A method for grain boundary display of an erosion agent, low-alloy high-strength steel

Abstract

The application provides an etchant and a grain boundary display method of low-alloy high-strength steel, and relates to the technical field of metallographic structure display.The etchant provided by the application can be stored for a long time, repeatedly used, has no strong corrosion and no danger of combustion and explosion, is used for grain boundary display of low-alloy high-strength steel, is safe and simple to operate, does not need heating and cooling, has wide applicability to low-alloy high-strength steel with different components and heat treatment states, and has the advantages that the grain boundary of the low-alloy high-strength steel obtained has clear contour and uniform etching, and can better meet the requirement of accurate determination of microcrystalline grain size.

Description

Technical Field

[0001] This invention relates to the field of metallographic structure display technology, and more particularly to a method for displaying grain boundaries using an etchant and low-alloy high-strength steel. Background Technology

[0002] Low-alloy high-strength steel has high yield strength, good plasticity, toughness, weldability, and resistance to atmospheric and seawater corrosion, so it is widely used in infrastructure construction, machinery manufacturing, national energy development, etc.

[0003] Grain size has a crucial impact on the hardness, tensile strength, and ductility of metals. Grain refinement achieves a fine-grain strengthening effect and is one of the important means to improve the performance of low-alloy high-strength steels. For the evaluation and optimization of metal processing technologies such as forging, rolling, heat treatment, and welding, grain size is a key performance indicator, playing a vital role in performance optimization and process improvement. Therefore, clearly displaying and accurately measuring grain size is particularly crucial. The primary task of grain size measurement is to identify grain boundaries, i.e., the contact interface between adjacent grains. Clearly displaying grain boundaries is a necessary prerequisite for the smooth progress of the measurement procedure and for the measurement results to effectively guide the process.

[0004] Chemical etching is a common method for revealing grain boundaries in low-alloy high-strength steel. Commonly used etching agent ①: 3-4% nitric acid in ethanol solution. This agent is effective for revealing the microstructure of tempered low-alloy high-strength steel, but poor for grain boundary revealing, especially for quenched or lightly tempered low-alloy high-strength steel, where grain boundaries are often not revealed or are only vaguely revealed. Commonly used etching agent ②: picric acid, sodium tridecylbenzenesulfinate, and water. Using this agent usually requires pre-heat treatment of the sample before etching by heating, making the procedure cumbersome, time-consuming, and labor-intensive. It also requires finding a suitable heat treatment regime or heating temperature, and is prone to problems such as no grain boundary reveal, vague revealing, and numerous corrosion pits, making accurate grain size determination difficult. Furthermore, this etching agent cannot be reused, resulting in reagent waste, and picric acid is flammable, explosive, and toxic, posing certain risks to human health and environmental safety. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide an etchant and a method for grain boundary display of low-alloy high-strength steel. The etchant provided by this invention can be stored for a long time, reused repeatedly, is non-corrosive, and poses no risk of combustion or explosion. It is used for grain boundary display of low-alloy high-strength steel, and the operation process is safe and simple, requiring no heating or cooling. It is also widely applicable to low-alloy high-strength steel with different compositions and heat treatment states. The obtained low-alloy high-strength steel grain boundaries have the advantages of clear contours and uniform etchation, which can better meet the requirements for accurate measurement of micrograin size.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] The present invention provides an etchant comprising phosphoric acid, methanol, and hydrochloric acid; wherein the mass fraction of the phosphoric acid is ≥85%; the mass fraction of the hydrochloric acid is 36-38%; the volume ratio of the phosphoric acid to methanol is 7-9:2-4; and the volume ratio of the phosphoric acid to hydrochloric acid is 70-90:1-10.

[0008] Preferably, the methanol has a mass fraction of ≥99.5%.

[0009] Preferably, the volume ratio of phosphoric acid to methanol is 8:2 to 4.

[0010] Preferably, the volume ratio of phosphoric acid to hydrochloric acid is 80:1 to 10.

[0011] This invention provides a method for grain boundary display of low-alloy high-strength steel, comprising the following steps:

[0012] Low-alloy high-strength steel is used as the anode, and electrolytic etching is carried out in the etchant described in the above scheme.

[0013] Preferably, the constant current used in the electrolytic etching is 1.5 to 2.5 A.

[0014] Preferably, the electrolytic erosion time is 100-300 seconds.

[0015] Preferably, the cathode used in the electrolytic etching is a nickel plate.

[0016] Preferably, the low-alloy high-strength steel includes: quenched and tempered low-alloy high-strength steel, quenched low-alloy high-strength steel, or quenched and low-temperature tempered low-alloy high-strength steel.

[0017] Preferably, before the electrolytic etching, the inspection surface of the low-alloy high-strength steel is further subjected to mechanical grinding and polishing.

[0018] The present invention provides an etchant comprising phosphoric acid, methanol, and hydrochloric acid; wherein the mass fraction of the phosphoric acid is ≥85%; the mass fraction of the hydrochloric acid is 36-38%; the volume ratio of the phosphoric acid to methanol is 7-9:2-4; and the volume ratio of the phosphoric acid to hydrochloric acid is 70-90:1-10.

[0019] The etchant provided by this invention can be stored for a long time and used repeatedly. There is no risk of strong corrosion or explosion during use. The resulting low-alloy high-strength steel grain boundaries have the advantages of clear outlines and uniform erosion, which can better meet the needs of accurately measuring micrograin size.

[0020] This invention provides a method for grain boundary display of low-alloy high-strength steel. The operation process is safe and simple, requires no heating or cooling, has high sample preparation efficiency, and has the advantages of wide applicability and high measurement efficiency for low-alloy high-strength steel with different compositions and heat treatment states. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the electrolytic etching device of the present invention, wherein: 1-DC power supply; 2-ammeter; 3-voltmeter; 4-insulating container; 5-etching agent; 6-cathode; 7-low alloy high-strength steel; 8-inspection surface;

[0022] Figure 2 The grain boundary morphology was observed by metallographic microscope after electrolytic etching of a tempered low-alloy high-strength steel sample of material 34CrNi3MoA using the etchant of Example 1.

[0023] Figure 3 The grain boundary morphology was observed by metallographic microscope to chemically etch a tempered low-alloy high-strength steel sample of material 34CrNi3MoA using the etchant of Comparative Example 1.

[0024] Figure 4 The grain boundary morphology was observed by metallographic microscope after electrolytic etching of a quenched low-alloy high-strength steel sample of material 38CrMoAl using the etchant of Example 2.

[0025] Figure 5 The grain boundary morphology was observed by metallographic microscope after chemical etching of a quenched low-alloy high-strength steel sample of material 38CrMoAl using the etchant of Comparative Example 2.

[0026] Figure 6 The grain boundary morphology was observed by metallographic microscope after electrolytic etching of a quenched and low-temperature tempered low-alloy high-strength steel sample of 25Cr2MoVA using the etchant of Example 3.

[0027] Figure 7 The grain boundary morphology was observed by metallographic microscope after chemical etching of a quenched and low-temperature tempered low-alloy high-strength steel sample of 25Cr2MoVA using the etchant of Comparative Example 3.

[0028] Figure 8 The grain boundary morphology was observed by metallographic microscope after electrolytic etching of a tempered low-alloy high-strength steel sample of material 16Cr3NiWMoVNbE using the etchant of Example 4.

[0029] Figure 9 The grain boundary morphology was observed by metallographic microscope after chemical etching of a tempered low-alloy high-strength steel sample of material 16Cr3NiWMoVNbE using the etchant of Comparative Example 4. Detailed Implementation

[0030] The present invention provides an etchant comprising phosphoric acid, methanol, and hydrochloric acid; wherein the mass fraction of the phosphoric acid is ≥85%; the mass fraction of the hydrochloric acid is 36-38%; the volume ratio of the phosphoric acid to methanol is 7-9:2-4; and the volume ratio of the phosphoric acid to hydrochloric acid is 70-90:1-10.

[0031] In this invention, the mass fraction of methanol is preferably ≥99.5%.

[0032] In this invention, the volume ratio of phosphoric acid to methanol is preferably 8:2 to 4, and in the embodiments of this invention, it is specifically 8:4.

[0033] In this invention, the volume ratio of phosphoric acid to hydrochloric acid is preferably 80:1 to 10, more preferably 80:2 to 6, and in the embodiments of this invention, it is specifically 80:2.

[0034] The present invention does not have special requirements for the preparation method of the etchant; the substances can be mixed evenly directly.

[0035] This invention provides a method for grain boundary display of low-alloy high-strength steel, comprising the following steps:

[0036] Low-alloy high-strength steel is used as the anode, and electrolytic etching is carried out in the etchant described in the above scheme.

[0037] This invention does not impose special requirements on the composition and heat treatment state of the low-alloy high-strength steel. The grain boundary display method of this invention is widely applicable to low-alloy high-strength steels with different compositions and heat treatment states. In the embodiments of this invention, the low-alloy high-strength steel specifically refers to: quenched and tempered low-alloy high-strength steel of material 34CrNi3MoA, quenched low-alloy high-strength steel of material 38CrMoAl, quenched and low-temperature tempered low-alloy high-strength steel of material 25Cr2MoVA, or quenched and tempered low-alloy high-strength steel of material 16Cr3NiWMoVNbE.

[0038] Before electrolytic etching, the present invention preferably performs mechanical grinding and polishing on the inspection surface of the low-alloy high-strength steel. The present invention has no special requirements for the mechanical grinding and polishing process; as long as a smooth and bright inspection surface is obtained, it is acceptable. In embodiments of the present invention, the mechanical grinding specifically involves sequentially using 150# and 800# wet sandpaper, and 1000#, 3000#, and 7000# metallographic sandpaper; the polishing specifically involves using a velvet cloth and a diamond polishing agent with a particle size of 1.0 μm.

[0039] In this invention, the cathode used for electrolytic etching is preferably a nickel plate. There are no special requirements for the amount of etchant used; it is sufficient to completely immerse the low-alloy high-strength steel. In this invention, the constant current used for electrolytic etching is preferably 1.5–2.5 A, more preferably 2 A; the electrolytic etching time is preferably 100–300 s, more preferably 130–250 s, and even more preferably 150–200 s.

[0040] Figure 1 This is a schematic diagram of the electrolytic etching device of the present invention, wherein: 1-DC power supply; 2-ammeter; 3-voltmeter; 4-insulating container; 5-etching agent; 6-cathode; 7-low alloy high-strength steel; 8-inspection surface. Figure 1 As shown, in this invention, low-alloy high-strength steel 7 is used as the anode and nickel plate is used as the cathode 6. Both are immersed in the prepared etching agent 5, and the inspection surface 8 is placed directly opposite the cathode 6. DC power supply 1 is connected to carry out electrolytic etching, and the solution is continuously stirred or the sample is shaken during the process.

[0041] After the electrolytic etching is completed, the inspection surface can be observed under a metallographic microscope.

[0042] The etchant provided by this invention can be stored for a long time, reused repeatedly, and is non-corrosive and non-flammable. It is used for grain boundary display of low-alloy high-strength steel. The operation process is safe and simple, requiring no heating or cooling. It is widely applicable to low-alloy high-strength steel with different compositions and heat treatment states. The obtained low-alloy high-strength steel grain boundaries have the advantages of clear outlines and uniform etch, which can better meet the needs of accurate measurement of micrograin size.

[0043] The following detailed description of the etchant and grain boundary display method for low-alloy high-strength steel provided by the present invention, with reference to the embodiments, should not be construed as limiting the scope of protection of the present invention.

[0044] Example 1

[0045] This embodiment uses a quenched and tempered low-alloy high-strength steel sample made of 34CrNi3MoA.

[0046] An etchant for preparing microscopic samples of low-alloy high-strength steel is composed of 80 mL of phosphoric acid (mass fraction ≥ 85%), 40 mL of methanol (mass fraction ≥ 99.5%), and 2 mL of hydrochloric acid (mass fraction 36%–38%).

[0047] Grain boundary visualization method for microscopic specimens of quenched and tempered low-alloy high-strength steel (34CrNi3MoA):

[0048] The samples were mechanically ground sequentially using 150# and 800# wet sandpaper, and 1000#, 3000# and 7000# metallographic sandpaper, and then polished using a gold velvet cloth and a diamond polishing agent with a particle size of 1.0μm.

[0049] The polished sample was used as the anode, and the nickel plate as the cathode, both immersed in the prepared electrolytic polishing solution, with the inspection surface facing the nickel plate. A DC power supply was applied for electrolytic etching, with continuous stirring of the solution or shaking of the sample during the process. A constant current of 2.0 A was maintained during the electrolytic etching process, and the sample was removed after 150 seconds. The etched 34CrNi3MoA sample was rinsed with running water and anhydrous ethanol at room temperature, and then quickly dried. The inspection surface was observed under a metallographic microscope under magnification; the grain boundary morphology was as follows: Figure 2 As shown.

[0050] Example 2

[0051] This embodiment uses a quenched low-alloy high-strength steel sample made of 38CrMoAl. The etchant and grain boundary display method used are the same as in Example 1. The electrolytic etching parameters are 2.2A and 160s. The grain boundary morphology is as follows: Figure 4 As shown.

[0052] Example 3

[0053] This embodiment uses a quenched and low-temperature tempered low-alloy high-strength steel sample of 25Cr2MoVA. The etchant and grain boundary display method used are the same as in Example 1. The electrolytic etching parameters are 1.6A and 200s. The grain boundary morphology is as follows: Figure 6 As shown.

[0054] Example 4

[0055] This embodiment uses a quenched and tempered low-alloy high-strength steel sample of material 16Cr3NiWMoVNbE. The etchant and grain boundary display method used are the same as in Example 1. The electrolytic etching parameters are 1.8A and 180s. The grain boundary morphology is as follows: Figure 8 As shown.

[0056] Comparative Example 1

[0057] This comparative example uses a quenched and tempered low-alloy high-strength steel sample made of 34CrNi3MoA.

[0058] An etchant for preparing microscopic samples of low-alloy high-strength steel is composed of 2g of picric acid (analytical grade), 1g of sodium tridecylbenzenesulfinate (analytical grade), and 100mL of distilled water.

[0059] Grain boundary display method for quenched and tempered low-alloy high-strength steel samples of material 34CrNi3MoA:

[0060] (1) The same mechanical grinding and mechanical polishing methods as in Example 1 were used;

[0061] (2) Heat the prepared etching agent to 80℃ until the picric acid is completely dissolved. Immerse the sample completely in the etching agent with the test surface facing upwards. Maintain the etching agent at 80℃ and etch the sample for 60 seconds, then remove it. After removal, rinse with running cold water and wipe with a degreased cotton ball until the etching product on the test surface is clean. Then rinse with anhydrous ethanol and quickly dry the sample. Observe the test surface under a metallographic microscope under magnification. The grain boundary morphology is as follows: Figure 3 As shown.

[0062] Comparative Example 2

[0063] This comparative example uses a quenched low-alloy high-strength steel sample made of 38CrMoAl. The etchant and grain boundary display methods used in sample preparation are the same as in Comparative Example 1. The grain boundary morphology is as follows: Figure 5 As shown.

[0064] Comparative Example 3

[0065] This comparative example uses a quenched and low-temperature tempered low-alloy high-strength steel sample made of 25Cr2MoVA.

[0066] The etching agent for preparing microscopic samples of low-alloy high-strength steel is composed of 4 mL of nitric acid (mass fraction 65%–68%) and 96 mL of anhydrous ethanol (mass fraction ≥99.7%).

[0067] Grain boundary indication method for quenched and low-temperature tempered low-alloy high-strength steel samples of 25Cr2MoVA:

[0068] (1) The same mechanical grinding and mechanical polishing methods as in Example 1 were used;

[0069] (2) Immerse the sample completely in the prepared etching solution with the test surface facing upwards, and etch the sample for 15 seconds. After etching, quickly remove the sample and rinse it thoroughly with running cold water. Then rinse it with anhydrous ethanol and quickly dry the sample. Observe its morphology under a metallographic microscope. The grain boundary morphology is as follows: Figure 7 As shown.

[0070] Comparative Example 4

[0071] This comparative example uses a tempered low-alloy high-strength steel sample of 16Cr3NiWMoVNbE. The etchant and grain boundary display methods used in sample preparation are the same as in Comparative Example 3. The grain boundary morphology is as follows: Figure 9 As shown.

[0072] A comparison of the grain boundary morphology of the examples and comparative examples reveals that, using the etchant and grain boundary display method in the comparative examples, Comparative Example 1 only shows some grain boundaries with poor clarity and connectivity, but grain size can still be measured; the grain boundary display in Comparative Example 3 is relatively blurry, with local corrosion pits, allowing only grain size estimation, not accurate measurement; the grain boundaries in Comparative Examples 2 and 4 are not displayed, making grain size estimation or measurement impossible. This indicates that the etchant and grain boundary display method in the comparative examples has poor applicability and unclear grain boundary display, which is not conducive to accurate measurement of micrograin size. In the microscopic samples prepared using the etchant and display method provided by this invention, the grain boundaries in Examples 1 to 4 are clearly displayed, with complete grain outlines, facilitating accurate measurement. This demonstrates that the etchant and display method of this invention has wide applicability, high efficiency, and simple operation, and can better meet the needs of accurate measurement of micrograin size for low-alloy high-strength steel under different heat treatment states.

[0073] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An etchant, characterized in that, It includes phosphoric acid, methanol, and hydrochloric acid; the mass fraction of the phosphoric acid is ≥85%; the mass fraction of the hydrochloric acid is 36-38%; the volume ratio of the phosphoric acid to methanol is 7-9:2-4; and the volume ratio of the phosphoric acid to hydrochloric acid is 70-90:1-10.

2. The etchant according to claim 1, characterized in that, The mass fraction of methanol is ≥99.5%.

3. The etchant according to claim 1, characterized in that, The volume ratio of phosphoric acid to methanol is 8:2 to 4.

4. The etchant according to claim 1 or 3, characterized in that, The volume ratio of phosphoric acid to hydrochloric acid is 80:1 to 10.

5. A method for grain boundary display of low-alloy high-strength steel, characterized in that, Includes the following steps: Using low-alloy high-strength steel as the anode, electrolytic etching is carried out in the etchant described in any one of claims 1 to 4.

6. The grain boundary display method according to claim 5, characterized in that, The constant current used in the electrolytic etching is 1.5 to 2.5 A.

7. The grain boundary display method according to claim 6, characterized in that, The electrolytic erosion time is 100–300 s.

8. The grain boundary display method according to claim 5, 6 or 7, characterized in that, The cathode used in the electrolytic etching is a nickel plate.

9. The grain boundary display method according to claim 5, characterized in that, The low-alloy high-strength steel includes: quenched and tempered low-alloy high-strength steel, quenched low-alloy high-strength steel, or quenched and low-temperature tempered low-alloy high-strength steel.

10. The grain boundary display method according to claim 5 or 9, characterized in that, Before the electrolytic etching, the inspection surface of the low-alloy high-strength steel is mechanically ground and polished.

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