Method for evaluating and classifying damage levels of gh783 high-temperature alloy bolts

Abstract

The application belongs to the technical field of high-temperature alloy bolts, and particularly relates to a method for evaluating and classifying management of damage levels of GH783 high-temperature alloy bolts, which comprises the following steps: S1, sample preparation; S2, data acquisition; and S3, establishment of the evaluation method. The method provided by the application can quickly determine the safe service state of the GH783 high-temperature alloy bolt. Based on the evaluation method, a method for classifying and managing the bolts according to the damage levels of the bolt materials is further provided. Specifically, the bolts with damage levels close to the end of the service life but without cracks and other defects exceeding the standard are timely replaced; and the bolts with moderate damage levels are subjected to strengthened nondestructive testing and supervised operation. The management method can improve the management level of the GH783 high-temperature alloy bolts of the unit and ensure the safe and stable operation of the unit.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high-temperature alloy bolts, and particularly relates to a method for evaluating and classifying management of damage levels of GH783 high-temperature alloy bolts. BACKGROUND

[0002] GH783 high-temperature alloy material is a high-temperature alloy with Co-Ni-Fe as a base and Nb, Al, Cr and other elements added to improve strength and oxidation resistance, has a low expansion coefficient and good comprehensive performance, and is characterized by high content of aluminum (5.5%) and a small amount of chromium, and is mainly used for manufacturing gap control parts such as compressor casings and sealing rings of aero-engine, and the service temperature is about 700 DEG C. In recent years, it has been applied to the field of energy. Siemens Company of Germany uses it for bolts of 600 DEG C grade ultra-supercritical steam turbine, and Shanghai Steam Turbine Plant has introduced the related technology of Siemens and also mass-produces and applies bolts of GH783 alloy material in the current production.

[0003] In recent years, the service period fracture accidents of high-temperature bolts of GH783 material have occurred in many power plants. Through chemical composition analysis, mechanical property analysis, fracture macro-morphology analysis and fracture microstructure analysis of the fractured bolts, the research shows that the internal cause of the fracture failure is related to insufficient oxidation resistance of the grain boundary. Long-term high-temperature service causes the grain boundary oxidation, the decrease of material plasticity and the gradual increase of brittleness, resulting in the intergranular fracture of the main crack and the secondary crack. GH783 alloy is a three-phase alloy, including austenitic matrix γ phase, γ' phase [Ni3(Al, Nb, Ti) phase] and β phase (NiAl phase). The γ' phase is the main strengthening phase and plays a decisive role in the strength of the alloy; the β phase is divided into granular primary β phase and grain boundary secondary β phase, which control the grain size and strengthen the grain boundary, respectively. Therefore, the main reasons for the bolt fracture failure are two aspects, the internal cause is the chain-like distribution of β phase at the grain boundary, which causes the increase of brittleness of the grain boundary and the insufficient oxidation resistance; and the external cause is related to the stress concentration at the root of the screw thread.

[0004] The existing GH783 high-temperature bolt can only guarantee the service life of several thousand hours. In order to ensure the safe and stable operation of the unit, the existing technology product generally replaces the product with excessive defects (such as cracks) according to the detection results of nondestructive testing and physical and chemical testing of the bolt during the shutdown maintenance. However, due to the imperfect detection technology or the aging stage of the bolt material being in the stage of about to produce cracks, the excessive defects are not found during the shutdown maintenance and the service continues. Therefore, the problem of cracks and expansion fracture due to the aging of the bolt organization and the stress concentration often occurs in the next cycle of operation. SUMMARY

[0005] The present application aims at the above-mentioned problems existing in the prior art, and provides a GH783 high-temperature alloy bolt damage level evaluation method and a classification management method based on the evaluation method.

[0006] The present application adopts the following technical solutions:

[0007] The present application provides a GH783 high-temperature alloy bolt damage level evaluation method, which establishes a quantitative analysis model representing the relationship between the GH783 high-temperature alloy bolt damage level and the non-linear ultrasonic nondestructive testing index, and a quantitative analysis model representing the relationship between the GH783 high-temperature alloy bolt damage level and the beta phase content of the metallographic structure, and then establishes a scientific and reasonable high-temperature bolt grading and classification management method or standard specification on this basis. Specifically, the evaluation method provided by the present application comprises the following steps:

[0008] S1, sample preparation:

[0009] A plurality of GH783 high-temperature alloy bolt samples with high-temperature creep test durations distributed in 0-x hours are prepared by a high-temperature creep test method simulating the actual operating conditions, wherein x is the creep duration that makes the sample just break;

[0010] S2, data acquisition:

[0011] S21, a non-linear ultrasonic detection system is used to perform non-linear ultrasonic detection on the samples to obtain the non-linear ultrasonic coefficients of each sample, and then a regression analysis method is used to establish a mapping relationship between the non-linear ultrasonic coefficients and the high-temperature creep test durations;

[0012] S22, a conventional ultrasonic detection system is used to perform conventional ultrasonic detection on the samples to obtain the crack conditions of each sample;

[0013] S23, metallographic examination is performed on each sample to obtain the area fraction of the beta phase in the same magnification microscope field of view of each sample, and then a regression analysis method is used to establish a mapping relationship between the area fraction of the beta phase and the high-temperature creep test durations;

[0014] S3, evaluation method establishment:

[0015] S31, according to the mapping relationship between the non-linear ultrasonic coefficients and the high-temperature creep test durations and the crack conditions of each sample, a damage level standard of the samples characterized by the ultrasonic coefficients is established as the main evaluation method: high damage level: ultrasonic coefficient >= a; medium damage level: b <= ultrasonic coefficient < a; low damage level: ultrasonic coefficient < b; wherein a is the non-linear ultrasonic coefficient of the sample in which the crack is first found; b is the non-linear ultrasonic coefficient corresponding to the high-temperature creep test duration of 1 / 2x or 2 / 3x obtained from the mapping relationship;

[0016] S32, according to the mapping relationship between the area fraction of the beta phase and the length of the high-temperature creep test, the damage level standard of the sample characterized by the area fraction of the beta phase is determined as an auxiliary evaluation method: high damage level: the area fraction of the beta phase is greater than or equal to c; low damage level: the area fraction of the beta phase is less than c; wherein, c: the area fraction of the beta phase corresponding to the length of the creep test of the sample in which the crack is first found.

[0017] Preferably, in step S1, the GH783 high-temperature alloy bolt samples are all test samples conforming to GB / T 2039 Metal Materials Uniaxial Tensile Creep Test Method.

[0018] Preferably, in step S1, there are at least 3 samples under each high-temperature creep test length to ensure the accuracy of the test results.

[0019] Preferably, in step S1, the high-temperature creep test temperature is equal to or slightly higher than the service temperature, and the test stress is the same as the actual service stress.

[0020] Preferably, in step S23, the magnification is a magnification that allows 4-5 complete grains to be visible in the field of view, and specifically, the field of view of a scanning electron microscope can be selected at a magnification of 5000 times, and the field of view of an optical microscope can be selected at a magnification of 1000 times.

[0021] The application also provides a classification management method for GH783 high-temperature alloy bolts based on the evaluation method, which uses a portable nonlinear ultrasonic device to perform nonlinear ultrasonic detection on in-service GH783 high-temperature alloy bolts during unit shutdown maintenance. At the same time, the bolts classified as medium damage level are further subjected to metallographic inspection to determine the area fraction of the beta phase. According to the detection results, the classification management method is further determined, so as to realize the safety management of the classification of GH783 high-temperature alloy bolts. Specifically, the classification management method comprises the following steps:

[0022] The sample to be tested is taken, nonlinear ultrasonic detection is first performed to obtain a nonlinear ultrasonic coefficient, then the corresponding damage level is found according to the damage level standard of the sample characterized by the ultrasonic coefficient, and the following classification management is performed: Ⅰ) the sample in the high damage level is replaced; Ⅱ) for the sample in the medium damage level, metallographic inspection is first performed to obtain the area fraction of the beta phase in the field of view of the microscope at the same magnification, then the corresponding damage level is found according to the damage level standard of the sample characterized by the area fraction of the beta phase, and the sample in the high damage level under the standard is replaced, and the remaining samples are not disposed of but are subjected to supervision and operation measures of inspection every time the unit is stopped; Ⅲ) the sample in the low damage level is not disposed of and continues to be regularly inspected according to the conventional maintenance regulations.

[0023] The application has the following beneficial effects:

[0024] The evaluation method provided by the application can realize rapid determination of the safe service state of the GH783 high-temperature alloy bolt. Based on the evaluation method, a grading and classification management method according to the damage level of the bolt material is further provided. Specifically, the bolt with a damage level close to the end of life but without cracks and other defects exceeding the standard is replaced in time, and the bolt with a moderate damage level is subjected to strengthened nondestructive testing and supervised operation management. The management method can improve the use and management level of the GH783 high-temperature alloy bolt of the unit and ensure the safe and stable operation of the unit. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 A drawing is prepared for the high-temperature creep test sample;

[0026] Figure 2 A photo is taken for part of the test sample;

[0027] Figure 3 A fitting curve of the nonlinear ultrasonic coefficient of the high-temperature creep test sample and the test duration is shown;

[0028] Figure 4 A curve showing the change of the area fraction of precipitated phase in the field of view of the high-temperature creep test sample and the test duration is shown;

[0029] Figure 5 The damage level of the sample is divided according to the ultrasonic detection result. DETAILED DESCRIPTION

[0030] To make the objectives, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be described in detail below. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application.

[0031] Embodiment 1

[0032] An evaluation method of the damage level of a GH783 high-temperature alloy bolt, comprising the following steps:

[0033] 1. New GH783 high-temperature alloy bolts are selected and processed into test samples in accordance with GB / T 2039 “Metallic Materials-Uniaxial Creep Rupture Testing in Tension” with a quantity of 50. The sample processing drawing is shown in Figure 1 The high-temperature creep test is started by simultaneously hanging all the samples under the test temperature of 650℃ and the test stress of 80MPa. Every certain time, 3 test samples are taken down simultaneously, and the last test sample is the sample that is broken and failed. Under the above conditions, 16 groups of samples with different aging times and 2 original state samples are obtained. Part of the sample photos are shown inFigure 2 .

[0034] 2. Using the nonlinear ultrasonic detection method to detect the above test samples, at the same time, using the ordinary ultrasonic detection system to detect the samples by the conventional ultrasonic detection to obtain the crack condition of each sample, and to conduct metallographic examination on each sample, and to obtain the area fraction of the beta phase (NiAl phase) in the microscope field of view with the same magnification (the scanning electron microscope field of view magnification is 5000 times). The mapping relationship between the nonlinear ultrasonic coefficient and the high temperature creep test time is established by the regression analysis method (see Figure 3 ) and the mapping relationship between the beta phase area fraction and the high temperature creep test time is established (see Figure 4 ).

[0035] 3. The high temperature bolts are divided into low, medium and high damage levels, as shown in Figure 5 .

[0036] Embodiment 2

[0037] A classification management method of a GH783 high temperature alloy bolt, comprising the following steps:

[0038] Taking the sample to be tested, first, the nonlinear ultrasonic detection is carried out to obtain the nonlinear ultrasonic coefficient, and then according to the grade divided in embodiment 1, the classification management is carried out: 1) for the high temperature bolt divided into the high damage level, the replacement treatment should be completed as soon as possible; 2) for the high temperature bolt divided into the medium damage level, the detection result of the beta phase area fraction should be further referred to for processing: if the aging state represented by the beta phase area fraction is in the high damage level range, the replacement treatment should be carried out; otherwise, the supervision operation measures should be taken according to the inspection; 3) for the high temperature bolt divided into the low damage level, the normal inspection can be completed according to the maintenance regulation.

[0039] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for evaluating the damage level of GH783 high-temperature alloy bolts, characterized in that, It includes the following steps: S1. Sample preparation: Prepare multiple GH783 superalloy bolt samples with the high-temperature creep test duration distributed from 0 to x hours by means of a high-temperature creep test method simulating the actual operating conditions; where x is the creep duration when the sample just fractures. S2. Data collection: S21. Use a non-linear ultrasonic detection system to perform non-linear ultrasonic detection on the samples to obtain the non-linear ultrasonic coefficients of each sample, and then establish a mapping relationship between the non-linear ultrasonic coefficient and the high-temperature creep test duration through regression analysis. S22. Use a conventional ultrasonic detection system to perform conventional ultrasonic detection on the samples to obtain the crack conditions of each sample. S23. Conduct metallographic inspection on each sample to obtain the area fraction of the β-phase in the same magnification microscope field of view of each sample, and then establish a mapping relationship between the area fraction of the β-phase and the high-temperature creep test duration through regression analysis. S3. Establishment of evaluation method: S31. Based on the mapping relationship between the non-linear ultrasonic coefficient and the high-temperature creep test duration and the crack conditions of each sample, establish a damage level standard of the sample characterized by the ultrasonic coefficient as the main evaluation method: High damage level: ultrasonic coefficient ≥ a; Medium damage level: b ≤ ultrasonic coefficient < a; Low damage level: ultrasonic coefficient < b; where a: the non-linear ultrasonic coefficient of the sample where cracks start to be found; b: the non-linear ultrasonic coefficient corresponding to the high-temperature creep test duration of 1 / 2x or 2 / 3x obtained from the mapping relationship. S32. Based on the mapping relationship between the area fraction of the β-phase and the high-temperature creep test duration, determine a damage level standard of the sample characterized by the area fraction of the β-phase as the auxiliary evaluation method: High damage level: area fraction of the β-phase ≥ c; Low damage level: area fraction of the β-phase < c; where c: the area fraction of the β-phase corresponding to the creep test duration of the sample where cracks start to be found. In step S1, there are at least 3 samples at each high-temperature creep test duration. In step S1, the high-temperature creep test temperature is equal to or higher than the service temperature, and the test stress is the same as the actual service stress. In step S23, the magnification is such that 4 - 5 complete grains can be seen in the field of view.

2. The evaluation method according to claim 1, characterized in that, In step S1, the GH783 superalloy bolt samples are all test samples conforming to GB / T 2039 "Metallic materials - Uniaxial tensile creep test method".

3. A method for classifying and managing GH783 high-temperature alloy bolts based on the evaluation method described in any one of claims 1-2, characterized in that, It includes the following steps: Take the sample to be tested, first perform nonlinear ultrasonic testing to obtain the nonlinear ultrasonic coefficient, then find the corresponding damage level according to the damage level standard characterized by the ultrasonic coefficient, and classify and manage it as follows: I) Replace samples with high damage level; II) For samples with medium damage level, further metallographic examination is performed to obtain the area fraction of the β phase in the same magnification microscope field of view, then find the corresponding damage level according to the damage level standard characterized by the area fraction of the β phase, and replace samples with high damage level under the above standard, while other samples are not treated but are subject to supervision and operation measures of mandatory inspection during shutdowns; III) For samples with low damage level, no treatment is required, and they continue to be inspected periodically according to the routine maintenance regulations.

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