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Surface Decarburization of Heat-treated Products and Preventive Measures

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

Decarburization is the result of mutual diffusion between atoms during the heat treatment process. On the one hand, oxygen diffuses into the steel; on the other hand, the 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 high, no obvious decarburization phenomenon may occur, that is, iron is oxidized into oxide scale after the decarburization layer is produced. Therefore, in an atmosphere where oxidation is relatively weak, a deeper decarburized layer can be formed.

The decarburization layer of steel includes two parts: a fully decarburized layer and a partially decarburized layer (transition layer). Partially decarburized layer refers to the structure where the carbon content of the steel is normal after the fully decarburized layer. In the case of not serious decarburization, sometimes only a partial decarburization layer is seen but not a full decarburization layer.

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

The main countermeasures 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; select the heating speed reasonably to shorten the total heating time.

2. Control the appropriate heating atmosphere to make it neutral or use protective gas for heating.

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

4. When performing cold deformation, reduce the number of intermediate annealing and lower the temperature of intermediate annealing as much as possible, or use softening and tempering instead of high-temperature annealing. When performing intermediate annealing or softening and tempering, heating should be carried out in a protective medium.

5. When heated at high temperature, the surface of the steel is protected by covering 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.