Cryogenic Processes And Their Applications zh4e

It is known the most important problems faced by the industries are the wear and tear of the machine parts.  This wear of the machine parts not only increases the cost of production but also the time wasted for the replacement process.  The strength and durability would be gained by cryogenically treating those machine parts.  The paper presents the purpose of cryogenic treatment and what happens in the metal structure along with its advantages and some of its applications 

Cryogenics have been derived from the Greek word “KRUOS” (frost) and “GENICS” meaning to produce very low temperatures.  Cryogenics is the ultra low temperature processing of materials to enhance their desired metallurgical and structural properties.  These ultra-cold temperatures, below -310°F, will greatly increase the strength and wear life of all types of vehicle components, castings and cutting tools. 

The theory was based on how heat-treating metal works and supposed that continuing the descent would allow for further strength increases.  The property of the cryogenic process is generally gained due to the conversion of austenite to martensite.  Proper heat treating can transform 85% of the retained austenite to martensite and the cryogenic treatment only transforms an addition of 8 to 15%.  But a more uniform, refined microstructure with greater density is formed as a result of cryogenic processing. 

These ultra cold temperatures are achieved using computer controls, a well-insulated treatment chamber and liquid nitrogen (LN2).  The part to be processed is placed in a processor. It is a computer controlled process the system is controlled with proven cooling curves programmed to the computer.  Liquid nitrogen is converted to a gas before it enters the chamber, so that a assuring that the dangers of cracking from too rapid cooling are eliminated.  . They are gradually cooled with nitrogen gas to 320 degrees Fahrenheit. That temperature is maintained for at least eight hours. 

After the cooling cycle is complete, the item is slowly warmed back to room temperature.  Then the object is heat with temperatures of 100 to 400 degrees Fahrenheit, depending on the composition of the item.  Finally, the item is gradually returned to room temperature.  The complete process takes a minimum of 24 hours to a maximum of 7 days.  Cryogenic processing aims to convert the entire structure to martensite, and it is not a heat treatment process that only affects the surface of the material. 

Martensite has a different crystalline structure (tetragonal) than the face-centered-cubic austenite from which it is formed, but identical chemical or alloy composition.  The transition between these two structures requires very little thermal activation energy because it occurs displacively or martensiticly by the subtle but rapid rearrangement of atomic positions, and has been known to occur even at cryogenic temperatures. 

Increases abrasive wear resistance.  Creates a denser molecular structure. The result is a larger surface area that reduces friction, heat and wear.  Increases durability or wear life.  Decreases residual stresses in tool steels.  Increases tensile strength, toughness and stability coupled with the release of internal stresses.  Insufficient soak time, cooling or warming too quickly, and skipping the post-soak temper can hamper the effectiveness of cryogenic treatment 

Steel spring – Fatigue life is increased from 800 to 2000 cycles.  Shaping – The tool lasts 4 times longer.  Slitting- cut 2.5 times more steel sheet.  Cutting of steel – Tools performance increased more than 50%.  Drilling – Number of holes is doubled.  Gear cutting- Double number of pieces cut between resharpening. 

From this paper it is clear that this process can create a more profitable tool line for a manufacturer.  It is also saving considerable tool expense for the end . Among the properties which define cutting qualities of tool steel, durability is the highest importance. 