Method for preparing super-thick compact infiltrated layer through interface reactivation type cyclic nitridation

Abstract

The invention relates to a method for preparing a super-thick compact infiltrated layer through interface reactivation type cyclic nitridation, and aims to solve the problems that an infiltrated layer obtained through existing salt bath nitridation treatment is insufficient in thickness and poor in performance. The method for preparing the super-thick compact infiltrated layer comprises the following steps: 1, putting a preheated workpiece in a nitriding furnace into a molten salt bath medium, and carrying out salt bath nitriding treatment at the temperature of 380-650 DEG C; 2, carrying out layer removal treatment through stress-assisted surface subtractive machining to remove a surface compound layer of the workpiece; thirdly, the salt bath nitriding treatment and the layer removing treatment are repeated for multiple times; and 4, carrying out surface ultrasonic rolling post-treatment on the workpiece subjected to circulating salt bath nitriding. According to the method, a compound layer formed in the nitriding process is removed, the nitrogen atom diffusion driving force of subsequent nitriding treatment is enhanced, the nitrided layer subjected to circulating salt bath nitriding treatment has higher nitrogen concentration, the thicker nitrided layer is prepared, the depth and compactness of the nitrided layer of the metal workpiece can be remarkably improved, and the comprehensive performance of the nitrided layer is enhanced.

Description

Technical Field

[0001] This invention belongs to the field of surface treatment of metallic materials, specifically involving an interface reactivation-type cyclic nitriding method for preparing an ultra-thick and dense diffusion layer. Background Technology

[0002] Heavy-duty gears operating in marine environments endure extremely complex conditions, including high contact stress, alternating impact loads, and multi-directional bending stress, while also being subject to seawater corrosion. This multi-stress coupling can easily lead to fatigue cracks, spalling, and even fracture on the gear surface. Therefore, heavy-duty gears for marine service must not only possess excellent fatigue resistance and load-bearing capacity but also outstanding corrosion resistance to ensure long-term stable operation.

[0003] Nitriding is a widely used technique for strengthening gear surfaces. Salt bath nitriding, in particular, boasts advantages such as process stability, high efficiency, and uniform diffusion layer. Furthermore, salt bath nitrided workpieces exhibit high corrosion resistance, making it a promising candidate for gear surface strengthening. However, the extreme service conditions of heavy-duty gears place more stringent demands on the diffusion layer performance: on the one hand, a sufficiently thick diffusion layer is required to provide adequate load-bearing capacity; on the other hand, the diffusion layer must possess excellent density and structural stability. This presents a fundamental challenge to traditional salt bath nitriding in heavy-duty gear applications. Therefore, innovative process design is urgently needed to achieve breakthroughs in nitriding kinetics control and composite strengthening. Summary of the Invention

[0004] The purpose of this invention is to solve the problems of insufficient thickness and poor performance of the infiltrated layer obtained by existing salt bath nitriding treatment, and to provide a method for preparing an ultra-thick and dense infiltrated layer by interface reactivation cyclic nitriding.

[0005] The method for preparing ultra-thick dense infiltrated layers by interface reactivation cyclic nitriding of the present invention is implemented according to the following steps:

[0006] Step 1: Place the preheated workpiece into a molten salt bath medium in a nitriding furnace. Nitrogen or ammonia gas is introduced into the nitriding furnace, and salt bath nitriding is performed at a temperature of 380~650℃ to obtain a workpiece with a surface compound layer and an inner nitriding layer.

[0007] Step 2: Cool the workpiece with the surface compound layer and the inner nitride layer to 80~100℃, and remove the surface compound layer of the workpiece by stress-assisted surface subtractive processing.

[0008] Step 3: Repeat the salt bath nitriding treatment in Step 1 and the delamination treatment in Step 2 multiple times, ending with salt bath nitriding treatment, to obtain the workpiece after cyclic salt bath nitriding.

[0009] Step 4: Perform surface ultrasonic rolling post-treatment on the workpiece after cyclic salt bath nitriding to obtain a workpiece with an ultra-thick and dense infiltrated layer.

[0010] The salt bath medium consists of 30%~40% potassium cyanate, 35%~45% sodium cyanate, 10%~20% sodium carbonate, 4%~8% sodium chloride, and 4%~8% sodium sulfate by mass percentage; in step two, stress-assisted surface subtraction is performed by grinding, ultrasonic vibration-assisted grinding, laser shot peening, or micro-blasting.

[0011] This invention provides a method for preparing an ultra-thick and dense nitrided layer using interface reactivation cyclic nitriding. After salt bath nitriding, stress-assisted surface subtractive processing removes the compound layer formed during nitriding, exposing the active surface. This overcomes the diffusion barrier of traditional nitriding, enhancing the driving force for nitrogen atom diffusion in subsequent nitriding processes. The removal of the compound layer on the workpiece surface reduces the obstruction to nitrogen atom diffusion in subsequent nitriding processes, resulting in a higher nitrogen concentration in the nitrided layer obtained using this method. Nitrogen atoms easily diffuse into the core, thus producing a thicker nitrided layer. Simultaneously, ultrasonic rolling post-treatment eliminates porosity in the nitrided layer, improving its quality and providing secondary strengthening, resulting in excellent overall performance. Therefore, this invention can significantly improve the depth and density of the nitrided layer in gear workpieces, enhancing its mechanical properties and corrosion resistance.

[0012] This invention can utilize existing industrial salt bath nitriding and ultrasonic rolling equipment, thus also having the advantages of simple equipment and process routes and low cost. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the circulating salt bath nitriding process of the present invention;

[0014] Figure 2 This is a cross-sectional metallographic image of the sample after a single salt bath nitriding in Example 1;

[0015] Figure 3 This is a cross-sectional scanning microstructure image of the sample after a single salt bath nitriding in Example 1;

[0016] Figure 4 The image shows a cross-sectional metallographic image of the sample after cyclic salt bath nitriding in Example 2.

[0017] Figure 5 This is a cross-sectional scanning microstructure image of the sample after cyclic salt bath nitriding in Example 2;

[0018] Figure 6 This is a scanning micrograph of the sample after cyclic salt bath nitriding-composite ultrasonic rolling treatment in Example 3;

[0019] Figure 7 The graphs show the friction and wear curves of the workpiece samples with ultra-thick dense infiltrated layers obtained in Examples 1, 2, and 3.

[0020] Figure 8 Corrosion curves of workpiece samples with ultra-thick dense infiltration layers obtained in Examples 1, 2, and 3;

[0021] Figure 9 The roughness test diagrams are of the workpiece samples with ultra-thick dense infiltration layers obtained in Examples 3 and 4. Detailed Implementation

[0022] Specific Implementation Method 1: The method for preparing an ultra-thick dense infiltration layer by interface reactivation-type cyclic nitriding in this implementation method is carried out according to the following steps:

[0023] Step 1: Place the preheated workpiece into a molten salt bath medium in a nitriding furnace. Nitrogen or ammonia gas is introduced into the nitriding furnace, and salt bath nitriding is performed at a temperature of 380~650℃ to obtain a workpiece with a surface compound layer and an inner nitriding layer.

[0024] Step 2: Cool the workpiece with the surface compound layer and the inner nitride layer to 80~100℃, and remove the surface compound layer of the workpiece by stress-assisted surface subtractive processing.

[0025] Step 3: Repeat the salt bath nitriding treatment in Step 1 and the delamination treatment in Step 2 multiple times, ending with salt bath nitriding treatment, to obtain the workpiece after cyclic salt bath nitriding.

[0026] Step 4: Perform surface ultrasonic rolling post-treatment on the workpiece after cyclic salt bath nitriding to obtain a workpiece with an ultra-thick and dense infiltrated layer.

[0027] The salt bath medium consists of 30%~40% potassium cyanate, 35%~45% sodium cyanate, 10%~20% sodium carbonate, 4%~8% sodium chloride, and 4%~8% sodium sulfate by mass percentage; in step two, stress-assisted surface subtraction is performed by grinding, ultrasonic vibration-assisted grinding, laser shot peening, or micro-blasting.

[0028] This embodiment optimizes the composition and formulation of the salt bath medium.

[0029] This embodiment proposes a method for preparing an ultra-thick, dense infiltrated layer using interface reactivation-type cyclic nitriding. First, the workpiece undergoes salt bath nitriding, followed by stress-assisted surface subtraction to remove the compound layer formed during nitriding, exposing the active surface. Simultaneously, stress-induced nanoscale grain boundaries are generated at the interface, promoting the formation of short-circuit paths for nitrogen diffusion and achieving interface reactivation. Repeated salt bath nitriding-interface activation processes are performed to achieve efficient cyclic salt bath nitriding, overcoming traditional nitriding diffusion barriers and enhancing the driving force for nitrogen atom diffusion in subsequent nitriding treatments, thereby significantly increasing the thickness of the nitrided layer. Finally, the cyclically nitrided workpiece undergoes ultrasonic rolling treatment. High-frequency impact-induced plastic deformation introduces residual compressive stress into the nitrided layer, while ultrasonic vibration generates cavitation to seal micron-level pores in the nitrided layer, improving the porous structure of the infiltrated layer and reducing the surface roughness of the workpiece.

[0030] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the material of the workpiece in step one is 32Cr3MoVE gear steel.

[0031] Specific Implementation Method 3: This implementation method differs from Specific Implementation Method 1 or 2 in that the preheating treatment of the workpiece in step 1 is carried out at a temperature of 320~480℃ for 30~240 minutes.

[0032] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that the workpiece in step one undergoes ultrasonic cleaning and acid pickling activation treatment in sequence before preheating.

[0033] The pickling solution in this embodiment includes 15 vol% hydrochloric acid, 5 vol% sulfuric acid and the balance water, and also contains a corrosion inhibitor.

[0034] Specific Implementation Method 5: This implementation method differs from Specific Implementation Methods 1 to 4 in that step 1 involves a salt bath nitriding treatment at a temperature of 550~600℃ for 100~200 minutes.

[0035] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that the salt bath medium in step one is composed of 35% potassium cyanate, 40% sodium cyanate, 15% sodium carbonate, 5% sodium chloride, and 5% sodium sulfate by mass percentage.

[0036] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that the salt bath nitriding treatment in Step One and the delamination treatment in Step Two are repeated 2 to 5 times in Step Three.

[0037] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in that the pressure of ultrasonic rolling in step four is controlled to be 0.4~0.8MPa and the ultrasonic frequency is 5~10Hz.

[0038] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Seven in that a pulsed electric field or magnetic field is applied during the ultrasonic rolling process in step four.

[0039] Specific Implementation Method 10: This implementation method differs from Specific Implementation Methods 1 to 9 in that the thickness of the ultra-thick dense infiltration layer on the workpiece in step 4 is 150~1000μm.

[0040] In this embodiment, the thickness of the ultra-thick dense infiltration layer on the workpiece is preferably 450μm~800μm.

[0041] Example 1: The method for preparing an ultra-thick dense infiltrated layer by interface reactivation cyclic nitriding in this example is implemented according to the following steps:

[0042] Step 1: The workpiece is 32Cr3MoVE gear steel. After sanding and mechanical polishing, it is cleaned with alcohol and preheated to 400℃ for 120 minutes. The preheated workpiece is placed in a molten salt bath medium in a nitriding furnace. Nitrogen gas is introduced into the nitriding furnace and the salt bath nitriding is carried out at 550℃ for 2 hours. Heating is stopped and the sample is taken out after the furnace temperature cools to room temperature. The workpiece with a surface compound layer and an inner nitriding layer is obtained.

[0043] Step 2: Cool the workpiece with the surface compound layer and the inner nitride layer to 80°C. First, use 60# sandpaper to polish and remove the layers until no white bright layer can be seen under a metallographic microscope. Then, use 400#~2000# sandpaper to polish and remove the surface compound layer of the workpiece.

[0044] Step 3: Repeat the salt bath nitriding treatment in Step 1 and the delamination treatment in Step 2 once, and then perform the salt bath nitriding treatment again to obtain the workpiece after cyclic salt bath nitriding.

[0045] The salt bath medium consists of 35% potassium cyanate, 40% sodium cyanate, 15% sodium carbonate, 5% sodium chloride, and 5% sodium sulfate by mass percentage.

[0046] In step two of this embodiment, observing the cross-section of the infiltrated layer with a metallographic microscope after removing the compound layer ensures that the compound layer is completely removed.

[0047] In this embodiment, the average thickness of the diffusion layer on the 32Cr3MoVE gear steel after treatment is 219 μm. Figure 2 As shown. Scanning electron microscopy observation of the cross-section reveals that the surface compound layer is loose and porous, and the infiltrated layer contains a large number of band-like structures, such as... Figure 3 As shown.

[0048] Example 2: The method for preparing an ultra-thick dense infiltrated layer by interface reactivation cyclic nitriding in this example is implemented according to the following steps:

[0049] Step 1: The workpiece is 32Cr3MoVE gear steel. After sanding and mechanical polishing, it is cleaned with alcohol and preheated to 400℃ for 120 minutes. The preheated workpiece is placed in a molten salt bath medium in a nitriding furnace. Nitrogen gas is introduced into the nitriding furnace and the salt bath nitriding is carried out at 600℃ for 4 hours. Heating is stopped and the sample is taken out after the furnace temperature cools down to room temperature. The workpiece with a surface compound layer and an inner nitriding layer is obtained.

[0050] Step 2: Cool the workpiece with the surface compound layer and the inner nitride layer to 80°C. First, use 60# sandpaper to polish and remove the layers until no white bright layer can be seen under a metallographic microscope. Then, use 400#~2000# sandpaper to polish and remove the surface compound layer of the workpiece.

[0051] Step 3: Repeat the salt bath nitriding treatment in Step 1 and the delamination treatment in Step 2 three times, and finally perform a salt bath nitriding treatment to obtain the workpiece after cyclic salt bath nitriding.

[0052] The salt bath medium consists of 35% potassium cyanate, 40% sodium cyanate, 15% sodium carbonate, 5% sodium chloride, and 5% sodium sulfate by mass percentage.

[0053] The average thickness of the carburized layer on the 32Cr3MoVE gear steel treated in Example 2 was 640 μm, which was nearly three times thicker than the carburized layer thickness treated in Example 1. Figure 4 As shown. However, cross-sectional scanning electron microscopy reveals that the surface compound layer is significantly looser, and numerous banded structures exist within the infiltrated layer, such as... Figure 5 As shown.

[0054] Example 3: The method for preparing an ultra-thick dense infiltrated layer by interface reactivation cyclic nitriding in this example is implemented according to the following steps:

[0055] Step 1: The workpiece is 32Cr3MoVE gear steel. After sanding and mechanical polishing, it is cleaned with alcohol and preheated to 400℃ for 120 minutes. The preheated workpiece is placed in a molten salt bath medium in a nitriding furnace. Nitrogen gas is introduced into the nitriding furnace and the salt bath nitriding is carried out at 570℃ for 4 hours. Heating is stopped and the sample is taken out after the furnace temperature cools down to room temperature. The workpiece with a surface compound layer and an inner nitriding layer is obtained.

[0056] Step 2: Cool the workpiece with the surface compound layer and the inner nitride layer to 80°C. First, use 60# sandpaper to polish and remove the layers until no white bright layer can be seen under a metallographic microscope. Then, use 400#~2000# sandpaper to polish and remove the surface compound layer of the workpiece.

[0057] Step 3: Repeat the salt bath nitriding treatment in Step 1 and the delamination treatment in Step 2 three times, and finally perform a salt bath nitriding treatment to obtain the workpiece after cyclic salt bath nitriding.

[0058] Step 4: Perform surface ultrasonic rolling post-treatment on the workpiece after cyclic salt bath nitriding. The ultrasonic rolling static pressure is 0.5MPa, the feed speed is 5mm / min, the ultrasonic frequency is 10Hz, and the amplitude is 5μm, thereby preparing a workpiece with an ultra-thick and dense infiltrated layer.

[0059] The salt bath medium consists of 35% potassium cyanate, 40% sodium cyanate, 15% sodium carbonate, 5% sodium chloride, and 5% sodium sulfate by mass percentage.

[0060] In Example 3, the 32Cr3MoVE gear steel treated with this method had a complete and dense compound layer, and the banded structure in the diffusion layer disappeared, with refined grains. Figure 6 As shown in the figure. In Implementation 2, after three cycles of nitriding, the friction coefficient decreased significantly, fluctuating around 0.6, as... Figure 7 As shown. In Example 3, the coefficient of friction of the sample after rolling treatment decreased to between 0.4 and 0.5, while the self-corrosion current density after three cycles of nitriding increased from 2.04 × 10⁻⁶. -6 μA / cm 2 Reduced to 9.13×10 -7 μA / cm 2 The self-corrosion current density of the sample after rolling treatment decreased to 8.26 × 10⁻⁶. -7 μA / cm 2 ,like Figure 8 As shown.

[0061] Example 4: This example differs from Example 3 in that the workpiece after cyclic salt bath nitriding undergoes surface ultrasonic rolling post-treatment. The ultrasonic rolling static pressure is 0.5 MPa, the feed speed is 5 mm / min, the ultrasonic frequency is 10 Hz, and the amplitude is 5 μm. Simultaneously, a pulsed electric field is applied to the workpiece, with the pulsed electric field direction perpendicular to the workpiece. The workpiece serves as one electrode, and the workpiece and the other electrode are connected to the two ends of the pulse power supply. The controlled electric pulse parameters are: peak current density 6 Jm / A mm. -2 The pulse frequency is 500Hz.

[0062] In Example 4, after pulsed electric field-assisted ultrasonic deep rolling, the surface roughness of the sample was 1.170 μm, a decrease of 78.2% compared to the roughness value of 5.368 μm after deep rolling without electric pulse. Figure 9As shown. This is because during conventional ultrasonic deep rolling, the material undergoes plastic flow from surface protrusions with higher pressure to micro-depressions with lower pressure under the rolling action of the tool head. However, if the material itself has a significant work hardening tendency or poor fluidity at room temperature, the material redistribution process is often insufficient and uneven, easily leading to incomplete flattening of protrusions and incomplete filling of micro-depressions, thus limiting the improvement of surface roughness. With the assistance of a pulsed electric field, the electroplastic effect induced by high-energy electric pulses significantly reduces the instantaneous rheological stress of the material, thereby dramatically enhancing its plastic flow capability. Under the same mechanical load, the material at protrusions is more likely to undergo coordinated and uniform plastic deformation and more smoothly and fully fill the surrounding micro-valve bottoms, effectively reducing surface profile fluctuations and achieving a significant reduction in surface roughness.

Claims

1. A method for preparing super-thick and dense diffusion layer by interface reactivation type cyclic nitridation, characterized in that The method for preparing the super-thick and dense permeation layer is realized according to the following steps: Step one, placing the preheating treated workpiece into a molten salt bath medium in a nitriding furnace, introducing nitrogen or ammonia into the nitriding furnace, and carrying out salt bath nitriding treatment at a temperature of 380-650 DEG C to obtain a workpiece with a surface compound layer and an inner layer nitriding layer; Step two, cooling the workpiece with the surface compound layer and the inner layer nitriding layer to 80-100 DEG C, and removing the surface compound layer of the workpiece by stress-assisted surface subtractive machining; Step three, repeating the salt bath nitriding treatment of step one and the layer removal treatment of step two for multiple times, and ending the salt bath nitriding treatment to obtain a workpiece after cyclic salt bath nitriding; Step four, carrying out surface ultrasonic rolling after-treatment on the workpiece after cyclic salt bath nitriding to prepare a workpiece with a super-thick and dense permeation layer; The salt bath medium is composed of 30-40% potassium cyanate, 35-45% sodium cyanate, 10-20% sodium carbonate, 4-8% sodium chloride and 4-8% sodium sulfate in terms of mass percentage; the stress-assisted surface subtractive machining in step two is grinding, ultrasonic vibration-assisted grinding, laser shot peening or micro-sand blasting.

2. The method for preparing super-thick and dense case depth by interface reactivation type cyclic nitriding according to claim 1, characterized in that The material of the workpiece in step one is 32Cr3MoVE gear steel.

3. The method for preparing super-thick and dense diffusion layer by interface reactivation type cyclic nitriding according to claim 1, characterized in that The preheating treatment of the workpiece in step one is carried out at a temperature of 320-480 DEG C for 30-240 min.

4. The method for preparing super-thick and dense diffusion layer by interface reactivation type cyclic nitriding according to claim 1, characterized in that The preheating treated workpiece is subjected to ultrasonic cleaning and acid pickling activation treatment in sequence before preheating.

5. The method for preparing super-thick and dense case depth by interface reactivation type cyclic nitriding according to claim 1, characterized in that The salt bath nitriding treatment in step one is carried out at a temperature of 550-600 DEG C for 100-200 min.

6. The method for preparing super-thick and dense case depth by interface reactivation type cyclic nitriding according to claim 1, characterized in that The salt bath medium in step one is composed of 35% potassium cyanate, 40% sodium cyanate, 15% sodium carbonate, 5% sodium chloride and 5% sodium sulfate in terms of mass percentage.

7. The method for preparing super-thick and dense case depth by interface reactivation type cyclic nitriding according to claim 1, characterized in that The salt bath nitriding treatment of step one and the layer removal treatment of step two are repeated for 2-5 times in step three.

8. The method of claim 1, wherein the interface reactivation type cyclic nitriding for preparing a super-thick and dense diffusion layer is characterized by The pressure of ultrasonic rolling in step four is controlled to be 0.4-0.8 MPa, and the ultrasonic frequency is 5-10 Hz.

9. The method of claim 1, wherein the interface reactivation type cyclic nitriding for preparing a super-thick and dense diffusion layer is characterized by A pulse electric field or a magnetic field is applied during the ultrasonic rolling in step four.

10. The method of claim 1, wherein the interface reactivation type cyclic nitriding for preparing a super-thick and dense diffusion layer is characterized by The thickness of the super-thick and dense permeation layer on the workpiece in step four is 150-1000 μm.

Images