Since ancient days it has been known that the world consists of solids, liquids and gases. In this issue I would like to limit my comments to the gas phase. In particular I would like to zoom in on the uses of gases in the heat treating world.
Without solids, the heat treating world would have no need of gases. However, in discussing solids, they too must be broken down further. Solids have what is called a core, or body, and also a surface. A good correlation might be our body, with corresponding skin as the protection for it. In the case of our body or that of a solid, such as a metal, each has different constituents that determine how each will function.
In the case of the body the properties required include the ability to withstand loads under static conditions, and to have sufficient ductility to warn of potential failure in the case of overload. This is the case in design of buildings and similar type structures.
However, these applications are but a small part of the use of metals. When we consider applications that are moving, then we consider what are characterized as dynamic properties. In this case, the body is moving and in doing so, it can require properties that resist impact, torsion, bending, compression, as well as tension, If used at elevated temperatures the additional consideration of stress rupture and creep becomes of importance.
I could continue, but the purpose of this post is to discuss gases important for heat treating.
We have discussed briefly the properties required of a metal structure. The properties of the surface are entirely different. Like the human body, the surface of the metal protects it from damage from the surrounding environment that could affect its performance by weakening the body through undesirable reaction with the constituents of the external environment. A classic example of this is rust on a car, or the green tinge that occurs with time on copper alloyed building fronts or statues. This is what you see with the eye. However, with the lapse of time what you see externally can occur throughout the entire body, resulting in inability of the body to function and do its job. This can happen quickly or over a significant period of time. However, when it does, structural damage is sufficient to cause either the structure to no longer function, often with damage, or termination of life to the people expecting its ability to do its job.
In respect to the body, the constituents of gases may be important to either the creation of the required properties or to its destruction. In steels, in particular, carbon is the major alloying element, providing the required strength to sustain the loading the component is to sustain. Many gases contain carbon, such as carbon monoxide, methane, acetylene, carbon dioxide, ethane and many more. In the case of powder metallurgy parts, these are compacted from powder and, depending on the compacting pressure, porosity exists throughout the part. Depending on the density, pores may be interconnected or segregated. In the case of parts containing interconnected porosity, a gas, such as methane, can enter the body through the interconnected porosity, thereby reacting with the alloying elements present to form carbides or phases that can markedly change the properties of the alloy being heat treated. Nitrogen is another element present in gases that can also infiltrate the porosity, contributing strengthening nitrides, or entering into solid solution to strengthen the alloy. You can therefore understand that in powder metallurgy it may be possible to compact a powder that is without carbon or nitrogen, therefore softer, permitting compaction to higher density, and then use a gaseous compound, such as methane, to nicrease the strength of the steel.
However, a steel, or any other alloy, is not going to maintain these properties from the environment unless a surface is produced that will prevent contamination, such as oxygen, nitrogen or reactive compounds of the elements, to deposit on the surface and diffuse inward with time along grain boundaries to react with alloyed elements or the base metal itself to weaken the alloy, causing subsequent failure. A comparison may be like the clogging of human arteries preventing the flow of blood in the human body. Even argon, an inert gas has been found to react in superalloys where it was used to protect the surface from corrosion to cause pore formation that in time, at high temperature actuates creep, causing stress corrosion.
However, the same carbon or nitrogen components of reactive gases may be used at the same time to advantage to react with the surface to form hard wear resistant non-reacting carbides or nitrides that prevent corrosive attack from the environment.These reactions are classified as carburization, nitriding or carbonitriding. There are other protective surfaces that may result from reaction of boron, silicon or oxygen, depending on the alloy composition.
Corrosion is one of the most expensive problems in the materials world today, causing billions of dollars loss of material and sometimes loss of life accordingly. Therefore much of the work that MagnaTech does is directed toward the resolution of problems of reaction of the environment to cause surface deterioration causing corrosion to result. remember that Magnatech does research and does not want to enter into production. We therefore are looking for customers that wish to use our developed technology to produce surfaces, without alteration of required core properties, to resist corrosion, wear, impact and fatigue of alloy parts. As such MagnaTech has five patents that provide economical, environmental friendly protection of surfaces of steels from the environment. In addition MagnaTech has fifty years of experience in the heat treatment and development of powder metallurgy steels and nonferrous alloys, specializing in the area of stainless steels, superalloys and alloys for magnetic applications. Should you have problems regarding heat treating or production of protective, wear resistant surfaces MagnaTech stands ready to quickly help you to resolve your problems economically in a clean and safe environment.