How to identify forging cracks and heating cracks

Generally, forging cracks will be formed at high temperature. Due to the expansion of the cracks and contact with air, under the microscope, the cracks are full of oxide skin and decarburized on both sides. The cracks are relatively thick, without detailed tips, relatively round and pure. Sometimes some forging cracks are relatively fine, and the cracks are semi decarburized rather than full decarburized.

There are obvious differences between the welding cracks in quenching process and the cracks in forging process. The welding heat treatment temperature is generally much lower than the forging temperature. Even for high speed steel and high alloy steel, the heating and holding time is much shorter than the forging temperature.

Due to the high heating temperature, long holding time or fast heating speed, early cracking will occur during the heating process, resulting in cracks distributed along the coarse grain boundaries. There is a slight decarburization structure on both sides of the crack. If the heating is too fast, early cracking will occur, without obvious decarburization, but the cracks and the tail are full of scale. Sometimes, due to high temperature instrument failure, the structure of the parts is extremely coarse, The cracks are distributed along the boundary of coarse grains.

Common defects of structural steel:

Forging defects

1.Overheating and overburning

The main characteristics are coarse grains, obvious widmanstatten structure, high heating temperature, inhomogeneous fracture grains and oxidation decarburization around grain boundaries.

2.Forging crack

It often occurs in the place of stress concentration or segregation of alloy elements, and the cracks are full of oxide scale, which is easy to produce cracks at high forging temperature.

3.Folding

Surface defects are caused by punching, cutting, wear of knife plate, rough forging, etc. in subsequent forging, defects such as surface oxide skin are involved in the forging body to form a fracture. It can be found that there is obvious decarburization around the fold by observing under microscope.

Heat treatment defects

Quenching crack is characterized by rigid and straight, transgranular distribution, wide starting point, slender and tortuous tail, which mostly occurs after martensitic transformation. There is no obvious difference in microstructure around the crack and no decarburization phenomenon.

The microstructure is coarse. If it is slightly overheated, it can be saved by secondary quenching. In addition to the coarse grains, some grains are easy to melt and the grain boundary is very coarse.

The microstructure is composed of massive or reticular troostite and undissolved ferrite. The soft spots will be produced if the heating is not enough, the holding time is not enough and the cooling is not uniform.