Low-temperature salt bath nitriding salt, preparation process and application

Abstract

The application belongs to the technical field of metal surface strengthening, and particularly relates to a low-temperature salt bath nitriding salt, a preparation process and application, which comprises the following components in percentage by mass: cyanate salt 45-60%; carbonate salt 25-40%; potassium chloride 5-14%; rare earth composite additive 0.3-1.5%; alkaline stabilizer 1-3%; the rare earth composite additive is composed of lanthanum oxide and yttrium oxide at a mass ratio of 1:1; and the molar ratio of Na+ to K+ in the salt bath system is 0.35-0.65. The low-temperature salt bath nitriding salt, the preparation process and the application, in the low-temperature high-efficiency easy-to-maintain salt bath nitriding salt, double rare earth catalysis is adopted, the infiltration layer is dense, the hardness is high, and the corrosion resistance is excellent. The production efficiency is increased by more than 40% by adopting potassium chloride to form a floating sludge, and in addition, the Na+ / K+ is accurately controlled, so that the quality of the salt bath nitriding salt is stable and consistent, and the nitriding in a 400 DEG C low-temperature environment is realized, and the workpiece is basically not deformed.

Description

Technical Field

[0001] This invention relates to the field of metal surface strengthening technology, and in particular to a low-temperature salt bath nitriding salt, its preparation process, and its application. Background Technology

[0002] Traditional salt bath nitriding treatments typically operate at temperatures above 480℃, which can easily cause deformation of precision workpieces. At low temperatures, the salt bath activity is insufficient, making it difficult to form an effective diffusion layer. Furthermore, the salt bath produces a lot of sediment, making cleaning difficult and hindering production continuity. Simultaneously, the porosity of the diffusion layer is uncontrollable, and the hardness fluctuates greatly, failing to meet the low-temperature strengthening requirements of high-precision components, stainless steel, ductile iron, and other materials. With the implementation of Euro 7 environmental regulations and the increasing requirements for new energy vehicle components, the industry urgently needs a new salt bath system that is low-temperature, highly active, easy to maintain, and provides stable diffusion layer quality. Summary of the Invention

[0003] Based on existing technical problems, this invention proposes a low-temperature salt bath nitriding salt, its preparation process and application, which can achieve nitrogen-carbon co-diffusion at a low temperature of 400℃, contains rare earth lanthanum and yttrium, is easy to remove slag, and allows the quality of the infiltration layer to be controlled by the ratio of sodium and potassium cations.

[0004] The present invention proposes a low-temperature salt bath nitriding salt, comprising, by mass percentage:

[0005] Cyanate 45%–60%;

[0006] Carbonates 25%–40%;

[0007] Potassium chloride 5%–14%;

[0008] Rare earth composite additives: 0.3%–1.5%;

[0009] Alkaline stabilizer 1%–3%;

[0010] The rare earth composite additive is composed of lanthanum oxide and yttrium oxide in a mass ratio of 1:1; the molar ratio of Na⁺ to K⁺ in the salt bath system is 0.35 to 0.65.

[0011] Preferably, the cyanate is a mixture of sodium cyanate and potassium cyanate, with a molar mass ratio of sodium cyanate 45:potassium cyanate 55.

[0012] The carbonate is a mixture of sodium carbonate and potassium carbonate, with a molar mass ratio of 45 parts sodium carbonate to 55 parts potassium carbonate.

[0013] Preferably, the potassium chloride converts the salt bath oxidation slag into floating slag, which is then removed online.

[0014] Preferably, the salt bath is used at a temperature of 400℃±5℃ to achieve low-temperature nitrogen-carbon co-infiltration.

[0015] Preferably, the infiltration layer formed by co-infiltration includes a matrix, a diffusion layer, a compound layer, and an oxide film;

[0016] The porosity of the infiltrated layer is ≤2, and the surface hardness is ≥1100HV.

[0017] A process for preparing low-temperature salt bath nitrided salt, wherein the infiltration layer is prepared sequentially according to the process of raw material → drying → mixing → melting → adding rare earth → cooling → crushing → sieving → finished product.

[0018] Preferably, the step of adding rare earth elements includes:

[0019] Step 1: Sprinkle rare earth lanthanum carbonate / cerium carbonate composite powder into the molten salt solution in 5 small batches at a uniform speed, with an 8-minute interval between each batch;

[0020] Step 2: After all the rare earth elements have been added, continue to aerate and stir for 120 minutes to allow the rare earth elements to fully dissociate into rare earth cations, which then react with Na. + / K + Formation of a composite alkali metal molten salt system;

[0021] Step 3, Endpoint Detection: Molten salt sampling and spectral analysis showed uniform distribution of rare earth ions, with no localized high-concentration areas. Rare earth elements weakened the Na+ content. + The high proportion of Na leads to increased porosity, widening the area by 35%–50%. + During the process window, the hardness of nitriding at 500℃ increases by 30-50 HV, and the salt bath slag removal cycle is extended by 2 times.

[0022] An application of a low-temperature salt bath nitriding salt, wherein the infiltration layer application includes QT450, QT500, N360, PH13-8Mo, 40Cr, and 42CrMo materials.

[0023] Preferably, when the infiltration layer is applied to a brake disc made of QT450 / QT500 material, the salt bath nitride is prepared in the following mass percentage ratio:

[0024] The composition is 48% cyanate, 32% carbonate, 10% potassium chloride, 1.0% rare earth additive, and 2% stabilizer, with Na⁺ / K⁺ = 0.45.

[0025] Preferably, when the diffusion layer is applied to stainless steel of material N360 / PH13-8Mo, the salt bath nitriding salt is prepared in the following mass percentage ratio:

[0026] The composition is as follows: 58% cyanate, 26% carbonate, 6% potassium chloride, 1.2% rare earth additives, 2% stabilizer, and Na⁺ / K⁺ = 0.60.

[0027] The beneficial effects of this invention are as follows:

[0028] 1. This low-temperature salt bath nitriding process utilizes dual rare earth catalysis, resulting in a dense, high-hardness, and highly corrosion-resistant infiltration layer. Furthermore, the use of potassium chloride to create scum increases production efficiency by over 40%. Precise control of the Na⁺ / K⁺ ratio ensures consistent and stable quality of the salt bath nitriding.

[0029] 2. By setting a low-temperature salt bath nitriding salt preparation process, this invention achieves nitriding in a low-temperature environment of 400℃ without deformation of the workpiece.

[0030] 3. This invention applies low-temperature salt bath nitriding salt to automotive brake discs and stainless steel, which has the advantages of being environmentally friendly, having a long service life, and being highly versatile, and meets the Euro 7 standard.

[0031] It is suitable for strengthening automotive brake discs, methanol nozzle valve seats, hydraulic valve cores, precision shafts, molds, gears and various metal parts with high precision and low deformation requirements. It can be widely used in new energy vehicles, engineering machinery, hydraulic equipment, high-end mold manufacturing and other fields. Attached Figure Description

[0032] Figure 1 This is a microscopic image of the infiltrated tissue structure treated with low-temperature salt bath nitriding salt according to the present invention.

[0033] Figure 2 This is a comparison diagram of floating slag and slag removal for a low-temperature salt bath nitriding salt, its preparation process, and its application proposed in this invention.

[0034] Figure 3 The diagram shows the Na⁺ / K⁺ ratio curves for controlling porosity and hardness in a low-temperature salt bath nitriding process proposed in this invention.

[0035] In the figure: 1. Substrate; 2. Diffusion layer; 3. Compound layer; 4. Oxide film. Detailed Implementation

[0036] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0037] Example 1

[0038] Reference Figure 1-2 As shown, a low-temperature salt bath nitriding salt comprises, by mass percentage:

[0039] Cyanate 45%–60%; cyanate is a mixture of sodium cyanate and potassium cyanate, with a molar mass ratio of sodium cyanate 45:potassium cyanate mixture 55.

[0040] The carbonate content is 25%–40%; the carbonate is a mixture of sodium carbonate and potassium carbonate, with a molar mass ratio of 45% sodium carbonate to 55% potassium carbonate mixture. Li₂CO₃ can be used as the carbonate, which can improve the flowability and stability of the exfoliating agent.

[0041] Potassium chloride 5%–14%; by adding potassium chloride as a base agent for seepage, the salt bath oxidation slag can be converted into scum, which can be easily removed online.

[0042] Rare earth composite additive 0.3% to 1.5%; the rare earth composite additive is composed of lanthanum oxide and yttrium oxide in a mass ratio of 1:1; the content of trace harmful substances (cyanide ions) generated during the process is controlled by rare earth elements.

[0043] Alkaline stabilizer 1%–3%; the molar ratio of Na⁺ to K⁺ in the salt bath system is 0.35–0.65.

[0044] The salt bath is used at a temperature of 400℃±5℃ to achieve low-temperature nitrogen-carbon co-infiltration.

[0045] The co-infiltration layer includes a substrate 1, a diffusion layer 2, a compound layer 3, and an oxide film 4; the porosity of the infiltration layer is ≤2 and the surface hardness is ≥1100HV.

[0046] K in the penetrant + and Na + Different ratios significantly affect the effective penetration depth and morphology of the infiltration layer formed after nitrocarburization. Specifically: K + and Na + It can increase the stability of cyanate ions, reduce the decomposition rate of cyanate ions, increase the concentration of effective ingredients, and significantly increase the N and C concentrations on the product surface. The increased diffusion rate of N and C prevents the formation of micropores in the effective penetration layer area, resulting in a denser effective penetration layer on the product surface. K + and Na + Adsorption on the product surface facilitates full contact between the product and the infiltrator. The addition of alkali metal cations alters the surface activity and stability of solid salt slag, solving the problems of nitrogen and carbon infiltration and cyanate ion stability at low temperatures. It also solves the problems of severe surface porosity of the effective infiltrated layer and the inability to remove slag in time, which affects continuous production. This results in a nitrogen and carbon infiltration technology that meets the requirements of ferrous metal matrix at 400℃.

[0047] This low-temperature salt bath nitriding process employs dual rare earth catalysis, resulting in a dense, high-hardness, and highly corrosion-resistant infiltration layer. The use of potassium chloride to create scum increases production efficiency by over 40%. Furthermore, precise control of the Na⁺ / K⁺ ratio ensures consistent and stable quality of the low-temperature salt bath nitriding.

[0048] Example 2

[0049] Reference Figure 1-3 As shown, a process for preparing low-temperature salt bath nitride includes: raw material → drying → mixing → melting → adding rare earth elements → cooling → crushing → sieving → finished product; wherein:

[0050] Drying: An intermittent hot air circulating vacuum drying oven and a tail gas condensation and recovery device are used. The first stage is a low-temperature pre-drying section at 80–100℃ for 2 hours to remove free adsorbed water from the surface of the raw materials. The second stage is a high-temperature deep dehydration section at 180±5℃ for 4–6 hours to remove the crystal water from the raw materials. The drying criteria are: after cooling, the raw materials should not clump or stick to the walls, and the Karl Fischer moisture analyzer should confirm compliance. After drying, the materials are temporarily stored in a sealed silo and introduced into the mixing process within 2 hours to prevent moisture regain.

[0051] Mixing: Employs a high-efficiency three-dimensional oscillating mixer with dual motion and no dead angles, using 316L stainless steel agitator.

[0052] Step 1: Premix the main bulk materials: KCNO + NaCNO + K2CO3 + Na2CO3 are added to the main mixing silo;

[0053] Step 2, medium-volume fluxing: lithium carbonate is added to the bulk mixture in three equal increments, each time for 10 minutes;

[0054] Step 3: Trace amounts of rare earth are added in stages: The total amount of rare earth added is 0.6 to 1.2 wt%, and it is increased in small amounts in 4 stages to prevent local rare earth enrichment from causing local over-permeation and excessive looseness in nitriding.

[0055] Mixing speed: 24-30 r / min (slow at the beginning and fast at the end); total mixing time: 120 min; uniformity judgment (core quality control): no white / light-colored rare earth spots visible to the naked eye, and no local agglomeration of sodium salts or potassium salts.

[0056] Melting: An externally heated stainless steel crucible molten salt reactor is used, equipped with a bottom-blowing agitation system, a segmented programmed heating system, and an ammonia absorption tower for waste gas. First stage heating: room temperature → 260℃, held for 90 minutes; residual moisture is completely vaporized and discharged. Second stage melting: 260℃ to 435±10℃ (the eutectic melting point of the system, low-temperature melting saves energy). Complete melting criteria: no solid floating material or suspended particles on the liquid surface, the liquid is transparent and uniform. Pre-melting homogenization holding: 435℃ for 2 hours, dry nitrogen is blown in and stirred at low speed (0.02MPa air pressure) to eliminate cation concentration gradients. Key: Nitrogen inert protection throughout the process to prevent CO2 and water vapor in the air from altering the Na⁺ / K⁺ ratio; a thin layer of dry potassium chloride is placed on the liquid surface to isolate it from air.

[0057] Adding rare earth elements: After complete melting and homogenization, add rare earth elements during the constant temperature holding stage at 450℃±10℃. It is strictly forbidden to mix rare earth elements into the solid raw material stage. Reason: Solid mixed rare earth elements have poor dispersion; liquid ion exchange after melting allows for uniform dissociation of rare earth elements, precisely controlling the surface porosity and hardness.

[0058] The steps for adding rare earth elements include:

[0059] Step 1: Sprinkle rare earth lanthanum carbonate / cerium carbonate composite powder into the molten salt solution in 5 small batches at a uniform speed, with an 8-minute interval between each batch;

[0060] Step 2: After all the rare earth elements have been added, continue to blow air and stir for 120 minutes to allow the rare earth elements to fully dissociate into rare earth cations and form a composite alkali metal molten salt system with Na⁺ / K⁺.

[0061] Step 3, Endpoint Detection: Molten salt sampling and spectral analysis showed uniform distribution of rare earth ions, with no localized high-concentration areas. Rare earth elements mitigated the increased porosity caused by a high Na⁺ content, broadened the 35%–50% Na⁺ process window, increased the hardness of nitriding at 500℃ by 30–50 HV, and doubled the salt bath slag removal cycle.

[0062] Cooling: Indirect water cooling is used in the stainless steel mold (water flows through the mold jacket, and molten salt is poured into the sealed mold). The molten salt is rapidly cooled from a liquid state of 435℃ to 25-35℃ before being discharged. The internal core temperature must not exceed 40℃. If the temperature is too high, the salt will break and absorb moisture. Quality control points: After cooling, the cross-section of the entire salt blank should have a uniform color and no black and white layered stripes.

[0063] Crushing: Jaw crusher (wear-resistant high manganese steel liner) crushes large solidified salt billets into 20-50mm blocks.

[0064] Screening: Stainless steel vibrating screen, equipped with dust removal and negative pressure recovery for fine powder. Over 92% of the finished salt particles fall within the 20-50mm range; there are no large particles, and the proportion of ultrafine powder is ≤5%.

[0065] The entire process involves strict control over three steps: drying, uniform mixing, and liquid rare earth addition. This ensures a stable Na⁺ molar ratio of 45% and a K⁺ molar ratio of 55%, resulting in uniform molten salt ion mixing. The finished salt exhibits a stable porous layer ≤2.1μm and a surface hardness of 750–1100 HV during use. Rare earth elements synergistically suppress nitride porosity, reducing salt bath sludge formation by 60% and extending the sludge cleaning and maintenance cycle. The salt is prepared using a low-temperature melting process at 435℃, resulting in a low melting point and enabling low-temperature nitrocarburization of the workpiece. The sieved, qualified salt is vacuum-sealed in moisture-proof bags, each weighing 25kg. Storage environment humidity should be ≤40% to prevent Na⁺ salt from absorbing moisture and disrupting the cation ratio.

[0066] The infiltration layer prepared according to the above process comprises a substrate 1, a diffusion layer 2, a compound layer 3, and an oxide film 4. Its porosity level is ≤2, and its surface hardness is ≥1100HV, preferably 1180HV.

[0067] This invention achieves nitriding in a low-temperature environment of 400℃ without deformation of the workpiece by setting a low-temperature salt bath nitriding process.

[0068] Example 3

[0069] Reference Figure 1-3 As shown, an application of a low-temperature salt bath nitriding salt is included, applicable to QT450, QT500, N360, PH13-8Mo, 40Cr, and 42CrMo materials. When applied to brake discs of models QT450 / QT500, the salt bath nitriding salt is formulated in the following mass percentage ratio:

[0070] Cyanate 48%, carbonate 32%, potassium chloride 10%, rare earth additives 1.0%, stabilizer 2%.

[0071] Where Na⁺ / K⁺=0.45.

[0072] The specific application processes for the workpieces include:

[0073] S1. Degrease brake discs → wash with water → dry.

[0074] S2. Heat the salt bath to 400℃±5℃ and maintain the temperature for at least 60 minutes.

[0075] S3. Immerse the brake disc in a salt bath and keep it warm for 90-180 minutes.

[0076] S4. Air cooling after furnace exit → cleaning → drying → oxidation treatment.

[0077] S5. Scum is removed every 2 to 4 hours to achieve continuous production.

[0078] The process parameters used were selected as 400℃×150min.

[0079] When applied to stainless steel of type N360 / PH13-8Mo, the salt bath nitriding salt is prepared in the following mass percentage ratio:

[0080] Cyanate 58%, carbonate 26%, potassium chloride 6%, rare earth additives 1.2%, stabilizer 2%.

[0081] Na⁺ / K⁺ = 0.60.

[0082] The specific application processes for the workpieces include:

[0083] P1. Degrease stainless steel tray → wash with water → dry.

[0084] P2. Heat the salt bath to 400℃±5℃ and maintain the temperature for at least 60 minutes.

[0085] P3 stainless steel is immersed in a salt bath and kept at that temperature for 90–180 minutes.

[0086] P4. Air cooling after furnace exit → cleaning → drying → oxidation treatment.

[0087] P5. Scum is removed every 2 to 4 hours to achieve continuous production.

[0088] The process parameters used were selected as 400℃×120min.

[0089] The infiltration layer formed by co-infiltration includes a matrix 1, a diffusion layer 2, a compound layer 3, and an oxide film 4.

[0090] This invention applies low-temperature salt bath nitriding salts to automotive brake discs and stainless steel, offering advantages such as environmental friendliness, long lifespan, and versatility, while meeting Euro 7 standards.

[0091] It is suitable for strengthening automotive brake discs, methanol nozzle valve seats, hydraulic valve cores, precision shafts, molds, gears and various metal parts with high precision and low deformation requirements. It can be widely used in new energy vehicles, engineering machinery, hydraulic equipment, high-end mold manufacturing and other fields.

[0092] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A low-temperature salt bath nitriding salt, characterized in that: Including by weight percentage: Cyanate 45%–60%; Carbonates 25%–40%; Potassium chloride 5%–14%; Rare earth composite additives: 0.3%–1.5%; Alkaline stabilizer 1%–3%; The rare earth composite additive is composed of lanthanum oxide and yttrium oxide in a mass ratio of 1:1; the molar ratio of Na⁺ to K⁺ in the salt bath system is 0.35 to 0.

65.

2. The low-temperature salt bath nitriding salt according to claim 1, characterized in that: The cyanate is a mixture of sodium cyanate and potassium cyanate, with a molar mass ratio of sodium cyanate 45:potassium cyanate mixture 55; The carbonate is a mixture of sodium carbonate and potassium carbonate, with a molar mass ratio of 45 parts sodium carbonate to 55 parts potassium carbonate.

3. The low-temperature salt bath nitriding salt according to claim 1, characterized in that: The potassium chloride transforms the salt bath oxidation slag into floating slag, which is then removed online.

4. The low-temperature salt bath nitriding salt according to claim 3, characterized in that: The salt bath is used at a temperature of 400℃±5℃ to achieve low-temperature nitrogen-carbon co-infiltration.

5. The low-temperature salt bath nitriding salt according to claim 4, characterized in that: The infiltration layer formed by co-infiltration includes a matrix (1), a diffusion layer (2), a compound layer (3), and an oxide film (4); The porosity of the infiltrated layer is ≤2, and the surface hardness is ≥1100HV.

6. The preparation process of a low-temperature salt bath nitrided salt according to claim 5, characterized in that: The infiltration layer is prepared sequentially according to the process of raw material → drying → mixing → melting → adding rare earth → cooling → crushing → screening → finished product.

7. The preparation process of a low-temperature salt bath nitrided salt according to claim 6, characterized in that: The step of adding rare earth elements includes: Step 1: Sprinkle rare earth lanthanum carbonate / cerium carbonate composite powder into the molten salt solution in 5 small batches at a uniform speed, with an 8-minute interval between each batch; Step 2: After all the rare earth elements have been added, continue to aerate and stir for 120 minutes to allow the rare earth elements to fully dissociate into rare earth cations, which then react with Na. + / K + Formation of a composite alkali metal molten salt system; Step 3, Endpoint Detection: Molten salt sampling and spectral analysis showed uniform distribution of rare earth ions, with no localized high-concentration areas. Rare earth elements weakened the Na+ content. + The high proportion of Na leads to increased porosity, widening the area by 35%–50%. + During the process window, the hardness of nitriding at 500℃ increases by 30-50 HV, and the salt bath slag removal cycle is extended by 2 times.

8. The application of a low-temperature salt bath nitriding salt according to claim 5, characterized in that: The infiltration layer applications include QT450, QT500, N360, PH13-8Mo, 40Cr, and 42CrMo materials.

9. The application of a low-temperature salt bath nitriding salt according to claim 8, characterized in that: When the infiltration layer is applied to brake discs made of QT450 / QT500 material, the salt bath nitriding salt is prepared in the following mass percentage ratio: The composition is 48% cyanate, 32% carbonate, 10% potassium chloride, 1.0% rare earth additive, and 2% stabilizer, with Na⁺ / K⁺ = 0.

45.

10. The application of a low-temperature salt bath nitriding salt according to claim 8, characterized in that: When the diffusion layer is applied to stainless steel of material N360 / PH13-8Mo, the salt bath nitriding salt is prepared in the following mass percentage ratio: The composition is as follows: 58% cyanate, 26% carbonate, 6% potassium chloride, 1.2% rare earth additives, 2% stabilizer, and Na⁺ / K⁺ = 0.60.

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