Forgings are the collective name of forging and stamping. It is a forming processing method that uses the hammer, anvil, punch or die of a forging machine to apply pressure to the blank to cause plastic deformation, thereby obtaining a part of the desired shape and size.
Forgings are formed by forging after metal is heated in the solid state. It requires the metal to have good thermoplasticity (also called forging property). Generally, forgings are steel parts. They have high strength and good plasticity, and are suitable for manufacturing important parts with high stress and high requirements. Such as bolts, shafts, gears, etc.
Casting metal is formed by pouring liquid into the mold. It requires liquid metal to have good fluidity and filling properties (also called casting properties). Steel castings are divided into cast steel and cast iron. Cast iron parts have a higher carbon content than steel, lower strength than steel, and poor plasticity. They are generally suitable for making parts with low bearing capacity, such as machine tool bodies, low-pressure valve seats, etc.; cast steel parts have higher strength and are used to make parts with complex shapes, such as chain track plates, high-pressure valve seats, etc.
The following are the specific steps of the forging process:
1. Material calculation and material feeding are one of the important links to improve material utilization and achieve blank refinement. Excessive material not only causes waste, but also aggravates die wear and energy consumption. If the material is not left with a little margin, it will increase the difficulty of process adjustment and increase the scrap rate. In addition, the quality of the end face of the material also affects the process and the quality of the forging.
2. The purpose of heating is to reduce the forging deformation force and improve the plasticity of the metal. However, heating also brings a series of problems, such as oxidation, decarburization, overheating and overburning. Accurately controlling the initial and final forging temperatures has a great impact on the product organization and performance.
3. Flame furnace heating has the advantages of low cost and strong applicability, but the heating time is long, oxidation and decarburization are easy to occur, and the working conditions also need to be continuously improved. Electric induction heating has the advantages of rapid heating and less oxidation, but it has poor adaptability to changes in product shape, size and material.
4. Forging is produced under the action of external force. Therefore, the correct calculation of deformation force is the basis for selecting equipment and conducting die calibration. Stress and strain analysis inside the deformed body is also indispensable for optimizing the process and controlling the microstructure and properties of forgings.
5. Therefore, the quality analysis of forgings can generally be divided into the on-site investigation stage, the test research and analysis stage, and the stage of proposing solutions and preventive measures. Before implementing these stages of work, it is best to formulate an implementation plan, which includes the work, work procedures, and completion time to be carried out in these three stages. The implementation plan can be appropriately supplemented and modified during the implementation process. Formulating an implementation plan is an important link in analyzing the quality problems of large and complex forgings and used parts.
During the on-site investigation stage, the main thing to investigate is the material brand, chemical composition, material specifications, test results on the material guarantee sheet, various physical and chemical tests and process performance test results of the re-inspection at the factory, and even the smelting and processing technology of the raw materials. At the same time, the forging process should also be investigated, including the forging materials, specifications, blanking process, initial and final forging temperatures of forging heating, forging equipment, heating equipment, heating process, forging operation, cooling method after forging, and preparatory heat treatment process. If necessary, the operator's situation and environmental conditions and the original records of the execution process should also be investigated. For the quality problems of forgings that occur in subsequent processes and use, the process of subsequent processes should also be investigated.
Depending on the type of steel and process requirements, forgings often use the following heat treatment methods: annealing, normalizing, quenching and tempering, quenching and aging, etc. Next, let's take a look at how they are done:
1. Annealing:
The annealing process for forgings includes full annealing, spheroidizing annealing, low-temperature annealing and isothermal annealing, etc., which need to be selected according to the material and deformation of the forgings.
After annealing, the forgings refine the grains due to recrystallization, eliminate or reduce residual stress, thereby reducing the hardness of the forgings, improving their plasticity and toughness, and improving cutting performance.
2. Normalizing:
Normalizing generally heats the forgings to 50-70℃ above the GSE line, and some high-alloy steel forgings are heated to 100-150℃ above the GSE line, and then cooled in the air after proper insulation. If the hardness of the forging is high after normalizing, in order to reduce the hardness of the forging, high-temperature tempering should also be performed, and the general tempering temperature is 560-660℃.
3. Quenching and tempering:
Quenching is to obtain an unbalanced structure to improve strength and hardness. The steel forging is heated to 30-50℃ above the Ac1 line. After heat preservation, it is rapidly cooled.
Tempering is to eliminate quenching stress and obtain a more stable structure. The forging is heated to a certain temperature below the Ac1 line, kept warm for a period of time, and then air-cooled or rapidly cooled.
4. Quenching and aging:
High-temperature alloys and alloys that can be strengthened by heat treatment are often treated with quenching and aging after forging. Among them, quenching is to heat the alloy to an appropriate temperature, keep it warm enough, so that certain structural products in the alloy are dissolved into the matrix to form a uniform solid solution, and then quickly cool it to become a supersaturated solid solution, so it is also called solid solution treatment. Its purpose is to improve the plasticity and toughness of the alloy and prepare the structure for further aging treatment. Aging treatment is to place the supersaturated solid solution or the alloy after cold working deformation at room temperature or heat it to a certain temperature, keep it warm for a period of time, so that the substances previously dissolved in the matrix are uniformly dispersed and precipitated. The purpose of aging treatment is to improve the strength and hardness of the alloy.
Forging heat treatment is carried out according to certain heat treatment specifications, based on the forging steel type, cross-sectional dimensions and technical requirements, and with reference to relevant manuals and materials. Its contents include: heating temperature, holding time and cooling method, etc. It is generally represented by a temperature-time change curve.
