With the increase in the weight and size of forgings required for steel, energy, and petrochemical industries, the weight of steel ingots continues to increase, making it more difficult to prevent or reduce metallurgical defects within steel ingots. On the other hand, due to the increase in the weight and size of steel ingots, the tonnage of hydraulic presses has been relatively reduced, and the quality standards of forgings have been continuously improved. The traditional upsetting and stretching deformation processes used to break the internal casting structure of steel ingots and repair internal metallurgical defects cannot meet the quality requirements of forgings. Large forging is the two most widely used steps in large forging. Compared to lengthening, due to the small volume, large deformation, and high stress in the defect area of the billet, lengthening is the main work step to break the casting structure and repair metallurgical defects. The following is a new process for large forging and lengthening: concave cutting and lengthening.
During forging deformation, due to the influence of friction and temperature gradient, there is always a large and small deformation area near the contact area between the tool and the forging blank. The size and shape of the deformation zone have an important impact on the deformation distribution and stress state inside the forging, thereby affecting the quality of the forging. During the stretching process, there is a region near the contact area between the cutting board and the forging blank that is difficult to deform, with the pressure direction perpendicular to the axis. Due to the existence of metallurgical defects in the steel ingot along the axis, large deformation and good stress state should be formed near the axis, which is conducive to the formation and good stress state, and is conducive to the repair of metallurgical defects in the steel ingot. From a deformation perspective, when the contact area between the forging blank and the cutting board has a region that is difficult to deform, the deformation of the heart region must be large.
From a stress perspective, during the stretching step, due to rigid constraints, when the metal flow velocity is high, in order to maintain the continuity of the deformable body, the upper and lower deformation regions must block the metal flow near the axis through the rigid ends, resulting in greater axial compressive stress in the heart. Therefore, the difficult deformation area in the contact area between the forging blank and the cutting board is conducive to the repair of metallurgical defects in the ingot. The larger the difficult deformation area, the more obvious the effect. According to the above analysis, changing the bottom plane of the cutting board to a slightly concave surface in the middle can increase the difficult deformation area at the bottom of the cutting board. The bottom surface of this cutting board is a concave surface stretching process, called concave cutting board stretching. Research has shown that concave cutting is better than ordinary cutting. Compared with other existing special forging methods, concave cutting has the advantages of convenient application and wide application.
The quality of heavy forgings is mainly reflected in three aspects: purity, uniformity, and density. Any defects can form defects that affect product quality. Common internal defects of large forgings include segregation, inclusions, cracks, looseness, coarse grains, and uneven grain size. The internal quality requirements of heavy forgings are very high, requiring not only mechanical properties and particle size testing, but also ultrasonic testing. When the defect exceeds the limit and cannot meet the technical requirements, the heavy forging product will be scrapped.
Therefore, excessive control of defects is the key to ensuring product quality. The goal of controlling forgings is to scientifically control the thermoplastic processing process of heavy forgings, eliminate defects in forgings, ensure good organization and performance, and obtain high-quality products. Better results can be achieved in eliminating internal cracks, improving partial analysis, mixing, loosening, and coarse crystals. The three elements of internal defects in welded forgings are temperature, stress state (reasonable distribution of strain within the forging), and pressure.