Medium carbon steel double-frequency composite induction strengthening process, gradient hardened layer part and production system

Abstract

This invention discloses a dual-frequency composite induction strengthening process for medium carbon steel, gradient hardened layer parts, and a production system. The process includes: selecting SAE 1045 medium carbon steel and performing quenching and tempering pretreatment; precision machining to form outer circular channels and inner ball channels; performing dual-frequency composite induction hardening on the outer circular channels and inner ball channels respectively, i.e., preheating to 680-750℃ with medium-frequency current, then rapidly heating to 880-920℃ with high-frequency current followed by spray cooling, and using thermal balance control to form a gentle transition zone with a width of 2-5mm between the two hardened zones, where the hardness gradient along the axial line connecting the two hardened zones does not exceed ΔHRC10 / mm; finally, low-temperature tempering. This invention also discloses wear-resistant parts with a dual-layer composite hardened layer structure and a gradient transition zone prepared using this process, as well as an online composite strengthening production system. This invention replaces low-carbon alloy steel with medium-carbon steel and replaces overall carburizing with local dual-frequency induction hardening, which can reduce raw material costs and energy consumption, reduce deformation, and obtain a hardened layer that meets the depth requirements, thus realizing online continuous production of heat treatment and machining.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for metallic materials, specifically to a process method for local surface strengthening of medium carbon steel parts using dual-frequency composite induction heating, and wear-resistant parts with a gradient hardened layer structure obtained by using this process method. Background Technology

[0002] In the automotive parts and construction machinery industries, there is a large number of parts that require localized surface hardening. Only certain specific working areas of the part (such as grooves, raceways, and mating surfaces) require high hardness and wear resistance, while the remaining areas need to maintain good toughness and machinability. To address this need, the widely adopted technical approach in the industry is to use low-carbon alloy steel as raw material and perform carburizing and quenching treatment on the entire part in a high-temperature carburizing furnace. This forms a high-carbon hardened layer on the surface of the part, thereby achieving comprehensive mechanical properties of high surface hardness while maintaining low-carbon toughness in the core.

[0003] For example, in a certain automotive transmission component, the outer groove area needs to accommodate bearing balls, and the inner ball groove area needs to move in conjunction with the lead screw. Therefore, these two areas require high surface hardness (HRC58-62) and a certain hardened layer depth (1.1mm-1.7mm). The traditional process involves manufacturing part blanks from low-carbon alloy steel (such as SAE J404 Grade 5120), then loading the entire part into a chamber furnace and heating it to 910-930℃ for high-temperature carburizing. Following this, it is held at approximately 865℃ and oil-quenched to harden the material surface. The carburizing process diffuses carbon atoms into the surface of the low-carbon steel at high temperatures, forming a high-hardness layer with a carbon content of 0.85%-1.05%, with a hardened layer thickness typically ranging from 0.5 to 2mm.

[0004] However, the above-mentioned traditional process route has the following problems: First, raw material costs are high. The procurement cost of low-carbon alloy steel (containing alloying elements such as Cr, Ni, and Mo) is significantly higher than that of ordinary medium-carbon steel, which puts considerable cost pressure on mass production.

[0005] Secondly, the heat treatment process is energy-intensive. Carburizing and quenching requires heating the entire part, along with the furnace body, to over 900°C and holding it at that temperature for an extended period. This requires high-power equipment, a long processing cycle, and enormous energy consumption. Traditional carburizing processes also require heating the entire workpiece, resulting in high electrical energy consumption.

[0006] Third, heat treatment causes significant deformation. Overall high-temperature heating leads to substantial thermal and structural stresses in the parts, resulting in noticeable dimensional changes after quenching. Subsequent deep grinding is often required to correct these heat treatment distortions. For precision parts, this deformation problem severely impacts dimensional accuracy and assembly consistency.

[0007] Fourth, the production process is fragmented. Carburizing heat treatment usually needs to be completed offline in a specialized heat treatment plant. Parts need to be packaged, transported and handled multiple times, which not only results in a long production cycle and many uncontrollable factors, but also makes them prone to bumps and damage during the process, increasing the scrap rate.

[0008] To address the aforementioned issues, those skilled in the art have attempted to replace carburizing with induction hardening. Induction surface hardening utilizes the principle of electromagnetic induction to rapidly heat the workpiece surface to its austenitizing temperature followed by rapid cooling. After hardening, the martensite grains are refined, and the surface hardness is 2–3 HRC higher than that of conventional hardening. Furthermore, the surface layer exhibits significant residual compressive stress, which is beneficial for improving fatigue strength. Induction hardening allows for localized heating, has low energy consumption, and is easily mechanized and automated. However, existing induction hardening alternatives suffer from the following technical bottlenecks: First, after conventional induction hardening of medium carbon steel, the hardened layer depth is typically shallow (0.5–1.0 mm), making it difficult to meet the technical requirement of over 1.1 mm. Second, the transition region between the hardened layer and the core matrix is ​​narrow, resulting in a steep hardness gradient, which makes the hardened layer prone to spalling under alternating loads. Third, the continuity of the hardened layer between different regions is difficult to guarantee, easily leading to soft bands or hardness troughs at geometric abrupt changes.

[0009] Therefore, how to obtain a hardened layer that meets the required depth while reducing raw material costs and processing energy consumption, and at the same time ensure good bonding between the hardened layer and the substrate, as well as excellent wear resistance and fatigue life, has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0010] (a) The technical problem to be solved by the present invention To address the problems of high raw material costs, high heat treatment energy consumption, severe part deformation, and fragmented production processes in existing low-carbon alloy steel carburizing and quenching processes, the present invention aims to provide a process method that can replace low-carbon alloy steel with medium-carbon steel and replace overall high-temperature furnace carburizing with local induction hardening. Simultaneously, it solves the problems of insufficient hardened layer depth, steep hardness gradient, and easy peeling of the hardened layer in existing induction hardening schemes. This reduces overall manufacturing costs while enabling parts to achieve surface hardness and a gentle hardness gradient that meet usage requirements.

[0011] (II) Technical Solution Option 1: A dual-frequency composite induction strengthening process for medium carbon steel, used to prepare a gradient hardening layer structure on parts with outer circular channels and inner ball tracks, comprising the following steps: S1. Tempering pretreatment: SAE 1045 medium carbon steel is selected as the raw material for the parts. The blank is subjected to tempering treatment. The tempering treatment includes heating the blank to 840-860℃ and holding it for 1-2 hours, then oil quenching, then holding it at 580-620℃ for 2-3 hours for high-temperature tempering, and then air cooling. S2. Part finishing: The tempered blank is cut to form the final geometry of the outer circular groove and the inner ball track; S3. Dual-frequency composite induction hardening of outer circular channels: First, preheat the outer circular channels with a medium-frequency current of 2500–8000 Hz, with a preheating power of 50%–60% of the rated power and a heating time of 2–4 seconds, raising the surface temperature of the outer circular channels to 680–750℃; then switch to a high-frequency current of 100–300 kHz, and rapidly heat at 90%–100% of the rated power for 1.5–3 seconds, raising the surface temperature of the outer circular channels to 880–920℃ and holding it for 0.2–0.8 seconds; immediately after heating, spray cooling quenching is performed on the outer circular channels to achieve a surface hardness of HRC58–62; S4. Dual-frequency composite induction hardening of the inner ball track: First, the inner ball track is preheated with a medium-frequency current of 2500-8000Hz, with a preheating power of 50%-60% of the rated power and a heating time of 1.5-3 seconds, raising the surface temperature of the inner ball track to 680-750℃; then, the current is switched to a high-frequency current of 100-300kHz, with a rapid heating of 90%-100% of the rated power, for 1-2 seconds, raising the surface temperature of the inner ball track to 880-920℃ and holding it for 0.2-0.8 seconds; immediately after heating, the inner ball track is spray-cooled and quenched to achieve a surface hardness of HRC58-62. S5. Low-temperature tempering: After quenching, the parts are held at 160-200℃ for 2-3 hours and then air-cooled. The process includes a thermal balance control step between steps S3 and S4: after the outer channel quenching is completed, the overall temperature distribution of the part is monitored in real time. Once the temperature of the outer channel quenching area drops below the lower limit of the tempering brittleness sensitive temperature zone of the material in that area, and the temperature at the monitoring point is not higher than 120°C, the inner ball track quenching program is started. This forms a transition zone with a width of 2-5 mm between the hardened area of ​​the outer channel and the hardened area of ​​the inner ball track, and the hardness change gradient along the axial line connecting the two hardened areas is not greater than ΔHRC10 / mm.

[0012] Furthermore, in this process method, the effective hardened layer depth after quenching of the outer circular channel in step S3 is 1.1 to 1.5 mm, and the effective hardened layer depth after quenching of the inner ball channel in step S4 is 1.0 to 1.4 mm.

[0013] Furthermore, in this process method, the cooling medium used in the spray cooling quenching in steps S3 and S4 is a PAG quenching liquid with a concentration of 8% to 12%, the quenching liquid temperature is controlled at 20 to 40°C, the spraying pressure is 0.3 to 0.5 MPa, and the cooling time is 10 to 20 seconds.

[0014] Furthermore, in this process method, during the quenching process in steps S3 and S4, an infrared temperature measurement closed-loop control system is used for real-time temperature monitoring and power adjustment. The infrared temperature measurement closed-loop control system includes an infrared temperature sensor, a signal processing unit, and a power control unit. The infrared temperature sensor acquires the channel surface temperature in real time at a sampling frequency of not less than 50Hz. The power control unit dynamically adjusts the output power of the induction heating power supply according to the deviation between the measured temperature and the preset temperature, so that the deviation between the actual heating temperature and the preset temperature is controlled within ±10℃.

[0015] Furthermore, in this process method, the coupling gap between the inductor used for quenching the outer circular channel in step S3 and the channel surface is 0.8 to 1.5 mm. In step S4, the inductor used for quenching the inner ball track adopts a contour-following structure. This inductor includes an arc-shaped heating section and a longitudinal connecting section. The contour of the arc-shaped heating section is adapted to the curved surface of the inner ball track, and the coupling gap between the arc-shaped heating section and the surface of the inner ball track is 0.5–1.2 mm. A ferrite magnetic conductor is mounted on the arc-shaped heating section to constrain and guide high-frequency magnetic lines of force to the surface of the inner ball track.

[0016] Option 2: A wear-resistant part with a gradient hardening layer structure, the part being manufactured using this process; the material of the part is SAE 1045 medium carbon steel; the structure of the part includes an outer circular groove hardening zone, an inner ball raceway hardening zone, a transition zone between the outer circular groove hardening zone and the inner ball raceway hardening zone, and an unhardened zone at the core of the part; the surface hardness of the outer circular groove hardening zone and the inner ball raceway hardening zone is both HRC58-62; the width of the transition zone is 2-5 mm, and the hardness value of the transition zone continuously decreases from HRC58-62 near the hardened zone to HRC25-32 near the core, and the hardness change gradient along the axial line connecting the two hardened zones is no greater than ΔHRC10 / mm; the hardness of the core of the part is HRC25-32.

[0017] Furthermore, in this wear-resistant part design, both the outer circular groove hardening zone and the inner ball track hardening zone have a double-layer composite hardening structure. This double-layer composite hardening structure consists of a full martensitic layer located on the surface and a transitional microstructure layer located on the subsurface. The hardness of the full martensitic layer is HRC58-62, and the depth is 0.6-1.0 mm. The hardness of the transitional microstructure layer is HRC45-58, and the depth is 0.4-0.7 mm.

[0018] Option 3: An online composite strengthening production system for implementing the process method of the present invention, comprising, in sequence along the part flow direction, a feeding device, an outer circular channel induction hardening station, a first infrared temperature measurement station, a thermal balance station, an inner ball channel induction hardening station, a second infrared temperature measurement station, a tempering furnace, and a feeding device. The above stations are connected in series by an automatic conveying device, which is used to sequentially convey parts from one station to the next, realizing continuous flow and online processing of parts.

[0019] (III) Beneficial Effects Compared with the prior art, the solution of the present invention has the following beneficial effects: First, raw material costs are significantly reduced. This invention replaces SAE 5120 low-carbon alloy steel with SAE 1045 medium-carbon steel, thus reducing material procurement costs compared to low-carbon alloy steel.

[0020] Second, the energy consumption of heat treatment is significantly reduced. This invention uses local induction heating, heating only the grooved areas on the part that need to be hardened, resulting in a heating volume much smaller than that of heating the entire furnace; at the same time, the heating time is measured in seconds, far shorter than the several hours of holding time required for carburizing.

[0021] Third, the deformation of parts is significantly reduced. This invention only performs short-term rapid heating on local areas, resulting in a small overall heated area and low heat input for the parts. The dimensional deformation after quenching is controlled within 0.05mm, which is far lower than the 0.10-0.20mm deformation commonly seen in traditional carburizing and quenching processes. This can significantly reduce the subsequent finishing allowance, and for some precision-grade parts, only surface finishing or light grinding is required after quenching before assembly and use.

[0022] Fourth, the hardened layer depth and hardness gradient are optimized. This invention employs a dual-frequency composite heating method, using a combined strategy of expanding heat penetration depth through medium-frequency preheating and achieving rapid surface austenitization through high-frequency heating. This increases the hardened layer depth of medium-carbon steel induction hardening from the conventional 0.5–1.0 mm to 1.0–1.7 mm, reaching or approaching the depth level of traditional carburizing and quenching. Simultaneously, by rationally controlling the thermal balance process of the two quenching processes, a gentle hardness gradient transition zone is formed between the two hardened areas. This avoids the risk of spalling caused by abrupt hardness changes between the hardened and unhardened areas in traditional induction hardening, significantly improving the rolling contact fatigue life of the parts. Attached Figure Description

[0023] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0024] Figure 1 This is a flowchart of the carbon steel dual-frequency composite induction strengthening process method in this invention. Detailed Implementation

[0025] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific illustrations.

[0026] This invention provides a dual-frequency composite induction strengthening process for carbon steel, which can be used to prepare a gradient hardening layer structure on parts with outer circular channels and inner ball channels. See [link to relevant documentation]. Figure 1 The method includes the following steps: S1. Quenching and tempering pretreatment: SAE 1045 medium carbon steel is selected as the raw material for the parts, and the blanks are quenched and tempered. The quenching and tempering treatment includes heating the blanks to 840-860℃ and holding for 1-2 hours, followed by oil quenching, then holding at 580-620℃ for 2-3 hours for high-temperature tempering, and finally air cooling, so that the core of the parts obtains a tempered sorbite structure and the hardness is controlled at HRC25-32; S2. Finishing of parts: The heat-treated blank is machined to form the final geometry of the outer circular groove and the inner ball track, with a fine grinding allowance of 0.05 to 0.10 mm reserved after quenching; S3. Dual-frequency composite induction hardening of outer circular channels: Dual-frequency composite heating is used to induction harden the outer circular channels. This step includes: a first heating stage, where the outer circular channels are preheated with a medium-frequency current of 2500–8000 Hz, with a preheating power of 50%–60% of the rated power of the induction heating power supply, for 2–4 seconds, so that the surface temperature of the outer circular channels rises uniformly to 680–750℃; a second heating stage, immediately following the first heating stage, switches to a high-frequency current of 100–300 kHz, with rapid heating at 90%–100% of the rated power, for 1.5–3 seconds, so that the surface temperature of the outer circular channels rises to 880–920℃, and is maintained within this temperature range for 0.2–0.8 seconds; immediately after the second heating stage, the outer circular channels are subjected to spray cooling quenching. S4. Dual-frequency composite induction hardening of the inner ball track: The inner ball track is induction hardened using a dual-frequency composite heating method based on the same principle as the outer circular channel hardening. The inductor used for inner ball track hardening has a contour-following structure, which includes an arc-shaped heating section and a longitudinal connecting section. The contour of the arc-shaped heating section is adapted to the curved surface of the inner ball track, and a ferrite magnetic conductor is installed on the arc-shaped heating section. The process includes: a third heating stage, where the inner raceway is preheated with a medium-frequency current of 2500–8000 Hz, with a preheating power of 50%–60% of the rated power of the induction heating power supply, and a heating time of 1.5–3 seconds, so that the surface temperature of the inner raceway rises uniformly to 680–750°C; a fourth heating stage, immediately following the third heating stage, switching to a high-frequency current of 100–300 kHz, with rapid heating at 90%–100% of the rated power, for a heating time of 1–2 seconds, so that the surface temperature of the inner raceway rises to 880–920°C, and is maintained for 0.2–0.8 seconds; and immediately after the fourth heating stage, the inner raceway is spray-cooled and quenched. S5. Low-temperature tempering: Place the parts that have been quenched in steps S3 and S4 into a tempering furnace, hold them at 160-200℃ for 2-3 hours, and then air cool them.

[0027] In this process, a thermal balance control step is included between steps S3 and S4: after the outer groove quenching is completed, the overall temperature distribution of the part is monitored in real time using an infrared temperature sensor. The inner ball track quenching program is only initiated after the temperature of the outer groove quenching area drops below the lower limit of the tempering brittleness sensitive temperature zone of the material in that area, and the temperature at the monitoring point does not exceed 120℃. This thermal balance control step creates a transition zone with a width of 2–5 mm between the outer groove quenching area and the inner ball track quenching area. The hardness of this transition zone gradually transitions from HRC58–62 to the core hardness of HRC25–32, and the hardness gradient along the axial line connecting the two hardened areas does not exceed ΔHRC10 / mm.

[0028] Furthermore, during the quenching processes in steps S3 and S4, an infrared temperature measurement closed-loop control system is employed to monitor the temperature and adjust the power of each heating section in real time. This infrared temperature measurement closed-loop control system includes an infrared temperature sensor, a signal processing unit, and a power control unit. The infrared temperature sensor acquires the channel surface temperature in real time at a sampling frequency of not less than 50Hz. The signal processing unit compares the measured temperature with a preset temperature curve. The power control unit dynamically adjusts the output power of the induction heating power supply based on the deviation signal, ensuring that the deviation between the actual heating temperature and the preset temperature is controlled within ±10℃.

[0029] Using the above-described process, the present invention enables the fabrication of a wear-resistant part with a gradient hardening layer structure on SAE 1045 medium carbon steel parts. This gradient hardening layer structure includes an outer circular groove hardening zone, an inner ball raceway hardening zone, a transition zone between the outer circular groove hardening zone and the inner ball raceway hardening zone, and an unhardened zone at the core of the part. The surface hardness of both the outer circular groove hardening zone and the inner ball raceway hardening zone is HRC 58–62. The width of the transition zone is 2–5 mm, and the hardness value of this transition zone continuously decreases from HRC 58–62 near the hardened zone to HRC 25–32 near the core. The hardness gradient along the axial line connecting the two hardened zones does not exceed ΔHRC10 / mm. The core hardness of the part is HRC 25–32. Both the outer circular groove hardening zone and the inner ball passage hardening zone have a double-layer composite hardening structure, which consists of a full martensite layer on the surface and a transitional structure layer on the subsurface. The hardness of the full martensite layer is HRC58-62 and the depth is 0.6-1.0 mm. The hardness of the transitional structure layer is HRC45-58 and the depth is 0.4-0.7 mm.

[0030] As further explained, the present invention employs a lower-frequency medium-frequency current in the first and third heating stages. Utilizing the greater penetration depth of the medium-frequency current, the metal within a certain depth range below the surface of the part is uniformly preheated to a temperature close to but below the phase transformation point. Subsequently, when switching to a high-frequency current, the skin effect of the high-frequency current rapidly heats the outermost layer of the part to its austenitizing temperature, while the subsurface layer, having already been preheated, also reaches its austenitizing temperature within a shorter high-frequency heating time. This combined effect of "deep preheating and rapid surface heating" significantly expands the depth range of the austenitizing region, thereby achieving the technical effect of obtaining a sufficiently deep and gently gradient hardened layer using induction hardening of medium carbon steel.

[0031] As further explanation, regarding the selection of frequency parameters, this invention sets the preheating frequency to 2500–8000 Hz. Within this frequency band, the current penetration depth is sufficient to cover the target hardened layer range of 1.0–2.0 mm. If the preheating frequency is below 2500 Hz, the heating efficiency is significantly reduced, resulting in excessively long preheating times. If it is above 8000 Hz, the penetration depth becomes shallower, failing to effectively preheat the subsurface layer. The heating frequency is set to 100–300 kHz. Within this frequency band, the skin effect is significant, allowing the surface layer to be heated to the austenitizing temperature in a very short time without causing excessive heat diffusion towards the core. If it is below 100 kHz, the surface heating rate is insufficient; if it is above 300 kHz, the risk of overheating of the extremely thin surface layer increases. The above frequency combination and power allocation strategy together ensure the controllability of the hardened layer depth and hardness gradient.

[0032] To address the aforementioned dual-frequency composite induction strengthening process for carbon steel, this invention also provides an online composite strengthening production system for implementing the above-mentioned process. This system, along the part flow direction, sequentially includes: a feeding device, an outer circular groove induction hardening station, a first infrared temperature measurement station, a thermal balance station, an inner ball track induction hardening station, a second infrared temperature measurement station, a tempering furnace, and a unloading device. The aforementioned stations are connected in series via an automatic conveying device, which sequentially transfers parts from one station to the next, enabling continuous part flow and online processing.

[0033] The thermal balance station includes a forced cooling device and / or a rotation mechanism with part buffering function. The automatic conveying device sends the parts that have completed outer circle quenching into the cooling buffer through parallel branch conveying paths at the thermal balance station. After the thermal balance conditions are met, the parts are transferred to the main conveying line of the inner ball track quenching station.

[0034] The implementation and effects of the present invention will be further illustrated by specific embodiments below.

[0035] Example 1 This embodiment focuses on a mating part in an automotive transmission system. The part is made of SAE 1045 medium carbon steel, with the following chemical composition (mass percentage): C 0.43%–0.50%, Si 0.15%–0.35%, Mn 0.60%–0.90%, P≤0.040%, S≤0.040%, and the balance being Fe. This part has two functional areas requiring hardening: an outer groove and an inner ball joint. The outer groove is used to mount bearing balls, and the inner ball joint is used for movement with the lead screw.

[0036] This example demonstrates the strengthening process of the mating parts using the dual-frequency composite induction strengthening method for carbon steel provided in this invention. The entire process includes the following steps: Step 1: Quenching and tempering pretreatment. The SAE 1045 medium carbon steel billet is heated to 850℃ and held for 1.5 hours, then oil-quenched; subsequently, it is tempered at 600℃ for 2.5 hours and air-cooled. After quenching and tempering, the core of the part obtains a tempered sorbite structure with a hardness of HRC28~30.

[0037] Step 2: Finishing of parts. The tempered blank is turned, milled and ground to form the final geometry of the outer circular groove and the inner ball track, leaving a 0.08mm allowance for finishing after quenching.

[0038] Step 3: Dual-frequency composite induction hardening of the outer circular channel. Position the part in the induction hardening station of the outer circular channel, with the inductor sleeved on the outside of the outer circular channel. The coupling gap between the inductor coil and the channel surface is controlled at 0.8–1.2 mm. The first heating stage preheats the outer circular channel with a medium-frequency current (frequency f1 = 5000 Hz) at a power of 6 kW (50% of the rated power of 12 kW) for 3 seconds, uniformly raising the channel surface temperature to approximately 700°C. The second heating stage switches to a high-frequency current (frequency f2 = 200 kHz) at 11.5 kW (96% of the rated power of 12 kW) for rapid heating for 2 seconds, raising the channel surface temperature to 900 ± 10°C and holding it for 0.5 seconds. During the heating process, an infrared temperature measurement closed-loop control system is used to monitor the surface temperature of the channel in real time. This system includes an infrared temperature sensor, a signal processing unit, and a power control unit. The sensor acquires temperature signals at a sampling frequency of 50Hz, and the power control unit dynamically adjusts the power output according to the temperature deviation, ensuring that the deviation between the actual temperature and the preset temperature is controlled within ±10℃. After heating, the spray cooling device immediately sprays PAG quenching liquid onto the outer circular channel for cooling. The PAG concentration is 10%, the quenching liquid temperature is 30±2℃, the spray pressure is 0.4MPa, and the cooling time is 15 seconds.

[0039] Step 4: Thermal Balance Control. After the outer groove quenching is completed, the part is transferred to the first infrared temperature measurement station via an automatic conveying device. The infrared temperature sensor scans the temperature of the entire part. When the temperature of the quenched area of ​​the outer groove drops to 175℃, the part is transferred to the thermal balance station and stays there for 45 seconds to homogenize the internal temperature field of the part.

[0040] Step 5: Dual-frequency composite induction hardening of the inner ball track. The part is transferred to the inner ball track induction hardening station, and the inner ball track inductor extends into the inner cavity of the inner ball track. The inner ball track induction coil adopts a contoured structure, its arc profile matching the curved surface of the inner ball track. The coupling gap between the arc-shaped heating section and the inner ball track surface is controlled at 0.6–1.0 mm. The arc-shaped heating section of the inductor is equipped with a ferrite magnetic conductor. The third heating section preheats the inner ball track with a medium-frequency current (frequency f1' = 5000 Hz) at a power of 4.5 kW (50% of the rated power of 9 kW) for 2 seconds, causing the surface temperature of the inner ball track to rise uniformly to approximately 690°C. The fourth heating section switches to a high-frequency current (frequency f2' = 200 kHz) at 8.5 kW (94% of the rated power of 9 kW) for rapid heating for 1.5 seconds, raising the surface temperature of the inner ball track to 900 ± 10°C and holding it for 0.5 seconds. Immediately after heating, PAG quenching liquid is sprayed for cooling, with the same cooling parameters as for the outer channel quenching.

[0041] Step Six: Low-Temperature Tempering. After the parts have undergone two quenching processes, place them in a tempering furnace and hold them at 180°C for 2 hours, then air cool them.

[0042] Testing revealed that the surface hardness of the outer circular groove of the part in this embodiment is HRC59-61, with an effective hardened layer depth (limiting hardness HRC50) of 1.3 mm; the surface hardness of the inner ball raceway is HRC59-61, with an effective hardened layer depth of 1.2 mm. The transition zone width between the two hardened areas is 3.5 mm, and the hardness gradient along the axial line connecting the two hardened areas is ΔHRC7.2 / mm. Both the outer circular groove hardened area and the inner ball raceway hardened area exhibit a double-layer composite hardened structure: the surface full martensite layer has a depth of approximately 0.8 mm and a hardness of HRC59-61; the subsurface transitional structure layer has a depth of approximately 0.5 mm and a hardness of HRC45-58. The core structure is tempered sorbite with a hardness of HRC28-30.

[0043] Example 2 The difference between this embodiment and Embodiment 1 is as follows: For the outer circular channel quenching, the frequency of the first heating section is adjusted to 2500Hz, the preheating power is 60% of the rated power, and the preheating time is 4 seconds; the frequency of the second heating section is adjusted to 100kHz, the heating time is 3 seconds, and the holding time is 0.8 seconds. For the inner ball channel quenching, the frequency of the third heating section is adjusted to 8000Hz, the preheating power is 55% of the rated power, and the preheating time is 1.5 seconds; the frequency of the fourth heating section is adjusted to 300kHz, the heating time is 1 second, and the holding time is 0.2 seconds. Other steps and parameters are the same as in Embodiment 1.

[0044] The performance indicators of the parts in this embodiment are as follows: the surface hardness of the outer groove is HRC59-62, and the effective hardened layer depth is 1.4mm; the surface hardness of the inner raceway is HRC58-61, and the effective hardened layer depth is 1.1mm. The width of the transition zone is approximately 4.2mm, and the hardness gradient along the axial line connecting the two hardened zones is approximately ΔHRC6.8 / mm. The deformation control is comparable to that of Embodiment 1.

[0045] Example 3 The difference between this embodiment and Embodiment 1 is that the quenching temperature in the tempering process is adjusted to 840℃, the tempering temperature is adjusted to 580℃, the low-temperature tempering temperature is adjusted to 160℃, and the holding time is extended to 3 hours. Other steps and parameters are the same as in Embodiment 1.

[0046] The performance indicators of the parts in this embodiment are as follows, as tested: the surface hardness of the outer groove is HRC60-62, with an effective hardened layer depth of 1.4mm; the surface hardness of the inner raceway is HRC60-62, with an effective hardened layer depth of 1.3mm; the core hardness is HRC26-30; and the deformation is controlled within 0.02mm.

[0047] Example 4 The difference between this embodiment and Embodiment 1 is that the PAG quenching fluid concentration is adjusted to 8%, the quenching fluid temperature is adjusted to 25±2℃, the spraying pressure is adjusted to 0.5MPa, and the cooling time is shortened to 10 seconds. Other steps and parameters are the same as in Embodiment 1.

[0048] Testing revealed that the surface hardness of the part in this embodiment reached HRC61-62, with a slight increase in the depth of the hardened layer (1.4 mm for the outer circular groove and 1.3 mm for the inner ball groove). The hardness gradient along the axial line connecting the transition areas between the two hardened zones was ΔHRC8.5 / mm. The deformation was basically the same as in Embodiment 1.

[0049] Comparative Example 1 (Traditional Carburizing and Quenching Process) Using SAE 5120 low-carbon alloy steel as raw material, the same parts were processed using the traditional carburizing and quenching process. Process parameters: carburizing temperature 920℃, holding time 4 hours; quenching temperature 865℃, oil cooling; tempering temperature 180℃, holding time 2 hours.

[0050] Testing revealed a surface hardness of HRC 58–61, an effective hardened layer depth of 1.5 mm, and a part deformation of 0.15 mm (in the direction of the outer diameter groove). Compared to Embodiment 1 of the present invention, Comparative Example 1 has approximately 25% higher raw material costs, approximately 120% higher energy consumption, and approximately 8 times higher deformation.

[0051] Comparative Example 2 (Conventional Single-Frequency Induction Hardening) Using SAE 1045 medium carbon steel as raw material, conventional single-frequency induction hardening process is adopted (frequency 200kHz, direct heating to 900℃ followed by spray cooling, no preheating section).

[0052] Testing revealed a surface hardness of HRC58–61, but the effective hardened layer depth was only 0.8 mm, failing to meet the technical requirement of 1.1 mm. Furthermore, the radial hardness gradient between the hardened area and the core was as high as ΔHRC18 / mm, posing a risk of hardened layer peeling.

[0053] As can be seen from the comparison of Examples 1 to 4 with Comparative Examples 1 and 2, the process method of the present invention achieves a controllable increase in the depth of the hardened layer and a significant mitigation of the hardness gradient while ensuring high surface hardness, thus overcoming the inherent defects of the existing single-frequency induction hardening scheme.

[0054] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A dual-frequency composite induction strengthening process for medium carbon steel, used to prepare a gradient hardening layer structure on parts with outer circular channels and inner ball channels, characterized in that, Includes the following steps: S1. Tempering pretreatment: SAE 1045 medium carbon steel is selected as the raw material for the parts. The blank is subjected to tempering treatment. The tempering treatment includes heating the blank to 840-860℃, holding it at the temperature for 1-2 hours, oil quenching, holding it at 580-620℃ for 2-3 hours for high-temperature tempering, and then air cooling. S2. Part finishing: The tempered blank is machined to form the final geometry of the outer circular groove and the inner ball track; S3. Dual-frequency composite induction hardening of outer circular channels: First, preheat the outer circular channels with a medium-frequency current of 2500–8000 Hz, with a preheating power of 50%–60% of the rated power and a heating time of 2–4 seconds, raising the surface temperature of the outer circular channels to 680–750℃; then switch to a high-frequency current of 100–300 kHz, and rapidly heat at 90%–100% of the rated power for 1.5–3 seconds, raising the surface temperature of the outer circular channels to 880–920℃ and holding it for 0.2–0.8 seconds; immediately after heating, spray cooling quenching is performed on the outer circular channels to achieve a surface hardness of HRC58–62; S4. Dual-frequency composite induction hardening of the inner ball track: First, the inner ball track is preheated with a medium-frequency current of 2500-8000Hz, with a preheating power of 50%-60% of the rated power and a heating time of 1.5-3 seconds, raising the surface temperature of the inner ball track to 650-750℃; then, the current is switched to a high-frequency current of 100-300kHz, with a rapid heating of 90%-100% of the rated power, for 1-2 seconds, raising the surface temperature of the inner ball track to 880-920℃ and holding it for 0.2-0.8 seconds; immediately after heating, the inner ball track is spray-cooled and quenched to achieve a surface hardness of HRC58-62. S5. Low-temperature tempering: After quenching, the parts are held at 160-200℃ for 2-3 hours and then air-cooled. The process includes a thermal balance control step between steps S3 and S4: after the outer channel quenching is completed, the overall temperature distribution of the part is monitored in real time. Once the temperature of the outer channel quenching area drops below the lower limit of the tempering brittleness sensitive temperature zone of the material in that area, and the temperature at the monitoring point is not higher than 120°C, the inner ball track quenching program is started. This forms a transition zone with a width of 2-5 mm between the hardened area of ​​the outer channel and the hardened area of ​​the inner ball track, and the hardness change gradient along the axial line connecting the two hardened areas is not greater than ΔHRC10 / mm.

2. The method for dual-frequency composite induction strengthening of medium carbon steel according to claim 1, characterized in that: In step S3, the effective hardened layer depth after quenching of the outer circular channel is 1.1 to 1.5 mm, and in step S4, the effective hardened layer depth after quenching of the inner ball channel is 1.0 to 1.4 mm.

3. The dual-frequency composite induction strengthening process for medium carbon steel according to claim 1, characterized in that: The cooling medium used in the spray cooling quenching in steps S3 and S4 is a PAG quenching liquid with a concentration of 8% to 12%, the quenching liquid temperature is controlled at 20 to 40°C, the spray pressure is 0.3 to 0.5 MPa, and the cooling time is 10 to 20 seconds.

4. The dual-frequency composite induction strengthening process for medium carbon steel according to claim 1, characterized in that: During the quenching process in steps S3 and S4, an infrared temperature measurement closed-loop control system is used for real-time temperature monitoring and power adjustment. The infrared temperature measurement closed-loop control system includes an infrared temperature sensor, a signal processing unit, and a power control unit. The infrared temperature sensor acquires the channel surface temperature in real time at a sampling frequency of not less than 50Hz. The power control unit dynamically adjusts the output power of the induction heating power supply according to the deviation between the measured temperature and the preset temperature.

5. The method for dual-frequency composite induction strengthening of medium carbon steel according to claim 1, characterized in that: In step S4, the inductor used for inner ball track quenching adopts a contour-following structure. The inductor includes an arc-shaped heating section and a longitudinal connecting section. The contour of the arc-shaped heating section is adapted to the inner ball track surface.

6. A wear-resistant part with a gradient hardening layer structure, characterized in that: The part is manufactured using the process method described in any one of claims 1 to 5; the material of the part is SAE 1045 medium carbon steel; the structure of the part includes an outer circular groove hardened area, an inner ball hardened area, a transition area located between the outer circular groove hardened area and the inner ball hardened area, and an unhardened area at the core of the part; the surface hardness of the outer circular groove hardened area and the inner ball hardened area are both HRC58-62; the width of the transition area is 2-5 mm, and the hardness value of the transition area continuously decreases from HRC58-62 near the hardened area to HRC25-32 near the core, and the hardness change gradient along the axial line connecting the two hardened areas is not greater than ΔHRC10 / mm; the hardness of the core of the part is HRC25-32.

7. The wear-resistant part with a gradient hardening layer structure according to claim 6, characterized in that: Both the outer circular groove hardening zone and the inner ball passage hardening zone have a double-layer composite hardening structure, which consists of a full martensite layer on the surface and a transitional structure layer on the subsurface. The hardness of the full martensite layer is HRC58-62 and the depth is 0.6-1.0 mm. The hardness of the transitional structure layer is HRC45-58 and the depth is 0.4-0.7 mm.

8. An online composite enhanced production system for implementing the process method according to any one of claims 1 to 5, characterized in that: Along the direction of part flow, the system includes, in sequence, a feeding device, an outer circular channel induction hardening station, a first infrared temperature measurement station, a thermal balance station, an inner ball channel induction hardening station, a second infrared temperature measurement station, a tempering furnace, and a feeding device. The above stations are connected in series by an automatic conveying device, which is used to sequentially convey parts from one station to the next, realizing continuous flow and online processing of parts.

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