When forging and compressing inner hole defects, people still prefer to consider the problem from the perspective of stress, often taking the three-dimensional compressive stress as the most powerful condition for forging inner holes. In fact, this is not a sufficient condition. In order to avoid tensile stress at the blank center, when using upper and lower flat cutting, it is necessary to have a cutting width ratio of ≥ 0.8; When using FM Farad length, the cutting width ratio must be ≥ 0.77. This harsh condition is difficult to achieve during the stretching process of large forgings. In addition, if the cutting is too wide, the capacity of the compressor is insufficient. In the rigid area under cutting, when the cutting width ratio is very large, RST effect internal tearing can also occur. At this point, it may be in a three-dimensional compressive stress state, but the hole here will not close. On the contrary, the state of constant tensile stress and two-way compressive stress during tensile molding is easy to close the internal hole. In addition, the stress state only represents an instantaneous state that constantly changes during deformation and cannot accumulate or overlap.
During the forging process, it is difficult to control and calculate the three-dimensional compressive stress state. If the stress state at the time of hole closure is a critical condition, it is definitely impossible, as this transient stress state is ubiquitous. Therefore, it is meaningless to discuss hole closure only from the perspective of three-dimensional compressive stress.
Conversely, the strain state can be calculated by superposition and accumulation. In order to change the compressive stress of the blank center, the cutting width ratio can exceed 0.51 by using up and down flat station stretching; When using FM faraday, the cutting width ratio should be ≥. Therefore, there is no need to pursue too large a cutting width ratio and a triaxial compressive stress state. The research results also found that there is a linear relationship between pore A and compressive strain and compressive strain. When the compressive strain in a certain direction is superimposed to a certain value, the hole here is completely compressed. Therefore, compressive strain plays a dominant role in the process of compressing internal holes.
Upsetting is only used as an auxiliary means to elongate the forging into three-dimensional deformation, which is the most effective means to compact internal holes, break coarse crystals, and improve segregation. The internal pore defects in steel ingots are basically axially distributed, which is also determined by the crystallization direction. Therefore, tensile deformation perpendicular to the axis of the ingot can compress and weld the holes as soon as possible. At the same time, the direction of pressure strain is often close to the direction of tension under pressure. Therefore, tensile forging is the shortest path to close the hole.
Upsetting is an axisymmetric deformation commonly referred to by scholars as two and a half dimensional deformation. At the beginning of upsetting, the hole in the center of the ingot expands horizontally, and does not begin to shrink until the height to diameter ratio reaches below 1:1. However, in order to press the holes in the center, the ratio of height to diameter needs to be above 8:3. Obviously, it is impossible and unnecessary to roll the steel ingot into such a flat shape. Upsetting can increase the cross-sectional area of the blank, essentially increasing the forging ratio for stretching. However, if it is judged from the technological standards of effective compaction methods that the direct stretching of steel ingots can meet the requirements of compaction and forging with a certain anvil width, compaction and forging are not required.
Only when the effective stretching times are insufficient, it is necessary to increase the diameter of the blank. As long as the upsetting reaches a satisfying diameter, there is no need to upset too large. Because it will prolong stretching time and reduce production efficiency. Therefore, when formulating a forging process, it is necessary to increase the diameter of the blank. Of course, during the forging process, the roughness must be increased. In the process of large-scale free forging, two key points must be considered first: 1. Effective compaction forging; 2. Prevent RST effect.
When the contact size between the forging tool and the blank in both directions is greater than the height of the blank, an internal sliding tear effect, known as the RST effect, occurs under larger deformation conditions. This phenomenon is common in thin cakes, thin plates, and thin shaft forgings. Therefore, when preparing or training a team, it should be emphasized that the anvil width ratio is always less than 1. In addition, when preparing pancakes and sheet forgings, it is necessary to provide a procedure for limiting the cutting width and pressing amount twice before molding. Then, when the forging has been compacted and forged, the density is very high, and there is no internal tear. After forging, the treatment and heat treatment are all normal, only the raw material itself has problems.