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Much has been written about the detail of what happens to materials when they are taken down towards ‘absolute zero’ and it is at this point that most people glaze over. So here is a brief summary of the effects that deep cryogenic treatment has.
In essence, with ferrous metals, the cryogenic process changes more of the retained austenite (a larger softer crystal) into martensite (a smaller harder crystal), the lower the temperature the greater the transformation rate. When we get to the deeper cryogenic temperatures of -
These photos are of a cutting tool (T4) that was sectioned, polished and then etched using a solution of 10mL 38% HCL and 50mL of methyl alcohol. The sample was then viewed under a Nikon Eclipse ME600P Microscope at a magnification of 1500X.
photographs courtesy of DANA CORP.
Both photographs are of the same part. The one one the top is prior to processing where the one on the bottom is post cryogenic treatment.
Non Ferrus Metals
The effect cryogenic processing has on some non ferrous metals is very useful to overcome some of the everyday issues that engineers face when working with these materials, especially in applications that require high levels of accuracy.
Deep cryogenic processing can be of benefit with aluminium and other non ferrous metals because when solidification takes place at the point metal is struck or cast, some of the molecules are fixed before they are able to form a uniform structure. This sets up ‘stress’ within the unit produced. In the same way, machining can also impart stress into a component, the effect is that some components can ‘move’ when temperature is applied. This may not be a problem unless we are looking at an engine component that needs to operate at a reasonable high temperature. The result is that the component may distort as it reaches operating temperature causing it to go out of true. In simple structure this may not be an issue but in more complex devises this may have a ‘knock-
Cryogenics is able to affect items made of copper and silver and so change the electrical characteristics of that conductor allowing the electrons to flow more freely producing a more efficient, cooler running component.
Materials made of brass show a change in the vibration characteristics so, for example, brass instruments will take on the tone of a prized older instrument. One Japanese sliver flute manufacturer routinely cryogenically processes its products. It is also a useful process to adopt when instruments are repaired to remove the stresses that are put into the instrument through fabrication repairs and welding/soldering.