Method for characterizing microcrystalline grain size of high thermal conductivity hot work die steel

Abstract

This invention provides a method for characterizing the microcrystalline grain size of high thermal conductivity hot work die steel, comprising the following steps: quenching and tempering a high thermal conductivity hot work die steel sample, grinding and polishing it, then etching it with an etchant, followed by observation of the original austenite grain boundaries and grain size rating under a metallographic microscope; wherein the composition and ratio of the etchant are: 3-9g oxalic acid, 4-6ml concentrated nitric acid, 4-6ml hydrochloric acid, 2-5ml acetic acid, 100ml anhydrous ethanol, and 10ml distilled water. The etchant provided by this invention is safe to prepare and can quickly, clearly, and completely display the original austenite grain boundaries, which helps to accurately assess the grain size and provides technical support for the formulation and optimization of high thermal conductivity die steel production processes.

Description

Technical Field

[0001] This invention relates to the field of metallographic analysis, and specifically to a method for characterizing the microcrystalline grain size of high thermal conductivity hot work die steel. Background Technology

[0002] Grain size is an important indicator for measuring the size of grains in a material. Grain size is related to processing techniques such as forging and heat treatment. From a materials science perspective, the properties of a material are determined by its microstructure and austenite grain size. Among these, austenite grain size is the main factor determining the material's properties. Refining austenite grains can improve the material's toughness and strength. Clearly displaying the original austenite grain boundaries has always been a challenge in the materials industry.

[0003] The internal microstructure of high thermal conductivity hot work die steel consists of numerous fine grains. For hot work die steels, especially high thermal conductivity series, grain size has a significant impact on the material's strength, plasticity, toughness, corrosion resistance, and other properties. Because high thermal conductivity hot work die steels contain a higher molybdenum content than other types of hot work die steels, the grains are extremely difficult to visualize during material evaluation. Furthermore, their metallographic structure contains a large number of primary and secondary carbides. Conventional metallographic etching methods can clearly reveal the carbides, but the precipitation of numerous dispersed fine secondary carbides makes it difficult to clearly visualize the austenite grain boundaries. Figure 1 As shown.

[0004] According to domestic industry surveys and consultations with international standards, the original method for characterizing the micrograin size of high thermal conductivity hot work die steels mainly used picric acid as the primary etchant. However, due to the toxicity and explosiveness of picric acid, its production and sale in China have been banned since 2018. Alternative etching methods are not yet mature, and there is no dedicated method. The oxidation method used in GB / T6394-2017 to determine austenite grain size involves oxidizing the sample surface, then lightly polishing the surface and observing the austenite grain size. This method is complex, requires extremely high polishing precision, and does not accurately reflect the actual grain size. Etching with ferric chloride + ethanol aqueous solution and picric acid-alcohol solution can simultaneously etch out the microstructure and grain boundaries, but the grain boundaries are not clearly visible. Martensite and large-sized primary carbides can affect the display of grain boundaries, introducing interference factors into the rating process.

[0005] Currently, the corrosion analysis of high thermal conductivity hot work die steel in China mainly relies on methods used for other materials. This approach suffers from several problems, including inconsistent pre-corrosion heat treatment methods, a lack of reproducibility in the corrosion process, and unclear grain size and grain boundary corrosion. Multiple tests are required to ensure testing quality, severely impacting the effectiveness and efficiency of assessing the microcrystalline grain size of high thermal conductivity hot work die steel. Therefore, clearly displaying the grain size of high thermal conductivity hot work die steel has been a persistent challenge for metallographic testing personnel. Clearly displaying the grain size and grain boundaries under different heat treatment processes is crucial for studying the amount of molybdenum-containing precipitates at grain boundaries in high thermal conductivity steels. Summary of the Invention

[0006] The purpose of this invention is to provide a method for characterizing the microcrystalline grain size of high thermal conductivity hot work die steel, which can achieve clear observation and efficient evaluation of the microcrystalline grain size of high thermal conductivity hot work die steel.

[0007] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0008] A method for characterizing the microcrystalline grain size of high thermal conductivity hot work die steel is provided, specifically as follows: the high thermal conductivity hot work die steel sample is subjected to quenching-tempering treatment, grinding and polishing, and then etched with an etchant. The original austenite grain boundaries are observed and the grain size is rated under a metallographic microscope. The composition and ratio of the etchant are as follows: oxalic acid 3-9g, concentrated nitric acid 4-6ml, hydrochloric acid 4-6ml, acetic acid 2-5ml, anhydrous ethanol 100ml, and distilled water 10ml.

[0009] Furthermore, the quenching process described in this invention is as follows: holding at 1030℃~1080℃ for 30min~60min, followed by cooling.

[0010] Furthermore, the cooling method described in this invention is oil cooling or air cooling.

[0011] Furthermore, the tempering temperature of the tempering treatment described in this invention is 560℃~620℃, and the tempering time is 1.5h~2.5h.

[0012] Furthermore, the tempering described in this invention employs a two-stage tempering process.

[0013] Furthermore, the corrosion solution described in this invention is used immediately after preparation.

[0014] Furthermore, the corrosion method described in this invention is to perform wiping corrosion at room temperature for 25–60 seconds.

[0015] Furthermore, the nitric acid, hydrochloric acid, and acetic acid used in this invention are all of analytical grade.

[0016] Furthermore, the composition and mass percentage of the high thermal conductivity hot work die steel of the present invention are as follows: C 0.05%~0.35%, Mo 2.6%~5.3%, Cr≤0.15%, Ni 0.01%~0.35%, with the remainder being iron and unavoidable impurities.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. The etching solution of this invention can effectively reduce the influence of carbide precipitation on grain size when used to etch the grain boundaries of high thermal conductivity hot work die steel at room temperature. The grain boundaries of the quenched and tempered high thermal conductivity hot work die steel after etching are obvious and completely unaffected by the microstructure. Each grain can be clearly observed, which has high accuracy and precision.

[0019] 2. The method of this invention can be used for corrosion at room temperature without heating, resulting in low energy consumption, low cost, and greater convenience and speed.

[0020] 3. The method of this invention involves observing the corrosion of the grain size of the sample after quenching and tempering, which can evaluate the grain size of the delivered material. Compared with the grain size of the quenched state alone, it cannot reflect the grain growth problem during tempering and tempering heat preservation. Attached Figure Description

[0021] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments:

[0022] Figure 1 Microstructure of high thermal conductivity hot work die steel after quenching;

[0023] Figure 2 The original austenite grains after treatment of the high thermal conductivity hot work die steel in Example 1;

[0024] Figure 3 The original austenite grains after treatment of the high thermal conductivity hot work die steel in Example 2;

[0025] Figure 4 The original austenite grains after treatment of the high thermal conductivity hot work die steel in Example 3. Detailed Implementation

[0026] To make the invention's objectives, technical solutions, and beneficial effects clearer, the invention will be further described in detail below with reference to embodiments. Example 1

[0027] The chemical composition of the high thermal conductivity hot work die steel in this embodiment is C 0.05%, Mo 2.6%, Cr 0.15%, Ni 0.01%, with the remainder being iron and unavoidable impurities; the method for characterizing its microcrystalline grain size includes the following steps:

[0028] (1) Take a wire cut sample of the high thermal conductivity mold steel to be inspected.

[0029] (2) The high thermal conductivity hot work die steel sample was subjected to quenching-tempering treatment. The quenching heat treatment regime was to hold at 1030℃ for 60 min and then oil cooling. The tempering was a two-stage tempering with a tempering temperature of 560℃ and a tempering time of 2 h.

[0030] (3) After heat treatment, the decarburized layer and oxide scale of the sample are removed, and then the sample is ground and polished as follows: The sample is ground by sandpaper of 60 mesh, 240 mesh, 400 mesh, 800 mesh and 1000 mesh in sequence. Each time, the next sandpaper is replaced and the sample is rotated 90 degrees along the grinding surface so that the new grinding mark is perpendicular to the previous grinding mark and covers it. The polished sample is placed on a velvet polishing cloth for polishing and 1.5μm diamond polishing agent is added to obtain a clean, flat, and scratch-free polished surface.

[0031] (4) Prepare the corrosion solution. The specific components and ratio of the corrosion solution are: 3g oxalic acid, 4ml concentrated nitric acid, 4ml hydrochloric acid, 2ml acetic acid, 100ml anhydrous ethanol, and 10ml distilled water.

[0032] (5) Use freshly prepared etching solution to rub and etch the polished high thermal conductivity hot work die steel sample for 25 seconds at room temperature, then rinse the surface with clean water, and finally rinse with alcohol and dry with a hair dryer.

[0033] (6) The original austenite grain boundaries and grain size were observed and rated under a metallographic microscope on the high thermal conductivity hot work die steel samples after corrosion. The results are as follows: Figure 2 As shown. By Figure 2 It can be seen that the grain etching is in place, the grain boundaries are clear, and the grain size is about 20μm. Example 2

[0034] The chemical composition of the high thermal conductivity hot work die steel in this embodiment is C 0.35%, Mo 5.3%, Cr 0.15%, Ni 0.22%, with the remainder being iron and unavoidable impurities; the method for characterizing its microcrystalline grain size includes the following steps:

[0035] (1) Take a wire cut sample of the high thermal conductivity mold steel to be inspected.

[0036] (2) The high thermal conductivity hot work die steel sample was subjected to quenching-tempering treatment. The quenching heat treatment regime was to hold at 1050℃ for 45 minutes and then air cool. The tempering was a two-stage tempering with a tempering temperature of 600℃ and a tempering time of 2 hours.

[0037] (3) After heat treatment, the decarburized layer and oxide scale of the sample are removed, and then the sample is ground and polished as follows: The sample is ground by sandpaper of 60 mesh, 240 mesh, 400 mesh, 800 mesh and 1000 mesh in sequence. Each time, the next sandpaper is replaced and the sample is rotated 90 degrees along the grinding surface so that the new grinding mark is perpendicular to the previous grinding mark and covers it. The polished sample is placed on a velvet polishing cloth for polishing and 1.5μm diamond polishing agent is added to obtain a clean, flat, and scratch-free polished surface.

[0038] (4) Prepare the corrosion solution. The specific components and ratio of the corrosion solution are: 9g oxalic acid, 6ml concentrated nitric acid, 6ml hydrochloric acid, 5ml acetic acid, 100ml anhydrous ethanol, and 10ml distilled water.

[0039] (5) Use freshly prepared etching solution to rub and etch the polished high thermal conductivity hot work die steel sample for 40 seconds at room temperature, then rinse the surface with clean water, and finally rinse with alcohol and dry with a hair dryer.

[0040] (6) The original austenite grain boundaries and grain size were observed and rated under a metallographic microscope on the high thermal conductivity hot work die steel samples after corrosion. The results are as follows: Figure 3 As shown. By Figure 3 It can be seen that the grain etching is in place, the grain boundaries are clear, and the grain size is about 45μm. Example 3

[0041] The chemical composition of the high thermal conductivity hot work die steel in this embodiment is C 0.19%, Mo 3.1%, Cr 0.11%, Ni 0.14%, with the remainder being iron and unavoidable impurities; the method for characterizing its microcrystalline grain size includes the following steps:

[0042] (1) Take a wire cut sample of the high thermal conductivity mold steel to be inspected.

[0043] (2) The high thermal conductivity hot work die steel sample was subjected to quenching-tempering treatment. The quenching heat treatment regime was to hold at 1080℃ for 30 min and then oil cooling. The tempering was a two-stage tempering with a tempering temperature of 620℃ and a tempering time of 2 h.

[0044] (3) After heat treatment, the decarburized layer and oxide scale of the sample are removed, and then the sample is ground and polished as follows: The sample is ground by sandpaper of 60 mesh, 240 mesh, 400 mesh, 800 mesh and 1000 mesh in sequence. Each time, the next sandpaper is replaced and the sample is rotated 90 degrees along the grinding surface so that the new grinding mark is perpendicular to the previous grinding mark and covers it. The polished sample is placed on a velvet polishing cloth for polishing and 1.5μm diamond polishing agent is added to obtain a clean, flat, and scratch-free polished surface.

[0045] (4) Prepare the corrosion solution. The specific components and ratio of the corrosion solution are: 6g oxalic acid, 5ml concentrated nitric acid, 5ml hydrochloric acid, 3.5ml acetic acid, 100ml anhydrous ethanol, and 10ml distilled water.

[0046] (5) Use freshly prepared etching solution to rub and etch the polished high thermal conductivity hot work die steel sample for 60s at room temperature, then rinse the surface with clean water, and finally rinse with alcohol and dry with a hair dryer.

[0047] (6) The original austenite grain boundaries and grain size were observed and rated under a metallographic microscope on the high thermal conductivity hot work die steel samples after corrosion. The results are as follows: Figure 4 As shown. By Figure 4 It can be seen that the grain etching is in place, the grain boundaries are clear, and the grain size is about 50μm.

Claims

1. A method of characterizing the micrograin size of a high thermal conductivity hot work die steel, characterized by, Includes the following steps: High thermal conductivity hot work die steel samples were quenched and tempered, ground and polished, and then etched with an etchant. The original austenite grain boundaries were observed and the grain size was rated under a metallographic microscope. The composition and ratio of the etchant were as follows: oxalic acid 3-9g, concentrated nitric acid 4-6ml, hydrochloric acid 4-6ml, acetic acid 2-5ml, anhydrous ethanol 100ml, and distilled water 10ml. The tempering temperature for the tempering treatment is 560℃~620℃, and the tempering time is 1.5h~2.5h; The tempering process used is a double tempering process.

2. The method for characterizing the microcrystalline grain size of high thermal conductivity hot work die steel according to claim 1, characterized in that, The quenching process is as follows: holding at 1030℃~1080℃ for 30min~60min, followed by cooling.

3. The method for characterizing the microcrystalline grain size of high thermal conductivity hot work die steel according to claim 2, characterized in that, The cooling method is either oil cooling or air cooling.

4. The method of claim 1, wherein the microcrystalline grain size of the high thermal conductivity hot work die steel is characterized by, The corrosive solution is used immediately after preparation.

5. The method for characterizing the microcrystalline grain size of high thermal conductivity hot work die steel according to claim 1, characterized in that, The corrosion method is to rub and corrode at room temperature for 25-60 seconds.

6. The method for characterizing the microcrystalline grain size of high thermal conductivity hot work die steel according to claim 1, characterized in that, The composition and mass percentage of the high thermal conductivity hot work die steel are as follows: C 0.05%~0.35%, Mo 2.6%~5.3%, Cr≤0.15%, Ni 0.01%~0.35%, with the remainder being iron and unavoidable impurities.

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