Four stages of tempering of forgings in Dalian

With the increase of tempering temperature, the quenching structure of Dalian forgings will change a series. According to the structure transformation, tempering is generally divided into four stages: martensite decomposition, residual austenite decomposition, carbide transformation, carbide aggregation and growth and ferrite recrystallization.

In the first stage of tempering, martensite decomposes. When tempering at the temperature below 80 ℃, the quenched steel does not have the microstructure transformation of bright S. at this time, only the carbon segregation in martensite occurs, but does not begin to decompose. When tempering at 80-200 ℃, martensite begins to decompose and precipitates very fine carbides, which reduces the mass fraction of carbon in martensite.

At this stage, due to the low tempering temperature, only a part of supersaturated carbon atoms are precipitated in martensite, so it is still the supersaturated solid solution of carbon in a-Fe. The precipitated very fine carbides are evenly distributed on the martensitic matrix. The mixed structure of martensite with low supersaturation and very fine carbide is called tempered martensite.

In the second stage of tempering, the retained austenite decomposes. When the temperature rises to 200-300 ℃, martensite decomposition continues, but the dominant transformation is the decomposition process of retained austenite. The decomposition of retained austenite is to form a segregation zone through the diffusion of carbon atoms, and then decompose into α The mixed structure of phase and carbide forms lower bainite. At this stage, the hardness of steel does not decrease significantly.

The third stage of forging tempering is carbide transformation. In this temperature range, due to the high temperature, the diffusion ability of carbon atoms is strong, and the diffusion ability of iron atoms is also restored. The transition carbide precipitated by martensite decomposition and residual austenite decomposition will be transformed into more stable cementite. With the precipitation and transformation of carbides, the mass fraction of carbon in martensite is continuously reduced, the lattice distortion of martensite disappears, and martensite is transformed into ferrite. A microstructure with fine granular (or flaky) cementite distributed in ferrite matrix is obtained, which is called tempered troostite. At this stage, the quenching stress is basically eliminated, the hardness decreases, and the plasticity and toughness are improved.

In the fourth stage of forging tempering, carbide aggregation and growth and ferrite recrystallization. Due to the high tempering temperature and strong diffusion ability of carbon and iron atoms, the cementite sheet formed in the third stage will continue to spheroidize and grow. Above 500-600 ° C, α The phase recrystallizes gradually, which makes the ferrite lose its original strip or sheet shape and form polygonal grains. At this time, the microstructure is ferrite and granular carbides are distributed on the matrix, which is called tempered sorbite. Tempered sorbite has good comprehensive mechanical properties. At this stage, the internal stress and lattice distortion are completely eliminated.