CN112213381A - Nondestructive testing method, system and medium for large-grain iron-based thin plate metallographic structure - Google Patents
Nondestructive testing method, system and medium for large-grain iron-based thin plate metallographic structure Download PDFInfo
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- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/72—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables
- G01N27/82—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws
- G01N27/83—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws by investigating stray magnetic fields
Abstract
The invention belongs to the technical field of metallographic structure nondestructive detection, and discloses a nondestructive detection method, a nondestructive detection system and a nondestructive detection medium for a metallographic structure of a large-grain iron-based thin plate. When the magnetic suspension liquid is not applied with a leakage magnetic field, the magnetic particles are uniformly dispersed in the liquid medium, and the magnetic suspension liquid is in a transparent state; after the magnetic leakage field is applied, under the action of magnetic force, magnetic particles near the magnetic leakage field can be rapidly gathered to form a visual image; after the leakage magnetic field is removed, the gathered magnetic particles can be rapidly dispersed in the liquid medium and become transparent again. The method realizes the reconstruction of the large-grain crystal boundary by adopting a special magnetic suspension liquid method for the large-grain iron-based sheet, achieves the purpose of nondestructive detection of metallographic structures, reduces the labor intensity and improves the detection efficiency.
Description
Technical Field
The invention belongs to the technical field of metallographic structure nondestructive detection, and particularly relates to a nondestructive detection method, a nondestructive detection system and a nondestructive detection medium for metallographic structures of large-grain iron-based thin plates.
Background
At present: the non-destructive Test (NDT) technique is to detect whether there is a defect or non-uniformity in an object to be detected by using the characteristics of a substance, such as sound, light, magnetism, and electricity, without damaging or affecting the performance of the object to be detected, and to give information on the size, position, properties, and number of the defect. The technology is not a production technology, but can obviously improve the economic benefit, and the level can reflect the industrial technical level of departments, industries, regions and even countries. The nondestructive testing method of the material is more, and the method is widely applied to ultrasonic testing, current accident testing, magnetic flux leakage testing and the like. The method for detecting the leakage magnetic flux by adopting the magnetic sensor is called a magnetic flux leakage detection method, and the method for detecting the leakage magnetic flux by adopting the magnetic powder is called a magnetic powder detection method. Through processing the magnetic leakage data detected by the sensor, the defects can be identified, quantized and reconstructed, and finally, the defect characteristics are displayed in the form of images. Because of the influence of the detection precision of the magnetic sensor, the minimum defect range detectable by the method at present is 1.6mm multiplied by 6 mm; in addition, because the stability and repeatability of the leakage magnetic signal are poor, and the leakage magnetic signal is influenced by various factors such as detection speed and detected material, and the defect shape parameter and the leakage magnetic field are in a nonlinear relationship, a strict and accurate mathematical relationship between the two is difficult to establish. Therefore, magnetic flux leakage imaging at the defect position is always a research hotspot and technical difficulty of magnetic flux leakage detection at home and abroad.
When the ferromagnetic material is magnetized, if the material is continuous and uniform, magnetic induction lines in the material are constrained in the material, the magnetic flux is parallel to the surface of the material, almost no magnetic induction lines penetrate out of the surface, and the detected surface has no magnetic field. When the material has the defect of cutting magnetic lines, the magnetic permeability is changed due to the defect or the structural state change of the surface of the material, the magnetic flux in a magnetic circuit is distorted, the magnetic lines can change the running path, a part of the magnetic flux can leave the surface of the material except that the part of the magnetic flux directly passes through the defect or bypasses the defect inside the material, and the part of the magnetic flux bypasses the defect through air and then reenters the material, so that a leakage magnetic field is formed at the defect position on the surface of the material. The position of the defect part can be well displayed by adopting a magnetic sensor (a magnetic probe) or magnetic powder, but the appearance characteristic of the defect is difficult to accurately display.
The magnetic powder detection method can be divided into a warm method and a dry method according to the type of magnetic powder, wherein the wet method adopts magnetic suspension and the dry method directly sprays dry powder to be uniformly distributed on the surface of a part for detection. The magnetic particle detection method is widely applied to nondestructive detection of pressure containers, petroleum pipelines, railway vehicle parts and the like, and mainly detects defects of cracks, gaps, incomplete penetration, inclusions, incomplete fusion, air holes and the like on the surfaces of the parts. Firstly, the surface of a component is strictly required, and the rough surface, the paint layer, the oxide layer, the dirt collapse, the rust and the like which influence the movement of magnetic powder cannot be influenced; secondly, the detection range is limited, only the horizontal upper surface defects can be detected, and the defects of the side surface and the bottom surface cannot be detected; in addition, the requirement on the environment is high, the operation field must be clean and tidy, the ventilation is good, the places such as electric welding, damp temperature and dust are far away or isolated, the environment temperature is not lower than 5 ℃, and the like. The nondestructive detection technology is mainly applied to the detection of material defects, and has no related application at home and abroad in the aspect of metal material tissue detection. The main reason is that the grain size of most metal material tissues is too small, usually in micron or even nanometer level, under the existing nondestructive detection precision condition, the tissue size of the grade is difficult to be effectively detected, and an accurate tissue image can not be obtained, and the sample can be observed under a metallographic microscope after being polished and corroded.
The oriented electrical steel is a unique soft magnetic functional material which is produced in batch by abnormal growth of crystal grains through a secondary recrystallization process to form a single-Goss texture, and is greatly used for manufacturing various transformer cores due to good magnetic conductivity in the rolling direction, and the domestic capacity reaches 128 million tons in 2015. The grain size range of the oriented electrical steel finished product is 3-50 mm, and because the material thickness is thin (generally 0.02-0.35 mm), the grain boundary penetrates through the oriented electrical steel in the thickness direction and is almost vertical to the surface of the matrix. The metallographic structure characteristic of the oriented silicon steel can be completely reflected by a leakage magnetic field by adopting a leakage magnetic method.
The grain boundary itself is also a structural defect, and theoretically, the grain boundary can be reconstructed by adopting a magnetic flux leakage detection technology. However, the dimension of the grain boundary in the width direction is far smaller than the minimum detectable range of the current magnetosensitive sensor, and the grain boundary of the oriented electrical steel cannot be accurately reconstructed by using the method. The magnetic powder method has certain advantages in displaying the grain boundary, but the grain boundary leakage detection can be caused by the influence of the movement of magnetic powder due to the existence of two bottom layers on the surface layer of the oriented electrical steel; in addition, magnetic powder scattered on the surface of the steel sheet but not attracted to the vicinity of the grain boundary by the leakage magnetic field seriously affects the judgment of the grain boundary. Therefore, the metallographic structure of the oriented silicon steel can be reconstructed only by adopting more advanced magnetic detection materials and methods, so that the nondestructive detection function of the metallographic structure is realized.
Through the above analysis, the problems and defects of the prior art are as follows: the magnetic powder which is scattered on the surface of the steel plate but not attracted to the vicinity of the grain boundary by the leakage magnetic field can seriously affect the judgment of the grain boundary, and the metallographic structure of the oriented silicon steel can be reconstructed only by adopting more advanced magnetic detection materials and methods, so that the nondestructive detection function of the metallographic structure is realized.
The difficulty in solving the above problems and defects is: firstly, the strength problem of a leakage magnetic field on a grain boundary needs to be solved, although the grain boundary is a defect, the size of the grain boundary is only 2-3 atomic distances generally, the leakage magnetic flux is less, the strength of the leakage magnetic field is very small, and in order to improve the reconstruction effect of the grain boundary, a very strong external magnetic field needs to be obtained so as to ensure that a strong leakage magnetic field can be generated at the grain boundary; and secondly, a material with better development effect must be found, and perfect reconstruction of the grain boundary is realized through a stronger leakage magnetic field.
The significance of solving the problems and the defects is as follows: if the reconstruction of the iron-based large-size grain boundary is realized under the condition of not damaging the product, on one hand, the labor intensity of detection personnel can be greatly reduced, and the harm of using concentrated acid to the body is reduced; on the other hand, the on-line detection of the metallographic structure of the large-grain iron-based sheet can be realized by combining the image acquisition and processing technology, and the method has an important guiding function on actual production. In addition, the technology is also suitable for detecting and imaging the defects of the iron-based material, and a simple, visual and reliable 3D imaging new method is provided for the magnetic flux leakage detection technology.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a nondestructive testing method, a nondestructive testing system and a nondestructive testing medium for a metallographic structure of a large-grain iron-based thin plate.
The invention is realized in this way, a nondestructive testing method for large-grain iron-based sheet metallographic structure comprises the following steps: by adopting an external magnetic field to the large-grain iron-based thin plate, a leakage magnetic field is generated near a grain boundary, magnetic suspension liquid packaged in a closed container is close to the leakage magnetic field, nano magnetic particles in the magnetic suspension liquid are gathered near the leakage magnetic field, large-grain boundary reconstruction is realized, and the purpose of metallographic structure nondestructive detection is achieved.
Further, when the magnetic suspension in the closed container is not applied with a leakage magnetic field, the magnetic particles are uniformly dispersed in the liquid medium, and the magnetic suspension is in a transparent state; after the magnetic leakage field is applied, under the action of magnetic force, magnetic particles near the magnetic leakage field can be rapidly gathered to form a visual image; after the leakage magnetic field is removed, the gathered magnetic particles can be rapidly dispersed in the liquid medium and become transparent again.
Further, the preparation method of the magnetic suspension comprises the following steps:
1) selection of magnetic particles: selecting magnetic material easy to be made into nano gradeThe material can be metal oxide (Fe) with nanometer level as the required magnetic particle3O4) Ferrite (CoFe)2O4、MnFe2O4、ZnFe2O4Etc.), metals (iron, cobalt, nickel, etc.), ferromagnetic iron nitride (Fe)xN(2<x<8))。
2) Size control of magnetic particles: the size of the magnetic particles is controlled to be in a nanometer level (10-100 nm), and after the magnetic particles are dispersed in the molten liquid, the magnetic suspension liquid observed by human eyes is in a transparent state; if the size of the magnetic particles exceeds 100nm, granular substances visible to human eyes are easily formed, on one hand, the moving speed of the magnetic particles is influenced, and the detection speed of the system is slowed; on the other hand, large particulate matter also affects the quality of the reconstructed image, causing unwanted dots or lines to appear in the reconstructed image. When the magnetic particles are smaller than 10nm, the magnetic saturation intensity is low, the manufacturing difficulty is high, the stability is poor and the production cost is high.
3) Concentration control of magnetic particles: the magnetic particles have different magnetic properties, and the concentration of the magnetic particles is different, but should be controlled within the range of 1-50 g/L. The concentration of the magnetic suspension liquid exceeding 50g/L can enable the magnetic suspension liquid to present a certain color instead of a transparent state, so that a visual image cannot be formed after the magnetic particles are rapidly aggregated, and the formed image cannot be clearly distinguished because the color of the formed image is the same as that of the original magnetic suspension liquid. However, the concentration of the magnetic particles cannot be too low, and less than 1g/L may result in too small amount of the magnetic particles being collected, resulting in unclear and incomplete displayed images, or greatly increasing the developing time required to display a clear and complete image.
4) Selection of base liquid: and selecting a proper base solution according to the magnetic particles and the surfactant, wherein the viscosity of the base solution is controlled to be 0.0050-0.0200 Pa.s. When the viscosity of the base liquid exceeds 0.0200Pa.s, the moving speed of the magnetic particles is slow, and the developing time is too long; when the viscosity of the base solution is less than 0.0050pa.s, the magnetic particles are not suspended for a long time and settle. The optional base fluids include: kerosene, synthetic oil, toluene, and the like.
5) Selection of surfactant: the surfactant is required to have a special molecular structure, one end of which has high affinity for magnetic particles, and the other end of which is easily dispersed in the base liquid. The proper surfactant is selected according to the types of the base liquid and the magnetic particles, the interface state of a solution system can be obviously changed by adding a small amount of surfactant, and the main function of the surfactant is to ensure that the nano-scale magnetic particles are in a dispersed state in the base liquid without aggregation under the condition of no magnetic leakage field. Meanwhile, the movement of the nano-scale magnetic particles under the condition of a certain intensity leakage magnetic field cannot be hindered, and the fast and efficient development is ensured. Optional surfactants include: oleic acid, polybutadiene succinimides, polyamines, and the like.
Further, the method of applying an external magnetic field to the large-grain iron-based thin plate to generate a leakage magnetic field near a grain boundary specifically includes: the magnetic flux is parallel to the surface of the material, almost no magnetic flux line penetrates out of the surface, the detected surface has no magnetic field, when the material has a grain boundary cutting magnetic lines, the grain boundary can change the magnetic permeability, the magnetic permeability of the grain boundary is very small, the magnetic resistance is very large, the magnetic flux in the magnetic circuit is distorted, the magnetic flux line can change the operation path, and besides a part of the magnetic flux directly passes through the grain boundary, a part of the magnetic flux can leave the material. Because the grain boundary defect is only 2-3 atomic distances, the generated leakage magnetic is less, the strength of the leakage magnetic field is very small, and in order to improve the reconstruction effect of the grain boundary, a very strong external magnetic field must be obtained to ensure that a strong leakage magnetic field is generated at the grain boundary. The magnetic induction intensity and the leakage magnetic field intensity generated by the external magnetic field are shown in figure 2.
As can be seen from fig. 2, the magnetic induction of the external magnetic field and the magnetic yoke spacing have a large influence on the magnetic induction of the leakage magnetic field. Under the condition that the distances between the magnetic yokes are equal, the magnetic induction intensity of an external magnetic field is higher, the magnetic induction intensity of a leakage magnetic field generated on the surface of the large-grain iron-based thin plate sample is higher, and otherwise, the magnetic induction intensity is lower; under the condition that the magnetic induction intensity of an external magnetic field is equal, the smaller the distance between the magnetic yokes is, the higher the magnetic induction intensity of a leakage magnetic field generated on the surface of the large-grain iron-based thin plate sample is, and vice versa.
The distance between the magnetic yokes should be controlled within the range of 300-1200 mm, and when the distance between the magnetic yokes of the U-shaped electromagnet is smaller than 300mm, a leakage magnetic field generated between the two magnetic yokes can generate magnetic force on the magnetic particles, so that the aggregation and formation of the magnetic particles along a crystal boundary are influenced; when the distance between the magnetic yokes exceeds 1200mm, the width of the large-grain iron-based material to be detected must exceed 1200mm, and the width of the produced large-grain iron-based material does not exceed 1200mm generally. The magnetic flux leakage field can generate larger magnetic force to the magnetic particles only when the magnetic induction intensity exceeds 20 mT. When the distance between the magnetic yokes is 300mm, 500mm, 8000mm, 1000mm and 1200mm, the minimum values of the magnetic induction intensity of the external magnetic field with the magnetic induction intensity of the leakage magnetic field exceeding 20mT are 1.27T, 1.40T, 1.55T, 1.72T and 1.87T respectively. The magnetic suspension liquid encapsulated in the closed container is close to a leakage magnetic field, and the nano magnetic particles in the magnetic suspension liquid rapidly move to the grain boundary position of the leakage magnetic field under the action of the leakage magnetic field, so that the grain boundary can be rapidly reconstructed. The larger the magnetic induction intensity of the external magnetic field is, the larger the magnetic induction intensity of the leakage magnetic field is, the larger the stress of the magnetic particles is, the faster the magnetic particles move in the suspension, and the shorter the time for reconstructing the grain boundary is.
It is a further object of the invention to provide a computer device comprising a memory and a processor, the memory storing a computer program which, when executed by the processor, causes the processor to perform the steps of: by applying an external magnetic field to the large-grain iron-based thin plate, a leakage magnetic field is generated near the grain boundary,
it is another object of the present invention to provide a computer-readable storage medium storing a computer program which, when executed by a processor, causes the processor to perform the steps of: by adopting an external magnetic field to the large-grain iron-based thin plate, a leakage magnetic field is generated near a grain boundary, the magnetic suspension liquid packaged in the closed container is close to the leakage magnetic field, and the nano magnetic particles in the magnetic suspension liquid are gathered near the leakage magnetic field, so that the reconstruction of the large-grain boundary is realized, and the nondestructive detection of a metallographic structure is achieved.
Another object of the present invention is to provide a nondestructive testing system of a large-grained iron-based sheet metallographic structure, which operates the nondestructive testing method of a large-grained iron-based sheet metallographic structure, the nondestructive testing system of a large-grained iron-based sheet metallographic structure including:
the leakage magnetic field generating module is used for generating a leakage magnetic field near a crystal boundary by applying an external magnetic field to the large-grain iron-based thin plate;
and the large-grain boundary reconstruction module is used for enabling the magnetic suspension liquid packaged in the closed container to approach a leakage magnetic field, and enabling the nano magnetic particles in the magnetic suspension liquid to be gathered near the leakage magnetic field, so that large-grain boundary reconstruction is realized, and the nondestructive detection of a metallographic structure is achieved.
Another object of the present invention is to provide a magnetic flux leakage testing system, which operates the nondestructive testing method for the metallographic structure of the large-grain iron-based thin plate.
By combining all the technical schemes, the invention has the advantages and positive effects that: the method realizes the reconstruction of the large-grain crystal boundary by adopting a special magnetic suspension liquid method for the large-grain iron-based sheet, achieves the purpose of nondestructive detection of metallographic structures, reduces the labor intensity and improves the detection efficiency.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments of the present application will be briefly described below, and it is obvious that the drawings described below are only some embodiments of the present application, and it is obvious for those skilled in the art that other drawings can be obtained from the drawings without creative efforts.
FIG. 1(a) is a schematic view of a material provided by an embodiment of the present invention without grain boundaries; (b) the material provided by the embodiment of the invention has a grain boundary, and the magnetic leakage detection by the magnetic suspension liquid is schematically shown.
Fig. 2 is a schematic diagram of a relationship between the magnetic induction intensity of the external magnetic field and the magnetic induction intensity of the leakage magnetic field according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Aiming at the problems in the prior art, the invention provides a nondestructive testing method, a nondestructive testing system and a nondestructive testing medium for a metallographic structure of a large-grain iron-based sheet, and the invention is described in detail with reference to the attached drawings.
The nondestructive testing method for the metallographic structure of the large-grain iron-based sheet provided by the embodiment of the invention comprises the following steps: by adopting an external magnetic field to the large-grain iron-based thin plate, a leakage magnetic field is generated near a grain boundary, magnetic suspension liquid packaged in a closed container is close to the leakage magnetic field, nano magnetic particles in the magnetic suspension liquid are gathered near the leakage magnetic field, large-grain boundary reconstruction is realized, and the purpose of metallographic structure nondestructive detection is achieved. Specific examples and comparative examples are shown in table 1.
TABLE 1 specific examples and comparative examples
The special magnetic suspension in the embodiment of the invention is different from the common magnetic suspension in the market, and the grain boundary can be displayed only by meeting the following 3 conditions:
1) when no leakage magnetic field is applied, the magnetic particles are uniformly dispersed in the liquid medium, and the magnetic suspension is in a transparent state;
2) after the magnetic leakage field is applied, under the action of magnetic force, magnetic particles near the magnetic leakage field can be rapidly gathered to form a visual image;
3) after the leakage magnetic field is removed, the gathered magnetic particles can be rapidly dispersed in the liquid medium and become transparent again.
The key to achieving the above three conditions is the selection of magnetic particles, the size and concentration control of the magnetic particles, and the selection of base fluid and surfactant.
1) Selection of magnetic particles
In principle, all magnetic substances that can be made to nanoscale dimensions can be used as the magnetic particles of this special magnetic suspension. But must be capable of being suspended in a suitable base fluid. At the same time, it can be uniformly dispersed in the base liquid without aggregation by a suitable surfactant. Further, considering the magnetic strength of the magnetic particles, the better the magnetic properties of the magnetic particles, the easier it is to reconstruct a grain boundary image under a constant-strength leakage magnetic field, but the magnetic particles are also likely to be adsorbed to each other and to be aggregated, and are not likely to be uniformly dispersed in the base liquid, so that it is considered comprehensively and selected as appropriate. In addition, the manufacturing cost and complexity of the magnetic particles are also considered, and the lower the cost and the simpler the manufacturing method, the better.
2) Size control of magnetic particles
The size of the magnetic particles must be controlled at nanometer level, the nanometer magnetic particles cannot be distinguished by human eyes, and the magnetic suspension observed by human eyes can be in a transparent state after the magnetic suspension is dispersed in the molten liquid; if the size of the magnetic particles is too large, granular substances visible to human eyes are easily formed, on one hand, the moving speed of the magnetic particles is influenced, and the detection speed of the system is slowed; on the other hand, large particulate matter also affects the quality of the reconstructed image, causing unwanted dots or lines to appear in the reconstructed image.
3) Concentration control of magnetic particles
The concentration of the magnetic particles cannot be too high, the magnetic suspension liquid can show a certain color instead of being transparent due to the too high concentration, and a visible image cannot be formed after the magnetic particles are rapidly aggregated, because the formed image has the same color as the original magnetic suspension liquid and cannot be clearly distinguished. However, the concentration of the magnetic particles cannot be too low, and too low a concentration may result in too small an amount of the magnetic particles being collected, resulting in unclear and incomplete displayed images, or a development time required to display a clear and complete image may be greatly increased.
4) Selection of base fluid
It is necessary to select an appropriate base liquid depending on the magnetic particles and the surfactant. The viscosity of the base liquid cannot be too high, and the too high viscosity of the base liquid can slow the moving speed of the magnetic particles, so that the developing time is too long; the viscosity of the base liquid cannot be too low, and too low viscosity of the base liquid can cause the magnetic particles to sink due to suspension for a long time.
5) Selection of surfactants
The appropriate surfactant is selected according to the types of the base liquid and the magnetic particles, the interface state of a solution system can be obviously changed by adding a small amount of surfactant, and the main function of the surfactant is to ensure that the nano-scale magnetic particles are in a dispersed state in the base liquid without aggregation under the condition of no magnetic leakage field. Meanwhile, the movement of the nano-scale magnetic particles under the condition of a certain intensity leakage magnetic field cannot be hindered, and the fast and efficient development is ensured.
As shown in fig. 1(a) and 1(b), the nondestructive examination of the metallographic structure of the large-grain iron-based magnetically permeable sheet is shown. FIG. 1(a) material without grain boundaries FIG. 1(b) material with grain boundaries, magnetic suspension was used to examine the leakage flux. An external magnetic field is applied to the thin plate material through the U-shaped magnet, the thin plate is magnetized due to the fact that the thin plate is made of an iron-based material, if a detected part does not have a grain boundary, magnetic induction lines in the material are restrained in the material, magnetic flux is parallel to the surface of the material, almost no magnetic induction lines penetrate out of the surface, and the detected surface does not have a magnetic field. When a grain boundary cutting magnetic lines exists in the material, the grain boundary can change the magnetic permeability, the magnetic flux in a magnetic circuit is distorted due to the fact that the magnetic permeability of the grain boundary is small and the magnetic resistance is large, the magnetic flux can change a running path, a part of the magnetic flux can leave the surface of the material besides directly passing through the grain boundary, and the air bypasses the grain boundary and then enters the material again, so that a leakage magnetic field is formed at the grain boundary on the surface of the material. Fig. 2 is a schematic diagram of a relationship between the magnetic induction intensity of the external magnetic field and the magnetic induction intensity of the leakage magnetic field according to an embodiment of the present invention.
The above description is only for the purpose of illustrating the present invention and the appended claims are not to be construed as limiting the scope of the invention, which is intended to cover all modifications, equivalents and improvements that are within the spirit and scope of the invention as defined by the appended claims.
Claims (10)
1. The nondestructive testing method for the metallographic structure of the large-grain iron-based thin plate is characterized in that an external magnetic field is applied to the large-grain iron-based thin plate, a leakage magnetic field is generated near a grain boundary, a magnetic suspension liquid packaged in a closed container is close to the leakage magnetic field, and nano magnetic particles in the magnetic suspension liquid are gathered near the leakage magnetic field, so that the grain boundary reconstruction of the large-grain iron-based thin plate is realized, and the nondestructive testing of the metallographic structure is realized.
2. The method for nondestructive inspection of large-grained iron-based sheet metallographic structure according to claim 1, wherein magnetic particles are uniformly dispersed in a liquid medium in the absence of a leakage magnetic field from the magnetic suspension in the closed container, and the magnetic suspension is transparent; after the magnetic leakage field is applied, under the action of magnetic force, magnetic particles near the magnetic leakage field can be rapidly gathered to form a visual image; after the leakage magnetic field is removed, the gathered magnetic particles can be rapidly dispersed in the liquid medium and become transparent again.
3. The method for non-destructive testing of large-grained iron-based sheet metallographic structures according to claim 1, wherein the magnetic suspension is prepared by a method comprising:
1) selection of magnetic particles: the nano-scale magnetic material is selected as the particles with the required magnetic performance and is the nano-scale metal oxide Fe3O4Ferrites, metals, ferromagnetic iron nitrides;
2) size control of magnetic particles: the size of the magnetic particles is controlled to be 10-100 nm at a nanometer level, and after the magnetic particles are dispersed in the molten liquid, the magnetic suspension liquid observed by human eyes is in a transparent state; if the size of the magnetic particles exceeds 100nm, granular substances visible to human eyes are easily formed, on one hand, the moving speed of the magnetic particles is influenced, and the detection speed of the system is slowed; on the other hand, the quality of the reconstructed image is also influenced by large granular substances, so that redundant points or lines are displayed in the reconstructed image;
3) concentration control of magnetic particles: the concentration of the magnetic particles is different, but the concentration of the magnetic particles is controlled within the range of 1-50 g/L; the concentration of the magnetic suspension liquid exceeds 50g/L, so that the magnetic suspension liquid presents certain color;
4) selection of base liquid: selecting a proper base solution according to the magnetic particles and the surfactant, wherein the viscosity of the base solution is controlled to be 0.0050-0.0200 Pa.s, and when the viscosity of the base solution exceeds 0.0200Pa.s, the magnetic particles are slow in moving speed, so that the developing time is too long; when the viscosity of the base solution is lower than 0.0050Pa.s, the magnetic particles can not suspend for a long time and sink; the selected base liquid comprises: kerosene, synthetic oil, toluene;
5) selection of surfactant: the surfactant is required to have a special molecular structure, one end of the surfactant has high affinity to the magnetic particles, the other end of the surfactant is extremely easy to disperse in the base liquid, the appropriate surfactant is selected according to the types of the base liquid and the magnetic particles, the interface state of a solution system can be obviously changed by adding the surfactant, and the nanoscale magnetic particles are ensured to be in a dispersed state in the base liquid without aggregation under the condition of no magnetic leakage field; meanwhile, the movement of the nano-scale magnetic particles under the condition of a certain intensity leakage magnetic field cannot be hindered.
4. The method of claim 3, wherein the ferrite is CoFe2O4、MnFe2O4、ZnFe2O4(ii) a The ferromagnetic iron nitride being FexN，2<x<8。
5. The method for nondestructive testing of large-grained iron-based sheet metallographic structure according to claim 3, characterized in that the selected base solution comprises: kerosene, synthetic oil, toluene; selected surfactants include: oleic acid, polybutadiene succinimide, polyamine.
6. The method for nondestructive testing of metallographic structure of a large-grained iron-based sheet as claimed in claim 1, wherein applying an external magnetic field to the large-grained iron-based sheet to generate a leakage magnetic field near the grain boundary comprises: applying an external magnetic field to an iron-based material sheet material through a U-shaped magnet, magnetizing the sheet, if a detected part has no grain boundary, restraining magnetic induction lines in the material, wherein magnetic flux is parallel to the surface of the material, almost no magnetic induction lines penetrate out of the surface, and the detected surface has no magnetic field; the lines of magnetic induction change the path of travel, and in addition to a portion of the flux passing directly through the grain boundary, a portion of the flux leaves the material.
7. A computer device, characterized in that the computer device comprises a memory and a processor, the memory storing a computer program which, when executed by the processor, causes the processor to carry out the steps of: by adopting an external magnetic field to the large-grain iron-based thin plate, a leakage magnetic field is generated near a grain boundary, the magnetic suspension liquid packaged in the closed container is close to the leakage magnetic field, and the nano magnetic particles in the magnetic suspension liquid are gathered near the leakage magnetic field, so that the reconstruction of the large-grain boundary is realized, and the nondestructive detection of a metallographic structure is achieved.
8. A computer-readable storage medium storing a computer program which, when executed by a processor, causes the processor to perform the steps of: by adopting an external magnetic field to the large-grain iron-based thin plate, a leakage magnetic field is generated near a grain boundary, the magnetic suspension liquid packaged in the closed container is close to the leakage magnetic field, and the nano magnetic particles in the magnetic suspension liquid are gathered near the leakage magnetic field, so that the reconstruction of the large-grain boundary is realized, and the nondestructive detection of a metallographic structure is achieved.
9. A nondestructive testing system for a large-grain iron-based sheet metallographic structure, which operates the nondestructive testing method for a large-grain iron-based sheet metallographic structure according to any one of claims 1 to 6, characterized by comprising:
the leakage magnetic field generating module is used for generating a leakage magnetic field near a crystal boundary by applying an external magnetic field to the large-grain iron-based thin plate;
and the large-grain boundary reconstruction module is used for enabling the magnetic suspension liquid packaged in the closed container to approach a leakage magnetic field, and enabling the nano magnetic particles in the magnetic suspension liquid to be gathered near the leakage magnetic field, so that large-grain boundary reconstruction is realized, and the nondestructive detection of a metallographic structure is achieved.
10. A magnetic flux leakage detection system is characterized in that the magnetic flux leakage detection system operates the nondestructive detection method of the large-grain iron-based thin plate metallographic structure according to any one of claims 1 to 6.
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