18CrNiMo7-6 steel for wind power gear parts and grain refinement heat treatment process and application thereof

Abstract

This invention provides a heat treatment process for refining the grain size of 18CrNiMo7-6 steel used in wind turbine gear components, belonging to the field of heat treatment technology for wind turbine gear steel materials. The 18CrNiMo7-6 steel used in wind turbine gear components undergoes sequential grain refinement treatments including preheating at 800-900℃, austenitization at 920-1050℃, initial cooling at 850-880℃, mid-stage cooling in a salt bath at 300-400℃, high-temperature tempering at 600-700℃, and finally cooling at room temperature. This process solves the problems of coarse grains and mixed grains after direct quenching following deep carburizing, ensuring the stability of the microstructure, achieving grain refinement, and improving mechanical properties, thus showing good application prospects.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for wind turbine gear steel materials, and provides a heat treatment process and application for 18CrNiMo7-6 steel for wind turbine gear parts and its grain refinement. Background Technology

[0002] 18CrNiMo7-6 steel is a grade of surface-hardening steel according to EN10084, which is the steel designation method of the European Committee for Standardization, and its numerical designation is 1.6587. 18CrNiMo7-6 steel is an excellent industrial material with high strength, high hardness and good toughness. It is commonly used to manufacture important components, such as wind turbine gears, bearings and mechanical transmission parts.

[0003] The 18CrNiMo7-6 steel used in wind turbine gear components requires carburizing and quenching to improve the surface hardness, wear resistance, and fatigue performance of the workpiece, while maintaining the toughness and plasticity of low-carbon steel in the internal regions to enable the workpiece to withstand certain impact loads during service. The carburizing process requires holding the carburizing process at a high temperature for an extended period of time.

[0004] Chinese invention patent CN114317928B (publication date: April 12, 2022) discloses a heat treatment method for 18CrNiMo7-6 wind turbine gear steel. The heat treatment method includes the following steps: heating for austenitization, placing the forging in a heating furnace and heating it to 900-1000℃ for austenitization; initial cooling, adjusting the temperature of the heating furnace to maintain at 860℃ and continuously holding it until the temperature of the forging is uniform; quenching and cooling, taking out the uniformly heated forging and quenching it until the temperature of the forging is room temperature. High-temperature tempering involves heating the forging, which has cooled to room temperature, to 650°C and holding it at that temperature. Air cooling forming involves removing the forging after holding it at that temperature and allowing it to cool naturally in the air. The heating furnace has a heating rate of 5°C / min, and the forging is held in the furnace for 30-60 minutes. The quenching time is 100-120 minutes. After quenching, the surface water is dried and the forging is left to stand in the air for 3-5 hours. This high-temperature holding, quenching, and high-temperature tempering heat treatment process shortens the total process time and improves production efficiency.

[0005] However, the most common problem with existing deep carburizing direct quenching techniques is coarse grains and mixed grains. This material, due to its high alloy content and the presence of various strong carbide-forming elements, has excellent hardenability and a very stubborn microstructure. Therefore, once the grains become coarse, it is difficult to refine the grain size. Coarse grains also reduce grain boundary length and area, leading to a relative increase in carbide concentration, thus increasing the susceptibility to temper brittleness. Furthermore, coarse grains result in relatively straight and simple crack propagation paths, reducing energy absorption and significantly impacting impact absorption energy.

[0006] Existing technologies (Zhang Luan, Research on the Characteristics of Salt Bath Staged Quenching Process for Carburized Gears [D]. Master's Thesis, Zhengzhou Mechanical Research Institute, June 2015) have conducted relatively systematic and in-depth research on the cooling characteristics of salt bath media and the process characteristics (core hardness, surface hardness, microstructure, reference carbon content, and distortion characteristics, etc.) of several commonly used materials such as 17CrNiMo6. The cooling capacity of the salt bath media can be adjusted, and for gears with different modules and tooth root cross-sectional thicknesses, the salt bath staged quenching process can effectively control the quenching quality of carburized gears, such as adjusting the core hardness and expansion / contraction deformation. However, the effect of salt bath quenching on grain size is still unclear.

[0007] Currently, most wind turbine gear parts are pretreated by normalizing and tempering or quenching and tempering after forging using 18CrNiMo7-6 steel. However, these processes are time-consuming, costly, and have poor grain size stability, with some grain sizes at level 2, which cannot meet the relevant production requirements of enterprises. Summary of the Invention

[0008] The purpose of this invention is to provide:

[0009] A heat treatment process for refining the grain size of 18CrNiMo7-6 steel used in wind turbine gear components, and related technologies, are disclosed to solve the problems of coarse grains and mixed grains, and to ensure the stability of the microstructure, or a combination thereof.

[0010] Terminology Explanation: Unless otherwise defined, all technical terms in this document have the same meaning as commonly understood by one of ordinary skill in the art to which the subject matter of the claims pertains. Unless otherwise stated, all patents, patent inventions, and disclosures cited in this document are incorporated herein by reference in their entirety. If multiple definitions exist for terms in this document, the definitions in this chapter shall prevail.

[0011] It should be understood that the above brief description and the following detailed description are exemplary and for illustrative purposes only, and do not limit the subject matter of the invention in any way. In this invention, the singular is used in conjunction with the plural unless otherwise specifically stated. It should also be noted that, unless otherwise stated, the use of “or” or “or” means “and / or”. Furthermore, the use of the term “comprising” and other forms such as “including,” “containing,” and “contains” are not limiting.

[0012] The definition of standard chemical terms can be found in the reference "Heat Treatment Handbook, 4th Edition", edited by the Heat Treatment Society of the Chinese Mechanical Engineering Society, Machinery Industry Press, January 2015.

[0013] Unless otherwise stated, conventional methods within the scope of the art shall be used.

[0014] Unless specifically defined herein, the use of all commercially available products herein employs standard techniques. For example, it may be carried out using the manufacturer's instructions for use with the kit, or in accordance with methods known in the art or the description of this invention. The techniques and methods described herein can generally be implemented according to conventional methods well known in the art, based on the descriptions in the various summary and more specific documents cited and discussed in this specification.

[0015] The terms “optional / arbitrary” or “optionally / arbitrarily” mean that the event or situation described below may or may not occur, including both the occurrence and non-occurrence of the event or situation.

[0016] The term "18CrNiMo7-6 steel" used in this article refers to steel that is air-cooled or stack-cooled after forging. It is prone to developing granular bainitic structure, has high hardness, is difficult to machine, and has severe mixed grains. It is also known in the industry as bainitic steel. In order to refine the grains, avoid the occurrence of non-equilibrium structure, and reduce hardness, the forging billet needs to be normalized to obtain a uniform equilibrium structure (ferrite + lamellar pearlite).

[0017] The term "austenite" as used herein refers to a high-temperature face-centered cubic lattice structure in steel, typically obtained by heating steel to a critical temperature (e.g., above Ac1 or Ac3), in which carbon atoms can dissolve. In the process of this invention, the original coarse, non-uniform room-temperature microstructure of the forging (such as granular bainite or martensite) will undergo a phase transformation during the heating and holding stage in step (2), transforming entirely into austenitic microstructure.

[0018] The term "pearlite" as used in this article refers to the layered eutectoid structure formed by alternating ferrite and cementite in steel during slow cooling after austenitization.

[0019] The term "martensite" used in this article refers to a body-centered cubic (bcc) structure, which is a supersaturated solid solution of carbon in α-Fe.

[0020] The term "bainite" as used in this article refers to the multiphase structure of ferrite and carbides formed when steel is isothermally cooled after austenitization within a range below the pearlite transformation temperature and above the martensite transformation temperature.

[0021] The term "sorbite" used in this article, also known as "fine pearlite," is a transformation product with an average lamellar spacing of 800-1500 Å obtained from the pearlite transformation of supercooled austenite at 650-600℃.

[0022] The term "ferrite" as used in this article refers to a body-centered cubic lattice structure in steel with extremely low carbon content, which usually precipitates eutectoidally during slow cooling or exists in the equilibrium structure.

[0023] The term “sample size D” as used in this article refers to the maximum dimension of the 18CrNiMo7-6 steel to be treated in all directions, such as the larger dimension of the diameter or length (thickness) in columnar 18CrNiMo7-6 steel.

[0024] In a first aspect, the present invention provides a heat treatment process for refining the grain size of 18CrNiMo7-6 steel used in wind turbine gear components, comprising the following steps: (1) Preheating: Heat the annealed 18CrNiMo7-6 steel to be treated to 800-900℃ for preheating; (2) Austenitization: The preheated 18CrNiMo7-6 steel is heated to 920-1050℃ and held at that temperature to austenitize it; (3) Initial cooling: Cool the austenitized 18CrNiMo7-6 steel to 850-880℃ and hold it at that temperature; (4) Intermediate cooling: The 18CrNiMo7-6 steel, after initial cooling, is cooled and kept warm in a salt bath at 300-400℃; (5) High-temperature tempering: After the 18CrNiMo7-6 steel cooled by salt bath is cooled at room temperature, it is heated to 600-700℃ and held at that temperature. (6) Cooling: The 18CrNiMo7-6 steel after high-temperature tempering is cooled at room temperature to obtain the 18CrNiMo7-6 steel for wind turbine gears.

[0025] In some preferred embodiments of the present invention, step (1) includes two stages of preheating: First stage: Increase the temperature to 400-600℃ at a rate of 50-100℃ / h; Second stage: Increase the temperature to 800-900℃ at a rate of 30-50℃ / h and maintain the temperature.

[0026] The heating rate in the first stage is 50-100℃ / h, including but not limited to 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100℃ / h; the heating temperature is 400-600℃, including but not limited to 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, 500, 510, 520, 530, 540, 550, 560, 570, 580, 590 or 600℃.

[0027] The second stage heating rate is 30-50℃ / h, including but not limited to 30, 35, 40, 45, or 50℃ / h; the heating temperature is 800-900℃, including but not limited to 800, 810, 820, 830, 840, 850, 860, 870, 880, 890, or 900℃; the holding time T1 and the sample size D satisfy the following relationship: T1 = K•αD; In the formula, T1 is the holding time in minutes; K is 1.0-1.3, dimensionless; α is 0.7-0.8 min / mm; and D is the sample size in mm. The sample size refers to the maximum dimension of the 18CrNiMo7-6 steel to be treated in all directions, such as the larger dimension of diameter or length (thickness) in columnar 18CrNiMo7-6 steel.

[0028] Step (1) employs a two-stage preheating process and limits the holding time T1 based on the sample size D. This effectively reduces the internal and external temperature difference of the large wind turbine gear forging (18CrNiMo7-6 steel) during the heating process, lowers thermal stress, and prevents workpiece cracking. It also provides a foundation for subsequent austenitization. If the temperature of the first stage of preheating is too low (below 400-600℃) or the heating rate is too fast (>100℃ / h), the temperature difference between the surface and the core of the workpiece will be too large, which can easily generate huge thermal stress inside the workpiece and even lead to microcracks. If the temperature of the second stage of preheating is too high (above 900℃), it will cause the proeutectoid ferrite and pearlite to undergo an uneven austenite transformation prematurely, which is not conducive to the final grain refinement. Determining the holding time based on the sample size can ensure that the core part just reaches the set temperature. If the holding time is too short, the center will not be fully heated. If the time is too long, it will increase energy consumption and may cause local grains to start growing.

[0029] In some preferred embodiments of the present invention, in step (2), the austenitizing holding temperature is 920-1050℃, including but not limited to: 920, 930, 940, 950, 960, 970, 980, 990, 1000, 1010, 1020, 1030, 0, 1040 or 1050℃.

[0030] In some preferred embodiments of the present invention, in step (2), the austenitizing holding time T2 is related to the sample size D by the following: T2 = K•αD; In the formula, T2 is the holding time in minutes; K is 1.0-1.3, dimensionless; α is 0.7-0.8 min / mm; and D is the sample size in mm. Similarly, the sample size refers to the maximum dimension of the 18CrNiMo7-6 steel to be treated in all directions, such as the larger dimension of the diameter or length (thickness) in columnar 18CrNiMo7-6 steel.

[0031] In step (2), during the austenitization process, holding the temperature at 920-1050℃ can completely transform the original microstructure (bainite / ferrite / carbides, etc.) in 18CrNiMo7-6 steel into a uniform face-centered cubic austenite structure, and allow the alloying elements (Cr, Ni, Mo) to fully dissolve. This is a prerequisite for achieving subsequent grain refinement and homogenization. If the holding temperature is lower than 920℃ or the holding time is insufficient, the alloy carbides cannot dissolve sufficiently, resulting in uneven austenite composition. After cooling, soft spots will appear locally, and the grain refinement effect will not be achieved. If the holding temperature is higher than 1050℃ or the holding time is too long, it will cause the austenite grains to become rapidly coarse (i.e., overheating). This coarse austenite will be inherited by the room temperature microstructure during subsequent cooling, causing a sharp decrease in the toughness of the material and seriously affecting its mechanical properties.

[0032] In some preferred embodiments of the present invention, in step (3), the initial cooling temperature is 850-880°C, including but not limited to 850, 855, 860, 865, 870, 875 or 880°C.

[0033] In some preferred embodiments of the present invention, in step (3), during the initial cooling process, the heat preservation time T3 is 20-30 min, including but not limited to 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30 min.

[0034] In some preferred embodiments of the present invention, in step (4), the temperature of the mid-section cooling is 300-400°C, including but not limited to: 300, 310, 320, 330, 340, 350, 360, 370, 380, 390 or 400°C.

[0035] In some preferred embodiments of the present invention, in step (4), the salt bath is a nitrate salt, wherein the nitrate salt is selected from sodium nitrate and / or potassium nitrate.

[0036] Preferably, the nitrate is selected from sodium nitrate and potassium nitrate.

[0037] More preferably, the nitrate is selected from sodium nitrate and potassium nitrate in a mass ratio of 1:1.

[0038] In some preferred embodiments of the present invention, in step (4), during the mid-section cooling process, the heat preservation time T4 is ≥3h, including but not limited to 3, 3.5, 4, 4.5 or 5h and any time ≥3.

[0039] Preferably, the heat preservation time T4 is ≥5h.

[0040] The multi-stage cooling process employed in this invention is the core of the entire heat treatment process. First, the material is held at 850-880℃ for a short time (20-30 min) to reduce the temperature gradient of the supercooled austenite before it enters the salt bath, without causing non-equilibrium transformation. This is equivalent to a "pre-cooling" treatment, which significantly reduces the thermal shock and structural stress generated during subsequent rapid salt bath quenching, ensuring the dimensional stability of large gear components. Next, it undergoes isothermal cooling (isothermal quenching) in a salt bath at 300-400℃. Within this temperature range, the supercooled austenite undergoes an isothermal transformation into a fine lower bainite structure. This fine acicular / lamellar lower bainite not only has extremely fine grains but also possesses excellent strength and toughness. Sufficient holding time (≥3 h) is required to ensure that the austenite in the core of the large forging completely transforms into bainite.

[0041] In some preferred embodiments of the present invention, in step (5), the high-temperature tempering temperature is 600-700°C, including but not limited to 600, 610, 620, 630, 640, 650, 660, 670, 680, 690 or 700°C.

[0042] In some preferred embodiments of the present invention, in step (5), the temperature for heat preservation is 630-680°C, including but not limited to: 630, 640, 650, 660, 670 or 680°C.

[0043] In some preferred embodiments of the present invention, in step (5), the holding time T5 during the high-temperature tempering process is greater than the holding time T2 during the austenitizing process.

[0044] This invention employs a high-temperature tempering process to ensure the transformation of bainite (or trace martensite) obtained in the previous process into tempered sorbite (composed of polygonal ferrite and fine spherical carbides). This process eliminates residual internal stress, significantly reduces hardness to meet subsequent machining requirements, and further cuts and refines the original grain morphology, solidifying the stability of grain size. A holding time T5 > T2 is set to ensure sufficient spheroidization and aggregation of carbides inside large workpieces.

[0045] Secondly, the present invention provides 18CrNiMo7-6 steel obtained by the above-mentioned heat treatment process.

[0046] Thirdly, the present invention also provides the application of the above-mentioned 18CrNiMo7-6 steel in wind turbine gears.

[0047] In this invention, the technical feature “in step (1), the preheating includes two stages: the first stage: heating to 400-600℃ at a rate of 50-100℃ / h; the second stage: heating to 800-900℃ at a rate of 30-50℃ / h and holding” is summarized from the foregoing explanation and / or the corresponding technical features in Examples 1-2 such as “heating to 500℃ at a rate of 50℃ / h, then heating to 850℃ at a rate of 30℃ / h and holding for 1h” and “heating to 500℃ at a rate of 50℃ / h, then heating to 830℃ at a rate of 30℃ / h and holding for 1h” (i.e., the specific process of step (1)). Therefore, those skilled in the art can reasonably infer that this technical feature, its subordinate concept, its essentially equivalent technical means, and technical means that can replace this technical feature based on existing technology and conventional technical means and common knowledge should all fall within the protection scope of claim 1 of this invention.

[0048] The technical feature “the relationship between the heat preservation time T1, the heat preservation time T2 and the sample size D” is summarized from the foregoing explanation and / or the corresponding technical feature “size of φ80mm, heat preservation for 1h” in Examples 1-2 (that is, the size of the sample to be processed and the heat preservation time of steps (1) and (2)). Therefore, those skilled in the art can reasonably infer that this technical feature, its subordinate concept, its essentially equivalent technical means, and technical means that can replace this technical feature based on the existing level of technology and conventional technical means and common knowledge should all fall within the protection scope of claim 1 of this invention.

[0049] The technical feature “austenitization: heating the preheated 18CrNiMo7-6 steel to 920-1050℃ and holding it at that temperature for austenitization” is summarized from the foregoing explanation and / or the corresponding technical feature in step (2) of Examples 1-2. Therefore, those skilled in the art can reasonably infer that this technical feature, its subordinate concepts, its substantially equivalent technical means, and technical means that can replace this technical feature based on existing technology and conventional technical means and common knowledge should all fall within the protection scope of claim 1 of this invention.

[0050] The technical features described above, namely "initial cooling: cooling the austenitized 18CrNiMo7-6 steel to 850-880℃ and holding it at that temperature" and "holding time T3 is 20-30 min," are summarized from the foregoing explanation and / or the corresponding technical features in step (3) of Examples 1-2. Therefore, those skilled in the art can reasonably infer that this technical feature, its subordinate concepts, its substantially equivalent technical means, and technical means that can replace this technical feature based on existing technology and conventional technical means and common knowledge, should all fall within the protection scope of claim 1 of this invention.

[0051] The technical features “mid-stage cooling: the 18CrNiMo7-6 steel after initial cooling is cooled and held in a salt bath at 300-400℃” and “holding time T4 is ≥3h” are summarized from the foregoing explanation and / or the technical features of step (4) of Examples 1-2. Therefore, those skilled in the art can reasonably infer that this technical feature, its subordinate concepts, its essentially equivalent technical means, and technical means that can replace this technical feature based on existing technology and conventional technical means and common knowledge should all fall within the protection scope of claim 1 of this invention.

[0052] The technical features described above, namely "high-temperature tempering: after the 18CrNiMo7-6 steel cooled in a salt bath is cooled at room temperature, it is then heated to 600-700℃ and held at that temperature" and "the holding time T5 is greater than the holding time T2 during the austenitization process," are summarized from the foregoing explanation and / or the technical features of step (5) in Examples 1-2. Therefore, those skilled in the art can reasonably infer that this technical feature, its subordinate concepts, its essentially equivalent technical means, and technical means that can replace this technical feature based on existing technology and conventional technical means and common knowledge, should all fall within the protection scope of claim 1 of this invention.

[0053] Compared with the prior art, the present invention has the following beneficial effects: After heat treatment according to the present invention, the tensile strength is increased to 952 MPa and 968 MPa, respectively; the elongation is increased to 36% and 35%, respectively; the average diameter is reduced to 0.04200 mm and 0.04230 mm, respectively; and the proportion of grains of grade 6 and above reaches 52.00% and 55.00%, respectively. This indicates that the present invention, through a combination of preheating, austenitization, initial cooling, intermediate salt bath cooling, and high-temperature tempering, can effectively refine grains, improve grain distribution uniformity, and simultaneously enhance the strength and plasticity of the material.

[0054] The present invention further optimizes the temperature conditions in each process of the heat treatment process, and finds that the heat treatment process of the present invention, combined with the setting of process parameters, as a whole, makes an undeniable contribution to refining the grains of 18CrNiMo7-6 steel, improving the uniformity of the microstructure and enhancing the mechanical properties. Attached Figure Description

[0055] Figure 1 This is a metallographic (OM) structure diagram of the heat-treated part of Embodiment 1 of the present invention; Figure 2 This is a scanning electron microscope (SEM) image of the tissue obtained from the heat treatment in Embodiment 1 of the present invention. Detailed Implementation

[0056] The following non-limiting embodiments are intended to enable those skilled in the art to gain a more comprehensive understanding of the present invention, but do not limit the invention in any way. The following content is merely an exemplary description of the scope of protection claimed by the present invention, and those skilled in the art can make various changes and modifications to the present invention based on the disclosed content, and such changes should also fall within the scope of protection claimed by the present invention.

[0057] The present invention will be further described below by way of specific embodiments. Unless otherwise specified, all instruments, devices, equipment, reagents, products, etc., used in the embodiments of the present invention are obtained through conventional commercial means.

[0058] The relevant performance indicators of the annealed 18CrNiMo7-6 steel (diameter φ=80mm, D=50mm) to be treated in the following examples are shown in Table 1.

[0059] Example 1 This embodiment provides a heat treatment process for refining the grain size of 18CrNiMo7-6 steel used in wind turbine gear components. The specific steps are as follows: (1) Preheating: The wind turbine gear parts made of 18CrNiMo7-6 steel are placed in a heating furnace and heated to 500℃ at a heating rate of 50℃ / h, and then heated to 850℃ at a heating rate of 30℃ / h and held for 1h. (2) Austenitization: The furnace was then heated to 950°C at a heating rate of 10°C / h and held for 1 hour to perform austenitization. (3) Initial cooling: The furnace is then cooled to 850°C and held for 30 minutes for initial cooling; (4) Mid-section cooling: Then remove and place in a salt bath furnace (NaNO3 and KNO3 with a mass ratio of 1:1) at a temperature of 350℃ for 5 hours; (5) High-temperature tempering: Then remove it and allow it to cool naturally to room temperature in the air, and then place it in a heating furnace for tempering at 630°C for 2 hours; (6) Finally, remove and cool at room temperature to obtain the 18CrNiMo7-6 steel used for wind turbine gear parts.

[0060] The metallographic structure and scanning electron microscope (SEM) images of the treated 18CrNiMo7-6 steel are shown below. Figure 1 and Figure 2 As shown, under macroscopic metallographic view, the original austenite grain boundaries are fine and the structure is evenly distributed. No obvious network carbide precipitation or severe structural segregation is observed. Under high magnification electron microscopy, a large number of fine, round granular carbides are clearly shown to be evenly and diffusely distributed on the ferrite matrix. There are no carbide aggregations and no microcracks at the grain boundaries. The overall microstructure is dense and stable.

[0061] Example 2 This embodiment provides a heat treatment process for refining the grain size of 18CrNiMo7-6 steel used in wind turbine gear components. The specific steps are as follows: (1) Preheating: The wind turbine gear parts made of 18CrNiMo7-6 steel are placed in a heating furnace and heated to 500℃ at a heating rate of 50℃ / h, and then heated to 830℃ at a heating rate of 30℃ / h and held for 1h. (2) Austenitization: The heating furnace is then heated to 1000℃ at a heating rate of 10℃ / h and held for 1h to perform austenitization; (3) Initial cooling: The furnace is then cooled to 850°C and held for 1 hour for initial cooling; (4) Mid-section cooling: Then remove and place in a salt bath furnace (NaNO3 and KNO3 with a mass ratio of 1:1) at a temperature of 300℃ for 6 hours; (5) High-temperature tempering: Then remove it and allow it to cool naturally to room temperature in the air, and then place it in a heating furnace for tempering at 650°C for 1.5 hours; (6) Finally, remove and cool at room temperature to obtain the 18CrNiMo7-6 steel used for wind turbine gear parts.

[0062] Comparative Example 1 The wind turbine gear components are heat-treated using 18CrNiMo7-6 steel according to method ZL202111556886.4. The specific steps are as follows: (1) Austenitization: The wind turbine gear parts made of 18CrNiMo7-6 steel are placed in a heating furnace and heated to 900℃ at a heating rate of 5℃ / h, and held for 30min to austenitize. (2) Initial cooling: The furnace is then cooled to 860℃ and held for 12 minutes for initial cooling; (3) Quenching and cooling: Take out the uniformly heated forging and quench it until the temperature of the forging is room temperature (the quenching medium is water, the water is kept at room temperature before quenching, the time from taking it out of the heating furnace to encountering water during quenching is controlled within 60 seconds, and the quenching time is 100 minutes). After quenching, dry the surface water stains and place it in the air for 3 hours. (4) High-temperature tempering: Place the quenched forgings back into the heating furnace and temper them at 650℃ for 1 hour (the heating rate of high-temperature tempering is the same as the heating rate of the heating furnace). (5) Finally, remove and cool at room temperature to obtain the 18CrNiMo7-6 steel used for wind turbine gear parts.

[0063] Comparative Example 2: Compared with Example 1, the only difference is that the preheating step of 830-850℃ is omitted, that is, the 18CrNiMo7-6 steel is directly heated to the austenitizing temperature for treatment, and the other process conditions are the same as those in Example 1.

[0064] Comparative Example 3 Compared with Example 1, the only difference is that the initial cooling (850-880℃ holding) step is omitted, that is, after austenitization, it directly enters the intermediate salt bath cooling. All other process conditions are the same as in Example 1.

[0065] Comparative Example 4 Compared with Example 1, the only difference is that the intermediate salt bath cooling (cooling and holding in a salt bath at 300-400°C) step is omitted. That is, after austenitization, only the initial cooling is performed and then high-temperature tempering is performed directly. All other process conditions are the same as in Example 1.

[0066] Comparative Example 5 Compared with Example 1, the only difference is that the high-temperature tempering (630-680℃ holding) step is omitted, that is, after austenitization, initial cooling and intermediate salt bath cooling, it is directly air-cooled. All other process conditions are the same as in Example 1.

[0067] Comparative Example 6 Compared with Example 1, the only difference is that the austenitizing temperature is 900°C, while all other process conditions are the same as in Example 1.

[0068] Comparative Example 7 Compared with Example 1, the only difference is that the austenitizing temperature is 1080°C, while all other process conditions are the same as in Example 1.

[0069] Comparative Example 8 Compared with Example 1, the only difference is that the mid-section salt bath temperature is 250°C, while all other process conditions are the same as in Example 1.

[0070] Comparative Example 9 Compared with Example 1, the only difference is that the mid-section salt bath temperature is 450°C, while all other process conditions are the same as in Example 1.

[0071] Comparative Example 10 Compared with Example 1, the only difference is that the high-temperature tempering temperature is 580°C, while all other process conditions are the same as in Example 1.

[0072] Comparative Example 11 Compared with Example 1, the only difference is that the high-temperature tempering temperature is 720°C, while all other process conditions are the same as in Example 1.

[0073] Test Example 1 The mechanical properties and grain size of the 18CrNiMo7-6 steel after heat treatment in the above embodiments were tested. The mechanical properties were tested according to the standard GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature"; the grain size was measured according to the standard GB / T6394-2017 "Metallic materials - Determination of average grain size". The results are shown in Table 1.

[0074] Table 1. Mechanical properties and grain size results of 18CrNiMo7-6 steel before and after heat treatment in each comparative example.

[0075] As shown in Table 1, the tensile strength of the annealed 18CrNiMo7-6 steel without heat treatment is 700MPa, the elongation is 20%, and the average diameter is 0.06200mm, indicating that the raw material has a relatively coarse structure and relatively low comprehensive performance.

[0076] After being treated by the heat treatment process of the present invention, the tensile strengths of Examples 1 and 2 increased to 952 MPa and 968 MPa, respectively; the elongation increased to 36% and 35%, respectively; the average diameter decreased to 0.04200 mm and 0.04230 mm, respectively; and the proportion of grains of grade 6 and above reached 52.00% and 55.00%, respectively. This demonstrates that the present invention, through a combination of preheating, austenitization, initial cooling, intermediate salt bath cooling, and high-temperature tempering, can effectively refine grains, improve grain distribution uniformity, and simultaneously enhance the strength and plasticity of the material.

[0077] Comparative Example 1, after undergoing conventional quenching and tempering processes as described in existing technologies, exhibited a tensile strength of 828 MPa, an elongation of 29%, and an average diameter of 0.05200 mm. The grain refinement effect and mechanical properties were significantly lower than those of Examples 1 and 2. These results demonstrate that the present invention is not a simple replacement of existing conventional heat treatment routes, but rather achieves superior microstructure control through staged cooling and mid-section salt bath heat preservation.

[0078] Compared with Example 1, after the preheating step or the initial cooling step was removed in Comparative Examples 2 and 3, the tensile strength and elongation of the materials decreased, the average diameter increased to 0.05600 mm to 0.05750 mm, and the proportion of grains of grade 6 and above decreased significantly. This indicates that the preheating and initial cooling steps play an important role in reducing the temperature gradient, avoiding microstructure segregation, and suppressing mixed crystals.

[0079] Compared with Example 1, the material properties of Comparative Examples 4 and 5 were lower than those of the Example after the mid-section salt bath cooling or high-temperature tempering was omitted. The average diameter was still maintained at 0.04950 mm to 0.05100 mm, indicating that the mid-section salt bath cooling and high-temperature tempering steps play a key role in further refining the grains, eliminating internal stress and stabilizing the microstructure.

[0080] Compared with Example 1, Comparative Examples 6 to 11 adjusted the austenitizing, intermediate salt bath, or high-temperature tempering temperatures. It was found that using the heat treatment process of the present invention, if the temperature deviated from a certain range in each treatment step, the average diameter of the material increased to varying degrees, reaching 0.05000 mm to 0.06500 mm, while the tensile strength and elongation decreased to varying degrees. This indicates that the heat treatment process of the present invention is not arbitrarily set, but rather an optimal window determined based on extensive experimentation. The specific heat treatment process, combined with the setting of process parameters, is an integral whole, indispensable for improving the strength, plasticity, and grain size of 18CrNiMo7-6 steel.

[0081] In summary, by rationally setting the process parameters for preheating, austenitization, initial cooling, intermediate salt bath cooling, and high-temperature tempering, this invention can significantly refine the grains of 18CrNiMo7-6 steel, improve the uniformity of the microstructure, and enhance its mechanical properties. It overcomes the problems of coarse grains, mixed grains, and insufficient microstructure stability in the prior art and has good application prospects.

[0082] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A heat treatment process for refining the grain size of 18CrNiMo7-6 steel used in wind turbine gear components, characterized in that, Includes the following steps: (1) Preheating: Heat the annealed 18CrNiMo7-6 steel to be treated to 800-900℃ for preheating; (2) Austenitization: The preheated 18CrNiMo7-6 steel is heated to 920-1050℃ and held at that temperature to austenitize it; (3) Initial cooling: Cool the austenitized 18CrNiMo7-6 steel to 850-880℃ and hold it at that temperature; (4) Intermediate cooling: The 18CrNiMo7-6 steel, after initial cooling, is cooled and kept warm in a salt bath at 300-400℃; (5) High-temperature tempering: After the 18CrNiMo7-6 steel cooled by salt bath is cooled at room temperature, it is heated to 600-700℃ and held at that temperature. (6) Cooling: The 18CrNiMo7-6 steel after high-temperature tempering is cooled at room temperature to obtain the 18CrNiMo7-6 steel for wind turbine gears.

2. The heat treatment process according to claim 1, characterized in that, In step (1), the preheating includes two stages: First stage: Increase the temperature to 400-600℃ at a rate of 50-100℃ / h; Second stage: Increase the temperature to 800-900℃ at a rate of 30-50℃ / h and maintain the temperature; The relationship between the holding time T1 and the sample size D satisfies: T1 = K•αD; In the formula, T1 is the heat preservation time in min; K is 1.0-1.3, dimensionless; α is 0.7-0.8 min / mm; and D is the sample size in mm.

3. The heat treatment process according to claim 1, characterized in that, In step (2), the austenitization holding time T2 and the sample size D are related by the following: T2 = K•αD; In the formula, T2 is the heat preservation time in min; K is 1.0-1.3, dimensionless; α is 0.7-0.8 min / mm; and D is the sample size in mm.

4. The heat treatment process according to claim 1, characterized in that, In step (3), the initial cooling process has a holding time of 20-30 minutes (T3).

5. The heat treatment process according to claim 1, characterized in that, In step (4), the salt bath uses nitrate, wherein the nitrate is selected from sodium nitrate and / or potassium nitrate.

6. The heat treatment process according to claim 1, characterized in that, In step (4), the heat preservation time T4 during the mid-section cooling process is ≥3h.

7. The heat treatment process according to claim 1, characterized in that, In step (5), the holding time T5 during the high-temperature tempering process is greater than the holding time T2 during the austenitization process.

8. The heat treatment process according to claim 1, characterized in that, In step (5), the holding temperature during the high-temperature tempering is 630-680℃.

9. 18CrNiMo7-6 steel obtained by the heat treatment process according to any one of claims 1-8.

10. The application of 18CrNiMo7-6 steel according to claim 9 in wind turbine gears.

Images