Decarburization of steel tube heat treatment surface and preventive measures

Decarburization is the phenomenon of reducing the surface carbon content of steel tubes during heat treatment. The essence of decarbonization is that the carbon atoms in the steel tube react with the atmosphere in the furnace such as hydrogen or oxygen to form methane or carbon monoxide at high temperature.

Decarbonization is the result of the diffusion of atoms during heat treatment. On the one hand, oxygen diffuses into the steel; on the other hand, carbon diffuses out of the steel. The decarburization layer can only be formed when the decarburization rate exceeds the oxidation rate. When the oxidation rate is very high, no obvious decarbonization can occur, that is, after the decarbonization layer is formed, iron is oxidized into oxide scale. Therefore, a relatively deep decarburization layer can be formed in a relatively weak atmosphere.

The decarburization layer of the steel pipe consists of two parts: full decarburization layer and part decarburization layer (transition layer). Part of the decarburization layer is after the whole decarburization layer to the normal carbon content of the steel. When decarburization is not serious, some decarburization layers are sometimes seen but no decarburization layers are found.

The decarburization depth can be determined by various methods based on the change of decarburization composition, microstructure and properties. The decarburization of steel is the most common method in the actual heat treatment production.

The countermeasures to prevent decarburization are mainly as follows:

1) When the workpiece is heated, the heating temperature and residence time at high temperature should be reduced as much as possible, and the heating speed should be reasonably selected to shorten the total heating time.

2) control the proper heating atmosphere to make neutral or protective gas heating.

3) In the process of hot pressing, if the production is interrupted by some accidental factors, the furnace temperature should be lowered to wait for production recovery. If the pause time is long, the billet should be removed from the furnace or cooled down with the furnace.

4) Reduce the number of intermediate annealing and the temperature of intermediate annealing as much as possible during cold deformation, or replace high temperature annealing with softening tempering. During intermediate annealing or softening tempering, the heating should be carried out in the protective medium.

5) when the steel is heated at high temperature, the surface of the steel is protected by coating and coating to prevent oxidation and decarbonization.

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

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