A method for displaying the nitride precipitation phase of economical duplex stainless steel

Abstract

The present invention belongs to the field of duplex stainless steel metallography, and specifically relates to an economical duplex stainless steel nitride precipitation phase display method, comprising the following steps: (1) preparing a metallographic sample; (2) preparing an etchant, according to a formula of 25-30 ml nitric acid, 270-290 ml anhydrous ethanol, and 0.5-1 ml hydrofluoric acid, wherein the nitric acid is analytically pure with a mass fraction of 65%-68%, the anhydrous ethanol is analytically pure with a mass fraction greater than or equal to 99.7%, and the hydrofluoric acid is analytically pure with a mass fraction greater than or equal to 40%; (3) etching the sample, completely immersing the prepared metallographic sample in the configured etchant, with the observation surface facing upward, and the etching time is 2-2.5 minutes; (4) microscopic observation. The method of the present invention can not only clearly display the two-phase structure of ferrite and austenite, but also can completely and clearly display the morphological characteristics of the nitride precipitation phase, which is easy to identify, and can use an X-ray energy spectrometer to accurately analyze and determine its composition.

Description

Technical Field

[0001] The invention belongs to the field of duplex stainless steel metallography, and in particular relates to an economical method for displaying a nitride precipitation phase of duplex stainless steel. Background Art

[0002] Economical duplex stainless steel is a new type of duplex stainless steel in which the expensive Ni element is replaced by the inexpensive austenitizing elements Mn and N while reducing the Mo content. It can replace 304 and 316 austenitic stainless steels to a certain extent in the fields of petrochemicals, storage tanks, bridges, etc. Economical duplex stainless steel often precipitates a nitride second phase during the processing and manufacturing process or when used at a temperature of 300℃ to 950℃ for a long time. The precipitation of nitrides will reduce the toughness and corrosion resistance of the material. Therefore, the corrosion analysis of the organization, especially the precipitation of the nitride second phase, is of great significance to the study of material properties.

[0003] At present, the economical duplex stainless steel microstructure display method often adopts hot alkaline potassium ferrocyanide solution, potassium hydroxide aqueous solution electrolysis or oxalic acid aqueous solution electrolysis and other etching methods; the first two methods use the principle of electrochemical etching deposition to display and distinguish the ferrite and austenite two-phase structure, the ferrite is dyed, if there is a nitride second phase in the microstructure, the nitride is also dyed, so the nitride and ferrite have the same color and degree, and the analyst cannot judge whether there is a nitride second phase precipitation; if the oxalic acid electrolysis method is used, the phase boundary of the ferrite and austenite phases can be displayed, under normal etching conditions, most of the nitride second phase will be dissolved to form pits, especially under the scanning electron microscope. The pits are extremely obvious. The above-mentioned etching methods cannot use X-ray spectrometers to accurately detect and analyze the composition of the nitride second phase due to the formation of a deposited film on the nitride dyed surface and the formation of pits by electrolysis, which seriously affects the research and analysis work. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a method for displaying the nitride precipitation phase of economical duplex stainless steel, so that the nitride precipitation phase in the microstructure of the economical duplex stainless steel after etching is easy to distinguish and identify, and the morphology is complete and clear, and no corrosion pits are formed due to erosion and shedding, which is convenient for accurate composition analysis and research by X-ray energy spectrometer.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for displaying an economical duplex stainless steel nitride precipitation phase comprises the following steps:

[0007] S1: Sample preparation

[0008] First, the metallographic specimen is cut by machining, and then the observation surface of the specimen is ground in sequence with metallographic sandpaper of different particle sizes; then the observation surface is fine-polished with woolen cloth or velvet fabric, and a high-efficiency diamond metallographic polishing agent with a particle size of 5μ and 1μ is used for fine polishing. After polishing, the observation surface of the specimen is bright, without deformation layer, scratches or holes; finally, the observation surface is cleaned with alcohol to confirm that the observation surface is clean and pollution-free.

[0009] S2: Prepare the etchant

[0010] Prepare the etching agent according to the formula of 25-30 ml nitric acid, 270-290 ml anhydrous ethanol, and 0.5-1 ml hydrofluoric acid, wherein the nitric acid is analytical grade with a mass fraction of 65%-68%, the anhydrous ethanol is analytical grade with a mass fraction greater than or equal to 99.7%, and the hydrofluoric acid is analytical grade with a mass fraction greater than or equal to 40%. Mix the three solutions in a plastic measuring cup and let it stand before use.

[0011] S3: Erosion sample

[0012] Completely immerse the prepared metallographic sample into the prepared etchant with the observation side facing upwards. The etching time is 2-2.5 minutes. When the surface of the sample turns light gray, take it out, rinse it with clean water and then with anhydrous ethanol, and then blow it dry with hot air.

[0013] S4: Microstructure observation

[0014] When the etched sample is observed under a microscope in bright field mode, the morphological characteristics of the precipitation of the nitride second phase can be clearly observed.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] (1) Through the erosion of specific etchants, not only can the ferrite and austenite two-phase structures be clearly displayed, but also the morphological characteristics of the nitride precipitation phase can be fully and clearly displayed, which is easy to identify, with a complete and clear morphology, and will not form pits due to erosion and shedding;

[0017] (2) The displayed nitride precipitate phase can be accurately analyzed and determined by X-ray energy spectrometer, which is convenient and direct, and avoids the problem that the composition of the nitride second phase cannot be detected and analyzed by X-ray energy spectrometer due to the formation of a deposited film on the nitride dyed surface and the formation of corrosion pits by electrolysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The microstructure of the economical duplex stainless steel 2101 after aging treatment at 750℃ for 2h is shown using this method. It can be clearly observed that the nitride precipitate phase is distributed on the interface between the austenite and ferrite phases. DETAILED DESCRIPTION

[0019] The present invention will be further described below in conjunction with embodiments, but the specific applicability of the present invention to economical duplex stainless steel materials is not limited to the steel types selected in the following examples.

[0020] The selected economical duplex stainless steel material is a plate with a thickness of 12 mm, and the state is aging treatment at 750 ° C for 2 hours. The specific chemical composition (mass percentage %) is: C: 0.045, Si: 0.78, Mn: 5.74, Cr: 21.58, Ni: 1.45, N: 0.22, and the balance is Fe and unavoidable impurities. The analysis equipment uses a Leica DM4000M optical microscope.

[0021] A method for displaying an economical duplex stainless steel nitride precipitation phase comprises the following steps:

[0022] (1) Sample preparation

[0023] First, machine and cut 20×20 mm square metallographic specimens. When sampling, pay attention to prevent changes in microstructure caused by temperature increase or plastic deformation caused by force. Then, the observation surface of the specimen is grinded in sequence with metallographic sandpapers of different grits, such as 120#, 240#, 320#, 500#, 800# and 1000#. Then, use woolen cloth or velvet fabric to fine-polish the observation surface. During fine-polishing, use high-efficiency diamond metallographic polishing agent with grit of 5μ and 1μ. During fine-polishing, control humidity and strength. After polishing, the observation surface of the specimen is bright, without deformation layer, scratches or holes. Finally, clean the observation surface with alcohol to confirm that the observation surface is clean and pollution-free.

[0024] (2) Preparation of etching agent

[0025] The preparation of the etchant should be carried out in a well-ventilated fume hood. The etchant should be prepared according to the formula of 25ml nitric acid, 270ml anhydrous ethanol, and 0.5ml hydrofluoric acid, wherein the nitric acid is analytically pure with a mass fraction of 65% to 68%, the anhydrous ethanol is analytically pure with a mass fraction greater than or equal to 99.7%, and the hydrofluoric acid is analytically pure with a mass fraction greater than or equal to 40%. Mix the three solutions in a plastic measuring cup and let it stand before use.

[0026] (3) Erosion samples

[0027] Use a bamboo sample clamp to completely immerse the prepared metallographic sample into the prepared etchant, with the observation side facing up. During the etching process, gently wipe the observation side of the sample with absorbent cotton. The etching time is 2 minutes. When the surface of the sample turns light gray, take it out, rinse it with clean water and then with anhydrous ethanol, and then blow it dry with hot air.

[0028] (4) Microstructure observation

[0029] Place the etched sample under a microscope and observe it in bright field mode. Figure 1 It can be seen that not only the ferrite and austenite two-phase structures can be observed, but also the granular nitride precipitation phase can be clearly observed to be distributed on the interface between the austenite and ferrite phases.

Claims

1. A method for displaying the nitride precipitation phase of an economical duplex stainless steel, characterized in that: The following steps are involved: S1: Prepare samples; First, the metallographic specimen is cut by machining, and then the observation surface of the specimen is ground in sequence with metallographic sandpaper of different particle sizes; then the observation surface is finely polished with woolen cloth or velvet fabric, and a high-efficiency metallographic polishing agent with a particle size of 5μ and 1μ diamond is used for fine polishing. After polishing, the observation surface of the specimen is bright, without deformation layer, scratches or pits; finally, the observation surface is cleaned with alcohol to confirm that the observation surface is clean and free of pollution; S2: prepare the etchant; Prepare the etchant according to the formula of 25-30 ml nitric acid, 270-290 ml anhydrous ethanol, and 0.5-1 ml hydrofluoric acid, wherein the nitric acid is analytically pure with a mass fraction of 65% to 68%, the anhydrous ethanol is analytically pure with a mass fraction greater than or equal to 99.7%, and the hydrofluoric acid is analytically pure with a mass fraction greater than or equal to 40%. Mix the three solutions in a plastic measuring cup and let it stand before use; S3: eroded sample; Immerse the prepared metallographic sample completely into the prepared etchant, with the observation side facing upwards. The etching time is 2-2.5 minutes. When the surface of the sample turns light gray, take it out, rinse it with clean water and then with anhydrous ethanol, and then blow it dry with hot air. S4: microstructure observation; When the etched sample is observed under a microscope in bright field mode, the morphological characteristics of the precipitation of the nitride second phase can be clearly observed.

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