A large number of application examples show that by reasonably controlling the cooling parameters after forging forming, the microstructure and properties of the forgings after the heat treatment of the residual heat reach the level of ordinary heat treatment, which has good process stability and reproducibility. The use of waste heat for heat treatment eliminates the heating process in the heat treatment process, and the energy saving effect is remarkable, and the investment and maintenance cost for the heat treatment equipment is reduced.
The meaning of waste heat utilization
The forging industry is a major energy consumer, and forging heat treatment is a major energy consumer in forging production, accounting for 30% to 35% of the total energy consumption of the entire forging production. The energy consumption per ton of die forgings in China is about 1.0t standard coal. Compared with foreign industrial developed countries, there is a big gap. For example, the energy consumption per ton of die forgings in Japan is about 0.515t standard coal. The energy consumption of forgings accounts for 8% to 10% of the cost of forgings. Reducing energy consumption can not only reduce the production cost of forgings, but also improve the economic benefits of enterprises. Moreover, the energy issue is an important issue related to the sustainable development of a country, or even a relationship. A major global problem in the survival of mankind. Therefore, making full use of forging waste heat for heat treatment has obvious advantages in energy saving, efficiency improvement, etc., saving energy, shortening process and protecting the environment.
Hot die forging heat treatment
After forging, the forging heat is directly used for heat treatment, that is, the residual heat treatment of the forging material omits the process of reheating the forging before the heat treatment after forging, and the residual heat treatment generally has the following three methods.
1. Forging heat treatment is carried out after forging. After the forging is formed directly into the heat treatment furnace, it is still carried out according to the conventional heat treatment process. After the temperature is uniform, the temperature of the different parts of the forging is the same, which can shorten the holding time. This method is called residual heat treatment. For the forgings with complex shapes, especially for large cross-section changes, this process ensures stable forging quality.
2. Direct heat treatment after forging. After the forging is formed, the forging heat is directly used for heat treatment, and the forging and heat treatment are closely combined, which saves a lot of energy waste caused by reheating of the ordinary heat treatment.
3. After forging, heat treatment is performed using part of the residual heat. After the forging is formed, the forging is cooled to about 600 to 650 ° C, and then the forging is heated to a desired temperature for heat treatment. The method can refine the grain and save the energy consumption of the forging from room temperature to 600-650 ° C, and is generally suitable for forgings with high grain size requirements.
Common heat treatment process
Forging residual heat quenching
Forging residual heat quenching is a process for obtaining a martensite or bainite structure when the temperature of the forging is higher than the temperature between Ar3 or Ar3 to Ar1 after forming the forging, and quenching into a suitable quenching medium.
After forging quenching and tempering, forgings not only can obtain better comprehensive mechanical properties, but also save energy, simplify the process, shorten the production cycle, reduce personnel and save investment costs of quenching furnace.
After forgings are quenched by forging heat and tempered at high temperature, their strength and hardness are generally higher than ordinary quenching and tempering, while plasticity and toughness are slightly lower than ordinary tempering (when both tempering temperatures are the same). If the forging heat is quenched, the plasticity and toughness are equivalent to or slightly higher than the ordinary quenching after adopting a higher tempering temperature (generally 40 to 80 ° C higher than the tempering temperature of the ordinary quenching and tempering). After the forging is quenched by forging heat, the strength and hardness are obviously improved under the premise of maintaining plasticity and toughness. In addition, since the grain is coarser than ordinary quenching, the cutting performance of the material can be improved.
Forging waste heat normalizing (annealing)
Forging waste heat normalizing (annealing) is a normalizing structure after the forging is formed, when the temperature is higher than Ar3 (for the hypoeutectic steel), it enters the normal furnace, the cooling box or the annealing furnace for normalizing or controlled cooling.
Due to the high heating temperature of the forging, the grain of the forging is coarse after being treated by this method, and is generally used for preliminary heat treatment, and forgings having high requirements on grain size are not applicable. At the same time, the microstructure obtained after the treatment is pearlite + ferrite equilibrium structure, the coarse grains have no tissue inheritance in the subsequent heat treatment, and the crystal grains can be re-refined.
Forging waste heat isothermal normalizing
Forging waste heat isothermal normalizing is the rapid cooling of the forging after forming, when the temperature is higher than Ar3 (for the hypoeutectic steel), cooling to the isothermal temperature, and then cooling to room temperature after a certain period of time.
The temperature after forging is generally 900-1000 ° C, the quenching speed is generally controlled at 30 ~ 42 ° C / min, the isothermal temperature is generally 550 ~ 680 ° C (specifically determined according to different materials). Quenching is the key process of the process. The cooling air volume, wind speed, wind temperature and wind direction can be adjusted to ensure uniform temperature after forging. The isothermal temperature is determined by the type of material and the required hardness, and is generally selected in the nose of the pearlite transformation curve to shorten the isothermal holding time. Forging waste heat isothermal normalizing is mostly used for carburized gear steels, such as SCM420H, SCM822H, SAE8620H and 20CrMnTiH.
Control points of waste heat treatment process
(1) Stable and controllable heating system. The heating system of the blank is a medium frequency induction heating, an infrared thermometer and a three-channel temperature sorting system, which can conveniently control the heating temperature and sort the blanks with unqualified heating temperature.
(2) Determine the appropriate quenching temperature and control it effectively. The suitable forging residual heat quenching temperature should be determined according to the test. In actual operation, it can be achieved by controlling the forging heating temperature and the forging time after forging. The forging time after forging is recommended to be no more than 60s for carbon steel and 20 to 60s for alloy steel.
The infrared thermometer and the temperature sorting system are arranged to sort out the forgings below the quenching temperature; when the forging heating temperature is stable and the forging process is stable, the process time measurement and alarm system can be configured to control the quenching temperature by controlling the process time. the goal of.
(3) Good quenching system. Under the premise of ensuring quenching effect, the quenching agent with slow cooling capacity is selected to prevent severe quenching deformation and cracking. Since the forging residual heat quenching temperature is higher than the ordinary quenching temperature, the forging has good hardenability, so carbon steel and alloy steel generally use oil or PAG quenching agent.
The quenching tank should have sufficient volume, the cooling time can be controlled, and the quenching medium circulation, cooling system and heating device should be arranged, the temperature of the quenching medium should be automatically controlled, and the drafting device should also be arranged. Strengthen the maintenance of the quenching medium, regularly check the cooling performance of the quenching medium, clean the impurities such as scale in the liquid tank and the circulation system, and keep the quenching medium clean.
(4) The location of the tempering and tempering furnace after quenching. After the forging is quenched, there is a large internal stress, which causes large deformation or even cracking during the placement process. In order to prevent deformation and cracking of the parts after quenching, the forgings after quenching should be tempered in time. The shelf life of the forging after quenching depends on the material, shape and ambient temperature of the forging and is determined by testing. In order to save energy and improve the utilization rate of the tempering furnace and reduce the energy consumption of the insulation, the forgings using the residual heat quenching are generally tempered in the heat treatment workshop.
Waste heat normalization (annealing)
(1) Properly control the temperature of the forging before entering the furnace. When the temperature of the part is high, the forging is required to be blown to cool, so that the temperature of the part is reduced to the required normalizing temperature, and the power of the heat treatment furnace needs to have a certain margin, and heating is performed before the start of production and when the temperature of a small amount of forging is low.
(2) Determine the reasonable holding time. Excessive holding time will result in coarse grains, and too short holding time will result in insufficient tissue transformation. It can be determined experimentally according to the material, shape and size of the forging.
(1) Temperature control of forgings after forging. The temperature after forming the forging must be above Ar3 (for the sub-eutectoid steel). When the temperature of the forged part is stable, direct quenching can be used. When the temperature of the forged part fluctuates greatly or the section of the forging changes greatly, the temperature equalization process must be increased. Make the temperature of the parts uniform before quenching, otherwise the forgings or different cross-section temperatures will be greatly different after quenching, resulting in abnormal microstructure (bainite or martensite).
(2) Quench cooling speed control. In the quenching process, the forgings are required to be rapidly cooled, and at the same time, the temperature of the same forgings and the same batch of forgings are uniform (or similar) after cooling. At the same time, the quenching speed needs to be controlled. The too fast quenching speed will produce the Wei's structure in the forging structure. Generally, the quenching speed is controlled at 30 to 42 ° C / min.
(3) Temperature control after quenching. After quenching, it must be ensured that the forging temperature is in the pearlite transformation zone and cannot be lower than the bainite transformation starting temperature (Bs). Otherwise, bainite (or granular bainite) structure will appear in the structure; if the temperature is too high after quenching, the temperature will be too high. The amount of ferrite is increased first, and the pitch of the pearlite sheets is large after the transformation, resulting in low hardness of the parts. After the forging is quenched, the temperature is generally controlled at 80 to 100 ° C above the material Bs temperature.
(4) Selection of isothermal temperature. The isothermal temperature directly affects the hardness of the forged part after isothermal normalizing, the hardness is low when the isothermal temperature is high, and the hardness is high when the isothermal temperature is low. The isothermal temperature is generally 50 to 80 ° C above the Bs temperature of the forging material, and the specific temperature is determined according to the material and shape of the forging.
(5) Determination of isothermal holding time. The pearlite transformation occurs during the isothermal process, so there must be sufficient holding time. For example, if the isothermal time is too short, the supercooled austenite will not be completely converted into pearlite, and it will be transformed into bainite or Markov in the subsequent cooling process. Body, resulting in unqualified tissue and high hardness after isothermal treatment. The isothermal time can be initially determined based on the isothermal transformation curve of the material and adjusted according to the test conditions.
Car gearbox gear part residual heat isothermal normalizing
The gears of the car gearbox are 20MnCr5JV and 27MnCr5JV, and the forgings are required to be isothermal normalized. After isothermal normalizing, the structure is ferrite + pearlite, and there must be no granular bainite, and the grain size is 6-9. In order to reduce energy consumption and meet the requirements for grain size, part of forging waste heat is used for isothermal normalizing treatment.
After testing, some partial heat isothermal normalizing process is determined. After the forging is formed, it is sent to the heating furnace through the conveyor belt. During this process, the forging temperature is lowered to 550-600 ° C, and the forging is reheated to 900-920 ° C in the heating furnace. Enter the quick-cooling chamber for rapid cooling; after the rapid cooling, the forging temperature is not lower than 600 °C, and then enter the isothermal furnace for isothermal, the isothermal temperature of the forging is 580-600 ° C, the isothermal time is 1 h, and then the air is cooled.
The metallurgical structure of the forged piece after heat treatment by this process is ferrite + pearlite, and no granular bainite structure appears. The hardness is appropriate, the cutting performance is good, and the deformation of the gear before and after the subsequent heat treatment meets the technical requirements. Partially forged waste heat isothermal normalizing saves some high-temperature heating process compared with conventional isothermal normalizing, saving about 150kWh/t.
Miniature vehicle crankshaft is quenched by residual heat
The material of a mini-car crankshaft forging is 40CrH (GB/T5216-2004). The forging is required for heat treatment. After the forging is quenched and tempered, the metallographic structure is between 1 and 4, and the hardness is 241-285 HBW. The ordinary quenching and tempering process is that the forging is air-cooled to room temperature after forming, and then heated to 850 ° C. After being kept for a certain period of time, it is quenched in a PAG quenching agent having a concentration of 10%, and then tempered, and quenched and tempered in a continuous quenching and tempering line.
The forging waste heat quenching process is quenching in the quenching oil after the forging is formed, and the quenched forgings are concentrated and tempered in the continuous tempering furnace. After inspection, it is produced by forging waste heat quenching process, and various performance indicators meet customer requirements. It is produced by waste heat quenching process, eliminating the quenching heating process of ordinary quenching and tempering, saving 259kWh/t of quenching heating electricity, simplifying the process and shortening the production cycle.
Miniature vehicle crankshaft using residual heat annealing
The forging material of a miniature car crankshaft is 40CrH, the heat treatment requirement is normalized, and the hardness requirement is 163~269HBW. The final heat treatment of the crankshaft forgings is quenched and tempered. The purpose of the normalizing is to reduce the hardness of the forgings, so as to facilitate the subsequent roughing and uniform organization, and prepare the structure for the subsequent quenching and tempering treatment.
The original process is to normalize to 860 ° C for heating; forging waste heat annealing is to put the crankshaft into the incubator after forging, and take it out after a period of heat preservation. The pearlite + ferrite structure can be obtained by forging residual heat annealing, and there is no abnormal structure such as bainite in the structure; at the same time, there is no serious Wei's structure in the tissue. The hardness is similar to the normalizing fire and has no effect on the rough machining of the crankshaft.
Affected by the inconsistent degree of deformation in different parts, the grain size of the part with smaller deformation is thicker, and the grain size of the crankshaft treated by the residual heat annealing is thicker than that of the normalizing treatment, which will be beneficial to the subsequent cutting process. At the same time, due to the forging waste heat annealing treatment, the equilibrium structure of pearlite + ferrite is obtained, which is not heritable, and the grains can be re-refined after quenching and tempering treatment.
The crankshaft subjected to forging residual heat annealing treatment is rough-processed and quenched and tempered by the customer. The metallurgical structure and mechanical properties after cutting performance and quenching and tempering treatment are the same as those in the original normalizing process, and there is no adverse reaction after installation. The crankshaft forging waste heat annealing process all utilizes forging waste heat, and does not need secondary heating of the forgings. Compared with the original normalizing process, it can save a lot of electric energy, reduce the starting time of the heat treatment furnace, and reduce the labor cost and equipment maintenance cost.
The production practice proves that it is feasible to use the forging waste heat for heat treatment. By reasonably controlling the post-forging cooling parameters, the forging structure and performance meet or exceed the ordinary heat treatment level. At the same time, the use of forging heat treatment during grain heat treatment can improve the cutting performance of forgings. The use of forging waste heat for heat treatment saves a lot of energy consumed by the heat treatment heating process, reduces production costs, has significant economic benefits, and has broad application prospects.