Gas annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery

Abstract

The invention provides a gas annealing kiln heat treatment method based on airflow adjustment and waste gas heat recovery. The gas annealing kiln heat treatment method comprises the steps that a casting to be annealed is placed on a rotatable workpiece bearing table in a kiln; the gas annealing kiln is divided into a preheating area, a strong heating area and a soaking and heat preservation area; pulse type high-speed burners are alternately arranged on the two sides of the strong heating area, an alternate ignition mode is adopted, and heat transfer is enhanced through periodic pulse airflow; high-temperature flue gas in the strong heating area flows back to the preheating area through the flue gas countercurrent flow guide channel to preheat the casting; low-pulse-ratio flat flame burners or wide burners are arranged on the two sides of the soaking heat preservation area, and convective heat transfer is enhanced by rotating the workpiece bearing table; and after heat treatment is finished, the burner is closed, waste heat in the kiln is used for heat preservation, and then natural cooling or controlled cooling is conducted. Energy waste can be effectively reduced, and the annealing quality and stability of the casting are improved.

Description

Technical Field

[0001] This application belongs to the field of heat treatment technology for metallic materials, specifically relating to a gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery. Background Technology

[0002] Gas-fired annealing furnaces are a common type of heat treatment equipment for castings, primarily used for the annealing of materials such as gray cast iron, ductile cast iron, malleable cast iron, and vermicular graphite cast iron. They utilize natural gas or liquefied petroleum gas as fuel, generating a high-temperature environment within the furnace to achieve graphitization, stress relief, and softening. These furnaces typically employ a continuous or tunnel structure, equipped with a multi-stage burner system and flue gas exhaust channels to achieve continuous heating, holding, and cooling processes for the castings. In the foundry industry, these furnaces are widely used for the heat treatment of high-end castings such as large machine tool beds, wind turbine hubs, automotive brake discs, and marine diesel engine frames, to improve the internal microstructure, reduce hardness, and enhance mechanical properties.

[0003] However, existing gas-fired annealing kilns have significant technical defects in actual operation, mainly manifested in uneven temperature field distribution within the kiln. This leads to uneven heating of castings, insufficient local stress release, or unsatisfactory graphite morphology, thus affecting the stability of annealing quality. For example, the burner arrangement and airflow control methods of traditional kilns cannot effectively disrupt the thermal boundary layer, resulting in a temperature difference of ±15 to 30°C between the surface and interior of the casting. Ultimately, this results in residual stress exceeding 80 MPa, hardness fluctuation greater than ±20 HBW, and graphitization degree less than 85% in the annealed castings, failing to meet the precision annealing requirements of high-end castings.

[0004] While existing technologies have attempted some improvements, such as adjusting burner power distribution or adding simple flue gas channels to alleviate temperature unevenness, these solutions are often limited to static heating modes and cannot achieve real-time matching between dynamic airflow disturbances and casting position, resulting in limited improvement in overall temperature uniformity. Therefore, it is necessary to develop a more comprehensive heat treatment method to solve the problem of uneven temperature field within the kiln and achieve high-quality annealing of castings. Summary of the Invention

[0005] This application provides a gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery, which solves the problem of uneven temperature field distribution inside the existing gas-fired annealing kiln, resulting in unstable annealing quality of castings.

[0006] This application provides a gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery, including the following steps: S1. Place the casting to be annealed on a rotatable workpiece support platform inside the kiln; S2. Divide the gas-fired annealing kiln along its length into a preheating zone, a strong heating zone, and a uniform heat preservation zone. S3. Pulsed high-speed burners are alternately arranged on both sides of the strong heating zone. The alternating ignition and combustion mode of the left and right burners is adopted. The duration of a single ignition and combustion of a single burner is 0.5 to 8 seconds, and the switching interval is 0.2 to 3 seconds. The heat transfer is enhanced by periodic pulsed airflow to achieve turbulence in the kiln. S4. The high-temperature flue gas generated in the strong heating zone is guided back to the preheating zone through the flue gas counter-current guide channel set on the top of the kiln to preheat the castings entering the preheating zone. S5. Low pulse ratio flat flame burners or wide flame burners are set on both sides of the heat preservation zone. The low pulse combustion mode is adopted. At the same time, the workpiece support platform is driven to rotate the casting continuously or intermittently at an angular velocity of 0.5 to 5° / min, so that the surface of the casting and the high temperature airflow in the kiln generate relative motion to enhance convective heat transfer. S6. After the heat treatment process is completed, close all burners and use the residual heat in the kiln to keep the castings warm for 0.5 to 2 hours. Then, allow them to cool naturally or under controlled cooling before exiting the kiln.

[0007] In one optional implementation, the ignition pulse frequency of the pulse-type high-speed burner is linked in real time with the rotational angular velocity of the workpiece carrier. The rotation angle of the workpiece carrier is monitored by a rotary encoder. When the workpiece carrier rotates a cumulative 90°±10°, the control system additionally triggers the ignition of the burner on the opposite side based on the basic pulse mode.

[0008] In one optional embodiment, the casting on the workpiece support platform is fixed to the workpiece support platform by a clamp or a tray, and rotates with the workpiece support platform as a whole without tipping over. The rotational angular velocity is controlled between 0.1 and 8° / min, and the relative linear velocity between the surface of the casting and the high-temperature airflow in the kiln during the rotation is not less than 0.8 m / s.

[0009] In one optional implementation, a controllable flue gas recirculation channel is provided on the kiln top of the heat-preserving zone, and the flue gas in the heat-preserving zone is recirculated back to the combustion air inlet of the burner in the strong heating zone at a volume ratio of 5% to 25%, so as to achieve staged combustion and secondary combustion of incomplete combustion products.

[0010] In one alternative implementation, the volumetric oxygen concentration inside the kiln is controlled at 1% to 6% throughout the entire heat treatment process.

[0011] In one optional embodiment, non-pulse high-speed burners or flat flame burners are provided on both sides of the preheating zone. The length ratio of the preheating zone, the strong heating zone, and the heat-preserving zone is 1:(1.5~2.5):(1~2). The ratio of the total heat load of the burners in the three zones is (10~20%):(50~65%):(20~35%).

[0012] In one optional embodiment, the controllable flue gas recirculation channel is equipped with a high-temperature resistant regulating butterfly valve, a temperature sensor, and an oxygen content sensor. The temperature of the recirculated flue gas in the controllable flue gas recirculation channel is not lower than 600°C, and the recirculation flow rate is adjusted and controlled by the control system according to the real-time temperature gradient and oxygen content in the kiln.

[0013] In one optional embodiment, the burners in the preheating zone, the strong heating zone, and the heat-preserving zone all use natural gas or liquefied petroleum gas as fuel; wherein, the outlet flow rate of the pulse-type high-speed burner is 50-150 m / s, the rated power of a single burner is 0.3-3 MW, and the pulse combustion frequency is 0.1-1 Hz.

[0014] Compared with the prior art, this application has the following beneficial effects: 1. This application provides a gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery. With a structural design of a preheating zone, a strong heating zone, and a homogenizing and heat-preserving zone, the heating process of the annealing kiln becomes more refined and efficient. By dividing the annealing kiln into three different zones along its length, targeted temperature control can be achieved. In the strong heating zone, the alternating ignition mode of pulsed high-speed burners ensures faster and more uniform heating, avoiding the temperature unevenness caused by localized heat concentration in traditional kilns. Specifically, the alternating pulsed airflow disturbance breaks the thermal boundary layer, reducing heat accumulation and temperature differences within the kiln, ensuring uniform temperature distribution of the casting during heating, and avoiding excessive temperature differences between the casting surface and interior, thereby improving the uniformity of the annealing effect. This temperature management method effectively avoids the problems of casting hardness fluctuations and excessive residual stress caused by uneven temperature field distribution in traditional technologies.

[0015] 2. This application, by incorporating the rotation of the workpiece support platform, can further enhance the convective heat transfer effect between the casting and the high-temperature airflow. The rotating workpiece support platform causes relative movement between the casting surface and the airflow inside the kiln, avoiding localized overheating or uneven cooling of the casting, reducing stress concentration problems caused by temperature differences, and also enhancing the degree of graphitization during the annealing process, resulting in a more uniform improvement in the performance of the casting, thereby improving the stability of the annealing quality.

[0016] 3. This application improves the energy efficiency of the annealing kiln by reusing waste gas heat. Through a counter-current flue gas guiding channel, the high-temperature waste gas generated in the strong heating zone is guided back to the preheating zone for preheating the castings. This not only reduces the demand for external energy but also fully utilizes the heat in the waste gas, thereby improving the energy efficiency of the heat treatment process. By recovering the waste gas and maintaining residual heat within the kiln, energy consumption can be further reduced, improving the economic and environmental benefits of heat treatment. This energy-saving and emission-reduction measure has significant cost-saving effects in large-scale production and promotes the sustainable development of the annealing process. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A flowchart of a gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery, provided as an embodiment of this application. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application are described clearly and completely below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are also within the scope of protection of this application.

[0020] like Figure 1 As shown in the figure, this application provides a gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery, including the following steps: S1. Place the casting to be annealed on a rotatable workpiece support platform inside the kiln; S2. Divide the gas-fired annealing kiln along its length into a preheating zone, a strong heating zone, and a uniform heat preservation zone. S3. Pulsed high-speed burners are alternately arranged on both sides of the strong heating zone. The alternating ignition and combustion mode of the left and right burners is adopted. The duration of a single ignition and combustion of a single burner is 0.5 to 8 seconds, and the switching interval is 0.2 to 3 seconds. The heat transfer is enhanced by periodic pulsed airflow to achieve turbulence in the kiln. S4. The high-temperature flue gas generated in the strong heating zone is guided back to the preheating zone through the flue gas counter-current guide channel set on the top of the kiln to preheat the castings entering the preheating zone. S5. Low pulse ratio flat flame burners or wide flame burners are set on both sides of the heat preservation zone. The low pulse combustion mode is adopted. At the same time, the workpiece support platform is driven to rotate the casting continuously or intermittently at an angular velocity of 0.5 to 5° / min, so that the surface of the casting and the high temperature airflow in the kiln generate relative motion to enhance convective heat transfer. S6. After the heat treatment process is completed, close all burners and use the residual heat in the kiln to keep the castings warm for 0.5 to 2 hours. Then, allow them to cool naturally or under controlled cooling before exiting the kiln.

[0021] The gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery provided in this application, with its structural design of a preheating zone, a strong heating zone, and a homogenizing and heat-preserving zone, makes the heating process of the annealing kiln more refined and efficient. By dividing the annealing kiln into three different zones along its length, targeted temperature control can be achieved. In the strong heating zone, the alternating ignition mode of pulsed high-speed burners makes heating faster and more uniform, avoiding the temperature unevenness caused by localized heat concentration in traditional kilns. Specifically, the alternating pulsed airflow disturbance breaks the thermal boundary layer, reducing heat accumulation and temperature differences within the kiln, ensuring uniform temperature distribution of the casting during heating, and avoiding excessive temperature differences between the surface and interior of the casting, thereby improving the uniformity of the annealing effect. This temperature management method effectively avoids the problems of casting hardness fluctuations and excessive residual stress caused by uneven temperature field distribution in traditional technologies.

[0022] Meanwhile, this embodiment, combined with the rotation of the workpiece support platform, can further enhance the convective heat transfer effect between the casting and the high-temperature airflow. The rotating workpiece support platform causes relative movement between the casting surface and the airflow inside the kiln, avoiding localized overheating or uneven cooling of the casting, reducing stress concentration problems caused by temperature differences, and also enhancing the degree of graphitization during the annealing process, resulting in a more uniform improvement in the performance of the casting, thereby improving the stability of the annealing quality.

[0023] Furthermore, the reuse of waste gas heat improves the energy efficiency of the annealing kiln. In this embodiment, a counter-current flue gas guide channel is used to guide the high-temperature waste gas generated in the strong heating zone back to the preheating zone for preheating the castings. This not only reduces the demand for external energy but also fully utilizes the heat in the waste gas, thereby improving the energy efficiency of the heat treatment process. By recovering waste gas and maintaining residual heat in the kiln, energy consumption can be further reduced, improving the economic and environmental benefits of heat treatment. This energy-saving and emission-reduction measure has significant cost-saving effects in large-scale production and promotes the sustainable development of the annealing process.

[0024] In practical applications, to achieve precise temperature control and optimized combustion, each heating zone within the kiln (including the preheating zone, high-intensity heating zone, and homogenization zone) is equipped with temperature and oxygen concentration sensors. These sensors monitor the temperature and oxygen concentration within the kiln in real time, ensuring that the operating status of each zone remains within predetermined ranges. Temperature sensors, positioned at key locations in each zone, accurately reflect temperature changes within the kiln, providing data support for the control system. This allows parameters such as burner ignition frequency and combustion duration to be adjusted according to temperature fluctuations. Oxygen concentration sensors continuously monitor the oxygen content within the kiln to ensure stable and sufficient combustion, thereby preventing incomplete or excessively rapid combustion due to insufficient or excessive oxygen supply.

[0025] In some embodiments, the ignition pulse frequency of the pulse-type high-speed burner is linked in real time with the rotational angular velocity of the workpiece carrier. The rotation angle of the workpiece carrier is monitored by a rotary encoder. When the workpiece carrier rotates a cumulative 90°±10°, the control system additionally triggers the ignition of the burner on the opposite side based on the basic pulse mode.

[0026] In this embodiment, the alternating ignition mode of the pulsed high-speed burner (0.5–8 seconds of combustion, 0.2–3 seconds of interval) constitutes the core rhythm of heating within the kiln. Rapid alternating ignition achieves periodic airflow disturbance, thereby enhancing heat transfer between the airflow and the casting surface. The goal of this design is to optimize heat transfer efficiency, i.e., by disturbing the thermal boundary layer, to make the heat distribution within the kiln more uniform, avoiding uneven heating caused by temperature inconsistencies. This basic pulse mode is independent of the rotation of the workpiece support platform and operates within a second-level timeframe, maintaining a continuous and stable heating rhythm to promote uniform heating of the casting and improve overall annealing quality. In this embodiment, the burner ignition pulse mode is linked in real-time with the rotational angular velocity of the workpiece support platform. The control system can dynamically adjust the burner's operating state based on the rotation angle of the workpiece support platform. Specifically, the control system uses a PID control algorithm to adjust the burner's ignition interval and combustion time to further optimize the heating process according to the temperature requirements of the casting at different locations, avoiding excessively high or low temperatures.

[0027] However, to further improve the accuracy and responsiveness of the heating process, this embodiment adds a rotation angle linkage mechanism. Specifically, during the rotation of the workpiece support platform, whenever the platform has rotated a cumulative 90°±10°, the control system additionally triggers the ignition of the burner on the opposite side in addition to the basic pulse mode. This additional triggering does not override or replace the basic pulse mode, but rather dynamically intervenes by adjusting the burner switching interval or extending the combustion time on one side to match the heating requirements of the casting's new position. This measure further optimizes the heating process in the basic mode, preventing excessive local temperature differences caused by changes in workpiece position, and avoiding problems such as over-concentrated heating or uneven temperature distribution.

[0028] Furthermore, the rotary encoder monitors the rotation angle of the workpiece support platform in real time, ensuring precise matching between ignition timing and rotational angular velocity. When the workpiece support platform rotates 90°, the ignition triggering mechanism of the opposite burner adjusts according to the current temperature requirements of the casting to further balance the temperature distribution within the kiln, avoiding overheating or underheating caused by asymmetrical ignition in traditional methods. This dynamic adjustment mechanism not only enhances the uniform distribution of airflow and reduces localized overheating but also helps reduce residual stress and hardness fluctuations, further improving the annealing quality of the casting, especially enhancing the degree of graphitization and the uniform softening effect of the casting. This precise temperature control method provides a more accurate annealing process for high-end castings, ensuring that the temperature difference between the surface and interior of the casting is minimized during annealing.

[0029] In some embodiments, the casting on the workpiece support platform is fixed to the platform by clamps or a tray, rotating with the platform without tipping over. The rotational angular velocity is controlled between 0.1 and 8° / min, and the relative linear velocity between the casting surface and the high-temperature airflow inside the kiln during rotation is not less than 0.8 m / s. In practical applications, the workpiece support platform achieves linear movement via a high-temperature resistant conveyor belt. Specifically, the workpiece support platform fixes the casting, securing it to the tool support platform with clamps or a tray to prevent tipping during rotation. The workpiece support platform itself, conveyed by the high-temperature resistant conveyor belt, travels linearly along the long axis of the kiln during heating.

[0030] During annealing, temperature inhomogeneity between the surface and interior of the casting has always been a major problem affecting heat treatment quality. Traditional heating methods often result in significant temperature differences between the casting surface and interior, leading to thermal stress and hardness fluctuations, thus affecting the annealing effect. This embodiment effectively solves this problem by rotating the workpiece support platform. Specifically, the rotation speed of the workpiece support platform is controlled within the range of 0.1–8° / min, which causes relative motion between the casting surface and the high-temperature airflow inside the kiln, resulting in more uniform heating of the casting. During rotation, the relative linear velocity between the casting surface and the airflow inside the kiln is not less than 0.8 m / s, which effectively promotes convective heat transfer between the airflow and the casting surface. By enhancing convective heat transfer, the temperature difference between the casting surface and interior is reduced, thereby reducing localized temperature inhomogeneity, helping to eliminate residual stress caused by excessive temperature differences, and improving the uniformity and stability of the annealing quality.

[0031] In addition, the castings on the workpiece support platform are fixed to the support platform by clamps or trays, which prevents the castings from tipping over during rotation. Combined with the control of the rotational angular velocity between 0.1 and 8° / min, the castings remain stable during rotation and will not become unstable due to excessive or uneven speed.

[0032] Through these designs, the castings not only maintain a stable posture during the annealing process, but the rotational motion further enhances the uniform distribution of heat, promotes temperature uniformity during the annealing process, and thus improves the stability of the annealing quality.

[0033] In this embodiment, the rotation control of the workpiece support platform is achieved by a control system, and the rotation speed can be dynamically adjusted according to the actual temperature inside the kiln and the state of the casting. The control system monitors the kiln temperature and oxygen concentration in real time, and adjusts the range and angle of the rotation speed based on feedback signals from temperature and oxygen concentration sensors, combined with the rotational angular velocity of the workpiece support platform, to ensure temperature uniformity during the heating process. Specifically, the control system employs a PID control algorithm to accurately calculate the appropriate rotation speed based on the actual rotation state of the workpiece support platform, and then adjusts the speed by driving the workpiece support platform with a motor. This process allows for timely adjustment of the rotation speed according to the temperature requirements at different locations within the kiln, addressing different heating needs and changes in the casting's position, thereby effectively improving the uniformity of temperature distribution and preventing excessive local temperature differences. Through this dynamic adjustment mechanism, even if the casting's position changes within the kiln, the control system can respond in real time, ensuring heating uniformity during the annealing process and avoiding temperature fluctuations caused by inconsistent rotation speeds or incompatibility with changes in the casting's position.

[0034] In some embodiments, a controllable flue gas recirculation channel is provided on the kiln top of the heat-preserving zone, and the flue gas in the heat-preserving zone is returned to the combustion air inlet of the burner in the strong heating zone at a volume ratio of 5% to 25%, so as to achieve staged combustion and secondary combustion of incomplete combustion products.

[0035] A controllable flue gas recirculation channel is installed on the kiln roof in the homogenization and insulation zone, recirculating 5%–25% of the flue gas back to the combustion air inlet of the burners in the strong heating zone. This design aims to achieve staged combustion and secondary combustion of incompletely burned products. By recirculating a portion of the flue gas from the homogenization and insulation zone, thermal efficiency can be effectively improved and energy consumption reduced. Because the recirculated flue gas is at a relatively high temperature, utilizing this waste gas as part of the combustion air reduces the amount of external air required for combustion, thereby lowering dependence on external energy sources.

[0036] Furthermore, the recirculated flue gas participates in combustion in the intensely heated zone, aiding in secondary combustion. This process not only reduces the emission of harmful unburned gases but also further improves combustion efficiency. After implementing this technology, the temperature distribution within the kiln becomes more uniform, and the thermal energy utilization rate during combustion is significantly improved. By adjusting the recirculation rate, optimization can be achieved based on temperature changes and oxygen content within the kiln, ensuring that combustion at each stage is in an optimal state.

[0037] In some embodiments, the volumetric oxygen concentration inside the kiln is controlled at 1% to 6% throughout the entire heat treatment process.

[0038] In the heat treatment process of an annealing furnace, controlling the oxygen concentration is crucial for the stability of the combustion process. This embodiment effectively avoids combustion problems caused by excessive or insufficient oxygen supply by maintaining the oxygen concentration within the furnace within the range of 1% to 6%. If the oxygen concentration is too high, the combustion rate will be too fast, easily leading to temperature fluctuations within the furnace and exacerbating energy waste. Conversely, if the oxygen concentration is too low, incomplete combustion will affect the full utilization of heat and may generate more pollutants. By controlling the oxygen concentration, the combustion state can be optimized, thereby improving combustion efficiency, reducing pollutant emissions, and enhancing energy utilization.

[0039] Furthermore, proper control of oxygen concentration within the kiln plays a crucial role in maintaining temperature stability during the annealing process. By adjusting the oxygen concentration, the combustion environment within the kiln is optimized, temperature fluctuations are effectively controlled, and the heating of the castings during heat treatment becomes more uniform. Stable oxygen concentration avoids temperature inconsistencies caused by oxygen content fluctuations, thereby improving the heating uniformity of the castings. This temperature uniformity not only improves the annealing quality but also ensures the mechanical properties and structural uniformity of the castings.

[0040] In some embodiments, non-pulse high-speed burners or flat flame burners are provided on both sides of the preheating zone. The length ratio of the preheating zone, the strong heating zone, and the heat preservation zone is 1:(1.5-2.5):(1-2). The ratio of the total heat load of the burners in the three zones is (10-20%):(50-65%):(20-35%).

[0041] In the design of annealing furnaces, the rational division and temperature control of the preheating zone, intense heating zone, and homogenization and holding zone are crucial for improving heating efficiency and annealing quality. Non-pulsating high-speed burners or flat-flame burners are installed on both sides of the preheating zone. Compared to traditional pulse burners, these burners provide a more stable and uniform heat distribution, avoiding temperature fluctuations that may occur with pulsed combustion. The main function of the preheating zone is to heat the casting from room temperature to the predetermined initial temperature. Using a stable combustion mode can more efficiently improve preheating efficiency while reducing the risk of overheating and preventing casting deformation or cracks caused by uneven heating.

[0042] Furthermore, the high-heating zone is configured with a higher heat load, while the preheating zone and the homogenization and holding zone have lower heat loads. This heat load distribution pattern, by rationally allocating the heating capacity of different zones, allows the casting to quickly reach the required temperature in the initial heating stage, while maintaining a stable temperature environment in the subsequent homogenization and holding stages. This temperature control design effectively reduces thermal stress, makes the casting heat more uniformly, and further improves the annealing quality.

[0043] Furthermore, this embodiment employs a length ratio of 1:(1.5-2.5):(1-2) for the preheating zone, the intense heating zone, and the homogenizing and heat-preserving zone, while using an optimized heat load ratio of (10-20%):(50-65%):(20-35%). This helps achieve a more balanced heat distribution across different zones. This heat distribution pattern results in more stable temperature changes within the kiln, avoiding overheating or undercooling, and ensuring uniform heating of the castings throughout the annealing process. This improves annealing efficiency and reduces energy consumption. Ultimately, this design not only enhances the heat treatment efficiency of the annealing kiln but also effectively solves the problems of uneven temperature and energy waste present in traditional annealing kilns, making the entire annealing process more energy-efficient and effective.

[0044] To verify the effectiveness of pulsed airflow disturbance in optimizing heat distribution within the kiln, this embodiment compares the temperature difference changes of castings under alternating ignition and conventional heating modes through simulation experiments. Experimental results show that the temperature difference between the casting surface and interior is significantly reduced using the pulsed airflow disturbance heating mode, with temperature fluctuations controlled within ±5℃, while under the conventional heating mode, the temperature difference can reach ±20℃. Furthermore, the hardness distribution of the annealed casting is more uniform; compared to the hardness fluctuations exceeding ±10 HBW in the conventional mode, the pulsed airflow disturbance mode reduces hardness fluctuations to within ±3 HBW. These experimental data demonstrate that pulsed airflow disturbance not only effectively improves temperature uniformity but also enhances heating accuracy during annealing, thereby ensuring the stability and performance consistency of the castings.

[0045] In some embodiments, the controllable flue gas recirculation channel is equipped with a high-temperature regulating butterfly valve, a temperature sensor, and an oxygen content sensor. The temperature of the recirculated flue gas in the controllable flue gas recirculation channel is not lower than 600°C, and the recirculation flow rate is adjusted and controlled by the control system according to the real-time temperature gradient and oxygen content in the kiln.

[0046] The high-temperature resistant regulating butterfly valve can flexibly adjust the flow rate of the recirculated flue gas in high-temperature environments, avoiding damage to the equipment caused by excessively high temperatures. This design allows the exhaust gas to effectively return to the combustion air inlet of the strong heating zone, improving combustion efficiency and reducing the demand for external air and fuel. Through the cooperation of temperature and oxygen content sensors, the system monitors the temperature and oxygen concentration inside the kiln in real time, and dynamically adjusts the temperature and flow rate of the recirculated flue gas according to the real-time temperature gradient and oxygen concentration inside the kiln, controlling the stability of the combustion process.

[0047] Furthermore, the temperature of the recirculated flue gas is controlled above 600℃, ensuring that the recirculated exhaust gas has sufficient heat for thermal energy recovery. This not only effectively improves the heat recovery efficiency of the exhaust gas but also maintains the combustion stability of the intense heating zone, avoiding problems such as excessive temperature fluctuations or incomplete combustion.

[0048] In some embodiments, the burners in the preheating zone, the strong heating zone, and the heat-preserving zone all use natural gas or liquefied petroleum gas as fuel; wherein, the outlet flow rate of the pulse high-speed burner is 50 to 150 m / s, the rated power of a single burner is 0.3 to 3 MW, and the pulse combustion frequency is 0.1 to 1 Hz.

[0049] In the design of the preheating zone, intense heating zone, and homogenization and insulation zone, burners using natural gas or liquefied petroleum gas as fuel can provide an efficient and stable combustion process. The outlet velocity of the pulse-type high-speed burner is in the range of 50–150 m / s. This velocity creates a strong disturbance in the combustion airflow within the kiln, promoting convective heat transfer between the airflow and the casting surface. The higher airflow velocity helps break the thermal boundary layer, enhancing heat transfer efficiency and resulting in a more uniform temperature difference between the casting surface and interior. This avoids temperature fluctuations caused by uneven airflow and improves the stability of the entire annealing process. In this way, the temperature distribution within the kiln is optimized, the heating effect on the casting is more uniform, the temperature difference is reduced, and stress concentration and hardness fluctuations caused by localized overheating or uneven cooling are avoided.

[0050] Furthermore, by adjusting the rated power (0.3–3 MW) and pulse combustion frequency (0.1–1 Hz) of a single burner, flexible energy output can be achieved according to the needs of different heating zones. In pulse combustion mode, the burner alternates between ignition and extinguishing, making the combustion process more stable and efficient. This periodic variation avoids the problems of overheating or uneven heating in traditional continuous combustion methods. By adjusting the burner's power and frequency, precise heat distribution can be achieved in different temperature zones, avoiding energy concentration or uneven distribution, thereby improving heating uniformity. This design enables more precise temperature control during annealing, especially for the heat treatment of high-end castings, thus improving the accuracy of the annealing effect.

[0051] The following embodiments further illustrate specific application examples of this application for graphitization, stress relief, and softening annealing of castings. These embodiments employ the technical solutions described above, taking into account the actual dimensions, material, and long-cycle annealing requirements of the castings. They optimize and adjust the burner parameters, rotation speed, flue gas recirculation ratio, and holding time for each zone, and provide repeatable operating conditions and measured technical effects so that those skilled in the art can directly implement them.

[0052] Example 1 (Applicable to graphitization and stress-relief annealing of gray cast iron machine tool beds, kiln length 42m, daily processing capacity 60t) This embodiment employs a roller hearth continuous gas-fired annealing kiln. A high-temperature resistant roller conveyor is installed at the kiln bottom, on which a high-temperature resistant conveyor belt runs. Workpiece support platforms are placed on the high-temperature conveyor belt, which is driven by an external variable frequency motor. This allows the workpiece support platforms, carrying castings, to sequentially pass through the preheating zone, the intense heating zone, and the homogenization and insulation zone. The overall longitudinal section structure of the gas-fired annealing kiln includes the kiln shell, the refractory insulation layer, and the three-zone division. The casting material is gray cast iron HT250, with individual dimensions of 4m in length, 2m in width, and 1.5m in height, weighing approximately 15t.

[0053] S1. Place the casting to be annealed on the rotatable workpiece support platform inside the kiln, and use a high-temperature resistant steel pallet to fix the casting to ensure stable placement.

[0054] S2. The gas-fired annealing kiln is divided into a 10m preheating zone, a 20m strong heating zone, and a 12m heat-preserving zone along the length of the kiln.

[0055] S3. Twelve pulse-type high-speed burners are alternately arranged on both sides of the strong heating zone. The burners on the left and right sides are used in an alternating ignition and combustion mode. The duration of a single ignition and combustion of a single burner on one side is 4 seconds, and the switching interval is 1 second. The heat transfer is enhanced by periodic pulse airflow to achieve kiln disturbance. The natural gas fuel pressure is 0.1 MPa, and the oxygen concentration is monitored by an online oxygen content sensor to adjust the combustion air ratio.

[0056] S4. The high-temperature flue gas generated in the strong heating zone is guided back to the preheating zone through the flue gas counter-flow guide channel set on the top of the kiln to preheat the castings entering the preheating zone; the flue gas temperature is controlled at 900℃, and the channel lining is made of high-temperature resistant alloy.

[0057] S5. Eight low-pulse-ratio flat flame burners are set on both sides of the heat-preserving zone. The low-pulse combustion mode is adopted, and the workpiece support platform is driven to continuously rotate the casting at an angular velocity of 2° / min. This causes the surface of the casting to move relative to the high-temperature airflow in the kiln to enhance convective heat transfer. The rotating fixture is made of high-temperature resistant steel.

[0058] S6. After the heat treatment process is completed, all burners are turned off, and the castings are kept warm for another hour using the residual heat in the kiln. Then they are allowed to cool naturally out of the kiln.

[0059] Results: Casting temperature uniformity ±3.5℃, residual stress ≤40MPa, Brinell hardness reduced by 60HBW, and graphitization degree (Type A graphite ≥92%); compared with traditional gas-fired annealing furnaces, temperature unevenness was reduced by 60%, residual stress was reduced by 75%, graphitization degree was increased by 20%, natural gas consumption was 0.85GJ / t (traditional 1.15GJ / t), and NOx emissions were 48mg / Nm³. 3 .

[0060] Example 2 (Applicable to stress relief and softening annealing of ductile iron wind turbine hubs, kiln length 34m, daily processing capacity 40t) This embodiment employs a roller hearth continuous gas-fired annealing kiln. A high-temperature resistant roller conveyor is installed at the kiln bottom, on which a high-temperature resistant conveyor belt runs. A workpiece support platform is placed on the high-temperature conveyor belt, which is driven by an external variable frequency motor. This allows the workpiece support platform, carrying the casting, to sequentially pass through the preheating zone, the intense heating zone, and the homogenization and insulation zone. The overall longitudinal section structure of the gas-fired annealing kiln includes the kiln shell, the refractory insulation layer, and the three-zone division. The intense heating zone accounts for approximately 47% of the length of the kiln structure. The casting material is ductile iron QT450, with a single piece having a diameter of 1.2m and a weight of 2t.

[0061] S1. Place the casting to be annealed on a rotatable workpiece support platform inside the kiln and fix the casting with a high-temperature resistant steel clamp.

[0062] S2. The gas-fired annealing kiln is divided into a preheating zone of 8m, a strong heating zone of 16m, and a heat-preserving zone of 10m along the length of the kiln.

[0063] S3. Ten pulse-type high-speed burners are alternately arranged on both sides of the strong heating zone. The combustion mode of alternating ignition of the left and right burners is adopted. The duration of a single ignition combustion of a single burner is 5s, and the switching interval is 1.5s. The periodic pulse airflow is used to achieve enhanced heat transfer by disturbing the kiln. The liquefied petroleum gas fuel pressure is 0.15MPa, and the oxygen concentration is monitored by an online oxygen content sensor and maintained at 3% by adjusting the excess combustion air coefficient.

[0064] S4. The high-temperature flue gas generated in the strong heating zone is guided back to the preheating zone through the flue gas counter-current guide channel set on the top of the kiln to preheat the castings entering the preheating zone; the flue gas temperature is controlled at 950℃.

[0065] S5. Six wide burners are set on both sides of the heat-preserving zone. They adopt a low-pulse combustion mode and drive the workpiece support platform to rotate the casting intermittently at an angular velocity of 1.5° / min. This causes the surface of the casting to move relative to the high-temperature airflow in the kiln to enhance convective heat transfer. The rotating fixture is made of high-temperature resistant steel pallet.

[0066] S6. After the heat treatment process is completed, all burners are turned off, and the castings are kept warm for another 1.5 hours using the residual heat in the kiln. Then, the castings are cooled and discharged from the kiln by controlling the cooling fan.

[0067] Results: Casting temperature uniformity ±3.2℃, residual stress ≤35MPa, Brinell hardness reduced by 50HBW, spheroidization rate ≥95%, and surface decarburized layer thickness ≤0.1mm; compared with traditional gas-fired annealing furnaces, temperature unevenness was reduced by 65%, residual stress was reduced by 80%, spheroidization rate was increased by 15%, natural gas consumption was 0.78GJ / t (traditional 1.1GJ / t), and NOx emissions were 52mg / Nm³. 3 .

[0068] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery, characterized in that, Includes the following steps: S1. Place the casting to be annealed on a rotatable workpiece support platform inside the kiln; S2. Divide the gas-fired annealing kiln along its length into a preheating zone, a strong heating zone, and a uniform heat preservation zone. S3. Pulsed high-speed burners are alternately arranged on both sides of the strong heating zone. The alternating ignition and combustion mode of the left and right burners is adopted. The duration of a single ignition and combustion of a single burner is 0.5 to 8 seconds, and the switching interval is 0.2 to 3 seconds. The heat transfer is enhanced by periodic pulsed airflow to achieve turbulence in the kiln. S4. The high-temperature flue gas generated in the strong heating zone is guided back to the preheating zone through the flue gas counter-current guide channel set on the top of the kiln to preheat the castings entering the preheating zone. S5. Low pulse ratio flat flame burners or wide flame burners are set on both sides of the heat preservation zone. The low pulse combustion mode is adopted. At the same time, the workpiece support platform is driven to rotate the casting continuously or intermittently at an angular velocity of 0.5 to 5° / min, so that the surface of the casting and the high temperature airflow in the kiln generate relative motion to enhance convective heat transfer. S6. After the heat treatment process is completed, close all burners and use the residual heat in the kiln to keep the castings warm for 0.5 to 2 hours. Then, allow them to cool naturally or under controlled cooling before exiting the kiln.

2. The gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery according to claim 1, characterized in that, The ignition pulse frequency of the pulse-type high-speed burner is linked in real time with the rotational angular velocity of the workpiece carrier. The rotation angle of the workpiece carrier is monitored by a rotary encoder. When the workpiece carrier rotates a cumulative 90°±10°, the control system additionally triggers the ignition of the burner on the opposite side based on the basic pulse mode.

3. The gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery according to claim 1, wherein the casting on the workpiece support platform is fixed to the workpiece support platform by a clamp or tray, and rotates with the workpiece support platform as a whole without tipping over, the rotation angular velocity is controlled between 0.1 and 8° / min, and the relative linear velocity between the surface of the casting and the high-temperature airflow in the kiln during the rotation is not less than 0.8 m / s.

4. The gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery according to claim 1, characterized in that, A controllable flue gas recirculation channel is set up on the kiln top of the heat-preserving zone, and the flue gas in the heat-preserving zone is returned to the combustion air inlet of the burner in the strong heating zone at a volume ratio of 5% to 25%, so as to achieve staged combustion and secondary combustion of incomplete combustion products.

5. The gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery according to claim 1, characterized in that, Throughout the heat treatment process, the volumetric oxygen concentration inside the kiln is controlled between 1% and 6%.

6. The gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery according to claim 1, characterized in that, Non-pulse high-speed burners or flat flame burners are installed on both sides of the preheating zone. The length ratio of the preheating zone, the strong heating zone, and the heat preservation zone is 1:(1.5~2.5):(1~2). The ratio of the total heat load of the burners in the three zones is (10~20%):(50~65%):(20~35%).

7. The gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery according to claim 4, characterized in that, The controllable flue gas recirculation channel is equipped with a high-temperature resistant regulating butterfly valve, as well as a temperature sensor and an oxygen content sensor. The temperature of the recirculated flue gas in the controllable flue gas recirculation channel is not lower than 600°C, and the recirculation flow rate is adjusted and controlled by the control system according to the real-time temperature gradient and oxygen content in the kiln.

8. The gas-fired annealing kiln heat treatment method based on airflow regulation and waste gas heat recovery according to any one of claims 1-7, characterized in that, The burners in the preheating zone, strong heating zone, and heat preservation zone all use natural gas or liquefied petroleum gas as fuel; wherein, the outlet flow velocity of the pulse high-speed burner is 50-150 m / s, the rated power of a single burner is 0.3-3 MW, and the pulse combustion frequency is 0.1-1 Hz.

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