The question “What is an atmosphere?” is often asked. A simple answer is a gas that can be introduced into a thermal process furnace with the objective of providing a surface-protection and controlled-oxidation environment and introducing elements for surface modification.
There are many ways that a heat-treatment atmosphere can be a generated. It can be a synthetic atmosphere or simply gas that is pre-mixed/pre-stored.
If one considers a wood fire and the flame being generated is seen as a bright yellow, then the flame is a carbon-rich flame. If one considers wood charcoal that is commonly used for a barbecue, then the gas generated as a result of the combustion is carbon monoxide, which is a carburizing gas, and carbon dioxide, which is considered as a decarburizing gas.
Because of the increasing demand for repeatable metallurgical results, great emphasis is being placed on the use of consistent gas analyses for various metallurgical processes (e.g., carburizing). It is necessary to understand the basic reactions of the industrial gases chosen for metallurgical processing. Depending on the application in question, one can use individual gases or combinations of gases. The principal gases used in the heat treatment of steel are as follows.
Oxygen: Oxygen is perhaps the most freely available gas. It is present in many generated gases such as the endothermically generated gases. Oxygen will react readily with iron in steel to produce iron oxide, more readily known as scale. It will also create intergranular oxidation in the surface of an atmosphere-carburized component. In addition to this, oxygen will react with the carbon present in the surface of the steel and cause surface decarburization. At this point, it must be said that some processes take advantage of the presence of oxygen in order to create a controlled surface oxidation. This is accomplished to provide a corrosion-resistant barrier on the surface of the steel.
- Nitrogen: Nitrogen is usually present in an atmosphere as molecular nitrogen, which is passive to ferrite and is most satisfactory for use in the processing of low-carbon steels for annealing. The grade of nitrogen must be chosen very carefully due to the potential for the presence of moisture in the gas. If it does contain moisture, surface oxidation and potential decarburization will take place. Therefore, clean, dry nitrogen is necessary for successful annealing of low-carbon steels. If atomic nitrogen is used (this means if the gas is cracked to provide atomic nitrogen for a fraction of a second), then the nitrogen will begin to react with the iron (and alloys) to form finely divided nitrides (iron nitrides) that will be present in the surface of the steel. This will cause an increase in hardness of the surface and in some cases brittleness, particularly at sharp corners. Nitrogen is often considered by many to be a neutral atmosphere. This is a misunderstanding of the action of nitrogen under heat. Nitrogen will prevent surface oxidation, but it will not stop surface decarburization. It should be remembered that in order to prevent surface decarburization, the carbon potential of the furnace atmosphere needs to be in equilibrium to the surface carbon potential of the steel.
Next time, we will cover the rest of the gases used in heat treatment.