Nodular cast iron cylinder block and method for manufacturing same and piston pump

Abstract

The application belongs to the technical field of preparation of cylinder block of plunger pump, and particularly relates to a nodular cast iron cylinder block, a preparation method thereof and a plunger pump. The preparation method of the nodular cast iron cylinder block comprises the following steps: obtaining a nodular cast iron cylinder block casting, the pearlite content in the metallographic structure of the nodular cast iron cylinder block casting is more than 70%, and the difference in the pearlite content at each part of the nodular cast iron cylinder block casting is less than or equal to 20%; the nodular cast iron cylinder block casting is subjected to stress relief annealing before rough machining, and is subjected to finish machining after rough machining, and is subjected to nitriding treatment after finish machining. The preparation method of the nodular cast iron cylinder block provided by the application realizes accurate control of nitriding deformation, the plunger hole has small nitriding deformation, and the plunger hole does not need to be honed after nitriding, thereby reducing the nitriding deformation out of tolerance caused by the difference in casting materials of the same grade in different processes, and effectively solving the problems of large nitriding deformation of the plunger hole of the nodular cast iron cylinder block and unstable process control.

Description

Technical Field

[0001] This invention belongs to the field of plunger pump cylinder block manufacturing technology, specifically relating to a ductile iron cylinder block and its manufacturing method, and a plunger pump. Background Technology

[0002] The papers "Design and Research of Cylinder Block for High Pressure and High Flow Direct Shaft Plunger Pump" and "Study on Microstructure and Properties of Horizontal Continuous Cast Ductile Iron Profiles for Plunger Pump Cylinder Block" mention the use of alloy ductile iron, such as QT500-7 or QT600-3, directly as friction pairs after gas soft nitriding.

[0003] However, although the strength and elastic modulus of different grades of alloy ductile iron meet the requirements of friction pairs after nitriding, in actual use, even cylinder blocks of the same grade still have material differences, which leads to different residual stress, high-temperature deformation and nitriding diffusion volume increases. After the same nitriding heat treatment, the dimensional deformation is unstable and the cylindricity is prone to exceed the tolerance.

[0004] Therefore, in view of the above shortcomings, this invention is proposed. Summary of the Invention

[0005] The main objective of this invention is to propose a ductile iron cylinder block, its preparation method, and a plunger pump. The preparation method of the ductile iron cylinder block provided by this invention achieves precise control of the pearlite content in the metallographic structure of the ductile iron cylinder block casting to be ≥70%, with the pearlite content range at different locations varying by less than 20%. This results in precise control of nitriding deformation, minimal nitriding deformation of the plunger holes, and eliminates the need for honing after nitriding. It also reduces the nitriding deformation deviation caused by material differences in castings of the same grade but from different processes, effectively solving the problems of large nitriding deformation and unstable process control in the plunger holes of ductile iron cylinder blocks.

[0006] The first aspect of the present invention provides a method for preparing a ductile iron cylinder block, the method comprising the following steps: obtaining a ductile iron cylinder block casting, wherein the pearlite content in the metallographic structure of the ductile iron cylinder block casting reaches 70% or more, and the difference in pearlite content at different parts of the ductile iron cylinder block casting is ≤20%; performing stress-relieving annealing on the ductile iron cylinder block casting before rough machining, and performing finish machining after the rough machining, and performing nitriding treatment after the finish machining.

[0007] In some embodiments of the present invention, the graphite spheroidization level in the metallographic structure of the ductile iron cylinder block casting is 1 to 3, the graphite spheroid size is 5 to 7, and the difference in graphite spheroid size at different parts of the ductile iron cylinder block casting does not exceed one level.

[0008] In some embodiments of the present invention, a casting process is employed, and the alloy content is controlled by alloying during the casting process to obtain the ductile iron cylinder block casting.

[0009] In some embodiments of the present invention, a casting process is used to obtain a casting, and the casting is subjected to normalizing heat treatment to obtain the ductile iron cylinder block casting.

[0010] In some embodiments of the present invention, the casting process includes sand casting or horizontal continuous casting.

[0011] In some embodiments of the present invention, the ductile iron cylinder block castings produced by the horizontal continuous casting process are first subjected to peeling and removal of the edge chilling layer before undergoing the stress-relieving annealing.

[0012] In some embodiments of the present invention, the alloying method includes adding 0.2% to 0.6% Cu and 0.020% to 0.060% Sn by mass percentage to molten iron.

[0013] In some embodiments of the present invention, the stress-relief annealing temperature is 530°C to 580°C and the time is 2 h to 5 h.

[0014] In some embodiments of the present invention, after the stress-relief annealing is completed, the temperature is reduced to 200°C at a rate of less than 60°C / h before the product is removed from the furnace.

[0015] In some embodiments of the present invention, the nitriding treatment adopts a nitrocarbon co-infiltration process, including a first-stage nitriding, wherein the nitrogen potential value of the first-stage nitriding is 1 to 3, the nitriding temperature of the first-stage nitriding is 540°C to 560°C, the nitriding time of the first-stage nitriding is set according to the target thickness of the nitrided layer to be obtained, and the layer is cooled to below 200°C and then air-cooled under nitrogen with a flow rate of 10 to 20 times the volume of the nitrogen furnace.

[0016] In some embodiments of the present invention, the nitriding treatment further includes pre-oxidation before the first nitriding stage, wherein the pre-oxidation temperature is 330°C to 380°C, the pre-oxidation time is 30 min to 120 min, and the pre-oxidation medium is air.

[0017] In some embodiments of the present invention, the preparation method further includes polishing the ductile iron cylinder block after the nitriding treatment.

[0018] The second aspect of the present invention provides a ductile iron cylinder body, which is prepared by the preparation method described in the first aspect; in the metallographic structure of the ductile iron cylinder body, the pearlite content reaches more than 70%, and the difference in pearlite content at different parts of the ductile iron cylinder body is ≤20%.

[0019] In some embodiments of the present invention, the graphite spheroidization level in the metallographic structure of the ductile iron cylinder body is 1 to 3, the graphite spheroid size is 5 to 7, and the difference in graphite spheroid size at different parts of the ductile iron cylinder body does not exceed 1 level.

[0020] A third aspect of the present invention provides a plunger pump comprising a ductile iron cylinder body prepared by the preparation method described in the first aspect or a ductile iron cylinder body described in the second aspect.

[0021] The beneficial effects of this invention are:

[0022] (1) In this invention, by quantitatively controlling the pearlite content, graphite sphere size and distribution uniformity, the rigidity of the material is ensured, the nitriding deformation is precisely controlled, and the nitriding deformation deviation caused by the difference in casting materials of the same grade in different processes is reduced.

[0023] (2) In this invention, the decomposition of pearlite is reduced by medium-low temperature nitriding and stress-relief annealing, thereby reducing high-temperature deformation. At the same time, the nitriding temperature is reduced, the temperature field uniformity in the nitriding furnace is improved, and the axial and radial deformation of hole-type parts is reduced.

[0024] (3) The present invention reduces the honing process after nitriding, ensuring the uniformity of surface hardness of the parts and reducing costs.

[0025] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described below. Attached Figure Description

[0026] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. In the drawings:

[0027] Figure 1 This is a metallographic diagram of the single-cast cylinder body in Embodiment 1 of the present invention. Wherein, Figure 1 a) in the diagram is the metallographic structure before nitriding; Figure 1 b) in the figure shows the metallographic structure after nitriding;

[0028] Figure 2 This is a metallographic diagram of the continuous casting cylinder block in Embodiment 2 of the present invention. Wherein, Figure 2 a) in the diagram is the metallographic structure before nitriding; Figure 2 b) in the figure is the metallographic structure after nitriding. Detailed Implementation

[0029] Exemplary embodiments of the present invention will now be described in more detail with reference to specific examples. It should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the invention, are intended to cover non-exclusive inclusion.

[0031] In the description of the embodiments of the present invention, the technical terms "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features.

[0032] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0033] In the description of the embodiments of this invention, the term "and / or" is merely a description of the association relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A exists, A and B exist simultaneously, and B exists. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0034] In the description of the embodiments of the present invention, the term "multiple" refers to two or more (including two), similarly, "multiple groups" refers to two or more (including two groups), and "multiple pieces" refers to two or more (including two pieces).

[0035] In the description of the embodiments of the present invention, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of the present invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of the present invention.

[0036] In the description of the embodiments of the present invention, unless otherwise explicitly specified and limited, the technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of the present invention according to the specific circumstances.

[0037] Terminology Explanation

[0038] Horizontal continuous casting: a continuous casting method in which molten iron enters a horizontally placed crystallizer from a ladle through a submerged entry nozzle, solidifies into a billet in the crystallizer, and is then pulled out of the crystallizer by a billet pulling machine at a certain speed.

[0039] Continuously cast cylinder block: A cylinder block formed by cutting horizontally continuously cast profiles into cylindrical blanks. The material cools rapidly and has fine grains.

[0040] Single-cast cylinder block: The cylinder block formed by machining after the initial structure of the cylinder block blank is formed by sand casting.

[0041] Pearlite: It is an important matrix structure in the metallographic structure of cast iron materials, and is a lamellar microstructure formed by alternating ferrite and cementite.

[0042] Nitrogen-carbon co-diffusion: a gas thermal diffusion process that simultaneously diffuses nitrogen and carbon into the surface of a workpiece.

[0043] The first aspect of this invention provides a method for preparing a ductile iron cylinder block. The key to this method is obtaining a ductile iron cylinder block casting in which the pearlite content reaches more than 70% in the microstructure, and the difference in pearlite content at different locations of the ductile iron cylinder block casting is ≤20%. The ductile iron cylinder block casting is subjected to stress-relieving annealing before rough machining, and then to finish machining after rough machining, and finally to nitriding treatment after finish machining to obtain the ductile iron cylinder block.

[0044] This invention provides a method for preparing ductile iron cylinder blocks with excellent nitriding performance, minimal nitriding deformation, and no need for honing after nitriding. The method for preparing ductile iron cylinder blocks for plunger pumps provided by this invention precisely controls the metallographic structure of the ductile iron cylinder block casting. Specifically, it controls the pearlite content to reach more than 70%, and the difference in pearlite content in different parts of the ductile iron cylinder block casting is ≤20%, so as to ensure material uniformity, improve rigidity, and reduce overall deformation and residual stress in the structure.

[0045] In some embodiments, the graphite spheroidization level in the metallographic structure of the ductile iron cylinder block casting is grade 1 to 3, and the graphite spheroid size is grade 5 to 7. The difference in graphite spheroid size at different locations in the ductile iron cylinder block casting does not exceed one grade. Specifically, the graphite spheroidization level can be grade 1, 2, or 3, and the graphite spheroid size can be grade 5, 6, or 7.

[0046] In some embodiments, the preparation method of the ductile iron cylinder block is carried out according to the following steps.

[0047] Obtain ductile iron cylinder block castings

[0048] In some embodiments, a casting process is employed, and the alloy content is controlled through alloying during the casting process to obtain the ductile iron cylinder block casting. By utilizing alloying, the pearlite content in the microstructure of the ductile iron cylinder block casting is controlled by adjusting the alloy content, achieving a pearlite content of 70% or higher, with a pearlite content difference of ≤20% across different parts of the ductile iron cylinder block casting; and achieving a graphite spheroidization level of 1-3, a graphite spheroid size of 5-7, and a graphite spheroid size difference of no more than one level across different parts of the ductile iron cylinder block casting.

[0049] In some embodiments, a casting process is used to obtain a casting, which is then subjected to normalizing heat treatment to obtain the ductile iron cylinder block casting. By performing normalizing heat treatment on the cast cylinder block casting, the pearlite content in the metallographic structure of the cylinder block casting is increased, resulting in a pearlite content of over 70% in the final ductile iron cylinder block casting, with a pearlite content difference of ≤20% across different parts of the ductile iron cylinder block casting; and achieving a graphite spheroidization level of 1-3, a graphite spheroid size of 5-7, and a graphite spheroid size difference of no more than one level across different parts of the ductile iron cylinder block casting.

[0050] In some embodiments, the casting process includes sand casting or horizontal continuous casting. Specifically, sand casting can be used, and the alloy content can be controlled by alloying during the casting process to obtain the ductile iron cylinder block casting. Horizontal continuous casting can also be used, and the alloy content can be controlled by alloying during the casting process to obtain the ductile iron cylinder block casting. Alternatively, a sand casting process can be used to obtain the casting, and then the casting can be normalized to obtain the ductile iron cylinder block casting. Horizontal continuous casting can also be used to obtain the casting, and then the casting can be normalized to obtain the ductile iron cylinder block casting.

[0051] In some embodiments, the alloying method described above includes adding 0.2% to 0.6% copper (Cu) and 0.020% to 0.060% tin (Sn) by mass percentage to the molten iron. Exemplarily, the mass percentage of Cu can be one of 0.2%, 0.3%, 0.4%, 0.5%, or 0.6%, or any value satisfying the above range. The mass percentage of Sn can be one of 0.020%, 0.030%, 0.040%, 0.050%, or 0.060%, or any value satisfying the above range.

[0052] In some embodiments, by alloying during the casting process, 0.2%~0.6% copper (Cu) and 0.020%~0.060% tin (Sn) by mass percentage are added, resulting in a ductile iron cylinder block casting with the following chemical composition (by mass percentage): C 3.0%~3.85%, Si 2.0%~2.9%, Mn≤0.7%, S 0.008%~0.020%, P≤0.06%, Cu 0.2%~0.8%, Sn 0.020%~0.080%, Mg 0.030%~0.050%, Re 0.010%~0.030%, with the balance being Fe. Exemplarily, the percentage of Cu added can be one of 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, or any value satisfying the above range. The percentage of Sn added can be one of 0.020%, 0.030%, 0.040%, 0.050%, 0.060%, or any value within the above range.

[0053] In some embodiments, the alloying method described above includes adding 0.4% to 0.7% Cu and 0.01% to 0.02% Sn to the molten iron, the amounts of which can be set according to actual needs. For example, the percentage of Cu added can be one of 0.4%, 0.5%, 0.6%, or 0.7%, or any value within the above range. The percentage of Sn added can be one of 0.010% or 0.020%, or any value within the above range.

[0054] In some embodiments, the normalizing heat treatment temperature can be 870℃~940℃, and the time can be 1~3 hours. Exemplarily, the normalizing heat treatment temperature can be one of 870℃, 875℃, 880℃, 885℃, 890℃, 895℃, 900℃, 905℃, 910℃, 915℃, 920℃, 925℃, 930℃, 935℃, or 940℃, or any value satisfying the above range. The normalizing heat treatment time can be one of 1 hour, 2 hours, or 3 hours, or any value satisfying the above range.

[0055] It is worth mentioning that the ductile iron cylinder block castings produced using the horizontal continuous casting process undergo a peeling process and the removal of the edge chilling layer before stress-relief annealing. The peeling process can be understood as removing oxide scale. This helps to ensure that the pearlite content difference in different parts of the ductile iron cylinder block is ≤20% and the graphite spheroid size difference does not exceed one level.

[0056] Stress-relief annealing

[0057] In some embodiments, stress-relief annealing is performed before rough machining of the ductile iron cylinder block casting, which can also be understood as residual stress annealing, mainly to eliminate residual stress in the structure.

[0058] In this embodiment of the invention, stress-relief annealing is performed at a medium-low temperature. It can be understood that by using medium-low temperature stress-relief annealing, the annealing temperature is kept as low as possible, which reduces the decomposition of pearlite structure and reduces structural changes.

[0059] In some implementations, the stress-relief annealing temperature is 530℃~580℃, which can also be understood as the stress-relief annealing holding temperature being 530℃~580℃. For example, the stress-relief annealing temperature can be one of 530℃, 540℃, 550℃, 560℃, 570℃, 580℃ or any value satisfying the above range.

[0060] In some embodiments, the stress-relief annealing time is 2 h to 5 h, which can also be understood as the stress-relief annealing holding time being 2 h to 5 h. Exemplarily, the stress-relief annealing time can be one of 2 h, 3 h, 4 h, 5 h, or any value satisfying the above range.

[0061] By controlling the stress-relief annealing temperature and time as described above, residual stress in the tissue can be eliminated, while reducing the decomposition of pearlite tissue and minimizing tissue changes.

[0062] In some embodiments, after the stress-relief annealing holding is completed, the temperature is reduced to 200°C at a rate lower than 60°C / h before being removed from the furnace. Exemplarily, the cooling rate can be one of 60°C / h, 55°C / h, 50°C / h, 40°C / h, 30°C / h, 20°C / h, 10°C / h, or any value satisfying the above range.

[0063] Rough and fine machining

[0064] In some embodiments, the cylinder casting obtained through the above steps is a blank, which needs to be roughed and finished to improve the surface quality and dimensional and shape accuracy of the cylinder casting.

[0065] Nitriding treatment

[0066] In some embodiments, the cylinder casting obtained after the above-mentioned finishing process is subjected to nitriding treatment to form a nitrided layer or a hardened layer on the surface of the cylinder casting.

[0067] In some embodiments, the nitriding treatment employs a nitrocarburizing process, which includes a first-stage nitriding process. The first-stage nitriding process uses a medium-low temperature nitriding temperature, which is below 560°C, to improve the uniformity of the temperature field inside the nitriding furnace and reduce the axial and radial deformation of the hole-type parts.

[0068] In some embodiments, the nitrogen potential (Kn) value of a nitriding stage is 1 to 3. Exemplarily, the nitrogen potential value of a nitriding stage can be one of 1, 2, or 3, or any value satisfying the above range.

[0069] In some embodiments, the nitriding temperature of a nitriding stage is 540°C to 560°C. Exemplarily, the nitriding temperature of a nitriding stage can be one of 540°C, 545°C, 550°C, 555°C, and 560°C, or any value satisfying the above range.

[0070] In some embodiments, the nitriding time for a nitriding process can be set according to the target thickness of the nitrided layer to be obtained, without being specifically limited.

[0071] In some embodiments, after a nitriding process is completed, the cylinder parts are cooled to below 200°C using a high flow rate of nitrogen before being air-cooled.

[0072] In some embodiments, the nitrogen flow rate during cooling is 10 to 20 times the volume of the nitrogen furnace.

[0073] In some embodiments, the nitriding process further includes pre-oxidation prior to the nitriding stage to form an oxide film on the surface of the ductile iron cylinder block casting.

[0074] In some embodiments, the pre-oxidation temperature is 330°C to 380°C. Exemplarily, the pre-oxidation temperature can be one of 330°C, 335°C, 340°C, 345°C, 350°C, 355°C, 360°C, 365°C, 370°C, 375°C, and 380°C, or any value that satisfies the above range.

[0075] In some embodiments, the pre-oxidation time is 30 min to 120 min. Exemplarily, the pre-oxidation time can be one of 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, and 120 min, or any value that satisfies the above range.

[0076] In some embodiments, the pre-oxidation medium is air.

[0077] Setting the pre-oxidation temperature, time, and medium as described above helps in the formation of the target oxide film.

[0078] Polishing

[0079] In some embodiments, the ductile iron cylinder block obtained after the above-described nitriding treatment is polished. Specifically, the plunger bore of the ductile iron cylinder block is polished, which helps to improve the shape accuracy of the plunger bore, such as cylindricity.

[0080] In this embodiment of the invention, the key to the preparation method of ductile iron cylinder blocks lies in the precise control of the metallographic structure of the ductile iron cylinder block casting, and the performance of medium-low temperature residual stress annealing before rough machining and medium-low temperature nitriding after finish machining. The preparation method of the ductile iron cylinder block includes the following steps: obtaining a ductile iron cylinder block casting, wherein the pearlite content in the metallographic structure of the ductile iron cylinder block casting reaches 70% or more, and the difference in pearlite content at different locations of the ductile iron cylinder block casting is ≤20%; performing stress-relief annealing on the ductile iron cylinder block casting before rough machining, and performing finish machining after rough machining, followed by nitriding and polishing.

[0081] The second aspect of the present invention provides a ductile iron cylinder body, which is prepared by the preparation method described in the first aspect.

[0082] In some embodiments, the pearlite content in the metallographic structure of the ductile iron cylinder body reaches 70% or more, and the difference in pearlite content at different parts of the ductile iron cylinder body, or understood as at different locations, is ≤20%.

[0083] In some embodiments, the graphite spheroidization level in the metallographic structure of the ductile iron cylinder body is 1 to 3, the graphite spheroid size is 5 to 7, and the difference in graphite spheroid size at different parts of the ductile iron cylinder body, or understood as at different locations, does not exceed one level.

[0084] In some embodiments, the ductile iron cylinder body comprises the following components by mass percentage: C 3.0%~3.85%, Si 2.0%~2.9%, Mn≤0.7%, S 0.008%~0.020%, P≤0.06%, Cu 0.2%~0.8%, Sn 0.020%~0.080%, Mg 0.030%~0.050%, Re 0.010%~0.030%, with the balance being Fe.

[0085] A third aspect of the present invention provides a plunger pump comprising a ductile iron cylinder body prepared by the preparation method described in the first aspect or a ductile iron cylinder body described in the second aspect.

[0086] Unless otherwise defined, the technical terms used in the following embodiments have the same meaning as commonly understood by those skilled in the art. Unless otherwise specified, the experimental reagents used in the following embodiments are all conventional biochemical reagents; the raw materials, instruments, and equipment used in the following embodiments can all be obtained commercially or through existing methods; unless otherwise specified, the amounts of experimental reagents used are the amounts used in conventional experimental operations; unless otherwise specified, the experimental methods are conventional methods. It should be further noted that the following description is merely exemplary and not a specific limitation of the present invention.

[0087] Example 1

[0088] The cylinder body of a plunger pump is manufactured, wherein the diameter of the plunger bore is 33.5 mm and the height is 140 mm.

[0089] The preparation method of the plunger pump cylinder body includes the following steps:

[0090] 1) A sand casting process is adopted, and alloying elements Cu and Sn are added during the casting process to obtain ductile iron cylinder block castings. The ductile iron cylinder block castings include the following composition by mass percentage: C 3.77%, Si 2.28%, Mn 0.38%, S 0.012%, P 0.024%, Cu 0.44%, Sn 0.020%, Mg 0.042%, Re 0.010%, with the balance being Fe.

[0091] In the metallographic structure of ductile iron cylinder block castings, the pearlite content is 75%~85%, and the difference in pearlite content in different parts of the ductile iron cylinder block casting is ≤20%; the graphite spheroidization level is 3, the graphite spheroid size is 5~6, and the difference in graphite spheroid size in different parts of the ductile iron cylinder block casting does not exceed 1 level.

[0092] 2) The ductile iron cylinder block casting is subjected to stress-relief annealing. After holding at 530℃ for 3 hours, it is cooled to 200℃ at a cooling rate of 55℃ / h before being taken out of the furnace.

[0093] 3) Rough and finish machining of the ductile iron cylinder block casting is performed, and the cylindricity of the plunger hole is 0.0032.

[0094] 4) Nitriding treatment was performed on the ductile iron cylinder block casting. The nitriding treatment employed a nitrocarburizing process, including pre-oxidation and a single-stage nitriding. The pre-oxidation temperature was 380℃, the time was 70 min, and the medium was air. The single-stage nitriding temperature was 560℃, the nitrogen potential Kn value was 2.5, the nitriding time was 18 h, the nitrided layer depth or thickness was 0.2 mm, the white bright layer thickness was 10 μm, and the cylindricity of the plunger hole after nitriding was 0.0062, with a cylindricity increment of 0.003.

[0095] 5) Polish the plunger holes of the ductile iron cylinder block parts obtained after nitriding treatment.

[0096] The ductile iron cylinder block prepared in Example 1 comprises the following components by mass percentage: C 3.77%, Si 2.28%, Mn 0.38%, S 0.012%, P 0.024%, Cu 0.44%, Sn 0.020%, Mg 0.042%, Re 0.010%, with the balance being Fe.

[0097] Metallographic structure of ductile iron cylinder block (see) Figure 1 As shown, the pearlite content is 75%, and the pearlite content varies by 5% in different parts of the ductile iron cylinder block; the graphite spheroidization level is 3, the graphite spheroid size is 5, and the graphite spheroid size varies by no more than 1 level in different parts of the ductile iron cylinder block casting.

[0098] Example 2

[0099] The cylinder body of a plunger pump is manufactured, wherein the diameter of the plunger bore is 33.5 mm and the height is 140 mm.

[0100] The preparation method of the plunger pump cylinder body includes the following steps:

[0101] 1) A horizontal continuous casting process is adopted, and alloying elements Cu and Sn are added during the casting process to obtain ductile iron cylinder block casting profiles. The ductile iron cylinder block casting profiles include the following composition by mass percentage: C 3.4%, Si 2.60%, Mn 0.30%, S 0.012%, P 0.030%, Cu 0.40%, Sn 0.050%, Mg 0.042%, Re 0.010%, with the balance being Fe.

[0102] In the metallographic structure of ductile iron cylinder block casting profiles, the pearlite content is 85%~95%, the graphite spheroidization level is 2, and the graphite spheroid size is 6~7.

[0103] 2) Peel the edges of the ductile iron cylinder block casting profiles and remove the edge chilling layer. Then cut the profiles into blanks according to the cylinder block height and process them into ductile iron cylinder block castings. The pearlite content difference between different parts of the ductile iron cylinder block casting is ≤20%, and the graphite spheroid size difference between different parts of the ductile iron cylinder block casting does not exceed one grade.

[0104] 3) The ductile iron cylinder block casting is subjected to stress-relief annealing. After holding at 570℃ for 3 hours, it is cooled to 200℃ at a cooling rate of 55℃ / h before being taken out of the furnace.

[0105] 4) Rough and finish machining of the ductile iron cylinder block casting, with a plunger hole cylindricity of 0.0032.

[0106] 5) Nitriding treatment was performed on the ductile iron cylinder block casting. The nitriding treatment employed a nitrocarburizing process, including pre-oxidation and a single-stage nitriding. The pre-oxidation temperature was 370℃, the time was 60 min, and the medium was air. The single-stage nitriding temperature was 560℃, the nitrogen potential Kn value was 2.5, the nitriding time was 18 h, the nitrided layer depth or thickness was 0.2 mm, the bright white layer thickness was 10 μm, and the cylindricity of the plunger hole after nitriding was 0.0060, with a cylindricity increment of 0.0028.

[0107] 6) Polish the plunger holes of the ductile iron cylinder block parts obtained after nitriding treatment.

[0108] The ductile iron cylinder block prepared in Example 2 comprises the following components by mass percentage: C 3.4%, Si 2.60%, Mn 0.30%, S 0.012%, P 0.030%, Cu 0.40%, Sn 0.050%, Mg 0.042%, Re 0.010%, with the balance being Fe.

[0109] Metallographic structure of ductile iron cylinder block (see) Figure 2As shown, the pearlite content is 85%, and the pearlite content varies by 5% in different parts of the ductile iron cylinder body; the graphite spheroidization level is 2, the graphite spheroid size is 6, and the graphite spheroid size varies by no more than 1 level in different parts of the ductile iron cylinder body.

[0110] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing a ductile iron cylinder block, characterized in that, Includes the following steps: The casting process is adopted, and the alloy content is controlled by alloying during the casting process to obtain ductile iron cylinder block castings; Alternatively, a casting process can be used to obtain a casting, which is then subjected to normalizing heat treatment to produce a ductile iron cylinder block casting. The chemical composition of the ductile iron cylinder block casting, by mass percentage, includes: C 3.0%~3.85%, Si 2.0%~2.9%, Mn≤0.7%, S 0.008%~0.020%, P≤0.06%, Cu 0.2%~0.8%, Sn 0.020%~0.080%, Mg 0.030%~0.050%, RE 0.010%~0.030%, balance being Fe; in the metallographic structure of the ductile iron cylinder block casting, the pearlite content reaches more than 70%, the difference in pearlite content at different parts of the ductile iron cylinder block casting is ≤20%, the graphite spheroidization level is 1~3, the graphite spheroid size is 5~7, and the difference in graphite spheroid size at different parts of the ductile iron cylinder block casting does not exceed 1 level; The ductile iron cylinder block casting is subjected to stress-relief annealing before rough machining at a temperature of 530℃~580℃ for 2 h~5 h; after rough machining, it is subjected to finish machining, followed by nitriding treatment, with the cylindricity increment of the plunger hole ≤0.003 after nitriding; the nitriding treatment adopts a nitrocarburizing process, which includes a first-stage nitriding and a pre-oxidation before the first-stage nitriding, wherein the pre-oxidation temperature is 330℃~380℃, the pre-oxidation time is 30 min~120 min, and the pre-oxidation medium is air; the nitrogen potential value of the first-stage nitriding is 1~3.

2. The preparation method according to claim 1, characterized in that, The casting process is either sand casting or horizontal continuous casting.

3. The preparation method according to claim 2, characterized in that, Before undergoing stress-relief annealing, the ductile iron cylinder block casting produced by horizontal continuous casting process is first subjected to peeling treatment and removal of the edge chilling layer.

4. The preparation method according to claim 1, characterized in that, After the stress-relief annealing is completed, the temperature is lowered to 200°C at a rate of less than 60°C / h before being removed from the furnace; and / or, The nitriding temperature of the first nitriding stage is 540℃~560℃, and the nitriding time of the first nitriding stage is set according to the target thickness of the nitrided layer to be obtained. The nitrided layer is cooled to below 200℃ under nitrogen gas with a flow rate of 10~20 times the volume of the nitrogen furnace and then air-cooled.

5. The preparation method according to claim 1, characterized in that, The preparation method further includes polishing the ductile iron cylinder block after the nitriding treatment.

6. A ductile iron cylinder block, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 5; In the metallographic structure of the ductile iron cylinder body, the pearlite content reaches more than 70%, the difference in pearlite content in different parts of the ductile iron cylinder body is ≤20%, the graphite spheroidization level is 1~3, the graphite spheroid size is 5~7, and the difference in graphite spheroid size in different parts of the ductile iron cylinder body casting does not exceed 1 level.

7. A plunger pump, characterized in that, The ductile iron cylinder body prepared by the preparation method of any one of claims 1 to 5 or the ductile iron cylinder body of claim 6.

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