Process for improving quenching performance of wind turbine main shaft during heat treatment

Abstract

This invention discloses a process for improving the hardenability of wind turbine main shafts during heat treatment, comprising the following steps: heating 42CrMo4 steel ingots to 1180~1220℃ for forging, with a forging ratio ≥4; air-cooling the forgings to 650~700℃ for the first time before loading them into a furnace, holding them at 300~350℃, then raising them to 640~660℃ for a second holding, with the holding time determined by the effective thickness × 1.5min / mm, followed by cooling to room temperature; vertically loading the forgings into tooling and heating them to 8... The surface is held at 40~860℃ for 1.2~1.5 min / mm (effective thickness × 1.2~1.5 min / mm) until the surface temperature drops to 780~800℃. Then, it is quenched in PAG solution within 3 minutes of quenching. After quenching, it is transferred to a tempering furnace and heated to 300℃ at a rate of ≤80℃ / h, held, and then heated to 540~580℃ at a rate of ≤100℃ / h, held for 2.0~2.5 min / mm (effective thickness × 2.0~2.5 min / mm). After tempering, it is cooled at a rate of ≤50℃ / h. This invention refines grains and carbides through forging and upsetting processes, controls pearlite cluster size through normalizing, and improves cooling uniformity through a density-layered dual-medium system during quenching. This method is suitable for improving the hardenability of large cross-sections in high-power wind turbine main shafts.

Description

Technical Field

[0001] This invention relates to the field of wind turbine main shaft technology, and more specifically to a process for improving the heat treatment hardenability of wind turbine main shafts. Background Technology

[0002] The wind turbine main shaft is the core load-bearing component of the wind turbine's transmission system. It connects the impeller and the gearbox and withstands complex alternating loads over long periods. Its reliability and lifespan directly determine the overall operational safety of the turbine. As the wind power industry develops towards larger and deeper-sea applications, the power output of wind turbines has increased to over 10MW. This has led to a dramatic increase in the size and load-bearing capacity of the main shaft, which in turn places higher demands on the yield strength, low-temperature toughness, and fatigue life of the materials.

[0003] Hardenability is a key intrinsic property determining the uniformity of the cross-sectional properties of a spindle. In actual production, insufficient hardenability in the spindle leads to inadequate core cooling, which easily results in the formation of non-martensitic structures (such as ferrite and pearlite), creating regions of abrupt strength changes. This reduces the internal hardness and toughness of the spindle, becoming a potential hazard for early fatigue fracture. Existing research indicates that commonly used materials for traditional spindles (such as 42CrMo4) have hardenability bottlenecks at ultra-large cross-sectional dimensions, making it difficult to meet the long-term reliable operation requirements of high-power wind power equipment.

[0004] In the heat treatment technology of key wind power components, matching materials and processes is challenging. There is a significant contradiction between reducing cooling intensity to prevent quenching cracks and implementing strong cooling to ensure hardenability. The lack of a precisely matched process window leads to substantial differences in the microstructure and properties between the core and surface. Existing carburizing and induction hardening technologies are mostly focused on bearings or gears and are difficult to directly apply to large forgings. The cooling gradient from the surface to the core of the spindle is complex.

[0005] Therefore, it is necessary to develop a precise hardenability control process for ultra-large cross-section wind turbine main shafts to achieve uniformity of the core and surface microstructure properties, thereby improving the reliability and service life of wind turbine main shafts. Summary of the Invention

[0006] The object of the present invention is to solve at least one of the technical problems shown in the background art.

[0007] To achieve the above objectives, the technical solution provided by the present invention is as follows.

[0008] A process for improving the heat treatment hardenability of wind turbine main shafts includes the following steps: S10. Heat 42CrMo4 steel ingots to 1180~1220℃ for forging to obtain forgings of wind turbine main shafts, with a forging ratio controlled at ≥4. S20. Perform the first air cooling on the forging. When the surface temperature drops to 650~700℃, put it in the furnace and heat it to 300~350℃ and hold it for 2~3 hours. Then raise the temperature to 640~660℃ for the second holding. The holding time is determined according to the effective thickness of the forging × 1.5min / mm. Finally, cool it to room temperature. S30. Place the forging vertically into the tooling and heat it to 840~860℃. Hold it at that temperature for 1.2~1.5 min / mm based on the effective thickness. When the surface temperature drops to 780~800℃, quench it in the PAG solution. S40. Immediately transfer the quenched forging to a tempering furnace, heat it to 300℃ at ≤80℃ / h and hold it for 1~2h, then heat it to 540~580℃ at ≤100℃ / h and hold it. The holding time is calculated as effective thickness × 2.0~2.5min / mm. After tempering, cool it at ≤50℃ / h.

[0009] As a preferred technical solution, in S10, the effective cross-sectional diameter of the forging is controlled within the range of 800~1200mm, and the content of Cr, Mo and Mn in its alloy composition satisfies the hardenability coefficient J factor ≥ 45 by mass percentage, wherein the J factor is determined by J = 30 + 5×(%Cr) + 8×(%Mo) + 3×(%Mn).

[0010] As a preferred technical solution, in S10, the components of the forging, by mass percentage, include: C: 0.38-0.45%; Cr: 0.90-1.20%; Mo: 0.15-0.30%; Mn: 0.60-0.90%; Ni≤0.80%; V≤0.6%; Si≤0.40%; Gu≤0.1%; P≤0.025%; S≤0.025%; N≤70ppm; H≤1.2ppm; O≤30ppm; and the content of any single element Pb, Sn, As, Sb, or Bi does not exceed 0.01%.

[0011] As a preferred technical solution, in S10, the forging process adopts an alternating upsetting and drawing process, the final drawing forging temperature is controlled at 1050~1080℃, the single reduction is controlled at 20~25%, the cumulative upsetting ratio is ≥2.0, and the drawing ratio is ≥3.0.

[0012] As a preferred technical solution, in S20, when the effective thickness of the forging is ≥500mm, the holding time is extended to 1.8min / mm of the effective thickness when the temperature of the second holding is in the range of 640~660℃. After the holding is completed, the forging is slowly cooled to room temperature at a rate of 15~25℃ / h, so that the size of the pearlite clusters in the core is controlled below ASTM grade 8.

[0013] As a preferred technical solution, in S30, the large end of the spindle and the step transition area use a 12-15% concentration PAG solution, while the spindle body and the small end use an 8-10% concentration PAG solution, and the medium temperature is controlled at 25-40℃.

[0014] As a preferred technical solution, in S30, a dual-medium system with density stratification is pre-placed in the quenching tank: the lower layer is a water-soluble PAG solution, and the upper layer is an oil-based PAG emulsion. The two media are immiscible and have a stable interface. During quenching, the large end of the spindle and the step transition zone first pass through the upper oil-based PAG emulsion to form a slow-cooling oil film, and then enter the lower water-soluble PAG solution to complete the quenching.

[0015] As a preferred technical solution, in S30, a transition layer with a height of 80~150mm is provided between the upper oil-based PAG emulsion and the lower water-soluble PAG solution. The transition layer is an oil-water emulsion, which is made by mixing oil-based PAG emulsion, water-soluble PAG solution and emulsifier in a mass ratio of 5:3:1.

[0016] As a preferred technical solution, in S30, a surfactant is added to the upper oil-based PAG emulsion. The composite surfactant is obtained by compounding polyoxyethylene dehydrated sorbitan monooleate, sodium dodecylbenzene sulfonate and fatty alcohol polyoxyethylene ether in a mass ratio of 2:1:1, and the amount added is 0.5~1.2% of the mass of the oil-based PAG emulsion.

[0017] As a preferred technical solution, in S30, a wetting accelerator is added to the lower water-soluble PAG solution. The wetting accelerator is a mixture of polyether-modified polysiloxane and alkyl glycoside in a mass ratio of 1:1, and the amount added is 0.3~0.6% of the mass of the water-soluble PAG solution.

[0018] The advantages and beneficial effects of this invention are as follows: Through the control of forging process and heat treatment parameters, the S10 stage adopts an alternating process of upsetting and drawing, and the forging temperature is controlled at 1050~1080℃ and the single reduction is 20~25% in the final heat treatment, so that the core carbides are fully broken and evenly distributed, and the grains are refined. This provides a short path for the diffusion of carbon elements during the subsequent austenitization process, and reduces the influence of local component segregation on phase transformation kinetics. The normalizing treatment in the S20 stage, combined with the differential heat treatment of the effective thickness of the forging, controls the pearlite cluster size, reduces the inheritance of ferrite / pearlite in the original microstructure, and improves the uniformity of austenite transformation.

[0019] The quenching process introduces a density-stratified dual-medium system. The upper layer, an oil-based PAG emulsion, forms a slow-cooling oil film at the large end of the spindle and in the step transition zone, delaying the initial cooling rate of this area entering the water-based medium and reducing thermal stress concentration at complex cross-sections. The lower layer, a water-soluble PAG solution, incorporates a wetting promoter composed of polyether-modified polysiloxane and alkyl glycosides. This reduces the interfacial tension between the liquid phase and the metal surface, improves the transition from film boiling to nucleation boiling in thick-walled regions, enhances the uniformity of vapor film rupture, and thus makes the core cooling rate more compatible with the surface cooling rate. The tempering stage employs a segmented heating and controlled cooling process. After holding at 300℃ to precipitate carbides, holding at 540–580℃ spheroidizes the carbide particles and controls their diameter to 0.2–0.5 μm, preventing the continuous precipitation of grain boundary carbides from weakening toughness. The wind turbine main shaft processed by the above process achieves a metallographic structure of Grade 1 sorbite according to GB / T 13320-2007, and the hardness fluctuation between the core and the surface is controlled within ±5HB. Detailed Implementation

[0020] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below. It is to be understood that the specific embodiments described herein are merely illustrative of this application and not intended to limit it. Furthermore, it should be noted that all other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0021] The terms “comprising” and “having”, and any variations thereof, used in this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such process, method, product, or apparatus.

[0022] 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 this application. 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 or implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0023] This invention provides a process for improving the hardenability of wind turbine main shafts after heat treatment, comprising the following steps: S10. Heat 42CrMo4 steel ingots to 1180~1220℃ for forging to obtain forgings of wind turbine main shafts, with a forging ratio controlled at ≥4. S20. Perform the first air cooling on the forging. When the surface temperature drops to 650~700℃, put it in the furnace and heat it to 300~350℃ and hold it for 2~3 hours. Then raise the temperature to 640~660℃ for the second holding. The holding time is determined according to the effective thickness of the forging × 1.5min / mm. Finally, cool it to room temperature. S30. Place the forging vertically into the tooling and heat it to 840~860℃. Hold it at that temperature for 1.2~1.5 min / mm based on the effective thickness. When the surface temperature drops to 780~800℃, quench it in the PAG solution. S40. Transfer the quenched forging to a tempering furnace within 3 minutes, heat it to 300℃ at ≤80℃ / h and hold it for 1~2 hours, then heat it to 540~580℃ at ≤100℃ / h and hold it. The holding time is calculated as effective thickness × 2.0~2.5 min / mm. After tempering, cool it at ≤50℃ / h.

[0024] In the forging process of step S10 above, the heating temperature design of 1180~1220℃ is based on the high-temperature plasticity diagram and grain growth kinetics curve of 42CrMo4 material. When the heating temperature is below 1180℃, the as-cast structure in the core of the ingot is difficult to deform sufficiently, dendrite segregation cannot be effectively broken, and the uneven distribution of alloying elements will be inherited by the finished forging, resulting in local differences in the dissolution kinetics of carbides during subsequent austenitization, which in turn causes cross-sectional fluctuations in hardenability. When the heating temperature exceeds 1220℃, the austenite grain boundaries begin to weaken, and the grains grow abnormally. These coarse grains are prone to forming a proeutectoid ferrite network at the grain boundaries during quenching and cooling, which in turn reduces the hardenability of the core.

[0025] A forging ratio ≥4 is based on the "critical deformation" theory in metal plastic forming. When the forging ratio is less than 3.5, the porosity defects in the core of the steel ingot cannot be completely welded, and the coarse columnar crystals still retain obvious orientation. This structural inheritance will lead to anisotropic phase transformation behavior during subsequent heat treatment. However, when the forging ratio reaches 4 or higher, dynamic recrystallization occurs fully, the core grains are refined, and the carbides are broken down to the submicron level and uniformly distributed in the matrix. These fine carbide particles can serve as a carbon source during the subsequent austenitization process, shortening the diffusion distance for austenite homogenization and thus improving the synchronicity of martensitic transformation during quenching.

[0026] In some preferred embodiments, the effective cross-sectional diameter of the forging in S10 is controlled within the range of 800~1200mm. The content of Cr, Mo, and Mn in its alloy composition, by mass percentage, satisfies the hardenability coefficient J factor ≥ 45, where the J factor is determined by J = 30 + 5×(%Cr) + 8×(%Mo) + 3×(%Mn). This empirical formula is derived from regression analysis of a large amount of experimental data. Cr element mainly improves hardenability by reducing the critical cooling rate, with an effect coefficient of 5; Mo element not only reduces the critical cooling rate but also inhibits pearlite transformation and delays ferrite precipitation, thus increasing the effect coefficient to 8; Mn element expands the austenite phase region and lowers the Ms point, with an effect coefficient of 3. When the J factor ≥ 45, for large spindles with a cross-sectional diameter of 800mm or more, the core can obtain ≥ 90% martensite structure.

[0027] In some other preferred embodiments, the components of the forging in S10, by mass percentage, include: C: 0.38-0.45%; Cr: 0.90-1.20%; Mo: 0.15-0.30%; Mn: 0.60-0.90%; Ni ≤ 0.80%; V ≤ 0.6%; Si ≤ 0.40%; Gu ≤ 0.1%; P ≤ 0.025%; S ≤ 0.025%; N ≤ 70ppm; H ≤ 1.2ppm; O ≤ 30ppm; and the content of any single element, Pb, Sn, As, Sb, or Bi, does not exceed 0.01%. This composition system is optimized based on the traditional 42CrMo4: the restriction of the five low-melting-point elements (Pb, Sn, As, Sb, Bi) is to avoid their segregation at grain boundaries to form a liquid film, which would lead to a decrease in high-temperature thermoplasticity.

[0028] In some preferred embodiments, the forging process in S10 employs an alternating upsetting and drawing process. The final drawing forging temperature is controlled at 1050~1080℃, the single reduction is controlled at 20~25%, the cumulative upsetting ratio is ≥2.0, and the drawing ratio is ≥3.0. The temperature range of 1050~1080℃ is precisely at the upper limit of the non-recrystallization region of austenite. Performing a single reduction deformation of 20~25% at this temperature can form high-density deformation bands inside the austenite grains. These deformation bands become preferential nucleation sites for ferrite during subsequent cooling, and also facilitate the dispersed precipitation of carbides. The alternating forging with a cumulative upsetting ratio ≥2.0 and a drawing ratio ≥3.0 can make the flow line distribution in the core of the ingot more complex, avoiding the formation of through-flow line structures. This twisting and interlacing of flow lines can effectively prevent quenching cracks from propagating along grain boundaries.

[0029] In the S20 stage normalizing pretreatment process, the furnace loading operation is carried out after the first air cooling to 650~700℃. During the air cooling process after final forging, the 42CrMo4 forging first undergoes the proeutectoid transformation from austenite to ferrite. When the surface temperature drops to about 700℃, the core temperature is still above 800℃. At this time, the furnace is immediately loaded for heating, which can utilize the residual heat of the core to shorten the holding time for re-austenitization, while avoiding the formation of a coarse proeutectoid ferrite layer on the surface due to excessive cooling.

[0030] The subsequent holding at 300-350℃ for 2-3 hours promotes the diffusion and redistribution of residual hydrogen atoms inside the forging towards grain boundaries and inclusion interfaces. The diffusion coefficient of hydrogen in α-Fe is much higher than that in γ-Fe, and 300-350℃ is precisely within the low-temperature diffusion window of hydrogen. This holding temperature can further reduce the hydrogen content in the forging, reducing defects that may occur during subsequent quenching.

[0031] The second holding temperature is 640-660℃, which corresponds to the subcritical annealing temperature range of 42CrMo4. When heated above 640℃, the pearlite in the forging begins to spheroidize, and the lamellar cementite gradually transforms into granular carbides. The lamellar cementite dissolves quickly, but this can easily lead to excessively high carbon concentrations at the austenite grain boundaries; while the granular carbides require a longer holding time to dissolve, but the dissolution process is more uniform, contributing to the formation of austenite with a homogeneous composition. The holding time is determined by the effective thickness of the forging × 1.5 min / mm. For large forgings with an effective thickness of 500 mm, a holding time of approximately 750 minutes allows for sufficient spheroidization of the carbides in the core while avoiding excessive grain coarsening.

[0032] In some preferred embodiments, during the second holding in S20, within the temperature range of 640~660℃, when the effective thickness of the forging is ≥500mm, the holding time is extended to 1.8 min / mm of the effective thickness. After holding, the forging is slowly cooled to room temperature at a rate of 15~25℃ / h to control the size of the pearlite clusters in the core to below ASTM grade 8. In thick-section forgings, there is a significant asynchronous phase transformation between the core and surface during the cooling process. If the cooling rate is too fast, the surface will enter the low-temperature region before the core has completed the pearlite transformation, resulting in significant structural stress. A slow cooling rate of 15~25℃ / h allows the core to remain above the Ar1 temperature for a sufficient time, allowing the pearlite clusters to grow and coarsen sufficiently. It should be noted that during the subsequent austenitization process, the austenite nuclei of the coarsened pearlite clusters preferentially form at the pearlite cluster interface. The interface area of ​​the coarse pearlite clusters is relatively small, which helps control the uniform growth of austenite grains and avoids the formation of mixed-grain structures.

[0033] In the S30 stage quenching process, the forging is vertically placed in a fixture and heated to 840~860℃ and held. The main carbides in 42CrMo4 are M23C6 and M2C types. Cr has a low diffusion coefficient in austenite, and the complete dissolution of M23C6 carbides requires a high temperature and sufficient time. 840~860℃ falls precisely within the rapid dissolution temperature range of these carbides. Holding at this temperature allows carbides with a diameter less than 0.5μm to completely dissolve into austenite, while a small amount of undissolved fine carbides can act as pinning phases for austenite grain growth, preventing excessive grain coarsening.

[0034] When the surface temperature drops to 780~800℃, the PAG solution is quenched, taking advantage of the selective precipitation characteristics of proeutectoid ferrite during the pre-cooling process. During the brief dwell time in the 780~800℃ range, a small amount of thin-film ferrite preferentially precipitates at the original austenite grain boundaries. These ferrite films undergo martensitic transformation first during the subsequent quenching and cooling process, and the volume expansion generates compressive stress, which offsets the tensile stress generated by the subsequent martensitic transformation in the core, thereby reducing the risk of quenching cracks.

[0035] In some preferred embodiments, the large end and stepped transition zone of the spindle in S30 use a 12-15% concentration PAG solution, while the spindle body and small end use an 8-10% concentration PAG solution, with the medium temperature controlled at 25-40°C. The physicochemical basis of this design lies in the reverse solubility mechanism of PAG aqueous solution: PAG polymers are completely soluble in water at room temperature, but when the solution comes into contact with a high-temperature workpiece, the solution temperature at the interface rapidly rises above the reverse melting point, causing the PAG polymer to precipitate from the solution and form a continuous polymer film on the workpiece surface. The presence of this film reduces the stability of the vapor film stage, allowing the cooling process to enter the boiling stage more quickly. The higher the PAG solution concentration, the thicker the polymer film formed on the workpiece surface, the lower the heat conduction rate, and the slower the cooling rate. The large end of the spindle and the stepped transition area have complex geometry and high stress concentration. Using a high-concentration PAG solution of 12-15% can appropriately reduce the cooling rate in these areas and reduce the peak thermal stress. On the other hand, the spindle body and the small end have relatively regular cross sections. Using a low-concentration PAG solution of 8-10% can achieve a higher cooling rate to ensure the quenching depth.

[0036] In some preferred embodiments, the quenching tank in S30 contains a pre-formed dual-medium system with stratified density: the lower layer is a water-soluble PAG solution, and the upper layer is an oil-based PAG emulsion. The two media are immiscible and have a stable interface. During quenching, the large end of the spindle and the stepped transition area first pass through the upper oil-based PAG emulsion to form a slow-cooling oil film before entering the lower water-soluble PAG solution to complete the quenching. The vapor film stage of the oil-based PAG emulsion is relatively long, and the cooling rate is slow. When the large end of the spindle and the stepped transition area first pass through the upper oil-based PAG emulsion, a slow-cooling oil film is first formed on the surface. This oil film plays a pre-cooling and coating role, making the temperature of the complex cross-sectional area uniform before entering the lower water-soluble PAG solution to complete the rapid quenching. This effectively avoids the initiation of microcracks caused by local rapid cooling when directly entering the water-based medium.

[0037] In some preferred embodiments, a transition layer with a height of 80-150 mm is provided between the upper oil-based PAG emulsion and the lower water-soluble PAG solution in S30. This transition layer is an oil-water emulsion, prepared by mixing the oil-based PAG emulsion, water-soluble PAG solution, and emulsifier in a mass ratio of 5:3:1. The function of the transition layer is to eliminate abrupt changes in the interface between the two media, preventing a sudden jump in cooling rate when the spindle passes through the interface. The thermal conductivity of the emulsion is between that of the oil-based and water-based media, thus acting as a buffer.

[0038] In some preferred embodiments, a surfactant is added to the upper oil-based PAG emulsion in S30. The composite surfactant comprises polyoxyethylene sorbitan monooleate, sodium dodecylbenzenesulfonate, and fatty alcohol polyoxyethylene ether in a mass ratio of 2:1:1, and the addition amount is 0.5-1.2% of the oil-based PAG emulsion mass. The hydrophilic groups of polyoxyethylene sorbitan monooleate have a large volume, forming a steric hindrance layer when adsorbed at the oil-solid interface, which can effectively prevent the oil film from becoming too thick. The anionic head groups of sodium dodecylbenzenesulfonate have electrostatic interactions with the oxide layer on the metal surface, promoting the uniform spreading of the oil film on the metal surface. The fatty alcohol polyoxyethylene ether acts as a solubilizer and stabilizes the emulsion. When these three components are compounded in a specific ratio, the spreading coefficient of the oil-based PAG emulsion on the spindle surface is optimized, avoiding the formation of discontinuous oil film patches.

[0039] In some preferred embodiments, a wetting promoter is added to the lower layer of the water-soluble PAG solution in S30. The wetting promoter is a mixture of polyether-modified polysiloxane and alkyl glycoside in a mass ratio of 1:1, and the amount added is 0.3-0.6% of the mass of the water-soluble PAG solution. The polyether-modified polysiloxane has extremely low surface tension, which can reduce the contact angle between the aqueous solution and the metal surface, allowing the solution to quickly penetrate into the micro-uneven structure of the spindle surface and disrupt the stability of the vapor film. The alkyl glycoside is a nonionic surfactant. The glycoside group in its molecule interacts with the ether bond of the PAG polymer through hydrogen bonding, which can promote the uniform adsorption of PAG molecules on the metal surface and avoid uneven cooling caused by local polymer enrichment.

[0040] In S40, the quenched forgings are transferred to the tempering furnace within 3 minutes. This time window requirement is based on the diffusion behavior of carbon atoms in martensite at room temperature. After quenching, the martensite is in a supersaturated state, and carbon atoms tend to agglomerate at dislocation lines and grain boundaries. If the transfer time is too long, carbon atoms will undergo short-range diffusion to form ε-carbides. These low-temperature precipitated carbides will coarsen during the subsequent tempering process and lead to a decrease in toughness.

[0041] Heating to 300℃ at a rate ≤80℃ / h and holding for 1-2 hours corresponds to the first and second stages of martensitic tempering. When the temperature rises to 100-200℃, metastable ε-carbides precipitate in the supersaturated martensite; when the temperature continues to rise to 250-300℃, the retained austenite begins to decompose into lower bainite, while the ε-carbides gradually transform into cementite. Holding at 300℃ for 1-2 hours allows this transformation process to be fully completed and the carbide distribution to become more uniform.

[0042] Heating to 540~580℃ at a rate of ≤100℃ / h and holding at this temperature falls within the mid-temperature range of the third stage of tempering, where the main transformations are the aggregation and growth of cementite and the recovery of dislocation substructures. The holding time is calculated as effective thickness × 2.0~2.5 min / mm; for a large spindle with an effective thickness of 500mm, the holding time is approximately 1000~1250 minutes. During this time, carbide particles grow through the Ostwald ripening mechanism, with larger particles engulfing smaller ones, gradually increasing the average diameter. After tempering, cooling to room temperature at a rate of ≤50℃ / h is performed. When 42CrMo4 steel is slowly cooled in the temperature range of 450~550℃, impurity elements such as P and Sn will segregate at the original austenite grain boundaries, leading to a decrease in impact toughness; however, if cooling is too rapid, new thermal stresses will be generated. A cooling rate of ≤50℃ / h allows the workpiece to avoid the tempering brittleness sensitive zone while maintaining a low residual stress level.

[0043] In some preferred embodiments, the cooling stage after tempering in S40 employs a segmented controlled cooling mode. When the surface temperature of the forging drops to the range of 400~450℃, the cooling rate is controlled at 30~40℃ / h, utilizing this temperature range for stress-relieving annealing. When the temperature drops to the range of 200~250℃, the cooling rate is increased to 50~60℃ / h, rapidly passing through the brittle-sensitive zone. This segmented controlled cooling mode can keep the residual austenite content in the core below 3%, and the hardness fluctuation along the thickness direction of the cross-section controlled within ±5HB.

[0044] The wind turbine main shaft processed by the above process achieves a metallographic structure of Grade 1 sorbite according to GB / T 13320-2007, an average grain size of Grade 8 according to GB / T 6394-2017, and non-metallic inclusions are rated as Class A fine series 0.5 and all other types 0 according to GB / T 10561-2005. The hardness fluctuation between the core and the surface is controlled within ±5HB. The comprehensive mechanical properties meet the long-term service requirements of high-power wind turbine main shafts.

[0045] The present invention will be further explained and illustrated below with reference to the embodiments.

[0046] Example 1 A process for improving the hardenability of wind turbine main shafts after heat treatment includes the following steps: S10. 42CrMo4 steel ingots are heated to 1180℃ and forged to obtain forgings for wind turbine main shafts, with a forging ratio controlled at 4.2. The composition of the forgings by mass percentage includes C: 0.38%, Cr: 0.90%, Mo: 0.15%, Mn: 0.60%, Ni: 0.50%, V: 0.2%, Si: 0.25%, Gu: 0.05%, P: 0.018%, S: 0.015%, N: 45ppm, H: 0.9ppm, O: 18ppm, and the contents of each element Pb, Sn, As, Sb, and Bi are all less than 0.005%. The forging process adopts an alternating upsetting and drawing process, with the final drawing forging temperature controlled at 1050℃, the single reduction controlled at 20%, the cumulative upsetting ratio at 2.2, and the drawing ratio at 3.2.

[0047] S20. Perform the first air cooling on the forging. When the surface temperature drops to 650℃, put it into the furnace, heat it to 300℃ and hold it for 3 hours. Then raise the temperature to 640℃ for the second holding. The holding time is determined according to the effective thickness of the forging × 1.5min / mm. Finally, cool it to room temperature at a rate of 20℃ / h.

[0048] S30. Place the forging vertically into the tooling and heat it to 840℃. The holding time is calculated as effective thickness × 1.2 min / mm. When the surface temperature drops to 780℃, quench it in PAG solution. Use 12% PAG solution for the large end of the spindle and the step transition area, and use 8% PAG solution for the spindle body and small end. The medium temperature is controlled at 25℃.

[0049] S40. Transfer the quenched forging to a tempering furnace within 3 minutes, heat it to 300℃ at 60℃ / h and hold it for 1.5 hours, then heat it to 540℃ at 80℃ / h and hold it. The holding time is calculated as effective thickness × 2.0 min / mm. After tempering, cool it to room temperature at 30℃ / h.

[0050] The wind turbine main shaft prepared in this embodiment has a metallographic structure that reaches grade 1 sorbite according to GB / T 13320-2007, an average grain size of grade 8 according to GB / T 6394-2017, and non-metallic inclusions are grade 0.5 for fine series A and grade 0 for all other types according to GB / T 10561-2005. The hardness fluctuation between the core and the surface is controlled within ±4HB.

[0051] Example 2 A process for improving the hardenability of wind turbine main shafts after heat treatment includes the following steps: S10. 42CrMo4 steel ingots are heated to 1220℃ and forged to obtain forgings for wind turbine main shafts. The forging ratio is controlled at 5.0. The effective cross-sectional diameter of the forging is controlled at 1000mm. The alloy composition contains 1.10% Cr, 0.25% Mo, and 0.80% Mn. The J factor is calculated as 48.5 according to J = 30 + 5×(%Cr) + 8×(%Mo) + 3×(%Mn). The forging process adopts an alternating upsetting and drawing process. The final drawing forging temperature is controlled at 1080℃, the single reduction is controlled at 25%, the cumulative upsetting ratio is 2.5, and the drawing ratio is 3.5.

[0052] S20. Perform the first air cooling on the forging. When the surface temperature drops to 700℃, put it into the furnace, heat it to 350℃ and hold it for 2 hours. Then raise the temperature to 660℃ for the second holding. The holding time is determined according to the effective thickness of the forging × 1.5min / mm. Finally, cool it to room temperature.

[0053] S30. Place the forging vertically into the tooling and heat it to 860℃ for holding. The holding time is calculated as effective thickness × 1.5 min / mm. When the surface temperature drops to 800℃, quench it in the PAG solution. The quenching tank is pre-filled with a dual-medium system with layered density: the lower layer is a water-soluble PAG solution, and the upper layer is an oil-based PAG emulsion. The two media are immiscible and have a stable interface. During quenching, the large end of the spindle and the step transition area first pass through the upper oil-based PAG emulsion to form a slow-cooling oil film, and then enter the lower water-soluble PAG solution to complete the quenching. A transition layer with a height of 100 mm is set between the upper oil-based PAG emulsion and the lower water-soluble PAG solution. The transition layer is an oil-water emulsion, which is made by mixing oil-based PAG emulsion, water-soluble PAG solution and emulsifier in a mass ratio of 5:3:1.

[0054] S40. Transfer the quenched forging to a tempering furnace within 3 minutes, heat it to 300℃ at 80℃ / h and hold it for 1 hour, then heat it to 580℃ at 100℃ / h and hold it. The holding time is calculated as effective thickness × 2.5 min / mm. After tempering, cool it to room temperature at 50℃ / h.

[0055] The wind turbine main shaft prepared in this embodiment has a metallographic structure that reaches grade 1 sorbite according to GB / T 13320-2007, an average grain size of grade 8.5 according to GB / T 6394-2017, and non-metallic inclusions are grade 0.5 for fine A series and grade 0 for all other types according to GB / T 10561-2005. The hardness fluctuation between the core and the surface is controlled within ±3HB.

[0056] Example 3 A process for improving the hardenability of wind turbine main shafts after heat treatment includes the following steps: S10. 42CrMo4 steel ingots are heated to 1200℃ and forged to obtain forgings for wind turbine main shafts, with a forging ratio controlled at 4.5. The effective cross-sectional diameter of the forging is controlled at 800mm, and its alloy composition contains 0.95% Cr, 0.18% Mo, and 0.65% Mn, calculated according to J = 30 + 5×(%Cr) + 8×(%Mo) + The J factor calculated by 3×(%Mn) is 45.2; the composition of the forging by mass percentage includes C: 0.42%, Cr: 0.95%, Mo: 0.18%, Mn: 0.65%, Ni: 0.60%, V: 0.3%, Si: 0.30%, Gu: 0.06%, P: 0.020%, S: 0.018%, N: 50ppm, H: 1.0ppm, O: 22ppm, and the contents of each element Pb, Sn, As, Sb, and Bi are all less than 0.008%; the forging process adopts an alternating upsetting and drawing process, the final drawing forging temperature is controlled at 1060℃, the single reduction is controlled at 22%, the cumulative upsetting ratio is 2.3, and the drawing ratio is 3.3.

[0057] S20. Perform the first air cooling on the forging. When the surface temperature drops to 680℃, put it into the furnace, heat it to 320℃ and hold it for 2.5 hours. Then raise the temperature to 650℃ for the second holding. The holding time is determined according to the effective thickness of the forging × 1.5min / mm. Finally, cool it to room temperature.

[0058] S30. Place the forging vertically into the tooling and heat it to 850℃. The holding time is calculated as effective thickness × 1.3 min / mm. When the surface temperature drops to 790℃, quench it in PAG solution. Use 13% PAG solution for the large end and step transition area of ​​the spindle, and 9% PAG solution for the spindle body and small end. The medium temperature is controlled at 30℃.

[0059] S40. Transfer the quenched forging to a tempering furnace within 3 minutes, heat it to 300℃ at 70℃ / h and hold it for 1.5 hours, then heat it to 560℃ at 90℃ / h and hold it. The holding time is calculated as effective thickness × 2.2 min / mm. After tempering, cool it to room temperature at 40℃ / h.

[0060] The wind turbine main shaft prepared in this embodiment has a metallographic structure that reaches grade 1 sorbite according to GB / T 13320-2007, an average grain size of grade 8 according to GB / T 6394-2017, and non-metallic inclusions are grade 0.5 for fine series A and grade 0 for all other types according to GB / T 10561-2005. The hardness fluctuation between the core and the surface is controlled within ±4HB.

[0061] Example 4 A process for improving the hardenability of wind turbine main shafts after heat treatment includes the following steps: S10. 42CrMo4 steel ingots are heated to 1190℃ and forged to obtain forgings for wind turbine main shafts. The forging ratio is controlled at 4.8. The effective cross-sectional diameter of the forging is controlled at 1200mm. The alloy composition contains 1.15% Cr, 0.28% Mo, and 0.85% Mn. The J factor is calculated to be 51.2 according to J = 30 + 5×(%Cr) + 8×(%Mo) + 3×(%Mn). The forging process adopts an alternating upsetting and drawing process. The final drawing forging temperature is controlled at 1070℃, the single reduction is controlled at 23%, the cumulative upsetting ratio is 2.4, and the drawing ratio is 3.4.

[0062] S20. Perform the first air cooling on the forging. When the surface temperature drops to 660℃, load it into the furnace and heat it to 330℃ for 2.5 hours. Then, raise the temperature to 655℃ for the second holding. When the effective thickness of the forging is ≥500mm, extend the holding time to 1.8min / mm of the effective thickness. After the holding is completed, slowly cool it to room temperature at a rate of 15℃ / h to control the size of the pearlite clusters in the core to below ASTM grade 8.

[0063] S30. Place the forging vertically into the tooling and heat it to 855℃ for holding. The holding time is calculated as effective thickness × 1.4 min / mm. When the surface temperature drops to 785℃, quench it in the PAG solution. The quenching tank is pre-filled with a dual-medium system with layered density: the lower layer is a water-soluble PAG solution, and the upper layer is an oil-based PAG emulsion. The two media are immiscible and have a stable interface. During quenching, the large end of the spindle and the step transition area first pass through the upper oil-based PAG emulsion to form a slow-cooling oil film, and then enter the lower water-soluble PAG solution to complete the quenching. The upper oil-based PAG emulsion contains a surfactant. The composite surfactant includes polyoxyethylene dehydrated sorbitan monooleate, sodium dodecylbenzene sulfonate, and fatty alcohol polyoxyethylene ether in a mass ratio of 2:1:1. The amount added is 0.8% of the mass of the oil-based PAG emulsion.

[0064] S40. Transfer the quenched forging to a tempering furnace within 3 minutes, heat it to 300℃ at 65℃ / h and hold it for 1.5 hours, then heat it to 570℃ at 85℃ / h and hold it. The holding time is calculated as effective thickness × 2.3 min / mm. After tempering, cool it to room temperature at 45℃ / h.

[0065] The wind turbine main shaft prepared in this embodiment has a metallographic structure that reaches grade 1 sorbite according to GB / T 13320-2007, an average grain size of grade 8 according to GB / T 6394-2017, and non-metallic inclusions are grade 0.5 for fine A series and grade 0 for all other types according to GB / T 10561-2005. The hardness fluctuation between the core and the surface is controlled within ±3HB.

[0066] Example 5 A process for improving the hardenability of wind turbine main shafts after heat treatment includes the following steps: S10. 42CrMo4 steel ingots are heated to 1210℃ and forged to obtain forgings for wind turbine main shafts, with a forging ratio controlled at 4.6. The effective cross-sectional diameter of the forging is controlled at 900mm, and its alloy composition contains 1.05% Cr, 0.22% Mo, and 0.75% Mn, calculated according to J = 30 + 5×(%Cr) + 8×(%Mo) + The J factor calculated by 3×(%Mn) is 47.3; the composition of the forging by mass percentage includes C: 0.43%, Cr: 1.05%, Mo: 0.22%, Mn: 0.75%, Ni: 0.70%, V: 0.4%, Si: 0.35%, Gu: 0.07%, P: 0.022%, S: 0.020%, N: 55ppm, H: 1.1ppm, O: 25ppm, and the contents of each element Pb, Sn, As, Sb, and Bi are all less than 0.009%; the forging process adopts an alternating upsetting and drawing process, the final drawing forging temperature is controlled at 1065℃, the single reduction is controlled at 24%, the cumulative upsetting ratio is 2.4, and the drawing ratio is 3.4.

[0067] S20. Perform the first air cooling on the forging. When the surface temperature drops to 690℃, put it into the furnace and heat it to 340℃ for 2 hours. Then raise the temperature to 645℃ for the second holding. When the effective thickness of the forging is ≥500mm, the holding time is extended to the effective thickness × 1.8min / mm. After the holding is completed, slowly cool it to room temperature at a rate of 25℃ / h to control the size of the pearlite clusters in the core to below ASTM grade 8.

[0068] S30. Place the forging vertically into the fixture and heat it to 845℃. Hold the temperature for 1.3 min / mm based on the effective thickness. When the surface temperature drops to 795℃, quench it in the PAG solution. The quenching tank contains a pre-prepared dual-medium system with stratified density: the lower layer is a water-soluble PAG solution, and the upper layer is an oil-based PAG emulsion. The two media are immiscible and have a stable interface. During quenching, the large end of the spindle and the step transition area first pass through the upper oil-based PAG emulsion to form a slow-cooling oil film before entering the lower water-soluble PAG solution. The solution is quenched; a transition layer with a height of 150 mm is set between the upper oil-based PAG emulsion and the lower water-soluble PAG solution. The transition layer is an oil-water emulsion, which is made by mixing oil-based PAG emulsion, water-soluble PAG solution and emulsifier in a mass ratio of 5:3:1; a wetting accelerator is added to the lower water-soluble PAG solution. The wetting accelerator is a mixture of polyether-modified polysiloxane and alkyl glycoside in a mass ratio of 1:1, and the amount added is 0.5% of the mass of the water-soluble PAG solution.

[0069] S40. Transfer the quenched forging to a tempering furnace within 3 minutes, heat it to 300℃ at 75℃ / h and hold it for 1.5 hours, then heat it to 550℃ at 95℃ / h and hold it. The holding time is calculated as effective thickness × 2.2 min / mm. After tempering, cool it to room temperature at 35℃ / h.

[0070] The wind turbine main shaft prepared in this embodiment has a metallographic structure that reaches grade 1 sorbite according to GB / T 13320-2007, an average grain size of grade 8.5 according to GB / T 6394-2017, and non-metallic inclusions are grade 0.5 for fine A series and grade 0 for all other types according to GB / T 10561-2005. The hardness fluctuation between the core and the surface is controlled within ±3HB.

[0071] Comparative Example 1 A heat treatment process for wind turbine main shaft includes the following steps: 42CrMo4 steel ingots were heated to 1150℃ for forging to obtain forgings for wind turbine main shafts, with a forging ratio controlled at 3.2. After the forgings were air-cooled to room temperature, they were directly heated to 860℃ and held for a holding time calculated as effective thickness × 1.0 min / mm. After holding, they were directly quenched in clean water. After quenching, the forgings were transferred to a tempering furnace and heated to 580℃ at a rate of 120℃ / h and held for a holding time calculated as effective thickness × 1.5 min / mm. After tempering, they were air-cooled to room temperature.

[0072] The wind turbine main shaft prepared in this comparative example has a mixed ferrite and pearlite microstructure according to GB / T 13320-2007, and is rated as grade 3. The hardness fluctuation between the core and the surface is controlled within ±18HB, and the core hardness is lower than the technical requirements.

[0073] Comparative Example 2 A heat treatment process for wind turbine main shaft includes the following steps: 42CrMo4 steel ingots were heated to 1250℃ and forged to obtain forgings for wind turbine main shafts, with a forging ratio controlled at 3.8. The forgings underwent a first air cooling, and when the surface temperature dropped to 600℃, they were placed in a furnace, heated to 400℃ and held for 1 hour, and then heated to 680℃ for a second holding. The holding time was determined by the effective thickness of the forging × 2.0 min / mm. Subsequently, they were cooled to room temperature at 50℃ / h. The forgings were then heated to 880℃ and held for 2.0 min / mm. When the surface temperature dropped to 760℃, they were quenched in a PAG solution (5% concentration, medium temperature 20℃). After quenching, they were transferred to a tempering furnace, heated to 500℃ at 150℃ / h and held for 1.5 min / mm. After tempering, they were air cooled.

[0074] The wind turbine main shaft prepared in this comparative example, according to GB / T 13320-2007, exhibits coarse martensite + retained austenite in its metallographic structure, and is rated as level 2. According to GB / T 6394-2017, the average grain size shows mixed crystals (local level 5, local level 8). The hardness fluctuation between the core and the surface is controlled within ±12HB, and the impact toughness is lower than the technical requirements.

[0075] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A process for improving the hardenability of wind turbine main shafts after heat treatment, characterized in that, It includes the following steps: S10. Heat 42CrMo4 steel ingots to 1180~1220℃ for forging to obtain forgings of wind turbine main shafts, with a forging ratio controlled at ≥4. S20. Perform the first air cooling on the forging. When the surface temperature drops to 650~700℃, put it in the furnace and heat it to 300~350℃ and hold it for 2~3 hours. Then raise the temperature to 640~660℃ for the second holding. The holding time is determined according to the effective thickness of the forging × 1.5min / mm. Finally, cool it to room temperature. S30. Place the forging vertically into the tooling and heat it to 840~860℃. Hold it at that temperature for 1.2~1.5 min / mm based on the effective thickness. When the surface temperature drops to 780~800℃, quench it in the PAG solution. S40. Immediately transfer the quenched forging to a tempering furnace, heat it to 300℃ at ≤80℃ / h and hold it for 1~2h, then heat it to 540~580℃ at ≤100℃ / h and hold it. The holding time is calculated as effective thickness × 2.0~2.5min / mm. After tempering, cool it at ≤50℃ / h.

2. The process for improving the hardenability of wind turbine main shaft heat treatment according to claim 1, characterized in that, In S10, the effective cross-sectional diameter of the forging is controlled within the range of 800~1200mm, and the content of Cr, Mo and Mn in its alloy composition meets the hardenability coefficient J factor ≥ 45 by mass percentage, wherein the J factor is determined by J = 30 + 5×(%Cr) + 8×(%Mo) + 3×(%Mn).

3. The process for improving the hardenability of wind turbine main shaft heat treatment according to claim 1, characterized in that, In S10, the forging comprises, by mass percentage, C: 0.38-0.45%; Cr: 0.90-1.20%; Mo: 0.15-0.30%; Mn: 0.60-0.90%; Ni≤0.80%; V≤0.6%; Si≤0.40%; Gu≤0.1%; P≤0.025%; S≤0.025%; N≤70ppm; H≤1.2ppm; O≤30ppm; and the content of any single element Pb, Sn, As, Sb, or Bi does not exceed 0.01%.

4. The process for improving the hardenability of wind turbine main shaft heat treatment according to claim 1, characterized in that, In S10, the forging process adopts an alternating upsetting and drawing process, the final drawing forging temperature is controlled at 1050~1080℃, the single reduction is controlled at 20~25%, the cumulative upsetting ratio is ≥2.0, and the drawing ratio is ≥3.

0.

5. The process for improving the hardenability of wind turbine main shaft heat treatment according to claim 1, characterized in that, In S20, the second holding temperature is in the range of 640~660℃. When the effective thickness of the forging is ≥500mm, the holding time is extended to 1.8min / mm of the effective thickness. After the holding is completed, the forging is slowly cooled to room temperature at a rate of 15~25℃ / h to control the size of the pearlite clusters in the core to below ASTM grade 8.

6. The process for improving the hardenability of wind turbine main shaft heat treatment according to claim 1, characterized in that, In S30, a 12-15% concentration PAG solution is used at the large end of the spindle and the step transition zone, while an 8-10% concentration PAG solution is used at the small end of the spindle. The medium temperature is controlled at 25-40℃.

7. The process for improving the hardenability of wind turbine main shaft heat treatment according to claim 1, characterized in that, In S30, a dual-medium system with pre-layered density is placed in the quenching tank: the lower layer is a water-soluble PAG solution and the upper layer is an oil-based PAG emulsion. The two media are immiscible and have a stable interface. During quenching, the large end of the spindle and the step transition zone first pass through the upper oil-based PAG emulsion to form a slow-cooling oil film, and then enter the lower water-soluble PAG solution to complete the quenching.

8. The process for improving the hardenability of wind turbine main shaft heat treatment according to claim 7, characterized in that, In S30, a transition layer with a height of 80~150mm is provided between the upper oil-based PAG emulsion and the lower water-soluble PAG solution. The transition layer is an oil-water emulsion, which is made by mixing oil-based PAG emulsion, water-soluble PAG solution and emulsifier in a mass ratio of 5:3:

1.

9. The process for improving the hardenability of wind turbine main shaft heat treatment according to claim 7, characterized in that, In S30, a surfactant is added to the upper oil-based PAG emulsion. The composite surfactant is obtained by compounding polyoxyethylene dehydrated sorbitan monooleate, sodium dodecylbenzene sulfonate and fatty alcohol polyoxyethylene ether in a mass ratio of 2:1:1, and the amount added is 0.5~1.2% of the mass of the oil-based PAG emulsion.

10. The process for improving the hardenability of wind turbine main shaft heat treatment according to claim 7, characterized in that, In S30, a wetting accelerator is added to the lower water-soluble PAG solution. The wetting accelerator is a mixture of polyether-modified polysiloxane and alkyl glycoside in a mass ratio of 1:1, and the amount added is 0.3~0.6% of the mass of the water-soluble PAG solution.