Cryogenics and Metallurgy of 300 Below Cryogenic Tempering from OneCryo

Cryogenic Tempering Research: Researchers at the National Bureau of Standards, speaking about cryogenic tempering, stated, "When carbon precipitates form, the internal stress in the martensite is reduced, which minimizes the susceptibility to micro cracking.

The wide distribution of very hard, fine carbides, from deep cryogenic tempering, also increases wear resistance." The study concludes, "... fine carbides and resultant tight lattice structures are precipitated from cryogenic treatment.

These particles are responsible for the exceptional wear characteristics imparted to materials by the process, due to a denser structure and resulting larger surface area of contact, reducing friction, heat and wear."

Deep cryogenic tempering is not a coating or a surface treatment, but a one-time, permanent, irreversible process that penetrates completely through the entire material structure.

Deep cryogenic temperatures are required to effect a complete molecular change in most alloy steels, converting the retained austenite into martensite ( a more refined grain structure, which is more uniform than austenite ).

Cryogenic tempering transforms the microstructure into a more uniform structure that is more durable, stronger, longer lasting, and more dimensionally stable.

Cryogenics: Deep cryogenic tempering can significantly reduce retained internal stresses on most alloys.

Stress imparted unequally can in most alloys cause a decrease in strength and durability.

Stress boundary areas are more susceptible to micro cracking, which can lead to premature fatigue and even eventual failure of the stressed part.

Residual stresses exist in all types of parts from engines to tooling. The stress is introduced into the parts at the time of forging, casting, heat treating and final machining.

These stresses create a complex, invisible ( to the naked eye ) random pattern in the alloy. As parts expand from the heat generated during operation, the retained stresses cause uneven expansion, increased dimensional instability, and increased wear as well as decreased performance.

Stress relief takes place when the entire mass of the part is at an equal temperature ( surface and core ), and then slowly cycled ( less than one degree per minute ) through a wide temperature range. By cycling parts to ultra low temperatures for a prolonged period a very dense molecular state is created. Absolute zero ( -459.67 Fahrenheit ) is known as the zero motion molecular state of mass. It is this slow rate of temperature change that allows what is known as thermal compression and thermal expansion to occur, which is what actually effects the release of stress. The result is a dimensionally stabilized part, which will resist distortion and warpage increasing performance and durability.

Precipitation of eta Carbide: In a study performed at the Jassy Institute in Romania, researchers used a scanning electron microscope with a microscopic particle counter to evaluate additional changes in the structure of cryogenically treated steel.

The study concluded that the number of countable small carbides increased throughout heat treatable steel, from 33,000 particles per square millimeter to over 80,000 particles per square millimeter as a result of the cryogenic treatment. The increase in the carbides adds greatly to the wear resistance of the part. The carbides make a refined flat "super hard" surface on the steel which is smoother and decreases friction and heat as well.

Metallurgy of Cryogenic Tempering