Defects caused by heating of forgings

Beryllium oxide: Beryllium oxide not only causes a large loss of steel, but also reduces the surface quality of forgings and the service life of forging dies. If it is pressed into the metal, it will cause the forgings to be scrapped. Failure to remove beryllium oxide will affect turning.

Decarburization: Decarburization refers to the phenomenon that all or part of the carbon on the surface of steel is burned off. Decarburization causes soft spots on the surface of the workpiece, reducing the hardness, wear resistance and fatigue strength of the surface.

Overheating and overburning: Overheating refers to the coarsening of grains after the steel exceeds the allowable temperature during heating. Overheating is not conducive to heat treatment, making the steel brittle and reducing mechanical properties, but it can be eliminated by normalizing or annealing after forging. Overburning refers to the phenomenon that oxides or partial melting of metals occur due to too long heating time and too high temperature. Overburning cannot be remedied.

Stress occurs: Due to the difference in heating inside and outside the metal, the expansion is uneven, and internal stress is generated, which is called thermal stress. The changes in the metallographic structure caused by heating also cause stress, which is called organizational stress. This will cause the workpiece to crack during heating, and cause the workpiece to crack after turning and be scrapped.

Fracture on the cross section: This defect destroys the chemical composition and uniformity of the steel, reduces the quenching hardness, and deteriorates the mechanical properties. If the temperature is too high during annealing and a graphite cross section is formed, it will not be easy to cut and will cause overheating and deformation during quenching. However, if the annealing is underheated or the temperature is low, the pearlite is not fully transformed, which is not conducive to cutting and subsequent heat treatment.

Hard and brittle network carbide: It weakens the bonding force between the crystal materials, significantly deteriorates the mechanical properties, especially reduces the impact toughness, but can be improved or eliminated by normalizing. If banded carbides appear, the hardness and structure after quenching and tempering will be uneven, and easy to deform. This is also a defect of banded structure in the direction of processing deformation of pearlite and ferrite. At the same time, it will also reduce the plasticity and toughness of the steel, make the turning size unstable, and the tool will wear quickly.