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Being a material scientist I research on different material patterns. In this article I’ve explained <a rel=”nofollow” onclick=”javascript:_gaq.push(['_trackPageview', '/outgoing/article_exit_link']);” href=” http://www.castechindia.com/steel-castings.html”>steel castings</a> and <a rel=”nofollow” onclick=”javascript:_gaq.push(['_trackPageview', '/outgoing/article_exit_link']);” href=”http://www.castechindia.com/”>metal castings</a>.

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Cryotron Corporation asked:


 
In order to fully appreciate and understand the process of cryogenics, one must have a basic understanding of the purpose of heat treating.
Parts to be hardened are heated by one of several different methods, none of which are important to the understanding of the cryogenics process. Tool steels are generally pre-heated to a fairly high temperature before they are then soaked (a certain temperature a metal is held in is referred to as a soak) in their final heat treat temperature. This allows for temperature equalization of the metals throughout as well as sets the grain structure to allow for better transformation in its austenizing temperature.
Lower alloy steels however, are not preheated but are soaked directly at their austenizing temperature. Austenite is the name of the grain structure that forms in ferrous steels at this critical temperature. It is this austenizing temperature, along with the correct amount of time that allows for the metal to transform to this austintenic grain structure. Austenite grain structure is a very large, coarse, irregular, loosely bonded structure. At this critical temperature the metal has essentially melted within its own physical structure. The molecules are now free floating with no bonds to one another.
There are many variables in this stage that can determine the amount of retained austenite in the next stage. However, it might also be noted that cryogenics can fix some of these problems, although it should never be used as a “band-aid” to repair sloppy heat treatment.
After the austenization soak the metals are then quenched. A quench is a removal of heat at a controlled rate; this rate is dependant on the alloys the metals posses. This is a deliberate action to motivate transformation of the austenitic structure to the preferred martensitic grain structure. This is the grain structure that is highly resistant to wear. The martensitic structure is a much more refined, hardened, smaller, tighter, and stronger bonding grain structure. To accomplish the transformation the heat must be removed from the metal. This is one of the most critical steps in heat treating, if the temperatures and times are off, then transformation will not be of ideal structuring.
The parts will be quenched to a temperature of approximately 65°C. At this stage the newly formed “raw” martensite is very unstable and needs to be tempered in a tempering oven to stabilize the structure. There is also a slight transformation of some retained austenite to martensite in the temper. Tempering steel after fresh martensite is formed is an absolute must as the highly unstable behavior of the grain structure can crack or shatter.
This ends the heat treatment process; however there still remains untransformed retained austenite in the metal. Even though a very high quality controlled heat treating may give up to 90% transformation, it is very highly unlikely in batch applications. The usual transformation for a good heat treatment is around the 60-80% mark, with poor treatments falling well below these percentages. However even with as little as 2-3% retained austenite, the difference after transforming the remainder can be very significant.
Cryogenic technology to continue the transformation to 99.8 – 100%, increasing wear resistance in heat treated steels.
Cryotron cryogenic equipment, designed to do a deep cryogenics process – often incorrectly referred to as “cryogenic tempering” – uses cryogenic temperatures (-196°C, -320°F) to transform the retained austenitic grain structure, into the smaller, stronger, more desirable martensitic grain structure. Hereby enhancing the molecular bond within the properties of the metals and giving them greater strength in several areas, including drastic increases in wear resistance. The transformation rate is much greater than that of heat treatment alone and will get a rate of about 99.8 to 100% transformation. However, Cryogenics should not ever be considered to replace the heat treatment, it is a complimentary treatment that enhances what took place during the heat process.
To add to the mixture; during the cryogenic process there is also a precipitation of fine eta carbides throughout the metal; this is in addition to the larger carbides already present in the ferrous metals. These fine carbides increase the bond mechanisms within the molecular structure of the metals, which in turn increases wear resistance again.
Once again, the metals are taken out of the cryogenic equipment and tempered in a proper tempering oven to stabilize the newly formed martensite.
When the process is complete, the metal is a tougher, stronger and a much more durable substance than before. Cryogenic processing is a thorough process and permanently alters the entire thickness of the wear resistant metal; it is not just a coating or surface treatment. This deep cryogenic treatment also relieves the stress built up by the heat treatment process, as well as any residual stresses that may subside from the manufacturing and/or machining process. Please click here to read more about cryogenic stress relief, and the array of materials the treatment will work on including castings.
The combination of the newly formed martensite and stabile stress relieved material makes the surface of the metal less porous, so there is more surface area to wear; again adding to the arsenal against wear.
However, the process will not work on all metals to improve wear characteristics as believed by some. If the carbon content is too low, or the proper heat treatment is not done correctly, the results may not show any value at all, or may even show the contrary. An assessment may be critical on some metals to find out more information in the chemistry and heat treatment practices done to properly determine the outcome of a cryogenic treatment.
As for the speculation over whether or not cryogenics can act as a stress relief, yes it can. As documented in the ASM “Handbook of Residual Stress and Deformation of Steel” it was proven at both the Technologic Institute of Louisiana, and the Technical University of Jassy, Romania that cryogenics will work to favorably redistribute the residual stresses throughout the entire part.
Cryogenic treatment has been documented throughout the scientific world as a means to improve wear resistance in many heat treated metals and thoroughly relieve any built up and residual stresses. It is not a cure all “snake oil” as is read in many places on the web, but it will significantly improve several characteristics in many metals in areas that are beneficial to some. There is no need to do a cryogenic treatment on your silverware at home, as it will not be beneficial. However, if you are having breakage problems with a high torque part, or abrasive wear problem at work, a cryogenic treatment is something to look at.

Metal Casting