A heat treatment method for a high-pressure fuel injection nozzle needle valve stem

Abstract

A kind of high-pressure fuel injection nozzle needle valve rod heat treatment method, using heat treatment furnace with continuous heating function, furnace has positive pressure nitrogen-containing atmosphere, after being radially constrained, needle valve rod blank is sent into heat treatment furnace transversely, in the heat treatment furnace heat preservation section, it is heated at 1080±5 ℃ for 4min-8min austenitizing, at the junction position of heat preservation section and cooling section, needle valve rod blank is cooled by using nitrogen-containing gas flow to spray, and the gas flow that separates heat preservation zone and cooling zone, needle valve rod blank is quenched at 140-150 ℃ / min in cooling section.The method can realize nitriding and quenching treatment to slender needle valve rod in low-cost mode by effectively controlling heat treatment parameters, improve the hardness, fatigue life and wear resistance of needle valve rod product, while effectively reducing the deformation caused by heat treatment.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment processing, and specifically relates to a heat treatment method for a high-pressure fuel injector needle valve stem. Background Technology

[0002] The needle valve stem of a high-pressure fuel injector is a core component that plays a decisive role in the performance level of the injector. During engine service, the needle valve stem needs to frequently reciprocate to open and close the fuel injection circuit. The working principle of the needle valve stem places high demands on its manufacturing process. On the one hand, the needle valve stem requires high-frequency reciprocating motion throughout its service life, necessitating high surface hardness, fatigue resistance, and wear resistance to prevent premature wear failure. On the other hand, the axial movement of the needle valve stem requires a tight fit with the slender flow channel in the nozzle, demanding high straightness of the stem body. Currently, improving the surface hardness of the needle valve stem typically requires heat treatment. Heat treatment of such slender rods usually requires expensive specialized equipment and control software to adjust heat treatment parameters, ensuring that the needle valve stem achieves high hardness and high fatigue resistance while meeting straightness standards. This results in complex production equipment and high processing costs. Summary of the Invention

[0003] The purpose of this invention is to provide a heat treatment method for the needle valve stem of a high-pressure fuel injector, which requires simple equipment and has low processing costs.

[0004] According to an embodiment of the present invention, a heat treatment method for a high-pressure fuel injector needle valve stem is provided, which utilizes a heat treatment furnace with continuous heating function to heat treat the needle valve stem blank of the high-pressure fuel injector. The method includes the following steps:

[0005] a) Provide a needle valve rod blank, provide radial fixing constraint to the needle valve rod blank and feed it laterally into a heat treatment furnace;

[0006] b) Provide a nitrogen-containing positive pressure gas atmosphere in the heat treatment furnace, and perform austenitizing heating on the needle valve rod blank at 1080±5℃ for 4min-8min in the heat treatment furnace holding section;

[0007] c) At the junction of the heat preservation section and the cooling section of the mesh belt furnace, a nitrogen-containing gas flow is used to spray the needle valve rod to separate the heat preservation section and the cooling section.

[0008] d) The needle valve stem blank is quenched at 140-150℃ / min using air cooling in the cooling section.

[0009] The needle valve stem of a high-pressure fuel injector has a slender structure and requires high surface hardness and axial straightness. During heat treatment, the heating, nitriding, and quenching processes interact with each other; improper process selection can lead to thermal deformation or unsatisfactory surface hardness. In this method, the needle valve stem blank enters the mesh belt furnace laterally, ensuring consistent temperature distribution along the length of each blank and preventing thermal deformation due to uneven heating during heat treatment. Simultaneously, nitrogen-containing gas is directly sprayed onto the blank, which, on the one hand, blocks gas flow between the heating and quenching zones, ensuring efficient cooling in the quenching zone; on the other hand, it directly generates gas-cooling effect on the blank, enabling rapid and uniform quenching. This method integrates the austenitizing, nitriding, and quenching processes of the needle valve stem blank into a single heat treatment furnace, avoiding the additional steps and thermal deformation risks associated with separate nitriding processes. It also eliminates the need for complex and expensive specialized production equipment and control software, reducing the production cost of high-pressure fuel injector needle valve stems and improving heat treatment efficiency.

[0010] Furthermore, the needle valve stem blank is made of X90CrMoV18 steel, and by weight, its composition includes 0.82%-0.98% C, no more than 1.05% Si, no more than 1.03% Mn, no more than 0.045% P, no more than 0.005% S, 16.8%-19.2% Cr, 0.85%-1.35% Mo, and 0.04%-0.15% V. The needle valve stem requires high structural strength and machinability, and must possess good wear resistance and surface hardness.

[0011] Furthermore, the heat treatment furnace is a mesh belt furnace. Mesh belt furnaces are commonly used heat treatment furnaces, with simple structure and low cost.

[0012] Further, in step a), the method for radially constraining the needle valve stem blank is to densely lay two layers of the needle valve stem blank in a flat-bottomed tray. The needle valve stem blanks are arranged laterally in the flat-bottomed tray so that adjacent needle valve stems abut against each other, forming a radial constraint on the needle valve stem blank, thereby suppressing bending deformation of the needle valve stem blank during heat treatment.

[0013] Furthermore, the nitrogen-containing gas atmosphere in step b) and the nitrogen-containing gas flow in step c) are configured as ammonia decomposition gas. Ammonia decomposition gas is easy to produce and can achieve nitriding processing relatively efficiently.

[0014] Furthermore, in step d), the inlet water temperature of the cooling section's water jacket is controlled between 8°C and 16°C. By controlling the water jacket temperature, rapid cooling of the needle valve rod blank is ensured during the quenching process.

[0015] Furthermore, the needle valve stem blank also includes a cryogenic treatment step after heat treatment, in which the needle valve stem blank is cooled and processed using a cryogenic furnace. Cryogenic treatment can promote the further transformation of residual austenite in the needle valve stem blank into martensite, thereby improving the strength of the finished needle valve stem.

[0016] Furthermore, the needle valve stem blank also includes a tempering process after the cryogenic treatment step. The tempering process eliminates residual stress generated during heat treatment.

[0017] Furthermore, after heat treatment, the needle valve stem blank meets the following requirements: one end of the needle valve stem blank is fixed and rotated around its axis, and the floating distance of the other end of the needle valve stem blank does not exceed 0.034% of its length. The needle valve stem needs to undergo high-frequency, long-cycle reciprocating motion in a slender, high-pressure fuel injector, thus requiring high straightness.

[0018] Furthermore, after heat treatment, the nitrogen content of the needle valve stem blank, by weight, is not less than 0.22% within a depth of 60 μm and not less than 0.08% within a depth of 160 μm. Surface nitrogen content is an important indicator for evaluating the effectiveness of nitriding treatment of the needle valve stem blank. Too low a nitrogen content will lead to insufficient fatigue performance and wear resistance of the needle valve stem, while a nitrogen content exceeding the specified range will result in abnormal softening.

[0019] Furthermore, the needle valve stem blank, after heat treatment, has a fatigue life of no less than 800 million cycles under actual working conditions. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a high-pressure fuel injector structure in one embodiment;

[0021] Figure 2 This is a schematic diagram of the arrangement structure of the needle valve stem blank in one embodiment;

[0022] Figure 3 This is a schematic diagram of the mesh belt furnace structure in one embodiment;

[0023] Figure 4 This is a schematic diagram showing the distribution of nitrogen content with depth in one embodiment.

[0024] The purpose of the above-described drawings is to provide a detailed description of the invention so that those skilled in the art can understand the technical concept of the invention, and not to limit the invention. For the sake of brevity, the above-described drawings only schematically depict the structures related to the technical features of the invention, and do not depict the complete structure and all details strictly according to actual scale. Detailed Implementation

[0025] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings.

[0026] The term "embodiment" as used herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment herein. The phrase appearing in various places in the specification does not necessarily refer to the same embodiment, nor is it limited to mutually exclusive, independent, or alternative embodiments. Those skilled in the art will understand that the embodiments herein can be combined with other embodiments without causing structural conflicts.

[0027] In this description, terms such as "upper," "lower," "left," "right," "lateral," "longitudinal," "height," "length," and "width," which indicate orientation or positional relationships, are intended to accurately describe the embodiments and simplify the description, rather than limiting the parts or structures involved to have a specific orientation, or to be installed or operated in a specific orientation, and should not be construed as limiting the embodiments in this document.

[0028] High-pressure fuel injectors are crucial components determining the overall performance of an engine, and the needle valve is the core component of a high-pressure fuel injector. In one embodiment, the high-pressure fuel injector structure is as follows: Figure 1 As shown, one end of the needle valve stem 1 is fixedly connected to the valve ball 2, and the other end is connected to the spring 4. Under the action of the spring 4, the valve ball 2 and the valve seat 3 are in contact. In the working state, the needle valve stem 1, driven by the spring 4, performs axial reciprocating motion in the valve seat 3: when the needle valve stem 1 moves to the right, a gap is created between the valve ball 2 and the valve seat 3, and fuel is injected into the engine cylinder through the valve seat 3; when the needle valve stem 1 moves to the left, the valve ball 2 and the valve seat 3 are in contact, and fuel injection stops. In order to ensure sufficient fuel atomization, the fuel injector has a slender structure, and the gap between the needle valve stem 1 and the guide structure inside the high-pressure fuel injector is very small (only a few μm). During the working process, the needle valve stem 1 needs to perform high-frequency reciprocating motion for a long time (up to hundreds of millions of times during its service life). Therefore, the needle valve stem 1 needs to have high straightness, centering and wear resistance to ensure that it will not jam or wear or shift during hundreds of millions of service cycles, thus affecting the sealing of the valve ball 2 and ensuring the normal operation of the high-pressure fuel injector.

[0029] The heat treatment of the needle valve stem 1 presents the following challenges: First, the needle valve stem 1 has a slender structure, approximately 44mm in length and 2mm in diameter, with a length-to-diameter ratio exceeding 20:1, making it prone to warping and deformation during heat treatment. Second, the needle valve stem 1 has high requirements for mechanical strength, fatigue performance, and wear resistance, necessitating nitriding and quenching. The effects of different heat treatment processes interfere with each other, and the complete heat treatment process is lengthy with numerous parameters, typically requiring expensive specialized heating equipment and equally expensive specialized software for control. Most manufacturers lack the necessary processing capabilities. Furthermore, splitting the heat treatment steps would increase equipment usage, and the repeated heating and cooling processes would further increase the risk of thermal deformation.

[0030] To address the aforementioned problems, the inventors, through long-term research and extensive practical experience, proposed a heat treatment method for the needle valve stem of a high-pressure fuel injector. This method allows for the heat treatment of the needle valve stem 1 using a conventional mesh belt furnace. The method includes the following steps:

[0031] First, such as Figure 2 As shown, a needle valve stem blank 1' is provided. The needle valve stem blanks 1' are arranged in two close layers in a flat-bottomed tray 5, so that adjacent needle valve stem blanks 1' abut against each other to form radial constraints, preventing radial deformation of the needle valve stem blanks 1'. In other embodiments, fixing devices such as fixing clips or shaping sleeves can also be added to the needle valve stem blanks 1' to form radial dimensional constraints. The needle valve stem blanks 1' are fed into the mesh belt furnace laterally. Laterally means that the needle valve stem blanks 1' are laid flat with their length direction perpendicular to the forward direction of the mesh belt in the furnace. This arrangement ensures that the temperature of each needle valve stem blank 1' remains consistent along its length, preventing temperature differences at both ends of the needle valve stem blank 1' that could lead to thermal deformation. In a preferred embodiment, the needle valve stem blank 1' is manufactured from X90CrMoV18 steel through hot rolling and cold drawing. Its specific composition, by weight ratio, includes 0.82%-0.98% C, no more than 1.05% Si, no more than 1.03% Mn, no more than 0.045% P, no more than 0.005% S, 16.8%-19.2% Cr, 0.85%-1.35% Mo, and 0.04%-0.15% V, and it possesses high hardness and good wear resistance.

[0032] Mesh belt furnace structure as follows Figure 3 As shown, the furnace includes a heat preservation section 6 and a cooling section 7. In the heat preservation section 6, the needle valve rod blank 1' is austenitized by heating and held at 1080±5℃ for 4-8 minutes. During this process, the specific temperature of the mesh belt furnace and the running speed of the mesh belt can be determined by thermocouple measurement to ensure that the heat preservation time of the needle valve rod blank 1' meets the requirements.

[0033] Three jet nozzles 8 are provided between the heat preservation section 6 and the cooling section 7 of the mesh belt furnace. The jet nozzles 8 spray nitrogen-containing gas towards the needle valve rod blank 1', providing a positive pressure atmosphere containing nitrogen inside the furnace. This prevents oxygen from entering from outside the furnace and allows the needle valve rod blank 1' to undergo surface nitriding treatment simultaneously within the furnace. Simultaneously, the nitrogen-containing gas curtain separates the heat preservation section 6 from the cooling section 7, blocking direct gas flow and reducing the impact of the hot gas flow from the heat preservation section 6 on the cooling section 7. Furthermore, the nitrogen-containing gas directly blowing onto the surface of the needle valve rod blank 1' creates a forced heat exchange effect, accelerating its cooling. The gas flow rate of each jet nozzle 8 can be individually adjusted to ensure uniform and stable airflow, while also facilitating precise adjustment of the intake volume according to processing parameters. In a preferred embodiment, the nitrogen-containing gas is ammonia decomposition gas, which forms a nitrogen-containing atmosphere within the mesh belt furnace.

[0034] A water-cooled jacket is installed on the outside of the cooling section of the mesh belt furnace. A cooling device is further installed upstream of the water inlet 9 of the water-cooled jacket's circulation system. This cooling device is configured as a refrigeration device capable of precisely controlling the water temperature. The needle valve rod blank 1' is quenched by air cooling under the combined action of the nitrogen-containing gas flow and the water-cooled jacket. The cooling rate is controlled at 140-150℃ / min during the quenching process. In a preferred embodiment, the inlet water temperature of the water-cooled jacket is controlled as a constant value within the range of 8℃-16℃ to ensure consistency in the heat treatment effects before and after treatment.

[0035] In the above processing, the effects of nitriding amount, austenitizing heating temperature and time, and quenching cooling rate are coupled and jointly affect the thermal deformation of the needle valve rod blank 1'. For different processing equipment, thermocouples can be used to measure the actual temperature change of the workpiece under different processing parameters in the mesh belt furnace. When the temperature range of the needle valve rod blank 1' exceeds the effective range during heat treatment, the finished needle valve rod is prone to deformation, resulting in a significant decrease in the product qualification rate. At the same time, the nitrided layer cannot effectively play its wear-resistant role, causing the durability test to fail. In other embodiments, other types of heat treatment furnaces with continuous heating function that can provide continuous heat preservation and cooling sections can also be used to implement the heat treatment process.

[0036] In a preferred embodiment, after quenching, the needle valve stem blank 1' is further transferred to a cryogenic furnace for cryogenic treatment at -85±5℃ for no less than 15 minutes to promote the further transformation of the retained austenite in its microstructure into martensite, thereby obtaining better mechanical properties. After cryogenic treatment, the content of retained austenite in the alloy microstructure does not exceed 2.5%.

[0037] In a preferred embodiment, after cryogenic treatment, the needle valve stem blank 1' is further tempered at 185±℃ for 135±15 minutes to eliminate residual stress generated during heat treatment, thereby obtaining the finished needle valve stem.

[0038] After machining is completed, the straightness of the finished needle valve rod 1 is inspected. In a preferred embodiment, the inspection method is as follows: one end of the needle valve rod blank 1' is fixed and rotated around its axis, and the distance of the other end of the needle valve rod blank 1' that floats is measured. The measured floating distance should not exceed 0.034% of the length of the needle valve rod blank 1'. For a 44mm long needle valve rod, in a preferred embodiment, this floating range does not exceed 0.015mm.

[0039] The nitrogen content on the surface of the needle valve stem blank 1' was detected using glow discharge testing. The distribution of nitrogen content with depth is as follows: Figure 4 As shown, the nitrogen content is not less than 0.22% within a depth of 60μm and not less than 0.08% (by weight) within a depth of 160μm. When the nitrogen content is too low, the wear resistance and fatigue performance are insufficient, while when the nitrogen content is too high, it will cause abnormal softening of the needle valve stem surface under service conditions.

[0040] The surface hardness of the finished needle valve stem can reach 680±70HV1, and in the preferred embodiment, it can achieve a fatigue life of no less than 800 million cycles under actual working conditions.

[0041] The purpose of the above embodiments is to provide a further detailed description of the present invention in conjunction with the accompanying drawings, so that those skilled in the art can understand the technical concept of the present invention. Within the scope of the claims of the present invention, optimization or equivalent substitution of the method steps involved, as well as combination of implementation methods in different embodiments without causing structural and principle conflicts, all fall within the protection scope of the present invention.

Claims

1. A heat treatment method for a high-pressure fuel injector needle valve stem, characterized in that, The needle valve rod blank of the high-pressure fuel injector is heat-treated using a heat treatment furnace with continuous heating function, including the following steps: a) Provide a needle valve rod blank, provide radial fixing constraint to the needle valve rod blank and feed it laterally into a heat treatment furnace; b) Provide a nitrogen-containing positive pressure gas atmosphere in the heat treatment furnace, and perform austenitizing heating on the needle valve rod blank at 1080±5℃ for 4min-8min in the heat treatment furnace holding section; c) At the junction of the heat treatment furnace's heat preservation section and cooling section, a nitrogen-containing gas stream is sprayed onto the needle valve rod to separate the heat preservation section and the cooling section. d) The needle valve stem blank is quenched at 140-150℃ / min using air cooling in the cooling section.

2. The heat treatment method for the high-pressure fuel injector needle valve stem according to claim 1, characterized in that, The needle valve stem blank is made of X90CrMoV18 steel, and by weight, its composition includes 0.82%-0.98% C, no more than 1.05% Si, no more than 1.03% Mn, no more than 0.045% P, no more than 0.005% S, 16.8%-19.2% Cr, 0.85%-1.35% Mo and 0.04%-0.15% V.

3. The heat treatment method for the high-pressure fuel injector needle valve stem according to claim 1 or 2, characterized in that, The heat treatment furnace is configured as a mesh belt furnace.

4. The heat treatment method for the high-pressure fuel injector needle valve stem according to claim 1 or 2, characterized in that, The method for radially constraining the needle valve stem blank in step a) is to lay two layers of the needle valve stem blank closely in a flat-bottomed tray.

5. The heat treatment method for the high-pressure fuel injector needle valve stem according to claim 1 or 2, characterized in that, The nitrogen-containing gas atmosphere in step b) and the nitrogen-containing gas flow in step c) are configured as ammonia decomposition gas.

6. The heat treatment method for the high-pressure fuel injector needle valve stem according to claim 1 or 2, characterized in that, In step d), the water inlet temperature of the cooling section is controlled between 8°C and 16°C.

7. The heat treatment method for the high-pressure fuel injector needle valve stem according to claim 1 or 2, characterized in that, The needle valve stem blank also includes a cryogenic treatment step after heat treatment, in which the needle valve stem blank is cooled and processed using a cryogenic furnace.

8. The heat treatment method for the high-pressure fuel injector needle valve stem according to claim 7, characterized in that, The needle valve stem blank also includes a tempering process after the cryogenic treatment step.

9. The heat treatment method for the high-pressure fuel injector needle valve stem according to claim 1 or 2, characterized in that, After heat treatment, the needle valve stem blank satisfies the following conditions: one end of the needle valve stem blank is fixed and rotated around the axis of the needle valve stem blank, and the floating distance of the other end of the needle valve stem blank does not exceed 0.034% of the length of the needle valve stem blank.

10. The heat treatment method for the high-pressure fuel injector needle valve stem according to claim 9, characterized in that, After heat treatment, the nitrogen content of the needle valve stem blank within a depth of 60 μm is not less than 0.22% and the nitrogen content within a depth of 160 μm is not less than 0.08% by weight.

11. The heat treatment method for the high-pressure fuel injector needle valve stem according to claim 10, characterized in that, The needle valve stem blank, after heat treatment, has a fatigue life of no less than 800 million cycles under actual working conditions.

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