Alloy steel fitting normalizing and annealing method based on porous medium combustion
Patent Information
- Application Number
- CN202611231400.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-08-14
- Publication Date
- 2026-10-27
Smart Images
Figure CN122979577A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of metal heat treatment technology, and in particular to a method for normalizing and annealing alloy steel fittings based on porous media combustion. Background Technology
[0002] Normalizing and annealing are among the most important heat treatment processes in the production of alloy steel parts, and are widely used in the automotive, machinery, and aerospace industries. In the manufacturing process of alloy steel parts, normalizing and annealing are crucial heat treatment processes that play a decisive role in improving the microstructure and enhancing the overall performance of alloy steel.
[0003] Traditional gas-fired heat treatment furnaces, when performing normalizing and annealing treatments on alloy steel parts, suffer from several drawbacks. First, the preheating process lacks an efficient energy recovery and utilization mechanism, resulting in insufficient preheating of the combustion air before it enters the burner. This leads to increased energy consumption as more energy is required to raise the temperature during combustion. Second, the high-temperature flue gas generated during heat treatment is directly emitted, and the large amount of heat energy contained within it is not effectively recovered and utilized, resulting in significant energy waste. This not only keeps production costs high but also contradicts the current advocacy of energy conservation and emission reduction. Third, during combustion, traditional heat treatment furnaces suffer from significant heat loss in flue gas due to improper control of the excess air coefficient. Furthermore, excess air promotes the formation of nitrogen oxides, causing serious environmental pollution. Finally, traditional combustion methods fail to effectively suppress the generation of thermal nitrogen oxides, exacerbating the problem of high pollution emissions. Summary of the Invention
[0004] The purpose of this invention is to provide a method for normalizing and annealing alloy steel fittings based on porous media combustion, thereby solving the technical problems existing in the prior art.
[0005] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:
[0006] A method for normalizing and annealing alloy steel parts based on porous media combustion includes the following steps: Workpiece loading and preheating: The alloy steel parts to be treated are loaded into a heat treatment furnace and preheated at a low temperature using a porous media burner. This provides the first stage of preheating for the combustion air, while simultaneously using the high-temperature flue gas generated during the normalizing process for a second stage of preheating. Heating: The power of the porous media burner is gradually increased, and the workpiece is heated to a normalizing temperature 30-80°C above Ac3 according to a preset heating rate. During the heating process, the temperature is adjusted according to the alloy steel's properties. The phase change endothermic characteristics dynamically adjust the combustion power, automatically increasing the heating power within the phase change temperature range; normalizing heat preservation: maintaining the furnace temperature within the normalizing temperature range, with the preservation time calculated based on the effective thickness of the workpiece; during the heat preservation process, a pulsed porous medium combustion control method is adopted, achieving precise temperature control by adjusting the duty cycle of the combustion pulse, ensuring furnace temperature uniformity within ±5℃; normalizing cooling and waste heat recovery: the normalized workpiece is transferred to a cooling chamber for air cooling, and the high-temperature flue gas generated in the normalizing process is recovered through a three-stage waste heat recovery mechanism; annealing heating. The workpiece after normalizing is returned to the furnace and heated in a low-temperature section using the residual heat recovered from the normalizing process, reaching an annealing temperature 20-50°C below Ac1. Annealing holding and slow cooling: The workpiece is held at the annealing temperature for 2-6 hours, then slowly cooled in the furnace at a rate of 10-30°C / h to 300-400°C. The cooling rate is controlled by reducing the combustion power of the porous media and adjusting the flue gas circulation. Furnace cooling: The workpiece is removed from the furnace and allowed to cool naturally to room temperature in the air. In the normalizing cooling and residual heat recovery step… The three-stage waste heat recovery mechanism in the process includes: the first stage waste heat recovery, porous media self-storage heat recovery, based on the high heat capacity characteristics of porous media materials, the heat stored during the combustion interval is used to preheat the mixed gas in the next cycle; the second stage waste heat recovery, combustion air preheating recovery, the high temperature flue gas is used to preheat the combustion air from room temperature to 300-500℃ through a gas-to-gas heat exchanger; the third stage waste heat recovery, annealing process preheating recovery, the 200-300℃ flue gas after the first two stages of recovery is introduced into the preheating section of the annealing furnace for low-temperature heating of the annealed workpiece.
[0007] Furthermore, in the workpiece loading and preheating step, the porous media burner uses silicon carbide foam ceramic as the porous media material, with a pore density of 10-30 PPI, a porosity of 70-90%, a porous media thickness of 20-50 mm, and a burner surface heat load of 200-500. .
[0008] Furthermore, in the workpiece loading and preheating step, the first stage of preheating involves the combustion air flowing through the heat storage layer of the porous medium burner, preheating the air to 300-500°C based on the thermal inertia and thermal conductivity of the porous medium; the second stage of preheating involves the high-temperature flue gas of 800-900°C discharged from the normalizing process entering the preheating chamber, preheating the workpiece through radiation and convection, and then entering the subsequent waste heat recovery mechanism after the flue gas temperature drops to 500-600°C.
[0009] Furthermore, the phase change heat absorption characteristics in the heating process are dynamically adjusted as follows: a database of phase change temperature heat absorption power for different steel grades is established in advance, the workpiece temperature is monitored in real time during the heating process, and when the workpiece temperature approaches the Ac1 and Ac3 phase change points, the combustion power is automatically increased by 10-30%.
[0010] Furthermore, the pulsed porous media combustion control method in the normalizing and heat preservation step is as follows: a periodic start-stop pulse combustion mode is adopted, with a pulse period of 30-120s and a duty cycle of 20-80%. The average heating power is adjusted by adjusting the duty cycle. During each pulse period, the porous media temperature rises to 1000-1200℃ during the combustion phase. During the combustion stop phase, the porous media maintains the furnace temperature through radiation and heat storage release.
[0011] Furthermore, the porous media combustion adopts low excess air coefficient combustion control, with the excess air coefficient α controlled between 1.02 and 1.08, and closed-loop control is achieved through online flue gas oxygen content analysis.
[0012] Furthermore, the porous media combustion employs a low-NOx combustion technology combining staged combustion and internal flue gas recirculation, dividing the combustion air into primary, secondary, and tertiary air; including the following steps: primary air accounts for 40-60%, premixed with fuel and then entering the inner layer of the porous media for combustion; secondary air accounts for 20-30%, introduced from the middle layer of the porous media; tertiary air accounts for 10-30%, introduced from the outer layer of the porous media or the furnace; a portion of the flue gas is recirculated to the burner inlet to reduce the combustion zone temperature and suppress thermal combustion. generate.
[0013] Furthermore, it also includes: deep recovery of waste heat from flue gas condensation. After three stages of waste heat recovery, the temperature of the flue gas drops to 100-150℃ and then enters a condensing heat exchanger to cool the flue gas to 40-60℃, recovering the latent heat of water vapor in the flue gas.
[0014] Furthermore, the slow cooling process control in the annealing, heat preservation and slow cooling steps is as follows: a composite control strategy of decreasing combustion power and increasing flue gas circulation volume is adopted; in the initial stage of slow cooling, the combustion power is maintained at 30-50% of the rated power and the flue gas circulation volume is 20-30%; as the furnace temperature decreases, the combustion power is gradually reduced while the flue gas circulation volume is increased to 50-70%.
[0015] Compared with the prior art, the present invention has the following beneficial effects:
[0016] (i) This invention achieves efficient utilization of thermal energy by combining porous media combustion technology with a three-stage waste heat recovery mechanism. The first stage of the three-stage waste heat recovery mechanism is porous media self-storage heat recovery, which uses the high heat capacity of porous media to store heat and preheat the mixed gas. The second stage is combustion air waste heat recovery, which preheats the combustion air to a higher temperature. The third stage is annealing process waste heat recovery, which uses waste heat to heat the annealed workpiece at a low temperature. In addition, the deep recovery step of flue gas condensation waste heat further recovers the latent heat of water vapor in the flue gas, which further reduces energy consumption. Compared with traditional gas heat treatment furnaces, the energy saving effect is obvious.
[0017] (ii) This invention adopts low excess air coefficient combustion control and achieves closed-loop control through online flue gas oxygen content analysis, which effectively reduces exhaust heat loss and reduces the generation of nitrogen oxides caused by excess air. At the same time, it adopts low-NOx combustion technology that combines staged combustion and flue gas internal recirculation, introduces combustion air in stages and recirculates part of the flue gas to reduce the temperature of the combustion zone, inhibiting the generation of thermal nitrogen oxides and reducing environmental pollution.
[0018] (iii) The present invention has uniform temperature and does not have local high temperature zones like those in traditional flames; the temperature uniformity of the porous medium surface can reach within ±30℃, and by reasonably arranging the burners, the temperature uniformity in the furnace can be controlled within ±5℃, so that the workpiece is heated uniformly and the structure and properties of the workpieces in the same furnace are consistent; and the thermal deformation of the workpiece is reduced. Attached Figure Description
[0019] Figure 1 This is a flowchart of a normalizing and annealing treatment method for alloy steel fittings based on porous media combustion, as disclosed in this invention.
[0020] Figure 2 This is a schematic diagram of the three-stage waste heat recovery mechanism of a normalizing and annealing treatment method for alloy steel fittings based on porous media combustion disclosed in this invention. Detailed Implementation
[0021] To make the content of this invention easier to understand, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Identical components are represented by the same reference numerals. It should be noted that the terms "front," "rear," "left," "right," "up," and "down" used in the following description refer to directions in the accompanying drawings, while the terms "inner" and "outer" refer to directions toward or away from the geometric center of a specific component, respectively.
[0022] like Figure 1-2As shown, this embodiment provides a method for normalizing and annealing alloy steel fittings based on porous media combustion, including the following steps:
[0023] Workpiece loading and preheating: The alloy steel parts to be treated are loaded into the heat treatment furnace. A porous media burner is used for low-temperature preheating, utilizing the heat storage characteristics of the porous media to preheat the combustion air in the first stage. Simultaneously, the high-temperature flue gas generated during the normalizing process is used for the second stage of preheating, raising the workpiece temperature from room temperature to 500-600℃. The porous media burner in the workpiece loading and preheating step uses silicon carbide foam ceramic as the porous media material, with a pore density of 10-30 PPI, a porosity of 70-90%, a porous media thickness of 20-50 mm, and a burner surface heat load of 200-500. The bulk density of the silicon carbide foam ceramic described in this embodiment is: Thermal conductivity is Compressive strength ≥2MPa, thermal shock resistance ≥100 cycles without cracking;
[0024] The first stage of preheating involves the combustion air flowing through the heat storage layer of the porous media burner, where the air is preheated to 300-500°C based on the thermal inertia and thermal conductivity of the porous media.
[0025] The second stage of preheating involves the high-temperature flue gas of 800-900℃ discharged from the normalizing process entering the preheating chamber and preheating the workpiece through radiation and convection. After the flue gas temperature drops to 500-600℃, it enters the subsequent waste heat recovery mechanism.
[0026] The fuel for the porous media burner includes one or more mixed fuels selected from natural gas, liquefied petroleum gas, hydrogen, ammonia, coke oven gas, producer gas, and petrochemical volatile organic waste gas.
[0027] The alloy steel parts mentioned are automotive alloy steel parts, including but not limited to gears, shafts, connecting rods, crankshafts, springs, fasteners, etc., and their materials include 20CrMnTi, 20CrMo, 40Cr, 42CrMo, 35CrMo, 50CrVA, 60Si2Mn, GCr15, etc.
[0028] Heating: Gradually increase the power of the porous media burner to heat the workpiece to a normalizing temperature 30-80°C above Ac3 according to a preset heating rate; during the heating process, dynamically adjust the combustion power according to the phase transformation heat absorption characteristics of the alloy steel, and automatically increase the heating power within the phase transformation temperature range to compensate for the latent heat of phase transformation; the dynamic adjustment of the phase transformation heat absorption characteristics in the heating process is as follows: a database of phase transformation temperature heat absorption power for different steel grades is established in advance, and the workpiece temperature is monitored in real time during the heating process. When the workpiece temperature approaches the Ac1 and Ac3 phase transformation points, the combustion power is automatically increased by 10-30%; since a large amount of latent heat of phase transformation is absorbed when alloy steel undergoes phase transformation, if the energy is not compensated in time, it may lead to uneven heating of the workpiece, affecting the final heat treatment effect. Therefore, this invention constructs a database of phase transformation temperature heat absorption power covering different steel grades; at the moment of phase transformation, the energy lost due to phase transformation heat absorption can be replenished in a timely and accurate manner, ensuring that the workpiece remains in a uniform heating state throughout the heating process, avoiding quality problems caused by temperature fluctuations.
[0029] Normalizing and holding: Maintaining the furnace temperature within the normalizing temperature range, the holding time is calculated based on the effective thickness of the workpiece to ensure the workpiece fully completes its microstructure transformation; during the holding process, a pulsed porous media combustion control method is adopted, and precise temperature control is achieved by adjusting the duty cycle of the combustion pulse, with the furnace temperature uniformity controlled within ±5℃; in this embodiment, the pulsed porous media combustion control method in the normalizing and holding step is: using a periodic start-stop pulse combustion mode, with a pulse period of 30-120s and a duty cycle of 20-80%, and the temperature is controlled by adjusting the duty cycle. The average heating power is adjusted to precisely control the temperature changes inside the furnace. During each pulse cycle, the temperature of the porous medium rises to 1000-1200℃ during the combustion phase, at which point the porous medium stores a large amount of thermal energy. During the combustion shutdown phase, the porous medium maintains the furnace temperature through radiation and heat storage release, thus keeping the furnace temperature stable. Through the above combustion control and heat release methods, not only is the uniformity of the furnace temperature ensured, avoiding local overheating or overcooling, but energy utilization efficiency is also improved, making the normalizing and heat preservation process more energy-efficient and effective.
[0030] Normalizing Cooling and Waste Heat Recovery: The workpiece after normalizing is transferred to the cooling chamber for air cooling. The high-temperature flue gas generated in the normalizing process is not directly emitted, but is recovered through a three-stage waste heat recovery mechanism, realizing efficient heat recovery and reuse.
[0031] The three-stage waste heat recovery mechanism in the normalizing cooling and waste heat recovery steps includes:
[0032] The first-stage waste heat recovery uses porous media self-storage heat recovery. Based on the high heat capacity of porous media materials, the heat stored during the combustion interval is used to preheat the mixture for the next cycle. By utilizing the intermittent heat during the combustion process, more favorable starting conditions are provided for the next combustion, effectively improving combustion efficiency and reducing energy waste.
[0033] The second stage of waste heat recovery involves preheating and recovering the combustion air. The high-temperature flue gas is preheated from room temperature to 300-500°C by a gas-to-gas heat exchanger. After the preheated combustion air enters the burner, it can participate more fully in the combustion reaction, improve the stability and efficiency of combustion, further reduce fuel consumption, and realize the secondary and efficient utilization of energy.
[0034] The third stage of waste heat recovery, namely the preheating recovery of the annealing process, involves passing the 200-300℃ flue gas after the first two stages of recovery into the preheating section of the annealing furnace for low-temperature heating of the annealed workpieces. This fully utilizes the remaining heat of the flue gas to preheat the workpieces in advance for the annealing process, reducing the additional energy input required during the annealing process. It also creates an organic cycle system for energy utilization in the entire heat treatment process, improving the comprehensive energy utilization rate and reducing production costs.
[0035] Annealing Heating: The normalized workpiece is reintroduced into the furnace, where the waste heat recovered from the normalizing process is used for low-temperature heating, improving energy utilization. Heating the workpiece to an annealing temperature 20-50°C below Ac1 effectively improves its toughness and ductility. Specifically, during the cooling process of the high-temperature workpiece from the normalizing process, a forced circulation fan transfers the heat dissipated by the workpiece to the preheating section of the annealing furnace. Simultaneously, the waste heat from the flue gas in the normalizing process heats the heat transfer oil or air through a heat exchanger, serving as an auxiliary heat source for the annealing furnace. In the low-temperature heating stage of the annealing process, the temperature is raised from room temperature to 500°C, with more than half of the heat coming from the waste heat recovery from the normalizing process. This step, utilizing the waste heat from normalizing for annealing heating, not only reduces dependence on external energy and lowers production costs but also optimizes the entire heat treatment process, fundamentally avoiding repeated energy consumption and ensuring more efficient energy utilization throughout the production process. It also reduces the environmental impact caused by energy consumption.
[0036] Annealing and slow cooling: The annealing temperature is held for 2-6 hours to allow sufficient time for atoms to arrange and diffuse in an orderly manner, eliminating internal stress generated during normalizing and ensuring a more uniform distribution of alloying elements. Then, the furnace is slowly cooled to 300-400°C at a rate of 10-30°C / h. Excessive cooling may cause internal stress to recur or even lead to cracks; conversely, excessively slow cooling may affect production efficiency. The cooling rate is controlled during slow cooling by reducing the combustion power of the porous medium and adjusting the flue gas circulation volume. The slow cooling process control in the annealing and slow cooling steps employs a composite control strategy of decreasing combustion power and increasing flue gas circulation volume. In the initial stage of slow cooling, the combustion power... The combustion power is maintained at 30-50% of the rated power, and the flue gas circulation volume is 20-30%. As the furnace temperature decreases, the combustion power is gradually reduced while the flue gas circulation volume is increased to 50-70%. The reduction in combustion power reduces the input of additional heat, while the increase in flue gas circulation volume accelerates the dissipation of heat, thereby ensuring that the furnace temperature decreases continuously and stably according to the predetermined cooling rate. Through the composite control strategy of decreasing combustion power and increasing flue gas circulation volume, the cooling rate is precisely controlled during the annealing and slow cooling process, ensuring that the internal microstructure of the alloy steel workpiece can be fully transformed and optimized, effectively improving the comprehensive properties of the workpiece such as toughness and ductility, and providing a guarantee for the production of high-quality alloy steel parts.
[0037] Furnace cooling: The workpiece is removed from the furnace and allowed to cool naturally to room temperature in the air. This allows the workpiece to release internal stress and stabilize its structure under relatively mild conditions. Natural cooling in the air allows the workpiece surface to exchange heat with the surrounding air through natural convection and radiation. Compared with other faster cooling methods, this natural cooling can reduce the temperature of different parts of the workpiece more evenly, reducing the risk of internal stress and deformation caused by uneven cooling rates.
[0038] Deep recovery of waste heat from flue gas condensation: After three stages of waste heat recovery, the temperature of the flue gas drops to 100-150℃ and then enters a condensing heat exchanger to cool the flue gas to 40-60℃, recovering the latent heat of water vapor in the flue gas. During this process, the water vapor in the flue gas is cooled and condensed into liquid water, releasing a large amount of latent heat. This released latent heat is recovered and reused. For example, the condensate produced by condensation is treated and recycled for use in workshop cleaning or equipment cooling, thereby effectively improving the energy utilization rate of the entire production process.
[0039] Furthermore, the porous media combustion employs low excess air coefficient combustion control, with the excess air coefficient α controlled between 1.02 and 1.08. Closed-loop control is achieved through online flue gas oxygen content analysis, reducing heat loss carried away by excess air while ensuring complete combustion and CO emissions ≤50ppm. When oxygen content fluctuates, feedback signals are rapidly transmitted to the control system, which then precisely adjusts the air supply to ensure the excess air coefficient remains stable within the set range. The program within the control system is preset by the operator; as it is existing technology, the specific workflow will not be elaborated here. This step reduces heat loss carried away by excess air, allowing the heat released during combustion to be used more effectively for heating the workpiece, significantly improving energy efficiency. In addition, while ensuring complete fuel combustion, strictly controlling CO emissions to a low level of 50ppm not only reduces environmental pollution but also demonstrates the process's excellent environmental performance.
[0040] The porous media combustion employs a low-NOx combustion technology combining staged combustion and internal flue gas recirculation, dividing the combustion air into primary air, secondary air, and tertiary air; including the following steps:
[0041] The primary air ratio is 40-60%, which is premixed with the fuel and then enters the inner layer of the porous medium for combustion; the premixing method can ensure that the fuel and air are initially and uniformly mixed.
[0042] The secondary air ratio is 20-30%, which is introduced from the middle layer of the porous medium. This part of the air can supplement the oxygen in the combustion zone and at the same time dilute the temperature of the combustion zone to avoid local high temperature.
[0043] The proportion of tertiary air is 10-30%, which is introduced from the outer layer of porous media or the furnace. At the same time, the airflow disturbance makes the combustion more complete and can further reduce the peak value of the overall temperature field of the furnace.
[0044] At the same time, some flue gas is recirculated to the burner inlet to reduce the temperature in the combustion zone and suppress thermal ignition. During combustion, a portion of the cooled flue gas is recirculated to the burner inlet, mixed with fresh fuel and combustion air, and then re-enters combustion. This measure effectively reduces the oxygen concentration and temperature in the combustion zone, fundamentally suppressing thermal combustion. Generate, the thermal type It is produced by the reaction of nitrogen and oxygen under high temperature conditions.
[0045] This embodiment also includes an intelligent energy-saving control system based on digital twins, comprising: establishing a digital twin model of the furnace temperature field, flow field, and combustion process; collecting real-time operating data such as furnace temperature, flue gas composition, and fuel flow rate; and performing online correction on the digital twin model; predicting the workpiece temperature change trend in the next 10-30 minutes based on the corrected model; adjusting combustion parameters in advance to avoid overshoot and achieve optimal energy-saving control; it should be noted that the above-mentioned digital twin technology is existing technology and will not be elaborated further here.
[0046] Example 1:
[0047] This embodiment uses a 20CrMnTi alloy steel gear for automotive transmissions as an example to illustrate in detail the normalizing and annealing treatment method based on porous media combustion of the present invention.
[0048] 20CrMnTi is a commonly used carburizing steel, widely used in automotive gears, shafts and other parts. Its specific chemical composition will not be elaborated on here.
[0049] The heat treatment equipment adopts a mesh belt type continuous normalizing and annealing furnace, which is 15m long and is divided into a preheating section, a heating section, a heat preservation section, and a slow cooling section.
[0050] The furnace top is equipped with 20 sets of porous medium burners, each with a power of 50kW, for a total power of 1000kW;
[0051] The porous medium is made of silicon carbide foam ceramic with a pore density of 20 PPI, a porosity of 85%, and a thickness of 30 mm.
[0052] The fuel used is natural gas, with a calorific value of approximately 8500 kcal. .
[0053] The specific process steps are as follows:
[0054] S1: Loading and preheating of workpieces: The cleaned 20CrMnTi gear blanks are evenly placed on the material tray and loaded into the mesh belt heat treatment furnace through the feeding end; In this embodiment, the mesh belt speed is set to 150mm / min; In the preheating section, a porous medium burner is used for low-temperature heating, while the high-temperature flue gas generated in the normalizing process is used for preheating.
[0055] The preheating section is divided into two zones:
[0056] First preheating zone: Preheating is carried out using the 700-800℃ high-temperature flue gas recovered from the normalizing process. The flue gas flows back from the heating section to the preheating section through the flue and heats the workpiece through convection and radiation. The burner is not turned on in this zone and the waste heat is fully utilized. The workpiece is heated from room temperature to about 400℃.
[0057] Second preheating zone: The porous medium burner is turned on for auxiliary heating to heat the workpiece from 400°C to 600°C. The burner is operated at low power with a duty cycle of about 30%. The combustion air first passes through the second-stage waste heat recovery gas-to-gas heat exchanger and is preheated to about 400°C before entering the burner. This step lasts for 40 minutes.
[0058] S2: Heating is performed by gradually increasing the power of the porous media burner to heat the workpiece from 600℃ to the normalizing temperature of 860℃ (approximately 35℃ above Ac3); the heating rate is approximately 6.5℃ / min; during the heating process, dynamic power adjustment is performed based on the phase transformation endothermic characteristics of 20CrMnTi steel: the phase transformation temperature and latent heat of phase transformation of 20CrMnTi steel are determined in advance by differential scanning calorimetry, and a phase transformation temperature-endothermic power database is established. The results show that the Ac1 of 20CrMnTi steel is approximately 740℃, the Ac3 is approximately 825℃, and the latent heat of phase transformation from pearlite to austenite is approximately 80kJ / kg.
[0059] During the heating process, the system monitors the workpiece temperature in real time: when the workpiece temperature is below 700℃, the combustion power is set according to the normal heating rate; when the workpiece temperature reaches 700℃ (close to Ac1), the system automatically increases the combustion power by 20% to compensate for the latent heat of phase transformation from pearlite to austenite; when the workpiece temperature reaches 780℃ (phase transformation is basically complete), the combustion power returns to normal; when the workpiece temperature reaches 800℃ (close to Ac3), since the transformation from ferrite to austenite also requires the absorption of heat, the combustion power is increased by 15% again; when the workpiece temperature reaches 850℃, the combustion power returns to normal.
[0060] The heating time for this step is approximately 40 minutes;
[0061] S3: Normalizing and heat preservation, maintain the furnace temperature at 860±5℃, and perform normalizing and heat preservation; the heat preservation time is calculated based on the effective thickness of the workpiece. The effective thickness of the gear is about 20mm, and the heat preservation time is calculated at 2min / mm, which is about 40min.
[0062] During the heat preservation process, a pulsed porous media combustion control method is adopted: the pulse cycle is set to 60s; the duty cycle is about 40%, that is, the combustion lasts for 24s and stops for 36s in each cycle; during the combustion stage, the surface temperature of the porous media rises to about 1100℃; during the combustion stop stage, the porous media maintains the furnace temperature through radiation and heat storage release, and the temperature slowly decreases by about 10-15℃.
[0063] The heat preservation time for this step is approximately 40 minutes.
[0064] S4: Normalizing cooling and waste heat recovery. A high-power fan is used to blow air to cool the workpiece at a rate of about 2-3℃ / s, cooling the workpiece from 860℃ to about 500℃. The microstructure after normalizing cooling is fine pearlite and a small amount of ferrite, with a hardness of about HB156-207, which meets the requirements of subsequent cutting and carburizing treatment.
[0065] The high-temperature flue gas generated during the normalizing process is recovered through a three-stage waste heat recovery system:
[0066] First-stage waste heat recovery: The porous media burner itself has a heat storage function; during the pulse combustion stage, the porous media is heated to a high temperature and stores a large amount of heat; during the combustion shutdown stage, the porous media releases the stored heat to the next batch of premixed gas, preheating it to a very high temperature.
[0067] Second-stage waste heat recovery: The flue gas discharged from the furnace has a temperature of about 800-900℃. It first enters a high-temperature gas-air heat exchanger to exchange heat with the combustion air. The combustion air is preheated from room temperature to about 400℃, and the flue gas temperature drops from 850℃ to about 400℃. The heat exchanger adopts a stainless steel finned tube heat exchanger with a heat exchange area of about 200m² and a heat exchange efficiency of about 90%.
[0068] Third-stage waste heat recovery: After the second-stage recovery, the flue gas temperature is approximately 400℃, and it is then introduced into the preheating section of the annealing furnace for low-temperature heating of the annealed workpieces; the flue gas temperature drops from 400℃ to approximately 150℃, and the recovery efficiency of this stage is approximately 15%;
[0069] Then, the flue gas at a temperature of about 150°C enters the condensing heat exchanger for deep recovery: the flue gas is cooled to about 50°C by cooling water as it passes through the condensing heat exchanger; the water vapor in the flue gas condenses and releases latent heat; the latent heat is absorbed by the cooling water and used to heat domestic hot water or for other purposes; the condensate produced by condensation is collected and treated and then used for workshop cleaning.
[0070] S5: Annealing heating. After cooling, the gears after normalizing are sent back to the annealing furnace for spheroidizing annealing. The purpose of spheroidizing annealing is to spheroidize the cementite, reduce hardness, improve machinability, and prepare the microstructure for subsequent carburizing and quenching. The annealing furnace also uses porous medium combustion heating and utilizes the residual heat recovered from the normalizing process for low-temperature heating.
[0071] Annealing heating is divided into two stages:
[0072] Low-temperature heating stage (room temperature to 600℃): 75% of the heat comes from the waste heat recovery of the normalizing process, including waste heat recovery of flue gas and workpiece; the heat from normalizing is transferred to the preheating section of the annealing furnace through the waste heat recovery mechanism to heat the workpiece from room temperature to 600℃.
[0073] High-temperature heating stage (600℃ to 720℃): After the workpiece temperature exceeds the waste heat recovery temperature level, the workpiece is heated to an annealing temperature of 720℃ (approximately 20℃ below Ac1) mainly by the porous medium burner.
[0074] The heating rate should be controlled at 50-80℃ / h to avoid excessively rapid heating that could lead to increased thermal stress.
[0075] The heating time for this step is approximately 10 hours.
[0076] S6: Annealing and slow cooling. The workpiece is held at 720℃ for 4 hours to fully spheroidize the lamellar cementite and form granular cementite distributed on the ferrite matrix. The holding process is also controlled by pulsed porous media combustion to maintain temperature uniformity within ±5℃. After the holding is completed, slow cooling is carried out in the furnace at a rate of 15-20℃ / h to slowly cool to 350℃.
[0077] The slow cooling process employs a combined control strategy of decreasing combustion power and increasing flue gas recirculation volume:
[0078] In the initial stage of slow cooling (720-600℃): the combustion power is maintained at about 40% of the rated power, and the flue gas circulation volume is 25%. At this time, the temperature is relatively high, and less heat needs to be added. The cooling rate is mainly adjusted by controlling the combustion power and flue gas circulation.
[0079] Mid-stage of slow cooling (600-500℃): Combustion power is gradually reduced to 25%, and flue gas circulation volume is increased to 45%; circulating flue gas is used to homogenize furnace temperature and ensure uniform cooling.
[0080] In the later stage of slow cooling (500-350℃): the combustion power is further reduced to 10%, and the flue gas circulation volume is increased to 65%; at this time, the temperature is relatively low, and the flue gas circulation is mainly used to maintain temperature uniformity and control the cooling rate.
[0081] After slow cooling to 350°C, the cooling rate can be accelerated because at this point it will no longer affect the spheroidization structure.
[0082] The insulation and slow cooling time for this step is approximately 20 hours.
[0083] S7: After the workpiece is removed from the furnace and cooled to room temperature in the air after the temperature drops to 350℃, it is taken out of the furnace and allowed to cool naturally to room temperature. The final microstructure is spherical pearlite with a hardness of about HB120-160 and a spheroidization level of 2-3, which meets the requirements of subsequent machining and carburizing treatment.
[0084] The process effects of this embodiment are compared with those of the traditional gas-fired normalizing and annealing process, and the results are shown in Table (I):
[0085] Table (1)
[0086]
[0087] The comparison results show that the present invention has significant improvements in energy saving, environmental protection, and product quality.
[0088] Example 2:
[0089] The parts that are the same as in Example 1 will not be repeated. The difference from Example 1 is that this example uses 50CrVA spring steel wire for automobile suspension as an example to illustrate the application of the present invention in the annealing of spring steel.
[0090] 50CrVA is a commonly used spring steel with high strength, elasticity and fatigue strength. It is widely used in automotive suspension springs, valve springs, etc. The specific chemical composition will not be elaborated here.
[0091] In this embodiment, the diameter of the spring steel wire is 12mm, each coil weighs about 500kg, and each furnace contains 4 coils, with a total weight of about 2000kg.
[0092] The heat treatment equipment adopts a pit-type annealing furnace with a furnace diameter of 1500mm and a depth of 2000mm; eight sets of porous media burners are arranged on the side of the furnace, each with a power of 40kW, and a total power of 320kW.
[0093] The purpose of annealing is to eliminate cold drawing stress, reduce hardness, improve plasticity, and prepare for the subsequent coiling process.
[0094] S1: Loading and preheating of workpieces: The cold-drawn 50CrVA spring steel wire coils are loaded into the pit-type annealing furnace, and the furnace cover is sealed. The porous medium burner is turned on for heating, and the heating rate is controlled at 80-100℃ / h, heating from room temperature to 500℃. During the preheating stage, the air is preheated using secondary waste heat recovery, and the combustion air is preheated to about 400℃, which improves the combustion efficiency.
[0095] S2: Heat the workpiece from 500℃ to 680℃ (approximately 72℃ below Ac1) for annealing. The heating rate is controlled at 50-60℃ / h. Since the stress-relief annealing temperature is below Ac1 and no phase transformation occurs, phase transformation power compensation is not required.
[0096] S3: Heat preservation, heat preservation at 680℃ for 4-6 hours to fully eliminate the work hardening caused by cold drawing and fully release the internal stress; the heat preservation process adopts pulsed porous medium combustion control, and the temperature uniformity is controlled within ±3℃.
[0097] S4: Slow cooling. After the heat preservation is completed, the furnace is slowly cooled to 300℃ at a cooling rate of 30-50℃ / h. The slow cooling process also adopts the control strategy of decreasing combustion power and increasing flue gas circulation volume to ensure uniform cooling.
[0098] S5: After being removed from the furnace and cooled, the temperature drops to 300℃, and then it can be air-cooled. The final hardness of the steel wire is about HB180-200, with good plasticity, which meets the requirements of coil springs.
[0099] The process effects of this embodiment are compared with those of the traditional gas annealing process, and the results are shown in Table (II):
[0100] Table (II)
[0101]
[0102] The comparison results show that, by using porous media combustion and waste heat recovery, this embodiment reduces energy consumption by more than half compared to traditional pit furnaces. Emissions are reduced by more than 90%, temperature uniformity is greatly improved, and the performance consistency of spring steel wire is better.
[0103] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.
Claims
1. A method for normalizing and annealing alloy steel fittings based on porous media combustion, characterized in that: Includes the following steps: Workpiece loading and preheating: The alloy steel parts to be processed are loaded into the heat treatment furnace and preheated at low temperature using a porous medium burner. The combustion air is preheated in the first stage, and the high-temperature flue gas generated in the normalizing process is used to preheat the workpiece in the second stage. Heating: Gradually increase the power of the porous medium burner and heat the workpiece to a normalizing temperature of 30-80°C above Ac3 according to the preset heating rate; during the heating process, dynamically adjust the combustion power according to the phase transformation heat absorption characteristics of alloy steel, and automatically increase the heating power within the phase transformation temperature range; Normalizing and heat preservation: Maintain the furnace temperature within the normalizing temperature range. The heat preservation time is calculated based on the effective thickness of the workpiece. During the heat preservation process, a pulsed porous medium combustion control method is adopted. Precise temperature control is achieved by adjusting the duty cycle of the combustion pulse. The furnace temperature uniformity is controlled within ±5℃. Normalizing Cooling and Waste Heat Recovery: The workpiece after normalizing is transferred to a cooling chamber for air cooling, and the high-temperature flue gas generated in the normalizing process is recovered through a three-stage waste heat recovery mechanism. Annealing heating: The workpiece after normalizing is sent back into the furnace and heated in a low-temperature section using the residual heat recovered from the normalizing process, so that the workpiece is heated to an annealing temperature of 20-50°C below Ac1. Annealing holding and slow cooling: Hold at the annealing temperature for 2-6 hours, then slowly cool with the furnace to 300-400℃ at a cooling rate of 10-30℃ / h. The cooling rate is controlled by reducing the combustion power of the porous medium and adjusting the flue gas circulation. Cooling after removal from the furnace: Remove the workpiece from the furnace and allow it to cool naturally to room temperature in the air; The three-stage waste heat recovery mechanism in the normalizing cooling and waste heat recovery step includes: The first stage of waste heat recovery is a porous media self-storage heat recovery. Based on the high heat capacity of porous media materials, the heat stored during the combustion interval is used to preheat the mixture for the next cycle. The second stage of waste heat recovery is combustion air preheating recovery. The high-temperature flue gas is preheated from room temperature to 300-500℃ by gas-to-gas heat exchangers. The third stage of waste heat recovery, the preheating recovery of the annealing process, involves passing the 200-300℃ flue gas after the first two stages of recovery into the preheating section of the annealing furnace for low-temperature heating of the annealed workpieces.
2. The method for normalizing and annealing alloy steel fittings based on porous media combustion according to claim 1, characterized in that: The porous media burner in the workpiece loading and preheating step uses silicon carbide foam ceramic as the porous media material, with a pore density of 10-30 PPI, a porosity of 70-90%, a porous media thickness of 20-50 mm, and a burner surface heat load of 200-500. .
3. The method for normalizing and annealing alloy steel fittings based on porous media combustion according to claim 1 or 2, characterized in that: The first stage of preheating in the workpiece loading and preheating process involves the combustion air flowing through the heat storage layer of a porous media burner, where the air is preheated to 300-500°C based on the thermal inertia and thermal conductivity of the porous media. The second stage of preheating involves the high-temperature flue gas of 800-900°C discharged from the normalizing process entering the preheating chamber, where the workpiece is preheated through radiation and convection. After the flue gas temperature drops to 500-600°C, it enters the subsequent waste heat recovery mechanism.
4. The method for normalizing and annealing alloy steel fittings based on porous media combustion according to claim 3, characterized in that: The dynamic adjustment of the phase change heat absorption characteristics in the heating process is as follows: a database of phase change temperature heat absorption power for different steel grades is established in advance, the workpiece temperature is monitored in real time during the heating process, and when the workpiece temperature approaches the Ac1 and Ac3 phase change points, the combustion power is automatically increased by 10-30%.
5. The method for normalizing and annealing alloy steel fittings based on porous media combustion according to claim 1, characterized in that: The pulsed porous media combustion control method in the normalizing and heat preservation step is as follows: a periodic start-stop pulse combustion mode is adopted, with a pulse period of 30-120s and a duty cycle of 20-80%. The average heating power is adjusted by adjusting the duty cycle. During each pulse period, the porous media temperature rises to 1000-1200℃ during the combustion phase. During the combustion stop phase, the porous media maintains the furnace temperature through radiation and heat storage release.
6. The method for normalizing and annealing alloy steel fittings based on porous media combustion according to claim 1, characterized in that: The porous media combustion employs low excess air coefficient combustion control, with the excess air coefficient α controlled between 1.02 and 1.08, and closed-loop control is achieved through online flue gas oxygen content analysis.
7. The method for normalizing and annealing alloy steel fittings based on porous media combustion according to claim 6, characterized in that: The porous media combustion employs a low-NOx combustion technology combining staged combustion and internal flue gas recirculation, dividing the combustion air into primary air, secondary air, and tertiary air; including the following steps: The primary air ratio is 40-60%, which is premixed with fuel and then enters the porous medium inner layer for combustion. The secondary air ratio is 20-30%, and it is introduced from the middle layer of the porous medium; The proportion of tertiary air is 10-30%, which is introduced from the outer layer of porous media or the furnace. By recirculating some of the flue gas to the burner inlet, the temperature in the combustion zone is reduced, thus suppressing thermal-type combustion. generate.
8. The method for normalizing and annealing alloy steel fittings based on porous media combustion according to claim 1, characterized in that: Also includes: The waste heat from flue gas condensation is deeply recovered. After three stages of waste heat recovery, the temperature of the flue gas drops to 100-150℃ and then enters a condensing heat exchanger to cool the flue gas to 40-60℃, recovering the latent heat of water vapor in the flue gas.
9. The method for normalizing and annealing alloy steel fittings based on porous media combustion according to claim 1, characterized in that: The slow cooling process control in the annealing, heat preservation and slow cooling steps is as follows: a composite control strategy of decreasing combustion power and increasing flue gas circulation volume is adopted; in the initial stage of slow cooling, the combustion power is maintained at 30-50% of the rated power and the flue gas circulation volume is 20-30%; as the furnace temperature decreases, the combustion power is gradually reduced while the flue gas circulation volume is increased to 50-70%.