Ion nitriding deformation control method for ball body of titanium alloy ball valve

Abstract

According to the ion nitriding deformation control method for the titanium alloy ball valve ball body, the titanium alloy ball body blank is subjected to high-temperature annealing, so that the thermal stability of a titanium alloy structure before ion nitriding is improved, and deformation induced by recrystallization stress release in the nitriding process is reduced; medium-low temperature annealing is carried out after the titanium alloy ball body is machined and ground, deformation caused by machining stress in the nitriding process is reduced, and then the roundness and the surface roughness of the titanium alloy ball body are guaranteed through match grinding. After ion nitriding, a hardening effect is generated on the surface of the titanium alloy ball body, meanwhile, the titanium alloy ball body has high dimensional precision, the sealing performance of the titanium alloy ball valve is guaranteed, the technological process is relatively simple, the ball body of the surface-hardened ball valve is stable in size and resistant to abrasion and corrosion, and the service life of the ball valve is greatly prolonged.

Description

Technical Field

[0001] The invention relates to the technical field of titanium alloy material surface engineering, in particular to a method for controlling the ion nitriding deformation of a titanium alloy ball valve ball. Background Art

[0002] Titanium alloys offer advantages such as strong corrosion resistance, low density, and high strength. Titanium alloy ball valves are increasingly used in industries such as shipbuilding, marine engineering, petroleum, chemical engineering, and metallurgy. Titanium alloy ball valves primarily consist of a valve body, ball, seat, and stem. The ball is a key component for ensuring a tight seal, and to ensure effective sealing and long service life, the roundness and surface hardness of the ball must be precisely controlled.

[0003] Titanium alloy has low hardness and poor wear resistance. The ball of the titanium alloy ball valve is very prone to wear and sticking during the opening and closing process of the valve, resulting in a decrease in the sealing performance of the titanium alloy ball valve.

[0004] Currently, common technologies for surface hardening modification of titanium alloys include micro-arc oxidation, thermal spraying, and physical vapor deposition. However, due to difficulties in controlling dimensional accuracy or insufficient coating adhesion, these methods are unable to meet the requirements of titanium alloy transmission components for high-precision, high-load, and high-impact applications. Plasma nitriding technology utilizes the glow discharge effect to introduce appropriate gases under vacuum electric field conditions. The workpiece surface is modified through a plasma process. After the process gas is ionized, nitrogen ions are accelerated to bombard the workpiece substrate surface under the action of the electric field, achieving diffusion of nitrogen ions into the workpiece substrate. A nitriding hardened layer composed of Ti-N solid solution, TiN, Ti2N, etc. is prepared. The layer penetrates into the surface of the substrate up to hundreds of microns, significantly improving the surface hardness while also having good toughness. It can achieve high hardness, wear resistance, corrosion resistance, and other functions. Compared with steel, titanium alloys are difficult to nitride. The surface passivation film has strong adhesion to the substrate, and the crystal structure of titanium is tightly arranged, making it difficult for nitrogen to diffuse. The ion nitriding temperature of titanium alloy is as high as 750-850℃ and the time is 10-30h. The workpiece is severely deformed and the subsequent grinding depth increases, which leads to a decrease in the surface hardness of the titanium alloy sphere. Therefore, the ion nitriding of the titanium alloy ball valve sphere needs to control deformation to ensure dimensional accuracy.

[0005] Invention patent CN109609894A discloses an annealing process to reduce deformation during ion nitriding of thin-walled titanium alloy annular parts. Prior to ion nitriding, the titanium alloy parts undergo a high-temperature annealing treatment at 900±50°C, which is then maintained at this high temperature for 2 to 8 hours. The parts are then cooled to below 200°C at a rate of 30 to 80°C / min before being removed from the furnace. This process reduces deformation by 50% compared to conventional processes. This method, which primarily targets thin-walled titanium alloy workpieces and involves only one high-temperature annealing step, is not suitable for titanium alloy ball valves requiring high dimensional accuracy.

[0006] Invention patent CN115584456B discloses a method for hardening the surface of titanium and titanium alloy ball valves. The titanium raw material is titanium or titanium alloy, with grades TA2 or F-2, and grades TA10, F-12, or TA31. The ball valve sphere has a diameter range of 40 to 240 mm and a diameter range of DN25 to DN200. The titanium raw material is annealed at 750 to 850°C for 1 to 3 hours and rough-machined to obtain a ball blank. The ball blank is then stress-annealed at 550 to 600°C for 1 to 3 hours and then machined to the final size to obtain the ball valve. The ball valve surface is polished and stabilized at -50 to -70°C for 1 to 2 hours. The stabilized ball valve sphere is then ion-nitrided. The roundness of the ball valve ball after nitriding is ≤0.02mm, the depth of the formed nitriding layer is ≥0.10mm, and the surface hardness of the nitriding layer is 500HV~1000HV. This method has complicated procedures and requires special low-temperature treatment. The applicable material grades are limited and the applicable specifications are small.

[0007] In summary, the existing technology usually performs fine machining and grinding on the titanium alloy ball valve sphere after ion nitriding, and performs secondary ion nitriding after the size reaches the requirement. This method requires two ion nitridings, and the size control effect is not significantly improved. In addition, the surface hardness increases after the first ion nitriding, and the difficulty of fine machining and grinding increases, resulting in a long processing cycle of the titanium alloy sphere, making it difficult to ensure the size accuracy and surface hardness of the titanium alloy ball valve sphere. Summary of the Invention

[0008] In view of this, the present invention aims to propose a method for controlling the deformation of a titanium alloy ball valve sphere during ion nitriding to solve the problem in the prior art that it is difficult to ensure the dimensional accuracy and surface hardness of the titanium alloy ball valve sphere during processing.

[0009] To achieve the above object, the technical solution of the present invention is achieved as follows:

[0010] A method for controlling deformation of a titanium alloy ball valve ball during ion nitriding, comprising the following steps:

[0011] S1: Before machining the part, the titanium alloy sphere blank is subjected to high temperature annealing treatment and then enters S2;

[0012] S2: machining the titanium alloy sphere blank to a set size to obtain a titanium alloy sphere, and then grinding the surface of the titanium alloy sphere, and then entering S3;

[0013] S3: performing a medium-low temperature annealing treatment on the titanium alloy sphere, and then proceeding to S4;

[0014] S4: applying abrasives on the matching valve seat or the tooling valve seat, grinding the surface of the titanium alloy sphere, and then proceeding to S5;

[0015] S5: performing ion nitriding treatment on the titanium alloy sphere to obtain a finished titanium alloy sphere.

[0016] Furthermore, in step S1, the high temperature annealing treatment temperature is 890-970°C, and the temperature is kept at this temperature for 1-4 hours, and then the product is taken out of the furnace and air-cooled.

[0017] Furthermore, in step S1, an anti-oxidation coating is applied on the surface of the titanium alloy sphere blank.

[0018] Furthermore, in step S2, after the surface of the titanium alloy sphere is ground, the spherical roundness is ≤0.02 mm.

[0019] Furthermore, in step S3, the medium-low temperature annealing treatment temperature is 450-750° C., and the temperature is kept at this temperature for 1-4 hours, and then cooled with the furnace.

[0020] Furthermore, in step S3, the medium and low temperature annealing treatment is carried out in a vacuum furnace, and the ultimate vacuum pressure of the vacuum furnace is ≤6.7×10 -3 Pa.

[0021] Furthermore, in step S4, the abrasive is 320-2000 mesh SiC or diamond grinding paste.

[0022] Furthermore, in step S5, the titanium alloy sphere is subjected to ion nitriding treatment using an ion nitriding furnace, the nitriding medium is NH3, the purity of NH3 is 99.99%, the nitriding temperature is 700-830°C, the holding time is 10-25h, and the sphere is removed from the furnace after cooling to a temperature of <100°C.

[0023] Furthermore, the size of the titanium alloy sphere is 1" to 10".

[0024] Compared with the prior art, the titanium alloy ball valve ion nitriding deformation control method of the present invention has the following advantages:

[0025] By performing high-temperature annealing on the titanium alloy sphere blank, the thermal stability of the titanium alloy structure before ion nitriding is improved, and the deformation induced by stress released by recrystallization during the nitriding process is reduced; after machining and grinding the titanium alloy sphere, medium and low temperature annealing is performed to reduce the deformation caused by machining stress during the nitriding process, and then the roundness and surface roughness of the titanium alloy sphere are ensured by matching grinding; after ion nitriding, the surface of the titanium alloy sphere produces a hardening effect and has a high dimensional accuracy, which ensures the sealing performance of the titanium alloy ball valve. The process flow of this application is relatively simple, and the ball valve ball after surface hardening is stable in size, wear-resistant and corrosion-resistant, which greatly extends its service life. DETAILED DESCRIPTION

[0026] To make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below with reference to the embodiments. It should be understood that the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. The specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention. It should be noted that, unless there is a conflict, the features in the embodiments and embodiments of the present invention may be combined with each other.

[0027] Example 1

[0028] This embodiment provides a method for controlling deformation of a titanium alloy ball valve during ion nitriding, comprising the following steps:

[0029] S1: Before machining the parts, the titanium alloy sphere blank is subjected to high-temperature annealing treatment. The titanium alloy sphere blank is a titanium alloy bar or a titanium alloy forging. The high-temperature annealing temperature is 890-970°C. The blank is kept at this temperature for 1-4 hours and then air-cooled. The high-temperature annealing process is carried out in a resistance furnace. Anti-oxidation coating is applied to the surface of the titanium alloy sphere blank, and then enters S2;

[0030] There are residual stresses in the titanium alloy blank during the preparation process. High-temperature annealing homogenizes the structure through atomic diffusion, reduces internal stress concentration, stabilizes the structure of the titanium alloy, and avoids large deformation of the titanium alloy sphere due to recrystallization during the ion nitriding process; applying anti-oxidation coating on the surface of the blank and combining it with resistance furnace annealing can achieve double protection. Titanium alloy easily reacts with oxygen at high temperature to form an oxide layer, which affects the subsequent machining accuracy and the bonding strength of the nitriding layer. The anti-oxidation coating forms a physical barrier to reduce the diffusion of oxygen atoms and avoid the formation of loose oxide scale on the surface; an oxide layer that is too deep will lead to an increase in the amount removed during machining, affecting the final size of the sphere. Coating protection can control the thickness of the oxide layer and reduce the amount of subsequent grinding processing.

[0031] Preferably, the glass coating is suitable for high temperature, strong acid, strong alkali and other environments, and can form a high-strength anti-oxidation protective layer on the surface of the titanium alloy to play an anti-oxidation role;

[0032] Ceramic coatings such as silicon carbide, silicon nitride, aluminum oxide, and zirconium oxide are single ceramic coatings and are a form of anti-oxidation coating on titanium alloy surfaces. They can be prepared by chemical vapor deposition, physical vapor deposition, and other methods. They have the advantages of high hardness, strong corrosion resistance, and long service life, and can provide anti-oxidation protection for titanium alloys at high temperatures.

[0033] S2: machining the titanium alloy sphere blank to a set size to obtain a titanium alloy sphere, and then grinding the surface of the titanium alloy sphere. After the surface grinding, the spherical roundness of the titanium alloy sphere is ≤0.02 mm, and then proceeding to S3;

[0034] Machining and grinding can eliminate machining marks and local stress concentration on the surface of the blank, making stress release more uniform during subsequent low-temperature annealing and reducing deformation caused by uneven initial stress. A smooth surface with satisfactory roundness can make nitrogen ion bombardment more uniform during ion nitriding, avoiding inconsistent nitriding layer thickness due to surface unevenness, which in turn affects hardness uniformity.

[0035] S3: The titanium alloy sphere is subjected to a medium-low temperature annealing treatment at a temperature of 450-750°C, kept at this temperature for 1-4 hours, and then cooled with the furnace. The medium-low temperature annealing treatment is carried out in a vacuum furnace with an ultimate vacuum pressure of ≤6.7×10 -3 Pa, enter S4;

[0036] The temperature range of 450-750℃ can activate the diffusion capacity of titanium alloy atoms, so that the residual stress generated by machining (such as turning and grinding) can be relaxed through lattice slip and dislocation movement. Keeping the temperature for 1-4 hours ensures that the stress is fully released and avoids deformation of the sphere due to stress concentration during nitriding.

[0037] The vacuum furnace can isolate oxygen, prevent oxidation of the titanium alloy surface, and avoid the formation of a brittle oxide layer on the surface during nitriding, thereby ensuring the bonding strength between the subsequent nitriding layer and the substrate. Slow cooling with the furnace can reduce the temperature difference between the inside and outside of the sphere, avoid the generation of new thermal stress during the cooling process, and further consolidate the annealing stress relief effect.

[0038] S4: Apply abrasives on the matching valve seat or the tooling valve seat, and grind the surface of the titanium alloy sphere. The matching valve seat is made of titanium alloy, and the tooling valve seat is made of titanium alloy or tool steel. The abrasive is 320-2000 mesh SiC or diamond grinding paste, and then proceed to S5;

[0039] Titanium alloy material consistent with the actual valve is used as the matching valve seat. The grinding process can directly simulate the assembly state of the sphere and the valve seat, ensuring that the curvature of the ground sphere is completely consistent with the valve seat, avoiding assembly errors caused by material differences, and ensuring valve sealing accuracy from the source.

[0040] S5: performing ion nitriding treatment on the titanium alloy sphere using an ion nitriding furnace, wherein the nitriding medium is NH3, the purity of NH3 is 99.99%, the nitriding temperature is 700-830°C, the holding time is 10-25h, and the sphere is cooled to a temperature less than 100°C before being taken out of the furnace to obtain a finished titanium alloy sphere;

[0041] The purity of NH3 reaches 99.99%, which can prevent impurity gases such as O2 and H2O from causing oxidation on the surface of titanium alloy, ensuring that only N +The chemical reaction between ions and Ti matrix forms a pure hardened layer mainly composed of TiN and Ti2N, avoiding contamination by impurities such as Fe and C that reduce the hardness; and the N generated by the ionization of high-purity NH3 in the electric field + The high ion concentration makes the energy more concentrated when bombarding the titanium alloy surface, accelerating the diffusion of nitrogen atoms into the matrix and improving the growth rate of the nitriding layer; cooling to <100°C with the furnace can avoid thermal stress cracks caused by the large temperature difference between the surface and the core of the sphere, ensuring the surface roundness accuracy of the sphere after nitriding.

[0042] The process of this embodiment has wide applicability and can be applied to titanium alloy materials of various specifications such as TA2, F-2, TC4, F-5, TA31, and the size of the titanium alloy sphere is 1"-10", which can meet the processing requirements of spheres of different specifications.

[0043] Example 2

[0044] This embodiment provides a method for controlling deformation of a 4"-300Lb TC4 titanium alloy ball valve during ion nitriding, comprising the following steps:

[0045] A1: Before machining the parts, the titanium alloy sphere blank is subjected to a high-temperature annealing treatment at 920±5℃, kept at this temperature for 1.5 hours, and then air-cooled. The high-temperature annealing process is carried out in a resistance furnace, and an anti-oxidation coating is applied to the surface of the titanium alloy sphere blank before entering A2;

[0046] A2: machining the titanium alloy sphere blank to a set size to obtain a titanium alloy sphere, and then grinding the surface of the titanium alloy sphere, and then proceeding to A3;

[0047] A3: placing the titanium alloy sphere in a vacuum furnace, performing a medium-low temperature annealing treatment at 650° C., maintaining the temperature at this temperature for 1.5 hours, and then cooling with the furnace before proceeding to A4;

[0048] A4: Apply abrasive to the matching valve seat or tooling valve seat, and grind the surface of the titanium alloy sphere. The abrasive is 320-2000 mesh SiC or diamond grinding paste. After the surface of the titanium alloy sphere is ground, the spherical roundness is 0.0128, and then proceed to A5;

[0049] S5: The titanium alloy sphere is subjected to ion nitriding treatment in an ion nitriding furnace, wherein the nitriding medium is NH3, the nitriding temperature is 820°C, the holding time is 18 hours, and the furnace is cooled to below 80°C and then taken out of the furnace to obtain a finished titanium alloy sphere.

[0050] After ion nitriding, testing revealed that the titanium alloy sphere had a spherical roundness of 0.0147 and a roundness deformation of 0.0019mm, significantly lower than conventional processes and capable of meeting high-precision sealing requirements. Testing also revealed that the surface hardness of the furnace sample was 1156HV0.1, and the nitrided layer depth was 129µm, giving the titanium alloy sphere excellent wear and corrosion resistance.

[0051] Example 3

[0052] This embodiment provides a deformation control method for ion nitriding of a 4"-300Lb TA2 titanium alloy ball valve, specifically as follows:

[0053] B1: The titanium alloy sphere blank is subjected to a high-temperature annealing treatment at 815±5°C, kept at this temperature for 1.5 hours, and then air-cooled. The high-temperature annealing process is carried out in a resistance furnace, and an anti-oxidation coating is applied to the surface of the titanium alloy sphere blank before entering B2;

[0054] B2: machining the titanium alloy sphere blank to a set size to obtain a titanium alloy sphere, and then grinding the surface of the titanium alloy sphere, and then proceeding to B3;

[0055] B3: placing the titanium alloy sphere in a vacuum furnace, performing a medium-low temperature annealing treatment at 600° C., maintaining the temperature at this temperature for 1.5 hours, and then cooling with the furnace, and proceeding to B4;

[0056] B4: Apply abrasive to the matching valve seat or tooling valve seat, and grind the surface of the titanium alloy sphere. The abrasive is 320-2000 mesh SiC or diamond grinding paste. After the surface of the titanium alloy sphere is ground, the spherical roundness is 0.0078, and then proceed to B5;

[0057] S5: The titanium alloy sphere is subjected to ion nitriding treatment in an ion nitriding furnace, wherein the nitriding medium is NH3, the nitriding temperature is 750°C, the holding time is 12 hours, and the furnace is cooled to below 80°C and then taken out of the furnace to obtain a finished titanium alloy sphere.

[0058] After ion nitriding, the titanium alloy sphere was tested to have a spherical roundness of 0.0187 and a roundness deformation of 0.0109 mm. The surface hardness of the furnace sample was tested to be 1109 HV0.1, and the depth of the nitrided layer was 81 µm.

[0059] The ion nitriding temperature is 750°C, which is lower than that of Example 2. While ensuring a certain surface hardness (1109HV0.1) and a nitriding layer depth (81μm), it can reduce energy consumption and reduce the adverse effects of high temperature on the performance of the sphere material.

[0060] For TA2 titanium alloy materials, by customizing process parameters, the titanium alloy spheres can still maintain good dimensional accuracy and surface properties after ion nitriding, broadening the applicability to different titanium alloy grades.

[0061] Comparative Example 1

[0062] This comparative example provides a method for controlling deformation of a 4"-300Lb TC4 titanium alloy ball valve during ion nitriding, as follows:

[0063] D1: Machining the titanium alloy sphere blank to a set size to obtain a titanium alloy sphere, and then grinding the surface of the titanium alloy sphere; applying abrasive on the matching valve seat or the tooling valve seat, and grinding the surface of the titanium alloy sphere. The abrasive is 320-2000 mesh SiC or diamond grinding paste. After the surface of the titanium alloy sphere is ground, the spherical roundness is 0.0190, and then proceeding to D2;

[0064] D2: The titanium alloy sphere is subjected to ion nitriding treatment in an ion nitriding furnace, the nitriding medium is NH3, the nitriding temperature is 820°C, the holding time is 18 hours, and the furnace is cooled to below 80°C and then taken out of the furnace to obtain a finished titanium alloy sphere.

[0065] After ion nitriding, the titanium alloy sphere was tested to have a spherical roundness of 0.0922 and a roundness deformation of 0.0732 mm. The surface hardness of the furnace sample was tested to be 1109 HV0.1, and the depth of the nitrided layer was 81 µm.

[0066] Compared with Examples 1-3, Comparative Example 1 did not undergo high-temperature annealing and medium-low temperature annealing treatment. The spherical roundness after ion nitriding was 0.0922 mm, and the roundness deformation was as high as 0.0732 mm, which far exceeded the deformation of Examples 1-3 and could not meet the dimensional accuracy requirements of the titanium alloy ball valve ball, proving that annealing treatment is a key step in controlling deformation.

[0067] Moreover, the lack of an annealing process fails to eliminate the structural stress and machining stress of the titanium alloy sphere blank, resulting in significant stress release during ion nitriding and increased deformation of the sphere. This shows that ignoring annealing treatment in the existing technology will seriously affect product quality.

[0068] The method for controlling the deformation of titanium alloy ball valve spheres during ion nitriding described in this application improves the thermal stability of the titanium alloy structure before ion nitriding and reduces the deformation induced by stress released by recrystallization during the nitriding process by performing high-temperature annealing on the titanium alloy sphere blank; performs medium-low temperature annealing after machining and grinding the titanium alloy sphere, reducing the deformation caused by machining stress during the nitriding process, and then ensures the roundness and surface roughness of the titanium alloy sphere through grinding; after ion nitriding, the surface of the titanium alloy sphere produces a hardening effect, while having high dimensional accuracy, ensuring the sealing performance of the titanium alloy ball valve. Experiments have shown that the deformation after treatment can be reduced by 50% compared with conventional processes, which has great practical value. The process flow of this application is relatively simple, and the surface-hardened ball valve sphere has the characteristics of dimensional stability, wear resistance, and corrosion resistance, which extends its service life.

[0069] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be based on the scope defined by the claims.

Claims

1. A method for controlling deformation of a titanium alloy ball valve during ion nitriding, characterized in that: The following steps are involved: S1: Before machining the part, the titanium alloy sphere blank is subjected to high temperature annealing treatment and then enters S2; S2: machining the titanium alloy sphere blank to a set size to obtain a titanium alloy sphere, and then grinding the surface of the titanium alloy sphere, and then entering S3; S3: performing a medium-low temperature annealing treatment on the titanium alloy sphere, and then proceeding to S4; S4: applying abrasives on the matching valve seat or the tooling valve seat, grinding the surface of the titanium alloy sphere, and then proceeding to S5; S5: performing ion nitriding treatment on the titanium alloy sphere to obtain a finished titanium alloy sphere.

2. The method for controlling deformation of a titanium alloy ball valve ball during ion nitriding according to claim 1, characterized in that: In step S1, the high temperature annealing treatment temperature is 890-970°C, and the temperature is kept at this temperature for 1-4 hours, and then the steel is taken out of the furnace and air-cooled.

3. The method for controlling deformation of a titanium alloy ball valve ball during ion nitriding according to claim 1, characterized in that: In step S1, anti-oxidation coating is applied on the surface of the titanium alloy sphere blank.

4. The method for controlling deformation of a titanium alloy ball valve during ion nitriding according to claim 1, characterized in that: In step S2, the titanium alloy sphere is surface ground to a spherical roundness of ≤0.02 mm.

5. The method for controlling deformation of titanium alloy ball valve sphere during ion nitriding according to claim 1, characterized in that: In step S3, the medium-low temperature annealing treatment temperature is 450-750° C., and the temperature is kept at this temperature for 1-4 hours, and then cooled with the furnace.

6. The method for controlling deformation of a titanium alloy ball valve ball during ion nitriding according to claim 1, characterized in that: In step S3, the medium and low temperature annealing treatment is carried out in a vacuum furnace, and the ultimate vacuum pressure of the vacuum furnace is ≤6.7×10 -3 Pa.

7. The method for controlling deformation of a titanium alloy ball valve during ion nitriding according to claim 1, characterized in that: In step S4, the abrasive is 320-2000 mesh SiC or diamond grinding paste.

8. The method for controlling deformation of a titanium alloy ball valve during ion nitriding according to claim 1, characterized in that: In step S5, the titanium alloy sphere is ion nitrided in an ion nitriding furnace. The nitriding medium is NH3 with a purity of 99.99%. The nitriding temperature is 700-830°C and the holding time is 10-25h. The sphere is then taken out of the furnace after cooling to a temperature less than 100°C.

9. The method for controlling deformation of a titanium alloy ball valve during ion nitriding according to claim 1, characterized in that: The size of the titanium alloy sphere is 1" to 10".