When the heating temperature is too high and the heat preservation time is long enough to cause the forging to be slightly overheated, the wheel forging will produce a coarse, crystalline, coarse-grain fracture. Coarse grains that are slightly overheated can be corrected by annealing or normalizing and recrystallization. When it is severely overheated, a teat-like fracture or a stone-like fracture will occur. The characteristic of the teat-shaped fracture is the appearance of fish scale-like bright spots, which break through the crystal. The reason for the teat-shaped fracture is that the coarse austenite grains form an intracrystalline texture, which has extremely high stability and is transformed into ferrite during the cooling process. Texture, still retains the characteristics of the texture, leading to fracture. The stone-like fracture has obvious coarse crystals, the surface has no metallic luster, the color is dark, and the crystal breaks. The reason for the stone-like fracture is that the solubility of non-metallic inclusions increases at the overheating temperature, and during the cooling process, the non-metallic inclusions pass through again. Precipitation occurs in the saturated coarse austenite, which surrounds the austenite grains to form a brittle crystalline shell, leading to fracture. Forged billets with severe overheating have extremely poor mechanical properties. The teat-shaped fracture can be normalized at high temperature to eliminate the intra-grain texture. The stone-like fracture is difficult to be corrected by heat treatment. Once the forging is formed, the stone-like fracture is irreparable.
The forging heating temperature is low. When it is not penetrated, cracks may propagate through the crystal and the tail end is 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 of the wheel forging is too high, the austenite will continue to grow after the final forging, even exceeding the original grain size. Fracture inspection can see the coarse-grained fracture, and high-power observation shows Weiss structure. If the final forging temperature is too low, the steel is in the dual phase region, and the surface is inclusions to form a band structure. The Weiss structure and the band structure reduce the mechanical properties of the forging, especially the impact toughness is more obvious. In order to refine the grains, improve the structure, and improve the mechanical properties, steels with such structures must be completely annealed to cause recrystallization.