Method for optimizing corrosion resistance of high-nitrogen steel based on powder metallurgy technology

Abstract

This invention relates to the field of powder metallurgy high-nitrogen steel preparation technology, specifically a method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology. The method includes: pre-treating gas-atomized high-nitrogen steel pre-alloyed powder by inert gas purging, constant-temperature heating for degassing, and mechanical stirring; then physically mixing it with a nano-chromium nitride, vanadium nitride, and niobium nitride composition; during filling, activating a variable-frequency mechanical vibration table to achieve gradient densification filling with a higher density at the bottom of the mold than at the top; after multi-stage vacuum and inert gas circulation, using stepped heating and simultaneous application of axial pressure and isotropic inert gas static pressure to complete solid-state sintering and preliminary densification. This invention, through gradient filling and multi-field coupled sintering process design, improves powder distribution and sintering densification state, reduces microscopic defects in the matrix, optimizes nitrogen element distribution, and enhances the corrosion resistance of high-nitrogen steel. The process has strong adaptability and is suitable for the powder metallurgy preparation of high-nitrogen steel components.

Description

Technical Field

[0001] This invention relates to the field of powder metallurgy high-nitrogen steel preparation technology, and in particular to a method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology. Background Technology

[0002] Conventional high-nitrogen steel powder metallurgy processes typically use gas-atomized high-nitrogen steel pre-alloyed powder as raw material. Simple inert gas purging and isothermal heating are performed in a vacuum environment. Powder mixing is achieved solely through conventional physical blending with micron-sized nitride additives. Mold fillers are uniformly filled using a constant vibration intensity method. Hot isostatic pressing (HIP) sintering often employs a single heating curve, applying isotropic inert gas static pressure to achieve powder densification and sintering. This type of process is currently the mainstream implementation method for high-nitrogen steel powder metallurgy forming and is widely used in the conventional preparation stages of high-nitrogen steel components.

[0003] Conventional pretreatment methods cannot effectively break up powder agglomerates, and isothermal degassing efficiency is limited. The mixed powder exhibits uneven component distribution. Uniform filling methods ensure consistent powder compaction density within the mold cavity, but the powder shrinkage behavior during sintering lacks gradient adaptability, easily leading to localized porosity and structural defects. Sintering modes using single gas static pressure combined with conventional heating cannot simultaneously address the temperature adaptability and multidimensional pressure effects of powder solid-state sintering. After sintering, the high-nitrogen steel matrix has numerous grain boundary defects, dispersed nitrogen distribution, and corrosion resistance that fails to meet the demands of harsh operating conditions.

[0004] In the powder metallurgy forming process of high nitrogen steel, the filler stage cannot form a gradient filling structure with a higher density at the bottom than at the top. The hot isostatic pressing sintering stage lacks a pressurization method that simultaneously couples step heating with axial pressure and inert gas static pressure. This leads to problems such as insufficient sintering densification, more matrix defects, and uneven nitrogen distribution, which directly restrict the improvement of the corrosion resistance of high nitrogen steel and have become a technical pain point that urgently needs to be addressed in the industry. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by proposing a method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology, comprising: A pre-alloyed high-nitrogen steel powder of a predetermined particle size is placed in a vacuum glove box for pretreatment. The pretreatment includes continuous inert gas purging, constant-temperature heating to degas the high-nitrogen steel powder, and mechanical stirring to break up powder agglomerates. The pre-alloyed high-nitrogen steel powder that has undergone pretreatment is physically mixed with nano-sized nitride forming agent powder to obtain a mixed powder, wherein the nano-sized nitride forming agent powder comprises a homogeneous composition of chromium nitride, vanadium nitride and niobium nitride. The obtained mixed powder is filled into the cavity of a high-strength graphite mold. During the filling process, a variable frequency mechanical vibration table is started to make the mixed powder achieve gradient densification filling in the cavity. The gradient densification filling is characterized by the powder compaction density at the bottom of the mold being higher than that at the top. The high-strength graphite mold that has been filled is moved into the working chamber of the hot isostatic pressing sintering furnace. The working chamber is subjected to multi-stage vacuuming and inert gas backfilling and circulation until the oxygen content and water content in the working chamber are lower than the preset threshold. The heating system of the hot isostatic pressing sintering furnace is started, and the high-strength graphite mold is heated according to the preset stepped heating program. During the heating process, axial pressure and isotropic inert gas static pressure are applied simultaneously, so that the mixed powder undergoes solid-state sintering and preliminary densification.

[0007] As a further aspect of the present invention, the pretreatment operation of placing the gas-atomized high-nitrogen steel pre-alloyed powder of a predetermined particle size in a vacuum glove box includes: The atomized high-nitrogen steel pre-alloyed powder is placed in the transition chamber of a vacuum glove box, and the transition chamber is evacuated. High-purity inert gas is backfilled into the vacuum-evacuated transition chamber to balance the pressure between the main working chamber and the transition chamber of the glove box. The high-nitrogen steel pre-alloyed powder is transferred from the transition chamber to the heating platform of the main working chamber of the glove box, and the heating platform is heated to the preset degassing temperature. At the degassing temperature, the powder in the main working chamber is statically degassed for a predetermined time, while the powder is intermittently stirred by a stirring paddle driven by a robotic arm. After degassing and stirring are completed, the heating platform is cooled, and the pretreated powder is transferred to a sealed container under the protective atmosphere of the main working chamber of the glove box for later use.

[0008] As a further aspect of the present invention, the nano-sized nitride forming agent powder comprises a homogeneous composition of chromium nitride, vanadium nitride, and niobium nitride, wherein the addition ratio and mixing method are as follows: Based on the matrix chemical composition of the target high-nitrogen steel, the theoretical ratio difference between the contents of chromium, vanadium, and niobium in the alloy matrix and the target total nitrogen content is calculated. Based on the theoretical ratio difference, weigh out the corresponding masses of nano-chromium nitride powder, nano-vanadium nitride powder and nano-niobium nitride powder respectively. The weighed nano-chromium nitride powder, nano-vanadium nitride powder and nano-niobium nitride powder are placed in a low-speed mixing tank for premixing to obtain a uniform composite nitride additive powder. In a vacuum glove box environment, the composite nitride additive powder and the pre-treated high-nitrogen steel pre-alloyed powder are added together into a double cone mixer; The rotation speed and mixing time of the double cone mixer are set, and three-dimensional rotational mixing is carried out under a protective atmosphere until the composite nitride additive powder is uniformly dispersed on the surface and in the gaps of the high nitrogen steel pre-alloyed powder particles to form the mixed powder.

[0009] As a further aspect of the present invention, the heating of the high-strength graphite mold according to a preset stepped heating program, wherein axial pressure and isotropic inert gas static pressure are applied simultaneously during the heating process, includes: During the heating phase from room temperature to the first intermediate temperature range, the static pressure of the inert gas is maintained at a low level, while a small pulsed axial pressure is applied to promote the rearrangement of the mixed powder particles and the expansion of the initial contact surface. When the temperature reaches the first intermediate temperature range, the static pressure of the inert gas is increased to a medium pressure level and kept constant. At the same time, the axial pressure is adjusted to a continuous constant pressure mode, and the heat preservation platform is started, so that the surface of the high nitrogen steel pre-alloyed powder particles undergoes local plastic deformation and neck growth. During the heating phase from the first intermediate temperature range to the second intermediate temperature range, the static pressure of the inert gas at a moderate pressure level is maintained, and the value of the axial pressure is increased linearly to promote the expansion of the metallurgical bonding interface and the spheroidization of pores between the high-nitrogen steel pre-alloyed powder particles. When the temperature reaches the peak sintering temperature, the static pressure of the inert gas is increased to the final high pressure level, and the axial pressure is simultaneously adjusted to the peak pressure. The temperature is maintained at the peak sintering temperature and the final high pressure level for a preset holding time to achieve complete densification of the powder and dissolution and diffusion of nano-sized nitride forming agent powder.

[0010] As a further aspect of the present invention, it also includes a step of performing in-situ plasma nitriding treatment on the high-strength graphite mold during the heat preservation and pressurization process: During the holding stage at the peak sintering temperature, a nitrogen-containing process gas mixture is introduced into the working chamber of the hot isostatic pressing furnace. A high-frequency pulsed electric field is applied inside the working chamber to ionize the nitrogen-containing process gas mixture, forming a uniform low-temperature nitrogen plasma; The concentration and spatial distribution density of active nitrogen atoms in the low-temperature nitrogen plasma are controlled by adjusting the duty cycle and voltage of the pulsed electric field. Under the combined constraints of the inert gas static pressure and the peak axial pressure at the final high pressure level, the active nitrogen atoms penetrate into the surface layer and grain boundaries of the high-nitrogen steel sintered body in a plastic state. The active nitrogen atoms combine with alloying elements within the sintered body to generate supplementary nitride strengthening phases in situ at grain boundaries and near-surface regions.

[0011] As a further aspect of the present invention, it also includes the step of implementing controlled nitrogen distribution and phase transformation regulation during the cooling stage of the sintered body: After holding at the peak sintering temperature, the heating system is turned off, and the sintered body is cooled from the peak sintering temperature to the nitride precipitation sensitive temperature range at the first cooling rate. Within the nitride precipitation sensitive temperature range, the cooling rate is adjusted to a second cooling rate lower than the first cooling rate, and an inert gas static pressure at a back pressure level is maintained during this stage. During the slow cooling process at the second cooling rate, nitrogen atoms supersaturated in the matrix diffuse and agglomerate with alloying elements toward grain boundaries and dislocations, precipitating a diffusely distributed secondary nitride phase with the elements dissolved from the nano-sized nitride forming agent powder as the core. When the temperature drops below the matrix phase transformation initiation point, the cooling rate is increased again, and the phase transformation zone is rapidly passed through at a third cooling rate to suppress the formation of coarse nitrides and grain growth, thereby obtaining a high-nitrogen steel material containing a fine-grained matrix and a uniformly dispersed nitride-reinforced phase.

[0012] As a further aspect of the present invention, after obtaining the high-nitrogen steel material containing a fine-grained matrix and a uniformly dispersed nitride-reinforcing phase, the method further includes subsequent surface densification and structural stabilization steps: The obtained high-nitrogen steel material was placed on a surface grinder and its surface was ground to remove the extremely thin oxide layer formed during sintering and cooling. The surface of the ground high-nitrogen steel material was subjected to non-melting scanning treatment using a high-energy beam scanning device. The scanning path of the high-energy beam scanning device was planned based on the microhardness distribution map of the material surface. During the non-melting scanning process, the energy input of the high-energy beam causes dynamic recovery and recrystallization in the extremely thin area of ​​the material surface, while promoting the healing of the micropores on the surface under thermal stress. After the surface scanning is completed, the high-nitrogen steel material is placed in a low-temperature tempering furnace and kept at a low temperature for a long time under a protective atmosphere to eliminate the residual stress generated on the surface during the non-melting scanning process and to further homogenize the distribution of nitrogen atoms.

[0013] As a further aspect of the present invention, the determination of the sintering peak temperature and the control of the holding time are based on the following: Differential thermal analysis was performed on the mixed powder to plot the heat flow curve of the mixed powder during the heating process; From the heat flow curve, the temperature at which the endothermic valley begins to open, corresponding to the solidus temperature of the matrix phase of the high-nitrogen steel pre-alloyed powder, and the temperature at which the exothermic peak begins to open, corresponding to the dissolution of the nano-sized nitride forming agent powder, are identified. The temperature range of 20 to 50 degrees Celsius above the exothermic peak initiation temperature is selected as the candidate sintering peak temperature range. Within the candidate sintering peak temperature range, a temperature value is selected as the initial sintering peak temperature, and the holding time at the initial sintering peak temperature is calculated based on the total amount of mixed powder, the size of the graphite mold, and the preset heating rate. In subsequent experimental verification, the initial sintering peak temperature and the initial holding time were fine-tuned based on the actual density and microstructure of the sintered body, and finally the optimized sintering peak temperature and the corresponding sintering peak temperature holding time were determined.

[0014] As a further aspect of the present invention, the determination of the nitride precipitation sensitive temperature range and the control of the second cooling rate include: By conducting a series of isothermal annealing experiments on samples that have been sintered but have not undergone the controlled nitrogen distribution and phase transformation regulation steps, statistical data on the morphology and size distribution of precipitated phases of samples after holding at different isothermal temperatures were obtained. Plot the relationship curves between the average size of the precipitated phase, the number density of the precipitated phase, and the isothermal temperature; From the relationship curves, the temperature range with the smallest average size of the precipitated phase and the highest number density of the precipitated phase is selected and defined as the optimal nitride precipitation sensitive temperature range. Through continuous cooling transformation experiments, the maximum allowable cooling rate required to avoid harmful phase precipitation and ensure sufficient diffusion precipitation within the optimal nitride precipitation sensitive temperature range was determined. The maximum allowable cooling rate is reduced by a safety factor as the control target value for the second cooling rate. In the subsequent cooling process, the flow rate and direction of the cooling airflow in the furnace are controlled by the program to achieve the second cooling rate.

[0015] As a further aspect of the present invention, the parameter setting method for the high-energy beam scanning device to perform non-melting scanning treatment on the surface of the ground high-nitrogen steel material includes: The micro Vickers hardness of the surface of the high-nitrogen steel material after grinding was measured, and the micro hardness distribution map characterizing the uniformity of hardness distribution was plotted. Identify localized softened areas with hardness below the average level from the microhardness distribution map; The basic energy density and scanning speed of the high-energy beam are set so that the beam energy input can only raise the surface temperature of the material to above its recrystallization temperature, but far below its solidus temperature. When planning the scanning path, the identified localized softened areas are processed using a mode that reduces the scanning speed or performs overlapping scans to provide higher energy input and promote tissue densification in the localized softened areas. After performing a complete scan of the entire surface, the surface is rapidly cooled, and the real-time temperature distribution of the surface is monitored by an infrared thermal imager to ensure that the temperature of any area does not exceed the preset upper limit of the non-melting temperature.

[0016] Compared with the prior art, the advantages and positive effects of the present invention are as follows: During the filling process, the variable frequency mechanical vibration table is activated, which enables the mixed powder to form a gradient densification state in the high-strength graphite mold cavity, with the bottom powder compaction density being higher than that at the top. The powder particles are arranged in a density gradient from top to bottom inside the cavity, and the contact gap between particles is gradually adjusted with the density change, avoiding the local pore concentration caused by uniform filling. The nitride forming agent is uniformly dispersed with the powder gradient arrangement, and the uniformity of nitrogen element distribution inside the high nitrogen steel matrix is ​​improved, and the number of micro-defects that are susceptible to corrosion at the grain boundaries is reduced.

[0017] In the hot isostatic pressing (HIP) sintering stage, heating is performed according to a preset stepped heating program, while axial pressure and isotropic inert gas static pressure are applied simultaneously. The stepped heating rhythm matches the temperature change pattern of the mixed powder solid-state sintering, avoiding abnormal grain growth caused by rapid heating. The axial pressure and inert gas static pressure act synergistically on the mixed powder from different directions, strengthening the bonding between powder particles, promoting the full progress of the solid-state sintering process, inhibiting the escape of nitrogen during sintering, refining the grain structure of the high-nitrogen steel matrix, reducing the generation of intergranular porosity and microcracks, blocking the penetration path of corrosive media inside the steel body, and optimizing the overall corrosion resistance of the high-nitrogen steel. Attached Figure Description

[0018] Figure 1 This is a flowchart of the method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology according to the present invention; Figure 2 A flowchart showing the addition ratio and mixing method of nano-nitride forming agent powder; Figure 3 The curves show the effects of isothermal temperature on the size, number density, and overall properties of precipitated phases in high-nitrogen steel. Figure 4 Temperature change curves for the entire high-energy beam treatment and tempering process; Figure 5The curve shows the effect of heating rate on the onset temperature of the DTA exothermic peak in high-nitrogen steel mixed powder. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0020] In the description of this invention, it should be understood that the terms "length," "width," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships, are based on the orientation or positional relationships shown in the accompanying drawings and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, in the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0021] See Figure 1 This invention provides a method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology, the method comprising: Pre-treated high-nitrogen steel pre-alloyed powder of predetermined particle size is placed in a vacuum glove box for pretreatment, including continuous inert gas purging, constant temperature heating degassing, and mechanical stirring to break up powder agglomerates. The pre-treated high-nitrogen steel pre-alloyed powder is then physically mixed with nano-sized nitride forming agent powder containing a uniform composition of chromium nitride, vanadium nitride, and niobium nitride to obtain a mixed powder. The obtained mixed powder is filled into the cavity of a high-strength graphite mold, and during the filling process, a variable frequency mechanical vibration table is activated to achieve gradient densification of the mixed powder in the cavity, with the bottom density higher than the top density. The filled mold is then moved into the working chamber of a hot isostatic pressing (HIP) sintering furnace, and the working chamber is subjected to multi-stage vacuuming and inert gas backfilling cycles until the oxygen and water content in the chamber is lower than the preset threshold. The HIP sintering furnace is then activated to heat the mold according to a preset stepped heating program, and during the heating process, axial pressure and isotropic inert gas static pressure are applied simultaneously to cause the mixed powder to undergo solid-state sintering and preliminary densification.

[0022] In one embodiment of the present invention, the atomized high-nitrogen steel pre-alloyed powder is placed in the transition chamber of a vacuum glove box and the transition chamber is evacuated. High-purity inert gas is backfilled into the evacuated transition chamber to balance the pressure between the main working chamber and the transition chamber. The high-nitrogen steel pre-alloyed powder is transferred from the transition chamber to the heating platform of the main working chamber of the glove box and the heating platform is heated to a preset degassing temperature. At the degassing temperature, the powder in the main working chamber is statically degassed for a predetermined time, while the powder is intermittently stirred by a stirring paddle driven by a robotic arm. After degassing and stirring are completed, the heating platform is cooled and the pretreated powder is transferred to a sealed container under the protective atmosphere of the main working chamber of the glove box for later use.

[0023] See Figure 2 Based on the matrix chemical composition of the target high-nitrogen steel, the theoretical ratio difference between the content of chromium, vanadium, and niobium elements in the alloy matrix and the target total nitrogen content is calculated. According to the theoretical ratio difference, the corresponding mass of nano-chromium nitride powder, nano-vanadium nitride powder, and nano-niobium nitride powder are weighed respectively. The weighed nano-chromium nitride powder, nano-vanadium nitride powder, and nano-niobium nitride powder are placed in a low-speed mixing tank for premixing to obtain a uniform composite nitride additive powder. In a vacuum glove box environment, the composite nitride additive powder and the pre-treated high-nitrogen steel pre-alloy powder are added together to a double-cone mixer. The speed and mixing time of the double-cone mixer are set to three-dimensional spatial rotation mixing under a protective atmosphere until the composite nitride additive powder is uniformly dispersed on the surface and gaps of the high-nitrogen steel pre-alloy powder particles to form the mixed powder.

[0024] In specific implementation, the method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology involves the pretreatment of gas-atomized high-nitrogen steel pre-alloyed powder and the mixing of nano-sized nitride-forming agent powder. The gas-atomized high-nitrogen steel pre-alloyed powder with a predetermined particle size of 15 to 45 micrometers is placed in a vacuum glove box for pretreatment. The pretreatment includes continuous argon purging, isothermal heating to degas the high-nitrogen steel pre-alloyed powder, and mechanical stirring to break up powder agglomerates. In some embodiments, the gas-atomized high-nitrogen steel pre-alloyed powder is placed in the transition chamber of the vacuum glove box, and the transition chamber is evacuated until the pressure is lower than [a certain value]. Pa, high-purity argon gas is backfilled into the evacuated transition chamber to balance the pressure between the main working chamber and the transition chamber at atmospheric pressure. The high-nitrogen steel pre-alloyed powder is then transferred from the transition chamber to the heating platform in the main working chamber of the glove box, and the platform is heated to a preset degassing temperature of 250°C. At this temperature, the powder in the main working chamber undergoes static degassing for a predetermined time of 2 hours. Simultaneously, a robotic arm drives a stirring paddle to intermittently stir the powder, with a stirring cycle of 30 seconds every 10 minutes. After degassing and stirring are completed, the heating platform is cooled to below 50°C, and the pretreated powder is transferred to a sealed container under the protective atmosphere of the main working chamber of the glove box for later use. It is understood that the degassing temperature can be adjusted within the range of 200 to 300°C depending on the composition of the high-nitrogen steel pre-alloyed powder, the static degassing time can vary from 1 to 3 hours, and the stirring cycle and duration can also be adjusted accordingly to adapt to the characteristics of different batches of powder.

[0025] In practical implementation, the nano-sized nitride forming agent powder comprises a homogeneous composition of chromium nitride, vanadium nitride, and niobium nitride. The addition ratio and mixing method are based on the matrix chemical composition of the target high-nitrogen steel. Based on the matrix chemical composition of the target high-nitrogen steel, the theoretical ratio difference between the chromium, vanadium, and niobium content in the alloy matrix and the target total nitrogen content is calculated. The theoretical ratio difference is expressed by the following formula: in: This represents the adjustment factor for the mass fraction of additives corresponding to the difference in theoretical proportions. This indicates the design value of the total nitrogen content in the target high-nitrogen steel. This indicates the measured value of the matrix nitrogen content in high-nitrogen steel pre-alloyed powder. This indicates the mass fraction of chromium, vanadium, and niobium in the nano-nitride forming agent powder. This indicates the nitride formation equivalent coefficient of the corresponding element. Based on the theoretical ratio difference, the corresponding masses of nano-chromium nitride powder, nano-vanadium nitride powder, and nano-niobium nitride powder are weighed out. For example, for a design with a target high-nitrogen steel matrix containing 18% chromium, 1.5% vanadium, and 0.5% niobium, the required mass fractions of nano-chromium nitride powder, nano-vanadium nitride powder, and nano-niobium nitride powder are calculated to be 0.3%, 0.1%, and 0.05%, respectively. The weighed nano-chromium nitride powder, nano-vanadium nitride powder, and nano-niobium nitride powder are placed in a low-speed mixing tank for premixing. The speed of the low-speed mixing tank is set to 30 rpm, and the mixing time is 1 hour to obtain a uniform composite nitride additive powder. In a vacuum glove box environment, the composite nitride additive powder and the pre-treated high-nitrogen steel pre-alloyed powder are added together to a double cone mixer. The speed of the double cone mixer is set to 20 revolutions per minute and the mixing time is 2 hours. The mixture is then rotated in three dimensions under a protective atmosphere until the composite nitride additive powder is evenly dispersed on the surface and in the gaps between the high-nitrogen steel pre-alloyed powder particles, forming a mixed powder.

[0026] In some embodiments, data comparisons reflect the impact of different pretreatment parameters on powder flowability and mixing uniformity. For example, comparing degassing temperatures of 200°C and 300°C, 300°C results in more thorough powder degassing but may increase energy consumption. Similarly, comparing mixing times of 2 hours and 3 hours, 3 hours improves uniformity but reduces efficiency. In specific implementations, the calculation of the addition ratio of nano-sized nitride forming agent powder can be based on data comparisons of different target high-nitrogen steel compositions. For example, for a design with a chromium content of 20%, the calculated addition amount of nano-chromium nitride powder may increase to 0.4%, while the addition amounts of vanadium and niobium are adjusted accordingly. This is dynamically determined using a theoretical ratio difference formula. It is understood that the pressure equilibrium point of the pretreatment operation in the vacuum glove box can be selected within the range of atmospheric pressure to slightly positive pressure, depending on the equipment performance. The heating rate of the heating platform is controlled within 5°C per minute to prevent thermal shock, and the rotation speed of the robotic-driven stirring paddle is adjustable between 10 and 50 revolutions per minute to accommodate the degree of powder agglomeration. Optionally, the particle size range of the nano-sized nitride forming agent powder is controlled between 20 and 100 nanometers. The material of the low-speed mixing tank in the premixing stage can be stainless steel or ceramic to avoid contamination. The protective atmosphere of the double cone mixer can be argon or nitrogen, and its purity must be maintained above 99.99% to ensure that there is no oxidation during the mixing process.

[0027] In one embodiment of the present invention, during the heating stage from room temperature to a first intermediate temperature range, the inert gas static pressure is maintained at a low pressure level while a small pulsed axial pressure is applied to promote the rearrangement of the mixed powder particles and the expansion of the initial contact surface. When the temperature reaches the first intermediate temperature range, the inert gas static pressure is increased to a medium pressure level and kept constant, while the axial pressure is adjusted to a continuous constant pressure mode and the heat preservation platform is activated to cause local plastic deformation and neck growth on the surface of the high-nitrogen steel pre-alloyed powder particles. During the heating stage from the first intermediate temperature range to the second intermediate temperature range, the inert gas static pressure is maintained at a medium pressure level and the value of the axial pressure is linearly increased to promote the expansion of the metallurgical bonding interface between the high-nitrogen steel pre-alloyed powder particles and the spheroidization of pores. When the temperature reaches the sintering peak temperature, the inert gas static pressure is increased to the final high pressure level and the axial pressure is simultaneously adjusted to the peak pressure. At the sintering peak temperature and the final high pressure level, a preset heat preservation time is maintained to achieve complete densification of the powder and dissolution and diffusion of nano-sized nitride forming agent powder. During the holding stage at the peak sintering temperature, a nitrogen-containing process gas mixture is introduced into the working chamber of the hot isostatic pressing furnace. A high-frequency pulsed electric field is applied in the working chamber to ionize the nitrogen-containing process gas mixture, forming a uniform low-temperature nitrogen plasma. The concentration and spatial distribution density of active nitrogen atoms in the low-temperature nitrogen plasma are controlled by adjusting the duty cycle and voltage of the pulsed electric field. Under the combined constraint of the inert gas static pressure and the peak axial pressure at the final high-pressure level, the active nitrogen atoms penetrate into the surface and grain boundaries of the high-nitrogen steel sintered body in a plastic state. The active nitrogen atoms combine with the alloying elements in the sintered body to generate supplementary nitride strengthening phases in situ at the grain boundaries and near the surface.

[0028] In specific implementation, the hot isostatic pressing (HIP) sintering stage of the method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology involves heating a high-strength graphite mold according to a preset stepped heating program. During heating, axial pressure and isotropic inert gas static pressure are applied simultaneously. In the heating stage from room temperature to the first intermediate temperature range of 500°C, the inert gas static pressure is maintained at a low level of 5 MPa, while pulsed axial pressure with an amplitude of ±2 MPa and a frequency of 0.5 Hz is applied to promote the rearrangement of the mixed powder particles and the expansion of the initial contact surface. When the temperature reaches the first intermediate temperature range of 500°C, the inert gas static pressure is increased to a medium level of 30 MPa and maintained constant. Simultaneously, the axial pressure is adjusted to a continuous constant pressure mode of 10 MPa, and the holding platform is activated. The temperature is held at 500°C for 20 minutes to induce localized plastic deformation and neck growth on the surface of the high-nitrogen steel pre-alloyed powder particles. During the heating phase from the first intermediate temperature range of 500°C to the second intermediate temperature range of 900°C, the inert gas static pressure at a moderate level of 30 MPa is maintained, and the axial pressure is linearly increased from 10 MPa to 25 MPa at a constant rate. This promotes the expansion of the metallurgical bonding interface and the spheroidization of pores between the high-nitrogen steel pre-alloyed powder particles. When the temperature reaches the sintering peak temperature of 1150°C, the inert gas static pressure is increased to the final high-pressure level of 80 MPa, and the axial pressure is simultaneously adjusted to the peak pressure of 35 MPa. The temperature is maintained at the sintering peak temperature of 1150°C and the final high-pressure level of 80 MPa for a preset holding time of 90 minutes to achieve complete densification of the powder and the dissolution and diffusion of nano-sized nitride forming agent powder. In some embodiments, the first intermediate temperature range can be selected from 450°C to 550°C, the moderate pressure level can be selected from 25 MPa to 35 MPa, the holding time can be adjusted from 15 to 30 minutes, and the rate of linear increase in axial pressure can be characterized by the following formula: in: This indicates the current axial pressure value. Indicates the initial temperature The axial pressure set at (i.e., the first intermediate temperature) The coefficient representing the linear increase rate of axial pressure with temperature. This indicates the current real-time temperature.

[0029] In practice, during the holding stage at the peak sintering temperature, a nitrogen-containing process gas mixture is introduced into the working chamber of the hot isostatic pressing furnace. This mixture consists of 80% argon and 20% nitrogen by volume. A high-frequency pulsed electric field with a frequency of 40 kHz and a peak voltage of 5 kV is applied within the working chamber to ionize the nitrogen-containing process gas mixture, forming a uniform low-temperature nitrogen plasma. The concentration and spatial distribution density of active nitrogen atoms in the low-temperature nitrogen plasma are controlled by adjusting the duty cycle and voltage of the pulsed electric field. For example, when the duty cycle is set to 30%, the concentration of active nitrogen atoms in the plasma is higher than that under a duty cycle of 10%, and when the pulse voltage is set to 6 kV, the spatial distribution density of the plasma is higher than that under a voltage of 4 kV. Under the combined constraints of an inert gas static pressure of 80 MPa and a peak axial pressure of 35 MPa, active nitrogen atoms penetrate into the surface and grain boundaries of the high-nitrogen steel sintered body in a plastic state. These active nitrogen atoms combine with alloying elements within the sintered body to generate supplementary nitride strengthening phases in situ at the grain boundaries and near-surface regions. It can be understood that the nitrogen component in the nitrogen-containing process gas mixture can be adjusted within the range of 10% to 30%, the frequency of the high-frequency pulsed electric field can be selected between 20 kHz and 60 kHz, the peak voltage can vary between 4 kV and 8 kV, and the plasma treatment time can be synchronized with or independently set between 30 and 120 minutes to the holding time at the sintering peak temperature. Optionally, the data comparison reflects the influence of different combinations of pressure and plasma parameters on the formation of the nitriding layer. For example, under the conditions of inert gas static pressure of 60 MPa and peak axial pressure of 30 MPa, the depth of the nitriding layer produced by the same plasma parameters is lower than that under the conditions of 80 MPa and 35 MPa. Under the conditions of pulse electric field duty cycle of 20% and voltage of 5 kV, the uniformity of the nitriding layer is better than that under the conditions of duty cycle of 40% and voltage of 7 kV.

[0030] In some embodiments, the temperature ranges and pressure parameters in the stepped heating program can be adjusted based on different high-nitrogen steel compositions. For example, for high-nitrogen steel pre-alloyed powders with higher chromium content, the first intermediate temperature range can be set to 480°C, the second intermediate temperature range can be set to 950°C, and the sintering peak temperature can be correspondingly increased to 1180°C. In specific implementations, the application mode of pulsed axial pressure can be compared using data. Applying a constant small pressure of 5 MPa and applying a pulsed pressure of ±2 MPa show differences in promoting powder rearrangement. The pulsed pressure mode exhibits higher powder packing density in the initial stage. Optionally, the inert gas static pressure level setting needs to match the equipment capacity and mold strength. Low pressure levels can be selected between 3 MPa and 8 MPa, medium pressure levels between 20 MPa and 40 MPa, and the final high pressure level between 60 MPa and 100 MPa. It is understandable that the penetration depth of active nitrogen atoms and the volume fraction of the supplementary nitride reinforcing phase in the in-situ plasma nitriding process are related to the holding time, gas mixture composition and electric field parameters. Longer holding time, higher nitrogen ratio and appropriate electric field parameters are conducive to the formation of a thicker and more uniform reinforcing layer.

[0031] In one embodiment of the present invention, after holding at the peak sintering temperature, the heating system is shut off and the sintered body is cooled from the peak sintering temperature to the nitride precipitation sensitive temperature range at a first cooling rate. Within the nitride precipitation sensitive temperature range, the cooling rate is adjusted to a second cooling rate lower than the first cooling rate, and an inert gas static pressure at a back pressure level is maintained during this stage. During the slow cooling process at the second cooling rate, nitrogen atoms supersaturated dissolved in the matrix diffuse and agglomerate with alloying elements to grain boundaries and dislocations, precipitating a diffusely distributed secondary nitride phase with elements dissolved from the nano-sized nitride forming agent powder as the core. When the temperature drops below the matrix phase transformation initiation point, the cooling rate is increased again to a third cooling rate to quickly pass through the phase transformation zone, suppressing the formation of coarse nitrides and grain growth, thereby obtaining a high-nitrogen steel material containing a fine-grained matrix and a uniformly diffusely distributed nitride strengthening phase. The determination of the nitride precipitation sensitive temperature range and the control of the second cooling rate include: obtaining statistical data on the morphology and size distribution of precipitated phases after holding at different isothermal temperatures by conducting a series of isothermal annealing experiments on samples that have been sintered but have not undergone the controlled nitrogen distribution and phase transformation regulation steps; plotting the relationship curves between the average size of the precipitated phase, the number density of the precipitated phase, and the isothermal temperature; selecting the temperature range with the smallest average size of the precipitated phase and the highest number density of the precipitated phase from the relationship curves as the optimal nitride precipitation sensitive temperature range; determining the maximum allowable cooling rate required to avoid harmful phase precipitation and ensure sufficient diffusion precipitation within the optimal nitride precipitation sensitive temperature range through continuous cooling transformation experiments; reducing the maximum allowable cooling rate by a safety factor as the control target value of the second cooling rate; and achieving the second cooling rate by controlling the flow rate and direction of the cooling airflow in the furnace through a program during the subsequent cooling process.

[0032] In specific implementation, the method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology includes the following steps after holding the sintering peak temperature: controlled nitrogen distribution and phase transformation regulation during the cooling stage of the sintered body. After holding the sintering peak temperature of 1150℃, the heating system is shut down, and the sintered body is cooled from the sintering peak temperature of 1150℃ to the nitride precipitation sensitive temperature range of 600℃ at a first cooling rate of 20℃ per minute. Within the nitride precipitation sensitive temperature range of 600℃ to 550℃, the cooling rate is adjusted to a second cooling rate of 3℃ per minute, which is lower than the first cooling rate, and an inert gas static pressure of 5MPa back pressure is maintained during this stage. During the slow cooling process at a second cooling rate of 3°C per minute, supersaturated nitrogen atoms dissolved in the matrix diffuse and agglomerate with alloying elements towards grain boundaries and dislocations. Secondary nitride phases precipitate, with elements dissolved from nano-sized nitride-forming agent powder as the core. When the temperature drops below the matrix phase transformation initiation point of 350°C, the cooling rate is increased again, rapidly passing through the phase transformation zone at a third cooling rate of 50°C per minute. This suppresses the formation of coarse nitrides and grain growth, resulting in a high-nitrogen steel material containing a fine-grained matrix and a uniformly dispersed nitride-reinforcing phase. In some embodiments, the first cooling rate can be selected from 15°C per minute to 25°C per minute, the nitride precipitation sensitive temperature range can be determined according to the alloy system within the range of 550°C to 650°C, the second cooling rate can be adjusted from 1°C per minute to 5°C per minute, the third cooling rate can be varied from 40°C per minute to 60°C per minute, and the inert gas static pressure of the back pressure level can be set from 2 MPa to 10 MPa.

[0033] In practical implementation, determining the nitride precipitation sensitive temperature range and controlling the second cooling rate involves conducting a series of isothermal annealing experiments on samples that have been sintered but have not undergone controlled nitrogen distribution and phase transformation regulation steps. This yields statistical data on the morphology and size distribution of the precipitated phases after holding at different isothermal temperatures, and plots the relationship curves between the average precipitated phase size, the precipitated phase number density, and the isothermal temperature. From these curves, the temperature range with the smallest average precipitated phase size and the highest precipitated phase number density is selected and defined as the optimal nitride precipitation sensitive temperature range. For example, when the average precipitated phase size in a sample isothermally annealed at 590℃ is 25 nm and the number density is... In each cubic meter of sample isothermally annealed at 610℃, the average size of the precipitated phase was 30 nanometers, and the number density was [missing information]. In each cubic meter of sample isothermally annealed at 570℃, the average size of the precipitated phase was 28 nanometers, and the number density was [missing information]. For each cubic meter, the range around 590℃ was selected as the optimal sensitive temperature range for nitride precipitation. Through continuous cooling transformation experiments, the maximum permissible cooling rate required to avoid harmful phase precipitation and ensure sufficient diffusion precipitation within the optimal sensitive temperature range for nitride precipitation was determined; for example, the experiments determined the maximum permissible cooling rate to be 5℃ per minute. The maximum permissible cooling rate of 5℃ per minute was reduced by a safety factor of 1.67, which was used as the control target value for the second cooling rate of 3℃ per minute. In subsequent cooling processes, the flow rate and direction of the cooling airflow within the furnace were controlled by a program to achieve the second cooling rate of 3℃ per minute. It can be understood that the maximum permissible cooling rate... With the second cooling rate control target value Relationship formula: in: A safety factor greater than 1 The results were determined through a continuous cooling transformation experiment. The value can range from 1.2 to 2.0.

[0034] In some embodiments, data comparisons reflect the impact of different second cooling rates on the final material microstructure. For example, samples cooled at a second cooling rate of 5°C per minute exhibit a larger average size of secondary nitride phases than samples cooled at a second cooling rate of 3°C per minute. While samples cooled at a second cooling rate of 1°C per minute show finer precipitates, potentially harmful phases may be present. Data comparisons also exist regarding the selection of the nitride precipitation-sensitive temperature range. Setting the optimal nitride precipitation-sensitive temperature range from 600°C to 550°C yields a higher number density and more dispersed distribution of secondary nitride phases compared to setting it from 650°C to 600°C. Optionally, the isothermal temperature points in the isothermal annealing series experiments can be selected at intervals of 20°C or 30°C. Continuous cooling transformation experiments can plot phase transformation and precipitation curves at different cooling rates, with a safety factor... The choice of cooling rate needs to consider process stability. A higher safety factor, such as 1.8, can provide a more stable process window but will extend the production cycle. In specific implementation, program-controlled cooling airflow in the furnace to achieve the second cooling rate involves adjusting the opening of the inlet and outlet valves. For example, setting the inlet flow rate to 30% of the standard flow rate and the outlet back pressure to 5 MPa can achieve a cooling rate of 3°C per minute in the furnace. It is understood that monitoring the temperature below the matrix phase transformation initiation point is crucial for starting the third cooling rate. The phase transformation initiation point temperature can be pre-determined by the expansion method or metallographic method. The rapid passage of the third cooling rate aims to suppress high-temperature phase transformations such as ferrite or pearlite. Optionally, the static pressure of the inert gas at the back pressure level not only provides constraint to prevent deformation, but its pressure value also affects the gas convection heat transfer coefficient during the cooling process, thus affecting the achievement of a constant second cooling rate. The matching relationship between back pressure and airflow velocity needs to be pre-calibrated experimentally.

[0035] See Figure 3 In the optimization method for the corrosion resistance of high-nitrogen steel based on powder metallurgy technology, the regulation law of isothermal temperature on the characteristics of precipitates and the comprehensive properties of the material can be analyzed through this curve system. In the figure, the isothermal temperature (550–650℃) is used as the horizontal axis, the left vertical axis represents the average size of the precipitates (nm) and hardness (HV), and the right vertical axis represents the number density of precipitates and the corrosion resistance score. The shaded area is marked as the optimal sensitive range (550–600℃). Average size of precipitates: It first decreases and then slowly increases with increasing isothermal temperature, reaching a minimum value (approximately 25nm) near 590℃. This reflects the sufficient diffusion and uniform nucleation of supersaturated nitrogen atoms during slow cooling, avoiding the formation of coarse nitride phases. Number density of precipitates: It maintains a high level within the 550–600℃ range, with a peak around 590℃. This synergizes with the minimum average size, indicating that the dispersion distribution of secondary nitride phases is optimal within this temperature range, providing a structural basis for grain boundary strengthening and corrosion resistance. Hardness (HV): The hardness initially increases and then decreases with increasing isothermal temperature, reaching a peak at 590℃. This peak closely matches the grain refinement and dispersion strengthening effects of the precipitated phases, verifying the gain in mechanical properties from the optimal sensitivity range. Corrosion resistance score: The trend is basically consistent with hardness and number density, reaching its highest value near 590℃. This indicates that the dispersed nano-nitride phases can effectively passivate grain boundaries and inhibit the penetration of corrosive media, thereby improving corrosion resistance. The figure visually reveals that 550–600℃ is the optimal sensitivity range for nitride precipitation. Within this range, a second cooling rate of 3℃ / min for slow cooling can achieve the goal of minimizing the average size and maximizing the number density of the precipitated phases, simultaneously optimizing the material's hardness and corrosion resistance. This provides direct experimental evidence for controlled nitrogen distribution and phase transformation control processes.

[0036] In one embodiment of the present invention, the obtained high-nitrogen steel material is placed on a surface grinder to grind its surface to remove the extremely thin oxide layer formed during sintering and cooling. A high-energy beam scanning device is used to perform non-melting scanning treatment on the surface of the ground high-nitrogen steel material. The scanning path of the high-energy beam scanning device is planned according to the microhardness distribution map of the material surface. During the non-melting scanning treatment, the energy input of the high-energy beam causes dynamic recovery and recrystallization in the extremely thin area of ​​the material surface, while promoting the healing of the micropores on the surface under thermal stress. After the surface scanning is completed, the high-nitrogen steel material is placed in a low-temperature tempering furnace and kept at low temperature for a long time under a protective atmosphere to eliminate the residual stress generated on the surface during the non-melting scanning treatment and further homogenize the nitrogen atom distribution. The parameter setting method for the high-energy beam scanning equipment to perform non-melting scanning treatment on the surface of the ground high-nitrogen steel material includes: measuring the micro Vickers hardness of the surface of the ground high-nitrogen steel material and drawing the microhardness distribution map characterizing the uniformity of hardness distribution; identifying local softened areas with hardness below the average level from the microhardness distribution map; setting the basic energy density and scanning speed of the high-energy beam so that the beam energy input can only raise the surface temperature of the material to above its recrystallization temperature but far below its solidus temperature; when planning the scanning path, the identified local softened areas are treated by reducing the scanning speed or using an overlapping scanning mode to provide higher energy input and promote the densification of the microstructure of the local softened areas; after performing a complete scanning path on the entire treated surface, the surface is rapidly cooled and the real-time distribution of surface temperature during the scanning process is monitored by an infrared thermal imager to ensure that the temperature of any area does not exceed the preset upper limit of the non-melting temperature.

[0037] In practical implementation, the method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology, after obtaining high-nitrogen steel material containing a fine-grained matrix and uniformly dispersed nitride-reinforcing phases, also includes subsequent surface densification and structural stabilization steps. The obtained high-nitrogen steel material is placed on a surface grinder for grinding, removing the extremely thin oxide layer formed during sintering and cooling. The grinding depth is controlled to be 10 to 20 micrometers. A high-energy beam scanning device is used to perform non-melting scanning treatment on the surface of the ground high-nitrogen steel material. The scanning path of the high-energy beam scanning device is planned according to the microhardness distribution map of the material surface. During the non-melting scanning process, the energy input of the high-energy beam causes dynamic recovery and recrystallization in the extremely thin area of ​​the material surface, while simultaneously promoting the healing of micropores on the surface under thermal stress. After surface scanning, the high-nitrogen steel material is placed in a low-temperature tempering furnace and held at a low temperature for an extended period under an argon protective atmosphere. The holding temperature is set at 300°C for 5 hours to eliminate residual stress generated during the non-melting scanning process and to further homogenize the nitrogen atom distribution. In some embodiments, the grinding depth can be adjusted within the range of 5 to 30 micrometers, the low-temperature tempering temperature can be selected between 250°C and 350°C, and the low-temperature tempering time can vary from 2 to 8 hours.

[0038] In practical implementation, the parameter setting method for non-melting scanning treatment of the surface of polished high-nitrogen steel by high-energy beam scanning equipment includes measuring the micro Vickers hardness of the surface of the polished high-nitrogen steel, drawing a microhardness distribution map characterizing the uniformity of hardness distribution, with the spacing of microhardness measurement points at 0.5 mm to form a grid data covering the entire surface to be treated. Locally softened areas with hardness below the average level are identified from the microhardness distribution map. The criterion for judging locally softened areas is a continuous area with a hardness value more than 15% lower than the average hardness value. The basic energy density and scanning speed of the high-energy beam are set so that the beam energy input can only raise the surface temperature of the material to above its recrystallization temperature but far below its solidus temperature. With respect to the thermophysical parameters of the material and the desired depth of the modified layer Related, their relationship: in: For material density, For the specific heat capacity of the material, The recrystallization temperature of the material. The initial room temperature, The energy absorption efficiency coefficient is the scanning speed. Based on the beam diameter and required overlap rate From the formula Linkage confirmed, The laser pulse frequency is used. When planning the scanning path, for identified locally softened areas, a reduced scanning speed or overlapping scanning mode is used to provide higher energy input and promote tissue densification in these areas. For example, a scanning speed of 100 mm / s is used in areas with normal hardness, while in locally softened areas, the scanning speed is reduced to 60 mm / s or an overlapping scan with a 30% overlap rate is used. After performing a complete scanning path on the entire treated surface, the surface is rapidly cooled using helium gas at a flow rate of 20 liters per minute. The real-time temperature distribution of the surface during the scanning process is monitored using an infrared thermal imager to ensure that the temperature of any area does not exceed the preset non-melting temperature limit of 1200°C. It can be understood that the spacing between microhardness measurement points can be set within the range of 0.2 mm to 1.0 mm, and the threshold for determining locally softened areas can be adjusted between 10% and 20% below the average value. The base energy density... The specific values ​​need to be calibrated through preliminary experiments, scanning speed With beam diameter overlap rate and pulse frequency The matching relationship needs to be determined through process debugging.

[0039] In some embodiments, the data comparison reflects the influence of different scanning parameters on the surface state. For the key parameters of non-melting scanning processing, please refer to Table 1.

[0040] Table 1: Examples and Comparison of High-Energy Beam Non-Melting Scanning Processing Parameters Optionally, the high-energy beam type can be a fiber laser or an electron beam. The monitoring frequency of the infrared thermal imager needs to be higher than the spatial resolution corresponding to the scanning speed. The protective atmosphere of the low-temperature tempering furnace can also use a nitrogen-hydrogen mixture. In specific implementations, data can be compared between reducing the scanning speed and using an overlapping scanning mode for locally softened areas, as shown in the table. Both parameter group A (reduced speed) and parameter group B (overlapping scanning) can increase local energy input, but the resulting temperature field distribution differs. It is understood that the upper limit of the non-melting temperature needs to be set at least 50°C lower than the solidus temperature of the material. The type, flow rate, and nozzle angle of the rapidly cooling gas will affect the final residual stress state of the surface layer. Optionally, the microhardness distribution map can be drawn using an automated indenter. The scanning path planning can be automatically generated by a computer numerical control system based on the hardness distribution map data. The energy absorption efficiency coefficient... It is necessary to determine the specific surface condition of the equipment and materials through calibration experiments.

[0041] See Figure 4In the entire process of high-energy beam non-melting scanning + tempering, the evolution of surface temperature with processing time intuitively reflects the control logic of the process window and the realization path of non-melting constraints. Specifically, the curve can be divided into three core stages: High-energy beam scanning heating stage (0~40min): The surface temperature rises rapidly from room temperature, reaching a peak of about 1200℃ at around 35min, and does not exceed the preset non-melting temperature upper limit (1200℃) throughout the process. This verifies the precise matching of the basic energy density and scanning speed parameters, ensuring that the material surface reaches above the recrystallization temperature to achieve dynamic recovery and pore healing, while strictly avoiding the risk of microstructure deterioration caused by local melting. Rapid cooling stage after scanning (40~60min): The temperature drops rapidly from the peak to about 300℃, corresponding to the helium rapid cooling link in the process. The rapid cooling rate in this stage can effectively suppress surface grain coarsening, and at the same time provide a stress control basis for subsequent low-temperature tempering. Low-temperature tempering stabilization stage (60~120min): The temperature slowly decreases to about 80℃, corresponding to a long-term holding tempering process at around 300℃. In this stage, the residual stress introduced by the high-energy beam scanning is eliminated through a slow temperature drop, while simultaneously promoting the homogenization of nitrogen atoms between the grain boundaries and the matrix, ultimately achieving a synergistic improvement in surface structure stability and corrosion resistance. The matching relationship between the temperature curve and the upper limit of the non-melting temperature shows that this process path, through precise energy input and cooling rate control, completes surface microstructure densification and stress relaxation while meeting non-melting constraints, providing a quantifiable temperature window for optimizing the surface properties of high-nitrogen steel materials.

[0042] In one embodiment of the present invention, the determination of the sintering peak temperature and the control of the holding time are based on the following: differential thermal analysis is performed on the mixed powder and a heat flow curve of the mixed powder during the heating process is plotted. From the heat flow curve, the temperature at which the endothermic valley begins to open and the temperature at which the exothermic peak begins to dissolve, corresponding to the solidus temperature of the high-nitrogen steel pre-alloyed powder matrix phase, are identified. A temperature range of 20 to 50 degrees Celsius above the exothermic peak opening temperature is selected as a candidate sintering peak temperature range. A temperature value is selected as the initial sintering peak temperature within the candidate sintering peak temperature range. The preliminary sintering peak temperature and holding time are calculated based on the total amount of mixed powder, the size of the graphite mold, and the preset heating rate. In subsequent experimental verification, the initial sintering peak temperature and the preliminary holding time are fine-tuned based on the actual density and microstructure of the sintered body. Finally, the optimized sintering peak temperature and the corresponding sintering peak temperature and holding time are determined.

[0043] In practical implementation, the determination of the sintering peak temperature and the control of the holding time in the optimization method for the corrosion resistance of high-nitrogen steel based on powder metallurgy technology are based on differential thermal analysis (DTA) of the mixed powder, plotting the heat flow curve of the mixed powder during the heating process. The DTA is conducted under argon protection, heating from room temperature to 1300℃ at a constant rate of 10℃ per minute. From the obtained heat flow curve, the endothermic valley initiation temperature corresponding to the solidus temperature of the high-nitrogen steel pre-alloyed powder matrix phase, and the exothermic peak initiation temperature corresponding to the dissolution of the nano-sized nitride forming agent powder, are identified. A temperature range of 20 to 50 degrees Celsius above the exothermic peak initiation temperature is selected as the candidate sintering peak temperature range. For example, when the endothermic valley initiation temperature is identified as 1100℃ and the exothermic peak initiation temperature as 1050℃, the candidate sintering peak temperature range is determined to be 1070℃ to 1100℃. Within the candidate sintering peak temperature range, a temperature value is selected as the initial sintering peak temperature. Based on the total amount of mixed powder, the graphite mold size, and the preset heating rate, the holding time at the initial sintering peak temperature is calculated. With the mass of mixed powder Mold cross-sectional area and heating rate Empirical relationship formula: in: This indicates the holding time at the initial sintering peak temperature. This indicates the total mass of the mixed powder. This represents the cross-sectional area of ​​the graphite mold cavity. This represents the average heating rate before reaching the peak sintering temperature. , , These are empirical coefficients obtained through prior process calibration. In some embodiments, the heating rate for differential thermal analysis can be selected between 5°C per minute and 20°C per minute, the identification of the endothermic valley and the onset temperature of the exothermic peak can be determined by the tangent method, and the candidate sintering peak temperature range can be selected within a range of 30 degrees Celsius above the onset temperature of the exothermic peak.

[0044] In practical implementation, during subsequent experimental verification, the initial sintering peak temperature and preliminary holding time were fine-tuned based on the actual density and microstructure of the sintered body. Ultimately, the optimized sintering peak temperature and corresponding holding time were determined. For example, 1080℃ was selected as the initial sintering peak temperature, and the preliminary holding time was calculated to be 120 minutes. After sintering, the relative density of the sintered body was measured to be 98.5%. Microscopic observation showed that the nitride forming agent powder had basically dissolved, but the grains had slightly grown. Based on this result, the initial sintering peak temperature was lowered to 1075℃ and the holding time was shortened to 110 minutes for the next verification experiment. Finally, a sintered body with a relative density of 99.2% and more uniform grain size was obtained, thus determining 1075℃ and 110 minutes as the optimized sintering peak temperature and corresponding holding time. It is understandable that the actual density achieved by the sintered body is measured using Archimedes' displacement method, and the microstructure is observed and evaluated using a metallographic microscope or scanning electron microscope. The fine-tuning process is iterative and may require multiple verification experiments. (Empirical coefficient) , , The calibration requires graphite molds of specific materials and specifications, as well as a fixed hot isostatic pressing (HIP) sintering furnace system, through a series of different... , , The parameters were obtained by fitting experimental data.

[0045] In some embodiments, data comparison reflects the impact of candidate temperature ranges selected based on different characteristic temperature points on the final sintering result. For example, using the exothermic peak initiation temperature of 1050℃ as a benchmark, selecting +20℃ to +50℃ (1070℃-1100℃) versus +10℃ to +40℃ (1060℃-1090℃) as candidate ranges, at the same initial sintering peak temperature of 1080℃, the former may calculate a longer initial holding time because it considers the time required for more diffusion. Data comparison also exists for identifying characteristic temperature points from heat flow curves. The initiation temperature determined by the tangent method and the initiation temperature determined by the extrapolation method may differ by 5℃ to 10℃, which will affect the starting point of the candidate temperature range. Optionally, the formula for calculating the holding time at the initial sintering peak temperature can be adjusted according to the richness of the process database. For example, a correction term related to powder particle size distribution can be introduced, and the preset heating rate usually refers to the average heating rate from room temperature to the sintering peak temperature range. In practice, the direction and magnitude of fine-tuning the initial sintering peak temperature and preliminary holding time are based on the evaluation results of the sintered body density and microstructure. If the density is insufficient but the grains have not grown, the temperature may be increased or the holding time extended. If the density meets the standard but the grains have grown excessively, the temperature may be decreased or the holding time shortened. It can be understood that the final optimized sintering peak temperature and the corresponding sintering peak temperature holding time are for a specific mixture of powders and a specific mold-equipment combination. When the powder batch or main equipment is changed, the determination process starting from differential thermal analysis needs to be re-executed.

[0046] See Figure 5In the process of determining the sintering peak temperature of high-nitrogen steel based on powder metallurgy technology, the influence of differential thermal analysis (DTA) heating rate on the identification of the exothermic peak initiation temperature can be quantitatively characterized by this curve. Specifically, the figure shows a significant positive linear correlation between the heating rate (°C / min) as the independent variable and the detected exothermic peak initiation temperature (°C) as the dependent variable: when the heating rate gradually increases from 5°C / min to 20°C / min, the exothermic peak initiation temperature monotonically increases from 1045°C to approximately 1058°C, an increase of about 13°C. This phenomenon stems from the kinetic hysteresis effect of differential thermal analysis. A higher heating rate causes the powder system to absorb more heat per unit time, leading to a shift in the exothermic reaction initiation point to a higher temperature range. Simultaneously, it makes the characteristic inflection point of the heat flow curve steeper, thus affecting the accuracy of the tangent method or extrapolation method in identifying the initiation temperature. In process practice, this data provides a key correction basis for defining the candidate range of sintering peak temperature: when using different heating rates for DTA testing, the exothermic peak initiation temperature needs to be rate-corrected according to the corresponding relationship in the figure to eliminate systematic errors caused by kinetic lag, and ensure the accuracy and repeatability of the candidate sintering peak temperature range (20–50℃ above the exothermic peak initiation temperature), laying the foundation for subsequent optimization of sintered body density and microstructure.

[0047] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments that can be applied to other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for optimizing the corrosion resistance of high-nitrogen steels based on powder metallurgy technology, characterized in that, The method includes: A pre-alloyed high-nitrogen steel powder of a predetermined particle size is placed in a vacuum glove box for pretreatment. The pretreatment includes continuous inert gas purging, constant-temperature heating to degas the high-nitrogen steel powder, and mechanical stirring to break up powder agglomerates. The pre-alloyed high-nitrogen steel powder that has undergone pretreatment is physically mixed with nano-sized nitride forming agent powder to obtain a mixed powder, wherein the nano-sized nitride forming agent powder comprises a homogeneous composition of chromium nitride, vanadium nitride and niobium nitride. The obtained mixed powder is filled into the cavity of a high-strength graphite mold. During the filling process, a variable frequency mechanical vibration table is started to make the mixed powder achieve gradient densification filling in the cavity. The gradient densification filling is characterized by the powder compaction density at the bottom of the mold being higher than that at the top. The high-strength graphite mold that has been filled is moved into the working chamber of the hot isostatic pressing sintering furnace. The working chamber is subjected to multi-stage vacuuming and inert gas backfilling and circulation until the oxygen content and water content in the working chamber are lower than the preset threshold. The heating system of the hot isostatic pressing sintering furnace is started, and the high-strength graphite mold is heated according to the preset stepped heating program. During the heating process, axial pressure and isotropic inert gas static pressure are applied simultaneously, so that the mixed powder undergoes solid-state sintering and preliminary densification.

2. The method for optimizing corrosion resistance of high-nitrogen steel based on powder metallurgy technology according to claim 1, characterized in that, The pretreatment operation of placing gas-atomized high-nitrogen steel pre-alloyed powder of a predetermined particle size in a vacuum glove box includes: The atomized high-nitrogen steel pre-alloyed powder is placed in the transition chamber of a vacuum glove box, and the transition chamber is evacuated. High-purity inert gas is backfilled into the vacuum-evacuated transition chamber to balance the pressure between the main working chamber and the transition chamber of the glove box. The high-nitrogen steel pre-alloyed powder is transferred from the transition chamber to the heating platform of the main working chamber of the glove box, and the heating platform is heated to the preset degassing temperature. At the degassing temperature, the powder in the main working chamber is statically degassed for a predetermined time, while the powder is intermittently stirred by a stirring paddle driven by a robotic arm. After degassing and stirring are completed, the heating platform is cooled, and the pretreated powder is transferred to a sealed container under the protective atmosphere of the main working chamber of the glove box for later use.

3. The method for optimizing corrosion resistance of high-nitrogen steel based on powder metallurgy technology according to claim 2, characterized in that, The nano-sized nitride forming agent powder comprises a homogeneous composition of chromium nitride, vanadium nitride, and niobium nitride, and its addition ratio and mixing method are as follows: Based on the matrix chemical composition of the target high-nitrogen steel, the theoretical ratio difference between the contents of chromium, vanadium, and niobium in the alloy matrix and the target total nitrogen content is calculated. Based on the theoretical ratio difference, weigh out the corresponding masses of nano-chromium nitride powder, nano-vanadium nitride powder and nano-niobium nitride powder respectively. The weighed nano-chromium nitride powder, nano-vanadium nitride powder and nano-niobium nitride powder are placed in a low-speed mixing tank for premixing to obtain a uniform composite nitride additive powder. In a vacuum glove box environment, the composite nitride additive powder and the pre-treated high-nitrogen steel pre-alloyed powder are added together into a double cone mixer; The rotation speed and mixing time of the double cone mixer are set, and three-dimensional rotational mixing is carried out under a protective atmosphere until the composite nitride additive powder is uniformly dispersed on the surface and in the gaps of the high nitrogen steel pre-alloyed powder particles to form the mixed powder.

4. The method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology according to claim 3, characterized in that, The process of heating the high-strength graphite mold according to a preset stepped heating program, simultaneously applying axial pressure and isotropic inert gas static pressure during the heating process, includes: During the heating phase from room temperature to the first intermediate temperature range, the static pressure of the inert gas is maintained at a low level, while a small pulsed axial pressure is applied to promote the rearrangement of the mixed powder particles and the expansion of the initial contact surface. When the temperature reaches the first intermediate temperature range, the static pressure of the inert gas is increased to a medium pressure level and kept constant. At the same time, the axial pressure is adjusted to a continuous constant pressure mode, and the heat preservation platform is started, so that the surface of the high nitrogen steel pre-alloyed powder particles undergoes local plastic deformation and neck growth. During the heating phase from the first intermediate temperature range to the second intermediate temperature range, the static pressure of the inert gas at a moderate pressure level is maintained, and the value of the axial pressure is increased linearly to promote the expansion of the metallurgical bonding interface and the spheroidization of pores between the high-nitrogen steel pre-alloyed powder particles. When the temperature reaches the peak sintering temperature, the static pressure of the inert gas is increased to the final high pressure level, and the axial pressure is simultaneously adjusted to the peak pressure. The temperature is maintained at the peak sintering temperature and the final high pressure level for a preset holding time to achieve complete densification of the powder and dissolution and diffusion of nano-sized nitride forming agent powder.

5. The method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology according to claim 4, characterized in that, This also includes the step of in-situ plasma nitriding treatment of high-strength graphite molds during the heat preservation and pressurization process: During the holding stage at the peak sintering temperature, a nitrogen-containing process gas mixture is introduced into the working chamber of the hot isostatic pressing furnace. A high-frequency pulsed electric field is applied inside the working chamber to ionize the nitrogen-containing process gas mixture, forming a uniform low-temperature nitrogen plasma; The concentration and spatial distribution density of active nitrogen atoms in the low-temperature nitrogen plasma are controlled by adjusting the duty cycle and voltage of the pulsed electric field. Under the combined constraints of the inert gas static pressure and the peak axial pressure at the final high pressure level, the active nitrogen atoms penetrate into the surface layer and grain boundaries of the high-nitrogen steel sintered body in a plastic state. The active nitrogen atoms combine with alloying elements within the sintered body to generate supplementary nitride strengthening phases in situ at grain boundaries and near-surface regions.

6. The method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology according to claim 5, characterized in that, It also includes steps for controlled nitrogen distribution and phase transformation regulation during the cooling stage of the sintered body: After holding at the peak sintering temperature, the heating system is turned off, and the sintered body is cooled from the peak sintering temperature to the nitride precipitation sensitive temperature range at the first cooling rate. Within the nitride precipitation sensitive temperature range, the cooling rate is adjusted to a second cooling rate lower than the first cooling rate, and an inert gas static pressure at a back pressure level is maintained during this stage. During the slow cooling process at the second cooling rate, nitrogen atoms supersaturated in the matrix diffuse and agglomerate with alloying elements toward grain boundaries and dislocations, precipitating a diffusely distributed secondary nitride phase with the elements dissolved from the nano-sized nitride forming agent powder as the core. When the temperature drops below the matrix phase transformation initiation point, the cooling rate is increased again, and the phase transformation zone is rapidly passed through at a third cooling rate to suppress the formation of coarse nitrides and grain growth, thereby obtaining a high-nitrogen steel material containing a fine-grained matrix and a uniformly dispersed nitride-reinforced phase.

7. The method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology according to claim 6, characterized in that, After obtaining the high-nitrogen steel material containing a fine-grained matrix and a uniformly dispersed nitride-reinforcing phase, the process further includes subsequent surface densification and structural stabilization steps: The obtained high-nitrogen steel material was placed on a surface grinder and its surface was ground to remove the extremely thin oxide layer formed during sintering and cooling. The surface of the ground high-nitrogen steel material was subjected to non-melting scanning treatment using a high-energy beam scanning device. The scanning path of the high-energy beam scanning device was planned based on the microhardness distribution map of the material surface. During the non-melting scanning process, the energy input of the high-energy beam causes dynamic recovery and recrystallization in the extremely thin area of ​​the material surface, while promoting the healing of the micropores on the surface under thermal stress. After the surface scanning is completed, the high-nitrogen steel material is placed in a low-temperature tempering furnace and kept at a low temperature for a long time under a protective atmosphere to eliminate the residual stress generated on the surface during the non-melting scanning process and to further homogenize the distribution of nitrogen atoms.

8. The method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology according to claim 7, characterized in that, The determination of the peak sintering temperature and the control of the holding time are based on the following: Differential thermal analysis was performed on the mixed powder to plot the heat flow curve of the mixed powder during the heating process; From the heat flow curve, the temperature at which the endothermic valley begins to open, corresponding to the solidus temperature of the matrix phase of the high-nitrogen steel pre-alloyed powder, and the temperature at which the exothermic peak begins to open, corresponding to the dissolution of the nano-sized nitride forming agent powder, are identified. The temperature range of 20 to 50 degrees Celsius above the exothermic peak initiation temperature is selected as the candidate sintering peak temperature range. Within the candidate sintering peak temperature range, a temperature value is selected as the initial sintering peak temperature, and the holding time at the initial sintering peak temperature is calculated based on the total amount of mixed powder, the size of the graphite mold, and the preset heating rate. In subsequent experimental verification, the initial sintering peak temperature and the initial holding time were fine-tuned based on the actual density and microstructure of the sintered body, and finally the optimized sintering peak temperature and the corresponding sintering peak temperature holding time were determined.

9. The method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology according to claim 8, characterized in that, The determination of the nitride precipitation sensitive temperature range and the control of the second cooling rate include: By conducting a series of isothermal annealing experiments on samples that have been sintered but have not undergone the controlled nitrogen distribution and phase transformation regulation steps, statistical data on the morphology and size distribution of precipitated phases of samples after holding at different isothermal temperatures were obtained. Plot the relationship curves between the average size of the precipitated phase, the number density of the precipitated phase, and the isothermal temperature; From the relationship curves, the temperature range with the smallest average size of the precipitated phase and the highest number density of the precipitated phase is selected and defined as the optimal nitride precipitation sensitive temperature range. Through continuous cooling transformation experiments, the maximum allowable cooling rate required to avoid harmful phase precipitation and ensure sufficient diffusion precipitation within the optimal nitride precipitation sensitive temperature range was determined. The maximum allowable cooling rate is reduced by a safety factor as the control target value for the second cooling rate. In the subsequent cooling process, the flow rate and direction of the cooling airflow in the furnace are controlled by the program to achieve the second cooling rate.

10. The method for optimizing the corrosion resistance of high-nitrogen steel based on powder metallurgy technology according to claim 9, characterized in that, The parameter setting method for the high-energy beam scanning equipment to perform non-melting scanning treatment on the surface of polished high-nitrogen steel includes: The micro Vickers hardness of the surface of the high-nitrogen steel material after grinding was measured, and the micro hardness distribution map characterizing the uniformity of hardness distribution was plotted. Identify localized softened areas with hardness below the average level from the microhardness distribution map; The basic energy density and scanning speed of the high-energy beam are set so that the beam energy input can only raise the surface temperature of the material to above its recrystallization temperature, but far below its solidus temperature. When planning the scanning path, the identified localized softened areas are processed using a mode that reduces the scanning speed or performs overlapping scans to provide higher energy input and promote tissue densification in the localized softened areas. After performing a complete scan of the entire surface, the surface is rapidly cooled, and the real-time temperature distribution of the surface is monitored by an infrared thermal imager to ensure that the temperature of any area does not exceed the preset upper limit of the non-melting temperature.

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