample in real time during the DIC test, the creep test furnace is modified by reserving opening slots so that the camera can capture the speckle pattern on the sample surface in real time and save it to the computer.

[0048] S3. Conduct accelerated creep rupture tests on standard specimens under high temperature and high pressure conditions to obtain the creep rupture life of austenitic heat-resistant steel and the first functional relationship between the creep strain value and time.

[0049] For details, see Figure 2 , carry out accelerated creep rupture test of Super304H steel, set the test temperature to be greater than 600℃, and the test stress is slightly less than 1 / 3 of the yield strength of austenitic heat-resistant steel. The requirements for the creep test machine are that its load error does not exceed ±1% and the temperature error does not exceed ±2℃. After the sample breaks, the creep rupture life t of Super304H steel under this condition is obtained. f , and the first functional relationship between the creep strain value and time calculated by DIC.

[0050] S4. Carry out creep interruption test under the same test conditions as step S3, with the time set to 20%, 40%, 60% and 80% of the fracture life in step S3, respectively, to obtain a group of creep specimens of austenitic heat-resistant steel with increasing creep damage degree, providing sample material for subsequent Kikuchi belt contrast quality test.

[0051] Specifically, the same temperature and stress conditions as those in step S3 are set, and a group of interrupted creep tests are carried out. The time is set to 20%, 40%, 60%, and 80% of the fracture life, respectively. After the test, a group of creep specimens of austenitic heat-resistant steel with increasing creep damage degree are obtained, which provide sample materials for the subsequent Kikuchi belt contrast quality test.

[0052] S5. Perform EBSD test on the creep sample to obtain the Kikuchi band parameters.

[0053] For details, see Figure 3-4 , EBSD test was carried out on the above samples. EBSD technology uses electron beam to hit atoms in the crystal to produce inelastic scattering, realize the identification and calibration of Kikuchi band, and solve the quality parameters of Kikuchi band. For example, Kikuchi band parameter BC (Band Contrast) is defined as the brightness contrast between Kikuchi band and background, reflecting the clarity of Kikuchi band, usually expressed as gray value or percentage. The higher the BC value, the clearer the Kikuchi band and the better the data quality.

[0054] This application utilizes high-performance EBSD probes based on CMOS sensor technology The test block scanning points ( Figure 3 , where the red dots represent the grain boundary pixels and the yellow dots represent the intracrystalline pixels) band contrast BC. Based on formula (1), the Kikuchi band parameter BC value is obtained;

[0055]

[0056] Where BC represents the Kikuchi belt parameter; I band Indicates the brightness of the Kikuchi belt; I background Indicates the background brightness.

[0057] S6. Perform data processing on the Kikuchi band parameters and use an exponential function fitting method to perform data fitting to obtain a second functional relationship between the Kikuchi band parameters and the creep strain value.

[0058] Specifically, the following three types of data processing are performed on the Kikuchi belt parameters:

[0059] S61. Sum and average all the pixels on a Kikuchi belt scan image, denoted as BC ave ;

[0060] S62. Randomly select some grains on a Kikuchi tape scanning image, sum and average the BC values ​​of some pixel points in the grain, and record it as

[0061] S63. Randomly select some grains on a Kikuchi tape scanning image, sum and average the pixel points at the grain boundary, and record it as

[0062] See also Figure 5-7 , the creep strain of the Super304H steel is set as the horizontal axis, with BC ave , The parameter is the ordinate, and the parameter curve is drawn using Origin software to obtain the variation law of the contrast of Super304H steel strip with the increase of its creep strain in actual conditions. The exponential function fitting method is used for data fitting to obtain the parameter BC of austenitic heat-resistant steel Kikuchi strip. ave , and creep strain value ε c The second functional relationship between The fitting function is Figure 5-7 As shown, the independent variable x is the creep strain, the dependent variable y is the BC value, and the focus is on the pixel points at the grain boundary. The law of change.

[0063] S7. Based on the second functional relationship, an EBSD test is performed on the actual austenitic heat-resistant steel to obtain the creep strain value of the actual austenitic heat-resistant steel.

[0064] Specifically, the creep damage degree of Super304H steel pipe components in actual service of the power station was evaluated based on contrast parameters. Four pipe components of the ultra-supercritical unit were selected as the evaluation objects, and EBSD tests were performed on them to obtain the creep damage degree of the steel in actual service. values; respectively substituted into the function formula in step S6 The creep strain value of the material is estimated to determine the creep damage degree of Super304H steel (see Table 1 below).

[0065] Table 1 is a life assessment table for steel pipe components with different service times.

[0066]

[0067] It can be seen from Table 1 that the creep strain values ​​of the three steel pipe components with service times of 38,000 hours, 75,000 hours and 135,000 hours respectively can be obtained as 0.02, 0.04 and 0.07 through the second functional relationship. Therefore, the damage degree and expected life estimation can be obtained based on the above creep strain values.

[0068] The present application proposes a method for evaluating the degree of creep damage of heat-resistant steel based on band contrast. Since the deformation of heat-resistant steel always shows a monotonically increasing change pattern during the creep process, the degree of creep damage of heat-resistant steel can be determined by deformation measurement. The present invention is based on the crystallographic level, utilizing the decrease in the quality of the Kikuchi band that occurs in the creep process of heat-resistant steel, exploring the intrinsic relationship between the Kikuchi band contrast and material strain, focusing on the parameter changes of the pixel points at the grain boundaries, and characterizing the degree of creep degradation of heat-resistant steel from a microscopic scale, thereby obtaining a more effective damage evaluation method and breaking through the problem of the difficulty in evaluating creep damage of important pressure-bearing components of ultra-supercritical units.

[0069] The above descriptions are merely embodiments of the present application and are not intended to limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A method for evaluating creep damage degree of heat-resistant steel based on contrast, characterized in that: include: S1. Based on austenitic heat-resistant steel material, the austenitic heat-resistant steel material is processed by wire cutting equipment to obtain a standard sample; S2. Performing a digital image correlation test on the standard sample to obtain a speckle paint pattern on the surface of the standard sample; S3. Performing an accelerated creep rupture test on the standard sample under high temperature and high pressure conditions to obtain the creep rupture life of the austenitic heat-resistant steel and the first functional relationship between the creep strain value and time; S4. Conduct a creep interruption test under the same test conditions as step S3, with the time being set to 20%, 40%, 60%, and 80% of the rupture life in step S3, respectively, to obtain a group of creep specimens of austenitic heat-resistant steel with increasing creep damage degrees, providing sample materials for subsequent Kikuchi belt contrast quality test; S5. Performing EBSD test on the creep specimen to obtain Kikuchi band parameters; S6. performing data processing on the Kikuchi belt parameters and performing data fitting using an exponential function fitting method to obtain a second functional relationship between the Kikuchi belt parameters and the creep strain value; S7. Based on the second functional relationship, perform EBSD test on the actual austenitic heat-resistant steel to obtain the creep strain value of the actual austenitic heat-resistant steel.

2. The method according to claim 1, characterized in that The method for performing digital image correlation testing in step S1 includes: Clean the surface of the standard sample to ensure that there is no oil or dust; select one side of the standard sample, spray high temperature resistant white paint as background primer, and leave it for 45 minutes to dry; Then, a high temperature resistant black paint is sprayed on the white primer using a spray gun, so that the black paint adheres to the white primer in spots, generating a speckled paint pattern of random black spots; During the creep experiment of the standard sample, a speckle paint image of the surface of the standard sample is photographed in real time to obtain a first functional relationship between the creep strain value and time.

3. The method according to claim 1, characterized in that In the creep test in step S3, the test temperature is greater than 600° C., and the test stress is less than 1 / 3 of the yield strength of the austenitic heat-resistant steel material.

4. The method according to claim 1, characterized in that The Kikuchi band parameter is the brightness contrast between the Kikuchi band and the background. Based on formula (1), the Kikuchi band parameter is obtained. Wherein, the formula (1) is: Where BC represents the Kikuchi belt parameter; I band Indicates the brightness of the Kikuchi belt; I background Indicates the background brightness.

5. The method according to claim 4, characterized in that The method for performing data processing on the Kikuchi belt parameters comprises: The following three types of data processing are performed on the Kikuchi belt parameters: S61. Sum and average all the pixels on a Kikuchi belt scan image, denoted as BC ave ; S62. Randomly select some grains on a Kikuchi tape scanning image, sum and average the BC values ​​of some pixel points in the grain, and record it as S63. Randomly select some grains on a Kikuchi tape scanning image, sum and average the pixel points at the grain boundary, and record it as 6. The method according to claim 5, characterized in that The method of using exponential function fitting method to fit data includes: The creep strain of the austenitic heat-resistant steel is set as the horizontal axis, BC ave , The parameter is the ordinate, and the parameter curve is drawn using Origin software to obtain the change law of the contrast of austenitic heat-resistant steel strip with the increase of its creep strain in actual conditions. The exponential function fitting method is used for data fitting to obtain the Kikuchi band parameter BC of austenitic heat-resistant steel ave , and the creep strain value.

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