Austenitic stainless steel metallographic etching agent and its preparation method and application

Abstract

The invention belongs to the field of metallographic inspection technology, and specifically relates to an austenitic stainless steel metallographic etchant and a preparation method and application thereof. The austenitic stainless steel metallographic etchant comprises the following components in parts by weight: 1 to 2 parts of ferric chloride, 1 to 3 parts of persulfate, 5 to 15 parts of acid reagent, and 80 to 120 parts of solvent. The etchant prepared by the invention has the characteristics of good corrosion effect, strong stability, high detection efficiency, high safety, etc., can conveniently and clearly display austenitic grain boundaries and σ phases, M23C6 and other harmful aging precipitation phases, can be used for on-site organization assessment of austenitic stainless steel pipes for (super)critical boilers, and the materials of austenitic stainless steel pipes that can be detected include but are not limited to S30432, TP347H, HR3C, etc., avoiding cutting pipe inspection, and greatly saving the maintenance period and cost of thermal power plants.

Description

Technical Field

[0001] The invention belongs to the technical field of metallographic inspection, and in particular relates to an austenitic stainless steel metallographic corrosive agent and a preparation method and application thereof. Background Art

[0002] In recent years, the deep peak-shaving operation of thermal power generating units has also brought great challenges to the wall temperature control of tube screens such as superheaters and reheaters of (super)critical units. Overtemperature of various heating surface tubes has become more and more frequent. On-site metallographic inspection of austenitic stainless steel has always been a difficult point in the industry, mainly manifested in the harsh on-site inspection conditions, high temperature and closed space, and high dust; conventional corrosive agents cannot display the organizational morphology well, and the grain boundaries and precipitated phases are not clear, which cannot match the on-site inspection environment well.

[0003] Aqua regia is one of the commonly used corrosive agents for metallographic inspection of austenitic stainless steel, but it is highly corrosive and volatile. The site is usually a closed space, so aqua regia is not suitable for on-site metallographic inspection conditions, especially when the temperature of the stainless steel pipe wall is high. The organization display effect is poor, the corrosion time is difficult to control, and the risk of use is high. Picric acid hydrochloric acid alcohol solution corrodes stainless steel at a slow rate, and picric acid is an explosive chemical, and its purchase and use are strictly controlled. Ferric chloride hydrochloric acid aqueous solution can be used in the laboratory to detect austenitic stainless steel structure, but in on-site metallographic inspection, it was found that the organization effect was not good, uneven corrosion was prone to uneven corrosion such as mottling, and the corrosion time was difficult to grasp.

[0004] Chinese patent CN114594098A discloses an optional metallographic etchant. Although this etchant can clearly display the σ phase (black under an optical microscope) in austenitic stainless steel, it cannot clearly display the grain boundary, and it is not easy to determine the grain size. When the sample is corroded, the reaction is violent, and it is not easy to operate. There are certain safety hazards. In addition, the above method is not suitable for the dusty detection environment of the austenitic stainless steel heating surface tube in the boiler furnace. Under the on-site metallographic microscope, the dust particles and the σ phase in this method are both black and difficult to distinguish. Therefore, in order to more conveniently carry out on-site austenitic stainless steel metallographic structure aging and grain size assessment inspection work, it is urgent to develop an etchant suitable for the site. Summary of the invention

[0005] The present invention aims to provide an austenitic stainless steel metallographic etchant and a preparation method and application thereof. The etchant prepared by the present invention has the characteristics of good corrosion effect, strong stability, high detection efficiency, high safety, etc., can conveniently and clearly display austenitic grain boundaries and harmful aging precipitation phases such as σ phase and M23C6, can be used for on-site organization assessment of austenitic stainless steel pipes for (super)critical boilers, and the materials of austenitic stainless steel pipes that can be detected include but are not limited to S30432, TP347H, HR3C, etc., avoiding pipe cutting inspection, and greatly saving the maintenance period and cost of thermal power plants.

[0006] In order to achieve the above purpose, the present invention adopts the following technical scheme: an austenitic stainless steel metallographic etching agent, comprising the following components in parts by weight: 1-2 parts of ferric chloride, 1-3 parts of persulfate, 5-15 parts of acid reagent, and 80-120 parts of solvent.

[0007] Preferably, the austenitic stainless steel metallographic etching agent comprises the following components in parts by weight: 1 part of ferric chloride, 2 parts of persulfate, 10 parts of acid reagent, and 100 parts of solvent.

[0008] Preferably, the ferric chloride is ferric chloride hexahydrate, with the molecular formula of FeCl 3 6H 2 O.

[0009] Preferably, the persulfate includes at least one of ammonium persulfate, potassium persulfate, and sodium persulfate. More preferably, the persulfate is ammonium persulfate.

[0010] Preferably, the acid reagent includes at least one of concentrated hydrochloric acid and sulfuric acid. More preferably, the concentration of the concentrated hydrochloric acid is 36% to 38%.

[0011] Preferably, the solvent is a mixed solution of hydrogen peroxide solution and ethanol solution (hydrogen peroxide ethanol solution), wherein the concentration of the hydrogen peroxide solution is 5% to 10%, and the purity of the ethanol solution is above 99.5%.

[0012] Preferably, the volume of the hydrogen peroxide solution is 40-60% of the total volume of the mixed solution.

[0013] Preferably, the austenitic stainless steel metallographic etching agent further comprises 0.05 to 0.1 parts of a surfactant component.

[0014] Preferably, the surfactant includes at least one of sodium alkyl sulfonate and sodium fatty alcohol ether sulfate. The surfactant in the present invention can also be replaced by detergent whose main components are sodium alkyl sulfonate and sodium fatty alcohol ether sulfate.

[0015] The present invention also claims a method for preparing the austenitic stainless steel metallographic corrosive agent, comprising the following steps:

[0016] All components except the acid reagent are dissolved in the solvent in sequence, and then the acid reagent is slowly added and allowed to stand for more than 1 hour to prepare the austenitic stainless steel metallographic etching agent.

[0017] The present invention also claims to protect the use of the austenitic stainless steel metallographic corrosive agent in detecting the aging condition and grain size of the austenitic stainless steel metallographic structure.

[0018] The etchant prepared by the present invention contains Fe3+ , S 2 O 8 2- , H + and H 2 O 2 When the metallographic sample is corroded, the main reaction is the joint reaction of acid corrosion and oxidation corrosion. 2 O 8 ) 2- and H 2 O 2 The higher the standard electrode potential, the stronger the ions (molecules) are. 4 ) 2 S 2 O 8 and H 2 O 2 Fe, Fe 2+ Oxidized to Fe 3+ , providing relatively stable Fe for the corrosive agent 3+ concentration environment, and the system contains H + Ions and H 2 O 2 The molecules can effectively inhibit (S 2 O 8 ) 2- ions, thereby achieving the hydrolysis of (S 2 O 8 ) 2- , Fe 3+ and H + The combined effect achieves a good presentation of the metallographic structure.

[0019] In addition, metallographic sample preparation points at production sites usually use oily diamond powder polishing paste, and oily dirt usually remains on the surface. The present invention uses hydrogen peroxide ethanol solution as a solvent and adds an appropriate amount of surfactant, which can effectively reduce the surface tension of the solvent liquid, avoid the phenomenon of uneven erosion caused by poor flushing of the metallographic sample preparation points, and make the erosion more uniform and easier to control.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) The etchant prepared by the present invention is stable, suitable for high-temperature closed environments on site, easy to operate, and the erosion time is easy to control. The effect of 100% displaying the good tissue morphology at one time can be achieved by using an absorbent cotton ball to dip the reagent and directly wipe the metallographic polishing point. At the same time, since the configured reagent is a low-concentration reagent, as the erosion time increases, the effective ingredients in the reagent are gradually consumed, so excessive erosion will not occur.

[0022] (2) The etchant of the present invention has an excellent corrosion effect, can clearly show the grain boundary, precipitated phase, especially the σ phase (pure white under an optical microscope), and has a significant color difference with the dust particles attached to the surface of the metallographic measurement point in a dusty environment, which is easy to distinguish, and the inspector can easily obtain key information such as the grain size level and the tissue aging level. At the same time, the etchant prepared by the present invention has a low acid concentration and a low risk of use. If it accidentally touches the skin during the corrosion process, it can be immediately washed with water or alcohol without causing damage to the skin.

[0023] (3) The etchant prepared by the present invention has the advantage of high detection efficiency. By using the etchant of the present invention, the speed and efficiency of metallographic detection of austenitic stainless steel on site can be greatly improved, and it is suitable for large-scale aging and grain size survey of austenitic stainless steel on site. Dip a cotton ball in the etchant, and 3 to 5 metallographic sample preparation points can be etched at the same time, and generally there will be no secondary rework due to too light or too heavy corrosion. Using the etchant of the present invention, each group of metallographic inspection personnel can detect more than 100 metallographic points per day (based on 7 hours of effective working time), and the detection efficiency is improved by more than 5 times. After actual testing of the metallographic survey of superheaters in a power plant, two groups of metallographic inspection personnel completed the metallographic inspection of about 1,700 points on site in 7 days using the etchant of the present invention. It was found that the grain size of about 30% of the metallographic measurement points did not meet the standard requirements, and about 5.7% had aging of more than level 4, which won more maintenance and construction time for the power plant. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a diagram showing the effect of using a corrosive agent in Example 1 of the present invention to detect the mildly aged structure of austenitic stainless steel.

[0025] Figure 2 This is a diagram showing the effect of using a corrosive agent in Example 1 of the present invention to detect the moderately aged structure of austenitic stainless steel.

[0026] Figure 3 This is a diagram showing the effect of using a corrosive agent in Example 1 of the present invention to detect the severely aged structure of austenitic stainless steel.

[0027] Figure 4 This is a diagram showing the effect of using a corrosive agent in Example 2 of the present invention to detect the severely aged structure of austenitic stainless steel.

[0028] Figure 5 This is a diagram showing the effect of the corrosive agent detecting austenitic stainless steel without obvious aging structure in Example 3 of the present invention.

[0029] Figure 6 This is a diagram showing the effect of using a corrosive agent in comparative example 1 of the present invention to detect the mildly aged structure of austenitic stainless steel.

[0030] Figure 7 This is a diagram showing the effect of using a corrosive agent in comparative example 1 of the present invention to detect the moderately aged structure of austenitic stainless steel.

[0031] Figure 8 This is a diagram showing the effect of using a corrosive agent in comparative example 1 of the present invention to detect the severely aged structure of austenitic stainless steel.

[0032] Fig. 9 This is a diagram showing the metallographic structure of austenitic stainless steel detected by the corrosive agent in comparative example 2 of the present invention.

[0033] Fig.10 This is a diagram showing the metallographic structure of austenitic stainless steel detected by the corrosive agent in comparative example 3 of the present invention.

[0034] Fig.11 This is a diagram showing the metallographic structure of austenitic stainless steel detected by the corrosive agent in comparative example 4 of the present invention.

[0035] Fig.12 This is a diagram showing the metallographic structure of austenitic stainless steel detected by the corrosive agent in comparative example 5 of the present invention.

[0036] Fig.13 This is a diagram showing the metallographic structure of austenitic stainless steel detected by the corrosive agent in comparative example 6 of the present invention. DETAILED DESCRIPTION

[0037] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0038] In the embodiments and comparative examples, the experimental methods used are conventional methods unless otherwise specified, and the materials, reagents, etc. used are all commercially available unless otherwise specified.

[0039] Example 1: Austenitic stainless steel metallographic etchant and preparation method thereof

[0040] Formula: 1g ferric chloride, 2g ammonium persulfate, 10ml concentrated hydrochloric acid, 100ml hydrogen peroxide ethanol solution (the volume of hydrogen peroxide is 50% of the total volume of hydrogen peroxide ethanol solution).

[0041] Preparation method: Dissolve ferric chloride and ammonium persulfate in hydrogen peroxide ethanol solution in sequence, then slowly add concentrated hydrochloric acid and let stand for 2 hours.

[0042] Example 2: Austenitic stainless steel metallographic etchant and preparation method thereof

[0043] Formula: 2g ferric chloride, 1g potassium persulfate, 5ml concentrated hydrochloric acid, 80ml hydrogen peroxide ethanol solution (the volume of hydrogen peroxide is 60% of the total volume of hydrogen peroxide ethanol solution).

[0044] Preparation method: Dissolve ferric chloride and ammonium persulfate in hydrogen peroxide ethanol solution in sequence, then slowly add concentrated hydrochloric acid and let stand for 2 hours.

[0045] Example 3: Austenitic stainless steel metallographic etchant and preparation method thereof

[0046] Formula: 2g ferric chloride, 3g sodium persulfate, 15ml concentrated hydrochloric acid, 120ml hydrogen peroxide ethanol solution (the volume of hydrogen peroxide is 40% of the total volume of hydrogen peroxide ethanol solution), and 0.1ml sodium alkyl sulfonate.

[0047] Preparation method: Dissolve ferric chloride, sodium persulfate and sodium alkyl sulfonate in hydrogen peroxide ethanol solution in sequence, then slowly add concentrated hydrochloric acid and let stand for 2 hours.

[0048] Comparative Example 1

[0049] Compared with Example 1, the difference of this comparative example is that a conventional corrosive agent in the art (DL / T884-2019) is selected.

[0050] Formula: 5g ferric chloride, 50ml concentrated hydrochloric acid, 100ml water.

[0051] The preparation method is similar to that of Example 1.

[0052] Comparative Example 2

[0053] Compared with Example 1, the only difference of this comparative example is that no ammonium persulfate component is added.

[0054] The preparation method is similar to that of Example 1.

[0055] Comparative Example 3

[0056] Compared with Example 1, the only difference of this comparative example is that the hydrogen peroxide solution component is replaced by an equal amount of water.

[0057] The preparation method is similar to that of Example 1.

[0058] Comparative Example 4

[0059] Compared with Example 1, the difference of this comparative example is that a higher concentration of the etchant is selected.

[0060] Formula: 3g ferric chloride, 5g ammonium persulfate, 10ml concentrated hydrochloric acid, 100ml hydrogen peroxide ethanol solution (the volume of hydrogen peroxide is 50% of the total volume of hydrogen peroxide ethanol solution).

[0061] The preparation method is similar to that of Example 1.

[0062] Comparative Example 5

[0063] Compared with Example 1, the difference of this comparative example is only that ferric chloride is replaced by an equal amount of cupric sulfate.

[0064] The preparation method is similar to that of Example 1.

[0065] Comparative Example 6

[0066] Compared with Example 1, the difference of this comparative example is only that ammonium persulfate is replaced by an equal amount of potassium permanganate.

[0067] The preparation method is similar to that of Example 1.

[0068] Test Example 1: Corrosion Effect Test

[0069] The etchants prepared in Examples 1 to 3 and Comparative Examples 1 to 6 were tested for corrosion effects.

[0070] Test method:

[0071] S1. Surface treatment before erosion: oily diamond powder polishing paste + mechanical polishing is used on site, and the particle size of the polishing paste is in the range of 2.5-5.0μm;

[0072] S2. After polishing the metallographic inspection point, use a squeezed alcohol cotton ball to wipe and remove the surface oil stains;

[0073] S3. Use an absorbent cotton ball to dip into the prepared etchant and wipe the metallographic sample points. It is appropriate to observe the color change of the polished surface in about 3 seconds. Etch until the test surface is uniformly gray. The total etching time is 10 to 30 seconds.

[0074] S4. After the etching is completed, spray the etched surface with alcohol, and use the squeezed alcohol cotton ball to wipe off the residual alcohol and dust on the surface;

[0075] S5. Use on-site metallographic testing equipment to conduct metallographic testing.

[0076] The experimental results are shown in Table 1 and Appendix Figures 1 to 13 shown.

[0077] Table 1

[0078]

[0079]

[0080] From Table 1 and the attached Figures 1 to 13It can be seen that the corrosion product prepared in the embodiment of the present invention has a good corrosion effect and clear grain boundaries. Under an optical microscope, the common aged σ phase and M23C6 precipitation phase in the organization are pure white and highly recognizable. The inspectors can easily obtain key information such as grain size level and organization aging level based on the organization picture.

[0081] In Comparative Example 1, a conventional etchant in the art is used for etching. For the moderately aged sample or above, the black block σ phase can be clearly displayed, and the second phase precipitated in the crystal can be identified, but the grain boundary display effect is not good; Comparative Example 2 lacks ammonium persulfate, and the hydrogen peroxide solution is replaced with an equal amount of water in Comparative Example 3, and the ferric chloride is replaced with an equal amount of copper sulfate in Comparative Example 5, and the ammonium persulfate is replaced with an equal amount of potassium permanganate in Comparative Example 6. The metallographic corrosion effect of the etchant prepared is worse than that of the embodiment; Comparative Example 4 uses a higher concentration of etchant, which can display the white block σ phase, but the grain boundary and the crystal display effect are not good.

[0082] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. An austenitic stainless steel metallographic etchant, characterized in that: The invention is composed of the following components: 1 g of ferric chloride, 2 g of ammonium persulfate, 10 ml of concentrated hydrochloric acid and 100 ml of hydrogen peroxide ethanol solution, wherein the volume of hydrogen peroxide in the hydrogen peroxide ethanol solution is 50% of the total volume of the hydrogen peroxide ethanol solution.

2. An austenitic stainless steel metallographic etchant, characterized in that: The invention is composed of the following components: 2 g of ferric chloride, 1 g of potassium persulfate, 5 ml of concentrated hydrochloric acid and 80 ml of hydrogen peroxide ethanol solution, wherein the volume of hydrogen peroxide in the hydrogen peroxide ethanol solution is 60% of the total volume of the hydrogen peroxide ethanol solution.

3. A method for preparing the metallographic etching agent for austenitic stainless steel as claimed in claim 1 or 2, characterized in that: The following steps are involved: Ferric chloride and ammonium persulfate were fully dissolved in hydrogen peroxide ethanol solution in sequence, and then concentrated hydrochloric acid was slowly added and allowed to stand for 2 hours.

4. Use of the austenitic stainless steel metallographic etching agent as claimed in claim 1 or 2 in detecting the aging of the metallographic structure and grain size of austenitic stainless steel.

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