Decarburization of steel pipe surface during heat treatment and its prevention measures

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

Decarburization is the result of mutual diffusion between atoms during heat treatment. On the one hand, oxygen diffuses into the steel; on the other hand, carbon in the steel diffuses outward. The decarburization layer can only be formed when the decarburization rate exceeds the oxidation rate. When the oxidation rate is very high, no obvious decarburization phenomenon may occur, that is, iron is oxidized to form an oxide scale after the decarburization layer is produced. Therefore, in an atmosphere with relatively weak oxidation, a deeper decarburization layer can be formed.

The decarburization layer of the steel pipe includes two parts: the full decarburization layer and the partial decarburization layer (transition layer). The partial decarburization layer refers to the organization after the full decarburization layer to the normal carbon content of the steel. In the case of not serious decarburization, sometimes only a partial decarburization layer is seen without a full decarburization layer.

The depth of the decarburization layer can be determined by a variety of methods according to the changes in decarburization composition, organization, and performance. In actual heat treatment production, the metallographic method is the most common method to determine the decarburization layer of steel.

Surface decarburization of steel pipe heat treatment

The countermeasures to prevent decarburization are mainly the following:

1) When heating the workpiece, reduce the heating temperature and the residence time at high temperatures as much as possible; reasonably select the heating speed to shorten the total heating time;

2) Control the appropriate heating atmosphere to make it neutral or use protective gas heating;

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

4) When cold deformation is performed, the number of intermediate annealing and the temperature of intermediate annealing should be reduced as much as possible, or softening and tempering should be used instead of 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 coverings and coatings to prevent oxidation and decarburization;

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