Surface gradient strengthening treatment process of wear-resistant and fatigue-resistant corrugated roller

Abstract

This application relates to the field of surface strengthening treatment technology for metallic materials, and discloses a surface gradient strengthening treatment process for wear-resistant and fatigue-resistant corrugated rolls. The process includes preheating the formed corrugated roll workpiece; providing a basic salt system by mass fraction, and adding anhydrous sodium metavanadate, anhydrous sodium molybdate, and anhydrous potassium tetraborate to prepare a composite co-infiltration salt bath; immersing the preheated workpiece in the composite co-infiltration salt bath for low-temperature constant-temperature immersion treatment, followed by high-temperature heat preservation treatment; during the heat preservation stage, continuously introducing a mixed gas composed of ammonia and dry nitrogen into the salt bath; finally, removing the workpiece for oxidation cooling treatment, cleaning, and drying. This invention, through the complexation and catalytic effects of specific additives, the synergistic effect of step temperature control and the ventilated micro-reducing atmosphere, inhibits the growth of brittle phases on the surface, forming a gentle hardness gradient, solving the problem of easy slagging failure in multi-element salt baths, and improving the wear resistance and contact fatigue resistance of corrugated rolls.

Description

Technical Field

[0001] This invention relates to the field of surface strengthening treatment technology for metal materials, specifically a surface gradient strengthening treatment process for wear-resistant and fatigue-resistant corrugated rolls. Background Technology

[0002] During corrugated cardboard production, corrugated rolls are subjected to high-load alternating contact stress and frictional wear for extended periods, making their tooth surfaces prone to abrasive wear and contact fatigue spalling. Therefore, surface strengthening treatment is essential. Currently, salt bath nitrocarburizing technology is commonly used in industry for surface modification of corrugated rolls. However, the traditional single-temperature isothermal salt bath nitrocarburizing process is difficult to precisely control in terms of growth kinetics, leading to the formation of a large amount of brittle and porous ε-phase compounds on the workpiece surface, with high porosity within the compound layer. This microstructure results in an excessively steep decrease in hardness towards the matrix, failing to create a gentle transition gradient.

[0003] When corrugated rolls are subjected to alternating contact stress in actual working conditions, steep hardness gradients can easily lead to severe stress concentration at the boundary of drastic hardness changes, causing subsurface yield deformation and the initiation of microcracks, ultimately resulting in large-area fatigue spalling of the surface hardened layer.

[0004] To suppress the formation of brittle phases and promote deep diffusion of carbon and nitrogen atoms, existing technologies attempt to add transition metal salts such as vanadium and molybdenum to the basic salt bath to form a multi-component composite co-diffusion system. However, in actual long-term continuous industrial production, these transition metal ions are extremely unstable in the high-temperature, strongly alkaline cyanate and carbonate melts, and are very prone to coordination imbalance and self-polymerization reactions.

[0005] Meanwhile, continuous contact between the salt bath surface and air causes active metal ions to be oxidized into high-valence ineffective polymers, which then combine with anions such as carbonate in the system to form a large amount of insoluble precipitates. This severe precipitation and slagging phenomenon not only rapidly consumes the active catalytic components in the salt bath, preventing the subsequent co-infiltration reaction from proceeding continuously and uniformly, but also causes a sharp increase in the viscosity of the salt bath or even its failure, shortening the service life of the salt bath and making it difficult to meet the stability requirements of continuous production. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a surface gradient strengthening process for wear-resistant and fatigue-resistant corrugated rolls. This process solves the problems of insufficient wear resistance and fatigue resistance caused by the easy formation of brittle phases in the surface diffusion layer during existing corrugated roll heat treatment processes, and the tendency of multi-component composite salt baths to precipitate and slag during long-term high-temperature operation.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a surface gradient strengthening treatment process for wear-resistant and fatigue-resistant corrugated rolls, employing the following technical solution: Preheat the corrugated roll workpiece that has been processed and shaped; The basic salt system is provided by mass fraction, and the composite co-infiltration salt bath is prepared by adding additives based on the total mass of the basic salt system. The additives include 1.5%-2.0% anhydrous sodium metavanadate, 0.8%-1.2% anhydrous sodium molybdate, and 2.5%-3.5% anhydrous potassium tetraborate by mass fraction. The preheated corrugated roll workpiece is suspended into a composite co-infiltration salt bath with a set temperature of 510℃~530℃ and immersed at a constant temperature for 1.5~2.5h. The temperature of the composite co-infiltration salt bath was raised to 580℃~600℃, and the bath was kept at 580℃~600℃ for 2.5~3.5h. During the heat preservation stage, a mixed gas is continuously introduced into the composite co-infiltration salt bath. The mixed gas consists of 4.0% to 6.0% ammonia and 94.0% to 96.0% dry nitrogen by volume. The corrugated roll workpiece that has undergone heat preservation treatment is removed from the composite co-infiltration salt bath and subjected to oxidation cooling treatment; The corrugated roll workpieces after oxidation and cooling treatment are cleaned and dried.

[0008] By adopting the above technical solution, the following beneficial effects are achieved: The composite co-diffusion salt bath decomposes at high temperatures, producing active nitrogen and carbon atoms that penetrate into the surface layer of the workpiece.

[0009] The addition of sodium metavanadate and sodium molybdate causes vanadium and molybdenum to dissolve in the surface layer during co-diffusion, altering the thermodynamic conditions of the phase transition and lowering the critical nitrogen concentration limit for the formation of the ductile γ'-Fe4N phase. This results in the surface layer being dominated by the ductile γ'-Fe4N phase, inhibiting the growth of the brittle and porous ε phase and thus reducing the porosity.

[0010] Meanwhile, the trace segregation of vanadium and molybdenum in the infiltration layer slows down the reverse desorption of carbon and nitrogen atoms to the outer surface, increases the diffusion activation energy of interstitial atoms in the solid solution, promotes the deep diffusion of active atoms to the core, and increases the effective hardening layer depth.

[0011] Potassium tetraborate dissociates into polyborate anions in a high-temperature, strongly alkaline salt bath, forming a spatial network structure within the melt and coordinating with vanadium and molybdenum ions. This complexation anchors the transition metal ions within the complex network, preventing their self-aggregation, sedimentation, and slagging failure in the melt, thus maintaining the continuous uniformity of the co-diffusion reaction.

[0012] The stepped temperature control process changes the traditional single isothermal co-infiltration growth mode. The low-temperature treatment of 510℃~530℃ induces microscopic lattice distortion on the surface of the workpiece, accumulates point defects in the diffusion channels, and provides conditions for nucleation; High-temperature treatment at 580℃~600℃ promotes deep diffusion of carbon and nitrogen atoms into the interior. This avoids vacancy accumulation caused by rapid growth in a single temperature zone, forms a gentle hardness gradient distribution, improves the shear yield strength of the subsurface layer, and prevents early fatigue spalling under alternating contact stress.

[0013] A mixture of ammonia and dry nitrogen is introduced, and the nitrogen acts as a physical stirrer, promoting uniform mass and heat transfer in the molten salt. Ammonia gas undergoes pyrolysis inside the salt bath to produce hydrogen atoms, maintaining a dynamic micro-reducing atmosphere within the melt. This reducing environment inhibits the oxidative transformation of vanadium and molybdenum ions to higher valence states ineffective polymers when the melt surface comes into contact with air, thus ensuring the activity of the catalytic ions.

[0014] Preferably, before preheating the processed corrugated roll workpiece, the process further includes steps of degreasing and rinsing the corrugated roll workpiece. The specific operation for preheating is as follows: transfer the rinsed corrugated roll workpiece into the drying oven and dry it at 150℃~180℃ for 60~90 minutes.

[0015] By adopting the above technical solution, surface processing residues are removed, and moisture is eliminated, preventing the workpiece from directly entering the high-temperature salt bath and causing splashing, thus ensuring the surface activity state in the initial stage of co-infiltration.

[0016] Preferably, the degreasing solution used for degreasing is composed of an aqueous solution of sodium hydroxide with a mass fraction of 10%-15% and sodium metasilicate with a mass fraction of 5%-10%; the degreasing temperature is controlled at 80℃~90℃ and the treatment time is 20~30min.

[0017] By adopting the above technical solution, the strong alkalinity and the saponification and emulsification effects of sodium metasilicate work together to remove grease and particulate deposits from the surface of the workpiece.

[0018] Preferably, the components and mass fractions of the basic salt system are: potassium cyanate 35.0%-45.0%, anhydrous sodium carbonate 35.0%-45.0%, and potassium chloride 15.0%-25.0%, with the sum of the mass fractions of the above three being 100%.

[0019] By adopting the above technical solution, a reasonable eutectic molten salt system ratio was set, the melting point of the system was reduced, and the decomposition rate and supply stability of cyanate were ensured.

[0020] Preferably, the specific preparation process of the composite co-infiltration salt bath is as follows: first, add the weighed potassium cyanate, anhydrous sodium carbonate and potassium chloride in sequence, and heat to 450℃~480℃ to allow the potassium cyanate, anhydrous sodium carbonate and potassium chloride to initially melt and obtain the basic base. After the basic base is completely liquid, raise the temperature to 600℃~620℃, and add anhydrous potassium tetraborate, anhydrous sodium molybdate and anhydrous sodium metavanadate in sequence under mechanical stirring at a speed of 30~50r / min. Maintain a constant temperature of 600℃~620℃ and stir for 40~60 minutes. Then stop stirring and lower the temperature of the composite co-infiltration salt bath to 510℃~530℃ for later use.

[0021] By adopting the above technical solution, a liquid base molten pool is first formed, and then additives are added under high temperature and mechanical shear force to allow high-melting-point borates and transition metal salts to fully dissolve in the melt and form a coordination complex network, thus ensuring the compositional uniformity of the salt bath.

[0022] Preferably, during the process of raising the temperature of the composite co-infiltration salt bath to 580℃~600℃, a linear heating rate of 1.5℃ / min~2.5℃ / min is adopted.

[0023] By adopting the above technical solution, workpiece deformation caused by thermal stress due to drastic temperature rise is avoided, and the smooth transition of defect evolution and atomic diffusion within the crystal lattice is ensured.

[0024] Preferably, the mixed gas is continuously introduced into the composite co-infiltration salt bath through a top-inserted air blowing pipe; The inlet of the top-insertion air blowing pipe is located 20-30 cm below the surface of the composite co-infiltration salt bath, and the flow rate of the mixed gas is stably controlled at 0.5-1.0 L / min.

[0025] By adopting the above technical solution, sufficient time for gas bubbles to stay and rise inside the salt bath is ensured, maximizing the physical stirring efficiency and chemical reduction area of ​​the gas.

[0026] Preferably, the top-insertion air tube is made of Inconel 600 corrosion-resistant alloy.

[0027] By adopting the above technical solutions, Inconel 600 alloy exhibits high resistance to high-temperature nitriding, oxidation and corrosion in high-temperature ammonia and cyanide-containing molten salt environments, preventing ordinary stainless steel pipes from becoming brittle and breaking under these harsh conditions.

[0028] Preferably, the specific operation of the oxidation cooling treatment is as follows: immerse the corrugated roll workpiece in an oxidizing salt bath preheated to 380℃~420℃ and keep it at that temperature for 20~40 minutes; The components and their mass fractions of the oxidizing salt bath are: sodium nitrate 45.0%-55.0% and potassium nitrate 45.0%-55.0%.

[0029] By adopting the above technical solution, the nitrate mixture undergoes a strong oxidation reaction on the surface of the workpiece, oxidizing and sealing the surface micropores to form a dense Fe3O4 passivation film. At the same time, it serves as an isothermal quenching medium to achieve stepped cooling of the workpiece and reduce quenching thermal stress.

[0030] Preferably, the specific operation of cleaning and drying is as follows: the corrugated roll after oxidation and cooling treatment is naturally cooled in the air to a surface temperature of 90°C to 110°C; Immerse in a boiling water bath at 95℃~100℃ for 30~40 minutes; Rinse with deionized water, then dry with hot air at 80℃~100℃ and apply rust-preventive oil to the surface of the workpiece.

[0031] By adopting the above technical solution, boiling water immersion dissolves the residual salt adhering to the surface, preventing the residual salt from corroding the substrate, and the coating of rust-preventive oil provides the final protection for storage and transportation.

[0032] This invention provides a surface gradient strengthening treatment process for wear-resistant and fatigue-resistant corrugated rolls. It has the following beneficial effects: 1. This invention adds anhydrous sodium metavanadate, anhydrous sodium molybdate, and anhydrous potassium tetraborate to the composite co-infiltration salt bath. This alters the phase transformation thermodynamics of vanadium and molybdenum due to solid solution, thereby inhibiting the excessive growth of the brittle and porous surface phase and promoting the deep diffusion of carbon and nitrogen atoms into the core. Simultaneously, the polyborate anions dissociated from potassium tetraborate construct a spatial complex network within the melt, preventing the agglomeration and slagging of transition metal ions. Therefore, this increases the effective hardened layer depth on the corrugated roll surface and solves the technical problem of easy failure during long-term high-temperature operation of multi-element salt baths.

[0033] 2. This invention employs a stepped temperature control process, first immersing at a low temperature and then raising the temperature to a high temperature and holding it. This allows the low-temperature treatment to induce microscopic lattice distortion on the surface of the workpiece, providing a physical channel for the deep diffusion of carbon and nitrogen atoms in the subsequent high-temperature treatment, and avoiding vacancy accumulation caused by rapid growth in a single temperature zone. As a result, a gentle hardness gradient distribution is formed on the subsurface of the corrugated roll, thus reducing the yield strength difference between the surface hardened layer and the core matrix to prevent fatigue spalling of the subsurface layer under alternating contact stress.

[0034] 3. In this invention, a mixed gas consisting of ammonia and dry nitrogen is continuously introduced into the composite co-infiltration salt bath during the heat preservation stage. This not only utilizes nitrogen to promote the uniformity of mass and heat transfer in the molten salt through physical stirring, but also maintains a dynamic micro-reducing atmosphere inside the melt through hydrogen atoms generated by the pyrolysis of ammonia inside the salt bath. This inhibits the oxidative transformation of vanadium and molybdenum ions to higher valence states of ineffective polymers when the surface of the melt comes into contact with air. Therefore, the active state of the catalytic ions is maintained and the porosity of the compound layer is further reduced. Detailed Implementation

[0035] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0036] Preparation Example 1: This preparation example provides a method for preparing a composite co-infiltration salt bath, including the following steps: Weigh the raw materials of the basic salt system according to their mass fractions. The components are 35.0% potassium cyanate, 45.0% anhydrous sodium carbonate, and 20.0% potassium chloride, with the sum of their mass fractions being 100%. Based on the total mass of this basic salt system, weigh out anhydrous sodium metavanadate (1.5%), anhydrous sodium molybdate (0.8%), and anhydrous potassium tetraborate (2.5%) by mass fraction.

[0037] First, add the weighed potassium cyanate, anhydrous sodium carbonate and potassium chloride to a heat-resistant crucible in sequence, and heat it to 450℃ to allow them to melt initially and obtain the basic base.

[0038] After the basic base is completely liquid, raise the temperature to 600℃ and add the prepared anhydrous potassium tetraborate, anhydrous sodium molybdate and anhydrous sodium metavanadate in sequence under mechanical stirring at 30 r / min.

[0039] Maintain a constant temperature of 600℃ and stir for 40 minutes, then stop stirring and lower the temperature of the salt bath to 510℃ before use.

[0040] Preparation Example 2: This preparation example provides a method for preparing a composite co-infiltration salt bath, including the following steps: Weigh the raw materials of the basic salt system according to their mass fractions. The components are 40.0% potassium cyanate, 40.0% anhydrous sodium carbonate, and 20.0% potassium chloride, with the sum of their mass fractions being 100%. Based on the total mass of this basic salt system, weigh out anhydrous sodium metavanadate (1.75%), anhydrous sodium molybdate (1.0%), and anhydrous potassium tetraborate (3.0%) by mass fraction.

[0041] First, add the weighed potassium cyanate, anhydrous sodium carbonate and potassium chloride to a heat-resistant crucible in sequence, and heat it to 465℃ to allow them to melt initially and obtain the basic base.

[0042] After the basic base is completely liquid, the temperature is raised to 610℃, and under mechanical stirring at 40 r / min, the prepared anhydrous potassium tetraborate, anhydrous sodium molybdate and anhydrous sodium metavanadate are added in sequence.

[0043] Maintain a constant temperature of 610℃ and stir for 50 minutes, then stop stirring and lower the temperature of the salt bath to 520℃ before use.

[0044] Preparation Example 3: This preparation example provides a method for preparing a composite co-infiltration salt bath, including the following steps: Weigh the raw materials of the basic salt system according to their mass fractions. The components are 45.0% potassium cyanate, 35.0% anhydrous sodium carbonate, and 20.0% potassium chloride, with the sum of their mass fractions being 100%. Based on the total mass of this basic salt system, weigh out anhydrous sodium metavanadate (2.0%), anhydrous sodium molybdate (1.2%), and anhydrous potassium tetraborate (3.5%) by mass fraction.

[0045] First, add the weighed potassium cyanate, anhydrous sodium carbonate and potassium chloride to a heat-resistant crucible in sequence, and heat it to 480℃ to make it initially melt and obtain the basic base.

[0046] After the basic base is completely liquid, raise the temperature to 620℃ and add the prepared anhydrous potassium tetraborate, anhydrous sodium molybdate and anhydrous sodium metavanadate in sequence under mechanical stirring at 50 r / min.

[0047] Maintain a constant temperature of 620℃ and stir for 60 minutes, then stop stirring and lower the temperature of the salt bath to 530℃ before use. Example

[0048] This embodiment provides a surface gradient strengthening treatment process for wear-resistant and fatigue-resistant corrugated rolls, including the following steps: The corrugated roll workpieces are degreased and rinsed. The degreasing solution is composed of an aqueous solution of 10% sodium hydroxide and 5% sodium metasilicate by mass. The degreasing temperature is controlled at 80℃ and the treatment time is 20 minutes.

[0049] After rinsing, the corrugated roll workpiece is transferred into a drying oven and preheated at 150℃ for 60 minutes.

[0050] The preheated corrugated roll workpiece was suspended into a composite co-infiltration salt bath prepared in Preparation Example 1 at a set temperature of 510°C and immersed at a constant temperature for 1.5 hours.

[0051] The temperature of the composite co-infiltration salt bath was then increased linearly at a rate of 1.5℃ / min to 580℃, and then kept at 580℃ for 2.5 hours.

[0052] During the heat preservation stage, a mixed gas is continuously introduced into the composite co-infiltration salt bath through a top-insertion air blowing pipe made of Inconel 600 corrosion-resistant alloy. The outlet of the air blowing pipe is located 20cm below the surface of the composite co-infiltration salt bath, and the flow rate of the mixed gas is stably controlled at 0.5L / min. The mixed gas consists of 4.0% ammonia and 96.0% dry nitrogen by volume.

[0053] The corrugated roll workpiece that has undergone heat preservation treatment is taken out of the composite co-infiltration salt bath and immersed in an oxidizing salt bath preheated to 380°C for 20 minutes for oxidation cooling treatment. The components and their mass fractions of the oxidizing salt bath are sodium nitrate 45.0% and potassium nitrate 55.0%.

[0054] After oxidation and cooling treatment, the corrugated roll workpiece is naturally cooled in air to a surface temperature of 90°C, and then immersed in a boiling water bath at 95°C for 30 minutes.

[0055] Finally, rinse with deionized water at a pressure of 0.3 MPa, then dry with hot air at 80°C and apply anti-rust oil to the surface of the workpiece to complete the cleaning and drying process. Example

[0056] This embodiment provides a surface gradient strengthening treatment process for wear-resistant and fatigue-resistant corrugated rolls, including the following steps: The corrugated roll workpieces are degreased and rinsed. The degreasing solution is composed of an aqueous solution of sodium hydroxide with a mass fraction of 12.5% ​​and sodium metasilicate of 7.5%. The degreasing temperature is controlled at 85℃ and the treatment time is 25 minutes.

[0057] After rinsing, the corrugated roll workpiece is transferred into a drying oven and preheated at 165℃ for 75 minutes.

[0058] The preheated corrugated roll workpiece was suspended into a composite co-infiltration salt bath prepared in Preparation Example 2 at a set temperature of 520°C and immersed at a constant temperature for 2.0 h.

[0059] The temperature of the composite co-infiltration salt bath was then increased linearly at a rate of 2.0℃ / min to 590℃, and then kept at 590℃ for 3.0h.

[0060] During the heat preservation stage, a mixed gas is continuously introduced into the composite co-infiltration salt bath through a top-insertion air blowing pipe made of Inconel 600 corrosion-resistant alloy. The outlet of the air blowing pipe is located 25cm below the surface of the composite co-infiltration salt bath, and the flow rate of the mixed gas is stably controlled at 0.75L / min. The mixed gas consists of 5.0% ammonia and 95.0% dry nitrogen by volume.

[0061] The corrugated roll workpiece that has undergone heat preservation treatment is taken out of the composite co-infiltration salt bath and immersed in an oxidizing salt bath preheated to 400℃ for 30 minutes for oxidation cooling treatment. The components and their mass fractions of the oxidizing salt bath are sodium nitrate 50.0% and potassium nitrate 50.0%.

[0062] After oxidation and cooling treatment, the corrugated roll workpiece is naturally cooled in air to a surface temperature of 100°C, and then immersed in a boiling water bath at 97.5°C for 35 minutes.

[0063] Finally, rinse with deionized water at a pressure of 0.4 MPa, then dry with hot air at 90°C and apply anti-rust oil to the surface of the workpiece to complete the cleaning and drying process. Example

[0064] This embodiment provides a surface gradient strengthening treatment process for wear-resistant and fatigue-resistant corrugated rolls, including the following steps: The corrugated roll workpieces are degreased and rinsed. The degreasing solution is composed of an aqueous solution of 15% sodium hydroxide and 10% sodium metasilicate by mass. The degreasing temperature is controlled at 90℃ and the treatment time is 30 minutes.

[0065] After rinsing, the corrugated roll workpiece is transferred into a drying oven and preheated at 180℃ for 90 minutes.

[0066] The preheated corrugated roll workpiece was suspended into a composite co-infiltration salt bath prepared in Preparation Example 3 at a set temperature of 530°C and immersed at a constant temperature for 2.5 hours.

[0067] The temperature of the composite co-infiltration salt bath was then increased linearly at a rate of 2.5℃ / min to 600℃, and then kept at 600℃ for 3.5 hours.

[0068] During the heat preservation stage, a mixed gas is continuously introduced into the composite co-infiltration salt bath through a top-insertion air blowing pipe made of Inconel 600 corrosion-resistant alloy. The outlet of the air blowing pipe is located 30cm below the surface of the composite co-infiltration salt bath, and the flow rate of the mixed gas is stably controlled at 1.0L / min. The mixed gas consists of 6.0% ammonia and 94.0% dry nitrogen by volume.

[0069] The corrugated roll workpiece that has undergone heat preservation treatment is taken out of the composite co-infiltration salt bath and immersed in an oxidizing salt bath preheated to 420°C for 40 minutes for oxidation cooling treatment. The components and their mass fractions of the oxidizing salt bath are sodium nitrate 55.0% and potassium nitrate 45.0%.

[0070] After oxidation and cooling treatment, the corrugated roll workpiece is naturally cooled in air to a surface temperature of 110°C, and then immersed in a boiling water bath at 100°C for 40 minutes.

[0071] Finally, rinse with deionized water at a pressure of 0.5 MPa, then dry with hot air at 100°C and apply anti-rust oil to the surface of the workpiece to complete the cleaning and drying process.

[0072] Comparative Example 1: Compared with Example 2, the difference is that anhydrous sodium metavanadate, anhydrous sodium molybdate, and anhydrous potassium tetraborate were not added to the composite co-infiltration salt bath, and the 520°C constant temperature immersion treatment step was omitted during the treatment process (it was directly kept at 590°C for 5.0h), no mixed gas was introduced, and no oxidation cooling treatment was performed. All other aspects were the same.

[0073] Comparative Example 2: Compared with Example 2, the difference is that anhydrous sodium metavanadate and anhydrous sodium molybdate were not added to the composite co-infiltration salt bath, while all other aspects are the same.

[0074] Comparative Example 3: Compared with Example 2, the difference is that anhydrous potassium tetraborate was not added to the composite co-infiltration salt bath, and all other aspects are the same.

[0075] Comparative Example 4: Compared with Example 2, the difference is that no mixed gas was introduced into the composite co-infiltration salt bath during the heat preservation stage; otherwise, they are the same.

[0076] Comparative Example 5: Compared with Example 2, the difference is that the 520℃ constant temperature immersion treatment step and the temperature rise transition stage were cancelled. The preheated corrugated roll workpiece was directly suspended into a composite co-infiltration salt bath with a set temperature of 590℃ for 5.0h for heat preservation treatment. All other aspects are the same.

[0077] Test Example 1: Wire-cut analytical blocks measuring 10mm × 10mm × 10mm were obtained by cutting the cross-sections of the samples treated in each embodiment and comparative example. After ultrasonic cleaning to remove oil, the blocks were mounted using thermosetting resin.

[0078] The inlaid sample cross-section was polished step by step using silicon carbide wet sandpaper with a particle size ranging from coarse to fine. Then, it was mechanically polished on a polishing machine using diamond polishing slurry with a particle size of 1.0μm until the cross-section was free of obvious scratches and had a mirror-like finish.

[0079] The polished sample cross-section was etched using a 4% (v / v) nitric acid alcohol solution, with the etching time controlled between 8 and 12 seconds. The sample was then rinsed with anhydrous ethanol and dried.

[0080] The cross-section of the sample was observed under an optical metallographic microscope, and the thickness of the compound layer was measured. Five different fields of view were randomly selected, and the percentage of the area of ​​pores in the surface compound layer to the total area of ​​the compound layer was calculated using image analysis software. The average value was taken as the porosity.

[0081] Uncorroded samples from the same batch were subjected to X-ray diffraction for phase analysis. Cu-Kα rays were used as the target material, with a scanning range of 2θ of 30°–90° and a scanning step size of 0.02°. The Rietveld full-spectrum fitting method was used to calculate the γ'-Fe₄N and ε-Fe phases in the surface layer. 2-3 The relative mass fraction of the N phase.

[0082] Table 1. Test results of the compound layer phase and micro-defects on the surface of the samples of each embodiment and comparative example. Example 1 18.7 1.4 86.2 13.8 Example 2 21.4 1.1 89.6 10.4 Example 3 20.2 1.6 85.3 14.7 Comparative Example 1 11.5 16.3 41.2 58.8 Comparative Example 2 15.6 7.9 59.4 40.6 Comparative Example 3 13.8 6.5 66.8 33.2 Comparative Example 4 16.1 9.4 55.7 44.3 Comparative Example 5 18.9 12.1 63.5 36.5 in conclusion: According to the data in Table 1, the thickness of the compound layer of the samples treated in Examples 1-3 all exceeded 18 μm, and the porosity remained below 1.6%. The phase composition was dominated by the ductile γ' phase, accounting for more than 85%. Comparative Example 1 used a traditional treatment process, and its compound layer was thinner with a porosity as high as 16.3%, and the phase was dominated by the brittle ε phase.

[0083] Comparative Examples 1 and 2 show that the amount of γ' phase generated is significantly reduced when sodium metavanadate and sodium molybdate are not added. This indicates that the solid solution of vanadium and molybdenum can change the thermodynamic conditions of phase transformation, reduce the critical nitrogen concentration for the formation of the γ' phase, and thus inhibit the excessive growth of the brittle and porous ε phase.

[0084] As can be seen from the comparison examples and Comparative Example 3, the lack of potassium tetraborate leads to a thinner compound layer. This is because borate ions act as complex network builders in this high-temperature, strongly alkaline salt bath, preventing the aggregation of transition metal ions such as vanadium and molybdenum, and ensuring the continuous uniformity of the co-diffusion reaction.

[0085] Comparative Example 4, which did not introduce a mixture of ammonia and nitrogen, resulted in an increase in porosity and a decrease in the proportion of the γ' phase. This indicates that introducing a specific ratio of mixed gas not only creates a physical stirring effect inside the salt bath, but its weak reducing atmosphere also maintains the catalytic state of the active metal ions.

[0086] Comparative Example 5 eliminated the low-temperature constant-temperature soaking step. Although the thickness of its compound layer increased, the porosity was relatively high. This indicates that the low-temperature section in the stepped temperature control process provides sufficient nucleation time for the adsorption of carbon and nitrogen atoms, making the diffusion growth in the subsequent high-temperature section more compact and avoiding vacancy accumulation and pore defects caused by rapid growth in a single temperature zone.

[0087] Test Example 2: Test specimens are cut along the cross-section of the sample, inlaid, and then the surface is sanded step by step with wet sandpaper and polished on a polishing machine until the cross-section is smooth and free of scratches.

[0088] The hardness gradient of the sample cross section was measured using a micro Vickers hardness tester. The test load was set to 1.96 N, and the holding time was 15 s.

[0089] The test path starts from the inner side near the surface compound layer and measures point by point perpendicular to the surface towards the substrate. Within a depth range of 0–100 μm from the surface, the spacing between adjacent test points is set to 20 μm; within a depth range greater than 100 μm, the spacing between adjacent test points is set to 50 μm.

[0090] Three parallel tests were conducted at each depth location, and the arithmetic mean was taken as the hardness value at that depth. The vertical distance from where the hardness value drops to 500 HV0.2 was taken as the effective hardened layer depth.

[0091] Table 2. Test results of cross-sectional hardness and effective hardened layer depth of specimens in each embodiment and comparative example. Example 1 1012 876 732 584 0.32 Example 2 1045 913 768 611 0.36 Example 3 1028 894 741 597 0.34 Comparative Example 1 864 632 487 374 0.09 Comparative Example 2 921 715 563 438 0.16 Comparative Example 3 896 684 522 415 0.13 Comparative Example 4 933 742 591 462 0.18 Comparative Example 5 1056 708 516 398 0.11 in conclusion: According to the data in Table 2, the surface hardness of the samples treated in Examples 1-3 reached above 1012 HV0.2, and the hardness decreased gradually towards the substrate, with an effective hardened layer depth of over 0.32 mm. Comparative Example 1, using a traditional process, had a lower surface hardness, with a steep decrease in hardness towards the substrate, and an effective hardened layer depth of only 0.09 mm.

[0092] Comparing the data of the comparative examples with those of Comparative Examples 2 and 3, the effective hardened layer depth was reduced to varying degrees in samples lacking sodium metavanadate, sodium molybdate, or potassium tetraborate. The trace segregation of vanadium and molybdenum in the diffusion layer slowed down the reverse desorption of carbon and nitrogen atoms to the outer surface, increased the diffusion activation energy of interstitial atoms in the solid solution, and enabled deep diffusion of carbon and nitrogen atoms towards the core.

[0093] Potassium tetraborate maintained the stability of the complex network in the system, preventing the precipitation of active catalytic ions and ensuring the diffusion driving force in the later stages of the co-diffusion process. In the comparative examples and Comparative Example 5, the sample that did not undergo the 520℃ low-temperature immersion treatment, although having higher surface hardness, showed a sharp decrease in subsurface hardness.

[0094] In the stepped temperature control process, the low-temperature treatment can induce microscopic lattice distortion on the surface of the workpiece, accumulating point defects in the diffusion channels. This provides a physical channel for the deep diffusion of carbon and nitrogen atoms into the interior during the subsequent high-temperature stage, forming a gentle hardness gradient.

[0095] A gentle hardness gradient can reduce the difference in yield strength between the surface hardened layer and the core matrix, preventing stress concentration and subsurface fatigue spalling at the interface when the corrugated roll is subjected to alternating contact stress.

[0096] Test Example 3: Cylindrical contact fatigue standard specimens with an outer diameter of 60 mm and a thickness of 20 mm were prepared according to the process conditions of each embodiment and comparative example. Before testing, the surface of the specimens was ultrasonically cleaned with anhydrous ethanol and dried, and the initial mass was weighed using a high-precision analytical balance.

[0097] The specimen is mounted on the drive shaft of the double-disc rolling contact fatigue testing machine, and the test wheel is made of GCr15 bearing steel with a hardness of 60HRC and is mounted on the driven shaft.

[0098] Test parameters were set as follows: the maximum Hertzian contact stress during cyclic operation was 1800 MPa, and the spindle speed was set to 1500 r / min. During the test, N32 mechanical lubricating oil was used for continuous drip lubrication, and the oil temperature was controlled at room temperature.

[0099] The testing machine is equipped with a vibration sensor. When the vibration acceleration exceeds a set threshold due to spalling of the sample surface, the machine automatically stops. The number of contact cycles at the time of shutdown is recorded as the contact fatigue life. If the number of cycles reaches 1.0 × 10⁻⁶, the machine will automatically stop. 7 If no peeling occurs after the second attempt, the test should be stopped.

[0100] After the test, the sample was removed, cleaned and dried, and weighed again. The difference in mass before and after the test was calculated as the wear amount.

[0101] Table 3. Test results of contact fatigue life and wear amount of each embodiment and comparative sample. Example 1 86.4 3.1 Example 2 94.2 2.5 Example 3 89.7 2.8 Comparative Example 1 14.3 15.6 Comparative Example 2 32.1 8.4 Comparative Example 3 41.5 6.7 Comparative Example 4 37.8 7.9 Comparative Example 5 22.6 11.2 in conclusion: According to the data in Table 3, the contact fatigue life of the samples treated in Examples 1-3 all exceeded 86.0 × 10⁻⁶. 5 The wear amount was less than 3.2 mg. The fatigue life of the sample in Comparative Example 1 was only 14.3 × 10⁻⁶. 5 The wear amount reached 15.6 mg.

[0102] Combining the results of Test Example 1 and Test Example 2, it can be seen that the compound layer formed on the surface of the embodiment is dominated by the ductile γ' phase, and the effective hardened layer is relatively deep with a gentle hardness gradient. Under high alternating contact stress, this microstructure allows the surface ductile phase to absorb deformation energy and delay microcrack initiation, while the gentle hardness gradient provides sufficient subsurface shear yield strength, preventing crack propagation at locations of abrupt hardness changes.

[0103] Comparative Example 2 lacked sodium metavanadate and sodium molybdate, resulting in an increase in the brittle ε phase on the surface. During rolling contact, the brittle phase on the surface rapidly developed microcracks and caused early abrasive wear, leading to increased wear. Comparative Example 5 omitted the low-temperature isothermal immersion treatment step, resulting in a steep decrease in hardness towards the center.

[0104] In rolling contact fatigue, the maximum shear stress usually occurs in the subsurface region. The steep hardness gradient causes the subsurface strength to be insufficient to resist the shear stress, inducing plastic deformation and internal microcracks, which ultimately leads to large-area fatigue spalling on the sample surface and shortens the fatigue life.

[0105] The composite co-infiltration salt bath formulation and the stepped temperature control process in the embodiment work synergistically to change the phase composition of the infiltrated layer and optimize the hardness distribution, thereby improving the wear resistance and contact fatigue resistance of the corrugated roll material.

[0106] Test Example 4: Weigh out 10.0 kg of each of the salt baths prepared in the examples and comparative examples, and place them in an externally heated crucible furnace. Heat the furnace to 590°C.

[0107] According to the gas introduction conditions set in each embodiment and comparative example, the corresponding gas was continuously introduced into the crucible to simulate continuous industrial production for 72 hours, during which no new salt was added.

[0108] After continuous operation, stop the gas supply, lower the crucible furnace temperature to 450℃ and let it stand for 4 hours to allow the insoluble suspended matter and failed precipitates inside the salt bath to settle fully to the bottom of the crucible.

[0109] Use an 80-mesh stainless steel mesh spoon to remove all the sediment from the bottom of the crucible.

[0110] The dredged sediment is repeatedly boiled and washed in boiling water to dissolve the soluble bases it carries, and then filtered to retain the insoluble sediment.

[0111] The sludge was dried in a drying oven at 120℃ until constant weight. The mass of the sludge was weighed using an electronic balance, and the sludge production rate was calculated.

[0112] Table 4. Results of slag production tests during continuous operation of salt baths in each embodiment and comparative example. Example 1 10.03 72 47.6 0.47 Example 2 10.01 72 41.2 0.41 Example 3 10.05 72 49.8 0.50 Comparative Example 1 10.00 72 185.3 1.85 Comparative Example 2 10.02 72 163.7 1.63 Comparative Example 3 10.04 72 421.5 4.20 Comparative Example 4 10.00 72 316.9 3.17 Comparative Example 5 10.03 72 45.1 0.45 in conclusion: According to the data in Table 4, after continuous operation at 590℃ for 72 hours, the slag production rate of the salt baths in Examples 1-3 was controlled at 0.50% or below, and the salt bath system remained clear and stable. In Comparative Example 3, without the addition of potassium tetraborate, the sludge quality increased sharply, with a slag production rate as high as 4.20%.

[0113] Because transition metal salts such as sodium metavanadate and sodium molybdate undergo self-polymerization or form insoluble precipitates with carbonate ions in high-temperature, strongly alkaline cyanate and carbonate melts, the addition of potassium tetraborate dissociates polyborate anions in the molten salt. These polyborate anions construct a spatial network structure inside the melt, coordinate with vanadium and molybdenum ions, anchor the transition metal ions in the complex network, and prevent their agglomeration and sedimentation in the melt.

[0114] Comparative Example 4, without the introduction of a mixed gas containing ammonia and nitrogen, achieved a slag production rate of 3.17%. Ammonia pyrolyzes inside the high-temperature salt bath to produce hydrogen atoms, maintaining a dynamic micro-reducing atmosphere within the melt. This micro-reducing environment inhibits the oxidative transformation of vanadium and molybdenum ions into higher valence states of ineffective polymers when the melt surface comes into contact with air, preventing the metal oxides from slagging and deactivating.

[0115] The comparative results show that the complexing network of potassium tetraborate and the micro-reducing atmosphere of the mixed gas work synergistically to maintain the activity and dispersion of transition metal ions, thus solving the technical problem of easy precipitation and slagging failure of multi-component composite salt baths during long-term high-temperature operation in industry.

Claims

1. A surface gradient strengthening treatment process for wear-resistant and fatigue-resistant corrugated rolls, characterized in that, Includes the following steps: Preheat the corrugated roll workpiece that has been processed and shaped; The basic salt system is provided by mass fraction, and the composite co-infiltration salt bath is prepared by adding additives based on the total mass of the basic salt system. The additives include 1.5%-2.0% anhydrous sodium metavanadate, 0.8%-1.2% anhydrous sodium molybdate, and 2.5%-3.5% anhydrous potassium tetraborate by mass fraction. The preheated corrugated roll workpiece is suspended into the composite co-infiltration salt bath with a set temperature of 510℃~530℃ and immersed at a constant temperature for 1.5~2.5h. The temperature of the composite co-infiltration salt bath is raised to 580℃~600℃, and the bath is kept at 580℃~600℃ for 2.5~3.5h. During the heat preservation stage, a mixed gas is continuously introduced into the composite co-infiltration salt bath. The mixed gas consists of 4.0% to 6.0% ammonia and 94.0% to 96.0% dry nitrogen by volume. The corrugated roll workpiece, after undergoing heat preservation treatment, is removed from the composite co-infiltration salt bath and subjected to oxidation cooling treatment; The corrugated roll workpiece after the oxidation and cooling treatment is cleaned and dried.

2. The process according to claim 1, characterized in that, Before preheating the processed corrugated roll workpiece, the process also includes steps of degreasing and rinsing the corrugated roll workpiece. The specific operation of preheating is as follows: After rinsing, the corrugated roll workpiece is transferred into a drying oven and dried at 150℃~180℃ for 60~90 minutes.

3. The process according to claim 2, characterized in that, The degreasing solution used for degreasing is composed of an aqueous solution of sodium hydroxide with a mass fraction of 10%-15% and sodium metasilicate with a mass fraction of 5%-10%. The degreasing temperature is controlled at 80℃~90℃, and the processing time is 20~30min.

4. The process according to claim 1, characterized in that, The components and mass fractions of the basic salt system are as follows: Potassium cyanate 35.0%-45.0%, anhydrous sodium carbonate 35.0%-45.0%, and potassium chloride 15.0%-25.0%, the sum of the mass fractions of the above three is 100%.

5. The process according to claim 4, characterized in that, The specific preparation process of the composite co-infiltration salt bath is as follows: First, add the weighed potassium cyanate, anhydrous sodium carbonate and potassium chloride in sequence, and heat to 450℃~480℃ to initially melt the potassium cyanate, anhydrous sodium carbonate and potassium chloride to obtain the basic salt system; After the basic salt system is completely liquid, the temperature is raised to 600℃~620℃, and under mechanical stirring at a speed of 30~50r / min, the anhydrous potassium tetraborate, the anhydrous sodium molybdate and the anhydrous sodium metavanadate are added in sequence. Maintain a constant temperature of 600℃~620℃ and stir for 40~60 minutes, then stop stirring and lower the temperature of the composite co-infiltration salt bath to 510℃~530℃ for later use.

6. The process according to claim 1, characterized in that, During the process of raising the temperature of the composite co-infiltration salt bath to 580℃~600℃, a linear heating rate of 1.5℃ / min~2.5℃ / min is adopted.

7. The process according to claim 1, characterized in that, The continuous introduction of mixed gas into the composite co-infiltration salt bath is carried out through a top-inserted air blowing pipe. The inlet of the top-inserted air blowing pipe is located 20-30 cm below the surface of the composite co-infiltration salt bath, and the flow rate of the mixed gas introduced is stably controlled at 0.5-1.0 L / min.

8. The process according to claim 7, characterized in that, The top-insertion air tube is made of Inconel 600 corrosion-resistant alloy.

9. The process according to claim 1, characterized in that, The specific operation of the oxidation cooling treatment is as follows: The corrugated roll workpiece is immersed in an oxide salt bath preheated to 380℃~420℃ and kept at that temperature for 20~40 minutes. The components and their mass fractions of the oxidizing salt bath are as follows: Sodium nitrate 45.0%-55.0%, potassium nitrate 45.0%-55.0%.

10. The process according to claim 1, characterized in that, The specific steps for cleaning and drying are as follows: The corrugated roll workpiece after oxidation and cooling treatment is naturally cooled in air to a surface temperature of 90℃~110℃; Immerse in a boiling water bath at 95℃~100℃ for 30~40 minutes; Rinse with deionized water, then dry with hot air at 80℃~100℃ and apply rust-preventive oil to the surface of the workpiece.