A stainless steel surface Cr2N chromizing nitriding layer and a preparation method thereof

Abstract

The application discloses a Cr2N chromizing nitriding layer on a stainless steel surface, which is composed of a nitriding layer with Cr2N as a main phase; and the application also discloses a preparation method of the chromizing nitriding layer. ‑4 Pa, and then argon gas is introduced to prepare a chromizing layer; step two, preparation of the chromizing nitriding layer: after the preparation of the chromizing layer in step one is completed, the surface metallurgy equipment is vacuumized to 1.0*10 ‑2 Pa again, and then nitrogen gas and argon gas are introduced to prepare the chromizing nitriding layer. The preparation method of the application can effectively reduce the reaction temperature and shorten the reaction time, removes the powder loading and powder removing processes in the embedding chromizing-solid solution nitriding technology, greatly shortens the production cycle, avoids the dust pollution problem, and is suitable for industrial production.

Description

Technical Field

[0001] This invention belongs to the field of protective coating preparation technology, and in particular relates to a Cr2N chromium nitriding layer on stainless steel surface and its preparation method. Background Technology

[0002] The stable operation of sodium-cooled fast reactors (NSCFRs) is closely related to the properties of the core structural materials. Practice has shown that austenitic stainless steel can be widely used as a key structural material for NSCFRs, such as sleeves, shrouds, and headers. However, the high core temperature, large radiation dose, and strong corrosiveness of liquid sodium containing impurities in NSCFRs make the key structural material, austenitic stainless steel, susceptible to corrosion. Frictional wear and fretting wear also occur when components move relative to each other. Therefore, to ensure an operational life of at least 30 years for NSCFRs, a Cr2N protective layer (i.e., a chromium-nitrided layer) with a thickness of at least 80 μm is typically prepared on the stainless steel surface to improve its corrosion resistance and wear resistance, ensuring the long-term safe operation of the reactor.

[0003] Currently, the method for preparing chromium-nitrided layers in sodium-cooled fast reactors in my country is the embedded chromium-nitriding-solution nitriding technique. This technique has significant drawbacks: 1) The reaction temperature is too high and the reaction time is too long, resulting in a significant increase in the grain size of the substrate, affecting the tensile strength, corrosion resistance, and other properties of the substrate; 2) The industrial production cycle is 400 hours per furnace, resulting in excessive energy consumption and a long cycle; 3) It involves extremely high labor intensity and severe dust pollution. Therefore, there is an urgent need to propose a method for preparing Cr2N chromium-nitrided layers with a lower reaction temperature and a shorter preparation cycle. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing a Cr2N chromium nitriding layer on a stainless steel surface. This method utilizes a high-power plasma surface metallurgy approach to prepare the Cr2N chromium nitriding layer. This method can obtain instantaneous ultra-high energy Cr and N plasmas, allowing the Cr and N plasmas to rapidly diffuse and react sequentially on the surface of the stainless steel workpiece at a relatively low reaction temperature. This completes the preparation of the Cr2N chromium nitriding layer on the stainless steel substrate surface within a short diffusion time, solving the problems of high workpiece reaction temperature and long production cycle in the preparation of Cr2N chromium nitriding layers in the prior art.

[0005] The technical solution adopted in this invention is: a method for preparing a Cr2N chromium nitriding layer on a stainless steel surface, characterized in that the method includes the following steps:

[0006] Step 1: Preparation of the chromium-impregnated layer: A clean stainless steel substrate and a Cr target are placed in a high-power plasma surface metallurgy device, and a vacuum of 1.0 × 10⁻⁶ is applied. -4Below Pa, argon gas is introduced, and Cr plasma is generated by bombarding the Cr target material with argon ions. Under the action of an electric field, the Cr plasma reaches the surface of the stainless steel substrate and undergoes a diffusion reaction to prepare the chromium-impregnated layer.

[0007] Step 2, Preparation of the Chromium Nitriding Layer: After the chromium nitriding layer is prepared in Step 1, the high-power plasma surface metallurgy equipment is evacuated again to 1.0 × 10⁻⁶. -2 Below Pa, nitrogen and argon are introduced to put the stainless steel substrate in the high-power plasma surface metallurgy equipment into a glow discharge state, so that the nitrogen is ionized to form N plasma. Under the action of electric field force, the N plasma reacts with the chromium-infiltrated layer on the surface of the stainless steel substrate to prepare the Cr2N chromium-infiltrated nitrided layer.

[0008] In this invention, a high-power plasma surface metallurgy method is used to prepare a Cr2N chromium nitriding layer. The preparation process only requires high-power plasma surface metallurgy equipment, which reduces the difficulty and time required to prepare the Cr2N chromium nitriding layer on the surface of stainless steel substrate.

[0009] It should be noted that the "high power" in the high-power plasma surface metallurgy equipment refers to the equipment power being between 1000W and 3000W.

[0010] The above-mentioned method for preparing a Cr2N chromium nitriding layer on a stainless steel surface is characterized in that the argon gas volume purity in step one is 99.999%, and the argon gas flow rate is 30 sccm to 50 sccm.

[0011] The above-mentioned method for preparing a Cr2N chromium nitriding layer on a stainless steel surface is characterized in that the Cr target material in step one has a purity greater than 99.95%.

[0012] The above-mentioned method for preparing a Cr2N chromium nitriding layer on a stainless steel surface is characterized in that the process parameters for preparing the chromium nitriding layer in step one are: gas pressure 5Pa~50Pa, target electrode voltage 600V~800V, target electrode frequency 50kHz~100kHz, target electrode duty cycle 5%~30%, workpiece electrode voltage 420V~650V, workpiece electrode frequency 50kHz~100kHz, workpiece electrode duty cycle 30%~60%, workpiece temperature 900℃~1000℃, and diffusion time 3h~10h.

[0013] Advantages of the chromium infiltration layer preparation process parameters of this invention: By setting the target electrode voltage to 600V~800V, the frequency to 50kHz~100kHz, and the duty cycle to 5%~30%, the peak power of the high-power plasma surface metallurgy equipment can reach over 1000W, and the plasma density can reach 10 ppm. 18The plasma exhibits an order-of-magnitude ionization rate of over 70%, enhancing its inward diffusion driving force and diffusion reaction rate. When the gas pressure is set to 5 Pa–50 Pa, the ion free path of the Cr plasma is large, and the discharge state is stable, facilitating the approach of the Cr plasma to the surface of the stainless steel substrate. Setting the workpiece electrode voltage to 420 V–650 V, the frequency to 50 kHz–100 kHz, and the duty cycle to 30%–60% places the stainless steel substrate in a state of instantaneous high-energy glow discharge. Maintaining the substrate temperature at 900 °C–1000 °C allows for the diffusion reaction of the Cr plasma, preventing severe grain growth. Simultaneously, this condition generates numerous discharge channels, accelerating the Cr plasma diffusion reaction rate. A chromium-infiltrated layer of 80 μm–140 μm can be formed within 3–10 hours of diffusion.

[0014] The above-mentioned method for preparing a Cr2N chromium nitriding layer on a stainless steel surface is characterized in that the chromium nitriding layer obtained in step one is mainly composed of Fe-Cr phase, and the mass percentage of Cr in the Fe-Cr phase is more than 80%.

[0015] The above-mentioned method for preparing a Cr2N chromium nitriding layer on a stainless steel surface is characterized in that, in step two, the nitrogen gas has a volume purity of 99.999% and a flow rate of 10 sccm to 50 sccm, and the argon gas has a volume purity of 99.999% and a flow rate of 10 sccm to 30 sccm.

[0016] The above-mentioned method for preparing a Cr2N chromium nitriding layer on a stainless steel surface is characterized in that the process parameters for preparing the Cr2N chromium nitriding layer in step two are: gas pressure 1.0 × 10⁻⁶. 2 Pa ~ 2.0 × 10 3 Pa, workpiece electrode voltage 320V~600V, frequency 50kHz~100kHz, duty cycle 30%~60%, auxiliary heating system temperature 300℃~600℃, workpiece temperature 700℃~1000℃, diffusion time 3h~10h.

[0017] Advantages of the Cr2N chromium nitriding layer preparation process parameters of this invention: setting the gas pressure to 1.0 × 10⁻⁶ 2 Pa ~ 2.0 × 10 3The setting of Pa ensures sufficient nitrogen concentration; setting the frequency to 50kHz~100kHz and the duty cycle to 30%~60% allows for instantaneous high-energy discharge under high-frequency, low-duty-cycle conditions, shortening the arc response time; setting the auxiliary heating system temperature to 300℃~600℃ ensures the thermal stability of the stainless steel substrate; setting the workpiece electrode voltage to 320V~600V and the workpiece temperature to 700℃~1000℃ ensures sufficient reaction energy for Cr2N formation; after 3h~10h of full reaction, the chromium-infiltrated layer formed on the stainless steel surface can be transformed into a Cr2N chromium-infiltrated nitriding layer.

[0018] The above-mentioned method for preparing a Cr2N chromium nitriding layer on a stainless steel surface is characterized in that the roughness Ra of the stainless steel substrate in step one is not greater than 2.5 μm, and the stainless steel substrate is 316 stainless steel, 304 stainless steel or 321 stainless steel.

[0019] The present invention sets the roughness Ra of the stainless steel substrate to be no greater than 2.5 μm. When the roughness Ra of the stainless steel substrate is greater than 2.5 μm, it will affect the discharge state of the stainless steel substrate surface, causing severe arcing and affecting the uniformity of the Cr2N chromium nitriding layer formation.

[0020] Meanwhile, the present invention also discloses a Cr2N chromium nitriding layer on the surface of stainless steel prepared by the above method, characterized in that the Cr2N chromium nitriding layer is a nitriding layer with Cr2N as the main phase.

[0021] The above-mentioned Cr2N chromium nitriding layer on the surface of stainless steel is characterized in that the thickness of the Cr2N chromium nitriding layer is 80μm to 140μm and the Vickers hardness is 400HV5 to 600HV5.

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

[0023] 1. This invention utilizes a high-power plasma surface metallurgy method to prepare a Cr2N chromium-nitrided layer on the surface of a stainless steel substrate. This method induces the stainless steel substrate to be in a glow discharge state. Simultaneously, the peak power of the high-power plasma surface metallurgy equipment reaches over 1000W, enabling the generation of instantaneous ultra-high-energy Cr and N plasmas with a plasma density of 10-1 per cubic meter. 18 The order of magnitude improvement effectively enhances the inward diffusion driving force and diffusion reaction rate of plasma, enabling Cr and N plasmas to diffuse and react rapidly on the surface of stainless steel substrates at lower reaction temperatures. This effectively shortens the diffusion time during the preparation of Cr2N chromium nitriding layers, eliminates the powder loading and removal processes in traditional embedded chromium nitriding-solution nitriding technology, greatly reduces the preparation steps of Cr2N chromium nitriding layers, shortens the production cycle, and avoids dust pollution problems, making it suitable for industrial production.

[0024] 2. The method for preparing the Cr2N chromium nitriding layer of the present invention transforms the chromium-diffused layer on the surface of stainless steel into a Cr2N chromium nitriding layer under conditions of lower reaction temperature and shorter reaction time, thereby avoiding the serious growth of stainless steel grains caused by excessively high temperature and long reaction time of the stainless steel substrate, which would damage the performance of the stainless steel substrate.

[0025] 3. This invention utilizes the characteristics of Cr plasma having a high inward diffusion driving force and a fast diffusion reaction rate to suppress the outward diffusion of C in stainless steel substrate, thereby avoiding the appearance of Cr-C phase on the surface of stainless steel substrate, reducing the driving energy of the nitriding reaction of the chromium-infiltrating layer, and affecting the formation of Cr2N chromium-infiltrating nitriding layer.

[0026] 4. The Cr2N chromium nitriding layer prepared by this invention has a thickness of 80μm to 140μm and a Vickers hardness of 400HV5 to 600HV5. It is metallurgically bonded to the stainless steel surface and has a dense structure, making it suitable for stainless steel substrates in fast neutron reactor environments. It can effectively improve the corrosion resistance and wear resistance of stainless steel substrates and ensure the long-term safe operation of fast neutron reactors.

[0027] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0028] Figure 1 This is a cross-sectional microstructure of the Cr2N chromium nitriding layer obtained in Example 1 of the present invention.

[0029] Figure 2 The image shows the XRD pattern of the Cr2N chromium nitriding layer obtained in Example 1 of this invention. Detailed Implementation

[0030] Example 1

[0031] This embodiment includes the following steps:

[0032] Step 1: Preparation of the chromium-impregnated layer: A clean 316 stainless steel substrate with a surface roughness Ra of no more than 2.5 μm and a Cr target with a purity greater than 99.95% are placed in a high-power plasma surface metallurgy device, and a vacuum of 1.0 × 10⁻⁶ is drawn. - 4 Below Pa, argon gas with a volume purity of 99.999% and a flow rate of 40 sccm is introduced. Under the conditions of maintaining a gas pressure of 10 Pa, a target electrode voltage of 700 V, a target electrode frequency of 70 kHz, a target electrode duty cycle of 15%, a workpiece electrode voltage of 600 V, a workpiece electrode frequency of 70 kHz, a workpiece electrode duty cycle of 50%, and a workpiece temperature of 950 °C, diffusion is carried out for 6 hours to obtain a chromium-diffused layer.

[0033] Step 2, Preparation of the Chromium Nitriding Layer: After the chromium nitriding layer is prepared in Step 1, the high-power plasma surface metallurgy equipment is evacuated again to 1.0 × 10⁻⁶. -2 Below Pa, nitrogen gas with a volume purity of 99.999% and a flow rate of 30 sccm and argon gas with a volume purity of 99.999% and a flow rate of 20 sccm are introduced, while maintaining a pressure of 1.0 × 10⁻⁶ Pa. 3 A Cr2N chromium nitriding layer was obtained by diffusion for 5 hours under the following conditions: Pa, workpiece electrode voltage 500V, frequency 70kHz, duty cycle 50%, auxiliary heating system temperature 500℃, and workpiece temperature 850℃.

[0034] In step one of this embodiment, the 316 stainless steel substrate can also be a 304 stainless steel substrate or a 321 stainless steel substrate.

[0035] Figure 1 The image shows the cross-sectional microstructure of the chromium-nitrided layer obtained in this embodiment, as shown below. Figure 1 As shown, the thickness of the chromium nitriding layer obtained in this embodiment is 98.8 μm; Figure 2 The XRD pattern of the chromium-nitrided layer obtained in this embodiment is shown below. Figure 2 As shown, the chromium-nitrided layer obtained in this embodiment is mainly composed of Cr2N.

[0036] The Vickers hardness test was performed on the chromium nitriding layer prepared in this embodiment, and the Vickers hardness of the chromium nitriding layer was measured to be 470HV5.

[0037] Example 2

[0038] This embodiment includes the following steps:

[0039] Step 1: Preparation of the chromium-impregnated layer: A clean 304 stainless steel substrate with a surface roughness Ra of no more than 2.5 μm and a Cr target with a purity greater than 99.95% are placed in a high-power plasma surface metallurgy device, and a vacuum of 1.0 × 10⁻⁶ is drawn. - 4 The pressure is below 5 Pa, and then argon gas with a volume purity of 99.999% and a flow rate of 30 sccm is introduced. Under the conditions of maintaining a gas pressure of 5 Pa, a target electrode voltage of 600 V, a target electrode frequency of 50 kHz, a target electrode duty cycle of 30%, a workpiece electrode voltage of 420 V, a workpiece electrode frequency of 50 kHz, a workpiece electrode duty cycle of 60%, and a workpiece temperature of 900 °C, diffusion is carried out for 3 hours to obtain a chromium-diffused layer.

[0040] Step 2, Preparation of the Chromium Nitriding Layer: After the chromium nitriding layer is prepared in Step 1, the high-power plasma surface metallurgy equipment is evacuated again to 1.0 × 10⁻⁶. -2Below Pa, nitrogen gas with a volume purity of 99.999% and a flow rate of 10 sccm and argon gas with a volume purity of 99.999% and a flow rate of 10 sccm are introduced, while maintaining a pressure of 1.0 × 10⁻⁶ Pa. 2 The Cr2N chromium nitriding layer was obtained by diffusion for 3 hours under the following conditions: Pa, workpiece electrode voltage 320V, frequency 50kHz, duty cycle 30%, auxiliary heating system temperature 300℃, and workpiece temperature 700℃.

[0041] In step one of this embodiment, the 304 stainless steel substrate can also be a 316 stainless steel substrate or a 321 stainless steel substrate.

[0042] The thickness of the chromium nitrided layer obtained in this embodiment was measured, and the thickness of the chromium nitrided layer was found to be 80.0 μm. The Vickers hardness of the chromium nitrided layer obtained in this embodiment was tested, and the Vickers hardness of the chromium nitrided layer was found to be 400 HV5.

[0043] Example 3

[0044] This embodiment includes the following steps:

[0045] Step 1: Preparation of the chromium-impregnated layer: A clean 321 stainless steel substrate with a surface roughness Ra not greater than 2.5 μm and a Cr target with a purity greater than 99.95% are placed in a high-power plasma surface metallurgy device, and a vacuum of 1.0 × 10⁻⁶ is drawn. - 4 The pressure is below Pa, and then argon gas with a volume purity of 99.999% and a flow rate of 50 sccm is introduced. Under the conditions of maintaining a gas pressure of 50 Pa, a target electrode voltage of 800 V, a target electrode frequency of 100 kHz, a target electrode duty cycle of 5%, a workpiece electrode voltage of 650 V, a workpiece electrode frequency of 100 kHz, a workpiece electrode duty cycle of 30%, and a workpiece temperature of 1000 °C, diffusion is carried out for 10 h to obtain a chromium-diffused layer.

[0046] Step 2, Preparation of the Chromium Nitriding Layer: After the chromium nitriding layer is prepared in Step 1, the high-power plasma surface metallurgy equipment is evacuated again to 1.0 × 10⁻⁶. -2 Below Pa, nitrogen gas with a volume purity of 99.999% and a flow rate of 50 sccm and argon gas with a volume purity of 99.999% and a flow rate of 30 sccm are introduced, while maintaining a pressure of 2.0 × 10⁻⁶ Pa. 3 A Cr2N chromium nitriding layer was obtained by performing diffusion for 10 hours under the following conditions: Pa, workpiece electrode voltage 600V, frequency 100kHz, duty cycle 60%, auxiliary heating system temperature 600℃, and workpiece temperature 1000℃.

[0047] In step one of this embodiment, the 321 stainless steel substrate can also be a 316 stainless steel substrate or a 304 stainless steel substrate.

[0048] The thickness of the chromium nitrided layer obtained in this embodiment was measured, and the thickness of the chromium nitrided layer was found to be 140.0 μm. The Vickers hardness of the chromium nitrided layer obtained in this embodiment was tested, and the Vickers hardness of the chromium nitrided layer was found to be 600 HV5.

[0049] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the present invention. Any simple modifications, alterations, or equivalent structural transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a Cr2N chromium nitriding layer on a stainless steel surface, characterized in that, The method includes the following steps: Step 1: Preparation of the chromium-impregnated layer: A clean stainless steel substrate and a Cr target are placed in a high-power plasma surface metallurgy device, and a vacuum of 1.0 × 10⁻⁶ is applied. -4 Below Pa, argon gas is introduced, and Cr plasma is generated by bombarding the Cr target with argon ions. Under the action of an electric field, the Cr plasma reaches the surface of the stainless steel substrate and undergoes a diffusion reaction to prepare a chromium-diffused layer. The chromium-diffused layer is mainly composed of Fe-Cr phase, and the Cr mass percentage in the Fe-Cr phase is more than 80%. The stainless steel substrate is 316 stainless steel, 304 stainless steel, or 321 stainless steel. The process parameters for preparing the chromium infiltrated layer are as follows: gas pressure 5Pa~50Pa, target electrode voltage 600V~800V, target electrode frequency 50kHz~100kHz, target electrode duty cycle 5%~30%, workpiece electrode voltage 420V~650V, workpiece electrode frequency 50kHz~100kHz, workpiece electrode duty cycle 30%~60%, workpiece temperature 900℃~1000℃, and diffusion time 3h~10h. Step 2: Preparation of the chromium-nitrided layer: After the chromium-nitrided layer is prepared in Step 1, the high-power plasma surface metallurgy equipment is evacuated again to 1.0 × 10⁻⁶. -2 Below Pa, nitrogen and argon are introduced to put the stainless steel substrate in the high-power plasma surface metallurgy equipment into a glow discharge state, so that the nitrogen is ionized to form N plasma, and under the action of electric field force, the N plasma reacts with the chromium-infiltrated layer on the surface of the stainless steel substrate to prepare the Cr2N chromium-infiltrated nitrided layer. The process parameters for preparing the Cr2N chromium nitriding layer are: gas pressure 1.0 × 10⁻⁶. 2 Pa ~ 2.0 × 10 3 Pa, workpiece electrode voltage 320V~600V, frequency 50kHz~100kHz, duty cycle 30%~60%, auxiliary heating system temperature 300℃~600℃, workpiece temperature 700℃~1000℃, diffusion time 3h~10h.

2. The method for preparing a Cr2N chromium nitriding layer on a stainless steel surface according to claim 1, characterized in that, The argon gas volume purity mentioned in step one is 99.999%, and the argon gas flow rate is 30 sccm~50 sccm.

3. The method for preparing a Cr2N chromium nitriding layer on a stainless steel surface according to claim 1, characterized in that, The Cr target material mentioned in step one has a purity greater than 99.95%.

4. The method for preparing a Cr2N chromium nitriding layer on a stainless steel surface according to claim 1, characterized in that, In step two, the nitrogen gas has a volume purity of 99.999% and a flow rate of 10 sccm to 50 sccm, and the argon gas has a volume purity of 99.999% and a flow rate of 10 sccm to 30 sccm.

5. The method for preparing a Cr2N chromium nitriding layer on a stainless steel surface according to claim 1, characterized in that, The roughness Ra of the stainless steel substrate mentioned in step one is no greater than 2.5 μm.

6. A Cr2N chromium nitriding layer on a stainless steel surface prepared by the method according to any one of claims 1 to 5, characterized in that, The Cr2N chromium-doped nitriding layer is a nitriding layer with Cr2N as the main phase.

7. The Cr2N chromium nitriding layer on the surface of stainless steel according to claim 6, characterized in that, The thickness of the Cr2N chromium nitriding layer is 80μm~140μm, and the Vickers hardness is 400HV5~600HV5.

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