A deep cooling method for increasing the service life of a hydraulic end valve box of a fracturing pump, a deep cooling system and the obtained product

Abstract

The present invention discloses a cryogenic method, a cryogenic system and a product obtained by improving the service life of the valve box at the hydraulic end of a fracturing pump, and belongs to the field of alloy technology. The present invention hoists a precipitation-hardened stainless steel valve box that has undergone fine processing into a cryogenic system; after sealing, dry ice is injected at a flow rate of 0.5~1.0 L / s, and after the injection of dry ice, the stop valve is closed and the pressure is maintained for 1~3 hours; finally, the valve box is disassembled and processed for storage. The present invention has a simple process, a small amount of cryogenic treatment materials, and a low cost; it can significantly reduce the content of inverted austenite produced by the "solid solution + aging" heat treatment, and improve the hardness and strength of the material with a thickness of about 1~3 cm in the inner cavity of the valve box; a large residual compressive stress is generated on the surface of the inner cavity of the valve box, while improving the roughness of the surface of the inner cavity of the valve box and reducing stress concentration; after the above-mentioned cryogenic treatment, the service life of the valve box fracturing operation is significantly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of alloys, and more specifically, relates to a cryogenic method and a cryogenic system for improving the service life of a hydraulic end valve box of a fracturing pump, and the obtained products. Background Art

[0002] Fracturing operation is the most technically demanding and most important part of the oil and gas production process. The hydraulic end valve box of the fracturing pump is an important device for injecting a large amount of fracturing medium (acidic liquid and proppant, etc.) into the oil and gas wells during oil and gas field operations to carry out geological stratum fracturing and acidizing operations and increase oil and gas production. The hydraulic end valve box of the fracturing pump must meet the performance requirements of high pressure resistance, corrosion resistance, long life and reliable sealing during the fracturing operation. In the 21st century, the development of unconventional oil and gas has made breakthrough progress, and fracturing technology has become the focus of attention, playing a vital role in the development of unconventional oil and gas such as shale gas and tight oil and gas. In this context, the fourth-generation valve box made of high-strength, high-toughness and corrosion-resistant precipitation-hardened stainless steel such as 17-4 PH and 15-5 PH has become the mainstream of the market.

[0003] The general manufacturing process of precipitation hardened stainless steel valve boxes is as follows: material collection, forging, annealing, rough machining, solution heat treatment, aging heat treatment, finishing, packaging and warehousing. Among them, the "solution + aging" heat treatment to obtain the "tempered martensite + dispersed precipitated ε-Cu" structure is the key to the high strength, high toughness and corrosion resistance of the valve box. However, long-term aging will cause the precipitation hardened stainless steel to undergo reverse transformation of martensite (α'-Fe) and produce reverse austenite (AR, γ-Fe). Reverse austenite is a soft phase, which will lead to a decrease in the strength and hardness of the valve box. During fracturing operations, cyclic stress preferentially nucleates and expands at the M / A interface, leading to early cracking of the valve box. In addition, the multiphase structure also reduces the corrosion resistance of the valve box. Therefore, the appearance of reverse austenite seriously restricts the service life of the precipitation hardened stainless steel valve box. The longer the aging time is, the more the content of reverse austenite in the valve box is, and the greater the impact on the service life of the valve box is. If the aging time is too short, ε-Cu cannot be obtained, which affects the hardness of the valve box.

[0004] Therefore, reducing the reverse austenite in the valve box material after "solid solution + aging" heat treatment is an effective means to increase the service life of the valve box. Low temperature treatment is a conventional method to increase the completeness of austenite transformation to martensite. The end temperature of martensite transformation of precipitation hardened stainless steel is low. Liquid nitrogen (-196 ℃) or dry ice (-78.5 ℃) is usually used in industry to reduce the reverse austenite content in the material.

[0005] After searching, CN108251759A discloses a reverse austenite toughened martensitic stainless steel and a manufacturing method thereof, wherein the components thereof are as follows by mass percentage: C 0.001~0.40, Cr 11.0~22.0, Ni 1.0~12.0, Mo 0~3.0, Ti0.07~1.50, Al 0.05~1.50, Si 0.10~2.0, and the remainder is Fe and inevitable impurities. The manufacturing steps of the stainless steel are: firstly smelting in an electric furnace, then refining in an AOD furnace and casting into a steel ingot; hot rolling or hot forging treatment; austenitizing treatment and air cooling quenching; cryogenic treatment; tempering, the cryogenic treatment temperature is -196~-50℃, and the holding time is 10~70min. However, the purpose of deep cryogenic treatment in this method is to completely transform the retained austenite into martensite and form a large number of fine carbides in the martensite, providing a large number of nucleation cores for the reversed austenite.

[0006] Patent CN110358983A discloses a precipitation hardening martensitic stainless steel and its preparation method. The chemical composition of the precipitation hardening martensitic stainless steel is as follows by weight percentage: C: 0.14-0.20%, Cr: 13.0-16.0%, Co: 12.0-15.0%, Mo: 4.5-5.5%, Ni: 0.5-2.0%, V: 0.40-0.60%, Mn≤0.5%, Si≤0.1%, P<0.01%, S<0.01%, O<0.01%, N≤0.1%, and the balance is iron. The preparation method of the precipitation hardening martensitic stainless steel is: batching, smelting, casting, forging, and heat treatment. The invention optimizes the content of Ni and N elements in the material, controls the content of reverse transformed austenite through heat treatment process, and reduces the amount of parasitism to less than 3% through liquid nitrogen cryogenic treatment.

[0007] In the above patents, the reversed austenite is treated by liquid nitrogen or dry ice, but the purpose is not to reduce the content of reversed austenite. Summary of the invention

[0008] 1. Problem to be solved

[0009] Aiming at the problem that the existing valve box structure has a high content of reverse austenite, resulting in low surface hardness of the inner cavity of the valve box and reduced service life, the first object of the present invention is to provide a deep cooling method for improving the service life of the hydraulic end valve box of a fracturing pump;

[0010] The second object of the present invention is to provide a cryogenic system for implementing the method;

[0011] A third object of the present invention is to provide a valve box made using the above method and system.

[0012] 2. Technical solution

[0013] In order to solve the above problems, the technical solution adopted by the present invention is as follows:

[0014] A first aspect of the present invention provides a deep cooling method for increasing the service life of a hydraulic end valve box of a fracturing pump. The method injects dry ice into a closed precipitation-hardened stainless steel valve box at a flow rate of 0.5 to 1.0 L / s, so that the inner cavity of the valve box is maintained at a temperature of -75°C and a pressure of 6 to 8 atmospheres for 1 to 3 hours.

[0015] The inner cavity of the valve box treated by the deep freezing method forms a hardened layer and a stress-strain layer. The hardened layer is obtained after the valve box is deep-cryogenically treated, and its structure consists of tempered martensite, ε-Cu and inverted austenite not exceeding 1wt%; the stress-strain layer is obtained by the gas pressure shock generated by the instantaneous gasification of dry ice, and is located at the outermost layer of the hardened layer, generating strain hardening and surface compressive stress.

[0016] Furthermore, the thickness of the hardened layer is 1-3 cm, and the thickness of the stress-strain layer is 80-120 μm.

[0017] Furthermore, the microhardness of the inner surface of the valve box at a depth of 1 to 3 mm is 380 to 400 HV, and the surface compressive stress of the stress-strain layer is 50 to 100 MPa.

[0018] Furthermore, in the hardened layer, the content of tempered martensite is 95wt%~96wt%, and the content of ε-Cu is 3wt%~4wt%.

[0019] The above-mentioned valve box refers to a valve box that has been processed through the steps of "taking materials, forging, annealing, rough machining, solution heat treatment, aging heat treatment, and finishing", which specifically includes the following steps:

[0020] S1. Material preparation: The materials are prepared according to the target stainless steel composition. The composition of the stainless steel is as follows by weight: C: 0.03%~0.08%, Si: 0.50%~1.0%, Mn: 0.70%~1.0%, Cr: 14.5%~18.0%, Ni: 3.0%~5.0%, Cu: 3.0%~5.0%, Nb: 0.20%~0.60%, P≤0.025%, S≤0.030%, and the balance is Fe.

[0021] S2. Forging: Forging valve box blank, the starting forging temperature of the steel ingot is 1180±10℃, the final forging temperature is ≥900℃, and the valve box forging blank is forged by three piers and three draws, and the forging ratio is 3~5;

[0022] S3, Annealing: Annealing treatment of valve box forgings. The forgings are heated to 610±10℃ in a box furnace and then annealed for 30~40 hours. After the insulation is completed, they are cooled with the furnace.

[0023] S4, Rough machining: Rough machining of annealed parts, rough machining of the shape and inner cavity of the valve box according to the size of the valve box, and the roughness of the rough machining surface is controlled within Ra25;

[0024] S5. Solution heat treatment: Solution treatment is performed on the rough machined parts. The workpiece is heated to 1050±10℃ in a box furnace and kept at this temperature for 48~72 hours. After the end of the heat preservation, the workpiece is water-cooled to room temperature.

[0025] S6. Aging heat treatment: Aging treatment is performed on the solution treated parts. The solution treated parts are placed in a box furnace, heated to 550 °C, kept at this temperature for 36-48 hours, and then air-cooled to room temperature.

[0026] S7, Finishing: Finish the aging parts, and further finish the shape and inner cavity of the valve box according to the size of the valve box. The surface roughness of the finishing is controlled within Ra3.2;

[0027] S8, cryogenic treatment: inject dry ice into the precipitation-hardened stainless steel valve box obtained by fine processing in step S7 at a flow rate of 0.5-1.0 L / s, so that the inner cavity of the valve box is kept at a temperature below -75°C and a pressure of 6-8 atmospheres for 1-3 hours.

[0028] The second aspect of the present invention is to provide a cryogenic system for implementing the method, the cryogenic system comprising an insulation device, a sealing device and a dry ice injection device, the valve box is placed in the insulation device, the valve box is connected to the dry ice injection device, and the sealing of the valve box is achieved by the sealing device.

[0029] The heat preservation device consists of a heat preservation barrel and a pit. The heat preservation barrel is placed in the pit, and the heat preservation barrel is used for placing the valve box after fine processing.

[0030] Furthermore, the insulation barrel is composed of a stainless steel inner liner, a quartz wool insulation blanket, a stainless steel outer liner and an insulation sealing cover. The quartz wool insulation blanket is arranged in the cavity between the stainless steel inner liner and the stainless steel outer liner, and the stainless steel inner liner and the outer liner are provided with insulation sealing covers, so that the insulation barrel forms a sealed space.

[0031] Furthermore, a heat-insulating sealing cover is provided on the pit.

[0032] The heat-insulating barrel and the heat-insulating sealing cover on the pit are composed of a stainless steel inner liner, a stainless steel outer liner and a quartz wool heat-insulating blanket arranged between the stainless steel inner liner and the stainless steel outer liner.

[0033] The sealing device comprises a sealing plug and a sealing rubber gasket respectively opened on the front and top of the valve box, and a stainless steel hose for dry ice injection is welded on the sealing plug.

[0034] The dry ice injection device includes a dry ice injection machine, which is connected to a valve box through a stainless steel hose. A stop valve is provided between the dry ice injection machine and the valve box to control the amount of dry ice injection, and also includes a pressure gauge and a flow meter.

[0035] The steps of treating the valve box using the cryogenic system described above include:

[0036] a. Encapsulate the valve box: hoist the precision-processed precipitation-hardened stainless steel valve box into the insulation barrel in the pit, seal the hole end of the valve box, then install the dry ice injection stainless steel hose in the middle hole on the top of the valve box and seal it, and finally close the insulation sealing cover of the insulation barrel and the pit;

[0037] b. Cooling and pressure maintenance of the valve box: Use a dry ice injection machine to inject dry ice into the valve box at a flow rate of 0.5~1.0 L / s. Inject dry ice until the pressure in the valve box reaches 6~8 atmospheres, close the stop valve and maintain the pressure for 1~3 hours;

[0038] c. Disassemble the valve box and process it for storage: open the sealing device, disassemble the dry ice injection device, lift out the valve box, spray paint, dry, and package it for storage.

[0039] Furthermore, in step b, the smaller the diameter of the dry ice and the smaller the aspect ratio, the better the gasification cooling effect, but the more expensive it is; considering the gasification cooling effect and economic benefits, the dry ice injected in the present invention is a solid particle with a diameter of about 1.5 mm and an aspect ratio of about 1 to 2.

[0040] The third object of the present invention is to provide a valve box made by using the above method and cryogenic system, wherein the inner cavity of the valve box obtained by treatment forms a hardened layer and a stress-strain layer, wherein the hardened layer is obtained by deep-cryogenic treatment of the valve box, and its structure consists of tempered martensite, ε-Cu and not more than 1wt% of inverted austenite; the stress-strain layer is obtained by the instantaneous air pressure shock generated by the instantaneous gasification of dry ice, and is located at the outermost layer of the hardened layer, generating strain hardening and surface compressive stress.

[0041] Furthermore, the thickness of the hardened layer is 1-3 cm, and the thickness of the stress-strain layer is 80-120 μm.

[0042] Furthermore, the microhardness of the inner surface of the valve box at a depth of 1 to 3 mm is 380 to 400 HV, and the surface compressive stress is 50 to 100 MPa.

[0043] Furthermore, in the hardened layer, the content of tempered martensite is 95wt%~96wt%, and the content of ε-Cu is 3wt%~4wt%.

[0044] Furthermore, it also includes a matrix, which is a valve box material after fine processing, and its structure consists of tempered martensite, ε-Cu and reverse austenite with a content higher than 1wt%, wherein the content of ε-Cu is 3wt%~4wt%, the content of reverse austenite is greater than 1wt%, and the rest is tempered martensite.

[0045] Furthermore, the valve box uses precipitation hardened stainless steel as a base material, and the components of the precipitation hardened stainless steel are as follows by mass percentage: C: 0.03%~0.08%, Si: 0.50%~1.0%, Mn: 0.70%~1.0%, Cr: 14.5%~18.0%, Ni: 3.0%~5.0%, Cu: 3.0%~5.0%, Nb: 0.20%~0.60%, P≤0.025%, S≤0.030%, and the balance is Fe.

[0046] It is worth noting that the present application is for cryogenic treatment of valve boxes with a volume exceeding that of a single valve box weighing 2 to 4 tons.

[0047] The present invention designs a deep cooling method and a deep cooling system for a valve box with a precision-processed precipitation-hardened stainless steel valve box. Through this method, the inverted austenite in the material with a thickness of 1 to 3 cm in the inner cavity of the valve box is significantly reduced; after the dry ice is injected, the volume expands 600 to 800 times in an instant, and a huge pressure is generated in the inner cavity, so that the inner cavity produces surface compressive stress, which increases the fatigue life of the valve box; the dry ice injection device realizes the injection of dry ice, and cooperates with the sealing system to achieve long-term pressure maintenance in the inner cavity, thereby increasing the service life of the valve box at the hydraulic end of the fracturing pump.

[0048] 3. Beneficial effects

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] (1) The present invention can significantly reduce the content of reversed austenite produced by the "solid solution + aging" heat treatment by injecting dry ice into the sealed valve box for cryogenic treatment, while improving the hardness, strength and tensile stress of the material with a depth of 1 to 3 cm in the valve box cavity;

[0051] (2) The present invention utilizes dry ice to sublimate and gasify in a sealed valve box, causing the volume to expand by 600 to 800 times, thereby generating a large residual compressive stress on the inner cavity surface, significantly improving the fatigue performance of the inner cavity surface and the service life of the valve box fracturing operation;

[0052] (3) The solid dry ice of the present invention sublimates within a few thousandths of a second, and its volume expands 600 to 800 times instantly, which produces a huge impact on the inner surface of the valve box. Combined with the volume contraction and material embrittlement caused by the sudden cooling on the protrusions, it can further improve the roughness of the inner surface, reduce stress concentration, increase the service life of the valve box, further remove surface dirt, and improve cleanliness;

[0053] (4) The present invention can improve the performance of the entire valve box by performing cryogenic treatment on the material with a depth of 1 to 3 cm in thickness in the inner cavity of the valve box, using a small amount of cryogenic material and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The technical solution of the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments, but it should be understood that these drawings are designed only for explanation purposes and are not intended to limit the scope of the present invention. In addition, unless otherwise specified, these drawings are intended only to conceptually illustrate the structural configurations described herein and are not necessarily drawn to scale.

[0055] Figure 1 This is the front view of the 5-cylinder (cavity) valve box of the hydraulic end of the fracturing pump;

[0056] Figure 2 This is a top view of the 5-cylinder (cavity) valve box of the hydraulic end of the fracturing pump;

[0057] Figure 3 It is the cross-sectional view of the A surface in the top view of the 5-cylinder (cavity) valve box of the hydraulic end of the fracturing pump;

[0058] Figure 4 This is the left view of the 5-cylinder (cavity) valve box of the hydraulic end of the fracturing pump;

[0059] Figure 5 This is the finite element stress analysis distribution diagram of the inner cavity section of the 5-cylinder (cavity) valve box of the hydraulic end of the fracturing pump;

[0060] Figure 6 This is a schematic diagram of the cryogenic system structure.

[0061] In the figure: 1. pit; 2. insulation barrel; 21. stainless steel outer liner; 22. stainless steel inner liner; 23. quartz wool insulation blanket; 3. valve box; 4. sealing plug; 5. sealing rubber gasket; 6. stainless steel hose; 7. pressure gauge; 8. flow meter; 9. stop valve; 10. dry ice injection machine; 11. insulation sealing cover;

[0062] Figure 7 This is a flow chart of cryogenic treatment operation;

[0063] Figure 8 This is the Rietveld phase analysis fitting diagram of the XRD test results of the inner cavity surface sample after the valve box is finely processed in Example 1;

[0064] Fig. 9 This is the Rietveld phase analysis fitting diagram of the XRD test results of the inner cavity surface sample of the valve box after deep cryogenic treatment in Example 1;

[0065] Fig.10 This is the Rietveld phase analysis fitting diagram of the XRD test results of the inner cavity surface sample after the valve box is finely processed in Example 2;

[0066] Fig.11 This is the Rietveld phase analysis fitting diagram of the XRD test results of the inner cavity surface sample of the valve box after deep cryogenic treatment in Example 2. DETAILED DESCRIPTION

[0067] The following detailed description of exemplary embodiments of the present invention refers to the accompanying drawings, which form a part of the description, and in which exemplary embodiments of the present invention that can be implemented are shown as examples. Although these exemplary embodiments are described in sufficient detail to enable those skilled in the art to implement the present invention, it should be understood that other embodiments can be implemented and various changes can be made to the present invention without departing from the spirit and scope of the present invention. The following more detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but is only for the purpose of illustrating and not limiting the description of the characteristics and features of the present invention, so as to propose the best mode for performing the present invention and to enable those skilled in the art to implement the present invention. Therefore, the scope of the present invention is limited only by the appended claims.

[0068] In order to improve the service life of the valve box 3, the present invention provides a deep cooling system and method for improving the service life of the valve box 3 of the hydraulic end of the fracturing pump. The system can achieve: (i) a significant reduction in the content of inverted austenite in the material with a thickness of about 1 to 3 cm in the inner cavity of the valve box 3, thereby improving the strength and hardness of the inner cavity of the valve box 3; (ii) generating surface compressive stress in the inner cavity of the valve box 3 to improve the fatigue life of the valve box 3; (iii) improving the smoothness and roughness of the inner cavity, reducing stress concentration and stress corrosion fatigue. The deep cooling method provided by the present invention reduces the use of cooling materials, significantly improves the service life of the valve box 3, and has the characteristics of easy operation and low cost.

[0069] a) Valve box

[0070] Long-term oil and gas field operation practice and a large number of studies have shown that during fracturing operations, the inner cavity of the valve box 3 at the hydraulic end of the fracturing pump is subjected to greater stress, especially the cross hole position, which is the weakest part of the valve box 3 and suffers the greatest stress. Most of the early failures of the valve box 3 are caused by cracks at the cross hole position. Figure 1-4 is a schematic diagram of the structure of the valve box 3, Figure 5 This is the finite element stress analysis distribution diagram of the inner cavity section of the 5-cylinder valve box 3. Figure 5 It is shown that during the fracturing operation, the stress at the "cross hole" position in the inner cavity is the largest, reaching a maximum of 800.67 MPa. Fatigue cracks are very likely to occur here, causing early failure and greatly shortening the service life of the valve box 3.

[0071] Therefore, improving the performance of the valve box 3, especially the performance at the cross hole position, is an effective way to increase the life of the valve box 3.

[0072] The valve box in the embodiment refers to a valve box that has been processed by the steps of "taking materials, forging, annealing, rough machining, solution heat treatment, aging heat treatment, and finishing", which specifically includes the steps:

[0073] S1. Material preparation: The materials are prepared according to the target stainless steel composition. The composition of the stainless steel is as follows by weight: C: 0.03%~0.08%, Si: 0.50%~1.0%, Mn: 0.70%~1.0%, Cr: 14.5%~18.0%, Ni: 3.0%~5.0%, Cu: 3.0%~5.0%, Nb: 0.20%~0.60%, P≤0.025%, S≤0.030%, and the balance is Fe.

[0074] S2. Forging: Forging valve box blank, the starting forging temperature of the steel ingot is 1180±10℃, the final forging temperature is ≥ 900℃, and the valve box forging blank is forged by three piers and three draws, and the forging ratio is 3~5;

[0075] S3, Annealing: Annealing treatment of valve box forgings. The forgings are heated to 610±10℃ in a box furnace and then annealed for 36 hours. After the insulation is completed, they are cooled with the furnace.

[0076] S4, Rough machining: Rough machining of annealed parts, rough machining of the shape and inner cavity of the valve box according to the size of the valve box, and the roughness of the rough machining surface is controlled within Ra25;

[0077] S5. Solution heat treatment: Solution treatment is performed on the rough machined parts. The workpieces are heated to 1050±10℃ in a box furnace and kept at this temperature for 60 hours. After the end of the heat preservation, they are water-cooled to room temperature.

[0078] S6. Aging heat treatment: Aging treatment is performed on the solution treated parts. The solution treated parts are placed in a box furnace, heated to 550 °C, kept at this temperature for 44 h, and then air-cooled to room temperature;

[0079] S7. Finishing: Finish the aging parts, and further finish the shape and inner cavity of the valve box according to the size of the valve box. The surface roughness of the finishing is controlled within Ra3.2.

[0080] b) Deep cooling system

[0081] The cryogenic system used in the present invention comprises: a heat preservation device, a sealing device and a dry ice injection device. The valve box 3 is placed in the heat preservation device, the valve box 3 is connected to the dry ice injection device, and the sealing of the valve box 3 is achieved by the sealing device.

[0082] The insulation device is composed of an insulation barrel 2 with a diameter of 3-4 m and a depth of 2-2.5 m and a pit 1 with a diameter of 5-6 m and a depth of 6-8 m. The insulation barrel 2 is placed in the pit 1 and is used to place the finely processed valve box 3. The insulation barrel 2 is composed of a 3-5 mm stainless steel inner liner 22, a 10-12 mm quartz wool insulation blanket 23, and a 3-5 mm stainless steel outer liner 21; the quartz wool insulation blanket 23 is arranged in the cavity between the stainless steel inner liner 22 and the stainless steel outer liner 21, and the stainless steel inner liner 22 and the stainless steel outer liner 21 are provided with an insulation sealing cover 11, and the insulation barrel 2 forms a sealed space through the insulation sealing cover 11. The pit 1 is also provided with an insulation sealing cover. The insulation sealing cover 11 of the pit 1 and the insulation barrel 2 is composed of a 3-5 mm stainless steel inner liner 22, a 3-5 mm stainless steel outer liner 21, and a 10-12 mm quartz wool insulation blanket 23 arranged between the stainless steel inner liner 22 and the stainless steel outer liner 21.

[0083] The sealing device comprises stainless steel sealing plugs 4 respectively opened on the front and top of the valve box 3 , and the stainless steel sealing plug 4 is sealed by a sealing rubber gasket 5 , wherein a stainless steel hose 6 for dry ice injection is welded on the stainless steel sealing plug 4 .

[0084] The dry ice injection device includes a dry ice injection stainless steel hose 6, a pressure gauge 7, a flow meter 8, a stop valve 9 and a dry ice injection machine 10. The stainless steel hose 6 is connected to the dry ice injection machine 10. The stainless steel hose 6 is provided with a pressure gauge 7, a flow meter 8 and a stop valve 9. The pressure in the valve box 3 is monitored by the pressure gauge 7. When the pressure in the valve box reaches 6 to 8 atmospheres, the injection of solid dry ice is stopped.

[0085] The above structure ensures that the workpiece is cryogenically treated at a constant temperature and pressure for a long time, so that the valve box 3 can be kept in a low temperature state for a long time.

[0086] c) Deep freezing method

[0087] The transformation of austenite to martensite is temperature-variable and incomplete, so low-temperature treatment is a conventional method to increase the completeness of the transformation of austenite to martensite. In the prior art, liquid nitrogen and dry ice are generally used for cooling. However, on the one hand, the valve box 3 is large in size and the box wall is thick. Liquid nitrogen and dry ice will quickly vaporize during deep-cold treatment, and the temperature will rise rapidly, resulting in poor effect of reducing the austenite content of the valve box 3 as a whole; on the other hand, liquid nitrogen and dry ice are expensive, and the hydraulic end valve box 3 of the fracturing pump is large in size. A single valve box 3 weighs 2 to 4 tons. In order to ensure a low-temperature environment, deep-cold treatment requires continuous injection of liquid nitrogen and dry ice, which consumes a lot and significantly increases the manufacturing cost. Therefore, the technology of deep cooling using liquid nitrogen and dry ice has not been widely used in the production of valve boxes 3.

[0088] The present invention utilizes the airtightness of the valve box 3 and the heat preservation of the cryogenic system, and injects solid dry ice into the airtight precipitation hardened stainless steel valve box 3 at a flow rate of 0.5-1.0 L / s, so that the inner cavity of the valve box 3 is kept at a temperature below -75°C and a pressure of 6-8 atmospheres for 1-3 hours. The airtightness of the valve box 3 keeps the inner cavity of the valve box 3 at a low temperature, especially for the valve box 3 with a large volume and thick box wall. After the dry ice is injected, the low temperature conditions required for the reaction can be provided, the consumption of dry ice is reduced, and the inversion austenite is conducive to the transformation of the inversion austenite into martensite, so that the content of the inversion austenite is less than 1wt%.

[0089] Surprisingly, it was found that the inner surface of the valve box 3 obtained by this method also produced strain hardening and surface compressive stress. This is because the dry ice injected into the valve box 3 instantly vaporizes and expands in volume by 600 to 800 times, which impacts the inner cavity of the valve box 3, causing surface compressive stress in the inner cavity and increasing the fatigue life of the valve box 3. The dry ice injection device realizes the injection of dry ice, and cooperates with the sealing system to maintain the inner cavity pressure for a long time, thereby increasing the service life of the valve box 3 at the hydraulic end of the fracturing pump.

[0090] The inner cavity of the valve box 3 obtained by the cryogenic treatment forms a hardened layer and a stress-strain layer. The hardened layer is obtained by the cryogenic treatment of the valve box 3, and its structure is composed of tempered martensite, ε-Cu and inverted austenite not higher than 1wt%; the stress-strain layer is obtained by the pressure shock generated by the instantaneous gasification of dry ice, which is located at the outermost layer of the hardened layer, generating strain hardening and surface compressive stress, which is more conducive to achieving the performance requirements of high pressure resistance, corrosion resistance, long life and reliable sealing in the fracturing operation, thereby improving the fatigue life of the valve box 3. Among them, the cross hole of the valve box 3 can withstand a stress greater than 810 MPa.

[0091] In addition, since dry ice can sublime within a few thousandths of a second, its volume instantly expands 600 to 800 times, which has a huge impact on the inner surface of the valve box 3. Combined with the volume contraction and material embrittlement caused by the sudden cooling on the protrusions, it can further improve the roughness of the inner surface, reduce stress concentration, increase the service life of the valve box 3, further remove surface dirt, and improve cleanliness.

[0092] d) Tempered martensite, ε-Cu and reversed austenite

[0093] A hardened layer and a stress-strain layer are formed on the inner surface of the valve box 3, which include tempered martensite with a content of 95wt% to 96wt%, ε-Cu with a content of 3wt% to 4wt% and reversed austenite with a content not higher than 1wt%.

[0094] Among them, the high content of tempered martensite and dispersed ε-Cu are the key to the high strength, high toughness and corrosion resistance of the valve box 3, while the reverse austenite is a soft phase, which will lead to a decrease in the strength and hardness of the valve box 3. During preparation, the generation of reverse austenite (AR, γ-Fe) should be minimized or avoided as much as possible. The generation of reverse austenite is caused by the reverse transformation of tempered martensite (α'-Fe) of precipitation-hardened stainless steel during over-aging. The longer the aging time, the more reverse austenite content in the valve box 3, and the greater the impact on the service life of the valve box 3. However, shortening the aging time will make it difficult to generate ε-Cu, which will affect the hardness of the valve box 3. The reason why the prior art requires a long aging time is generally limited by the cryogenic treatment environment. Since a stable low temperature needs to be maintained all the time, not only does it require continuous introduction of cooling medium, but the final cryogenic treatment effect is also not ideal. Therefore, the present application optimizes the cryogenic process and system, utilizes the airtightness of the valve box 3 and the thermal insulation of the cryogenic system, and injects solid dry ice into the closed precipitation-hardened stainless steel valve box 3 at a flow rate of 0.5~1.0 L / s, so that the inner cavity of the valve box 3 is kept at a temperature below -75°C and a pressure of 6~8 atmospheres for 1~3 hours. The valve box 3 is cryogenically treated in a low temperature and high pressure environment, which is beneficial to improving the effect of the cryogenic treatment, so that the reverse transformation of martensite (α'-Fe) in the precipitation-hardened stainless steel is effectively suppressed. The content of tempered martensite in the final valve box 3 is increased, and the content of reversed austenite is effectively reduced, the content is less than 1wt%, and ε-Cu is not affected, so that the valve box 3 still maintains a relatively high hardness, and the microhardness of the inner cavity surface of the valve box 3 at a depth of 1~3 mm is 380~400. HV, surface compressive stress is 50~100MPa.

[0095] e) Testing standards

[0096] The hardness test in this application is carried out according to GB / T4340.1-2009 "Metal Vickers hardness test Part 1: Test method" standard;

[0097] The structure of the inner cavity of the valve box 3 is tested by XRD diffraction spectrum.

[0098] Example 1

[0099] The 5-cylinder valve box 3 of embodiment 1 is as follows Figure 1-4 As shown, it is made of precipitation hardening stainless steel, and its specific composition is: C: 0.06%, Si: 0.55%, Mn: 0.80%, Cr: 15.5%, Ni: 4.0%, Cu: 3.2%, Nb: 0.60%, P: 0.017%, S: 0.008%, and the balance is Fe.

[0100] The finite element stress analysis distribution diagram of the inner cavity section of the 5-cylinder valve box 3 is as follows Figure 5As shown, during the fracturing operation, the stress generated at the "cross hole" position of the inner cavity is the largest, which can reach a maximum of 800.67 MPa. Fatigue cracks are very likely to occur here, causing early failure and greatly shortening the service life of the valve box 3.

[0101] In order to improve the life of the valve box 3, the 5-cylinder valve box 3 that has been processed by the steps of "taking materials, forging, annealing, rough machining, solution heat treatment, aging heat treatment, and finishing" is hoisted into the deep cooling system (see Figure 6 ), and cryogenic treatment (see the flowchart for Figure 7 ). The specific steps of cryogenic treatment are: after encapsulating the valve box 3 hung in the stainless steel insulation barrel 2, inject dry ice with a diameter of 1.5 mm and an aspect ratio of 1 to 2 at a flow rate of 0.5 L / s. When the pressure in the inner cavity of the valve box reaches 6 to 8 atmospheres, the dry ice injection is completed, close the stop valve 9 and maintain the pressure for 3 hours, then disassemble the valve box 3 and process it for storage.

[0102] Among them, the inner surface of the valve box 3 after fine processing is as follows Figure 8 As shown, it includes tempered martensite (α'-Fe) 89wt%, ε-Cu 3wt%, and reversed austenite (γ-Fe) 8wt%.

[0103] The inner surface of the valve box 3 after cryogenic treatment is as follows Fig. 9 As shown, it includes tempered martensite (α'-Fe) 96wt%, ε-Cu 3wt%, and reversed austenite (γ-Fe) 1wt%.

[0104] It can be seen that after cryogenic treatment, the content of reversed austenite (γ-Fe) on the inner surface of valve box 3 is significantly reduced from 8wt% of the finely processed sample without cryogenic treatment to 1wt%, while the content of tempered martensite (α'-Fe) is significantly increased from 89wt% to 96wt%, and the microhardness of the inner surface of valve box 3 is increased from 365 HV of the finely processed sample without cryogenic treatment to 385 HV, and the surface roughness is further improved. After comparison under actual working conditions in the oil field, the service life of valve box 3 fracturing operation is increased from 982 h without cryogenic treatment to 1258 h.

[0105] Example 2

[0106] The 5-cylinder valve box 3 of Example 2 is made of precipitation-hardened stainless steel, and the specific composition is C: 0.07%, Si: 0.45%, Mn: 0.75%, Cr: 17.2%, Ni: 4.1wt%, Cu: 4.2%, Nb: 0.45%, P: 0.016%, S: 0.010%, and the balance is Fe.

[0107] The 5-cylinder valve box 3 that has been processed through the steps of "taking materials, forging, annealing, rough machining, solution heat treatment, aging heat treatment, and finishing" is hoisted into the cryogenic system for cryogenic treatment. The specific steps of cryogenic treatment are: after the valve box 3 hoisted into the stainless steel insulation barrel 2 is packaged, dry ice with a diameter of 1.5 mm and an aspect ratio of 1 to 2 is injected at a flow rate of 1.0 L / s. When the pressure in the inner cavity of the valve box reaches 6 to 8 atmospheres, the dry ice injection is completed, the stop valve 9 is closed and the pressure is maintained for 1 hour, and then the valve box 3 is disassembled and processed for storage.

[0108] Among them, the inner surface of the valve box 3 after fine processing is as follows Fig.10 As shown, it includes tempered martensite (α'-Fe) 90wt%, ε-Cu 4wt%, and reversed austenite (γ-Fe) 6wt%.

[0109] The inner surface of the valve box 3 after cryogenic treatment is as follows Fig.11 As shown, it includes tempered martensite (α'-Fe) 95wt%, ε-Cu 4wt%, and reversed austenite (γ-Fe) 1wt%.

[0110] It can be seen that after cryogenic treatment, the content of reversed austenite (γ-Fe) on the inner surface of valve box 3 is significantly reduced from 6wt% of the finely processed sample without cryogenic treatment to 1wt%, while the content of tempered martensite (α'-Fe) is significantly increased from 90wt% to 95wt%, and the microhardness of the inner surface of valve box 3 is increased from 372 HV of the finely processed sample without cryogenic treatment to 391 HV, and the surface roughness is further improved. After comparison under actual working conditions in the oil field, the service life of valve box 3 fracturing operation is increased from 957 h without cryogenic treatment to 1189 h.

[0111] The above description is only a preferred embodiment of the present application and an explanation of the technical principles used. Those skilled in the art should understand that the scope involved in the present application is not limited to the technical solution formed by a specific combination of the above-mentioned technical features, but should also cover other technical solutions formed by any combination of the above-mentioned technical features or their equivalent features without departing from the inventive concept, such as a technical solution formed by replacing the above-mentioned features with (but not limited to) technical features with similar functions disclosed in this application.

[0112] Except for the technical features described in the specification, the remaining technical features are known technologies to those skilled in the art. In order to highlight the innovative features of the present invention, the remaining technical features will not be described here in detail.

Claims

1. A deep cooling method for increasing the service life of a hydraulic end valve box of a fracturing pump, characterized in that: Solid dry ice is injected into the closed precipitation-hardened stainless steel valve box (3) at a flow rate of 0.5 to 1.0 L / s, so that the inner cavity of the valve box (3) is kept at a temperature below -75°C and a pressure of 6 to 8 atmospheres for 1 to 3 hours; The inner cavity of the treated valve box (3) forms a hardened layer and a stress-strain layer, wherein the hardened layer is obtained after the valve box (3) is cryogenically treated, and its structure consists of tempered martensite, ε-Cu and not more than 1wt% of reversed austenite; the stress-strain layer is obtained by the pressure shock generated by the instantaneous gasification of dry ice, and is located at the outermost layer of the hardened layer, generating strain hardening and surface compressive stress. The inner cavity surface of the treated valve box (3) has a microhardness of 380-400 HV at a depth of 1-3 mm, and a surface compressive stress of 50-100 MPa.

2. The cryogenic method according to claim 1, characterized in that: In the hardened layer, the content of tempered martensite is 95wt%~96wt%, and the content of ε-Cu is 3wt%~4wt%.

3. The cryogenic method according to claim 1, characterized in that: The thickness of the hardened layer is 1-3 cm, and the thickness of the stress-strain layer is 80-120 μm.

4. The cryogenic method according to claim 1, characterized in that: The components of the precipitation hardening stainless steel are as follows by mass percentage: C: 0.03%~0.08%, Si: 0.50%~1.0%, Mn: 0.70%~1.0%, Cr: 14.5%~18.0%, Ni: 3.0%~5.0%, Cu: 3.0%~5.0%, Nb: 0.20%~0.60%, P≤0.025%, S≤0.030%, and the balance is Fe.

5. The cryogenic method according to claim 1, characterized in that: The valve box (3) is processed through the steps of "material collection, forging, annealing, rough machining, solution heat treatment, aging heat treatment and finishing" before cryogenic treatment.

6. A valve box, characterized in that: The valve box (3) is a valve box (3) prepared by the cryogenic method according to any one of claims 1 to 5; A hardened layer and a stress-strain layer are formed in the inner cavity of the valve box (3), wherein the hardened layer is obtained by deep-cryogenic treatment of the valve box (3), and its structure consists of tempered martensite, ε-Cu and not more than 1wt% of reversed austenite; The stress-strain layer is obtained by the pressure impact generated by the instantaneous gasification of dry ice. It is located at the outermost layer of the hardened layer and produces strain hardening and surface compressive stress.

7. The valve box according to claim 6, characterized in that: The microhardness of the inner cavity surface of the treated valve box (3) at a depth of 1 to 3 mm is 380 to 400 HV, and the surface compressive stress of the stress-strain layer is 50 to 100 MPa.

8. The valve box according to claim 6, characterized in that: In the hardened layer, the content of tempered martensite is 95wt%~96wt%, and the content of ε-Cu is 3wt%~4wt%.

9. The valve box according to claim 6, characterized in that: The thickness of the hardened layer is 1-3 cm, and the thickness of the stress-strain layer is 80-120 μm.

10. The valve box according to claim 6, characterized in that: Precipitation hardening stainless steel is used as the base material, and the components of the precipitation hardening stainless steel are as follows by mass percentage: C: 0.03%~0.08%, Si: 0.50%~1.0%, Mn: 0.70%~1.0%, Cr: 14.5%~18.0%, Ni: 3.0%~5.0%, Cu: 3.0%~5.0%, Nb: 0.20%~0.60%, P≤0.025%, S≤0.030%, and the balance is Fe.

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