No matter whether quenching + high temperature tempering or energy-saving waste heat quenching + high temperature tempering are used, the microstructure defects of parts are inevitable due to improper process or operation during the heat treatment of quenched and tempered steel. The microstructure defect of steel after heat treatment refers to the microstructure defect that can only be identified by metallographic microscope. The common structural defects of heat treatment include deformation, network carbide, poor spheroidization, overheating and overburning, oxidation and decarburization of steel surface, incomplete quenching and quenching cracks. The existence of these structural defects will have an adverse impact on the mechanical properties and process properties of the steel, and even make the parts invalid and scrapped in the service process. In order to ensure product quality and service life, it is necessary to understand and identify the heat treatment structural defects of steel and their causes, and to seek ways to prevent and eliminate these defects.
Oxidation refers to the process that iron in the surface layer of quenched and tempered steel reacts with oxygen, oxidizing gas or oxidizing impurities in the heating medium to form iron oxide. The surface of quenched and tempered steel is prone to oxidation and decarburization during heat treatment. When the heating temperature is below 600 ℃, a relatively dense Fe3O4 oxide film is formed, and the oxidation speed is relatively slow. However, with the increase of the heating temperature, the atomic diffusion speed increases, and the oxidation speed of the steel increases sharply. Especially when the heating temperature is above 600 ℃, a loose FeO based oxide is formed. Oxygen and iron atoms are easy to penetrate into the steel parts through this oxide film, and the brittle oxide layer is easier to thicken and peel off.
Oxidized parts will reduce their size, increase their surface hardness and roughness, and even affect the cooling speed during quenching, resulting in waste products. Therefore, oxidation should be avoided as far as possible.