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The stress field and strain field inside the billet are very uneven [1] due to the frictional contact between the anvil and the ingot in the process of the crack. Due to the square 370mm steel ingot H/L2, the metal part of the 1/4 height of the first twisting square has the largest degree of metal deformation, and the appearance of the steel has the same drum shape; the upper and lower end metal deformation degree is the smallest, and the end area flanging increases. The degree of lateral metal deformation is centered. The unevenness of the deformation easily enlarges the voids and pores in the region where the metal deformation is the greatest. At the same time, the as-cast dendrites of the large steel ingots are smaller, the outer layer of the cross-section is a fine-grained area, the middle is a columnar crystal area, and the core is an equiaxed coarse-grained area [2]. The diagonal of the square ingot is the columnar crystal junction, the bonding force is very low, and the low melting point impurities are gathered, which is a fragile interface. When forging up, the metal dots flow along the shortest normal I and m directions, tending to be round [3]. This flow mode tears the fragile interface of the core along the direction of the columnar crystal. Since the amount of deformation at the center is 113118 times the average deformation amount [4], the fragile interface with the largest lateral deformation in the middle of the square drum and the isometric thick The tear at the intersection of the crystals creates a source of cracks, forming intergranular cracks.
Cross-section structure of steel ingotsFig. Flat anvil rounding and long crack propagation [5] When the upper and lower flat anvils are rounded and rounded, the shaded portion of the triangle ABC that is in contact with the upper and lower anvils is a non-deformed area, as the wedges are driven up and down into the forging metal, resulting in hammering force. F is decomposed into RH by vertical wedge surface, and RH forms a flattened anvil. The force of the Fig. 14 force RR causes the forging to be subjected to a large lateral tensile stress, which increases from the surface of the blank to the maximum in the horizontal direction. The result of the deformation is that the longitudinal flow of the metal shaft is small, and the lateral flow is large. This radial tensile stress can not only forge the intergranular intergranular crack generated during the piercing process, but the transverse tensile stress acts on the original crack in different directions during the inversion process to form a penetrating axial fracture. At the same time, the rounding of the flat anvil will produce a large free stretch, which reduces the efficiency of the drawing, and causes the journal end to be concave when the stepped shaft is forged, which tends to cause the length of the shaft to be insufficient.
Solution From the above analysis, it is known that the inter-axial intergranular cracks are generated in the initial pier steps, and are not completely forged during the subsequent piercing process, and are expanded in the process of flattening the flat anvil. Therefore, the solution is mainly to improve the uniformity of the ingot and reduce the transverse tensile stress of the forging.
The blank raw material should be changed from square steel ingot to octagonal steel ingot forging. The more the number of ingots, the smaller the angle between the columnar crystals, and the more uniform the solidification of the metal 1, which can effectively prevent the hazard of angular segregation. For square ingots, they should be chamfered into octagonal cylinders and then thickened. When chamfering, the hammering is light and brittle. As far as possible, the deformation only appears in the minimum range of contact with the anvil. It is strictly forbidden to strike hard to prevent the metal from flowing laterally. Diagonal crack [6]. When the octagonal cylinder is upset, the metal is spread along the horizontally facing sides, reducing the tearing force on the fragile interface.
In the final forming stage, the long round billet is drawn using a 120b upper and lower V-shaped anvil, and the side pressure of the V-shaped anvil can limit the lateral flow of the metal, forcing the metal to elongate in the axial direction. At the same time, the maximum deformation is at the center, the stress state is very good, the direction of defect shrinkage is basically unchanged during the turning process, the lateral tensile stress is greatly reduced, the axial defect is well forged, and the internal longitudinal crack is prevented from being generated. Long efficiency.
Through the improvement of the above forging process, the recurrence of the inter-axial intergranular cracks is effectively eliminated in the process of repairing and forging the scrap shaft.
Conclusion The intergranular cracks in the shaft of the gear shaft are mainly due to the tearing of the fragile interface during the upsetting of the square ingot and the further tearing of the core defect by the transverse tensile stress during the flattening of the flat anvil. The use of octagonal steel ingots for forging shaft parts and the use of upper and lower V-shaped anvils to pull long round billets can effectively prevent the occurrence of inter-axis intergranular cracks. Through the discussion on the causes and preventive measures of the intergranular cracks in the shaft of the gear shaft, it has certain guiding significance for improving the forging quality of the shaft products and improving the production efficiency.