Decarburization on steel pipe

Decarburization is a phenomenon in which the surface carbon content of a steel pipe is lowered during heat treatment. The essence of decarburization is that carbon atoms in steel pipes react with furnace atmospheres such as hydrogen or oxygen at high temperatures to form methane or carbon monoxide.

Decarburization is the result of interdiffusion between atoms during heat treatment. On the one hand, oxygen diffuses into the steel; on the other hand, the carbon in the steel diffuses outward. The decarburization layer can be formed only when the decarburization rate exceeds the oxidation rate. When the oxidation rate is large, the obvious decarburization phenomenon may not occur, that is, the iron is oxidized to form an oxide scale after the decarburization layer is produced. Therefore, in an atmosphere in which oxidation is relatively weak, a deep decarburization layer can be formed.

The decarburization layer of the steel pipe includes a full decarburization layer and a partial decarburization layer (transition layer). Partially decarburized layer refers to the structure of the steel with normal carbon content after the full decarburization layer. In the case where the decarburization is not severe, sometimes only a part of the decarburized layer is seen without a full decarburization layer.

The depth of the decarburization layer can be determined by various methods depending on the change in decarburization composition, structure and properties. It is most common to measure the decarburization layer of steel by metallographic method in actual heat treatment production.

The main measures to prevent decarbonization are as follows:
1) When the workpiece is heated, reduce the heating temperature and the residence time at high temperature as much as possible; reasonably select the heating speed to shorten the total heating time;

2) Controlling the appropriate heating atmosphere to make it neutral or heated with a protective gas;

3) During the hot pressure processing, if the production is interrupted due to some accidental factors, the furnace temperature should be lowered to resume production. If the pause time is long, the billet should be taken out of the furnace or cooled down with the furnace;

4) When the cold deformation is performed, the number of intermediate annealings is reduced as much as possible, and the temperature of the intermediate annealing is lowered, or the soft annealing is used instead of the high temperature annealing. When performing intermediate annealing or softening and tempering, heating should be carried out in a protective medium;

5) When heating at high temperature, the surface of the steel is protected by coating and coating to prevent oxidation and decarburization;

6) Correct heat treatment process operation and increase the machining allowance of the workpiece so that the decarburized layer can be completely removed during processing.