Method for distinguishing ferrite and cementite structures in metallographic specimen

Abstract

The invention relates to the technical field of metal material detection, and discloses a method for distinguishing ferrite and cementite structures in a metallographic sample. Comprising the following steps: (1) pretreating a metallographic specimen, and then immersing the metallographic specimen into a corrosive agent for etching; (2) observing the etched metallographic specimen by using a scanning electron microscope, and distinguishing ferrite and cementite tissues; the metallographic structure of the metallographic sample contains ferrite and / or cementite; the corrosive agent contains sodium metabisulfite. According to the method disclosed by the invention, the sample is etched by using the corrosive containing sodium metabisulfite, so that a clear morphology structure can be obtained, and ferrite and cementite structures can be accurately judged by observing in a secondary electron image mode of a scanning electron microscope; under the condition that a small amount of chemical reagent is used, the corrosion degree is appropriate, the clear morphology of ferrite and cementite structures is obtained, the ferrite and cementite structures are distinguished, and the method is an environment-friendly and accurate test method.

Description

Technical Field

[0001] The invention relates to the technical field of metal material detection, and in particular to a method for distinguishing ferrite and cementite structures in a metallographic sample. Background Art

[0002] Ferrite is an interstitial solid solution of carbon dissolved in α-Fe, usually represented by the symbol F. It has a body-centered cubic lattice and its carbon solubility is very low. It can only dissolve 0.0008% of carbon at room temperature, and the maximum carbon solubility is 0.02% at 727°C. It is called ferrite or α solid solution, and the formation process is that the austenite of hypoeutectoid composition forms ferrite through proeutectoid precipitation.

[0003] Cementite is a metal compound formed by iron and carbon, with a chemical formula of Fe3C. The carbon content of cementite is ωc = 6.67[3]%, and the melting point is 1227°C. Its crystal lattice is a complex orthorhombic lattice, with a very high hardness of HBW = 800, almost zero plasticity and toughness, and very high brittleness.

[0004] CN113702124A proposes a test method for obtaining the dendrite morphology of medium and low carbon steel ingots: the medium and low carbon steel ingot samples are first heat treated to obtain the martensitic structure and then treated with a corrosion mixed solution. The method obtains the martensitic structure through the heat treatment process, and can obtain clear photos of the dendrite structure of the ingot using a small amount of chemical reagents, and quantitatively evaluate the dendrite structure.

[0005] CN 116106304A provides a method for identifying the structure of a high nickel-chromium roll, comprising: A. selecting the outer ring of the head roller body of the high nickel-chromium roll and cutting it into a sample block, and performing rough grinding, fine grinding and polishing on the side of the sample block close to the core to obtain a mirror surface; B. preheating the sample block; C. then pre-etching the mirror surface, and observing the etching degree with a metallographic microscope; D. finally dyeing the pre-etched sample, and observing the dyeing effect under a microscope. The method is simple to operate and accurate in detection. The evaluation result of the metallographic structure of the high nickel-chromium roll can meet the requirements, thereby providing an important basis for optimizing the preparation process of the high nickel-chromium roll.

[0006] Although the above schemes all disclose the operation of observing the microstructure, they are not related to ferrite and cementite structures. Both structures appear white under a metallographic microscope at room temperature. Currently, they are mainly judged and analyzed based on their morphology and carbon content in steel. However, since the difference between the two structures is small under a microscope, it is impossible to clearly distinguish them through very accurate characteristics, and there is a risk of misjudgment in the structure judgment. Summary of the invention

[0007] The purpose of the present invention is to overcome the problem that the prior art cannot clearly distinguish ferrite and cementite structures in a sample, and to provide a method for distinguishing ferrite and cementite structures in a metallographic sample. The method can obtain a perfect morphological structure by corroding the sample with sodium metabisulfite, and combined with a scanning electron microscope, it can provide an accurate basis for the determination of ferrite and cementite structures.

[0008] In order to achieve the above object, the present invention provides a method for distinguishing ferrite and cementite structures in a metallographic sample, the method comprising the following steps:

[0009] (1) Pre-treating the metallographic sample and then immersing it in a corrosive agent for etching;

[0010] (2) Use scanning electron microscopy to observe the metallographic specimen after etching and distinguish between ferrite and cementite structures;

[0011] The metallographic structure of the metallographic sample contains ferrite and / or cementite;

[0012] The corrosive agent contains sodium metabisulfite.

[0013] Preferably, in step (1), pre-treating the metallographic sample is a conventional operation in the art, and the specific steps include: grinding and polishing the metallographic sample to a mirror surface.

[0014] In step (1) of the present invention, the content of sodium metabisulfite in the etchant is 5-12wt%, more preferably 6-9wt%; because if the concentration is too high, the corrosiveness will be too strong and it will be difficult to operate, while if the concentration is too low, the corrosiveness will be insufficient and the requirements cannot be met; in a specific embodiment, the content of sodium metabisulfite in the etchant is 8wt%.

[0015] Furthermore, in step (1), sodium metabisulfite and water are mixed and stirred uniformly to obtain the corrosive agent of the present invention.

[0016] In step (1), after the metallographic sample is pretreated, it is immersed in an etchant, and the amount of the etchant used is sufficient to completely immerse the pretreated metallographic sample.

[0017] In step (1), the pretreated metallographic sample is immersed in a corrosive agent for etching, and then the sample is taken out after observing the color change on the surface of the sample (the color change indicates that the etching is suitable). In order to achieve the corresponding effect, the etching time is usually controlled to 10-20 seconds, more preferably 10-15 seconds.

[0018] In the present invention, after the etching in step (1) is completed, a cleaning agent is used to clean the etched metallographic sample to remove the corrosive agent, and then the cleaned metallographic sample is dried.

[0019] Further preferably, in step (1), the cleaning agent is water and / or alcohol.

[0020] In step (2) of the present invention, the SE secondary electron image observation mode is used to observe the metallographic sample after drying, so that the ferrite and cementite structures can be distinguished. The principle is as follows:

[0021] Since ferrite is an interstitial solid solution of carbon dissolved in α-Fe, its corrosion resistance is relatively low, and its corrosion resistance to sodium metabisulfite is relatively low. Under a scanning electron microscope, the microstructure presents a smooth plane that is significantly lower than that of other organizations. Cementite is a metallic compound formed by iron and carbon, and its corrosion resistance to sodium metabisulfite is significantly higher. The microstructure presents a smooth plane that is significantly higher than that of other organizations. Therefore, the two metallographic organizations can be clearly distinguished.

[0022] Furthermore, the scanning electron microscope used in the present invention is a scanning electron microscope with a puzzle shooting function. The metallographic sample is photographed in the SE secondary electron image observation mode to obtain tissue photos, which can be used to further measure the relevant parameters of ferrite and cementite structures, facilitating other research.

[0023] The present invention utilizes an etchant containing sodium metabisulfite to etch a sample, and can obtain a clear morphological structure. By observing in a secondary electron image mode of a scanning electron microscope, the ferrite and cementite structures can be accurately determined. The present invention has the advantage that a small amount of chemical reagents are used when the etchant is configured. When a small amount of chemical reagents is used, the corrosion degree is appropriate, and a clear morphology of ferrite and cementite structures is obtained, and the ferrite and cementite structures are distinguished. The present invention is an environmentally friendly and accurate test method. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a microstructure photograph of the metallographic sample in Example 1;

[0025] Figure 2 This is a microstructure photograph of the metallographic sample in Example 2. DETAILED DESCRIPTION

[0026] The specific implementation of the present invention is described in detail below in conjunction with the accompanying drawings and examples. It should be understood that the specific implementation described herein is only used to illustrate and explain the present invention, and is not used to limit the present invention.

[0027] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0028] Example 1

[0029] The metallographic sample used in this embodiment is AAR-C low-carbon wheel steel, whose inherent metallographic structure is ferrite and pearlite;

[0030] (1) Sodium metabisulfite and distilled water are mixed and stirred to obtain an etchant, wherein the content of sodium metabisulfite in the etchant is 8 wt %; a cross section of a metallographic sample is ground and polished to a mirror surface, and then completely immersed in 10 mL of the etchant, and the surface changes of the metallographic sample are observed, and the surface color of the metallographic sample is obviously changed after etching for 15 seconds, and then the etched metallographic sample is taken out and cleaned with alcohol to clean off the etchant, and then dried;

[0031] (2) Use a scanning electron microscope with a puzzle shooting function and SE secondary electron image observation mode to observe the metallographic sample after drying and take photos of the metallographic sample to obtain the organizational photos of the metallographic sample, such as Figure 1 As shown, it can be clearly seen Figure 1 The blocky structure shown in area A in the middle shows obvious concave characteristics and is a ferrite structure. Figure 1 All block structures similar to or identical to the block structure in zone A correspond to ferrite structures.

[0032] Example 2

[0033] The metallographic sample used in this embodiment is GCr15 high carbon bearing steel, and its inherent metallographic structure is cementite and pearlite;

[0034] (1) Sodium metabisulfite and distilled water are mixed and stirred to obtain an etchant, wherein the content of sodium metabisulfite in the etchant is 8 wt %; a cross section of a metallographic sample is ground and polished to a mirror surface, and then completely immersed in 10 mL of the etchant, and the surface changes of the metallographic sample are observed, and the surface color of the metallographic sample is obviously changed after etching for 10 seconds, and then the etched metallographic sample is taken out and cleaned with alcohol to clean off the etchant, and then dried;

[0035] (2) Use a scanning electron microscope with a puzzle shooting function and SE secondary electron image observation mode to observe the metallographic sample after drying and take photos of the metallographic sample to obtain the organizational photos of the metallographic sample, such as Figure 2 As shown, it can be clearly seen Figure 2The blocky structure shown in area B in the middle shows obvious convexity and is cementite structure. Figure 2 All the blocky structures in the region that are similar or identical to the blocky structures in region B correspond to cementite structures.

[0036] It should be understood that parts not elaborated in detail in this specification belong to the prior art.

[0037] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for distinguishing ferrite and cementite structures in a metallographic sample, characterized in that: The method comprises the following steps: (1) Pre-treating the metallographic sample and then immersing it in a corrosive agent for etching; (2) Use scanning electron microscopy to observe the metallographic specimen after etching and distinguish between ferrite and cementite structures; The metallographic structure of the metallographic sample contains ferrite and / or cementite; The corrosive agent contains sodium metabisulfite.

2. The method according to claim 1, characterized in that In step (1), the step of pre-treating the metallographic sample includes grinding and polishing the metallographic sample to a mirror surface.

3. The method according to claim 1, characterized in that In step (1), the content of sodium metabisulfite in the etchant is 5-12 wt %.

4. The method according to claim 3, characterized in that In step (1), the content of sodium metabisulfite in the etchant is 6-9wt%.

5. The method according to any one of claims 1, 3 or 4, characterized in that: In step (1), sodium metabisulfite is mixed with water to obtain a corrosive agent.

6. The method according to claim 1, characterized in that In step (1), the etching time is 10-20 seconds.

7. The method according to claim 6, characterized in that In step (1), the etching time is 10-15 seconds.

8. The method according to claim 1, characterized in that In step (1), after etching is completed, the etched metallographic sample is cleaned with a cleaning agent and then dried.

9. The method according to claim 8, characterized in that In step (1), the cleaning agent is water and / or alcohol.

10. The method according to claim 1, characterized in that In step (2), observation is performed using the SE secondary electron image observation mode.

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