Free forging steel parts have the following three critical characteristics under quenching conditions.
(1) Structural characteristics
According to the steel size, heating temperature, time, transformation characteristics and the cooling method, the structure of the steel after quenching is composed of martensite or martensite + residual austenite. In addition, there may be a little undissolved carbonization. Things. Martensite and residual austenite are metastable at room temperature, and they all have a tendency to convert to the stable state of the cement body plus cementite.
(2) hardness characteristics
The lattice distortion caused by the carbon atoms is revealed by the hardness, which increases as the degree of supersaturation (i.e., carbon content) increases. The quenched structure has high hardness and strength, and low plasticity and toughness.
(3) Stress characteristics
Including microscopic stress and macroscopic stress, the former is related to the lattice distortion caused by carbon atoms, especially related to the very large value of the martensite, and the martensite is in the state of stress during quenching; the latter is due to When the temperature difference formed on the cross section is obtained during quenching, the stress state of the surface or the core of the workpiece is different, and there is tensile stress or compressive stress to maintain balance in the workpiece. If the internal stress of the hardened steel is not removed in time, further deformation or even cracking of the part may occur.
In summary, although the quenched workpiece has 髙 hardness and 髙 strength, but the 跪 is large, the structure is unstable, and there is a large quenching internal stress, which must be applied after tempering. Generally speaking, the tempering process is a follow-up process that is indispensable after quenching steel. It is also a very late process of the heat treatment process, which gives the workpiece a very demanding function.