Decarburization defect of quenched and tempered steel connecting rod during heat treatment

Decarburization refers to the phenomenon that CO2, H2O, O2, H2 and other gases in the heating medium interact with the carbon in the steel surface layer with oxidizing impurities, so that part or all of the carbon is oxidized. The decarburized surface layer is called decarburized layer. It is the part where the carbon concentration in the surface layer of steel parts decreases. The depth of decarburized layer refers to the distance from the surface of decarburized layer to the place where there is no difference in carbon content with the original matrix metallographic structure. The decarburization layer is generally composed of full decarburization layer and partial decarburization layer (transition zone). After quenching and tempering, the carbides in the surface decarburization zone are significantly reduced.

The higher the heating temperature and the longer the holding time, the more serious the decarburization. The higher the carbon content in the steel, especially the higher the content of Si, Mo and other elements, the easier it is to decarburize. Due to the rapid diffusion of carbon, the decarburization rate of steel is always greater than its oxidation rate. Therefore, there is always a certain depth of decarburization layer under the oxidation layer of steel.

Due to the decarburization of steel parts, the surface hardness and wear resistance of the material are reduced, the fatigue strength is reduced, and even quenching cracks are formed. Therefore, important mechanical parts are not allowed to have decarburization defects, or it is specified that they are within the process tolerance, so that the decarburization layer can be removed during processing.

In order to prevent oxidation and decarburization, protective atmosphere heating, vacuum heating and anti-oxidation paste coating on the surface of the workpiece can be used; When heating in the salt bath, attention should be paid to the good deoxidation of the salt bath.