CN111678933A - Method for analyzing influence of pulsed magnetic field treatment on microstructure of metal part - Google Patents
Method for analyzing influence of pulsed magnetic field treatment on microstructure of metal part Download PDFInfo
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Abstract
The invention discloses an analysis method for the influence of pulsed magnetic field treatment on the microstructure of a metal part, which changes the magnetic field intensity of one of the magnetic treatment parameters, performs microstructure observation on 20Cr2Ni4A gear steel samples with the same pulse magnetic field discharge frequency and the same sample size, and analyzes the change of the microstructure of the material under different magnetic field intensities. First, the material was tested for grain structure change with EBSD and the cause of grain structure change was analyzed. Then, the sample was tested by EDS, and the element type and content of the material were analyzed by a spectrum analysis chart. And finally, analyzing the change reasons of the grain structure and the element species content of the material under different field strengths. The scheme lays a foundation for the performance of magnetic field treatment of the more complicated 20Cr2Ni4A gear steel parts in the future by analyzing the change of the microstructure of the material under different magnetic field strengths.
Description
Technical Field
The invention relates to the technical field of material mechanics, in particular to an analysis method for the influence of pulsed magnetic field treatment on the microstructure of a metal part.
Background
The grain orientation and dislocation density inside the material are the main factors for evaluating the material properties. At present, there are many methods for changing the crystal grain orientation and dislocation density in the material, but there are crystal grain breakage, orientation disorder and dislocation disorder caused in the experimental process, which causes the external performance of the material or parts to be reduced, resulting in the service performance such as fatigue and abrasion to be greatly reduced. Magnetic treatment is used as a novel non-contact microstructure regulation and control technology, and the microstructure represented by a grain curve and a microscopic dislocation in a material can be regulated and controlled through the Lorentz force of a magnetic field. Improve the fatigue performance of the material.
The pulse magnetic field adjusts the characteristic parameters of the pulse magnetic field acting on the ferromagnetic material, so that the residual stress generated in the processing process of the material can be obviously reduced, and the fatigue performance of the material is improved. The process research shows that the service life of the material 55SiMnMo with lower carbon content can be obviously prolonged by pulse magnetic treatment (2.25T/5Hz), and the service life of the material 95CrMo with higher carbon content is improved to a limited extent; the corrosion rate of the low-carbon steel welding joint is obviously reduced. Meanwhile, when the direction of the magnetic field is perpendicular to the direction of the maximum main stress of the residual stress, the reduction of the residual stress is most obvious, and after the magnetoelectric composite treatment is carried out on the welding seam in the same direction, the stress reduction level is more obvious. The pulse magnetic treatment device is developed by Innovex corporation in America, so that the residual stress of the cutter is relaxed, and the service life of the cutter after treatment can be improved by 20-50%.
The research on the mechanism of the modification of the pulsed magnetic field material is still in the exploration stage, and the research on the aspect focuses on the micro scale and takes the coupling effect of crystal grains, dislocation and magnetic domains as the starting point. The level of stress induced in the material by the magnetic vibration in the magnetic treatment process is low, and the phenomenon of residual stress reduction after the magnetic treatment is difficult to explain by similar stress relaxation, so that the original grain boundary is moved due to the non-uniformity of magnetostriction on the grain scale at present, and the original grain boundary is rearranged after dislocation migration, so that the lattice distortion is reduced, and the second type of residual stress, namely the microscopic stress, is released, so that the residual stress on the macroscopic scale is reduced.
The existing gear manufacturing strengthening process is mainly divided into two types according to strengthening internal factors: phase change/modification strengthening and strain strengthening of the material.
Phase change/modification strengthening refers to that the whole gear or the strengthening layer material is subjected to a high-temperature cooling process, and a phase structure or hard particles with higher strength/hardness are generated through material phase change or introduction of strengthening elements. The process needs to go through a high-temperature phase change-rapid cooling process, and has the prominent problems that the original structure (such as forging streamline structure) is easy to damage, the internal stress is large due to uneven cooling, harmful phases are easy to generate due to improper temperature control, the deformation control difficulty is large, and the like. Although surface quenching (such as induction, laser, electron beam quenching, etc.) helps to improve deformation, the high temperature-rapid cooling method still causes problems of local stress concentration, low process control precision, etc.
The strain strengthening process means that the surface of the gear undergoes certain plastic deformation at room temperature, the yield strength is improved through strain strengthening, and appropriate compressive stress is introduced. The process effectively avoids the problem of phase change caused by temperature change, but has the problems of difficult effective control of the strengthening layer (shot blasting and ultrasonic shot blasting), low strengthening efficiency (laser shot blasting and ultrasonic extrusion), expensive equipment (laser shot blasting), poor roughness and the like.
The high-end manufacturing technology of the heavy-duty gear is always one of the key contents of domestic and foreign industries, military departments and research institutions, and a batch of new technologies are continuously developed and applied to engineering in recent 20 years, such as vacuum carburization, high-pressure gas quenching, laser shot blasting, double-frequency induction quenching and the like. The American Gear Manufacturing Association (AGMA)2010 north american heat treatment society (asmheattreatsocietymenting) proposed that the gear high-end manufacturing enhancement technology must satisfy the following characteristics: excellent microstructure is maintained or generated, stress evolution and final state are accurately controlled, deformation is minimized, and adaptability efficiency is achieved. Along with the rapid development of strong magnetic field technology and material technology in recent years, besides the traditional enhanced driving force of heat, force and the like, the pulse magnetic field provides a brand new method for the high-end manufacturing and strengthening of the gear, can effectively avoid the defects of the process, and has obvious advantages and great potential.
Disclosure of Invention
In view of the above, the invention provides an analysis method for the influence of pulse magnetic field treatment on the microstructure of a metal part, which is used for analyzing the change of the microstructure of a material under different magnetic field strengths and lays a foundation for the performance of magnetic field treatment on a more complicated 20Cr2Ni4A gear steel part in the future.
In order to achieve the purpose, the invention provides the following technical scheme:
a method for analyzing the influence of pulsed magnetic field treatment on the microstructure of a metal part comprises the following steps:
respectively carrying out magnetic treatment on a plurality of groups of samples with different field strengths;
respectively testing the microstructure data of the samples which are subjected to the strong magnetic treatment and are not subjected to the magnetic treatment in different fields; the microstructure data includes: the structure and the form of a grain structure, the element types and the element contents;
and analyzing the influence of different magnetic field strong magnetic treatment on the sample according to the microstructure data.
Preferably, the testing of the microstructure data of the samples which are respectively treated by the magnetic field and not treated by the magnetic field comprises the following steps:
EBSD is used for respectively testing the grain structure change of samples which are magnetically treated with different field strengths and are not magnetically treated;
and testing the samples subjected to the strong magnetic treatment and the samples not subjected to the magnetic treatment in different fields by using the EDS, and analyzing the element types and the element contents of the materials by using an energy spectrum analysis chart.
Preferably, before the EDS test, the method further comprises: and polishing the surface of the sample to be tested.
Preferably, the sample to be tested is electropolished prior to performing the EBSD test.
Preferably, the analyzing the influence of the different magnetic field strong magnetic treatment on the sample according to the microstructure data comprises:
and analyzing the change rule of the grain size according to the grain structure data.
Preferably, the analyzing the influence of the different magnetic field strong magnetic treatment on the sample according to the microstructure data comprises:
and analyzing the change rule of the nonmetallic inclusion.
Preferably, the performing the magnetic treatment with different field strengths on the plurality of groups of samples respectively includes:
all the tests are numbered and grouped, four different magnetic field strengths are used as grouping marks which are respectively 0T, 1T, 4T and 6T, and the number of times of pulse magnetic field discharge is 10.
Preferably, before the magnetic treatment with different field strengths is performed on the plurality of groups of samples, the method further comprises:
and carrying out a heat treatment process on the sample.
Preferably, before the magnetic treatment with different field strengths is performed on the plurality of groups of samples, the method further comprises: preparing a test sample;
the preparing a test sample comprises: the rectangular sample is processed by wire cutting from a raw 20Cr2Ni4A gear steel bar stock which is not subjected to a heat treatment process.
According to the technical scheme, on the basis of preliminarily exploring that the pulse magnetic field can reduce the size of the internal crystal grains of the material on a 20Cr2Ni4A pinion steel sample, the pulse magnetic field parameters are changed, the EBSD is used for testing the size of the internal crystal grains of the material under different magnetic field strengths, and the test result shows that the average size of the internal crystal grains of the material is reduced along with the increase of the pulse magnetic field strength and is mainly related to the nucleation rate of martensite of the material, and the driving force of martensite phase transformation of the material is larger when the pulse magnetic field strength is larger, so that the growth speed is reduced, and the average size of the crystal grains of the material is reduced;
when observing the microstructure in the material, the performance of the analysis material is influenced by the pulse magnetic field from the viewpoint of the change of the content and the type of the element of the material. The EDS tests show that the content and the type of elements in the 20Cr2Ni4A gear steel sample under four magnetic field strengths are not greatly different, but the content and the type of the elements under 0T field strength are different from those of the materials under 1T, 4T and 6T field strengths, namely the element change of the materials is not greatly related to the magnitude of the magnetic field strength, but the pulse magnetic field treatment is performed or not, the internal dislocation density of the materials is increased after the pulse magnetic field treatment, the size and the number of nonmetallic inclusions existing in the original sample are reduced, so that the yield strength of the 20Cr2Ni4A gear steel is improved, and the plasticity and toughness of the steel are improved due to the reduction of the S-phase inclusions.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
FIG. 1 is a schematic view of a thermal processing process provided by an embodiment of the present invention;
FIG. 2a is a grain structure at 0T according to an embodiment of the present invention;
FIG. 2b is a grain structure morphology at 1T according to an embodiment of the present invention;
FIG. 2c shows the grain structure at 4T according to an embodiment of the present invention;
FIG. 2d is a grain structure morphology at 6T according to an embodiment of the present invention;
FIG. 3a is a diagram of an energy spectrum analysis at 0T according to an embodiment of the present invention;
FIG. 3b is a graph of energy spectrum analysis at 1T according to an embodiment of the present invention;
FIG. 3c is a diagram of energy spectrum analysis at 4T according to an embodiment of the present invention;
FIG. 3d is a graph of energy spectrum analysis at 6T according to an embodiment of the present invention;
FIGS. 4a and 4b illustrate a dislocation structure prior to magnetic treatment provided by an embodiment of the present invention;
FIGS. 4c and 4d are dislocation structures after magnetic treatment as provided by embodiments of the present invention;
FIG. 5 shows XRD spectra before and after magnetic treatment provided by an embodiment of the present invention;
FIG. 6 illustrates dislocation density variations before and after processing provided by an embodiment of the present invention;
FIG. 7 is a graph showing the average fatigue life of samples at different magnetic field strengths provided by an embodiment of the present invention;
FIG. 8 is a schematic diagram of Frank-Read dislocation source mechanism.
Detailed Description
The invention provides an analysis method for the influence of pulse magnetic field treatment on the microstructure of a metal part, which changes the magnetic field intensity of one of the magnetic treatment parameters, performs microstructure observation on 20Cr2Ni4A gear steel samples with the same pulse magnetic field discharge frequency and the same sample size, and analyzes the change of the material microstructure under different magnetic field intensities. First, the material was tested for grain structure change with EBSD and the cause of grain structure change was analyzed. Then, the sample was tested by EDS, and the element type and content of the material were analyzed by a spectrum analysis chart. And finally, analyzing the change reasons of the grain structure and the element species content of the material under different field strengths.
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The method for analyzing the influence of the pulsed magnetic field treatment on the microstructure of the metal part, provided by the embodiment of the invention, comprises the following steps:
respectively carrying out magnetic treatment on a plurality of groups of samples with different field strengths;
respectively testing the microstructure data of the samples which are subjected to the strong magnetic treatment and are not subjected to the magnetic treatment in different fields; the microstructure data includes: the structure and the form of a grain structure, the element types and the element contents;
and analyzing the influence of different magnetic field strong magnetic treatment on the sample according to the microstructure data.
According to the technical scheme, the method for analyzing the influence of the pulse magnetic field treatment on the microstructure of the metal part, provided by the embodiment of the invention, is used for testing the grain structure of the material and the content and the type of elements in the material of the gear steel sample under different magnetic field strengths, and analyzing the change of the microstructure of the material under different magnetic field strengths, so that a foundation is laid for the later more complicated performance of the magnetic field treatment on the 20Cr2Ni4A gear steel part.
1.1 sample microstructure testing
1.1.1 test sample preparation
The samples tested in different field strengths are all cuboid samples with the length of 20mm, the width of 10mm and the height of 15mm which are processed by wire cutting from original 20Cr2Ni4A pinion steel bars without heat treatment process.
1.1.2 test protocol design
The protocol for microstructure testing of 20Cr2Ni4A pinion steel coupons at different magnetic field strengths was designed as follows.
First, all the samples were subjected to a heat treatment process, i.e., a thermal refining process, before the microstructure test, as shown in fig. 1, the purpose of the thermal refining was to obtain 20Cr2Ni4A gear steel samples with good overall mechanical properties.
Second, all trials are numbered and grouped. 24 quenched and tempered 20Cr2Ni4A gear steel samples in total are divided into two groups, each group comprises 12 samples, wherein the samples used for the EBSD test and the EDS test are respectively divided into 4 groups, each group comprises 3 samples, four different magnetic field strengths are used as grouping marks and are respectively 0T, 1T, 4T and 6T, and the pulse magnetic field discharge times are all 10 times.
And finally, polishing all samples by using a polishing machine before carrying out the crystal grain structure test and the element type and content test, so as to reduce the surface roughness of the samples, thereby ensuring that the surfaces of the samples are smooth and flat during the test.
1.2 EBSD grain structure change analysis of 20Cr2Ni4A gear steel samples with different field strengths
EBSD is short for electron back scattering diffraction, is widely applied to microstructure of materials, and can combine the microstructure with related analysis of crystallography to observe the tissue of the materials more accurately. Therefore, in order to observe the grain structure of 20Cr2Ni4A pinion steel samples with different magnetic field strengths, EBSD is used for observing the grain morphology of the material, and a transmission electron microscope is not used, because the transmission electron microscope is difficult to prepare samples, the test speed and the test accuracy are lower than those of electron back scattering diffraction, and the transmission electron microscope is more complicated than the electron back scattering diffraction.
Before the EBSD test, the sample to be tested is subjected to electrolytic polishing, so that interference factors are reduced during the test of the sample, and the result is more accurate. The results of the EBSD testing of the gear steel coupons at four field strengths 0T, 1T, 4T, 6T are shown in fig. 2a, 2b, 2c and 2 d. In all the crystal grains shown in the figure, the crystal grains of the same color represent the same crystal grain orientation, for example, blue represents the crystal grain orientation parallel to the (111) direction, green represents the crystal grain orientation parallel to the (101) direction, and red represents the crystal grain orientation parallel to the (001) direction.
As is apparent from fig. 2a, 2b, 2c and 2d, the grain structure and morphology did not vary greatly at the field strengths of 0T, 1T, 4T and 6T, but the average size of the grains at different field strengths was different as can be seen from the EBSD measurement results, wherein the average size of the grains at 0T was 2.098 μm, the average size of the grains at 1T was 1.816 μm, the average size of the grains at 4T was 1.603 μm and the average size of the grains at 6T was 1.268 μm. That is, the average size of the crystal grains of the 20Cr2Ni4A pinion steel sample after magnetic treatment is reduced along with the increase of the magnetic field intensity, wherein the average size of the crystal grains of the sample is the smallest when the magnetic field intensity is 6T.
The average size of the grains of the 20Cr2Ni4A pinion steel sample after magnetic treatment is smaller than that of the sample without the action of a magnetic field, because the martensite grains in the material are refined under the action of the magnetic field, the grains are crushed, and the grain boundary shows a tendency to increase. And the different grain sizes under different magnetic field strengths indicate that the variation conditions of the grains and the grain boundaries of different magnetic field strengths are different. The higher the magnetic field strength, the faster the magnetic field transforms austenite to martensite upon quench cooling of the material, i.e., the greater the driving force for martensite transformation, thereby increasing the nucleation rate of martensite and growing it more slowly, and hence the smaller the average size of the grains.
In conclusion, an increase in the magnetic field strength favors an increase in the nucleation rate of martensite, causing a reduction in the grain size of the material.
1.3 EDS elemental change analysis of 20Cr2Ni4A gear steel samples of different field strengths
Inside the material, all elements have the characteristic wavelength of X-rays belonging to the elements, the size of the characteristic wavelength of the X-rays of each element is determined by the characteristic energy released in the process of energy level transition, and an EDS (electron-discharge spectroscopy) analyzes the elements by utilizing the difference of the characteristic energy.
Therefore, in order to observe the change of element types and content of 20Cr2Ni4A gear steel samples after magnetic treatment, analyze the element content and the change of the types of non-metallic inclusions of materials under different magnetic field strengths, respectively carry out EDS tests on the samples under the field strengths of 0T, 1T, 4T and 6T, carry out the analysis of the element types and the element content, and carry out polishing treatment on the surfaces of the tested samples by using a polishing machine before the test so as to reduce the surface roughness of the samples and obtain a flat surface. The change of the element types and the contents of the nonmetallic inclusions of the gear steel samples at four magnetic field strengths are shown in fig. 3a, 3b, 3c and 3 d.
As can be seen from the material spectrum analysis diagrams of FIG. 3a, FIG. 3b, FIG. 3c and FIG. 3d under different magnetic field strengths, the content and species of the sample elements have certain differences under different magnetic field strengths. Under the field intensity of 0T, Fe and S are mainly contained in the material, the element content of S accounts for 35.9 percent, the element content of Fe accounts for 53.5 percent, and the element content of Cr accounts for 0.6 percent; under the field intensity of 1T, the interior of the material is mainly composed of Fe and Cr elements, the content of the Fe element accounts for 87.6%, and the content of the Cr element accounts for 3.3%; under the field intensity of 4T, the interior of the material also takes Fe and Cr as main elements, the content of the Fe element accounts for 89.9 percent, and the content of the Cr element accounts for 3.0 percent; under the field intensity of 6T, the interior of the material is mainly composed of Fe and Cr elements, the content of Fe accounts for 90.7%, and the content of Cr element accounts for 3.1%.
The test result shows that the types and the contents of the elements in the sample materials under different magnetic field strengths have certain differences, the content of Fe in the steel changes greatly at first and then changes slightly with the increase of the magnetic field strength, the content of Cr changes slightly, and the existence of the S element is a nonmetallic inclusion serving as sulfide. The reason for the increase of the content of the Fe element of the sample under the field intensity of 0T is the existence of non-metallic inclusions, and when the non-metallic inclusions mainly containing the S element are not added with a magnetic field, the content of the S-phase non-metallic inclusions is more, so that the fatigue property of the material is reduced. After the magnetization of the magnetic field, the number of the non-metallic inclusions is reduced, the ductility and toughness of the steel are improved, the content of Fe is increased, but the increase is only increased to a certain extent, and the increase speed of the content of Fe is reduced from 1T magnetic field intensity to 6T magnetic field intensity.
Therefore, the influence of the pulsed magnetic field strength on the element change is small, but the element change is greatly related to the existence or nonexistence of the pulsed magnetic field treatment on the material, namely when the pulsed magnetic field is increased to a certain strength, the element type in the material is not influenced, but the number of the inclusions in the material is less, because the dislocation density in the material is increased under the action of the magnetic field, so that the sizes of the inclusions are smaller, and the yield strength and the fatigue performance in the material are improved.
In summary, the EBSD test and the EDS test are carried out on the grain structure of the material and the element content and the element type of the non-metallic inclusions in the material of the 20Cr2Ni4A pinion steel sample under different magnetic field strengths by aiming at the pulsed magnetic field. After testing, the following conclusions were reached:
(1) on the basis of preliminarily exploring that the pulsed magnetic field can reduce the size of the internal crystal grains of the material on a 20Cr2Ni4A pinion steel sample, the parameters of the pulsed magnetic field are changed, and the EBSD tests show that the size of the internal crystal grains of the material under different magnetic field strengths are large, and the test result shows that the average size of the internal crystal grains of the material is reduced along with the increase of the pulsed magnetic field strength, mainly related to the nucleation rate of martensite of the material, the larger the pulsed magnetic field strength is, the larger the driving force of martensite phase transformation of the material is, so that the growth speed is reduced, and the average size of the crystal grains of the.
(2) When observing the microstructure in the material, the performance of the analysis material is influenced by the pulse magnetic field from the viewpoint of the change of the content and the type of the element of the material. The EDS tests show that the content and the type of elements in the 20Cr2Ni4A gear steel sample under four magnetic field strengths are not greatly different, but the content and the type of the elements under 0T field strength are different from those of the materials under 1T, 4T and 6T field strengths, namely the element change of the materials is not greatly related to the magnitude of the magnetic field strength, but the pulse magnetic field treatment is performed or not, the internal dislocation density of the materials is increased after the pulse magnetic field treatment, the size and the number of nonmetallic inclusions existing in the original sample are reduced, so that the yield strength of the 20Cr2Ni4A gear steel is improved, and the plasticity and toughness of the steel are improved due to the reduction of the S-phase inclusions.
In addition, this scheme still includes:
2.1TEM dislocation texture Change
The grain refinement of the material after magnetic treatment is due to dislocation slip, propagation and entanglement within the material. Dislocations are a typical line defect that is actually a boundary between the slip and non-slip regions of the crystal and which has a large effect on the mechanical properties of the material.
Therefore, the dislocation texture change is observed herein using the transmitted electrons on the sample before and after the magnetic treatment. Moreover, because the TEM test requires that the electron beam can pass through the sample, the sample preparation process is complicated, and the sample is required to be thin, so in the experimental scheme before the test, the number of 0T samples and 6T samples is larger than that of the previous test, and finally the most suitable sample is found out and observed under the transmission electron microscope.
The TEM results are shown in fig. 4a, 4b, 4c and 4d, and it is evident from the graphs that the 20Cr2Ni4A pinion steel sample before magnetic treatment has a lath martensite structure in the surface layer under the TEM, and the dislocation density is low overall, the dislocation morphology is single, and there is no obvious dislocation entanglement and dislocation product. However, the 20Cr2Ni4A pinion steel sample after magnetic treatment has obvious dislocation entanglement phenomenon, the dislocation density of the sample is increased, a large amount of dislocations exist in the material, and the dislocation slip and dislocation entanglement phenomenon is obvious.
The explanation for dislocation propagation can be based on the frank-reed dislocation source mechanism, i.e. when the material is subjected to a magnetic field, the force of the dislocations under the action of the magnetic field can overcome the force caused by the dislocation line tension, i.e. the applied magnetic field can drive the dislocation source inside the material, thus causing the dislocations to multiply.
The influence mechanism of the pulse magnetic field on the dislocation structure, which is one of the factors influencing the residual stress of the 20Cr2Ni4A gear steel sample, can be explained as that under the action of the pulse magnetic field, new distortion power is given to crystal lattices in the crystal, so that under the impact of the continuous magnetic field on the material, the original equilibrium state of the crystal lattices is broken, and a certain degree of lattice distortion occurs in the material, so that a new lattice defect, namely dislocation, is generated in the material, and the dislocation gradually proliferates and slips along with the continuous development of the dislocation, the dislocation entanglement phenomenon is obvious, the density of dislocation cells in the material is increased, and under the condition, the residual stress of the material is reduced, the mechanical property is improved, and the material is strengthened. Namely, the strengthening mechanism of the pulsed magnetic field strengthened 20Cr2Ni4A pinion steel sample is supported by the theory of dislocation strengthening.
2.2 XRD dislocation Density Change
The dislocation structure can observe that the sample is obviously dislocation tangled after being magnetically treated under a transmission electron microscope, but the dislocation density change before and after the magnetic treatment can not be intuitively observed through data, so that the section tests the 20Cr2Ni4A pinion steel sample before and after the magnetic treatment through an X-ray diffractometer to obtain XRD (X-ray diffraction) spectrums before and after the magnetic treatment, calculates the dislocation density of the sample through a formula, and more intuitively represents the dislocation density change of the sample after the magnetic treatment.
The XRD spectra before and after the magnetic treatment are shown in FIG. 5. It can be seen from the figure that the change in the position of the diffraction peak of the sample after the magnetic treatment is not significant, that is, no new phase is generated in the 20Cr2Ni4A pinion steel sample after the magnetic treatment, but the full width at half maximum, FWHM, of the diffraction peak related to the dislocation density is changed.
Therefore, in order to examine the change in dislocation density of the gear steel sample after the magnetic treatment, the full widths at half maximum of 3 diffraction peaks corresponding to 3 crystal planes (110), (220), and (200) were formulated. The dislocation density before and after magnetic treatment was calculated and analyzed by Dunn's formula, as shown in equation (2.1).
In the formula, beta represents the full width at half maximum of a diffraction peak, b represents a Burgers vector, and D represents dislocation density.
The FWHM value can be obtained by an integration method, that is, when the full width at half maximum of a diffraction peak corresponding to a certain crystal plane is obtained, a tangent is taken to the bottom of the diffraction peak, and the area of a triangle formed by the tangent and the full width at half maximum is divided by the height of the triangle to obtain the full width at half maximum.
The Burgers vector is related to the material properties of the sample, and the material of the sample before and after magnetic treatment is necessarily the same, so the Burgers vector can be regarded as a constant, namely the dislocation density of the material is proportional to the square of FWHM. Therefore, the calculation analysis of the dislocation density of the material can be realized by using the full width at half maximum of a diffraction peak in an XRD spectrogram.
The change in the full width at half maximum of the diffraction peak corresponding to 3 crystal planes before and after the magnetic treatment was calculated and arranged as shown in fig. 6. It is obvious from the figure that the dislocation density of the 20Cr2Ni4A pinion steel sample after magnetic treatment is increased, and the FWHM of the corresponding diffraction peak at the (220) crystal plane is changed most, i.e., the dislocation density thereof is changed most remarkably.
2.3 three-point bending fatigue test results
20Cr2Ni4A pinion steel specimens were carburized and pulsed magnetic field treated, and then placed in a fatigue testing machine to be tested, and the bending fatigue life data of the tested specimens are shown in Table 1, and the average fatigue life at 5 magnetic field strengths is shown in FIG. 7. As can be seen from the figure, the fatigue life of the 20Cr2Ni4A pinion steel sample after magnetic treatment is higher than that of the sample without magnetic treatment, the fatigue life of the sample is increased along with the increase of the magnetic field intensity, but the change size is not more than one order of magnitude and can be ignored because the data of the fatigue test are more discrete, but obviously, the fatigue life value of the 20Cr2Ni4A pinion steel sample is the largest at the field intensity of 6T.
TABLE 1 flexural fatigue life data of 20Cr2Ni4A pinion steel specimens of different field strengths
As can be seen from Table 1, the fatigue lives of 6 samples of 0T 20Cr2Ni4A pinion steel are all about 1 ten thousand cycles and can not exceed 5 ten thousand cycles at most, and the samples with the fatigue lives of about 5 ten thousand cycles exist under the field strengths of 1T, 2T and 4T, because the number of the samples for testing the fatigue lives is small, and the fatigue life of the samples subjected to magnetic treatment is not improved to a new order of magnitude, but the fatigue life of the samples under the field strength of 6T reaches 14 ten thousand cycles, namely, the bending fatigue life of the 20Cr2Ni4A pinion steel sample under the field strength of 6T is obviously improved compared with that of the samples before magnetic treatment. It is also apparent from fig. 7 that the fatigue life of the test piece increases with increasing magnetic field strength, with the fatigue life value of the test piece being at a maximum at 6T field strength.
The fatigue life data of the samples under different field strengths show that the magnetic treatment has a certain influence on the fatigue performance of the 20Cr2Ni4A pinion steel sample, and the fatigue performance of the sample reaches the optimal state under the condition that the field strength is 6T.
Therefore, the fatigue strength of the specimen after the magnetic treatment at 6T field strength and the specimen without the magnetic treatment were compared. The fatigue life test was performed on the sample before magnetic treatment and the sample at 6T field strength on a three-point bending fatigue testing machine at 4 different stress levels, respectively, and when the sample broke, the fatigue testing machine stopped the test, at which time the fatigue life data at the different stress levels were recorded, as shown in tables 2 and 3.
TABLE 2 bending fatigue life data of 20Cr2Ni4A gear steel samples under different stresses under 0T field strength
TABLE 3 bending fatigue life data of 20Cr2Ni4A gear steel samples under different stresses under 6T field strength
As can be seen from Table 2, the fatigue life of the 20Cr2Ni4A pinion steel sample under the field strength of 0T is also changed under four different stress levels, the fatigue life value is reduced along with the increase of the stress value, and the fatigue life value is reduced from the maximum life value of 789021 cycles to the minimum life of 11050 cycles; as can be seen from Table 3, the life of the 20Cr2Ni4A pinion steel specimen at 6T field strength was 1679893 cycles at a maximum stress of 316MPa, which is about 80 ten thousand cycles higher than that of the specimen at 0T field strength under the same stress condition.
The force of the magnetic field acting on the dislocation in the material is related to the strength of the magnetic field, the direction of the magnetic field and the directions of the easy magnetization axes of two adjacent crystal planes. In fact, the force of the magnetic field acting on the internal dislocations of the material is the driving force of the magnetic field on the internal dislocations of the material, this force being caused by the magnetoplastic deformation of the material. According to the Frank-Read dislocation source mechanism, as shown in fig. 8, when the force of the magnetic field acting on the dislocations is greater than the resistance force caused by the tension of the dislocation lines, the dislocations start to return to the original state from the innermost black line to the outermost black line, and the process is repeated to continuously generate dislocation multiplication. Therefore, the 20Cr2Ni4A pinion steel sample is subjected to magnetic field treatment, and then the fatigue performance is remarkably improved under the action of a dislocation strengthening mechanism.
The scheme is applied to the metal crystal grain orientation and dislocation regulation and control technology, and has the following advantages:
the novel material structure performance regulation and control technology does not need to contact the surface of a part, and reduces surface damage and initiation;
secondly, the time is short and the speed is high;
thirdly, the crystal grain orientation is obvious and the dislocation is uniform.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (9)
1. A method for analyzing the influence of pulsed magnetic field treatment on the microstructure of a metal part is characterized by comprising the following steps:
respectively carrying out magnetic treatment on a plurality of groups of samples with different field strengths;
respectively testing the microstructure data of the samples which are subjected to the strong magnetic treatment and are not subjected to the magnetic treatment in different fields; the microstructure data includes: the structure and the form of a grain structure, the element types and the element contents;
and analyzing the influence of different magnetic field strong magnetic treatment on the sample according to the microstructure data.
2. The method of claim 1, wherein said separately testing the microstructure data of different field magnetically treated and non-magnetically treated samples comprises:
EBSD is used for respectively testing the grain structure change of samples which are magnetically treated with different field strengths and are not magnetically treated;
and testing the samples subjected to the strong magnetic treatment and the samples not subjected to the magnetic treatment in different fields by using the EDS, and analyzing the element types and the element contents of the materials by using an energy spectrum analysis chart.
3. The analytical method of claim 2, further comprising, prior to the EDS test: and polishing the surface of the sample to be tested.
4. The analytical method of claim 2, wherein the sample to be tested is electropolished prior to performing the EBSD test.
5. The method of claim 1, wherein analyzing the effect of different field intensity magnetic treatment on the sample based on the microstructure data comprises:
and analyzing the change rule of the grain size according to the grain structure data.
6. The method of claim 1, wherein analyzing the effect of different field intensity magnetic treatment on the sample based on the microstructure data comprises:
and analyzing the change rule of the nonmetallic inclusion.
7. The analysis method according to claim 1, wherein said performing magnetic treatment of different field strengths on a plurality of sets of samples respectively comprises:
all the tests are numbered and grouped, four different magnetic field strengths are used as grouping marks which are respectively 0T, 1T, 4T and 6T, and the number of times of pulse magnetic field discharge is 10.
8. The analysis method according to claim 1, further comprising, before the magnetic treatment at different field strengths is performed on the plurality of sets of samples, respectively:
and carrying out a heat treatment process on the sample.
9. The analysis method according to claim 1, further comprising, before the magnetic treatment at different field strengths is performed on the plurality of sets of samples, respectively: preparing a test sample;
the preparing a test sample comprises: the rectangular sample is processed by wire cutting from a raw 20Cr2Ni4A gear steel bar stock which is not subjected to a heat treatment process.
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