A method for nondestructive determination of hardness distribution of a dual-phase gradient structure metal material
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- CHONGQING UNIV
- Filing Date
- 2023-08-09
- Publication Date
- 2026-06-19
Smart Images
Figure CN117074525B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the technical field of internal hardness testing of metals, and specifically to a non-destructive method for determining the hardness distribution of a two-phase gradient structure metallic material. Background Technology
[0002] In the production of metal workpieces, surface heat treatment is commonly used. This method mainly involves heating, holding, and cooling the surface layer of the workpiece to obtain a high-hardness surface with a gradient distribution structure from martensite (or bainite) to the matrix. This gradient distribution structure can significantly improve the comprehensive mechanical properties of metal workpieces, such as hardness and ultimate tensile strength. However, excessive heat treatment can lead to increased material brittleness, making it prone to cracks and fractures, affecting the service life and safety of the workpiece. In addition to employing more refined processing techniques to strictly control the hardening depth, accurately measuring the hardness distribution of the internal material of the metal workpiece to ensure that its performance indicators meet the usage requirements is also crucial.
[0003] In existing technologies, commonly used methods for measuring the hardness of metallic materials include destructive indentation, non-destructive ultrasonic testing, and eddy current testing. Indentation involves applying a load to an indenter of different shapes, causing it to penetrate the surface of the workpiece to be tested, and the hardness value is obtained based on the surface area or depth of the indentation. Ultrasonic testing utilizes the characteristic that the resonant frequency of an ultrasonic vibrator of a certain length varies with the degree of tightening of the free end of the vibrator, employing a comparative measurement method to inspect the surface hardness of the workpiece. Eddy current testing uses the principle of electromagnetic induction, first magnetizing the workpiece by passing it through an alternating magnetic field, and then evaluating the hardness value by measuring the magnetic field strength of the residual magnetic field on the workpiece surface. However, all of the above methods can only measure the surface hardness of the workpiece; to know its internal hardness distribution, the workpiece must be cut open along its cross-section. This undoubtedly leads to workpiece waste, makes it impossible to perform full inspection of parts before leaving the factory, and the testing process is time-consuming and labor-intensive. Summary of the Invention
[0004] In view of this, the present invention provides a non-destructive method for measuring the hardness distribution of a two-phase gradient structure metallic material, which can measure the hardness distribution of the material inside the metallic workpiece without damaging the workpiece.
[0005] To achieve the above objectives, the technical solution of the present invention is as follows:
[0006] A non-destructive method for determining the hardness distribution of a two-phase gradient structure metallic material, characterized by comprising the following steps:
[0007] S1: The grain size and hardness at the corresponding location of the reference specimen were measured using metallographic methods and a microhardness tester;
[0008] S2: Build a liquid immersion ultrasonic measurement system that supports signal acquisition and waveform data output, and obtain the ultrasonic backscatter signal of the reference specimen as a reference signal;
[0009] S3: Collect and export n sets of ultrasonic backscattered signals from the workpiece under test, and calculate their time-domain spatial variance curves. ;
[0010] Spatial variance curve for:
[0011] (2)
[0012] In the formula, The peak voltage of the first surface echo in the reference signal obtained in step S2 is denoted as n, and n is the number of signal groups derived in step S3. Let be the voltage value of the i-th backscattered wave at time t;
[0013] S4: Based on the measurement data from steps S1 and S2, the transducer parameters of the constructed measurement system, the ultrasonic sound field propagation characteristics, and the property function of the material under test, derive the theoretical time-domain ultrasonic backscattering model.
[0014] The original equations of the theoretical time-domain ultrasound backscattering model are as follows:
[0015] (3)
[0016] The above equation represents the source function and the receiver function. With the eighth-order tensor of the microstructure of quantized materials The double convolution is the source transducer. Starting from the material's internal microstructure, it is scattered once, passing through... Finally, it is received by the transducer. The integral of all possible energy states received;
[0017] S5: Theoretical Model Spatial variance curve of measured data By performing a fitting and matching process, the hardness variation curve with ultrasonic propagation time is obtained, and then it is converted into a hardness variation curve with depth.
[0018] Preferably, the reference specimen is made of the same material as the workpiece to be tested.
[0019] Preferably, in step S2, when acquiring the reference signal, the transducer should be perpendicular to the surface of the reference specimen, and the distance between the transducer and the reference specimen should be equal to its focal length. It is also required that the incident surface of the ultrasonic signal on the reference specimen should be a plane.
[0020] Preferably, in step S4, based on the Gaussian beam model and according to the composition of the measurement system, equation (3) can be quantified as follows:
[0021] (4)
[0022] In the formula, the subscripts S and R represent the source transducer parameters and the receiving transducer parameters, respectively. The first two lines of the formula characterize the parameters of the ultrasonic testing system. This represents the width of the Gaussian beam along the z-axis. These represent the densities of metallic materials and liquid media, respectively. These represent the transverse wave velocity in the material and the sound velocity in the liquid medium, respectively. These represent the transmission coefficients from the liquid medium to the material and from the material to the liquid medium, respectively. It is the reflection coefficient. It is a diffraction correction. It is the transducer angular frequency. These represent the incident angle and the refraction angle of the sound beam entering the material, respectively. The transducer pulse width, It is the attenuation coefficient of ultrasound in a liquid medium. It is the focal length of the transducer in the liquid medium. The first line represents the path distance of ultrasound in the liquid medium, and the third line represents the backscattering coefficient related to the internal microstructure of the material. It is the shape factor of the scattering grains, which expresses the orientation and shape variations of grains within the material. H represents the angle between the incident wave vector p and the scattered wave vector s, and H represents the local hardness. It is the inner product of the eighth-order covariance tensor; the remaining part represents the Gaussian sound beam of the transducer, where t is the sound beam propagation time, and the independent variable is... For transverse wave attenuation, X, Y, and Z are coordinate transformations related to the transducer position.
[0023] Preferably, in step S5, a series of hardness values are substituted into the monotonic variation range of hardness according to the theoretical model, and the results are first processed at each sampling time. Pick The corresponding hardness value is used as the initial hardness. Then, by iteratively reducing the matching error, the hardness change curve related to the ultrasonic propagation time was obtained. .
[0024] Preferably, in step S2, the immersion ultrasonic measurement system includes a water-immersion ultrasonic focusing probe, an ultrasonic pulse transmitter / receiver, a digital oscilloscope, and a three-dimensional displacement stage. The three-dimensional displacement stage is used to control the relative positional relationship between the water-immersion ultrasonic focusing probe and the workpiece, and to test and calibrate the focal length of the probe in water. Pulse width .
[0025] Preferably, the electromagnetic wave frequency of the water immersion ultrasonic focusing probe is 20MHz.
[0026] Preferably, in step S1, the reference specimen is first cut along the quenching depth direction using a cold cutting method. After rough grinding, fine grinding, polishing, and acid etching, the cross-section is photographed using a metallographic microscope from the edge of the reference specimen inwards at 5 mm increments of 0.2 mm. The average grain size is obtained according to the method of GB / T6394-2017. Subsequently, the corresponding hardness value is obtained at the same location using a micro Vickers hardness tester to calibrate the relationship between grain size and hardness.
[0027] (1)
[0028] In the formula, H0 is the hardness of a single crystal. These are material-related constants.
[0029] Preferably, in step S5, the physical quantities involved include the test system parameter liquid focal length. Pulse width Center frequency probe diameter Liquid density Velocity of sound in liquids Liquid attenuation Angle of incidence underwater acoustic distance Workpiece parameters: transverse wave velocity attenuation coefficient ,density Stiffness matrix parameters , , ;
[0030] First let
[0031] (12)
[0032] Then there is
[0033] (13)
[0034] Based on the root interval of H in equation (13), H is taken as [100, 1000]HV with a step size of 1, for a total of 900 points. Then, these points are substituted into equation (13) for each sampling time. The hardness value that minimizes the absolute value of the calculation result is taken as the initial hardness value for the inverse solution. ; Use Newton's iteration method to further obtain an exact solution, when the following conditions are met. The iteration terminates when the time is right, and finally based on...
[0035] (14)
[0036] Then the hardness variation curve with depth can be obtained. .
[0037] Compared with the prior art, the beneficial effects of the present invention are:
[0038] 1. This invention provides a non-destructive method for determining the hardness distribution of a two-phase gradient structure metallic material. Based on the fundamental theory of ultrasonic backscattering, a Gaussian acoustic beam model is used to quantify the sound field energy propagation state, deriving specific ultrasonic backscattering model equations. By combining the theoretical equations with the measured spatial variance curves through a combination of matched search and iterative calculation, the internal variation curve of the workpiece is obtained, improving calculation accuracy and shortening calculation time. The entire process effectively solves the problem that traditional methods for measuring internal hardness require prior workpiece destruction to measure the internal hardness distribution of metallic materials, reducing testing costs.
[0039] 2. It has the advantage of being easy to use, especially for workpieces with high performance requirements, it can perform full inspection before leaving the factory and effectively control product quality.
[0040] 3. The measurement results are accurate. This method can be used to measure the gradient hardness distribution and compare the measurement results with the results of direct measurement on the cross-section using a microhardness tester after cold cutting. The average error is 4.9%. Attached Figure Description
[0041] Figure 1 A flowchart of a non-destructive method for determining the hardness distribution of a two-phase gradient structure metallic material;
[0042] Figure 2 This is a comparison chart of the measured spatial variance of the ultrasonic backscattering signal and the calculated value of the theoretical ultrasonic backscattering model (where the short line is the measured spatial variance of the signal and the solid line is the calculated result of the theoretical ultrasonic backscattering model).
[0043] Figure 3 This is a comparison chart of the theoretical hardness distribution inside an example workpiece calculated using the method of the present invention and the actual hardness obtained by destructive measurement (where the dotted line represents the theoretically calculated hardness value, the solid line represents the fitted theoretical hardness curve, and the circled dots represent the actual hardness value obtained by destructive measurement). Detailed Implementation
[0044] The present invention will be further described below with reference to the embodiments and accompanying drawings.
[0045] This embodiment uses the measurement of the gradient hardness of 40Cr after induction hardening as an example for explanation. Please refer to... Figure 1 The specific operating steps are as follows:
[0046] Step S1: Take a specimen of the same material as the workpiece to be tested and with a flat surface as a reference specimen. The size of the reference specimen is... First, the workpiece is cut along the quenching depth direction using a cold cutting method. After rough grinding, fine grinding, polishing, and acid etching, the cross-section is photographed using a metallographic microscope, starting from the edge of the workpiece and moving inwards at 5 mm increments of 0.2 mm. The average grain size is then obtained according to the method in GB / T6394-2017. Subsequently, the corresponding hardness value is obtained at the same location using a micro Vickers hardness tester to calibrate the relationship between grain size and hardness.
[0047] The relationship between grain size and hardness is modeled as follows:
[0048] (1)
[0049] In the formula, H0 is the hardness of a single crystal. These are material-related constants.
[0050] In particular, step one need not be repeated for the same material.
[0051] Step S2: Construct a water immersion ultrasonic measurement system, including a 20MHz water immersion ultrasonic focusing probe, an ultrasonic pulse transmitter / receiver, a digital oscilloscope with waveform output function, and a three-dimensional displacement stage for controlling the relative position of the ultrasonic probe and the workpiece. Then, test and calibrate the focal length of the probe in water. Pulse width Then, using a reference specimen, the probe is controlled to be perpendicular to the specimen surface and the distance between them is equal to... Acquire a set of ultrasonic backscattered reference signals and extract the peak voltage of the first surface echo from the signals. .
[0052] Step S3: Replace the reference specimen in the testing system with the workpiece to be tested. The workpiece to be tested is a cylindrical part with a size of [missing information]. The probe position is adjusted by controlling the three-dimensional displacement stage so that the probe normal is located within the axial section of the workpiece under test, the probe focus is located inside the workpiece near the surface, and the ultrasonic signal incident angle is between the first and second critical angles of mode conversion (approximately 15-27 degrees in this embodiment). Then, the probe position is fixed, and the cylindrical surface of the workpiece is scanned by axial rotation and translation. Within a given range, 2500 sets of time-domain ultrasound backscatter signals were acquired and exported using a scan step size of 0.2 mm. The spatial variance curve of the measured signals was then calculated, with the following equation:
[0053] (2)
[0054] In the formula, The peak voltage of the first surface echo in the reference signal obtained in step two is given, and n=2500 is the number of derived ultrasonic signal groups. Let be the voltage value of the i-th backscattered wave at time t. The calculation result is as follows: Figure 2 As shown in the short to medium term.
[0055] Step S4: Since the spatial variance curve contains rich physical information such as the characteristics of the test system, the propagation characteristics of the ultrasonic field, and the material properties, a detailed ultrasonic backscattering model needs to be constructed to obtain the material hardness distribution. Based on the Gaussian beam model, the original equations of the backscattering model, which characterizes the entire transmission process of ultrasonic energy from emission to reception, are as follows:
[0056] (3)
[0057] Based on the specific circumstances of this example, it can be quantified as follows:
[0058] (4)
[0059] In the formula, the first row represents the parameters of the ultrasonic testing system. This represents the width of the Gaussian beam along the z-axis. These represent the densities of metallic materials and liquid media, respectively. These represent the transverse wave velocity in the material and the sound velocity in the liquid medium, respectively. These represent the transmission coefficients from the liquid medium to the material and from the material to the liquid medium, respectively. It is the reflection coefficient. It is a diffraction correction. It is the transducer angular frequency. These represent the incident angle and the refraction angle of the sound beam entering the material, respectively. It is the attenuation coefficient of ultrasound in a liquid medium. It is the focal length of the transducer in the liquid medium. This represents the path distance of ultrasound in a liquid medium. The left part of the second row characterizes the backscattering coefficient related to the material's internal microstructure. It is the shape factor of the scattering grains, which expresses the orientation and shape variations of grains within the material. H represents the angle between the incident wave vector p and the scattered wave vector s, and H represents the local hardness. It is the inner product of the eighth-order covariance tensor. The remaining part represents the Gaussian sound beam of the transducer, where t is the sound beam propagation time (independent variable). The transducer pulse width, For transverse wave attenuation, X, Y, and Z are coordinate transformations related to the transducer position.
[0060] The Gaussian beamwidth along the z-axis can be expressed as:
[0061] (5)
[0062] In the formula, the complex Gaussian beam parameters This can be expressed as:
[0063] (6)
[0064] In the formula, k f The wavenumber of ultrasound in water is denoted as . Let be the radius of curvature of the wavefront. The curvature of the workpiece surface within the ultrasonic incident plane. The workpiece surface curvature is the direction orthogonal to the incident surface and the medium interface.
[0065] Diffraction correction factor This can be expressed as:
[0066] (7)
[0067] In the formula Let be the i-th order Bessel function.
[0068] Shape factor of scattering grains This can be expressed as:
[0069] (8)
[0070] In the formula, P(s) is the probability that any vector s falls within a single grain, which can be approximated by an exponential function. Then there is
[0071] (9)
[0072] In the formula
[0073] Eighth-order covariance tensor inner product This can be expressed as:
[0074] (10)
[0075] In the formula < > Or This represents the average value of the grain orientation. Specifically, for this embodiment... The crystal system is cubic symmetric, and the above formula can be further expressed as:
[0076] (11)
[0077] In the formula c 11 c 12 c 44 These are the parameters of the elastic stiffness matrix of the single crystal of the material.
[0078] Step S5: Finally, based on the theoretical model... Spatial variance curve of measured data The material hardness distribution of the test specimen is obtained by inverse solving.
[0079] The physical quantities involved in step S5 include: test system parameters (liquid focal length) Pulse width Center frequency probe diameter Liquid density Velocity of sound in liquids Liquid attenuation Angle of incidence underwater acoustic distance ), workpiece parameters (transverse wave velocity) attenuation coefficient ,density Stiffness matrix parameters , , ).
[0080] First let
[0081] (12)
[0082] Then there is
[0083] (13)
[0084] Based on the root interval of H in equation (13), H is taken as [100, 1000]HV with a step size of 1, for a total of 900 points. Then, these points are substituted into equation (13) for each sampling time. The hardness value that minimizes the absolute value of the calculation result is taken as the initial hardness value for the inverse solution. To obtain an exact solution, Newton's iteration method is used, when the following conditions are met. The iteration terminates when the time is right, and finally based on...
[0085] (14)
[0086] Then the hardness variation curve of the workpiece under test with depth can be obtained. ,like Figure 3 As shown by the midpoint line.
[0087] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention. Those skilled in the art, under the guidance of the present invention, can make various similar representations without departing from the spirit and claims of the present invention, and such modifications all fall within the protection scope of the present invention.
Claims
1. A method for non-destructive determination of hardness distribution in a dual-phase gradient structured metal material, characterized in that, Includes the following steps: S1: The grain size and hardness at the corresponding location of the reference specimen were measured using metallographic methods and a microhardness tester; S2: Build a liquid immersion ultrasonic measurement system that supports signal acquisition and waveform data output, and obtain the ultrasonic backscatter signal of the reference specimen as a reference signal; S3: Collect and export the ultrasonic backscattering signals of n groups of workpieces to be tested to calculate the time-domain spatial variance curve thereof ; Spatial variance curve for: （2） In the formula, The peak voltage of the first surface echo in the reference signal obtained in step S2 is denoted as n, and n is the number of signal groups derived in step S3. Let be the voltage value of the i-th backscattered wave at time t; S4: Based on the measurement data from steps S1 and S2, the transducer parameters of the constructed measurement system, the ultrasonic sound field propagation characteristics, and the property function of the material under test, derive the theoretical time-domain ultrasonic backscattering model. The original equations of the theoretical time-domain ultrasound backscattering model are as follows: （3） The above equation represents the source function and the receiver function. With the eighth-order tensor of the microstructure of quantized materials The double convolution is the source transducer. Starting from the material's internal microstructure, it is scattered once, passing through... Finally, it is received by the transducer. The integral of all possible energy states received; Based on the Gaussian beam model and according to the composition of the measurement system, equation (3) can be quantified as: （4） In the formula, the subscripts S and R represent the source transducer parameters and the receiving transducer parameters, respectively. The first two lines of the formula characterize the parameters of the ultrasonic testing system. This represents the width of the Gaussian beam along the z-axis. These represent the densities of metallic materials and liquid media, respectively. These represent the transverse wave velocity in the material and the sound velocity in the liquid medium, respectively. These represent the transmission coefficients from the liquid medium to the material and from the material to the liquid medium, respectively. It is the reflection coefficient. It is a diffraction correction. It is the transducer angular frequency. These represent the incident angle and the refraction angle of the sound beam entering the material, respectively. The transducer pulse width, It is the attenuation coefficient of ultrasound in a liquid medium. It is the focal length of the transducer in the liquid medium. The first line represents the path distance of ultrasound in the liquid medium, and the third line represents the backscattering coefficient related to the internal microstructure of the material. It is the shape factor of the scattering grains, which expresses the orientation and shape variations of grains within the material. H represents the angle between the incident wave vector p and the scattered wave vector s, and H represents the local hardness. It is the inner product of the eighth-order covariance tensor; the remaining part represents the Gaussian sound beam of the transducer, where t is the sound beam propagation time, and the independent variable is... For transverse wave attenuation, X, Y, and Z are coordinate transformations involving the transducer position; S5: Theoretical Model Spatial variance curve of measured data By performing a fitting and matching process, the hardness variation curve with ultrasonic propagation time is obtained, and then it is converted into a hardness variation curve with depth.
2. The non-destructive testing method for hardness distribution of dual-phase gradient structure metallic materials according to claim 1, characterized in that: The reference specimen is made of the same material as the workpiece to be tested.
3. The non-destructive testing method for hardness distribution of dual-phase gradient structure metallic materials according to claim 1, characterized in that: In step S2, when acquiring the reference signal, the transducer should be perpendicular to the surface of the reference specimen, and the distance between the transducer and the reference specimen should be equal to its focal length. The incident surface of the ultrasonic signal on the reference specimen should be a plane.
4. The non-destructive testing method for hardness distribution of dual-phase gradient structure metallic materials according to claim 1, characterized in that: In step S5, a series of hardness values are substituted into the monotonic variation range of hardness according to the theoretical model, and the results are first processed at each sampling time. Pick The corresponding hardness value is used as the initial hardness. Then, by iteratively reducing the matching error, the hardness change curve related to the ultrasonic propagation time was obtained. .
5. The non-destructive testing method for hardness distribution of dual-phase gradient structure metallic materials according to claim 3, characterized in that: In step S2, the immersion ultrasonic measurement system includes a water-immersion ultrasonic focusing probe, an ultrasonic pulse transmitter / receiver, a digital oscilloscope, and a three-dimensional displacement stage. The three-dimensional displacement stage is used to control the relative positional relationship between the water-immersion ultrasonic focusing probe and the workpiece, and to test and calibrate the focal length of the probe in water. Pulse width .
6. The non-destructive testing method for hardness distribution of dual-phase gradient structure metallic materials according to claim 5, characterized in that: The electromagnetic wave frequency of the water immersion ultrasonic focusing probe is 20MHz.
7. The non-destructive testing method for hardness distribution of dual-phase gradient structure metallic materials according to claim 1, characterized in that: In step S1, the reference specimen is first cut along the quenching depth direction using a cold cutting method. After rough grinding, fine grinding, polishing, and acid etching, the cross-section is photographed using a metallographic microscope from the edge of the reference specimen inwards at 5 mm increments of 0.2 mm. The average grain size is obtained according to the method of GB / T6394-2017. Subsequently, the corresponding hardness value is obtained at the same location using a micro Vickers hardness tester to calibrate the relationship between grain size and hardness. （1） In the formula, H0 is the hardness of a single crystal. These are material-related constants.
8. The non-destructive testing method for hardness distribution of dual-phase gradient structure metallic materials according to claim 1, characterized in that: In step S5, the physical quantities involved include the test system parameter liquid focal length. Pulse width Center frequency probe diameter Liquid density Velocity of sound in liquids Liquid attenuation Angle of incidence underwater acoustic distance Workpiece parameters: transverse wave velocity attenuation coefficient ,density Stiffness matrix parameters , , ; First let （12） Then there is （13） Based on the root interval of H in equation (13), H is taken as [100, 1000]HV with a step size of 1, for a total of 900 points. Then, these points are substituted into equation (13) for each sampling time. The hardness value that minimizes the absolute value of the calculation result is taken as the initial hardness value for the inverse solution. ; Use Newton's iteration method to further obtain an exact solution, when the following conditions are met. The iteration terminates when the time is right, and finally based on... （14） Then the hardness variation curve with depth can be obtained. .