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Forgings are objects in which metal is subjected to pressure and shaped into the desired form through plastic deformation, using appropriate compressive forces. This force is typically applied using a hammer or pressure. The forging process creates a fine grain structure and improves the physical properties of the metal. In the actual use of components, a proper design can ensure that the particle flow moves in the direction of the main pressure. Each forging piece must be uniform, with no porosity, excess space, inclusions, or any other defects. The significance of waste heat utilization: The forging industry is a major consumer of energy, and the heat treatment of forgings represents another significant source of energy consumption in forging production, accounting for approximately 30% to 35% of the total energy used in forging production. The energy consumption per ton of die-forged parts in our country is approximately 1.0 tons of standard coal, which represents a significant gap compared to industrialized countries abroad; for example, Japan’s energy consumption per ton of die-forged parts is around 0.515 tons of standard coal. The energy consumption associated with forging accounts for approximately 8% to 10% of the cost of forged products. Reducing energy consumption not only helps to lower the production costs of these products and improve a company’s economic performance, but it is also an important factor in ensuring sustainable development; in fact, it is a global issue that affects human survival. Therefore, making full use of the residual heat from forging for heat treatment offers obvious advantages in terms of energy savings, reduced consumption, and improved efficiency; it not only saves energy and shortens the processing workflow but also helps protect the environment. Heat treatment using the residual heat from hot die forging involves carrying out heat treatment directly using the heat remaining in the forged part after forging. In other words, this method eliminates the need to reheat the forged part before heat treatment. There are generally three ways to carry out heat treatment using residual heat. Post-forging, residual heat homogenization treatment is carried out. After forging, the forgings are directly sent to the heat treatment furnace, where they undergo conventional heat treatment processes. Once the temperature is uniform throughout the forgings, the holding time can be reduced; this method is known as residual heat homogenization heat treatment. For forgings with complex shapes, especially those with large variations in cross-section, using this process ensures stable forging quality. Direct post-forging residual heat treatment. After forging, heat treatment is carried out directly using the residual heat from forging, thereby integrating forging and heat treatment together and saving a significant amount of energy that would otherwise be wasted due to the need for reheating in conventional heat treatment processes. Heat treatment is carried out after forging using some of the residual heat. After the forging is shaped, it is cooled to around 600–650°C, and then reheated to the desired temperature for heat treatment. This method can refine the grain structure, while also saving energy required to heat the forgings from room temperature to 600–650°C; it is generally suitable for forgings that require a fine grain size. Common processes for waste heat treatment: forging waste heat quenching. Forging waste heat quenching is a process in which, after the forging has been shaped and its temperature is above Ar3 or some temperature within the range of Ar3 to Ar1, it is plunged into an appropriate quenching medium to obtain a martensitic or bainitic structure. After forging, residual heat quenching and tempering of the forgings not only enable the attainment of good overall mechanical properties, but also help save energy, simplify the manufacturing process, shorten the production cycle, reduce labor costs, and lower the investment required for quenching furnaces. After being quenched using the residual heat from forging and followed by high-temperature tempering, forgings generally exhibit higher strength and hardness compared to those that are subjected to conventional quenching and tempering; however, their ductility and toughness are slightly lower (when the tempering temperatures are the same for both). If, after forging and residual heat quenching, a higher tempering temperature is used (generally 40–80°C higher than that for conventional quenching and tempering), its plasticity and toughness will be comparable to or slightly higher than those of parts subjected to conventional quenching. After forging and residual heat quenching, the forgings exhibit significantly improved strength and hardness while maintaining their plasticity and toughness. Additionally, due to their coarser grains compared to those resulting from conventional quenching, their machinability is enhanced. Forging residual heat normalizing (annealing): Forging residual heat normalizing (annealing) involves placing the forged part in a normalizing furnace, cooling box, or annealing furnace after it has been shaped, when its temperature is above Ar3 (for hypoeutectoid steels), so as to carry out normalizing or controlled cooling and obtain a normalized microstructure. Due to the high heating temperature during forging, the grains of the forgings become coarse after treatment with this method; it is generally used for pre-heat treatment and is not suitable for forgings that require a fine grain structure. At the same time, the tissue obtained after treatment is a pearlite + ferrite equilibrium structure; the coarse grains do not exhibit structural inheritance in subsequent heat treatments, allowing the grains to be refined. Isothermal normalizing using forging residual heat: Isothermal normalizing using forging residual heat involves rapidly cooling the forged part after it has been shaped, when the temperature is above Ar3 (for hypoeutectoid steel); after cooling to the isothermal temperature, the part is held at that temperature for a certain period of time before being cooled to room temperature. After forging, the temperature is generally between 900 and 1000°C. The cooling rate is usually controlled at 30 to 42°C/minute, while the isothermal temperature ranges from 550 to 680°C (the exact value depends on the material used). Quenching is a key step in this process; by adjusting the volume, speed, temperature, and direction of the cooling air, it is possible to ensure uniform temperature of the forged part after cooling. The isothermal temperature is determined based on the type of material and the desired hardness; it is generally chosen at the peak of the pearlite transformation curve in order to reduce the isothermal holding time. Isothermal normalizing using forging residual heat is commonly used for carburized gear steels, such as SCM420H, SCM822H, SAE8620H, and 20CrMnTiH, etc. Key control points of the waste heat treatment process – Waste heat quenching: (1) A stable and controllable heating system. The heating system for the blanks consists of medium-frequency induction heating, an infrared temperature sensor, and a three-channel temperature sorting system, which enables easy control of the heating temperature and the sorting out of blanks with unsuitable heating temperatures. ⑵ Determine the appropriate quenching temperature and ensure its effective control. The appropriate quenching temperature after forging must be determined through testing; in practice, it can be achieved by controlling the forging heating temperature and the holding time after forging. It is recommended that the holding time for carbon steel not exceed 60 seconds, while for alloy steel it should be between 20 and 60 seconds. An infrared thermometer and a temperature sorting system are installed to sort out forgings that are below the quenching temperature; when the forging heating temperature is stable and the forging process is also stable, a process time measurement and alarm system can be installed, so as to control the quenching temperature by regulating the process time. ⑶ A good quenching system. While ensuring the desired quenching effect, a quenching agent with a slower cooling capacity should be chosen to prevent severe quenching deformation and cracking. Since the quenching temperature for forging with residual heat is higher than that for ordinary quenching, forgings achieve good hardening properties; therefore, oil or PAG quenching agents are generally used for carbon steel and alloy steel. The quenching tank should have sufficient capacity, with a controllable cooling time. It must also be equipped with a system for circulating the quenching medium, a cooling system, and a heating device; the temperature of the quenching medium should be automatically controlled, and an exhaust system should also be provided. Strengthen the maintenance of the quenching medium, regularly test its cooling performance, remove impurities such as scale from the liquid tanks and circulation systems, and keep the quenching medium clean. ⑷ Tempering after quenching and the placement of the tempering furnace. After quenching, the forgings have high internal stresses, which cause significant deformation or even cracking during storage. To prevent deformation and cracking of the parts after quenching, the forgings should be tempered promptly after quenching. The time during which a forged part can be left after quenching depends on the material of the part, its shape, and the ambient temperature; it must be determined through testing. To save energy, improve the utilization rate of tempering furnaces, and reduce insulation energy consumption, forgings treated by waste heat quenching are generally tempered in a centralized heat treatment workshop. Residual heat normalizing (annealing) ⑴ Properly control the temperature of the forging before it is placed in the furnace. When the temperature of the parts is high, it is necessary to use air blowing to cool the forgings, thereby reducing their temperature to the desired normalizing temperature. At the same time, the power of the heat treatment furnace must have a certain margin, so that heating can take place before production starts or when the temperature of a small number of forgings is low. ⑵Determine an appropriate insulation time. Excessive holding time can lead to coarse grains, while too short a holding time results in insufficient structural transformation. It can be determined through testing based on the forging material, shape, and size. Isothermal normalizing of residual heat ⑴ Temperature control of the forging after forging. The temperature of the forged part after shaping must be above Ar3 (for hypoeutectoid steel). When the temperature of the part is stable after forging, direct rapid cooling can be employed; however, when there are large fluctuations in the part’s temperature or significant variations in its cross-section, it is necessary to include a homogenization step to ensure that the temperature of the part is uniform before rapid cooling. Otherwise, large temperature differences between different sections of the forging after rapid cooling may occur, leading to abnormal microstructures such as bainite or martensite. ⑵ Quenching cooling rate control. The rapid cooling process requires that the forgings be cooled quickly, while after cooling, the temperature of each forging and those within the same batch should be uniform (or similar). At the same time, it is necessary to control the quenching rate; an excessively fast quenching rate can result in Widmanstatten structure in the forge product’s microstructure. Generally, the quenching rate is controlled at 30–42°C/min. ⑶ Temperature control after rapid cooling. After rapid cooling, it is essential to keep the temperature of the forging within the pearlite transformation range, and it must not be lower than the starting temperature for bainite transformation (Bs); otherwise, bainite (or granular bainite) structures will form in the material. If the temperature is too high after rapid cooling, the amount of pro-eutectoid ferrite increases, and the spacing between pearlite layers becomes larger as a result of the structural transformation, leading to low hardness in the part. After rapid cooling, the temperature of the forgings is generally maintained at 80–100°C, above the material’s Bs temperature. ⑷ Selection of the isothermal temperature. The level of the isothermal temperature has a direct impact on the hardness of the forgings after isothermal normalizing; a higher isothermal temperature results in lower hardness, while a lower isothermal temperature leads to higher hardness. The isothermal temperature is generally 50–80°C above the Bs temperature of the forging material; the specific temperature must be determined through testing based on the material and shape of the forging. ⑸ Determination of the isothermal holding time. The pearlite transformation occurs during the isothermal process; therefore, sufficient holding time is necessary. If the isothermal time is too short, not all of the supercooled austenite will transform into pearlite, and it will instead transform into bainite or martensite during the subsequent cooling process, resulting in an unsatisfactory microstructure and high hardness after the isothermal treatment. The isothermal time can be initially determined based on the material’s isothermal transformation curve, and adjusted according to the experimental conditions. Application example: Isothermal normalizing of the residual heat in the gears of automobile transmissions. The materials used for these gears are 20MnCr5JV and 27MnCr5JV, and the forgings require isothermal normalizing treatment. After isothermal normalizing, the microstructure consists of ferrite + pearlite; granular bainite is not allowed, and the grain size is grade 6–9. To reduce energy consumption while meeting the requirements for grain size, isothermal normalizing is carried out using some of the residual heat from forging. Through testing, it was determined that a partial waste heat isothermal normalizing process involves transporting the forged parts to a heating furnace via a conveyor belt after they are formed. During this process, the temperature of the forged parts drops to 550–600°C; they are then reheated in the heating furnace to 900–920°C. After holding at this temperature, they are sent to a rapid cooling chamber for fast cooling. The temperature of the forged parts remains above 600°C after rapid cooling, after which they are transferred to an isothermal furnace for isothermal treatment. The isothermal temperature for the forged parts is 580–600°C, with an isothermal holding time of 1 hour, followed by cooling in the air outside the furnace. After heat treatment using this process, the microstructure of the forgings is ferrite + pearlite, with no granular bainite structure present. It has appropriate hardness and good cutting performance, and the deformation of the gears before and after subsequent heat treatment meets the technical requirements. Compared with conventional isothermal normalizing, partial forging residual heat isothermal normalizing eliminates part of the high-temperature heating process, saving approximately 150 kWh/t in electricity consumption. The crankshaft of a microcar is quenched using waste heat. The material of the crankshaft forging for this microcar is 40CrH (GB/T5216-2004). According to the heat treatment requirements for such forgings, after quenching and tempering, the microstructural grade should be between 1 and 4, with a hardness of 241–285 HBW. The conventional quenching and tempering process involves cooling the forged part to room temperature after shaping, then heating it to 850°C and holding it at that temperature for a certain period of time before quenching it in a 10% PAG quenching medium. Subsequently, tempering is carried out, with the quenching and tempering treatment taking place on a continuous quenching and tempering line. The forging residual heat quenching process involves quenching the forged parts in quenching oil after they have been shaped, followed by centralized tempering of the quenched parts in a continuous tempering furnace. Tests have shown that products manufactured using the forging residual heat quenching process meet the customer’s requirements in terms of various performance indicators. Produced using the waste heat quenching process, which eliminates the quenching heating step required in conventional tempering; this saves 259 kWh/t of electricity used for quenching heating, while also simplifying the process and reducing the production cycle. The crankshaft of a microcar is annealed using waste heat. The material of the crankshaft forging for this microcar is 40CrH; the required heat treatment is normalizing, with a hardness range of 163–269 HBW. The final heat treatment for crankshaft forgings is quenching and tempering. The purpose of normalizing is to reduce the hardness of the forgings, thereby facilitating subsequent rough machining, as well as to homogenize the microstructure and prepare it for the subsequent quenching and tempering process. The original process involved heating to 860°C for normalizing; forging residual heat annealing entailed placing the crankshaft in an insulated box after forging, leaving it there for a certain period of time before removing it. Tests have shown that annealing using the residual heat from forging can result in a pearlite + ferrite microstructure, with no abnormal phases such as bainite present; simultaneously, there is no severe Widmanstätten structure in the microstructure. The hardness is similar to that after normalizing, and it has no impact on the rough machining of the crankshaft. Due to the varying degrees of deformation in different areas, the grains in those areas with less deformation are coarser. The grain size of the crankshaft after heat treatment is coarser than that after normalizing, which is advantageous for subsequent machining. At the same time, since the forging residual heat annealing process yields a balanced pearlite + ferrite structure that lacks heritability, the grain size can be refined again after quenching and tempering. The crankshafts that underwent annealing using the forging residual heat were further subjected to rough machining and quenching and tempering by the customer. Their cutting performance, microstructure after quenching and tempering, and mechanical properties were identical to those of those processed using the normalizing method, and no adverse effects were observed after installation. The residual heat annealing process for crankshaft forging makes full use of the heat remaining from the forging process, eliminating the need for secondary heating of the forgings. Compared to the conventional normalizing process, this method saves a significant amount of electrical energy, reduces the time required to operate the heat treatment furnaces, and lowers labor costs as well as equipment maintenance expenses. Conclusion Production practice has shown that heat treatment using the residual heat from forging is feasible; by properly controlling the cooling parameters after forging, the microstructure and properties of the forgings can reach or even exceed those achieved through conventional heat treatment. At the same time, taking advantage of the coarse grain structure resulting from forging residual heat during heat treatment, the machinability of forgings can be improved. Using the residual heat from forging for heat treatment saves a large amount of energy consumed in the heating process for heat treatment, reduces production costs, and offers significant economic benefits, thus holding broad application prospects.