CN112763524B - Three-dimensional corrosion method for carbide in GCr15 bearing steel - Google Patents

Abstract

The invention discloses a three-dimensional etching method for carbides in GCr15 bearing steel, which belongs to the field of metal material detection. Under a specific etching method, the present invention performs current directional etching by configuring a given etching solution and setting different etching parameters, so that the GCr15 steel matrix is partially dissolved, and under the premise of ensuring the complete morphology of the carbide , so that it is fully exposed. The advantage of this method is that: under the given etching solution of the present invention, by changing the current density and the etching time, the primary carbides and secondary carbides in the steel can be etched separately, and the whole process can be etched at room temperature. It can be carried out under low temperature, no extraction process is required, the operation is simple, the time-consuming is short (≤20min), and the in-situ electrolysis and in-situ analysis are carried out on the metal substrate. After the etching, the three-dimensional morphology of different carbides can be completely observed by scanning electron microscope, so as to accurately grasp the spatial distribution and morphological characteristics of carbides, and deepen the further analysis of carbides.

Description

A three-dimensional etching method of carbides in GCr15 bearing steel

technical field

The invention relates to the field of metal material detection, in particular to a three-dimensional etching method for carbides in GCr15 bearing steel

Background technique

GCr15 is a kind of high carbon bearing steel with good performance, the most widely used and the largest output in the world. After quenching and tempering, it has high and uniform hardness, good wear resistance and high contact fatigue performance. Due to its high carbon content (0.95-1.05%), its hardness and strength are high, so it is often used in the manufacture of tools such as drill bits and reamers. However, the high carbon content also leads to the precipitation of a large amount of carbides during solidification and rolling. These carbides with high hardness and brittleness will not only become the origin of fatigue cracks, but also cause the surface of the product to peel off. Phenomenon.

GCr15 bearing steel will undergo severe dendrite segregation during solidification, which will increase the concentration of carbon and other metal elements in local areas. When the conditions for forming eutectic components are reached, they will segregate to form ledeburite eutectic carbides. Precipitated directly from the liquid phase. This kind of liquefied carbide is also called primary carbide. Primary carbide has higher chromium content, larger hardness and size, and is also the most harmful in carbide defects.

However, if the concentration of carbon and other metal elements in local areas does not reach the precipitation conditions during the solidification process, these areas will be elongated into high-concentration segregation zones after hot working deformation, and a large amount of precipitation will be precipitated during the subsequent cooling process. The carbide forms a banded structure, so it is also called banded carbide. In addition, during the rolling process, under the influence of liquefied carbides and banded carbides, the bearing steel will precipitate pro-eutectoid carbides along the grain boundaries during the cooling of the austenite, and these carbides will surround the austenite. The bulk grains are networked under the microscope, so they are also called network carbides. Banded carbides and reticulated carbides are essentially carbides precipitated from austenite, so they are also collectively referred to as secondary carbides. The existence of secondary carbides weakens the bonding force between metals and makes steel. The mechanical properties are reduced, especially the impact toughness is reduced, and it is easy to cause grain boundary cracking, which reduces the wear resistance of bearing steel.

Therefore, how to control the precipitation of carbides in GCr15 bearing steel is of great significance for improving the quality of bearing steel, so how to accurately and completely obtain the morphology of carbides in bearing steel is a key area of research. There are two methods for extracting non-metallic inclusions that have been reported so far: acid-dissolving method and electrolysis method. The acid-dissolving method developed very early and has a wide range of applications. It uses various concentrations of HNO ₃ , H ₂ SO ₄ , HCl and aqueous solutions to dissolve the metal matrix, and some stable inclusions that are not dissolved by acid can be retained. For the acid dissolution method, please refer to the literature "Study on the Three-dimensional Morphology of Ultrafine Oxide Inclusions in Steel by Acid Dissolving Method" (Journal of Iron and Steel Research, 2007, (04) 85-89.)

The electrolysis method also includes aqueous electrolysis and non-aqueous electrolysis, both of which are electrolysis to separate the inclusions from the steel, then filter, extract, weigh and analyze the particle size and chemical composition, which can give the content of non-metallic inclusions in the steel, Type and granularity information. The electrolyte usually used in the aqueous electrolysis method is an acidic aqueous solution. Similar to the acid solution method, the acidic aqueous solution will destroy many inclusions in the steel, and the electrolysis method can only completely retain the inclusions with large size (≥50 μm). Inclusions of small size (2-20 μm) with high content in steel cannot be accurately evaluated, so this method also needs to be improved. In the book "Non-metallic Inclusions in Steel", a total of 7 kinds of non-aqueous electrolytes are mentioned, but the types of steel and inclusions that can be extracted by these electrolytes have great limitations, or for one steel, or For a class of inclusions.

Therefore, if these methods are used to extract the inclusions in the steel, it is not only time-consuming but also inefficient, and because the electrical conductivity of GCr15 bearing steel is weak and the corrosion resistance of carbides is not high, the existing electrolytic formula and parameters are not applicable. For this type of steel, there are large limitations, which limit the further analysis of carbides.

Chinese patent CN 106840802 A discloses an original appearance analysis method for electrolytic separation of inclusions in high carbon steel, which is characterized in that the sample is preheated in a heating furnace, and the electrolyte is anhydrous methanol or ethanol-(5-15 )% acetylacetone-(0.5～1.5)% tetramethylammonium chloride; the voltage control range during electrolysis is 120mV～160mV, the current is kept in the range of 0.04A～0.07A cm ² , and the electrolysis time is 2～5h; Afterwards, the rinsing solution and electrolyte were collected and poured into a suction filter bottle, and the inclusions were graded and filtered by filter membranes with pore diameters of 2 μm and 0.45 μm and dried, and then the extracted inclusions were analyzed by scanning electron microscopy and energy spectrum. The difference between the present invention and the above-mentioned invention is that: the present invention adopts the method of in-situ electrolysis, does not need extraction and collection process, in-situ electrolysis and in-situ analysis on the metal substrate, and the present invention does not require preheating treatment, which is time-consuming It only takes 10 to 20 minutes, and the operation is simpler and more efficient. And the carbides in the bearing steel are mainly (Fe, Cr) ₃ C, and the complexing agent acetylacetone in the above invention has a strong ability to complex reactions with Fe ²⁺ , so carbides will occur during the electrolysis process. Partial dissolution, thereby destroying the complete morphology of carbides.

Chinese patent CN 110161066 A discloses a method for extracting inclusions in steel from a non-aqueous solution. 1～5% ammonium thiocyanate, the rest are anhydrous methanol, the voltage is 2～5V, the current is 0.04～0.05A cm ² , and the electrolysis time is 2.5h; after electrolysis, the electrolyte is moved into a centrifuge tube for high-speed centrifugal motion, Make the inclusions in the electrolyte stick to the side wall of the centrifuge tube, pour out the electrolyte, add absolute ethanol, mix well and drop it onto the single crystal silicon wafer, evaporate to dryness naturally, and observe the inclusions on a scanning electron microscope. The difference between the present invention and the above-mentioned invention is that: the present invention adopts the method of in-situ electrolysis, and does not need extraction and collection process, and in-situ electrolysis and in-situ analysis are performed on the metal substrate. In addition, acetylacetone is also selected as the complexing agent in the above invention, and its strong complexing ability still has the problem that the damage in the electrolysis process will destroy the carbide morphology.

Chinese patent CN111596094 A discloses a three-dimensional engraving device and engraving method for non-metallic inclusions in steel (9 electrolytic formulas are provided), through a given engraving device, engraving is performed according to a given method and engraving parameters. Operation, current directional etching is carried out in the etching solution prepared by neutral solvent, complexing agent and conductive agent, so that the steel matrix is partially dissolved, and the inclusions are not dissolved, thereby exposing the spatial morphology of the inclusions and retaining the inclusions. Spatial position information, the electrolysis temperature is -15°C to 45°C, and the current density is 20 to 300 mA/cm ² . The difference between the present invention and the above-mentioned invention is: the etching solution adopted in the present invention, the etching current density and the corresponding etching time are different from it, and the electrolyte in the above-mentioned invention is suitable for the electrolytic matrix with a ferrite structure. Steel is not suitable for weakly conductive bearing steel with pearlite + cementite structure. In addition, the current density is small, which further limits the corrosion of the bearing steel matrix, so that the carbides in the steel cannot be fully exposed.

SUMMARY OF THE INVENTION

In order to observe the three-dimensional morphology of carbides in GCr15 bearing steel, the present invention provides a three-dimensional etching method of carbides in GCr15 bearing steel. By configuring a specific electrolyte and setting different etching parameters, the steel matrix is formed. Corrosion, carbide exposure, so that the three-dimensional morphology of different carbides in steel can be observed comprehensively and meticulously, no toxic substances are produced during the whole electrolysis process, and the operation process is simple and efficient.

A three-dimensional etching method for carbides in GCr15 bearing steel, specifically:

a. The method is aimed at GCr15 bearing steel, the chemical composition mass percentage of the material and the microstructure of the material meet: GB/T18254-2016;

b. The composition of the etching solution used in the method is: tetramethyl ammonium chloride, potassium chloride, acetylacetone, sodium tripolyphosphate, methanol solution, by volume percentage: 10-15% (m/V) Tetramethylammonium chloride, 8-15% (m/V) potassium chloride, 2-5% acetylacetone, 10-15% sodium tripolyphosphate, balance methanol;

c. The initial temperature of electrolysis used in the method is the normal temperature condition of 10-30 ° C, the etching current density is 500-800 mA/cm ² , and the etching time is controlled at 10-20 min; the current density selected for different carbides should be In conjunction with the etching time, for primary carbides with a size of 50-200 μm, that is, M ₇ C ₃ carbides directly precipitated from the liquid phase during solidification, the current density should be controlled at 700-800 mA/cm ² , The etching time should be controlled within 16 to 20 minutes; for secondary carbides with a size of 5 to 30 μm, that is, M ₂₃ C ₆ carbides precipitated from the austenite grain boundaries during cooling, the current density should be controlled within 500 to 500 μm. 600mA/cm ² , the etching time is controlled at 10-15min.

The preferred composition of the etching solution is: 13% (m/V) tetramethylammonium chloride, 10% (m/V) potassium chloride, 3% acetylacetone, 13% sodium tripolyphosphate, and the remainder methanol.

The preferred etching current density and corresponding etching time of different carbides are: primary carbide, the current density is 780mA/cm ² , the etching time is 19min; secondary carbide, the current density is 530mA/cm ² , the etching time is 19min The time is 11 minutes.

Preferably, the three-dimensional etching method of carbides in GCr15 bearing steel is not only suitable for GCr15 bearing steel, but also for wear-resistant steel, cord steel or other high carbon steel.

The method of the invention mainly includes sample preparation, etching solution configuration, connection of etching device, etching parameter setting, sample processing and observation. The specific operation process is as follows:

(1) Sample preparation

There are no special requirements for the shape of the sample. The observation surface is ground and polished with sandpaper, rinsed with absolute ethanol, and dried, and the observation surface is facing the cathode.

(2) Configuration of etching solution (volume ratio)

The etching solution is mainly configured with neutral solvent, electrolyte and matrix element complexing agent in a certain proportion. The electrolyte formula of the present invention is: 10-15% (m/V) tetramethyl ammonium chloride + 8-15% (m/V) potassium chloride + 2-5% acetylacetone + 10-15% tripolyphosphoric acid Sodium + balance methanol solution. The neutral solvent chooses high-purity methanol, which has better solubility for inorganic salts and complexes than ethanol, and is the preferred solvent for neutral electrolytes; due to the weak conductivity of bearing steel, in order to make the electrolyte provide more The electroconductive particles of the present invention enhance its electrical conductivity, and the electrolyte selected in the present invention is 10-15% (m/V) tetramethyl ammonium chloride + 8-15% (m/V) potassium chloride; The carbide is (Fe, Cr) _x C _y . In order to slow down the reaction of the strong complexing agent acetylacetone and Fe ⁺² to protect the complete appearance of the carbide, the content of acetylacetone is controlled at 2-5% in the present invention, At the same time, in order to prevent the precipitation of Fe(OH) ₃ caused by the low content of the complexing agent, thereby destroying the stability of the electrolyte, sodium tripolyphosphate with weak complexing ability is added in the present invention, and its content is controlled at 10-15% .

(3) Connect the etch device

The anode rectangular steel sheet and cathode clamp of the etching device are made of stainless steel, which can maintain good current stability. They are fixed with insulating rubber respectively. The distance between the cathode and anode is 20-30mm, and the anode clamp at the other end is connected to the cathode. The bottom position is on the same horizontal line to ensure that the anode and cathode do not touch the inner wall of the electrolytic etching tank.

(4) Rotation parameter setting

①Corrosion temperature

The electrolytic corrosion temperature of GCr15 bearing steel is controlled within the room temperature range, 10 to 30 °C, but it should not be too high or too low. If the temperature is too high, the volatilization rate of the etching solution will become faster, which will increase the resistivity and reduce the electrical conductivity during the electrolysis process, thus prolonging the electrolysis time. If the temperature is too low, the diffusion of ions in the etching solution will be slowed down, and the crystallization of the etching solution will be formed, resulting in incomplete corrosion of the steel matrix and incomplete exposure of carbides.

②Current density and corrosion time

When electrolyzing GCr15 bearing steel, a DC regulated power supply should be used. The carbon content of GCr15 bearing steel is high, and the structure is cementite + pearlite. Compared with low carbon steel, the conductivity is poor. Therefore, a larger current density is required to fully corrode the steel matrix, so that it is easier to observe the carbonization. Therefore, the current density should be controlled at 500 ~ 800mA/cm ² ; because the carbides in GCr15 bearing steel are often metal carbides formed by carbon atom chains passing through the deformed metal structure with Cr, Mn, and Fe, these metals are carbides. The physical properties are between the ionic type and the simple filling type, and the corrosion resistance is not high, so the electrolysis time is controlled at 10-20min. If the time is too long, the carbide will be partially dissolved, thereby destroying the integrity and making it impossible to observe the whole picture. .

The current density selected for different carbides should be matched with the etching time. For primary carbides, which are directly precipitated from the liquid phase during the solidification process, the alloying elements Cr and Fe and C will combine to form M ₇ C ₃ type carbides, which are larger in size, ranging from 50 to 200 μm, and due to the Due to the influence of dendrite segregation, the content of Cr in M ₇ C ₃ carbides is higher, which is expressed as Cr-rich carbides, so the content of Fe is lower, which makes M ₇ C ₃ carbides have certain corrosion resistance. , Under high-density current and long-term etching, the complete morphology can still be maintained. Therefore, for M ₇ C ₃ type carbides with large size and certain corrosion resistance, the current density should be controlled at 700-800 mA/cm ² , and the etching time should be controlled at 16-20 min. The long etching time can make the steel matrix more fully corroded, so that the large size M ₇ C ₃ carbides are completely exposed.

As for the secondary carbide, it is precipitated from the austenite grain boundary during the cooling process. Since most of the Cr has formed M ₇ C ₃ type carbide with Fe and C in the liquid phase, it is The carbides precipitated in the austenite are M ₂₃ C ₆ with low Cr content, the size of which is 5-30 μm, and the Fe content is high, so the corrosion resistance of M ₂₃ C ₆ type secondary carbides is relatively high. Poor, for this secondary carbide, it is necessary to choose a relatively small current density and a short etching time, the current density should be controlled at 500 ~ 600mA/cm ² , and the etching time should be controlled at 10 ~ 15min to prevent Its complete morphology is destroyed during the electrolysis process.

(5) Sample processing and observation

After the etching is completed, use anhydrous ethanol to spray the residual solvent on the surface. Before placing it under the scanning electron microscope, it needs to be dried in a drying oven to prevent the residual solvent from contaminating the scanning electron microscope. , you can repeat the above engraving process by adjusting the parameters.

Description of drawings

Figure 1 Example 1: GCr15 bearing steel - 780mA/cm ² , SEM pictures of primary carbides etched for 19 minutes.

Fig. 2 Example 2: GCr15 bearing steel -780mA/cm ² , SEM image of primary carbide after 17min etching.

Figure 3 Example 3: GCr15 bearing steel - 720mA/cm ² , SEM pictures of primary carbides etched for 19 minutes.

Figure 4 Example 4: GCr15 bearing steel - 720mA/cm ² , SEM pictures of primary carbides etched for 17 minutes.

Fig. 5 Example 5: GCr15 bearing steel - 590mA/cm ² , SEM picture of secondary carbides etched for 13 minutes.

Fig. 6 Example 6: GCr15 bearing steel - 590mA/cm ² , SEM picture of secondary carbides etched for 11 minutes.

Figure 7 Example 7: GCr15 bearing steel - 530mA/cm ² , SEM picture of secondary carbides etched for 13 minutes.

Fig. 8 Example 8: GCr15 bearing steel - 530mA/cm ² , SEM picture of secondary carbides etched for 11 minutes.

Fig. 9 Comparative example 1: SEM image of etched carbide of GCr15 bearing steel

Detailed ways

Further detailed description will be given below in conjunction with the accompanying drawings, examples and comparative patents.

Example:

Electrolytic engraving of GCr15 bearing steel (chemical composition: C: 1.0%, Si: 0.25%, Mn: 0.34%, P: 0.013%, S: 0.002%, Cr: 1.48%);

Carry out engraving according to the steps in the content of the invention, the size of the sample is 10*10*10mm, and the engraving parameters are:

The volume percentage of the etching solution is: 13% (m/V) tetramethyl ammonium chloride + 10% (m/V) potassium chloride + 3% acetylacetone + 13% sodium tripolyphosphate + balance methanol;

Corrosion temperature: 24-26℃;

The current density and the corresponding etching time in each embodiment are shown in the table:

It can be seen from Figures 1 to 4 that with the increase of the current density and the increase of the etching time, the large-sized secondary carbides are gradually etched out of the iron matrix, and their three-dimensional morphology becomes more and more Therefore, in order to accurately observe the three-dimensional morphology of secondary carbides in GCr15 bearing steel, the current density should be 780mA/cm ² and the etching time should be 19min; and it can be seen from Figures 5 to 8 that for small size For secondary carbides that are not resistant to corrosion, their three-dimensional morphology can be clearly observed at lower current and shorter etching time, but with the increase of current density and etching time, these secondary carbides Partial dissolution begins to occur and merges with the surrounding carbides and structures, which has a certain impact on the observation of the real morphology of carbides. Therefore, in order to accurately observe the three-dimensional morphology of secondary carbides in GCr15 bearing steel, the current density It should be 530mA/cm ² , and the etching time is 11min.

The above-mentioned embodiments are only used to illustrate the technical solutions of the present invention and not to limit them, and the contents covered by the present invention are given in the claims.

Comparative example 1:

The electrolytic etching operation steps of GCr15 bearing steel (chemical composition: C: 1.0%, Si: 0.25%, Mn: 0.34%, P: 0.013%, S: 0.002%, Cr: 1.48%) are the same as those in Chinese patent CN111596094A Consistent as described in Example 2; the etched parameters are:

Sample size: 10*10*10mm;

Etching liquid (volume percentage): 6% (m/V) tetramethylammonium chloride + 18% acetylacetone + balance methanol;

Etching temperature: 5℃;

Current density: 300mA/cm ² ;

Carving time: 45-80min.

The implementation effect is shown in Figure 9. It can be seen from the figure that under the electrolytic etching conditions, the large-sized primary carbides are not fully exposed, resulting in a blurred overall appearance, which cannot be accurately observed, and the secondary carbides are not fully exposed. Completely failed to be electrolyzed, and the effect of electrolytic corrosion is far inferior to that of the embodiment.

Claims (3)

1. A three-dimensional corrosion method for carbide in GCr15 bearing steel is characterized by comprising the following steps:

a. aiming at GCr15 bearing steel, the method has the advantages that the chemical component mass percentages and the microscopic structures of the material both meet the following requirements: GB/T18254-2016;

b. the etching solution adopted by the method comprises the following components: tetramethylammonium chloride, potassium chloride, acetylacetone, sodium tripolyphosphate and methanol solution, wherein the components in percentage by volume are as follows: 10-15% (m/V) tetramethylammonium chloride, 8-15% (m/V) potassium chloride, 2-5% acetylacetone, 10-15% sodium tripolyphosphate and the balance methanol;

c. the method adopts the normal temperature condition that the initial electrolysis temperature is 10-30 ℃ and the etching current density is 500-800mA/cm ² Etching time is controlled at 10 ℃20min; the current density selected for different carbides is matched with the etching time, and for the primary carbides with the size of 50-200 μ M, namely M directly precipitated from the liquid phase in the solidification process ₇ C ₃ The current density of the type carbide is controlled between 700 and 800mA/cm ² The etching time is controlled to be 16-20 min; for secondary carbides of size 5-30 μ M, i.e. M precipitated from the austenite grain boundaries during cooling ₂₃ C ₆ The current density of the carbide is controlled between 500 and 600mA/cm ² The etching time is controlled to be 10-15 min;

the three-dimensional etching method for carbides in GCr15 bearing steel is not only suitable for GCr15 bearing steel, but also suitable for wear-resistant steel, cord steel or other high-carbon steel.

2. The three-dimensional etching method for carbides in GCr15 bearing steel according to claim 1, wherein: the etching solution comprises the following components: 13% (m/V) tetramethylammonium chloride, 10% (m/V) potassium chloride, 3% acetylacetone, 13% sodium tripolyphosphate and the balance of methanol.

3. The three-dimensional etching method for carbides in GCr15 bearing steel according to any one of claims 1-2, wherein: the etching current density and the corresponding etching time of different carbides are as follows: primary carbide with current density of 780mA/cm ² Etching time is 19min; secondary carbide with current density of 530mA/cm ² The etching time is 11min.

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