CN118186337A - Stainless steel surface Cr2N chromizing nitride layer and preparation method thereof - Google Patents

Abstract

The invention discloses a _Cr2N chromized nitriding layer on the surface of stainless steel, which is composed of a nitriding layer with _Cr2N as the main phase; the invention also discloses a preparation method of the chromized nitriding layer: step one, preparation of the chromized layer: placing a stainless steel substrate with a clean surface in a surface metallurgical device, evacuating to below 1.0×10 ^‑4 Pa, and then introducing argon to prepare the chromized layer; step two, preparation of the chromized nitriding layer: after the chromized layer is prepared in step one, the surface metallurgical device is evacuated again to below 1.0×10 ^‑2 Pa, and then nitrogen and argon are introduced to prepare the chromized nitriding layer. The preparation method of the invention prepares the _Cr2N chromized nitriding layer on the surface of stainless steel by high-power plasma surface metallurgy technology, can effectively reduce the reaction temperature, shorten the reaction time, remove the powder loading, powder removal and other processes in the embedded chromized solid solution nitriding technology, greatly shorten the production cycle, avoid the dust pollution problem, and is suitable for industrial production.

Description

A Cr2N chromized nitrided layer on stainless steel surface and preparation method thereof

Technical Field

The invention belongs to the technical field of protective coating preparation, and in particular relates to a Cr ₂ N chromized nitrided layer on the surface of stainless steel and a preparation method thereof.

Background technique

The stable operation of sodium-cooled fast reactors is closely related to the characteristics of core structural materials. Practice has proved that austenitic stainless steel can be widely used as key structural materials such as sleeves, enclosures, grid headers, etc. of sodium-cooled fast reactors. However, the core temperature of sodium-cooled fast reactors is high, the irradiation dose is large, and the liquid sodium containing impurities is highly corrosive. The key structural material austenitic stainless steel is prone to corrosion, and friction wear and fretting wear will occur when the component materials move against each other. Therefore, in order to ensure that the sodium-cooled fast reactor reaches an operating life of at least 30 years, it is usually necessary to prepare a Cr ₂ N protective layer (i.e., chromized nitriding layer) with a thickness of more than 80μm on the stainless steel surface to improve its corrosion resistance and wear resistance, and ensure the long-term safe operation of the reactor.

At present, the preparation method of chromized nitriding layer in sodium-cooled fast reactor in China is embedded chromized-solid solution nitriding technology, which has significant disadvantages: 1) the reaction temperature is too high and the time is too long, which leads to serious 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 400h/furnace, the energy consumption is too high and the cycle is too long; 3) the labor intensity is extremely high and the dust pollution is serious. Therefore, it is urgent to propose a preparation method of Cr ₂ N chromized nitriding layer with low reaction temperature and short preparation cycle.

Summary of the invention

The purpose of the present invention is to overcome the deficiencies in the above-mentioned prior art and provide a method for preparing a Cr ₂ N chromized nitriding layer on the surface of stainless steel. The preparation method prepares the Cr ₂ N chromized nitriding layer by adopting a high-power plasma surface metallurgy method. The method can obtain instantaneous ultra-high energy Cr and N plasmas, and in a relatively low reaction temperature environment, the Cr and N plasmas are successively rapidly diffused and reacted on the surface of the stainless steel workpiece, and the preparation of the Cr ₂ N chromized nitriding layer on the surface of the stainless steel substrate is completed in a relatively short diffusion time, thereby solving the problems of high reaction temperature of the workpiece and long production cycle when preparing the Cr ₂ N chromized nitriding layer in the prior art.

The technical solution adopted by the present invention is: a method for preparing a Cr ₂ N chromized nitriding layer on the surface of stainless steel, characterized in that the method comprises the following steps:

Step 1: Preparation of chromized layer: Place a clean stainless steel substrate and a Cr target in a high-power plasma surface metallurgy device, evacuate to below 1.0×10 ^-4 Pa, then introduce argon gas, bombard the Cr target with argon ions to generate Cr plasma, and under the action of the electric field, the Cr plasma reaches the surface of the stainless steel substrate and undergoes a diffusion reaction to prepare the chromized layer;

Step 2, preparation of chromized nitriding layer: After the chromized layer is prepared in step 1, the high-power plasma surface metallurgy equipment is evacuated again to below 1.0×10 ^-2 Pa, and then nitrogen and argon are introduced to make the stainless steel substrate in the high-power plasma surface metallurgy equipment in a glow discharge state, so that the nitrogen is ionized to form N plasma, and under the action of the electric field force, the N plasma and the chromized layer on the surface of the stainless steel substrate undergo a nitriding reaction, thereby preparing the Cr ₂ N chromized nitriding layer.

In the present invention, a high-power plasma surface metallurgy method is used to prepare the Cr ₂ N chromized nitriding layer. Only high-power plasma surface metallurgy equipment is needed in the preparation process, thereby reducing the difficulty and time of preparing the Cr ₂ N chromized nitriding layer on the surface of the stainless steel substrate.

It should be noted that the high power in the high power plasma surface metallurgy equipment refers to equipment power of 1000W to 3000W.

The above method for preparing a Cr ₂ N chromized nitriding layer on a stainless steel surface is characterized in that the volume purity of the argon gas in step 1 is 99.999% and the flow rate of the argon gas is 30 sccm to 50 sccm.

The above method for preparing a Cr ₂ N chromized nitrided layer on a stainless steel surface is characterized in that the mass purity of the Cr target material in step 1 is greater than 99.95%.

The above-mentioned method for preparing a Cr ₂ N chromized nitriding layer on a stainless steel surface is characterized in that the process parameters for preparing the chromized layer in step 1 are: air pressure 5Pa-50Pa, target pole voltage 600V-800V, target pole frequency 50kHz-100kHz, target pole duty cycle 5%-30%, workpiece pole voltage 420V-650V, workpiece pole frequency 50kHz-100kHz, workpiece pole duty cycle 30%-60%, workpiece temperature 900℃-1000℃, and diffusion time 3h-10h.

The advantages of the chromized layer preparation process parameters of the present invention are: the target electrode voltage is set to 600V-800V, the frequency is set to 50kHz-100kHz, and the duty cycle is set to 5%-30%, which can make the peak power of high-power plasma surface metallurgy equipment reach more than 1000W, and the plasma density reaches 10 per cubic meter. ¹⁸ orders of magnitude, generating Cr plasma with an ionization rate of more than 70%, improving its inward diffusion driving force and diffusion reaction rate; when the gas pressure is set to 5Pa~50Pa, the ion free path of the Cr plasma is large and the discharge state is stable, which is conducive to the Cr plasma approaching the surface of the stainless steel substrate; the workpiece pole voltage is set to 420V~650V, the frequency is set to 50kHz~100kHz, and the duty cycle is set to 30%~60%. Under this condition, the stainless steel substrate is in an instantaneous high-energy glow discharge state, and the temperature of the stainless steel substrate is kept at 900℃~1000℃ to carry out the diffusion reaction of Cr plasma, avoiding serious growth of grain size. At the same time, a large number of discharge channels can be generated under this condition to accelerate the diffusion reaction rate of Cr plasma. A chromized layer of 80μm~140μm can be formed within a diffusion time of 3h~10h.

The above method for preparing a Cr ₂ N chromized nitrided layer on a stainless steel surface is characterized in that the chromized layer prepared in step 1 has a Fe-Cr phase as a main phase, and the mass percentage of Cr in the Fe-Cr phase is more than 80%.

The above method for preparing a Cr ₂ N chromized nitriding layer on a stainless steel surface is characterized in that the nitrogen volume purity in step 2 is 99.999%, the nitrogen flow rate is 10 sccm-50 sccm, and the argon volume purity is 99.999%, the argon flow rate is 10 sccm-30 sccm.

The above-mentioned method for preparing a Cr ₂ N chromized nitriding layer on a stainless steel surface is characterized in that the process parameters for preparing the Cr ₂ N chromized nitriding layer in step 2 are: air pressure 1.0×10 ² Pa～2.0×10 ³ Pa, workpiece pole voltage 320V～600V, frequency 50kHz～100kHz, duty cycle 30%～60%, auxiliary heating system temperature 300℃～600℃, workpiece temperature 700℃～1000℃, and diffusion time 3h～10h.

The advantages of the _Cr2N chromized nitriding layer preparation process parameters of the present invention are as follows: the gas pressure is set at 1.0× ^102Pa -2.0× ^103Pa , which can ensure sufficient nitrogen concentration; the frequency is set at 50kHz-100kHz, and the duty cycle is set at 30%-60%, and under the conditions of high frequency and low duty cycle, instantaneous high-energy discharge is obtained, and the arc response time is shortened; the temperature of the auxiliary heating system is set at 300℃-600℃, which can ensure the thermal stability of the stainless steel substrate; the workpiece pole voltage is set at 320V-600V, and the workpiece temperature is set at 700℃-1000℃, which can ensure sufficient reaction energy for the formation of _Cr2N ; after a sufficient reaction of 3h-10h, the chromized layer formed on the surface of the stainless steel can be converted into the _Cr2N chromized nitriding layer.

The above method for preparing a Cr ₂ N chromized nitriding layer on a stainless steel surface is characterized in that the roughness Ra of the stainless steel substrate in step 1 is not greater than 2.5 μm, and the stainless steel substrate is 316 stainless steel, 304 stainless steel or 321 stainless steel.

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, the discharge state of the stainless steel substrate surface will be affected, causing serious sparking and affecting the uniformity of the Cr ₂ N chromized nitriding layer.

Meanwhile, the present invention also discloses a Cr ₂ N chromized nitriding layer on the surface of stainless steel prepared by the above method, characterized in that the Cr ₂ N chromized nitriding layer is a nitriding layer with Cr ₂ N as a main phase.

The above-mentioned Cr ₂ N chromized nitriding layer on the surface of stainless steel is characterized in that the Cr ₂ N chromized nitriding layer has a thickness of 80 μm to 140 μm and a Vickers hardness of 400 HV ₅ to 600 HV ₅ .

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

1. The present invention adopts a high-power plasma surface metallurgy method to prepare a _Cr2N chromized nitriding layer on the surface of a stainless steel substrate, so that the stainless steel substrate is in a glow discharge state. At the same time, the peak power of the high-power plasma surface metallurgy equipment reaches more than 1000W, and instantaneous ultra-high energy Cr and N plasma can be obtained, and the plasma density reaches the order of ¹⁰¹⁸ per cubic meter, which effectively improves the inward diffusion driving force and diffusion reaction rate of the plasma, so that the Cr and N plasmas can quickly diffuse and react on the surface of the stainless steel substrate under a relatively low reaction temperature condition, effectively shortening the diffusion time when preparing the _Cr2N chromized nitriding layer, eliminating the powder loading and powder removal processes in the traditional embedded chromizing-solid solution nitriding technology, greatly reducing the preparation process of the _Cr2N chromized nitriding layer, shortening the production cycle, and avoiding the dust pollution problem, and being suitable for industrial production.

2. The method for preparing the Cr ₂ N chromized nitriding layer of the present invention converts the chromized layer on the surface of stainless steel into the Cr ₂ N chromized nitriding layer under the conditions of relatively low reaction temperature and relatively short reaction time, thereby avoiding the serious growth of stainless steel grains due to excessively high temperature of the stainless steel substrate and excessively long reaction time, thereby damaging the performance of the stainless steel substrate.

3. The present invention uses the characteristics of Cr plasma with higher inward diffusion driving force and faster diffusion reaction rate to inhibit the outward diffusion of C in the stainless steel substrate, thereby avoiding the appearance of Cr-C phase on the surface of the stainless steel substrate, reducing the nitriding reaction driving energy of the chromized layer, and affecting the formation of the Cr ₂ N chromized nitriding layer.

4. The Cr ₂ N chromized nitriding layer prepared by the present invention has a thickness of 80 μm to 140 μm and a Vickers hardness of 400 HV ₅ to 600 HV ₅ . It is metallurgically bonded to the stainless steel surface and has a dense structure. It is suitable for stainless steel substrates in a fast neutron reactor environment and can effectively improve the corrosion resistance and wear resistance of the stainless steel substrate, thereby ensuring the long-term safe operation of the fast neutron reactor.

The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a cross-sectional microscopic morphology of a Cr ₂ N chromized nitrided layer prepared in Example 1 of the present invention.

FIG. 2 is an XRD spectrum of the Cr ₂ N chromized nitrided layer prepared in Example 1 of the present invention.

Detailed ways

Example 1

This embodiment includes the following steps:

Step 1, preparation of chromized layer: a 316 stainless steel substrate with a clean surface and a roughness Ra not greater than 2.5 μm and a Cr target with a mass purity greater than 99.95% are placed in a high-power plasma surface metallurgy device, evacuated to below 1.0×10 ^{- 4} Pa, and then introduced with argon gas with a volume purity of 99.999% and a flow rate of 40 sccm, and the chromized layer is obtained by diffusing for 6 hours under the conditions of maintaining a gas pressure of 10 Pa, a target pole voltage of 700 V, a target pole frequency of 70 KHz, a target pole duty cycle of 15%, a workpiece pole voltage of 600 V, a workpiece pole frequency of 70 KHz, a workpiece pole duty cycle of 50%, and a workpiece temperature of 950°C;

Step 2, preparation of chromized nitriding layer: After the preparation of chromized layer in step 1 is completed, the high-power plasma surface metallurgy equipment is evacuated again to below 1.0×10 ^-2 Pa, and then nitrogen with a volume purity of 99.999% and a flow rate of 30 sccm and argon with a volume purity of 99.999% and a flow rate of 20 sccm are introduced, and the chromized nitriding layer is expanded for 5 hours under the conditions of maintaining a gas pressure of 1.0×10 ³ Pa, a workpiece pole voltage of 500V, a frequency of 70kHz, a duty cycle of 50%, an auxiliary heating system temperature of 500°C, and a workpiece temperature of 850°C, to obtain a Cr ₂ N chromized nitriding layer.

The 316 stainless steel substrate in step 1 of this embodiment may also be a 304 stainless steel substrate or a 321 stainless steel substrate.

FIG1 is a cross-sectional microscopic morphology of the chromized nitriding layer prepared in this embodiment. As shown in FIG1 , the thickness of the chromized nitriding layer prepared in this embodiment is 98.8 μm. FIG2 is an XRD spectrum of the chromized nitriding layer prepared in this embodiment. As shown in FIG2 , the chromized nitriding layer prepared in this embodiment is mainly composed of Cr ₂ N.

The Vickers hardness test was performed on the chromized nitrided layer prepared in this embodiment, and the Vickers hardness of the chromized nitrided layer was measured to be 470 HV ₅ .

Example 2

This embodiment includes the following steps:

Step 1, preparation of chromized layer: a 304 stainless steel substrate with a clean surface and a roughness Ra not greater than 2.5 μm and a Cr target with a mass purity greater than 99.95% are placed in a high-power plasma surface metallurgy device, evacuated to below 1.0×10 ^{- 4} Pa, and then introduced with argon gas with a volume purity of 99.999% and a flow rate of 30 sccm, and the chromized layer is obtained by diffusing for 3 hours under the conditions of maintaining a gas pressure of 5 Pa, a target pole voltage of 600 V, a target pole frequency of 50 KHz, a target pole duty cycle of 30%, a workpiece pole voltage of 420 V, a workpiece pole frequency of 50 KHz, a workpiece pole duty cycle of 60%, and a workpiece temperature of 900°C;

Step 2, preparation of chromized nitriding layer: After the preparation of chromized layer in step 1 is completed, the high-power plasma surface metallurgy equipment is evacuated again to below 1.0× ^10-2 Pa, and then nitrogen with a volume purity of 99.999% and a flow rate of 10sccm and argon with a volume purity of 99.999% and a flow rate of 10sccm are introduced, and the chromized nitriding layer is expanded for 3 hours under the conditions of maintaining a gas pressure of 1.0× ^102Pa , a workpiece pole voltage of 320V, a frequency of 50kHz, a duty cycle of 30%, an auxiliary heating system temperature of 300℃, and a workpiece temperature of 700℃, to obtain a _Cr2N chromized nitriding layer.

The 304 stainless steel substrate in step 1 of this embodiment may also be a 316 stainless steel substrate or a 321 stainless steel substrate.

The thickness of the cross section of the chromized nitriding layer prepared in this embodiment was measured, and the thickness of the chromized nitriding layer was measured to be 80.0 μm. The Vickers hardness of the chromized nitriding layer prepared in this embodiment was tested, and the Vickers hardness of the chromized nitriding layer was measured to be 400 HV ₅ .

Example 3

This embodiment includes the following steps:

Step 1, preparation of chromized layer: a 321 stainless steel substrate with a clean surface and a roughness Ra not greater than 2.5 μm and a Cr target with a mass purity greater than 99.95% are placed in a high-power plasma surface metallurgy device, evacuated to below 1.0×10 ^{- 4} Pa, and then introduced with argon gas with a volume purity of 99.999% and a flow rate of 50 sccm, and the chromized layer is obtained by diffusing for 10 hours under the conditions of maintaining a gas pressure of 50 Pa, a target pole voltage of 800 V, a target pole frequency of 100 KHz, a target pole duty cycle of 5%, a workpiece pole voltage of 650 V, a workpiece pole frequency of 100 KHz, a workpiece pole duty cycle of 30%, and a workpiece temperature of 1000°C;

Step 2, preparation of chromized nitriding layer: After the preparation of chromized layer in step 1, the high power plasma surface metallurgy equipment is evacuated again to below 1.0×10 ^-2 Pa, and then nitrogen with a volume purity of 99.999% and a flow rate of 50 sccm and argon with a volume purity of 99.999% and a flow rate of 30 sccm are introduced, and the chromized nitriding layer is expanded for 10 hours under the conditions of maintaining a gas pressure of 2.0×10 ³ Pa, a workpiece pole voltage of 600V, a frequency of 100kHz, a duty cycle of 60%, an auxiliary heating system temperature of 600°C, and a workpiece temperature of 1000°C to obtain a Cr ₂ N chromized nitriding layer.

The 321 stainless steel substrate in step 1 of this embodiment may also be a 316 stainless steel substrate or a 304 stainless steel substrate.

The thickness of the cross section of the chromized nitriding layer prepared in this embodiment was measured, and the thickness of the chromized nitriding layer was measured to be 140.0 μm. The Vickers hardness of the chromized nitriding layer prepared in this embodiment was tested, and the Vickers hardness of the chromized nitriding layer was measured to be 600 HV ₅ .

The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent structural transformation made to the above embodiment based on the technical essence of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims (10)

1. The preparation method of the Cr ₂ N chromizing nitride layer on the surface of the stainless steel is characterized by comprising the following steps:

Step one, preparing a chromizing layer: placing a stainless steel substrate with clean surface and a Cr target material in high-power plasma surface metallurgical equipment, vacuumizing to less than 1.0X10 ^-4 Pa, then introducing argon, bombarding the Cr target material by argon ions to generate Cr plasma, enabling the Cr plasma to reach the surface of the stainless steel substrate under the action of an electric field, performing diffusion reaction, and preparing a chromizing layer;

Preparing a chromizing nitriding layer: after the chromizing layer is prepared in the first step, vacuumizing the high-power plasma surface metallurgical equipment again to below 1.0X10 ^-2 Pa, then introducing nitrogen and argon to enable the stainless steel substrate in the high-power plasma surface metallurgical equipment to be in a glow discharge state, ionizing the nitrogen to form N plasma, and enabling the N plasma to be subjected to nitriding reaction with the chromizing layer on the surface of the stainless steel substrate under the action of an electric field force to prepare the Cr ₂ N chromizing nitride layer.

2. The method for preparing a Cr ₂ N chromizing nitride layer on a stainless steel surface according to claim 1, wherein in the first step, the argon gas volume purity is 99.999%, and the argon gas flow is 30 sccm-50 sccm.

3. The method for producing a Cr ₂ N chromizing nitride layer on a stainless steel surface according to claim 1, wherein the mass purity of said Cr target material in step one is greater than 99.95%.

4. The method for preparing a chromium-doped nitride layer on a stainless steel surface Cr ₂ N according to claim 1, wherein the technological parameters of the preparation of the chromium-doped nitride layer in the first step are as follows: the air pressure is 5 Pa-50 Pa, the target electrode voltage is 600V-800V, the target electrode frequency is 50 kHz-100 kHz, the target electrode duty ratio is 5-30%, the workpiece electrode voltage is 420V-650V, the workpiece electrode frequency is 50 kHz-100 kHz, the workpiece electrode duty ratio is 30-60%, the workpiece temperature is 900-1000 ℃, and the diffusion time is 3-10 h.

5. The method for preparing a Cr ₂ N chromizing nitride layer on a stainless steel surface according to claim 1, wherein the chromizing layer prepared in the first step uses a Fe-Cr phase as a main phase, and the mass percentage of Cr in the Fe-Cr phase is 80% or more.

6. The method for preparing a Cr ₂ N chromizing nitride layer on a stainless steel surface according to claim 1, wherein in the second step, the volume purity of nitrogen is 99.999%, the flow rate of nitrogen is 10 sccm-50 sccm, the volume purity of argon is 99.999%, and the flow rate of argon is 10 sccm-30 sccm.

7. The method for preparing a Cr ₂ N chromizing nitride layer on a stainless steel surface according to claim 1, wherein the technological parameters of the preparation of the Cr ₂ N chromizing nitride layer in the second step are as follows: the air pressure is 1.0 multiplied by 10 ²Pa～2.0×10³ Pa, the workpiece electrode voltage is 320V-600V, the frequency is 50 kHz-100 kHz, the duty ratio is 30% -60%, the auxiliary heating system temperature is 300-600 ℃, the workpiece temperature is 700-1000 ℃, and the diffusion time is 3-10 h.

8. The method for preparing a Cr ₂ N chromizing nitride layer on a stainless steel surface according to claim 1, wherein in the first step, the roughness Ra of the stainless steel substrate is not more than 2.5 μm, and the stainless steel substrate is 316 stainless steel, 304 stainless steel or 321 stainless steel.

9. A Cr ₂ N chromizing nitride layer on the surface of stainless steel prepared by the method as claimed in any one of claims 1 to 8, wherein the Cr ₂ N chromizing nitride layer is a nitride layer with Cr ₂ N as the main phase.

10. The Cr ₂ N chromizing nitride layer on the surface of stainless steel according to claim 9, wherein the thickness of the Cr ₂ N chromizing nitride layer is 80 μm-140 μm, and the vickers hardness is 400HV ₅～600HV₅.