Get Started With Cryogenics
If you are looking for a technical explanation of Deep Cryogenic Treatment of Metals (or Deep Cryogenic Processing or Deep Cryogenic Tempering) stop reading. This is for the layman or the person who just wants to know the basics. Believe me, the basics are complicated enough as I will quickly demonstrate.
Deep Cryogenic Treatment or DCT for short is a process that uses cold to modify metals and other materials. It results in improvements in mechanical parts and electrical parts. These improvements result in considerably longer life of automobile components, and industrial tooling. For instance, valve springs last over six times longer, brake rotors over three times as long, industrial dies and tools over three times as long. DCT can also greatly improve electronics and musical instruments. The process works on most metals, some plastics, carbide and diamonds. Use of DCT can have a distinct effect on industrial plant efficiency.
- DCT is a process where the temperature of an item is reduced slowly to 300oF below zero, held there for a while and then slowly brought back to room temperature. There may be an above room temperature tempering cycle added.
The main benefits of DCT are increased wear resistance, increased fatigue life, and a change in the vibrational characteristics of the material.
Most people know that heat can be used to change metals. You can use heat to harden, soften, and relieve stresses among other things. The thought that cold can make changes to metal is not so self-evident. For one thing, extreme cold was not easy to get until the early 1900’s. For another, people’s perception of a solid metal is that it is solid and nothing more could change without adding energy in the form of heat. While this perception seems logical, closer study of the structure of metals shows that it is not. Metals are crystals. That is how they get their metallic characteristics. Changing the temperature of a crystal changes the way the atoms in the crystal relate to each other. Some of those changes can be more or less permanent.
Most metallurgists will tell you the only thing freezing does to metals is that it causes retained austenite to become martensite. Hold on there. What are martensite and austenite? They are names of patterns you see when you polish a piece of steel and then etch it and look at it under a microscope. Martensite is a structure that results in steel being hard. It is made by heating the steel up until the steel forms an austenite structure and then cooling it quickly. Other patterns are called bainite, cementite, ferrite and pearlite. Don’t worry, there will not be a quiz.
- Most hardened steel contains martensite, which is named after Adolf Martens, a German metallurgist. Martensite is formed by quickly cooling steel that is heated above a certain point. The faster the metal is cooled (or quenched as it is called), the more of the steel changes to martensite. Some austenite usually survives and is referred to as retained austenite. DCT converts most of the retained austenite to martensite.
All together, these patterns are called the microstructure of the metal, basically because they can be seen through a microscope. A lot of the properties of metals are caused by what microstructure they have. You can cause changes in the microstructure through heat, vibration, deformation, and get this, cold. The patterns are caused by how the atoms in the metals relate to each other. That is called the crystal lattice structure.
Let’s talk a bit about crystalline structure. This simply means that the atoms align themselves in an orderly pattern when the material is solid.
- This also means that there are no molecules. If someone starts to tell you about the ‘molecular structure’ of a metal just walk away quietly.
Ideally, the pattern is perfect. Nothing is perfect and in the real world most metals are really pretty imperfect. In most metal crystals you will find:
- Places where an atom is missing
- Places that have an inappropriate atom
- Places where the atoms are too close together
- Places where the atoms are too far apart
- Places where there is an atom where one shouldn’t be
Each of these raises the energy in the crystal. By slowly lowering the temperature, many of these defects are corrected or modified. The simplified version is that it takes the extra energy out of the crystal. The net result is that the crystal becomes more wear resistant and failure resistant. The real result is increased component life that creates many manufacturing efficiency solutions.