Nitrofreeze® Cryogenic Treatment
Nitrofreeze provides Cryogenic Treatment services to customers who want to enhance the overall performance of their metal components. Cryogenic Treatment, which is also known as Cryogenic Processing, modifies the micro-structure of metals by subjecting them to ultra-cold temperatures (down to –300ºF).
Cryogenic Treatment as a whole, promotes three transformations in heat-treated steels, cast irons and other metals:
Relieve Stresses | Reduce Fatigue Failures
All metals – not just steel, (aluminum, copper, cast alloys, etc.) will benefit from residual stress relieving as a result of Deep Cryogenic Treatment. All metals have residual stresses; they are created from the moment the metal “freezes” from its molten form into its solid form. Molten metal freezes – or transforms from its liquid phase to its solid phase – like water. As heat is extracted through cooling, dendrites (or crystals) form from the coolest areas first. Typically, these are at the surfaces and edges. Irregular freezing results in natural stress lines, where the dendrites collide or along the boundaries of the liquid (molten) metal and the solid metal.
After the metal is cast in its raw stock form, (e. g. block, billet, plate, round, etc.), it is heat treated to normalize the material and modify its properties (e.g. hardness, tensile strength, etc.). Once the raw stock is further modified, additional stresses are induced (through its machining, cutting, grinding, forging, etc.) during the manufacturing process. When combined, all of these stresses form weak areas that are prone to fail through propagation of the stress lines, resulting in cracks. These are often characterized as fatigue failures or more simply “metal fatigue”. Cryogenic Treatment attacks the root cause – the residual stresses. Cryogenic Treatment greatly reduces or eliminates fatigue failures, as well as the propagation of cracks in the metal components.
Increase Durability | More Consistent Grain Structure
All steels need to be heat treated after being forged or cast. Heat treating is used to obtain desired characteristics in the steel including the conversion of the “unwanted” austenite crystal structure to the more desirable crystal structure called martensite. While it is near impossible to get a 100% martensitic crystal structure, all steels have a certain allowable percentage of “retained austenite” or “RA” that is not converted to martensite. It is widely accepted in the heat-treating industry that all heat-treated steels will have some percentage of RA after heat treatment, recipes routinely specify an RA will “not exceed” a certain percentage. Cryogenic Treatment can help promote the additional transformation of retained austenite into martensite.
Utilizing Cryogenic Treatment helps to further eliminate voids or imperfections in the steel’s microstructure. This is widely accepted and well-documented fact that is evident in X-Ray and SEM (scanning electron microscope) analysis of steels before and after Cryogenic Treatment.
Improve Wear Resistance | Increase Carbon Structure In Steel
Steel is an alloy made by combining iron and other elements, the most common being carbon. By varying the amount of carbon and other elements and how they form in the steel, you can control the qualities of the steel such as the hardness, ductility, and tensile strength. When carbon is used, it’s content in the steel is generally between 0.2% and 2.1% by weight, depending on the grade of steel. The greater the carbon content the harder and stronger the steel will be, i.e. tool steels.
So, knowing this, why would you be interested in further processing the steel? When steels are cryogenically treated, the carbon structure is modified through a mechanism that is technically described as “the precipitation of eta-carbides”. After Cryogenic Treatment, more eta-carbides particles are formed. While it is not fully understood why this occurs, it is undisputed that it does happen and can also be seen through SEM (scanning electron microscope) analysis of steels that are cryogenically treated versus those that are not. The population of these eta-carbides – both brilliant ones (white ones) and dark ones (black ones) – is dramatically increased after Cryogenic Treatment.