Change of microstructure and properties of quenched steel during tempering

(1) Decomposition of martensite

When tempered from room temperature to about 200 ℃, a part of supersaturated carbon and fine ε - carbide (fexc or fe2.4c) in martensite precipitated and distributed on martensite matrix, which decreased the carbon content in martensite, the square degree C / a of the square of the body center (i.e. the degree of national saturation) and the brittleness and hardness of martensite heat treatment. At this time, the microstructure is a mixture of martensite with a slightly lower degree of supersaturation and very small ε - carbide, which is called "tempered martensite structure", M-cycle.

ε - Carbide: it is a non-equilibrium phase, which makes the transition to Fe3C.

(2) Transformation of retained austenite

At about 200-300 ℃, as the martensite continues to decompose, the retained austenite also transforms into lower bainite structure. At this time, the main structure is still tempered martensite, but due to the higher heating temperature, the degree of supersaturation of martensite is further reduced, the hardness of the structure is reduced, and the plasticity is improved. As the retained austenite transforms into lower bainite with higher hardness, the hardness of steel decreases little. At this time, the structure is "tempered martensite + lower bainite"

(3) Cementite formation and ferrite recovery

At about 300-400 ℃, the supersaturated Heat-Treated Carbon in the α solid solution gradually precipitates, and the ε - carbide transforms into stable and smaller Fe3C particles. The carbon content in the α solid solution almost reaches the equilibrium composition, so the martensite becomes ferrite (C / a ≈ 1), the body centered square lattice becomes the body centered cubic lattice, and the structure is a mixture of ferrite and fine-grained cementite dispersed in it ”, called "tempered troostite", t-circuit.

(4) Aggregation and growth of cementite and recrystallization of ferrite

The structure is a mixture of ferrite and spheroidized cementite, which is called "tempered sorbite", s-Cycle. At this time, the strengthening effect of carbon solution disappears, and the strength depends on the particle size and dispersion of Fe3C. The higher the tempering temperature, the larger the cementite particle, the lower the dispersion reading and the lower the strength.

The tempered sorbite structure has good comprehensive mechanical properties, i.e. both strength and toughness. If the temperature continues to rise above 650 ℃, the cementite will continue to coarsen, and the structure will become spherical pearlite with lower strength. The comprehensive mechanical properties will be reduced, which is generally not needed.

The tempering structure has higher strength and toughness than the normalizing structure (the main reason is the different morphology of Fe3C).

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