1. Raw material defects
For example, there are residual shrinkage holes, bubbles, looseness, inclusions, etc. in the steel ingot or steel, which may cause the forging to crack. Forging cracks caused by metallurgical reasons are often accompanied by a large number of oxides, sulfides, silicates and other inclusions. The raw materials of high-carbon and high-alloy steel are prone to severe segregation of second phases such as carbides. If they are not crushed and distributed evenly during forging, the mechanical properties of the forging will be reduced, and the forging may be cracked or cracked during heat treatment. distortion. Scratches, scars, folds, and cracks on the surface of the raw material will cause defects in the forging. Therefore, raw materials must be inspected during die forging production.
2. Heating specification
When forging large die forgings and alloy steel die forgings, if the heating speed is too fast, the temperature difference between the inner and outer layers will be large, and the center part will be cracked due to temperature stress and structural stress.
When the heating temperature is too high and the holding time is too long to cause slight overheating, a shiny, crystalline, coarse-grain fracture will be produced. Slightly overheated coarse grains can be corrected by annealing or normalizing treatment and recrystallization. In severe overheating, naphthalene-like fractures or stone-like fractures will occur. The naphthalene-like fracture is characterized by the appearance of fish-scale bright spots and transcrystalline fracture; the cause of the naphthalene-like fracture is that the coarse austenite grains form intragranular texture, which is extremely stable and transforms into ferrite during cooling. When body, it will still retain the characteristics of texture. The stone-shaped fracture has obvious coarse crystals, the surface has no metallic luster, the color is gray, and the crystal fractures; the cause of the stone-shaped fracture is that the solubility of non-metallic inclusions increases under overheating temperature, and during the cooling process, non-metallic inclusions pass Saturated coarse austenite precipitates out and surrounds the austenite grains to form a brittle crystal shell. Forging billets with severe overheating have extremely poor mechanical properties. Naphthalene-like fractures can be normalized at high temperature to eliminate intragranular texture, while stone-like fractures are difficult to correct with heat treatment. After forgings are formed, once stone-like fractures are found, there is no salvation.
The forging heating temperature is low, and when it is not heated through, cracks that spread through the crystal may occur, and the tail ends are sharp. When there is no subsequent heating process, there is no oxidation and decarburization on the crack surface.
For alloy structural steel, if the final forging temperature is too high, the austenite will continue to grow after the final forging, and even exceed the original grain size. Fracture inspection can see coarse-grained fractures, while high magnification observation shows Widmanstatten structure. If the final forging temperature is too low, the steel is in the dual phase zone, the inclusions are distributed along the main deformation direction of the blank, and the ferrite precipitated from the austenite preferentially adheres to the surface of the inclusions to form a banded structure. Widmanstatten structure and banded structure decrease the mechanical properties of forgings, especially the decrease of impact toughness. In order to refine the grains, improve the structure, and improve the mechanical properties, the steel with this type of structure must be fully annealed to produce recrystallization.