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


Decarbonization is the result of atom diffusion during heat treatment. On the one hand, oxygen diffuses into steel; on the other hand, carbon in steel diffuses outward. Only when the decarbonization rate exceeds the oxidation rate can the decarbonization layer be formed. When the oxidation rate is high, no obvious decarbonization may occur, that is, after the decarbonization layer is produced, iron is oxidized to form oxide scale. Therefore, deep decarbonization layer can be formed in relatively weak oxidation atmosphere.


The decarbonization layer of steel pipe includes complete decarbonization layer and partial decarbonization layer (transition layer). Partial decarbonization layer refers to the structure of steel with normal carbon content after complete decarbonization layer. When decarbonization is not serious, sometimes only part of the decarbonization layer is not completely decarbonized.


The depth of decarbonization layer can be determined by various methods according to the composition, structure and properties of decarbonization. In actual heat treatment production, metallographic method is most commonly used to measure decarburized layer of steel.


The main measures to prevent decarbonization are as follows:
1) When heating the workpiece, the heating temperature and high temperature residence time should be reduced as much as possible, and the heating speed should be reasonably selected to shorten the total heating time.


2) Control appropriate heating atmosphere to keep it neutral or heated with protective gas;


3) In the process of hot pressing, if the production is interrupted due to some unexpected factors, the furnace temperature should be lowered to resume production. If the suspension time is long, the billet should be removed from the furnace or cooled by the furnace.


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


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


6) Correct heat treatment process operation, increase the workpiece processing allowance, in order to completely remove decarbonization layer in the process.