CN110940595A - Test method for simplifying actual service load in stress corrosion process and application thereof - Google Patents
Test method for simplifying actual service load in stress corrosion process and application thereof Download PDFInfo
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- CN110940595A CN110940595A CN201811109457.0A CN201811109457A CN110940595A CN 110940595 A CN110940595 A CN 110940595A CN 201811109457 A CN201811109457 A CN 201811109457A CN 110940595 A CN110940595 A CN 110940595A
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Abstract
The invention discloses a test method for simplifying actual service load in a stress corrosion process, which comprises the following steps: step 1, preparing a test solution simulating a PWR environment; step 2, placing the stress corrosion sample in a test solution, and measuring the stress corrosion sample by (0.5-1.5) multiplied by 10‑ 4s‑1Loading to a constant load at the strain rate of (2), wherein the constant load is 85-95% of the yield strength of the stress corrosion test sample, and then keeping the constant load for 100-150 hours; step 3, in the test solution, the stress corrosion sample is expressed by (0.5-1.5) multiplied by 10‑4s‑1The overload rate is 1.2-1.6 respectively, then the load is reduced to the constant load value of the step 1, and the corrosion is continued in the test solution for 100-150 hours; step 4, observing the microcosmic state of the oxide film on the surface of the stress corrosion sample by using a scanning electron microscope and/or a transmission electron microscopeIn this way, the oxidation behavior of the load mode on stress corrosion was examined. The invention can evaluate the stress corrosion behavior of the material more accurately.
Description
Technical Field
The invention relates to the technical field of stress corrosion, in particular to a test method for simplifying the actual service load in the stress corrosion process and application thereof.
Background
Stress corrosion refers to the process of hysteresis cracking or brittle fracture of a metal material under the action of stress less than yield strength in a specific environment medium, and relates to three factors of metal, medium environment and stress. The stresses referred to herein may be applied stresses or residual stresses present during machining and heat treatment, as well as wedging forces of corrosion products, and are typically much lower in magnitude than the fracture stresses. The effect of the factors affecting stress corrosion on the stress corrosion resistance of the material will ultimately be reflected in altering the performance and structure of the oxide film. The composition and structure of the oxide film have an important influence on the stress corrosion resistance of the oxide film.
In a laboratory environment, techniques for assessing the susceptibility of materials to stress corrosion cracking can be categorized as constant load, constant deflection, and constant displacement tensile tests. The dead load test requires a long time without disturbing the load to break the oxide film, and therefore, the stress corrosion sensitivity data obtained from the dead load stress corrosion test and the test results of the oxide film structure are greatly different from the actual use cases. The constant deflection technique is easy to implement, but has a disadvantage in that stress relaxation occurs with the passage of time under a high temperature environment. The constant displacement stretching technology is a simple and easy method, but the constant stretching speed causes the continuous fracture of the oxide film, which is not in accordance with the actual situation, and the method can only be used for qualitatively evaluating the stress corrosion cracking resistance tendency of the material. During the service process of the nuclear power pipeline, the pipeline is subjected to neither constant load nor discontinuous plastic strain, such as axial stress generated by pipeline positioning in the installation stage of the nuclear power plant, fluctuating load generated by cooling water flowing in the operation process of the nuclear power plant, and occasionally natural disasters such as earthquake waves and the like. Therefore, the results obtained by these test methods can only be used for comparing the stress corrosion resistance of different materials, and cannot give a response relationship between the material and the load under actual service load.
Disclosure of Invention
The invention aims to provide a test method for simplifying the actual service load in the stress corrosion process and application thereof aiming at the technical defects in the prior art.
The technical scheme adopted for realizing the purpose of the invention is as follows:
a test method for simplifying actual service load in a stress corrosion process comprises the following steps:
and 4, observing the microscopic morphology of the oxide film on the surface of the stress corrosion sample by using a scanning electron microscope and/or a transmission electron microscope to investigate the stress corrosion oxidation behavior in a load mode.
In the above technical solution, the preparation process of the test solution in step 1 is as follows:
2.2ppm LiOH and 1200ppm H3BO3Adding into water to obtain mixed solution, adding into high temperature autoclave, heating to 300 deg.C at a heating rate of 80-120 deg.C/h, and continuously introducing nitrogen gas to maintain the test solution at 5ppbThe following steps.
In the technical scheme, the gauge length section of the stress corrosion sample in the step 2 is 6-7mm in diameter and 20-30mm in length.
In the technical scheme, the stress corrosion test sample in the step 2 is pretreated before the experiment, wherein the pretreatment process comprises the steps of polishing the initial surface of the test sample by using a gravel grinding wheel, eliminating surface defects and processing residual stress, ultrasonically cleaning the sample by using alcohol, drying and storing in a drying oven.
In another aspect of the invention, the application of the test method for simplifying the actual service load in the stress corrosion process in judging the response relationship between the material and the load under the actual service load is further included.
Compared with the prior art, the invention has the beneficial effects that:
compared with the traditional stress corrosion detection method, the method for simplifying the actual service load is easier to realize and is closer to the actual service working condition of the component, so that the stress corrosion behavior of the material is more accurately evaluated.
Drawings
FIG. 1 stress corrosion coupon geometry (mm);
FIG. 2 is a schematic view of a stress corrosion sample loading mode;
FIG. 3 shows the appearance of an oxide film on the surface of a sample under constant load;
FIG. 4 shows the appearance of the oxide film on the surface of the test piece when the test piece is overloaded;
FIG. 5 is a cross-sectional view of an oxide film on a surface of a sample under constant load and element distribution;
FIG. 6 shows the cross-sectional morphology and the element distribution of the oxide film on the surface of the sample with an overload ratio of 1.5.
Detailed Description
The invention is described in further detail below with reference to the figures and specific examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The following examples evaluate the typical oxidation behavior of CF8A SS under uniaxial tension at different overload ratios in a simulated Pressurized Water Reactor (PWR) environment.
To simulate PWR environment, a test solution was prepared using high purity water, and 2.2ppm Li (LiOH) and 1200ppm B (H) were added3BO3). 1.7L of the solution was charged into a 3.5L high-temperature autoclave and heated, and nitrogen (99.999%) was continuously introduced during the heating to maintain the test solution at 5ppb, and the test solution was heated to 300 ℃ at a heating rate of 100 ℃/h, and the temperature fluctuation of the test solution during the heating was within. + -. 1 ℃.
Comparative example 1
When the environment parameters of the simulated PWR are stable, a test for enabling the sample to be always under the constant load is carried out in the PWR environment, and specifically, the sample is 1 multiplied by 10-4s-1Is loaded to 150MPa (90% of the yield strength at 300 ℃ CF8A SS) and then held under constant load for 240 hours, yielding a constant load sample.
The morphology of the oxide film on the surface of CF8A SS of the constant load sample of comparative example 1 was observed by a scanning electron microscope. Fig. 3a is an SEM image of an oxide film formed under a constant load, and fig. 3b is an enlarged image from fig. 3 a. The oxide film on the surface of the sample is dense and continuous and mainly consists of two kinds of crystal grains: dense bulk oxides and particulate oxides.
FIG. 5a is a cross-sectional profile of an oxide film of a permanent sample. The plate-like oxide films were observed to be vertically distributed on the inner oxide film, which corresponds to the surface morphology observed by SEM. FIGS. 5b-e are EDS maps of O, Fe, Cr, and Ni in the frame region of FIG. 5 a. From the results of the EDS analysis, it is apparent that the oxide film has a double-layer structure. The distribution of the elements in the outer layer of the plate oxide is very non-uniform. Thicker plate oxides (middle vertical plate oxide in box in fig. 5 a) are Fe-rich inside and Cr-rich outside; the thinner plate oxide (the tilted plate oxide to the right of the vertical plate oxide in the box in fig. 5 a) is Ni-rich; the inner oxide film is Cr-rich (fig. 5c shows a clear curve with higher brightness, indicating that the Cr content is high here). In addition, the Ni content at the interface of the inner oxide film/substrate is significantly higher than the Ni content in the substrate (FIG. 5e shows a clear higher brightness curve indicating the Ni content at this point).
Example 1
When simulating a PWRWhen the environmental parameters are stable, the sample is at 1X 10-4s-1Is loaded to 150MPa (90% of the yield strength of CF8ASS at 300 ℃) and then kept under constant load for 120 hours, after which the test specimen is at 1X 10-3s-1Is overloaded at a rate of 1.2, 1.4 and 1.6 respectively, and then the load is reduced to 150MPa and is continuously corroded in a PWR environment for 120 hours to obtain an overloaded sample.
Fig. 4 shows typical morphology of oxide generated on the CF8A SS surface by the overloaded sample at different overload ratios (OLR 1.2, OLR 1.4, OLR 1.6), fig. 4a shows an oxide film with OLR 1.2, and fig. 4b shows an enlargement of the central region in fig. 4 a. Notably, the overload sample of example 1 exhibited a gem-like oxide on the outside compared to the constant load sample of comparative example 1. Fig. 4c shows the oxide film formed when OLR is 1.4, and fig. 4d is an enlargement of the central region of fig. 4c, and it can be seen that the number of gem-like oxides is significantly increased and the size becomes uniform. When OLR ═ 1.4, the flaky oxide was densely distributed over the entire surface of the sample. When the OLR is increased to 1.6 (fig. 4e and 4f), the gem-like and plate-like oxides develop irregular shapes, and the number of both increases sharply.
In the case of OLR ═ 1.4, the oxide film formed on the surface of CF8A SS had a cross-sectional morphology as shown in fig. 6a, with a clearly visible interface (black pinhole in cross section) between the outer oxide film and the inner oxide film. Fig. 6b-e show the results of EDS analysis of the area enclosed by the rectangle in fig. 6a, and it can be seen that the gem-like oxide (at the approximately diamond-shaped structures in the black box of 6 a) is Fe-rich. The inner Cr-rich oxide film disappeared compared to the dead load (no line of apparent higher brightness at fig. 6c relative to the bright line in fig. 5 c).
Comparing comparative example 1 with example 1, it can be seen that the loading mode has an important influence on the oxide film structure and composition of the stress corrosion oxidation behavior.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (5)
1. A test method for simplifying actual service load in a stress corrosion process is characterized by comprising the following steps:
step 1, preparing a test solution simulating a PWR environment;
step 2, placing the stress corrosion sample in a test solution, and measuring the stress corrosion sample by (0.5-1.5) multiplied by 10-4s-1Loading to a constant load at the strain rate of (2), wherein the constant load is 85-95% of the yield strength of the stress corrosion test sample, and then keeping the constant load for 100-150 hours;
step 3, in the test solution, the stress corrosion sample is expressed by (0.5-1.5) multiplied by 10-4s-1The overload rate is 1.2-1.6 respectively, then the load is reduced to the constant load value of the step 1, and the corrosion is continued in the test solution for 100-150 hours;
and 4, observing the microscopic morphology of the oxide film on the surface of the stress corrosion sample by using a scanning electron microscope and/or a transmission electron microscope to investigate the stress corrosion oxidation behavior in a load mode.
2. The method for testing the reduction of the actual service load during stress corrosion according to claim 1, wherein the test solution in step 1 is prepared by the following steps:
2.2ppm LiOH and 1200ppm H3BO3Adding the mixed solution into water to obtain a mixed solution, adding the mixed solution into a high-temperature autoclave, heating the high-temperature autoclave to 300 ℃ at a heating speed of 80-120 ℃/h, and continuously introducing nitrogen during heating to keep the test solution at 5 ppb.
3. The method for testing the simplification of the actual service load in the stress corrosion process as claimed in claim 1, wherein the gauge length section of the stress corrosion test specimen in the step 2 is 6-7mm in diameter and 20-30mm in length.
4. The method for testing the simplification of the actual service load in the stress corrosion process according to claim 1, wherein the stress corrosion test specimen in the step 2 is pretreated before the test, and the pretreatment comprises the steps of polishing the initial surface of the test specimen by a grit grinding wheel, eliminating surface defects and processing residual stress, ultrasonically cleaning the sample by alcohol, drying and storing in a drying oven.
5. Use of the test method for reduction of actual service load during stress corrosion according to claim 1 for evaluating the response of a material to a load under actual service load.
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