Material selection and surface strengthening process for over-flow part of concentration tower circulating pump

Abstract

This invention relates to the field of surface treatment of duplex stainless steel, and discloses a material selection and surface strengthening process for the flow components of a concentration tower circulating pump. The process includes: continuously bubbling carbon dioxide into a specific ternary carbonate molten salt to steadily anchor the active donor using the common ion effect, inducing microscopic pre-expansion of the substrate lattice without carbide precipitation; subsequently, pulse injection of preheated ammonia and liquid formamide through a bottom porous dispersion ring, utilizing the explosive kinetic energy generated by the microporous confined flash evaporation phase change to form a high-pressure microjets, tearing apart the flow field boundary layer formed in the dead zone region of the complex components by the high-viscosity salt bath; and finally, in-situ cleaning with residual enthalpy to complete the process. This effectively avoids corrosion resistance degradation caused by chromium desolvation during the strengthening process, eliminates strengthening blind spots in components such as pump impellers, and enables the components to obtain a high-hardness surface layer while possessing excellent hardness uniformity and resistance to erosion corrosion.

Description

Technical Field

[0001] This invention relates to the field of duplex stainless steel surface treatment technology, specifically to a material selection and surface strengthening process for the flow components of a concentration tower circulating pump. Background Technology

[0002] 2507 duplex stainless steel is widely used in flow-through components in industries such as chemical concentration and seawater desalination due to its excellent mechanical strength and resistance to pitting corrosion. In the high-chlorine environment of concentration towers and under conditions of high-speed scouring by hard particles, the material faces not only chemical corrosion but also severe mechanical abrasion. Current technologies often employ high-temperature salt bath co-infiltration at 500-600℃ for surface strengthening. However, duplex steel is highly susceptible to microstructural sensitization within this temperature range, where chromium atoms react with infiltrated atoms to form brittle precipitates, leading to chromium depletion in the matrix and a significant reduction in the corrosion resistance of the strengthened material.

[0003] If the process temperature is lowered to below 450℃, mass transfer obstacles arise due to the increased viscosity of the salt bath. Especially for pump impellers with complex three-dimensional flow channels, a deep flow field boundary layer forms at low Reynolds numbers in areas such as the blade roots. Traditional gravity-based media supply or pure diffusion methods struggle to overcome the physical flow resistance generated by high-viscosity molten salt. This spatial difference in mass transfer efficiency prevents active atoms from effectively reaching geometrically confined areas, resulting in significant differences in the depth and hardness of the reinforced layer across different parts of the component. Premature peeling at dead zones directly limits the overall service life of the equipment. Furthermore, the method of directly adding liquid organic media to a high-temperature melt lacks spatial constraints in its pyrolysis phase change process, easily leading to molten salt splashing and failing to provide directional kinetic energy for deep mass transfer, making it difficult to achieve uniform solid solution reinforcement across the entire surface of large flow components. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a material selection and surface strengthening process for the flow components of a concentration tower circulating pump. This solves the problems of corrosion resistance degradation caused by high-temperature sensitization and uneven distribution of the strengthening layer due to mass transfer obstruction in complex geometric dead zones under low-temperature, high-viscosity environments during the surface strengthening process of 2507 duplex stainless steel flow components.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a material selection and surface strengthening process for the flow components of a concentration tower circulating pump, comprising: 2507 duplex stainless steel was selected as the base material for the flow-through components, and the surface was degreased, cleaned and dried.

[0006] The flow-through components are immersed in a composite co-diffusion medium placed in a salt bath furnace. The temperature is controlled, and carbon dioxide gas is continuously bubbled into the salt bath furnace. The active donor is anchored in a stable low supersaturation range by utilizing the common ion effect, which induces microscopic distortion and pre-expansion of the substrate lattice.

[0007] The processing temperature is lowered, and then preheated anhydrous ammonia gas is introduced into the composite co-diffusion medium in the salt bath furnace, and liquid formamide is injected in a pulsed manner. This causes the gas-liquid blend to flash in the porous confined space and induces high-pressure micro-jet flow, followed by isothermal treatment.

[0008] The reinforced flow components are lifted out of the salt bath furnace and quenched in hot water for in-situ cleaning using residual enthalpy.

[0009] By employing the above technical solution, the use of carbon dioxide bubbling intervention in the homoionic steady-state pre-expansion treatment establishes an extremely low and stable carbon potential environment in stage one. In the 450-70℃ range of 2507 duplex stainless steel, smaller atomic radii of carbon atoms preferentially occupy the interstitial spaces between austenite and ferrite. This gentle pre-expansion process does not involve long-range diffusion of chromium atoms, thus avoiding the precipitation of chromium carbide and protecting the intrinsic corrosion resistance of the matrix. Simultaneously, through the microporous confined flash evaporation treatment in stage two, the chemical energy of formamide is converted into the kinetic energy of a high-frequency cavitation jet, forcibly tearing the thick flow field boundary layer formed by the high-viscosity salt bath in dead-angle areas such as the impeller root, ensuring equivalent mass transfer of active atoms on the complex three-dimensional surface. Therefore, a strengthening layer with high hardness, high uniformity, and well-preserved passivation properties is obtained.

[0010] Preferably, the surface degreasing specifically includes the following steps: using a sodium hydroxide aqueous solution with a mass fraction of 3% to 8% to perform ultrasonic treatment at 55-65°C for 15 to 30 minutes to complete the degreasing, wherein the ultrasonic frequency of the ultrasonic treatment is set to 28-40kHz.

[0011] By adopting the above technical solution, high-frequency ultrasound combined with a specific concentration of alkaline solution can completely remove trace amounts of oil residues left on the surface of duplex stainless steel after rolling or machining, thus clearing the physical barrier for the subsequent efficient adsorption and penetration of heteroatoms at the solid-liquid interface.

[0012] Preferably, the controlled temperature is 450-470℃, the flow rate of the injected carbon dioxide gas is 1-3L / min, and the injection time of the carbon dioxide gas is 3-6 hours.

[0013] By adopting the above technical solution, the process parameters ensure a saturated carbonate concentration in the molten pool. Its innovative principle is as follows: In the molten salt bath system involved in this invention, the pyrolytic decay of the effective active donor cyanate ions follows the following chemical equilibrium: ; By continuously and forcibly replenishing the system with carbon dioxide gas, according to Le Chatelier's principle, the high partial pressure of carbon dioxide and the high concentration of carbonate ions work together to significantly suppress the forward progression of the aforementioned side reactions. This common ion effect successfully locks the mass fraction of cyanate ions within an extremely narrow fluctuation range, avoiding the brittleness of the infiltration layer induced by excessively high initial concentrations.

[0014] Preferably, the treatment temperature is 395-410℃, the flow rate of the anhydrous ammonia gas is 2L / min to 5L / min, the pulse injection frequency of the injected liquid formamide is 0.5 to 2Hz, the flow rate is 2-5 vol% of the flow rate of the anhydrous ammonia gas, and the isothermal treatment time is 5-8 hours.

[0015] By adopting the above technical solution, the 475℃ brittle zone of duplex stainless steel is effectively avoided through cooling treatment. In this stage, the microscale confined flash evaporation mechanism plays a crucial role, and its specific reaction process is as follows: Step 1: Liquid formamide enters the preheated ammonia gas flow in the form of micron-sized droplets and reaches the bottom confined space with the gas flow.

[0016] Step 2: After absorbing heat from the environment, the droplet undergoes an instantaneous decomposition reaction accompanied by a violent phase transition and expansion; Step 3: Under the constraint of micropores of 5-15 micrometers, the explosive expansion of volume generates a high-pressure microjet. This jet directly pumps the active amino group to the surface of the workpiece, while carbon monoxide establishes a local equilibrium carbon potential at the solid-liquid interface, preventing the carbon atoms that penetrated in the first stage from undergoing reverse desorption under a low chemical potential environment.

[0017] Preferably, the in-situ cleaning specifically includes the following steps: after the flow-through component is suspended in the air to leach salt for 10 to 20 seconds, it is completely immersed in hot water and then rinsed with mechanical water flow for 5 to 15 minutes; the temperature of the preheated anhydrous ammonia gas is 400-430℃, and the temperature of the hot water when quenching is 80-95℃.

[0018] By adopting the above technical solution, the residual enthalpy of the component when it is taken out of the furnace can be used to induce micro-boiling in the water, which can quickly peel off the solid salt shell remaining in the impeller channel under thermal stress and bubble scouring, avoiding the problem of difficult removal of residual salt by subsequent cold cleaning. At the same time, the appropriate preheating temperature ensures that the gas does not cause a large local temperature drop when it enters the molten pool.

[0019] This invention provides a material selection and surface strengthening process for the flow components of a concentration tower circulating pump. It offers the following advantages: 1. This invention continuously injects carbon dioxide gas into a composite co-diffusion medium, utilizing the common ion effect to reversely regulate the chemical equilibrium within the molten pool, thereby locking the concentration of the active donor within a stable range of low supersaturation for a long period. In conjunction with a specific ternary carbonate system, this allows the lattice of 2507 duplex stainless steel to pre-expand without long-range diffusion of chromium, fundamentally avoiding the risk of pitting corrosion in chromium-depleted areas caused by chromium carbide precipitation in traditional high-temperature treatment, and achieving deep compatibility between the high-hardness strengthening layer and the intrinsic corrosion resistance of the substrate.

[0020] 2. This invention induces microporous confined flash evaporation by pulsed injection of formamide, and utilizes the explosive expansion kinetic energy generated by gas-liquid phase change to form a directional high-pressure microjet. This allows it to forcefully penetrate the laminar boundary layer formed by the high-viscosity salt bath on the surface of the complex pump impeller, effectively solving the technical bottleneck of difficult mass transfer in geometric dead corner areas such as the back of the blade root. It also suppresses the spatial attenuation rate of hardness across the entire surface to an extremely low level, ensuring a high degree of consistency in the overall reinforcement quality of the flow-through components.

[0021] 3. This invention utilizes the synergistic effect of lattice pre-expansion and low-temperature co-infiltration to construct a dense, single-phase, supersaturated solid solution-expanded phase on the surface of the component. The carbon monoxide generated by the in-situ pyrolysis of formamide within the microscale pores establishes a crucial carbon potential at the solid-liquid interface, preventing reverse desorption of carbon atoms and promoting deep penetration of nitrogen atoms. This allows the reinforcing layer to maintain high toughness while possessing excellent resistance to particle erosion, significantly reducing mechanical spalling and mass loss under high-salt mortar conditions in the thickening tower. Attached Figure Description

[0022] Figure 1 This is a diagram illustrating the hydrodynamics and system stability verification of the microporous confined flash evaporation process in this invention. Figure 1 Neutron plot (a) is a 10-second time-domain curve of the back pressure fluctuation in the intake pipeline during the co-infiltration treatment. Figure 1 Neutron plot (b) is a bar chart of macroscopic salt bath splash mass loss; Figure 2 This is a chemical thermodynamic verification diagram of the common ion steady-state control mechanism of the present invention, wherein... Figure 2 Neutron plot (a) is a curve showing the decay of the mass fraction of cyanate, the effective active ingredient in the bath solution, over time during the first 3 hours of pre-expansion treatment. Figure 2 Neutron plot (b) is a bar chart of the time-averaged decay rate of cyanate concentration; Figure 3 This is a test diagram showing the spatial distribution uniformity of surface hardness on a complex impeller workpiece according to the present invention. Figure 3 Neutron plot (a) shows the attenuation trend of Vickers hardness values ​​on the water-facing side and the back of the blade root of the workpiece. Figure 3 Neutron plot (b) is a scatter plot of the macroscopic hardness spatial decay rate; Figure 4 This is a test diagram of the two-phase flow under extreme conditions of scour and weightlessness according to the present invention. Figure 4 Neutron plot (a) is a line graph showing the evolution trend of cumulative absolute weightlessness. Figure 4 Neutron diagram (b) is a distribution diagram of the steady-state macroscopic scour weight loss rate of the needles; Figure 5 This is a comparison chart of pure electrical characteristic parameters under simulated high-chlorine environment according to the present invention, wherein... Figure 5 Neutron diagram (a) shows the stepped distribution of pitting corrosion breakdown potential. Figure 5 Neutron plot (b) is a scatter plot of self-corrosion current density. Detailed Implementation

[0023] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Please see the appendix Figure 1 -Appendix Figure 5 : Preparation Example 1: This preparation example provides a method for preparing a composite co-diffusion medium for surface strengthening, comprising the following steps: Weigh out 35 parts by mass of anhydrous potassium carbonate, 35 parts by mass of anhydrous sodium carbonate and 30 parts by mass of anhydrous lithium carbonate and mix them mechanically to obtain a basic ternary carbonate.

[0025] The mixed basic ternary carbonate was placed in a well-type resistance-heated salt bath furnace equipped with a bottom sintered metal porous dispersion ring (made of 316L alloy with an average pore size of 5μm). The heating program was turned on to raise the temperature to 480℃ and held at that temperature for 1 hour to allow it to melt and homogenize completely.

[0026] Subsequently, 5 parts by weight of urea granules were slowly and evenly added to the surface of the molten salt bath. After the urea was completely dissolved and there were no obvious bubbles on the surface of the salt bath system, the temperature control system was adjusted to steadily reduce the temperature of the salt bath furnace to 450°C for later use.

[0027] Preparation Example 2: This preparation example provides a method for preparing a composite co-diffusion medium for surface strengthening, comprising the following steps: Weigh 40 parts by mass of anhydrous potassium carbonate, 30 parts by mass of anhydrous sodium carbonate and 30 parts by mass of anhydrous lithium carbonate and mix them mechanically to obtain a basic ternary carbonate.

[0028] The mixed basic ternary carbonate was placed in a well-type resistance-heated salt bath furnace equipped with a bottom sintered metal porous dispersion ring (made of 316L alloy with an average pore size of 10μm). The heating program was turned on to raise the temperature to 500℃ and hold it at that temperature for 1.5 hours to allow it to melt and homogenize completely.

[0029] Subsequently, 8 parts by weight of urea granules were slowly and evenly added to the surface of the molten salt bath. After the urea was completely dissolved and there were no obvious bubbles on the surface of the salt bath system, the temperature control system was adjusted to steadily reduce the temperature of the salt bath furnace to 460℃ for later use.

[0030] Preparation Example 3: This preparation example provides a method for preparing a composite co-diffusion medium for surface strengthening, comprising the following steps: Weigh 45 parts by mass of anhydrous potassium carbonate, 25 parts by mass of anhydrous sodium carbonate and 30 parts by mass of anhydrous lithium carbonate and mechanically mix them to obtain a basic ternary carbonate.

[0031] The mixed basic ternary carbonate was placed in a well-type resistance-heated salt bath furnace equipped with a bottom sintered metal porous dispersion ring (made of 316L alloy with an average pore size of 15μm). The heating program was turned on to raise the temperature to 520℃ and held at that temperature for 2 hours to allow it to melt and homogenize completely.

[0032] Then, slowly and evenly add 10 parts by weight of urea granules to the surface of the molten salt bath. After the urea has completely dissolved and there are no obvious bubbles rising on the surface of the salt bath system, adjust the temperature control system to steadily reduce the temperature of the salt bath furnace to 470℃ for later use.

[0033] Example 1: This embodiment provides a surface strengthening process for the flow components of a concentration tower circulating pump, including the following steps: S1, Substrate Pretreatment: The machined 2507 duplex stainless steel flow parts are placed in an ultrasonic cleaning tank and degreased for 15 minutes at 55°C using a 3% sodium hydroxide aqueous solution. The ultrasonic frequency is set to 28kHz. The parts are then removed, rinsed with deionized water, and thoroughly dried in an 80°C hot air circulating oven.

[0034] S2, Stage Ion Steady-State Pre-expansion Treatment: The pretreated flow component is completely immersed in the composite co-diffusion medium prepared in Preparation Example 1 and kept at 450°C; the bottom gas pipeline is opened, and carbon dioxide gas is continuously bubbled into the porous dispersion ring at a flow rate of 1L / min, and the temperature is kept constant for 3 hours.

[0035] S3, Stage Two Microporous Confined Flash Co-osmosis Treatment: Adjust the temperature control system to reduce the overall temperature of the salt bath to 395℃, while simultaneously cutting off the carbon dioxide gas source and switching to anhydrous ammonia gas; activate the electric heating cable on the outside of the main inlet pipe to preheat the ammonia gas to 400℃, and set the main ammonia gas flow rate to 2L / min; inject room temperature liquid formamide into the preheated ammonia gas flow in a pulsed manner through a diaphragm metering pump, with the formamide injection flow rate set to 2vol% of the ammonia gas flow rate and the pulse injection frequency at 0.5Hz; maintain constant temperature treatment for 5 hours under the condition of gas-liquid co-injection.

[0036] S4, In-situ cleaning of the furnace and residual enthalpy: After the heat preservation is completed, the parts are slowly and vertically lifted out of the salt bath and suspended in the air to drain the salt for 10 seconds; then, before the parts have cooled down, they are directly immersed in a cleaning tank containing 80°C flowing industrial water, continuously soaked and rinsed with mechanical water flow for 5 minutes; finally, the parts are taken out, rinsed with deionized water, and dried with compressed air at 0.4MPa.

[0037] Example 2: This embodiment provides a surface strengthening process for the flow components of a concentration tower circulating pump, including the following steps: S1, Substrate Pretreatment: The machined 2507 duplex stainless steel flow parts are placed in an ultrasonic cleaning tank and degreased for 20 minutes at 60°C using a 5% sodium hydroxide aqueous solution. The ultrasonic frequency is set to 35kHz. The parts are then removed, rinsed with deionized water, and thoroughly dried in a 90°C hot air circulating oven.

[0038] S2, Stage Ion Steady-State Pre-expansion Treatment: The pretreated flow component is completely immersed in the composite co-diffusion medium prepared in Preparation Example 2 and kept at 460°C; the bottom gas pipeline is opened, and carbon dioxide gas is continuously bubbled into the porous dispersion ring at a flow rate of 2L / min, and the temperature is kept constant for 4.5 hours.

[0039] S3, Stage Two Microporous Confined Flash Co-osmosis Treatment: Adjust the temperature control system to reduce the overall temperature of the salt bath to 400℃, while simultaneously cutting off the carbon dioxide gas source and switching to anhydrous ammonia gas; activate the electric heating cable on the outside of the main inlet pipe to preheat the ammonia gas to 420℃, and set the main ammonia gas flow rate to 3.5L / min; inject room temperature liquid formamide into the preheated ammonia gas flow in a pulsed manner through a diaphragm metering pump, with the formamide injection flow rate set to 3.5 vol% of the ammonia gas flow rate and the pulse injection frequency set to 1Hz; maintain constant temperature treatment for 6.5 hours under the condition of gas-liquid co-injection.

[0040] S4, In-situ cleaning of the furnace and residual enthalpy: After the heat preservation is completed, the parts are slowly and vertically lifted out of the salt bath and suspended in the air to drain the salt for 15 seconds; then, before the parts have cooled down, they are directly immersed in a cleaning tank containing 88°C flowing industrial water, continuously soaked and rinsed with mechanical water flow for 10 minutes; finally, the parts are taken out, rinsed with deionized water, and dried with compressed air at 0.5MPa.

[0041] Example 3: This embodiment provides a surface strengthening process for the flow components of a concentration tower circulating pump, including the following steps: S1, Substrate Pretreatment: The machined 2507 duplex stainless steel flow parts are placed in an ultrasonic cleaning tank and degreased for 30 minutes at 65°C using an 8% sodium hydroxide aqueous solution. The ultrasonic frequency is set to 40kHz. The parts are then removed, rinsed with deionized water, and thoroughly dried in a 100°C hot air circulating oven.

[0042] S2, Stage Ion Steady-State Pre-expansion Treatment: The pretreated flow component is completely immersed in the composite co-diffusion medium prepared in Preparation Example 3 and kept at 470°C; the bottom gas pipeline is opened, and carbon dioxide gas is continuously bubbled into the porous dispersion ring at a flow rate of 3L / min, and the temperature is kept constant for 6 hours.

[0043] S3, Stage Two Microporous Confined Flash Co-osmosis Treatment: Adjust the temperature control system to reduce the overall temperature of the salt bath to 410℃, while simultaneously cutting off the carbon dioxide gas source and switching to anhydrous ammonia gas; activate the electric heating cable on the outside of the main inlet pipe to preheat the ammonia gas to 430℃, and set the main ammonia gas flow rate to 5L / min; inject room temperature liquid formamide into the preheated ammonia gas flow in a pulsed manner through a diaphragm metering pump, with the formamide injection flow rate set to 5 vol% of the ammonia gas flow rate and the pulse injection frequency to 2Hz; maintain constant temperature treatment for 8 hours under the condition of gas-liquid co-injection.

[0044] S4, In-situ cleaning of the furnace and residual enthalpy: After the heat preservation is completed, the parts are slowly and vertically lifted out of the salt bath and suspended in the air to drain the salt for 20 seconds; then, before the parts have cooled down, they are directly immersed in a cleaning tank containing 95°C flowing industrial water, continuously soaked and rinsed with mechanical water flow for 15 minutes; finally, the parts are taken out, rinsed with deionized water, and dried with compressed air at 0.6MPa.

[0045] Comparative Example 1: Compared to Example 1, the difference is that the two-stage temperature-changing treatment process S2 and S3 is omitted. The flow-through component is directly immersed in a composite co-diffusion medium with a set temperature of 550°C, and pure ammonia gas is continuously introduced for constant temperature treatment for 8 hours without the addition of formamide. All other aspects are the same.

[0046] Comparative Example 2: The difference from Example 1 is that in the common ion steady-state pre-expansion treatment of step S2, the continuously injected carbon dioxide gas is replaced with an equal flow rate of ordinary industrial nitrogen (N2). All other aspects are the same.

[0047] Comparative Example 3: Compared with Example 1, the difference is that the common ion steady-state pre-expansion treatment in step S2 is omitted, and the substrate is directly subjected to the 395°C salt bath in step S3 after pretreatment, extending the total treatment time to 8 hours. All other aspects are the same.

[0048] Comparative Example 4: The difference from Example 1 is that in the microporous confined flash co-osmosis treatment of step S3, only preheated anhydrous ammonia gas is introduced, and liquid formamide is not injected through a diaphragm metering pump. All other aspects are the same.

[0049] Comparative Example 5: Compared to Example 1, the difference lies in that, in the microporous confined flash co-diffusion treatment of step S3, the formamide is not mixed with the preheated ammonia gas at the bottom during injection. Instead, a dripping device is used to directly drip an equal amount of room-temperature liquid formamide from above the salt bath furnace onto the liquid phase surface of the molten salt bath. All other aspects are the same.

[0050] Test Example 1: Experimental steps: S11. In the stage two microporous confined flash co-diffusion treatment process of Examples 1-3 and Comparative Examples 4-5, a bypass is led out at the node of the main air intake near the bottom porous dispersion ring, and a high-frequency piezoresistive pressure sensor with a range of 0-0.5MPa is connected to record the back pressure fluctuation of the pipeline throughout the process at a sampling rate of 50Hz.

[0051] S12. Install a weighing collection tray with condensation baffles around the exhaust port and furnace flange of the salt bath furnace to capture splashed salt droplets caused by local boiling or bubble rupture during constant temperature co-infiltration.

[0052] S13. After the constant temperature treatment for the specified time is completed and the machine is stopped to cool down, export the time domain signal of the pressure sensor and extract the back pressure fluctuation amplitude and main frequency in the interception window. At the same time, collect the solidified residual salt in the collection pan and weigh the total mass of splash loss using an electronic balance with an accuracy of 0.1g.

[0053] S14. Remove the bottom porous dispersion ring, cut it along the axial direction and observe whether there is any blockage of crystallized salt in the internal flow channel.

[0054] The test data for the above experimental steps are detailed in Table 1 below: Table 1

[0055] Summarize: According to the data in Table 1, the gas mass transfer stability in the high-viscosity salt bath system directly determines the engineering feasibility of the co-infiltration process. Conventional gas-solid interface reactions often exhibit channeling or gas distributor freezing due to the rapid increase in kinematic viscosity of the salt bath near the critical temperature. In Examples 1-3, controlled microscale confined flash boiling was induced in the pipeline by pulsed injection of trace amounts of liquid formamide, manifested as regular back pressure fluctuations of 2.14-5.62 kPa. This fluctuation frequency, which closely matches the injection frequency of the metering pump, verifies that a stable alternating high-pressure microjet and hydrodynamic cavitation were indeed formed inside the micropores. The jet, carrying a large phase change energy, effectively overcomes the solidification resistance of the molten salt at the capillary end, which is objectively confirmed by the absence of crystal adhesion after disassembling the porous dispersion ring.

[0056] In Comparative Example 4, only preheated anhydrous ammonia gas was introduced, and the back pressure in the pipeline remained stable without a clear cycle. However, a large amount of solidified salt blocks were observed inside the nozzle, resulting in severe diameter reduction. This indicates that relying solely on the sensible heat of the gas cannot provide sufficient hydrodynamic agitation energy, and boundary layer condensation or even physical blockage of the pipeline is highly likely to occur in a high-viscosity molten pool at around 400°C.

[0057] Industrial field experience shows that directly dripping liquid organic matter onto a high-temperature melt can easily trigger macroscopic thermal runaway. The splash loss of 845.7g in Comparative Example 5 directly reflects the explosive boilover that occurs when unconfined liquid formamide comes into large-area contact with high-temperature molten salt. This invention confines the phase change physical space within the 3-15μm pores of sintered metal, using the robust base metal wall constraint to transform the disordered thermodynamic phase change expansion into an ordered high-frequency kinetic jet, controlling the splash amount to below 30g, within the industrially permissible range.

[0058] This mechanism, having eliminated the risk of macroscopic phase change explosion, relies on jet kinetic energy to forcefully penetrate the laminar boundary layer on the surface of the complex pump impeller, clearing the geometric mass transfer dead zone during cryogenic processing.

[0059] Test Example 2: Experimental steps: S21. The isothermal molten salt baths in the same ion steady-state pre-expansion treatment process in Examples 1-3 and Comparative Example 2 were used as experimental objects.

[0060] S22. Starting from the beginning of the heat preservation time (0 hours) of this stage, every hour, use a specially made high-temperature resistant nickel-based alloy sampling spoon to extract about 5g of molten salt solution from 10cm below the center of the salt bath furnace.

[0061] S23. Quickly pour the extracted salt solution onto a water-cooled copper plate for rapid cooling and solidification. After cooling to room temperature, transfer it to an agate mortar and grind it into fine powder. Then, transfer it to a desiccator and seal it for storage.

[0062] S24. Accurately weigh 2g of the ground solidified salt powder and place it in a beaker. Add 100ml of deionized water to dissolve it completely. Add dilute nitric acid to adjust the pH of the solution to neutral to slightly acidic. Heat the solution on a hot plate until it boils gently to completely remove the dissolved free carbon dioxide and residual ammonia from the system.

[0063] S25. After cooling to room temperature, perform automatic potentiometric titration using a standard silver nitrate solution with a concentration of 0.1 mol / L. Calculate the mass fraction of the effective active donor cyanate in the extracted sample based on the volume of titrant consumed at the equivalence point, and calculate the average decay rate based on the concentration difference at each time point.

[0064] The test data for the above experimental steps are detailed in Table 2 below: Table 2

[0065] Summarize: According to the data in Table 2, conventional medium-temperature liquid co-diffusion systems inevitably face the thermodynamic dilemma of rapid pyrolysis and dissipation of active components under continuous heating. Comparative Example 2, using conventional inert nitrogen as the carrier gas, showed that after only 3 hours of holding at 450℃, the mass fraction of cyanate in the bath solution plummeted from 3.16% to 0.39%, with an average decay rate as high as 0.923% / h. This typical concentration avalanche phenomenon is extremely common in past industrial field monitoring. The scarcity of free cyanate often leads to stagnation of gas-solid mass transfer on the workpiece surface, while the extremely high initial concentration easily induces the sudden agglomeration and precipitation of hard and brittle phases such as chromium carbide on the surface of duplex steel.

[0066] Shifting the analytical perspective to the example groups, the chemical equilibrium of the system underwent a substantial reversal after continuous injection of carbon dioxide gas. The cyanate concentrations in Examples 1-3 exhibited extremely strong anti-decay characteristics over a monitoring period of up to 3 hours, with the concentration evolution tending towards a gradual asymptote, and the overall decay rate suppressed to a low level of 0.093-0.143% / h.

[0067] The common ion effect plays a crucial chemical anchoring role here. The high concentration of dissolved carbonate in the molten pool, under the control of Le Chatelier's principle, effectively suppresses the irreversible forward reaction of cyanate to carbonate. The donor environment, where activity is locked in a stable low-supersaturation range for a long time, creates mild diffusion kinetics for the preferential infiltration of smaller atomic radius carbon atoms into the interstitial spaces between the austenite and ferrite lattice of the dual-phase steel. Under the driving force of low chemical potential and the drag of long-term solute atoms, the lattice channels undergo microscopic distortion and pre-expansion, completely avoiding the risk of forced precipitation of supersaturated compounds under high concentration gradients. This lays the physical crystallographic basis for the deep synergistic substitution of large-sized nitrogen atoms in the subsequent cooling stage.

[0068] This steady-state process, based on the reverse regulation of the equilibrium constant, eliminates the thermal shock and flow field disturbance caused by frequent artificial salt replenishment in traditional salt baths, and macroscopically endows the multi-component co-infiltration medium with excellent service life and process reproducibility.

[0069] Test Example 3: Experimental steps: S31. The 2507 duplex stainless steel circulating pump impellers of the thickening tower, which have been processed by the processes of Examples 1-3 and Comparative Examples 4-5, are selected as the test objects.

[0070] S32. Immerse the impeller to be tested entirely in anhydrous ethanol for ultrasonic cleaning to remove residual salt and oil stains on the surface, and then use a cold air blower to thoroughly dry the surface.

[0071] S33. Define two test areas with typical hydrodynamic differences on the three-dimensional geometry of each impeller. Select the front of the impeller facing the flow as the "water-facing surface" (representing the area with extremely easy mass transfer) and select the narrow flow channel area near the shaft on the back of the impeller as the "blade root back" (representing the high viscosity mass transfer dead zone).

[0072] S34. Use a digital display micro Vickers hardness tester to calibrate the hardness of the above-defined area. Set the test load to 0.5 kgf (i.e., HV0.5) and the holding time to 15 seconds.

[0073] S35. Randomly select 10 non-overlapping valid test points within each defined area to record the hardness values. After removing abnormal extreme values, calculate the arithmetic mean and calculate the spatial attenuation rate based on the average hardness difference between the two areas.

[0074] The detailed data of the above experimental steps are shown in Table 3: Table 3

[0075] Summarize: According to the data in Table 3, the processing quality of complex three-dimensional components in a low-temperature salt bath system is directly limited by the mass transfer limit of the flow field boundary layer. In the actual service environment of industrial pump impellers, the back surface of the blade root often becomes a major area of ​​cavitation and erosion wear due to the presence of eddies. This requires that the strengthening process must overcome the geometric obstacles of complex three-dimensional flow channels to achieve equivalent solid solution strengthening of the entire surface.

[0076] In past cases of fluid machinery failure, equipment often faced total scrapping due to premature spalling in these hidden areas. The data from Comparative Examples 4 and 5 vividly illustrate the inadequacy of traditional methods when dealing with critically high-viscosity melts. While the hardness on the upstream surface could barely reach around 1000 HV0.5, the hardness on the underside of the blade root experienced a precipitous drop, with spatial attenuation rates as high as 58.17% and 52.31%, respectively. This enormous hardness difference exceeding 500 HV0.5 confirms that pure gaseous media without additional kinetic energy injection or traditional gravity-feeding methods are simply unable to break through the thick flow field boundary layer in dead zones. The retention of high-viscosity salt baths in these areas leads to complete stagnation of deep mass transfer, leaving the matrix in a vulnerable and easily damaged state.

[0077] To overcome the spatial barrier caused by high viscosity, the data from Examples 1-3 of this invention fully demonstrate the effectiveness of microporous confined flash evaporation-induced high-pressure microjets. Compared to the severe imbalance in the comparative examples, the hardness on both sides of the example groups maintained a high degree of consistency. Even at the blade root, where mass transfer is extremely difficult, the hardness still steadily increased to above 1015.7 HV0.5, and the overall spatial attenuation rate was forcibly suppressed to an extremely low level of less than 4.42%.

[0078] The controlled phase transition of liquid formamide after absorbing the sensible heat of ammonia gas within the porous sintered ring transforms the disordered thermal expansion into a directional high-frequency jet. This cavitation flow, carrying enormous kinetic energy, forcibly pumps fresh cyanate ions and free amino groups into the geometric dead zone. Simultaneously, carbon monoxide, a formamide cracking product accompanying the microjet reaching the dead zone, establishes a crucial carbon potential maintenance layer at the microscopic solid-liquid interface. This thin carbon potential boundary precisely counteracts the reverse desorption driving force of carbon atoms, firmly locking in the lattice pre-expansion state already formed during the intermediate temperature stage.

[0079] This deep coupling of kinetic energy penetration in the physical dimension and thermodynamic anchoring in the chemical dimension fundamentally eliminates the reinforcement blind zone on large flow components, ensuring the long service life of the components under complex two-phase flow scouring conditions.

[0080] Test Example 4: Experimental steps: S41. The same batch of 2507 duplex stainless steel bars were wire-cut into standard square test blocks with a size of 30mm×30mm×5mm. Surface strengthening treatment was carried out using the process parameters of Examples 1 to 3 and Comparative Examples 1 and 3, respectively, and these were used as test objects.

[0081] S42. The treated test block was ultrasonically cleaned in acetone and anhydrous ethanol for 15 minutes each, dried with cold air, placed in a glass desiccator and left to stand for 24 hours. Then, the initial mass was weighed using an analytical balance with an accuracy of 0.1 mg / 100,000.

[0082] S43. In a high-speed rotating mortar erosion and wear tester, a corrosive solid-liquid two-phase flow medium simulating the working conditions of a thickening tower is prepared. The liquid phase is a sodium chloride aqueous solution with a mass fraction of 3.5%, and the solid phase is angular quartz sand particles with a mass fraction of 15% (particle size range of 60 to 80 mesh). The temperature of the test medium is dynamically maintained at 85℃ through a constant temperature water bath jacket.

[0083] S44. Fix the weighed test block onto the PTFE insulating clamp of the testing machine, adjust the water-facing surface of the test block to form a 45-degree scouring angle with the main shaft liquid flow direction, set the impeller speed to 1200 r / min, and start the equipment to conduct a continuous scouring wear test for 72 hours.

[0084] S45. After the test cycle is completed, the test block is disassembled, rinsed with deionized water and the residual sand particles embedded in the surface are removed with a soft brush, ultrasonically cleaned again and dried in a hot air oven, the final mass is weighed, and the macroscopic scouring weight loss rate is calculated based on the mass difference.

[0085] The data details for the above experimental steps are shown in Table 4 below: Table 4

[0086] Summarize: According to the data in Table 4, the hard solid particles carried in the high-salt fluid of the industrial concentration tower pose an extreme threat of mechanical cutting and fatigue spalling to the flow components of the equipment. Conventional wear resistance assessments are often limited to static indentation hardness indicators, but in real fluid dynamic scouring scenarios, the penetration depth of the reinforcement layer and the ratio of lattice toughness are often more fatal than the absolute surface hardness.

[0087] Comparative Example 3 omitted the crucial intermediate-temperature steady-state pre-expansion treatment, resulting in a final scour weight loss rate soaring to 2.799 mg / h. Directly initiating the carbonitriding process at nearly 400°C created significant steric hindrance in the narrow, unexpanded lattice channels, severely restricting the diffusion of the active medium into deeper layers. This thin and shallow hardened zone was easily penetrated by the periodic stress cutting of high-speed quartz sand, exposing the soft, dual-phase steel matrix within and triggering an accelerated upward shift in the later stages of the weight loss curve.

[0088] Moreover, in actual field fracture analysis, it was frequently observed that the traditional 550℃ high-temperature co-diffusion process also failed to withstand the stress, with the weight loss rate of Comparative Example 1 reaching 2.217 mg / h. This high-temperature environment inevitably induces long-range desolvation diffusion of chromium atoms at the grain boundaries of austenite and ferrite, and a large amount of hard and brittle phases such as chromium carbide are locally agglomerated and precipitated. These brittle phases interrupt the original continuous tough network of the matrix and become microcrack initiation sources under the action of 45-degree cutting stress, ultimately leading to grain-level bulk brittle micro-scraping.

[0089] To address the service life challenges of this material, Examples 1 to 3 employed a phase-one method of continuously introducing carbon dioxide to lock in the low supersaturation carbon potential. This method utilizes carbon atoms to gently expand the lattice, clearing the physical steric hindrance for subsequent deep co-diffusion of large-sized nitrogen atoms at low temperatures. Test data demonstrated the superior performance of this technical approach, with the weight loss rate of the example groups firmly suppressed within a narrow range of 0.265-0.324 mg / h. The high-pressure microjets generated by confined flash evaporation, combined with the pre-expanded lattice channels, constructed a deep expanded phase (S phase) with high concentration of interstitial solid solution characteristics while completely suppressing the precipitation of brittle compounds.

[0090] This dense single-phase solid solution perfectly inherits the intrinsic plasticity of face-centered cubic and body-centered cubic structures. It can efficiently absorb and dissipate the impact kinetic energy of hard sand particles by relying on the elastic energy of micro-lattice distortion, thus completely blocking the mechanical peeling path.

[0091] Test Example 5: Experimental steps: S51. The 2507 duplex stainless steel standard electrochemical test block that has been processed by Examples 1-3 and Comparative Examples 1 and 2 is selected as the experimental object.

[0092] S52. Fully solder conductive copper wires onto the non-working surface of the test block, and then cold-mount and encapsulate it using insulating epoxy resin, leaving only 1cm. 2 The working area is precisely exposed, and then the working surface is polished step by step with 400-2000 grit silicon carbide wet sandpaper. It is then ultrasonically cleaned with anhydrous ethanol and thoroughly dried with cold air.

[0093] S53. Assemble a standard three-electrode system electrochemical workstation test cell, using a packaged test block as the working electrode, a saturated calomel electrode as the reference electrode, and a 2cm×2cm high-purity platinum sheet as the auxiliary electrode. The test electrolyte is a 3.5% sodium chloride aqueous solution prepared with deionized water. The entire test process is conducted in a constant temperature water bath at 25±1℃.

[0094] S54. Immerse the working electrode in the electrolyte for 60 minutes to monitor the open circuit potential until the potential drift rate with time approaches a very small steady-state range.

[0095] S55. Using the steady-state open-circuit potential as a reference, set -0.5V towards the anode direction to the polarization range where the passivation film on the electrode surface undergoes local breakdown, and perform potentiodynamic polarization scanning at a potential scanning rate of 1mV / s.

[0096] S56. Extract the polarization curve obtained from the scan, use the built-in electrochemical analysis software to perform extrapolation fitting of the Tafel section and local peak finding of the anode curve jump point, and extract and record the self-corrosion current density and pitting breakdown potential.

[0097] The data details for the above experimental steps are shown in Table 5 below: Table 5

[0098] Summarize: According to the data in Table 5, the long-term service life of duplex stainless steel in chlorine-rich liquid phases such as thickening towers is entirely dependent on the dense oxide passivation film spontaneously constructed by free chromium elements on its substrate surface. In our field investigations of corrosion failures, we frequently find that once this steel encounters the sensitization temperature range or inappropriate surface strengthening, the chromium elements within and at the grain boundaries readily undergo irreversible affinity reactions with externally infiltrated interstitial atoms. The extracted electrical polarization signals precisely confirm the macroscopic electrochemical destructive force brought about by this microscopic solid-state phase transition.

[0099] Comparative Example 1 employed conventional 550℃ high-temperature co-diffusion intervention, where extremely high thermodynamic activity directly drove the precipitation of chromium carbide and chromium nitride. Due to the rapid consumption of chromium, a large area of ​​the matrix fell into a chromium-depleted state. This was reflected in the polarization parameters, with the pitting breakdown potential plummeting to 215.3 mV, which easily induces spontaneous pitting corrosion, and the self-corrosion current density soaring to 8.741 µA / cm². 2 The material had essentially lost its self-passivation ability. Even with reduced processing temperature, Comparative Example 2 also succumbed to corrosion in an environment lacking common ion steady-state control. The highly supersaturated carbon potential gradient forcibly triggered sudden compound condensation on the surface, interrupting the continuity of the anti-corrosion network, causing its breakdown potential to remain weak at 342.8 mV.

[0100] Examples 1-3 achieved lattice pre-expansion through mild thermodynamic anchoring in the early stage, and then used microporous flash evaporation to establish and maintain the carbon potential after cooling, cleverly avoiding the long-range diffusion trap of chromium atoms. Carbon and nitrogen atoms filled the expanded interstitial channels with extremely low reaction steric hindrance, ultimately forming a single-phase supersaturated solid solution. This intervention method, which perfectly preserves the native chromium element in the lattice, exhibited remarkable stability in electrochemical tests. The breakdown potential of all examples jumped to a high passivation region of over 1000mV, and the corrosion current density was significantly suppressed to 0.086µA / cm. 2 The following is the passivation maintenance state. The exponential increase in charge transfer impedance objectively confirms that this process successfully unifies the high-hardness wear-resistant layer and the extremely low-activity anti-corrosion passivation layer within the same crystal structure.

[0101] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A material selection and surface strengthening process for the flow components of a concentration tower circulating pump, characterized in that, Includes the following steps: 2507 duplex stainless steel was selected as the base material for the flow-through components, and the surface was degreased, cleaned and dried. The flow-through component is immersed in a composite co-diffusion medium placed in a salt bath furnace. The temperature is controlled and carbon dioxide gas is continuously bubbled into the salt bath furnace. The active donor is anchored in a stable low supersaturation range by utilizing the common ion effect, which induces microscopic distortion and pre-expansion of the substrate lattice. The processing temperature is lowered, and then preheated anhydrous ammonia gas is introduced into the composite co-diffusion medium in the salt bath furnace, and liquid formamide is injected in a pulsed manner, so that the gas-liquid blend undergoes flash evaporation in the porous confined space and induces high-pressure micro-jet flow, and then isothermal treatment is performed. The reinforced flow-through components are lifted out of the salt bath furnace and quenched in hot water for in-situ cleaning using residual enthalpy.

2. The material selection and surface strengthening process for the flow components of a concentration tower circulating pump according to claim 1, characterized in that, The composite co-permeation medium comprises the following raw materials in parts by weight: Anhydrous potassium carbonate 35-45 parts, anhydrous sodium carbonate 25-35 parts, anhydrous lithium carbonate 30 parts, urea 5-10 parts.

3. The material selection and surface strengthening process for the flow components of a concentration tower circulating pump according to claim 1, characterized in that, The preparation of the composite co-permeation medium specifically includes the following steps: The basic ternary carbonates are mechanically mixed and then melted and homogenized at 480-520℃. Then add urea granules to the liquid surface and wait until there are no more bubbles on the surface of the system before using it.

4. The material selection and surface strengthening process for the flow components of a concentration tower circulating pump according to claim 1, characterized in that, The surface degreasing specifically includes the following steps: Degreasing is achieved by ultrasonic treatment at 55-65°C for 15-30 minutes using a sodium hydroxide aqueous solution with a mass fraction of 3%-8%. The ultrasonic frequency of the ultrasonic treatment is set to 28-40kHz.

5. The material selection and surface strengthening process for the flow components of a concentration tower circulating pump according to claim 1, characterized in that, The controlled temperature is 450-470℃, the flow rate of the injected carbon dioxide gas is 1-3L / min, and the injection time of the carbon dioxide gas is 3-6 hours.

6. The material selection and surface strengthening process for the flow components of a concentration tower circulating pump according to claim 1, characterized in that, The processing temperature is 395-410℃, the flow rate of anhydrous ammonia is 2-5L / min, the pulse injection frequency of the injected liquid formamide is 0.5-2Hz, and the flow rate is 2vol%-5vol% of the flow rate of anhydrous ammonia. The isothermal treatment time is 5-8 hours.

7. The material selection and surface strengthening process for the flow components of a thickening tower circulating pump according to claim 1, characterized in that, Both the carbon dioxide gas and the gas-liquid blend are injected into the salt bath through a porous 316L sintered metal ring with an average pore size of 5-15 μm located at the bottom of the salt bath furnace.

8. The material selection and surface strengthening process for the flow components of a concentration tower circulating pump according to claim 1, characterized in that, The temperature of the preheated anhydrous ammonia gas is 400-430℃, and the temperature of the hot water used for quenching is 80-95℃.

9. The material selection and surface strengthening process for the flow components of a concentration tower circulating pump according to claim 1, characterized in that, The in-situ cleaning specifically includes the following steps: The flow-through component is suspended in the air for 10-20 seconds to allow salt to leach out; The entire sample is immersed in the hot water and then rinsed with mechanical water for 5-15 minutes.

10. The material selection and surface strengthening process for the flow components of a concentration tower circulating pump according to claim 1, characterized in that, The injected liquid formamide is injected in a pulsed manner through a diaphragm metering pump. Before entering the porous confined space, the liquid formamide is kept in a suspended state of micron-sized droplets in the anhydrous ammonia gas flow.

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