A method for evaluating microstructure damage of heat-resistant steel welded joint based on grain boundary integrity

Abstract

A method for assessing microstructural damage in heat-resistant steel welded joints based on grain boundary integrity is proposed. First, standard metallographic samples of the fine-grained heat-affected zone of the heat-resistant steel welded joint are prepared. Then, M images are taken using a scanning electron microscope, and the total length L of all grain boundaries in each image is measured. i and the perimeter p of all precipitated phases on grain boundaries i‑1 , ..., p i‑m If there are voids at the grain boundaries, measure the perimeter h of all voids at all grain boundaries. i‑1 , ..., h i‑n Tissue damage parameter S in each image i =[f1(p i‑1 +…+p i‑m )+f2(h i‑1 +…+h i‑n )] / L i f1 is the harmonizing factor for precipitation-type damage, and f2 is the harmonizing factor for cavitation-type damage; the average tissue damage parameter S is calculated based on M images. 服役时长 =(S1+…+S M ) / M; High-temperature, high-stress creep tests were conducted on the original state heat-resistant steel welded joints, and the average tissue damage parameter S was calculated under different fracture durations. TD The threshold value S of the tissue damage parameter was obtained. th Comparison S 服役时长 With S th Issue a safety warning.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of industrial equipment failure detection, and particularly relates to a microstructure damage evaluation method for a heat-resistant steel welded joint. BACKGROUND

[0002] Thermal power generation is the main power generation method in China, and low pollution and high efficiency of power generation make the service conditions of ultra-supercritical units more demanding. Higher service temperature and steam pressure put higher requirements on the performance of the welded pipe materials in the boiler. Martensitic heat-resistant steel is often used for welded pipe materials in the boiler due to its good high-temperature mechanical properties. Type IV cracking often occurs in the fine-grain heat-affected zone of such a welded joint during service, which is easy to cause major safety accidents. Therefore, it is necessary to regularly inspect the fine-grain heat-affected zone of such a service state joint. Before type IV cracking, the microstructure damage characteristics of the fine-grain heat-affected zone mainly include an increase in the number and size of creep cavities, an increase in the types and size of precipitated phases, and an increase in the number of precipitated phases as the service time increases. Existing microstructure damage evaluation methods mainly involve statistics of the average size, area ratio, hole spacing and number density of all precipitated phases and cavities in the microstructure, and the average size, area ratio and number density of all second phases. These methods have the following problems: 1. Statistics of all precipitated phases and cavities in the microstructure of the martensitic heat-resistant steel lack pertinence to the main failure position (grain boundary) of the martensitic heat-resistant steel. 2. Precipitated phases and small cavities in the grains are difficult to accurately and effectively count due to the influence of the original lath martensite lath marks. SUMMARY

[0003] In order to overcome the deficiencies of the existing evaluation methods, the present application provides a microstructure damage evaluation method for a heat-resistant steel welded joint based on the grain boundary integrity, and the specific steps are as follows:

[0004] (1) Standard metallographic sample preparation; a standard metallographic sample is prepared for the fine-grain heat-affected zone of the heat-resistant steel in the welded joint to be evaluated, i.e. a welded joint with a certain service time;

[0005] (2) Microstructure measurement of the standard metallographic sample; the above metallographic sample is continuously photographed under the field of view of a scanning electron microscope, and a total of M images are obtained; for the ith image, the total length L of all grain boundaries is measured i ; the perimeters p i-1 , p i-2 , …, p i-m of the precipitated phases on all grain boundaries are measured; wherein m is the total number of precipitated phases on the grain boundaries in the corresponding image; if there are cavities on the grain boundaries, the perimeters h i-1 , h i-2 , …, h i-n of all cavities on the grain boundaries are measured; wherein n is the total number of cavities on the grain boundaries in the corresponding image; and the average microstructure damage parameter S i is Si = [f1(p i-1 +p i-2 +p i-3 +…p i-m )+f2(h i-1 +h i-2 +h i-3 +…h i-n )] / L i , wherein i = 1, 2, …, M, f1 is a precipitation phase damage harmonic factor; f2 is a void damage harmonic factor; for the standard metallographic sample, S 服役时长 = (S1+S2+…+S M ) / M;

[0006] (3) High temperature and large stress creep step test of original state welded joint test bar; a test bar is taken from an original state, i.e. unserved, welded joint of the same process as the welded joint to be evaluated, and a high temperature and large stress creep test is performed until the test bar is broken, and the breaking time is TD, and a high temperature and large stress creep test is performed at different time periods in the interval of [50%TD, TD);

[0007] (4) Measurement of creep metallographic sample structure; the structure of the metallographic sample after the creep test at different time periods described in step (3) is measured by the method described in step (2), and the average structure damage parameters of the creep test bar at the corresponding time period are calculated in turn;

[0008] (5) Damage early warning evaluation of the welded joint to be evaluated; the average of the average structure damage parameters of the creep test bar at each time period in step (4) is taken as S th , S 服役时长 is compared with S th , if S 服役时长 <S th , the served joint is safe; if S 服役时长 >S th , the served joint is safe.

[0009] Further, in step (2), the field of view magnification of the image is 500 to 5000.

[0010] Further, in step (2), M>=20.

[0011] Further, in step (4), the high temperature and large stress creep test of the welded joint to be evaluated is a high temperature and large stress step creep test, and the high temperature and large stress creep test is performed at 50%, 60%, 70%, 80% step time periods of TD.

[0012] Further, in step (2), f1 is taken as [1, 2], and f2 is taken as [3, 4].

[0013] Further, the step (5) to be evaluated is the welding joint damage early warning evaluation, and the average tissue damage parameters of the creep test bar at each period in step (4) are recorded as S 50％TD , S 60％TD , S 70％TD , S 80％TD , S th =(1-0.5)*0.5*S 50％TD +(1-0.6)*0.6*S 60％TD +(1-0.7)*0.7*S 70％TD +(1-0.8)*0.8*S 80％TD , S 服役时长 and S th are compared, if S 服役时长 <S th , the service joint is safe; if S 服役时长 >S th , the service joint is safe early warning.

[0014] The precipitated phase and the cavity are the signs of aging of the martensitic heat-resistant steel in the service process, and the precipitated phase and the cavity on the grain boundary are often the positions of service failure. The method provides early warning evaluation of the safety state of the martensitic heat-resistant steel based on the damage degree of the grain boundary integrity caused by the change of the precipitated phase and the cavity on the grain boundary of the welding joint in the service process of the heat-resistant steel, compared with the prior art, the beneficial effects of the present application are:

[0015] 1. The grain boundary strength of the martensitic heat-resistant steel welding joint at the service temperature is reduced more than the grain, and the grain boundary strength is greatly weakened. The irregularity of the grain boundary structure attracts the precipitated phase solute atoms to continuously segregate to the grain boundary at high service temperature, forms the precipitated phase and continuously coarsens. Under the steam pressure, the vacancies diffuse to the interface of the grain boundary precipitated phase, causing the grain boundary precipitated phase to fall off and form the creep cavity. Therefore, the grain boundary of the heat-resistant steel is often the place where the crack is generated. According to the failure mechanism in the above service process, the tissue damage parameter S i =[f1(p i-1 +p i-2 +p i-3 +…p i-m )+f2(h i-1 +h i-2 +h i-3 +…h i-n )] / L iAccording to the different damage degrees of different damage types on the tissue, different harmonic factors are introduced for precipitated phase and cavity type damage, wherein f1 is a precipitated phase type damage harmonic factor, f1 takes [1, 2]; f2 is a cavity type damage harmonic factor, f2 takes [3, 4]. The failure mechanism and easy failure position of the martensitic heat-resistant steel are considered in the application, and the type of microstructure damage is considered by setting the harmonic factor, which provides a more targeted safety warning method for the martensitic heat-resistant steel under service conditions from the perspective of evaluating the microstructure damage.

[0016] 2. The method considers safety warning during service, and scientifically defines the critical average tissue damage parameter threshold S th . The creep rupture time TD of the original state welded joint fracture creep sample is taken as the benchmark, and high temperature and large stress creep tests are carried out on the original state welded joint at different time periods such as [50%, 100%) TD. The tissue damage parameter threshold is defined by the above-mentioned tissue damage parameter; since the statistics are only for the grain boundary perimeter, and the perimeter of the cavity and precipitated phase near the grain boundary, the statistical object is easy to locate, the parameter type is less, and the statistical work is easy to operate, and the statistical workload is greatly reduced compared with the existing method.

[0017] 3. The method focuses on measuring and counting the perimeter of the precipitated phase and cavity on the grain boundary, effectively avoiding the problem that the influence of the trace of the original lath martensite in the intercrystalline heat affected zone of the welded joint martensitic heat-resistant steel causes inaccurate statistical results of the intercrystalline precipitated phase and microcavity, and the statistical object is easy to identify. The threshold comparison method is fast and simple. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 The step block diagram of the evaluation method described in the application.

[0019] Figure 2 5000X field of view under the typical tissue map. DETAILED DESCRIPTION

[0020] The technical solutions of the application will be described in detail below in combination with the drawings and specific embodiments.

[0021] Example 1

[0022] It is known that a certain dissimilar steel welded joint has been in service for 70,000 hours, and the safety warning evaluation of the joint is carried out; the specific steps are as follows:

[0023] (1) Standard metallographic sample preparation; the standard metallographic sample of the fine grain heat affected zone of the martensitic heat-resistant steel in the welded joint to be evaluated, i.e. the welded joint with a service time of 70,000 hours, is prepared;

[0024] (2) Measurement of the microstructure of standard metallographic samples; 1000X images (M=30 images) were continuously taken of the above metallographic samples under the field of view of a certain type of field emission scanning electron microscope; for the i-th image, the sum of all grain boundary lengths L was measured. i ; Measure the perimeter p of the precipitated phases at all grain boundaries. i-1 p i-2 , ..., p i-m Where m is the total number of precipitates on the grain boundaries in the corresponding image; if voids exist on the grain boundaries, the perimeter h of all voids on the grain boundaries is measured. i-1 h i-2 , ..., h i-n Where n is the total number of voids on the grain boundaries in the corresponding image; then, based on the above measurement data, the tissue damage parameter S derived from the grain boundary incompleteness in the corresponding scanned image is calculated. i For S i =[f1(p i-1 +p i-2 +p i-3 +…+p i-m )+f2(h i-1 +h i-2 +h i-3 +…h i-n )] / L i Where i = 1, 2, ..., M, f1 is the precipitation-type damage harmonizing factor, which is taken as f1 = 2 in this embodiment; f2 is the void-type damage harmonizing factor, which is taken as f2 = 4 in this embodiment; for the standard metallographic sample, based on the statistical calculation results of 30 scanned images, the average microstructural damage parameter S of the heat-resistant steel welded joint after 70,000 hours of service is calculated. 7万h =(S1+S2+…+S 30 ) / 30;

[0025] As the operating temperature of ultra-supercritical units continuously increases, the grain boundary strength of welded joints in heat-resistant steel decreases more significantly than that within the grains at service temperatures, resulting in a substantial weakening of grain boundary strength. The irregularity of the grain boundary structure attracts solute atoms of the precipitated phase to continuously agglomerate towards the grain boundaries at high service temperatures, forming precipitates that continuously coarsen. Under vapor pressure, vacancies diffuse towards the interface of the precipitated phases at the grain boundaries, leading to the shedding of these precipitates and the formation of creep cavities. Therefore, grain boundaries in heat-resistant steel are often the sites of crack initiation. This application focuses on statistically measuring the perimeter of grain boundaries at locations prone to failure, specifically measuring the perimeter of grain boundaries, precipitates, and cavities in the fine-grained region and critical heat-affected zone of martensitic heat-resistant steel. Based on the aforementioned failure mechanisms during service, the microstructural evolution law of martensitic heat-resistant steel during service (formation of grain boundary precipitates and cavities) is analyzed using the mathematical concept of average statistics. Reasonable damage harmonization factors are introduced based on different damage types (precipitates and cavities) to make corresponding corrections for grain boundary incompleteness.

[0026] (3) Original state welded joint test bar high temperature large stress creep step test; take the test bar of the original state, i.e. the same process as the welded joint to be evaluated, and perform high temperature large stress creep test until the test bar breaks, and the fracture time is TD. In the interval of [50%TD, TD), high temperature large stress creep test is performed at different time periods; in this embodiment, high temperature large stress creep test is performed at 50%, 60%, 70%, and 80% of TD.

[0027] (4) Creep metallographic sample organization measurement; according to the method described in step (2), the metallographic sample organization after the creep test at different time periods described in step (3) is measured, and the average organization damage parameter of each creep test bar at the corresponding step period is calculated in turn, S 50％TD 、S 60％TD 、S 70％TD 、S 80％TD ; below 50%TD, corresponding to the early and middle stages of the first creep stage (creep rate reduction) and the second creep stage (steady creep rate) in the creep curve, at this time there are almost no cavities and precipitates in the organization, the organization is stable, and the safety margin is high. At 50%TD, grain boundary precipitates are observed; at 60%TD and 70%TD, the number of grain boundary precipitates increases, the size becomes larger, and small grain boundary cavities appear; at 80%TD, corresponding to the end of the steady creep stage, the grain boundary precipitates are significantly coarsened and accompanied by cavities of a certain size; and the joint enters the third creep stage (accelerated stage) which will produce the risk of fracture, so a certain safety margin needs to be left in actual engineering evaluation.

[0028] (5) Damage early warning evaluation of the welded joint to be evaluated; take the average value of S 50％D 、S 60％D 、S 70％D 、S 80％D as S th , compare S 服役时长 and S th , if S 服役时长 <S th , the service joint is safe; if S 服役时长 >S th , the service joint is safe and early warning.

[0029] Example 2

[0030] A certain dissimilar steel welded joint has been in service for 100,000 hours, and the safety early warning evaluation of the joint is performed; the specific steps are as follows:

[0031] The steps (1)-(4) are the same as those of Example 1, and 2000X images (M=50) are continuously taken under the field of view of a certain type of field emission scanning electron microscope when the standard metallographic sample is measured; when the microstructure changes are observed to be severe with the step period in the high-temperature large-stress step creep test, the step (5) is to evaluate the damage warning of the welded joint, and the average microstructure damage parameters of the creep test bars at each period in the step (4) are recorded as S 50％TD , S 60％TD , S 70％TD , S 80％TD , S th =(1-0.5)*0.5*S 50％TD +(1-0.6)*0.6*S 60％TD +(1-0.7)*0.7*S 70％TD +(1-0.8)*0.8*S 80％TD , S 服役时长 and S th are compared, if S 服役时长 <S th , the service joint is safe; if S 服役时长 >S th , the service joint is safe and warning.

[0032] The above is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited to this, any skilled person in the art can make equivalent replacement or change according to the technical scheme and the inventive concept of the present application within the technical range disclosed by the present application, which should be covered in the protection scope of the present application.

Claims

1. A method for assessing microstructural damage in heat-resistant steel welded joints based on grain boundary integrity, characterized in that, The steps are as follows: (1) Preparation of standard metallographic samples: Prepare standard metallographic samples of the fine-grained heat-affected zone of the heat-resistant steel in the welded joint to be evaluated, i.e., a welded joint with a certain service time; (2) Measurement of the microstructure of standard metallographic samples; The above metallographic samples were continuously photographed under the field of view of a scanning electron microscope, for a total of M images; For the i-th image, measure the sum L of all grain boundary lengths. i ; Measure the perimeter p of the precipitated phases at all grain boundaries. i-1 p i-2 , ..., p i-m Where m is the total number of precipitates on the grain boundaries in the corresponding image; if voids exist on the grain boundaries, the perimeter h of all voids on the grain boundaries is measured. i-1 h i-2 , ..., h i-n Where n is the total number of voids on the grain boundaries in the corresponding image; then the average tissue damage parameter S i For S i =[f1(p i-1 +p i-2 +p i-3 +…p i-m )+f2(h i-1 +h i-2 +h i-3 +…h i-n )] / L i Where i = 1, 2, ..., M, f1 is the precipitate phase damage harmonization factor; f2 is the void damage harmonization factor; for the standard metallographic sample, S 服役时长 =(S1+S2+…+S M ) / M; (3) High-temperature high-stress creep step test of the original state welded joint test bar; Test bars are taken from the original state, i.e., the welded joint that is not in service and has the same process as the welded joint to be evaluated, and high-temperature high-stress creep test is carried out until the test bar breaks. The breaking time is TD. High-temperature high-stress creep test is carried out at different time periods in the range of [50%TD, TD). (4) Measurement of creep metallographic sample structure; referring to the method described in step (2), the metallographic sample structure after creep test at different time periods described in step (3) is measured in the same way, and the average tissue damage parameters of the creep test bar at the corresponding time period are calculated in turn. (5) Early warning assessment of welded joint damage; the mean value of the average tissue damage parameters of the creep test bar under each time period in step (4) is S. th Comparison S 服役时长 With S th If S 服役时长 th Then the service joint is safe; if S 服役时长 >S th Then, a safety warning will be issued for the service joint.​ 2. The method for assessing microstructural damage of heat-resistant steel welded joints based on grain boundary integrity according to claim 1, characterized in that, In step (2), the magnification of the field of view of the captured image is 500 to 5000.

3. The method for assessing microstructural damage of heat-resistant steel welded joints based on grain boundary integrity according to claim 2, characterized in that, In step (2), M>=20.

4. The method for assessing microstructural damage of heat-resistant steel welded joints based on grain boundary integrity according to claim 1, characterized in that, The original state welded joint high temperature and high stress creep test in step (3) is a high temperature and high stress stepped creep test. The welded joint to be evaluated is subjected to high temperature and high stress creep test under the test conditions of step (3) at 50%, 60%, 70% and 80% stepped time periods of TD.

5. A method for assessing microstructural damage of heat-resistant steel welded joints based on grain boundary integrity according to any one of claims 1 to 4, characterized in that, In step (2), f1 takes the value [1, 2] and f2 takes the value [3, 4].

6. The method for assessing microstructural damage of heat-resistant steel welded joints based on grain boundary integrity according to claim 4, characterized in that, The damage warning assessment of the welded joint to be evaluated in step (5) is to record the average tissue damage parameters of the creep test bar at each time period in step (4), which are S. 50%TD S 60%TD S 70%TD S 80%TD S th = (1-0.5)*0.5*S 50%TD +(1-0.6)*0.6* S 60%TD +(1-0.7)*0.7* S 70%TD +(1-0.8)*0.8* S 80%TD Comparison S 服役时长 With S th If S 服役时长 th Then the service joint is safe; if S 服役时长 >S th Then, a safety warning will be issued for the service joint.​ 7. A method for assessing microstructural damage of heat-resistant steel welded joints based on grain boundary integrity according to any one of claims 1 to 4, characterized in that, The heat-resistant steel is a martensitic heat-resistant steel.

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