Analysis of tempering transformation process of quenched steel

(1) martensite decomposition in the tempering stage (≤200°C)

When the tempering temperature of 100 ° C above, martensite begins to decompose, M in the supersaturated carbon atoms in the form of epsilon carbide precipitation, precipitation of carbon to reduce the carbon in the M of the degree of supersaturation, the tetragonal degrees C/a it also decreases, and low temperature, at this stage in the martensite precipitation only part of the supersaturated carbon atoms, it is still in alpha - Fe carbon supersaturated solid solution, in the phase of tempering steel hardness does not reduce, but as a result of epsilon carbide precipitation, the lattice distortion is reduced, less quenched internal stress.

(2) transformation of residual A in the tempering stage (200 ~ 300°C)

Residual decomposition of A 200 ° C to 300 ° C basic end, residual broken down into A bainite, in the stage of the tempering, residual into bainite at the same time, under A M continues to decompose, M will continue to break down to the hardness of steel is reduced, but due to the weaker the remnants of A shift into more should lower bainite, so the hardness of steel was not significantly reduced, but the quenching further reduce internal stress.

(3) conversion of carbide in the third stage of tempering (300 ~ 400°C)

In the third stage, tempering carbon atoms from supersaturated alpha continue to precipitate in solid solution, epsilon carbide at the same time also gradually become associated with alpha solid solution no more lattice cementite (Fe3C), alpha almost have to balance the carbon content, carbon content in the solid solution for degrees c/a close to 1, after the third stage, the organization of the steel is composed of ferrite and granular cementite, reduce the hardness of steel quenching stress to this basically eliminated.

(4) in the fourth stage of tempering (>400°C), the cementite accumulates and grows up and recrystallization of the lep phase

After the third stage of tempering, the structure of the steel was composed of ferrite and granular cementite, but the lubenite (ferrite) still retained the original M's strip or sheet, and became polygonal grain. Indicates that the deformation of quenched steel in tempering increases with the increase of temperature, the size of carburized body increases ↑, internal stress decreases ↓, residual quantity of A ↑, carbon content of M ↓.