CN111766257A - Steel austenite grain boundary display method and steel austenite grain size evaluation method - Google Patents
Steel austenite grain boundary display method and steel austenite grain size evaluation method Download PDFInfo
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- 229910001566 austenite Inorganic materials 0.000 title claims abstract description 62
- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 55
- 239000010959 steel Substances 0.000 title claims abstract description 55
- 238000000034 method Methods 0.000 title claims abstract description 51
- 238000011156 evaluation Methods 0.000 title abstract description 7
- 239000000523 sample Substances 0.000 claims abstract description 145
- 238000010438 heat treatment Methods 0.000 claims abstract description 45
- 239000000110 cooling liquid Substances 0.000 claims abstract description 24
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims abstract description 16
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 16
- 238000005520 cutting process Methods 0.000 claims abstract description 16
- 238000005336 cracking Methods 0.000 claims abstract description 15
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 claims description 60
- 238000001816 cooling Methods 0.000 claims description 40
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- 238000012360 testing method Methods 0.000 claims description 6
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- 239000002826 coolant Substances 0.000 claims description 2
- 238000002360 preparation method Methods 0.000 abstract description 7
- 238000005260 corrosion Methods 0.000 abstract description 5
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- 230000000694 effects Effects 0.000 description 4
- OXNIZHLAWKMVMX-UHFFFAOYSA-N picric acid Chemical group OC1=C([N+]([O-])=O)C=C([N+]([O-])=O)C=C1[N+]([O-])=O OXNIZHLAWKMVMX-UHFFFAOYSA-N 0.000 description 4
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- 229910000639 Spring steel Inorganic materials 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
- G01N23/20—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by using diffraction of the radiation by the materials, e.g. for investigating crystal structure; by using scattering of the radiation by the materials, e.g. for investigating non-crystalline materials; by using reflection of the radiation by the materials
- G01N23/203—Measuring back scattering
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
- G01N23/20—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by using diffraction of the radiation by the materials, e.g. for investigating crystal structure; by using scattering of the radiation by the materials, e.g. for investigating non-crystalline materials; by using reflection of the radiation by the materials
- G01N23/20008—Constructional details of analysers, e.g. characterised by X-ray source, detector or optical system; Accessories therefor; Preparing specimens therefor
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2223/00—Investigating materials by wave or particle radiation
- G01N2223/05—Investigating materials by wave or particle radiation by diffraction, scatter or reflection
- G01N2223/053—Investigating materials by wave or particle radiation by diffraction, scatter or reflection back scatter
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- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2223/00—Investigating materials by wave or particle radiation
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Abstract
The application provides a steel austenite grain boundary display method and a steel austenite grain size evaluation method, and belongs to the technical field of steel structure analysis. The method for displaying the austenite grain boundary of the steel comprises the following steps: heating a sample to be tested with the carbon content of 0.95-1.10 wt% to a temperature higher than a preset critical temperature, preserving heat to enable the sample to be tested to be completely austenitized, and then putting the sample to be tested into cooling liquid within 5s to be cooled at a speed of not less than 150 ℃/s, so as to obtain the sample to be tested with surface cracks. And cutting the sample to be tested with the cracked surface to expose the fracture surface formed by cracking, and shooting the back scattering electron image of the fracture surface by using a back scattering electron probe. The method for evaluating the austenite grain size of the steel comprises the following steps: the back-scattered electron image is obtained by the display method. The backscattered electron image is evaluated for austenite grain size by using an area method or a cut-off point method. Metallographic phase sample preparation and corrosion treatment are not needed, and the austenite grain boundary line can be clearly displayed.
Description
Technical Field
The application relates to the technical field of steel structure analysis, in particular to a steel austenite grain boundary display method and a steel austenite grain size evaluation method.
Background
The grain size is a measure for representing the grain size, and the austenite grain size influences the size and the structure of a structure after the phase transformation of a subsequent heat treatment structure, and further influences the mechanical property of the steel, so that the austenite grain size is an important index for measuring the performance of the steel for the steel needing heat treatment subsequently. The metallographic method is used as a currently common austenite grain size detection method, and the existing metallographic method needs to heat steel to a certain temperature, then preserve heat for a certain time, then cool the steel to room temperature in a certain mode, then carry out metallographic sample preparation, namely, use a grinding machine to grind off a decarburized layer and then carry out fine grinding and polishing to prepare a metallographic sample, and then use a specific corrosive agent to display the prior austenite grain boundary.
At present, when the metallographic method is adopted for austenite grain size detection, grain boundaries and tissues are often displayed simultaneously due to the limitation of corrosive agents, so that the grain boundaries are difficult to distinguish, and the grain boundaries are difficult to show when a part of steel types such as Si-containing spring steel and the like are subjected to conventional corrosive agents, so that the observation and evaluation of the grain sizes are influenced. The current ideal crystal boundary display corrosive agent is a saturated picric acid corrosive agent, but picric acid belongs to flammable and explosive reagents, is difficult to purchase and has higher requirement on storage environment.
Disclosure of Invention
The purpose of the present application is to provide a method for displaying austenite grain boundaries of a steel material and a method for evaluating austenite grain size of a steel material, which do not require metallographic sample preparation and corrosion treatment and can clearly display austenite grain boundary boundaries.
The embodiment of the application is realized as follows:
in a first aspect, an embodiment of the present application provides a method for displaying austenite grain boundaries of a steel material, including:
heating a sample to be tested with the carbon content of 0.95-1.10 wt% to a temperature higher than the critical temperature by 60 ℃, preserving the heat to ensure that the sample to be tested is completely austenitized, then taking a NaCl aqueous solution as a cooling liquid, putting the sample to be tested into the cooling liquid within 5s, and cooling at the speed of not less than 150 ℃/s to obtain the sample to be tested with cracked surface.
And cutting the sample to be tested with the cracked surface to expose the fracture surface formed by cracking, and shooting the back scattering electron image of the fracture surface by using a back scattering electron probe.
In a second aspect, embodiments of the present application provide a method for displaying austenite grain boundaries of a steel material, including:
heating a sample to be tested with the carbon content of 0.95-1.10 wt% to a temperature higher than the critical temperature by 100 ℃, preserving the heat to ensure that the sample to be tested is completely austenitized, and then putting the sample to be tested into cooling liquid in 5s to be cooled at a speed of not less than 150 ℃/s, so as to obtain the sample to be tested with cracked surface.
And cutting the sample to be tested with the cracked surface to expose the fracture surface formed by cracking, and shooting the back scattering electron image of the fracture surface by using a back scattering electron probe.
In a third aspect, embodiments of the present application provide a method for evaluating austenite grain size of a steel material, including:
the display method provided by the embodiment of the first aspect or the second aspect is adopted to obtain the backscattered electron image.
The backscattered electron image is evaluated for austenite grain size by using an area method or a cut-off point method.
The steel austenite grain boundary display method and the steel austenite grain size evaluation method provided by the embodiment of the application have the beneficial effects that:
the inventor researches and discovers that when a steel material with high carbon content is quenched, after the quenching temperature is high to a certain degree, the quenching produces huge stress which is larger than the strength of the steel material and exceeds the plastic deformation limit, so that quenching cracks can be generated, at the moment, the steel material is rapidly cooled at a certain cooling speed, the cracks generated by quenching can crack, and the cracks generated by rapidly cooling the steel material after the high-temperature quenching often crack along grain boundaries. The research also finds that because the fracture surface cracked along the crack has a stretching form corresponding to the structure of the grain boundary, when the fracture surface after cracking is observed, the grain boundary line in a back scattering electron image is dark black, the crystal grains are grey white, the obvious difference of the colors makes the grain boundary line and the grain contrast obvious, the surface characteristic matching of the grain boundary stretching corresponding to the fracture surface is good, and the austenite grain boundary line of the fracture surface naturally cracked along the grain boundary can be clearly displayed. In the application, a sample to be tested is rapidly cooled and naturally cracked after high-temperature quenching, and metallographic sample preparation and corrosion treatment are not required; the generated cracks crack along the grain boundary, the austenite grain boundary can be truly reflected, and meanwhile, the back scattering electron image can clearly display the austenite grain boundary line of the fracture surface naturally cracked along the grain boundary, so that the austenite grain size can be conveniently and accurately evaluated.
In the embodiment of the application, the sample to be tested with the carbon content of 0.95-1.10 wt% is quenched, the carbon content of the steel is high, and cracks cracking along the grain boundary are easily generated by rapid cooling after high-temperature quenching. Heating a sample to be tested to the critical temperature of more than 60 ℃ and preserving heat to ensure that the sample to be tested is completely austenitized, then cooling the sample by adopting a NaCl aqueous solution at the speed of not less than 150 ℃/s, or heating the sample to be tested to the critical temperature of more than 100 ℃ and preserving heat to ensure that the sample to be tested is completely austenitized, then cooling the sample at the speed of not less than 150 ℃/s: the heating temperature and the cooling speed are matched with the carbon content of the sample to be measured, so that cracks cracked along the grain boundary can be reliably generated when the steel with specific carbon content is rapidly cooled after high-temperature quenching.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings that are required to be used in the embodiments will be briefly described below, it should be understood that the following drawings only illustrate some embodiments of the present application and therefore should not be considered as limiting the scope, and for those skilled in the art, other related drawings can be obtained from the drawings without inventive effort.
FIG. 1 is a schematic diagram of a sample to be tested being cut in a steel austenite grain boundary display method provided in an embodiment of the present application;
FIG. 2 is a graph showing the morphology of austenite grain boundaries in example 8 of the present application;
FIG. 3 is a graph showing the morphology of austenite grain boundaries in example 9 of the present application;
FIG. 4 is a graph showing the morphology of austenite grain boundaries in example 10 of the present application;
FIG. 5 is a graph showing the morphology of austenite grain boundaries in example 12 of the present application;
FIG. 6 is a graph showing the morphology of austenite grain boundaries in comparative example 17 of the present application;
FIG. 7 is a graph showing the morphology of austenite grain boundaries in comparative example 18 of the present application;
FIG. 8 is a graph showing the morphology of austenite grain boundaries in comparative example 19 of the present application.
Icon: 100-a cutting line; 200-crack extension plane.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions of the embodiments of the present application will be clearly and completely described below. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.
The steel austenite grain boundary display method and the steel austenite grain size evaluation method according to the examples of the present application will be specifically described below.
The embodiment of the application provides a steel austenite grain boundary display method, which comprises the following steps: heating a sample to be tested with the carbon content of 0.95-1.10 wt% to a temperature higher than the critical temperature by 60 ℃, preserving the heat to ensure that the sample to be tested is completely austenitized, then taking a NaCl aqueous solution as a cooling liquid, putting the sample to be tested into the cooling liquid within 5s, and cooling at the speed of not less than 150 ℃/s to obtain the sample to be tested with cracked surface. Or heating a sample to be tested with the carbon content of 0.95-1.10 wt% to a temperature higher than the critical temperature by 100 ℃, preserving the heat to ensure that the sample to be tested is completely austenitized, and then putting the sample to be tested into cooling liquid in 5s to be cooled at a speed of not less than 150 ℃/s, so as to obtain the sample to be tested with cracked surface.
And cutting the sample to be tested with the cracked surface to expose the fracture surface formed by cracking, and shooting the back scattering electron image of the fracture surface by using a back scattering electron probe.
The austenite grain boundary display method of the steel provided by the embodiment of the application is exemplarily suitable for GCr15 round steel, such as GCr15 round steel with the diameter of 80mm, and the chemical components of the GCr15 round steel comprise the following components in percentage by mass: 0.95-1.05% of C, 0.15-0.35% of Si, 0.25-0.45% of Mn, 1.40-1.65% of Cr and less than or equal to 0.08% of Mo.
In the embodiment of the application, the sample to be measured with the carbon content of 0.95-1.10 wt% is quenched, the carbon content of the steel is high, and cracks cracking along grain boundaries are easily generated by rapid cooling after high-temperature quenching.
Research shows that when the heating temperature of a sample to be measured is low, quenching cracks cannot be generated during sample quenching, or the generated quenching cracks are transgranular cleavage cracks and quasi-cleavage cracks, and crystal boundaries cannot be observed. When the cooling speed of the sample to be measured is low, the sample cannot crack along the quenching crack after cooling.
It has also been found that the higher the heating temperature of the sample to be tested, the more likely the sample to crack after cooling, under the same cooling conditions. Under the same heating condition, the sample to be measured is more easily cracked after cooling when the cooling speed of the sample to be measured is higher. When the heating temperature of the heating operation is more than 100 ℃, the water and the NaCl aqueous solution can well crack the sample to be tested, and the cooling liquid is exemplified by water or the NaCl aqueous solution; when the heating temperature of the heating operation is more than 60 ℃ higher than the critical temperature but not more than 100 ℃, the NaCl aqueous solution can well crack the sample to be detected, and the water can not crack the sample to be detected, and the cooling liquid is selected to be the NaCl aqueous solution.
Heating the sample to be measured to exceed the critical temperature AC1Keeping the temperature above 60 ℃ to ensure that the sample to be tested is completely austenitized, cooling the sample to be tested by adopting a NaCl aqueous solution at the speed of not less than 150 ℃/s under the condition, or heating the sample to be tested to exceed the critical temperature AC1Keeping the temperature above 100 ℃ to ensure that the sample to be tested is completely austenitized, and cooling at the speed of not less than 150 ℃/s under the condition: the heating temperature and the cooling speed are matched with the carbon content of the sample to be measured, so that cracks cracked along the grain boundary can be reliably generated when the steel with specific carbon content is rapidly cooled after high-temperature quenching.
It has been found that the secondary electron image has high resolution, large depth of field, and strong stereoscopic impression, but the grain boundary lines are not clearly displayed when the fractured fracture surface is observed. In the back scattering electron image, the grain boundary line is dark black and the crystal grain is grey white, and the obvious difference of the colors makes the grain boundary line and the crystal grain contrast obvious, so that the austenite grain boundary line can be clearly displayed.
In the application, a sample to be tested is rapidly cooled and naturally cracked after high-temperature quenching, and metallographic sample preparation and corrosion treatment are not required; the generated cracks crack along the grain boundary, the austenite grain boundary can be truly reflected, and meanwhile, the back scattering electron image can clearly display the austenite grain boundary line of the fracture surface naturally cracked along the grain boundary, so that the austenite grain size can be conveniently and accurately evaluated.
It will be appreciated that in the embodiments of the present application, the heating is carried out when the critical temperature A is exceededC1The selection of the cooling liquid is not limited to 100 ℃ or higher, as long as the sample to be measured can be cooled at a rate of not less than 150 ℃/s by being placed in the cooling liquid. Research shows that the water and NaCl aqueous solution can better meet the cooling requirement, wherein the NaCl aqueous solution has better cooling effect than water; the oil cooling method is found not to beThe cooling requirements can be met.
When the cooling liquid is selected to be an NaCl aqueous solution, the cooling liquid is an NaCl aqueous solution with a mass concentration of 1-10%, or an NaCl aqueous solution with a mass concentration of 4-6%, the mass concentration of the NaCl aqueous solution is, for example and without limitation, any one of 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% and 10%, or a range between any two of the two, so that the sample to be tested can be naturally cracked when the heating temperature exceeds the critical temperature by more than 60 ℃, and the cooling effect is reduced when the mass concentration of the NaCl aqueous solution is too high or too low.
Further studies have also found that when the coolant is chosen to be an aqueous solution of NaCl: when the heating temperature reaches 60 ℃ exceeding the critical temperature, the cracking rate of the NaCl aqueous solution with the mass concentration of 1% and 10% is less than 50% and the generated cracks are small, and the cracking rate of the NaCl aqueous solution with the mass concentration of 5% is about 50%; when the heating temperature reaches 80 ℃ which exceeds the critical temperature, the NaCl aqueous solution with the mass concentration of 1% and 10% can only crack the part of the sample to be detected, and the crack generated by the crack is smaller, and the cracking rate of the NaCl aqueous solution with the mass concentration of 5% can basically reach 100%; when the heating temperature reaches 100 ℃ higher than the critical temperature, the NaCl aqueous solution with the mass concentration of 1-10% can enable the sample to be detected to crack well, and the cracking rate can reach 100% basically.
In some exemplary embodiments, when the cooling liquid is an aqueous NaCl solution, particularly when the cooling liquid is an aqueous NaCl solution with a mass concentration of 1-10%, the heating temperature of the heating operation is above 100 ℃ above the critical temperature, so as to ensure that the surface of the sample to be tested can be well cracked when the sample to be tested is cooled at a speed of not less than 150 ℃/s by using the aqueous NaCl solutions with different mass concentrations as the cooling liquid. Furthermore, under the heating condition, the cooling liquid can be selected from a NaCl aqueous solution with the mass concentration of 1-10% and can also be selected from water.
In some possible embodiments, the cooling is performed until the temperature of the sample to be tested is reduced to below 40 ℃, for example to room temperature, ensuring that the cooling is sufficient.
In some possible embodiments, the sample to be measured has a specification of (10-25) mm × 10-15 mm × 20-30 mm, for example, 20mm × 15mm × 30mm, and the sample to be measured has a suitable specification to ensure that the sample to be measured has a suitable heating effect and cooling effect on the core of the sample to be measured when the sample to be measured is heated and cooled.
In the embodiment of the present application, the holding time after the heat treatment may be selected according to the specification of the sample to be measured in order to completely austenitize the sample to be measured. Illustratively, the time of the heat preservation operation is more than 10min, for example, 30min, so as to ensure that the sample to be measured with the specification of (10-25) mm (10-15) mm (20-30) mm can be well completely austenitized.
As shown in fig. 1, it can be understood that, in the embodiment of the present application, when the sample to be tested with a cracked surface is cut so that the fracture surface formed by the crack is exposed, since the two fracture surfaces that are cracked are relatively distributed in the interior of the sample to be tested, it is necessary to cut the cracked sample portion from the whole sample to be tested so that the fracture surface can be exposed for further observation. Illustratively, the cutting operation is performed in a direction perpendicular to the crack, i.e., the plane of the cut line 100 is perpendicular to the crack propagation plane 200, to cut the cracked sample portion from the entirety of the sample to be tested.
In some possible embodiments, the cooling treatment and the cutting operation further comprise: and cleaning and drying the sample to be tested with cracked surface, and facilitating cutting operation.
The embodiment of the application provides a method for evaluating austenite grain size of steel, which comprises the following steps: obtaining a back scattering electron image by adopting the display method provided by the embodiment; the backscattered electron image is evaluated for austenite grain size by using an area method or a cut-off point method.
The features and properties of the present application are described in further detail below with reference to examples.
Examples 1 to 16
A method for evaluating austenite grain size of steel comprises the following steps:
s1, sample processing: the test sample to be tested with the length, width and height of 20mm 15mm 30mm is sawed at the position of 1/4 diameter of 80mm GCr15 round steel by using a sawing machine, and the critical temperature of the GCr15 round steel is 760 ℃.
S2, sample heat treatment: heating the sample to be measured to a preset temperature, preserving the temperature for 30min, and cooling the sample to be measured to room temperature in cooling liquid within 5 s.
S3, preparing a fracture: and cleaning and drying the sample to be tested after the heat treatment is finished, and then cutting the sample to be tested by using a metallographic cutting machine along the direction of quenching cracks generated on the sample to be tested during the vertical heat treatment so as to expose the fracture surface formed by the cracks.
S4, fracture observation: and observing the fracture surface by using a scanning electron microscope, finding out the area of the fracture along the crystal in the original quenched fracture, then shooting the backscattered electron image of the fracture along the crystal by using a backscattering probe, and evaluating the austenite grain size by using an intercept point method.
In examples 1 to 16, the heating temperature and the cooling method are shown in Table 1, wherein the brine cooling means cooling with an aqueous NaCl solution.
Comparative examples 1 to 16
A method for evaluating austenite grain size of a steel material, which is different from example 1 in at least one of heating temperature and cooling manner in step S2, is shown in Table 1.
Comparative example 17
A method for evaluating austenite grain size of a steel material, which is different from that of example 1, is: in step S4, a secondary electron image along the fracture is captured using the secondary electron probe.
Comparative example 18
A method for evaluating austenite grain size of steel comprises the following steps:
s1, sample processing: a sawing machine is used for sawing a sample to be tested with the length and the width of 10mm by 20mm at the position of 80mm diameter GCr15 round steel 1/4, and the critical temperature of the GCr15 round steel is 760 ℃.
S2, sample heat treatment: heating the sample to be tested to 860 ℃, preserving heat for 30min, and putting the sample into a NaCl aqueous solution with the mass concentration of 5% in 5s for cooling to room temperature.
S3, sample preparation: the sample after heat treatment was ground to remove the decarburized layer by a grinder and then finely ground and polished.
S4, sample corrosion: putting the sample into a saturated picric acid and detergent solution with the volume ratio of 1:1, etching at normal temperature for 10min, then washing the corrosive substances on the surface of the sample with clear water, then spraying absolute ethyl alcohol on the sample, and finally drying the sample by using a blower.
S5, crystal boundary observation: and observing the corroded surface of the sample by using a scanning electron microscope, and evaluating the austenite grain size by using a comparison method or an intercept method.
Comparative example 19
A method for evaluating austenite grain size of steel comprises the following steps:
s1, sample processing: a sawing machine is used for sawing a sample to be tested with the length, the width and the height of 20mm 15mm 30mm at the position of 1/4 diameter of 80mm GCr15 round steel, and a V-shaped groove is formed in the sample to be tested, and the critical temperature of the GCr15 round steel is 760 ℃.
S2, sample heat treatment: heating a sample to be tested to a preset temperature, then preserving heat for 30min, and putting the sample into a NaCl aqueous solution with the mass concentration of less than 30 ℃ for 5s to cool to room temperature.
S3, preparing a fracture: and breaking the sample along the gap by using a hammer.
S4, fracture observation: and observing the fracture surface by using a scanning electron microscope, then shooting a back scattering electron image along the fracture of the crystal by using a back scattering probe, and measuring the size of the crystal grain along the grain boundary.
Test example 1
The results of statistics on the cracking conditions of examples 1 to 16 and comparative examples 1 to 16 are shown in table 1.
TABLE 1 Experimental conditions and cracking conditions
According to table 1, in the embodiment, the sample to be tested is placed into the cooling liquid to be cooled at a speed of not less than 150 ℃/s in a salt water cooling mode, and the sample to be tested can be naturally cracked when being heated to be higher than the critical temperature of 60 ℃; when the sample to be measured is heated to be higher than the critical temperature by 100 ℃, the sample to be measured is placed into the cooling liquid to be cooled at the speed of not less than 150 ℃/s by adopting a water cooling or salt water cooling mode, and the sample to be measured can crack naturally. In the comparative example, the sample to be measured is placed into the cooling liquid in an oil cooling mode and cooled at the speed of less than 150 ℃/s, and the sample to be measured begins to crack naturally when the sample to be measured is heated to be higher than the critical temperature of 200 ℃.
Test example 2
The austenite grain boundaries of example 8, example 9, example 10, example 12, comparative example 17, comparative example 18, and comparative example 19 were observed and shown, and the results are shown in fig. 2 to 8.
As can be seen from fig. 2, 3, 4, 5 and 7, the austenitic grain boundary lines can be clearly shown in the examples of the present application as compared with the case of the treatment with the saturated picric acid etching agent in comparative example 18 without the need for the metallographic preparation and the etching treatment.
As can be seen from fig. 6, in comparative example 17, the fracture surface was displayed using the secondary electron image, and the grain boundary lines were not clearly displayed. In the application, the fracture surface is displayed by adopting the back scattering electron image, so that the austenite grain boundary line can be clearly displayed.
As can be seen from fig. 8, in comparative example 19, the provision of the V-shaped groove on the sample to be tested not only increases the operation steps, but also the fracture surface obtained by knocking and breaking along the notch is limited by the position of the V-shaped groove, and does not crack at the weakest crystal grain, and cannot reflect the poor field of view of the crystal grain. The sample to be measured cracks naturally in the application, and the generated cracks crack along the grain boundary, so that the austenite grain boundary can be reflected really.
The embodiments described above are some, but not all embodiments of the present application. The detailed description of the embodiments of the present application is not intended to limit the scope of the claimed application, but is merely representative of selected embodiments of the application. 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 application.
Claims (10)
1. A method for displaying austenite grain boundaries of a steel material is characterized by comprising the following steps:
heating a sample to be tested with the carbon content of 0.95-1.10 wt% to a temperature higher than the critical temperature by 60 ℃, preserving the heat to ensure that the sample to be tested is completely austenitized, then taking a NaCl aqueous solution as a cooling liquid, putting the sample to be tested into the cooling liquid in 5s, and cooling at the speed of not less than 150 ℃/s to obtain the sample to be tested with cracked surface;
and cutting the sample to be tested with the cracked surface to expose the fracture surface formed by cracking, and shooting the back scattering electron image of the fracture surface by using a back scattering electron probe.
2. The display method according to claim 1, wherein a heating temperature of the heating operation is 100 ℃ or more above the critical temperature.
3. The display method according to claim 1, wherein the coolant is an aqueous solution of NaCl having a mass concentration of 1 to 10%.
4. A method for displaying austenite grain boundaries of a steel material is characterized by comprising the following steps:
heating a sample to be tested with the carbon content of 0.95-1.10 wt% to a temperature higher than the critical temperature by 100 ℃, preserving the heat to ensure that the sample to be tested is completely austenitized, and then putting the sample to be tested into cooling liquid in 5s to be cooled at a speed of not less than 150 ℃/s to obtain the sample to be tested with cracked surface;
and cutting the sample to be tested with the cracked surface to expose the fracture surface formed by cracking, and shooting the back scattering electron image of the fracture surface by using a back scattering electron probe.
5. The display method according to any one of claims 1 to 4, wherein the sample to be measured has a specification of (10 to 25) mm (10 to 15) mm (20 to 30) mm.
6. The display method according to claim 5, wherein the time for the heat-retaining operation is 10min or more.
7. The display method according to any one of claims 1 to 4, wherein the cooling operation is performed until the temperature of the sample to be measured is reduced to 40 ℃ or lower.
8. The display method according to any one of claims 1 to 4, wherein the cutting the test sample with a cracked surface so that a fracture surface formed by the crack is exposed comprises: and cutting the part of the test sample with the fracture surface from the whole test sample, wherein the cutting operation is performed along the direction vertical to the fracture.
9. The display method according to any one of claims 1 to 4, wherein the cooling process and the cutting operation further comprise: and cleaning the to-be-detected sample with the cracked surface and then drying the to-be-detected sample.
10. A method for evaluating austenite grain size of a steel material, comprising:
obtaining the backscattered electron image by the display method according to any one of claims 1 to 9;
and evaluating the austenite grain size of the back scattering electron image by using an area method or a point intercept method.
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