A method for removing chromium-based coatings from tool surfaces

Abstract

This invention discloses a method for removing chromium-based coatings from the surface of cutting tools. The tool to be removed is contacted with an aqueous solution containing a stripping solution, and the reaction is carried out by heating. The stripping solution is an alkaline stripping solution containing potassium ferricyanide and an alkali, with a concentration of 140-160 g per liter of water. The reaction temperature is controlled between 40°C and 55°C, and a constant temperature water bath is used to maintain this temperature. Reaction temperature is a critical factor: below 40°C, the reaction rate decreases significantly; above 55°C, the metal protective agent in the stripping solution fails, leading to corrosion of the tool substrate. The stripping time is adjusted according to the coating thickness and temperature: the greater the thickness or the lower the temperature, the longer the time required; and vice versa. This invention has high stripping efficiency, effectively protects the substrate, and is suitable for various chromium-based coatings such as Cr, CrAl, CrN, and AlCrN on high-speed steel, cemented carbide, or powder metallurgy steel cutting tools.

Description

Technical Field

[0001] This invention relates to the field of surface treatment technology, and more specifically, to a method for removing chromium-based coatings from the surface of cutting tools, particularly a chemical removal process for chromium-based hard coatings deposited on cutting tools. Background Technology

[0002] In the field of cutting tools, to extend tool life, chromium-based hard coatings, such as Cr, CrAl, CrN, and AlCrN, are often deposited on the surface of substrates such as high-speed steel, cemented carbide, or powder metallurgy steel. When the tool wears out or needs to be recoated, the original coating must be completely removed.

[0003] Existing technologies disclose methods for chemically stripping cutting tools using an alkaline stripping solution containing potassium ferricyanide and alkali. During operation, the tool is immersed in the aqueous stripping solution and heated according to conventional processes. However, in actual production, operators often control reaction conditions based on general instructions from the stripping solution supplier (such as "appropriate heating can accelerate the reaction") or personal experience, resulting in highly unstable stripping effects. Specifically, some batches of tools still have coating residue on the surface after stripping, indicating incomplete stripping and requiring repeated treatment; other batches of tools, although the coating fades quickly, show corrosion spots, oxidation discoloration, or even dimensional deviations on the tool substrate surface, rendering the tools unusable. This coexistence of "insufficient stripping" and "excessive damage" has long troubled those skilled in the art.

[0004] Based on the above findings and through systematic analysis of numerous failed cases, this invention resolves the contradiction between stripping effect and substrate protection with minimal process improvements, while keeping the existing stripping solution system unchanged. Summary of the Invention

[0005] The present invention aims to provide a method for removing chromium-based coatings from the surface of cutting tools, in order to solve the problems of unstable removal effect and easy corrosion or damage to the substrate caused by improper methods in existing methods for removing chromium-based coatings from the surface of cutting tools.

[0006] Technical solution

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A method for removing a chromium-based coating from the surface of a cutting tool includes:

[0009] The cutting tool to be stripped is brought into contact with an aqueous solution containing stripping solution, and the stripping reaction is carried out under heating conditions;

[0010] The stripping solution is an alkaline stripping solution containing potassium ferricyanide and an alkali;

[0011] The reaction temperature is controlled at 40℃~55℃.

[0012] Preferably, the stripping solution also contains a metal protectant that remains effective within the reaction temperature range, but fails and causes corrosion of the tool substrate when the reaction temperature exceeds 55°C.

[0013] Preferably, the concentration of the stripping solution in the aqueous solution is 140g to 160g per liter of water, more preferably 150g / L.

[0014] As a preferred method, a constant temperature water bath is used for heating to keep the temperature of the reaction system constant at the set value.

[0015] Preferably, the stripping reaction rate is significantly reduced when the reaction temperature is below 40°C, and the stripping solution corrodes the tool substrate when the reaction temperature is above 55°C.

[0016] Preferably, the time for the stripping reaction is adjusted according to the thickness of the chromium-based coating and the reaction temperature: the greater the coating thickness and the lower the reaction temperature, the longer the time required; and vice versa.

[0017] Furthermore, when the thickness of the chromium-based coating is 2μm to 5μm, the stripping reaction time within the temperature range of 40℃ to 55℃ is 20 minutes to 45 minutes.

[0018] Preferably, the cutting tool is selected from high-speed steel cutting tools, cemented carbide cutting tools, or powder metallurgy steel cutting tools; the chromium-based coating is selected from Cr coating, CrAl coating, CrN coating, AlCrN coating, CrAlSiN coating, or chromium-containing multilayer composite coating.

[0019] Preferably, the stripping solution is a powdered mixture, the pH value of its aqueous solution is ≥13, and the potassium ferricyanide content and sodium hydroxide content in the stripping solution are ≥30wt%.

[0020] Preferably, the method further includes cleaning and drying the cutting tool after the stripping reaction is completed, and collecting and treating the waste liquid separately as a cyanide-containing compound.

[0021] Beneficial effects of the present invention

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. By precisely controlling the stripping reaction temperature between 40℃ and 55℃, the stripping reaction is ensured to have a sufficiently fast rate, while avoiding corrosion of the tool substrate due to the failure of the metal protective agent caused by excessively high temperature.

[0024] 2. The constant temperature water bath method is adopted, which ensures uniform and stable temperature of the reaction system and good consistency of coating removal effect.

[0025] 3. The adjustment rules between stripping time, coating thickness, and reaction temperature have been clarified, allowing operators to flexibly adjust process parameters according to actual conditions, making it widely applicable.

[0026] 4. It is particularly suitable for alkaline stripping solutions containing potassium ferrocyanide and sodium hydroxide, which can effectively protect the integrity of the substrate of precision cutting tools such as high-speed steel and cemented carbide, and extend the service life of the tools.

[0027] 5. This improvement does not require changes to the existing stripping solution formula. It solves the two major problems of incomplete stripping and substrate damage by precisely controlling only one process parameter. It has low implementation cost and is easy to promote and apply. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0029] The present invention will be further described in detail below through specific embodiments, but the scope of protection of the present invention is not limited to the embodiments described.

[0030] I. Materials and Equipment

[0031] Coating stripper: deconexde-coat231 (main components: potassium ferricyanide ≥30wt%, sodium hydroxide ≥30wt%), powder, aqueous solution pH≈13.

[0032] Cutting tool: Carbide end mill with a chromium-based coating deposited on the surface.

[0033] Thermostatic water bath: Equipped with a microcomputer thermostatic control board, with a temperature control accuracy of ±0.5℃.

[0034] Deionized water.

[0035] II. Preparation of stripping solution

[0036] Weigh 150g of stripping solution powder, add it to 1L of deionized water, and stir until completely dissolved to obtain the stripping working solution (concentration 150g / L).

[0037] III. Examples

[0038] Example 1 (Optimal Conditions)

[0039] Tool surface coating: CrAl coating, approximately 3μm thick.

[0040] Add the prepared stripping solution to a constant temperature water bath and heat to 55°C, maintaining this temperature. Completely immerse the cutting tool in the stripping solution and react at this constant temperature for 25 minutes. After the reaction is complete, remove the tool, rinse thoroughly with deionized water, and dry with hot air.

[0041] Test results:

[0042] Surface observation: The coating has been completely removed, the tool surface is clean, and there is no coating residue.

[0043] Matrix condition: No corrosion spots, no oxidation discoloration, and tool dimensions are qualified.

[0044] Repeatability: All 10 knives in the same batch achieved the same results.

[0045] Example 2

[0046] It is basically the same as Example 1, except that the stripping reaction temperature is 50°C and the reaction time is 30 minutes.

[0047] Test results: The coating was completely removed, the substrate was free of corrosion, and the tool dimensions were within acceptable limits.

[0048] Example 3

[0049] It is basically the same as Example 1, except that the stripping reaction temperature is 45°C and the reaction time is 35 minutes.

[0050] Test results: The coating was completely removed, and the substrate showed no corrosion.

[0051] Example 4

[0052] It is basically the same as Example 1, except that the stripping reaction temperature is 40°C and the reaction time is 40 minutes.

[0053] Test results: The coating was completely removed, and the substrate showed no corrosion.

[0054] Example 5

[0055] Tool surface coating: CrN coating, approximately 2 μm thick. Stripping reaction temperature: 55℃, reaction time: 20 minutes.

[0056] Test results: The coating was completely removed, and the substrate showed no corrosion.

[0057] Example 6

[0058] Tool surface coating: AlCrN multilayer coating, approximately 5 μm thick. Stripping reaction temperature: 55℃, reaction time: 40 minutes.

[0059] Test results: The coating was completely removed, and the substrate showed no corrosion.

[0060] IV. Comparative Example

[0061] Comparative Example 1 (Temperature Too High)

[0062] It is basically the same as Example 1, except that the stripping reaction temperature is 60°C.

[0063] After 15 minutes of reaction, a grayish-black oxide layer was observed on the tool surface, and the stripping solution darkened in color. The reaction was continued for 25 minutes, and upon inspection, it was found that although the coating had been removed, the tool substrate surface showed obvious corrosion pits, edge passivation, and dimensional deviations, rendering the tool unusable.

[0064] Comparative Example 2 (Temperature Too High)

[0065] It is basically the same as Example 1, except that the stripping reaction temperature is 58°C.

[0066] After 25 minutes of reaction, the coating was removed, but slight corrosion spots appeared on the surface of the tool, and pitting was visible under a microscope, which did not meet the requirements for recoating.

[0067] Comparative Example 3 (Temperature Too Low)

[0068] It is basically the same as Example 1, except that the stripping reaction temperature is 35°C.

[0069] After 40 minutes of reaction, about 30% of the coating remained on the tool surface; after 60 minutes, about 10% remained, and the reaction rate was extremely low, making production efficiency unacceptable.

[0070] Comparative Example 4 (Concentration Deviation)

[0071] It is basically the same as Example 1, except that the concentration of the stripping solution is 100 g / L (below the scope of this invention).

[0072] After reacting at 55℃ for 25 minutes, the coating was not completely removed, with about 15% of the coating remaining; it was only completely removed after extending the reaction time to 40 minutes, but slight corrosion of the substrate was observed.

[0073] V. Results Analysis

[0074]

[0075] As shown in the table above, when the reaction temperature is controlled within the range of 40℃ to 55℃ and the stripping solution concentration is 140 to 160 g / L, the chromium-based coating can be effectively removed while protecting the tool substrate from corrosion. Temperatures exceeding 55℃ will cause the metal protectant to fail and the substrate to corrode; temperatures below 40℃ will result in an excessively low reaction rate and poor practicality.

[0076] VI. Industrial Application Examples

[0077] A cutting tool manufacturer used the method of this invention to strip the coating from a batch of recycled cemented carbide hobs (with a CrAl coating on the surface, 3-4 μm thick). The stripping temperature was set at 52℃, using a constant-temperature water bath for 30 minutes. Each batch consisted of 50 tools, and 10 batches were processed consecutively. Results showed that all tools achieved 100% coating removal, with no corrosion on the substrate. The tools could directly proceed to the recoating process after stripping. Compared to the previous experience-based operation, the stripping failure rate decreased from 25% to below 2%, and the tool scrap rate decreased from 15% to 1%.

[0078] Industrial applicability

[0079] The method of this invention can be used in the coating removal process of various precision cutting tools, especially high-value cemented carbide cutting tools that require multiple recoatings, and has significant industrial application value and economic benefits.

[0080] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for removing a chromium-based coating from the surface of a cutting tool, characterized in that, include: The cutting tool to be stripped is brought into contact with an aqueous solution containing stripping solution, and the stripping reaction is carried out under heating conditions; The stripping solution is an alkaline stripping solution containing potassium ferricyanide and an alkali; The reaction temperature is controlled at 40℃~55℃.

2. The method according to claim 1, characterized in that, The stripping solution also contains a metal protectant, which remains effective within the reaction temperature range. When the reaction temperature exceeds 55°C, the metal protectant fails and causes corrosion of the tool substrate.

3. The method according to claim 1, characterized in that, The concentration of the stripping solution in the aqueous solution is 140g to 160g per liter of water.

4. The method according to claim 1, characterized in that, A constant temperature water bath is used for heating to keep the temperature of the reaction system constant at the set value.

5. The method according to claim 1, characterized in that, When the reaction temperature is below 40℃, the stripping reaction rate decreases significantly; when the reaction temperature is above 55℃, the stripping solution corrodes the tool substrate.

6. The method according to claim 1, characterized in that, The time for the stripping reaction is adjusted according to the thickness of the chromium-based coating and the reaction temperature: the greater the coating thickness and the lower the reaction temperature, the longer the time required; and vice versa.

7. The method according to claim 6, characterized in that, When the thickness of the chromium-based coating is 2μm to 5μm, the stripping reaction time is 20 minutes to 45 minutes within a temperature range of 40℃ to 55℃.

8. The method according to claim 1, characterized in that, The cutting tool is selected from high-speed steel cutting tools, cemented carbide cutting tools, or powder metallurgy steel cutting tools; the chromium-based coating is selected from Cr coating, CrAl coating, CrN coating, AlCrN coating, CrAlSiN coating, or chromium-containing multilayer composite coating.

9. The method according to claim 1, characterized in that, The stripping solution is a powdered mixture with an aqueous solution pH value ≥13, and the potassium ferricyanide content and sodium hydroxide content in the stripping solution are ≥30wt%.

10. The method according to claim 1, characterized in that, The method also includes cleaning and drying the cutting tool after the stripping reaction is completed, and collecting and treating the waste liquid separately as a cyanide compound.

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