Prevention of decarburization and oxidation by quenching and heating

The inner oxide layer caused by quenching and heating is very thin, generally only a few microns, which can be worn away after quenching, without affecting the use. However, the internal oxidation in carburizing or carbonitriding layer is deeper. If it can not be worn off after quenching, the surface performance will be affected.

There are many ways to prevent oxidation, such as:

① Deoxidize the salt bath.

② Heating in a protective atmosphere.

③ Remove water stains and rust spots on the surface of the workpiece.

④ High temperature and short time heating are adopted.

⑤ The surface is protected by coating, etc.

Decarburization refers to the phenomenon that when the steel is heated in decarburization medium (oxygen, carbon dioxide, water and hydrogen, etc.), the solid solution carbon in the steel surface reacts with it and the generated gas escapes out of the steel to reduce the carbon concentration of the steel surface. When decarburization is serious, the surface layer can become ferrite. After the surface decarburization, the carbon in the inner layer diffuses to the surface, which makes the decarburization layer deepen gradually. The longer the heating time is, the deeper the decarburization layer is.

Decarburization and carburization are a pair of reversible reactions. The direction of the reaction depends on the carbon potential of the medium and the carbon concentration in the steel. When the carbon potential of the medium is equal to the carbon concentration in the steel, the two reach a balance, neither decarburization nor carburization, so the fundamental way to prevent decarburization is to use controlled atmosphere furnace heating. In addition, decarburization can also be prevented by heating in high purity nitrogen or inert atmosphere and vacuum heat treatment. The above mentioned anti-oxidation measures can also be used to prevent decarbonization. The decarburized workpiece can be heated in a controlled atmosphere furnace for recarburization and quenching to restore the original carbon concentration.