Measurement of temperature
To determine the initial forging temperature of steel, we must first ensure that the steel is not over-burned. Therefore, for carbon steel, the initial forging temperature should be 150-250 °C lower than the initial melting point of the iron-carbon equilibrium diagram, as shown in Figure 2. In addition, factors such as the structure of the blank, the forging method, and the deformation process should also be considered.
图2 碳钢锻造温度范围
Figure 2 Forging temperature range of carbon steel
Final forging temperature
When determining the final forging temperature, we must ensure that the steel has sufficient plasticity before the final forging and that the forging can obtain good structural properties. Therefore, in order to ensure complete recrystallization after forging and obtain fine grain structure during forging, the final forging temperature of the steel should be higher than the recrystallization temperature.
For carbon steel, the final forging temperature cannot be lower than the A1 line of the iron-carbon equilibrium diagram. Otherwise, the plasticity will be significantly reduced, the deformation resistance will increase, the work hardening phenomenon will be serious, and it will be easy to cause forging cracks.
For hypoeutectoid steels, the final forging temperature should be 15-50 °C higher than the A3 line, since they are located in the single-phase austenite region. The structure is uniform and has good plasticity. However, for low-carbon steels (carbon content less than 0.3%), the final forging temperature can be lowered below the A3 line. Although they are in the (γ+α) two-phase region, they have sufficient plasticity, their deformation resistance is not too high, and the forging temperature range is also wider.
For hypereutectoid steels, the final forging temperature should be lower than the Acm line and 50-100 °C higher than the A1 line. This is because if the final forging temperature is selected higher than the Acm line, secondary network cementite will precipitate along the grain boundaries during the cooling process after forging, which will significantly reduce the mechanical properties of the forged product. If forging is performed between the Acm line and the A1 line, the precipitated secondary cementite can be dispersed due to the mechanical crushing effect caused by plastic deformation.
It should also be pointed out that the final forging temperature of steel is also related to the structure of the steel, the forging process, and the subsequent processes. For steels without phase change, the grain size can only be controlled by forging, because the grains cannot be refined by heat treatment. In order to obtain fine grains in the forging, the final forging temperature of this type of steel is generally low. If the forging is subjected to residual heat treatment immediately after forging, the final forging temperature should meet the requirements of the residual heat treatment. If the forging is made of low carbon steel, the final forging temperature should be slightly higher than A. Wire. [2]
Powder hot forging
Ordinary powder metallurgy products have a certain amount of porosity, low strength, and limited scope of application. Practice has proven that the density of powder materials or products can reach or approach the theoretical value through the hot forging process. Figure 3 shows the process flow of powder hot forging.
Figure 3 Powder high temperature forging process flow
As shown in Figure 3, there are two types of hot forging processes. One is the process of hot forging the powder preform without pre-sintering, which is called powder forging. The other is the process of hot forging the powder preform after pre-sintering, which is called powder sintering forging. Most of them adopt the latter, sintering in a protective atmosphere to make it have a certain strength, and then heating the preform to the forging temperature. After keeping warm, it can be quickly put into the forging die and forged in one go to meet the design requirements.
Compared with general forging, powder hot forging absorbs the characteristics of the ordinary die forging process, and improves the density of the product by heating the powder preform by forging, so that the performance of the product approaches or exceeds the level of similar molten cast products. Meanwhile, powder hot forging maintains the characteristics of the powder metallurgy process.
Powder preform contains about 80% porosity, so the forging flow stress is much lower than that of ordinary molten cast materials. Therefore, it can be formed with lower forging energy, and at the same time, by rationally designing the shape and size of the preform, its weight can be accurately controlled, and die forging without or with less burrs can be achieved, improving the material utilization rate. Generally, the utilization rate of powder hot forging materials is more than 80%, while the utilization rate in ordinary forging is only about 50%. Compared with general forged products, powder forged products have the characteristics of high dimensional accuracy, uniform organizational structure, and no segregation of parts. Another important feature is that powder hot forging technology can forge metals and alloys that are generally difficult to forge, such as high-temperature casting alloys that are difficult to deform, into various products with complex shapes.
Powder hot forging technology was developed based on general powder metallurgy and precision die forging processes. The powder hot forging process has attracted widespread attention because it can improve the quality of metal products while reducing or eliminating cutting, simplify the machining process, and save valuable materials and processing time. Powder hot forging products are widely used in many fields of industry and agriculture. However, the technology is still in the early stages of development and needs to be gradually improved and perfected in scientific research and production.
