Cryogenics process and heat treatment of tools steels and other hardened metals
The cryogenics process is quickly catching on as an extension of the heat treating process to continue the transformation of austenite to the preferred martensite grain structure. As significant as the heat treatment of metals is in today’s industrialized nations, there is plenty of room for improvement in increasing wear resistance in tool steels and other hardened metals. With the use of Cryotron’s cryogenic equipment the transformation process can be completed to within 99.8 to 100% martensitic structure. The “cryogenic tempering” treatment does not only increase performance in high wear applications, but also provides a complete stress relief process to remove any residual stress that may have been built up in the heat treatment, manufacturing, or machining stages.
Conventional heat treatment processes precedes cryogenic processing
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 process however. 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 for a good heat treatment is around 60-80% transformation, with poor treatments falling well below these marks. 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 referred to as an extension of the heat treatment process - 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 treated metals 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. Tools can be cryo treated when new or used, sharp or dull. Cryo'd tools are easier to re-sharpen, and the edge will last much longer. The tool can also be re-sharpened many more times, because of the fact that less material is lost on each sharpening. 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.
The combination of the newly formed martensite and stable 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.
Worth mentioning…
It may be worthy to include the fact that contrary to popular belief, cryogenic tempering is not the correct terminology. To call the deep cryogenics process a cryogenic tempering is rather misleading as tempering is always done at a positive value temperature and there is no such thing as a cold tempering. Nor will cryogenic temperatures increase the hardness value. The final temper in the heat treat process sets the hardness value and cryogenics will not alter this.