Mechanism of cryogenic treatment

1. Eliminate retained austenite:

Generally, the retained austenite after quenching and tempering is about 8-20%. The retained austenite will be further martensitized over time. During the martensite transformation process, it will cause volume expansion, which will affect The dimensional accuracy increases the internal stress of the crystal lattice, which seriously affects the performance of the metal. Cryogenic treatment can generally reduce the retained austenite to less than 2% and eliminate the influence of the retained austenite. If there is more retained austenite, the strength will decrease. Under the action of cyclic stress, it is easy to fatigue and fall off, causing nearby carbide particles to hang in the air and quickly fall off from the matrix, resulting in spalling pits and forming a relatively rough surface.

2. Fill the internal gaps to increase the metal surface area, namely the wear-resistant surface:

Cryogenic treatment makes the martensite fill the internal voids, makes the metal surface more dense, increases the wear resistance area, smaller crystal lattice, uniform alloy composition, increases the depth of the quenched layer, and not only the surface, but also increases the number of renovations and life improve.

3. Precipitation of carbide particles:

Cryogenic treatment not only reduces residual martensite, but also precipitates carbide particles, and can refine martensite twins. The shrinkage of martensite during cryogenic cooling forces the reduction of the crystal lattice and drives the precipitation of carbon atoms. Diffusion of carbon atoms is difficult, so the carbides formed are nano-sized and adhere to the martensite twin zone to increase hardness and toughness. The wear morphology of metals after cryogenic treatment is significantly different from that of metals that have not been cryogenically treated, indicating that their wear mechanisms are different.

Cryogenic treatment can martensitize most of the retained austenite, and precipitate high-dispersion carbide particles in the martensite. Accompanied by the refinement of the matrix structure, this change cannot be made with traditional metallography, phase transformation The theoretical explanation is not carried out in the form of atom diffusion. Generally, at -150℃～-180℃, atoms have lost their ability to diffuse and can only be explained from the viewpoint of physical energy. The transformation mechanism has not yet been studied clearly. So it needs to be discussed further.

4. Reduce residual stress;

5. Make the metal matrix more stable;

6. Increase the strength and toughness of metal materials;

7. Increase the metal hardness by about HRC1～2;

8. The redness and hardness increase significantly.