Stress corrosion crack initiation high-throughput test method based on variable cross-section sample

Abstract

The invention belongs to the technical field of metal material corrosion testing, and discloses a stress corrosion crack initiation high-throughput testing method based on a variable cross-section sample. The method comprises the following steps: 1) dividing a variable cross-section plate-shaped sample into equidistant intervals in a scale distance section; 2) carrying out a constant load test on the variable cross-section plate-shaped sample in a corrosion environment; (3) taking out the sample when the sample is subjected to constant load until the crack is observed for the first time, finding out a crack initiation interval in a scanning electron microscope, shooting pictures in each crack initiation interval, counting the cracking grain boundary length and the total grain boundary length of each interval, and calculating the grain boundary cracking proportion; (4) repeating the step (2) and the step (3), and setting a critical cracking proportion according to the grain boundary cracking condition on the surface of the sample; the critical cracking stress under each constant load time node is obtained through an internal difference method. The method is simple, the crack initiation time under multiple groups of stress loads is obtained in a high-throughput manner, and the problems of low efficiency and poor precision of crack initiation time and critical crack initiation stress testing in an existing method are solved.

Description

[0001] This invention belongs to the field of metal material corrosion testing technology, specifically relating to a high-throughput testing method for stress corrosion crack initiation based on variable cross-section specimens. Background Technology

[0002] Stress corrosion cracking (SCC) is a failure mode of materials under the combined effects of stress and corrosion. It is prevalent in fields such as nuclear energy and petrochemicals, posing a significant threat to the service safety of equipment. The crack initiation process in SCC accounts for the majority of the material's effective service life and is a crucial stage in SCC research. Studying the stress corrosion crack initiation behavior of materials and understanding the influence of stress on crack initiation time is of great significance for evaluating the SCC performance of materials under service environments and predicting their safe service life.

[0003] Current methods for studying SCC crack initiation, such as constant strain, constant load, and slow strain rate tensile testing, typically employ specimens with uniform cross-sections for crack initiation evaluation. However, the constant strain and constant load methods for uniform cross-section specimens struggle to accurately capture crack initiation time, and are characterized by long experimental cycles and low efficiency, requiring multiple sampling observations to obtain a single set of crack initiation data. While the slow strain rate tensile method can accelerate crack initiation evaluation, the experimental results are prone to deviation from the material's actual service performance and are also inefficient. Therefore, there is an urgent need to develop a method that can efficiently and accurately test SCC crack initiation in materials, effectively measuring the critical initiation stress and crack initiation time within a reasonable timeframe. Summary of the Invention

[0004] The purpose of this invention is to provide a high-throughput testing method for stress corrosion crack initiation based on variable cross-section specimens. This invention simultaneously performs crack initiation tests at multiple stress levels on a single sample, acquiring crack initiation times under multiple stress loads with high throughput, thus solving the problems of low efficiency and poor accuracy in existing methods for testing crack initiation time and critical initiation stress.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A high-throughput testing method for stress corrosion crack initiation based on variable cross-section specimens includes the following steps: 1) Divide the variable cross-section plate specimen with a gauge length of isosceles trapezoid into 10-20 equidistant intervals, measure the width range of each interval and mark it; the cross-section of the gauge length of the variable cross-section plate specimen is rectangular and perpendicular to the isosceles trapezoidal surface; 2) Fix both ends of the variable cross-section plate specimen to the device that provides stress load, and conduct a constant load test in a corrosive environment; the stress load is a tensile stress load; 3) When the constant load is applied to the point where the stress of the sample is at its maximum and a crack is first observed, the sample is removed. The crack initiation interval is located from the point of maximum stress in the scanning electron microscope. At least 20 images are taken in each interval where the crack initiation occurs. The initiation of grain boundary cracks in each interval is analyzed. The cracked grain boundary length and total grain boundary length (excluding twin boundaries) in each interval are counted. The grain boundary cracking ratio is calculated. 4) Extend the test and repeat steps 2) and 3). Crack initiation in the sample gradually progresses from the high-stress area to the low-stress area, i.e., crack initiation progresses from the narrowest end of the gauge length to the widest end. Each time a sample is taken out, the distribution of crack initiation degree at the corresponding time point is obtained. The critical cracking ratio is set according to the overall cracking of the grain boundaries on the sample surface as the critical value for crack initiation. The stress corresponding to this ratio is the critical cracking stress. The time point when the sample is taken out refers to the time point at which cracks are observed to propagate to a lower stress area compared to the previous time point. The critical crack initiation stress at each constant load time node is obtained by the interpolation method. Multiple sampling observations are performed to obtain the critical crack initiation stress at multiple time nodes on a single sample, thereby efficiently obtaining the correspondence between the material crack initiation time and the critical crack initiation stress.

[0006] In step 4), repeating steps 2) and 3) means performing a constant load test on the sample from step 3) according to step 2), and then maintaining the constant load until a crack just appears in the lower stress region of the sample. The sample is then removed, and a scanning electron microscope is used to locate the crack initiation zone. At least 20 images are taken in each crack initiation zone, and the grain boundary crack initiation situation in each zone is analyzed. The cracked grain boundary length and total grain boundary length (excluding twin boundaries) are statistically analyzed, and the grain boundary cracking ratio is calculated. The zone corresponding to the previous time node is defined as the high stress zone, and the zone corresponding to the next time node as the lower stress zone. The lower stress zone is considered to be the zone where the crack just appeared in step 3). This process is repeated to extend the experiment.

[0007] The sample is an alloy in which preferential oxidation occurs at grain boundaries.

[0008] The corrosive environment is provided in an autoclave.

[0009] The conditions of the corrosive environment can be set according to actual needs, such as a high-temperature water environment of 300~600℃ and 10~25MPa.

[0010] The cross-sectional area of ​​the gauge length section of the variable cross-section specimen changes continuously along the tensile direction.

[0011] The variable cross-section specimen is a tensile specimen of uniform thickness, with a rectangular cross-section in the gauge length section and a continuously varying width along the tensile direction.

[0012] The width ratio of the narrow end to the wide end of the gauge length of the variable cross-section specimen is 1:(1.5~4), preferably 1:2, so that the stress level in the gauge length can change continuously within the range of (0.5~1.0)σ.

[0013] In step 4), the time node for taking out the sample refers to the interval corresponding to the later time node being the interval corresponding to the previous time node when the crack was just observed, and the interval corresponding to the later time node did not have the crack at the previous time node. At this time, the sample is taken out for analysis.

[0014] In this invention, setting the critical cracking ratio based on the overall grain boundary cracking of the sample surface refers to setting the critical cracking ratio using scanning electron microscopy and the overall grain boundary cracking of the sample surface. The critical cracking ratio of grain boundaries is determined based on the results of all constant load experiments on the sample. The basis is that after each constant load experiment, the grain boundary cracking ratio is high and the cracking characteristics are obvious in the high stress range. However, as it develops towards the low stress range, the grain boundary cracking ratio decreases and the cracking characteristics gradually become less obvious. At this time, the crack is in the incubation period or early stage of initiation. Therefore, the grain boundary cracking ratio corresponding to the point where the cracking characteristics are not easily identifiable is set as the critical cracking ratio. In stress ranges where the grain boundary cracking ratio is higher than this value, the cracking characteristics are obvious and easily identifiable under a scanning electron microscope; conversely, the cracking characteristics are not obvious, and it is impossible to effectively distinguish whether the grain boundaries are cracked.

[0015] In the method of the present invention, when performing a constant load test, the load applied to the specimen is such that the stress is at its maximum near the yield point, and the entire gauge length of the specimen is within the range of elastic deformation.

[0016] The device that provides stress load is a tensioning device.

[0017] The variable cross-section specimen has two clamping ends, each end of which is designed with two rounded shoulders. The two ends of the specimen are connected to the specimen clamp of the tensile device through two positioning pins passing through the rounded shoulders, ensuring good specimen alignment and avoiding drilling.

[0018] The gauge length of the variable cross-section specimen has a transition arc with the clamping end, and the clamping end does not undergo plastic deformation during the sample stretching process.

[0019] Compared with the prior art, the present invention has the following advantages: (1) The variable cross-section specimen provided by the present invention can achieve stress change within a certain range on a single sample. According to the number of intervals, the degree of crack initiation under multiple stress levels can be tested at the same time. Each sampling observation can obtain the critical stress of crack initiation at the corresponding time node. (2) Variable cross-section specimens can show different stages of crack initiation. At the same time, they can realize quasi-in-situ observation of the crack initiation process under different constant load times at the same grain boundary, which can be used to reveal the crack initiation mechanism (the effect here corresponds to the extended test in the method). (3) The stress distribution in the gauge length section of the variable cross section specimen changes linearly and continuously along the tensile direction, and there are no abnormal stress points. (4) The sample clamping end does not require drilling and can be processed by wire cutting. The rounded shoulders on both sides of the clamping end are connected to the fixture by pins, which can ensure that the sample center is aligned with the stretching axis. (5) There is no strain concentration in the transition arc between the clamping end of the variable cross-section specimen and the gauge length section during the application of load; (6) The variable cross-section specimen is easy to install and disassemble. The specimen and the fixture are connected by pins, which has good centering. (7) By loading samples in series and using them in conjunction with a multi-axis servo loading device (multiple tensile axes in parallel), the testing efficiency can be further improved. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the assembly of a plate-shaped variable cross-section specimen and a tensile device. Figure 2 A schematic diagram showing the shape and dimensions of a Ni15Cr binary alloy plate-shaped variable cross-section specimen. Figure 3 Figure 1 shows the results of a constant load tensile crack initiation test on a Ni15Cr alloy plate-shaped variable cross-section specimen. Figure 4 The graph shows the relationship between crack initiation time and critical cracking stress in Ni15Cr alloy under a high-temperature water environment of 320℃ and 15MPa. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto.

[0022] The schematic diagram of the assembly of the variable cross-section specimen and the tensile device in the high-throughput testing method for stress corrosion crack initiation based on variable cross-section specimens of the present invention is shown below. Figure 1 As shown, the tensile device includes a positioning pin 2, a pair of sample clamps 3, a centering pin 4, a centering cylinder 5, a fixed frame 6, a connecting clamp 7, and a tensile shaft 8; the pair of sample clamps 3 are divided into sample clamp A and sample clamp B. Sample clamp A is connected to the fixed frame 6, and sample clamp B is connected to the tensile shaft 8 through the centering cylinder 5 and the connecting clamp 7. Sample clamp B is fixed by the centering pin 4, and the connecting clamp 7 is connected to the tensile shaft 8 by threads.

[0023] During assembly, the two ends of the variable cross-section specimen 1 are clamping ends, and the middle is the gauge length section. The clamping ends of the variable cross-section specimen 1 are connected to the specimen clamp 3 through the positioning pins 2. The cross-sectional area of ​​the gauge length section of the variable cross-section specimen 1 changes continuously along the tensile direction. The variable cross-section specimen 1 is placed inside the autoclave. The servo loading system of the tensile device applies a load to the variable cross-section specimen 1 inside the autoclave through the tensile shaft 8. Under the coupled effect of stress and corrosive environment, crack initiation occurs in the specimen 1. As the test time increases, the crack initiation gradually develops from the high stress area to the low stress area.

[0024] The variable cross-section specimen 1 is a tensile specimen of uniform thickness, with a rectangular cross-section in the gauge length section and a continuously varying width along the tensile direction.

[0025] The surface of the variable cross-section specimen 1 is polished.

[0026] The variable cross-section specimen is designed with rounded shoulders at both clamping ends, and each side is connected to the specimen clamp 3 by two positioning pins, ensuring good specimen alignment.

[0027] A small arc is provided between the narrow end of the gauge length section of the variable cross-section specimen and the clamping end.

[0028] The gauge length of the variable cross-section specimen has a transition arc with the clamping end, and the clamping end does not undergo plastic deformation during the sample stretching process.

[0029] The sample clamps are connected to the equipment mounting frame and the tension shaft of the servo loading device, respectively.

[0030] This invention provides a high-throughput testing method for stress corrosion crack initiation based on variable cross-section specimens, comprising the following steps: Step 1: Process the material to be tested into a plate-shaped variable cross-section specimen (the gauge length of the specimen is an isosceles trapezoid with a rectangular cross-section). After grinding and polishing the surface of the specimen, divide the gauge length into 10-20 equidistant intervals (intervals are numbered, starting from the narrowest point, as 1, 2, 3...). Measure the width of each interval and calculate the corresponding stress interval. Take the average stress at both ends of each interval as the stress of that interval. The formula for calculating the stress level is σ=F / A, where σ is the stress, F is the applied load, and A is the cross-sectional area at the test location. Step Two: The variable cross-section specimen obtained in Step One is fixed in the tensile apparatus using a clamp. Simultaneously, the specimen is placed in an autoclave, where a constant load test is conducted under corrosive conditions. The load is applied using a servo loading system, ensuring the stress at the narrowest point of the gauge length reaches 1.0σ. y At this point, the stress level within the gauge length section is (0.5~1.0)σ. y The range changes continuously; Step 3: Take out the sample after constant load for a certain period of time (cracks are first observed at the point of maximum stress). In the scanning electron microscope, find the region where cracks initiate from the point of maximum stress, and take at least 20 pictures in each region where cracks initiate. Analyze the grain boundary crack initiation in each region, count the cracked grain boundary length and total grain boundary length (excluding twin boundaries) in each region, and calculate the grain boundary cracking ratio. Step 4: Extend the test time and repeat steps 2 and 3. Crack initiation gradually progresses from the high-stress area to the low-stress area (meaning crack initiation progresses from the narrowest point of the gauge length to the widest point; when a crack just appears in the low-stress area relative to the previous high-stress area, the sample is removed for analysis). Each sample analysis yields the distribution of crack initiation degree at the corresponding time point. Based on the grain boundary cracking on the sample surface, a critical cracking ratio is set as the critical value for crack initiation (the critical grain boundary cracking ratio is set based on the results of all constant load tests on the sample). The determination is based on the following: After each constant load experiment, the proportion of grain boundary cracking is high and the cracking characteristics are obvious in the high stress range. However, as the stress range decreases, the proportion of grain boundary cracking decreases and the cracking characteristics gradually become less obvious. At this point, the crack is in the incubation period or early stage of initiation. Therefore, the grain boundary cracking proportion corresponding to the point where the cracking characteristics are not easily identifiable is set as the critical cracking proportion. In stress ranges where the grain boundary cracking proportion is higher than this value, the cracking characteristics are obvious and easily identifiable under a scanning electron microscope; conversely, the cracking characteristics are not obvious, and it is impossible to effectively distinguish whether the grain boundary has cracked. The stress corresponding to this proportion is the critical cracking stress. The critical cracking stress at each constant load time point can be calculated using the following interpolation method:

[0031] In the formula X The critical stress for crack initiation; Y The critical grain boundary cracking ratio for crack initiation; Y 1 and Y 2 represents the two crack ratio values ​​that are closest to the critical crack ratio; X 1 and X 2 are respectively Y 1 and Y 2 corresponds to the interval average stress ( Y 1> Y 2); where “·” represents a product. By taking multiple samples and observing, the critical crack initiation stress at multiple time points can be obtained on a single sample, thus efficiently obtaining the correspondence between the material crack initiation time and the critical crack initiation stress.

[0032] In the method of this invention, the time node for sampling and analysis refers to the time node at which the crack distribution range observed in the later time node develops towards a lower stress region compared to the previous time node. For example, at the first time node, cracks are only observed in stress ranges 1 and 2; after extending to the second time node, the cracks have developed to the 6th range, i.e., the cracked range is ranges 1 to 6. At the same time, compared with the first time node, the proportion of grain boundary cracking in ranges 1 and 2, which had already cracked, further increases at the second time node.

[0033] In this invention, the resolution of the scanning electron microscope should be no less than 1 nm, and the same pixel should be used to photograph the crack morphology during the experiment. Example

[0034] In this embodiment, a high-throughput testing method for stress corrosion crack initiation based on variable cross-section specimens is applied to the testing of the relationship between the critical stress and time for crack initiation in Ni15Cr binary alloy. The specific implementation steps are as follows: Step 1: Process the Ni15Cr binary alloy into a plate-shaped variable cross-section specimen (see schematic diagram of specimen shape and dimensions). Figure 2 As shown, after grinding and vibratory polishing of the sample surface, the gauge length is divided into 20 equidistant intervals, each 1 mm long. The width of each interval is measured and the corresponding stress level is calculated. The average stress at both ends of each interval is taken as the stress of that interval. The variable cross-section sample is 41.15 mm long, with a gauge length of 20 mm and widths of 2 mm and 4 mm at both ends. The stress level within the gauge length is (0.5~1.0)σ. y The range changes continuously; Step Two: Fix the variable cross-section specimen obtained in Step One using a tensile device, and conduct a constant load test in a corrosive environment (320℃, 15MPa high-temperature water) in an autoclave. The load is applied under controlled conditions using a servo loading system, ensuring the stress at the narrowest point of the gauge length reaches 1.0σ. y (Approximately 165 MPa), at this point, the stress level within the gauge length is (0.5~1.0) σ. y The range changes continuously; Step 3: Starting from 608 hours after crack initiation, locate the crack initiation zone in the scanning electron microscope from the point of maximum stress. Take 30 images in each crack initiation zone and count the length of the cracked grain boundary and the total length of grain boundaries (excluding twin boundaries) in each zone. Calculate the grain boundary cracking ratio: Grain boundary cracking ratio = cracked grain boundary length / total grain boundary length. Step 4: Extend the test time and repeat steps 2 and 3, taking samples approximately every 200 hours. Crack initiation gradually progresses from the high-stress area to the low-stress area. Each sample is analyzed to obtain the critical stress for crack initiation at the corresponding time point. After three constant load tests, three sets of crack initiation time data under stress loads were efficiently obtained on a single variable cross-section sample, such as... Figure 3 As shown. Based on the grain boundary cracking on the sample surface, this experiment set a 2% grain boundary cracking ratio as the critical value for crack initiation (all three constant load results showed that in the range where the grain boundary cracking ratio was below 2%, the crack was in the incubation period or early initiation stage, and the cracking characteristics were not obvious under scanning electron microscopy at this time). The critical cracking stress at each time point when the grain boundary cracking ratio reached the critical value was calculated using the linear interpolation method. The calculated critical cracking stresses at constant load time points of 608h, 816h, and 1117h (the constant load experimental time interval was determined according to the characteristics of the actual test material, with the standard being that each sampling observation could allow the crack to develop to a lower stress range) were 158.8MPa, 143.9MPa, and 131.4MPa, respectively. Figure 4 As shown. Finally, through three constant load tests on variable cross-section specimens, three sets of data on the relationship between material crack initiation time and critical cracking stress were efficiently obtained.

[0035] Figure 2 A schematic diagram showing the shape and dimensions of a Ni15Cr binary alloy plate-shaped variable cross-section specimen. Figure 3 Figure 1 shows the results of a constant load tensile crack initiation test on a Ni15Cr alloy plate-shaped variable cross-section specimen. Figure 4 The graph shows the relationship between crack initiation time and critical cracking stress in Ni15Cr alloy under a high-temperature water environment of 320℃ and 15MPa.

[0036] Table 1 shows the statistical results of grain boundary cracking in Ni15Cr alloy under different constant load times in various stress ranges.

[0037] Table 1. Statistical results of grain boundary cracking in Ni15Cr alloy under different constant load times in various stress ranges.

Claims

1. A high-throughput testing method for stress corrosion crack initiation based on variable cross-section specimens, characterized in that: Includes the following steps: 1) Divide the gauge length of the variable cross-section plate specimen, which has an isosceles trapezoidal gauge length, into 10-20 equidistant intervals, measure the width of each interval and mark it; the cross-section of the gauge length of the variable cross-section plate specimen is rectangular; 2) Fix both ends of the variable cross-section plate specimen to the device that provides stress load, and conduct a constant load test in a corrosive environment; The stress load is a tensile stress load; 3) When a constant load is applied to the point where the stress of the specimen is at its maximum and a crack is first observed, the specimen is removed. Using a scanning electron microscope, the region where the crack initiation occurs is identified, starting from the point of maximum stress. At least 20 images are taken in each region where the crack initiation occurs. The grain boundary crack initiation situation in each region is analyzed. The length of the cracked grain boundary and the total grain boundary length in each region are calculated, excluding twin boundaries. The grain boundary cracking ratio is calculated as follows: Grain boundary cracking ratio = (cracked grain boundary length / total grain boundary length) × 100%. 4) Extend the test and repeat steps 2) and 3). Crack initiation in the sample gradually progresses from the high-stress area to the low-stress area, i.e., crack initiation progresses from the narrowest end of the gauge length to the widest end. Each time a sample is taken out, the distribution of crack initiation degree at the corresponding time point is obtained. The critical cracking ratio is set according to the overall cracking of the grain boundaries on the sample surface as the critical value for crack initiation. The stress corresponding to this ratio is the critical cracking stress. The time point when the sample is taken out refers to the time point at which the crack development towards the lower stress area is observed compared to the previous time point. The critical crack initiation stress at each constant load time node was obtained by the interpolation method. Multiple sampling observations were conducted to obtain the critical crack initiation stress at multiple time nodes on a single sample, thereby obtaining the correspondence between the material crack initiation time and the critical crack initiation stress.

2. The high-throughput testing method for stress corrosion crack initiation based on variable cross-section specimens according to claim 1, characterized in that: The width ratio of the narrow end to the wide end of the gauge length of the variable cross-section specimen is 1:(1.5~4).

3. The high-throughput testing method for stress corrosion crack initiation based on variable cross-section specimens according to claim 2, characterized in that: The width ratio of the narrow end to the wide end of the gauge length of the variable cross-section specimen is 1:2, which allows the stress level within the gauge length to change continuously within the range of (0.5~1.0)σ.

4. The high-throughput testing method for stress corrosion crack initiation based on variable cross-section specimens according to claim 1, characterized in that: The critical cracking stress at each constant load time node was calculated using the following interpolation method: In the formula X The critical stress for crack initiation; Y The critical grain boundary cracking ratio for crack initiation; Y 1 and Y 2 represents the two nearest neighboring crack ratio values ​​to the critical crack ratio. Y 1> Y 2; X 1 and X 2 are respectively Y 1 and Y The interval average stress corresponding to 2.

5. The high-throughput testing method for stress corrosion crack initiation based on variable cross-section specimens according to claim 1, characterized in that: In step 4), repeating steps 2) and 3) means performing a constant load test on the sample from step 3) according to step 2), and then maintaining the constant load until a crack just appears in the lower stress region of the sample. The sample is then removed, and a scanning electron microscope is used to locate the crack initiation zone. At least 20 images are taken in each crack initiation zone, and the grain boundary crack initiation situation in each zone is analyzed. The cracked grain boundary length and total grain boundary length (excluding twin boundaries) are statistically analyzed, and the grain boundary cracking ratio is calculated. The zone corresponding to the previous time node is defined as the high stress zone, and the zone corresponding to the next time node as the lower stress zone. The lower stress zone is considered to be the zone where the crack just appeared in step 3). This process is repeated to extend the experiment.

6. The high-throughput testing method for stress corrosion crack initiation based on variable cross-section specimens according to claim 1, characterized in that: In step 4), the time point at which the sample is taken out refers to the interval corresponding to the later time point being the interval corresponding to the previous time point, at which the crack was just observed, and the interval corresponding to the later time point did not have the crack at the previous time point. At this time, the sample is taken out for analysis. The sample is an alloy whose grain boundaries are prone to oxidation; The corrosive environment is provided in an autoclave; The corrosive environment is a high-temperature water environment with a temperature of 300~600℃ and a pressure of 10~25MPa. The variable cross-section specimen is a tensile specimen of uniform thickness; During constant load testing, the load applied to the specimen is such that the stress is maximized near the yield point, and the entire gauge length of the specimen is within the range of elastic deformation.

7. The high-throughput testing method for stress corrosion crack initiation based on variable cross-section specimens according to claim 1, characterized in that: The device that provides stress load is a tensioning device; The variable cross-section specimen has two clamping ends, and each end is designed with two rounded shoulders. The two ends of the specimen are connected to the specimen clamp of the tensile device through two positioning pins passing through the rounded shoulders. The narrow end of the gauge length of the variable cross-section specimen is provided with a transition arc between the arc shoulder of the clamping end and the gauge length.

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