Ferrous metals, those that are rich in iron (Fe), often require normalizing. Why should this be the case? Is the grain size really that inconsistent and material-coarse after an iron workpiece has been heat-treated? Actually, yes, the grain size will alter in an iron workpiece if it’s continually processed. This effect worsens when the material is alloyed. Restoring uniformity, the workpiece’s atomic structure does require normalizing.
Carbon Steels Retain Memories
That’s not quite true. In place of memories, it’s work stress that gets locked within the microcrystalline lattices of a ferrous metal part. The internal stresses are there because of a welding operation, or a forging service, or because of a cold-work process. Carbon content, the amount of alloying carbon that diffuses into an iron workpiece, seems to be the primary offender here, for low carbon steels don’t typically require normalizing. Anyway, the internal stress, those trapped “memories” of welding and forging, are swept away by the slow cooking and air cooling treatment. With those internalized atomic tensions leeched away, workpieces won’t distort or deform when exposed to more heat treatment work.
Incorporating Grain Refined Toughness
Here’s another problem that occurs in carbon and iron alloys. As the grains are worked or exposed to supplementary thermal treatment operations, they become coarse. The alloy crystals change in size and adopt an irregular form. The loss of grain uniformity causes a matching loss of workpiece toughness. By taking the part into a normalizing furnace and then into an air cooling room, the grains homogenize. They become smaller, finer, and they produce harder workpiece structures. Of some concern, the process is usually conducted in an air-charged furnace atmosphere, so parts scale and decarburized contaminants can form during the process. That’s not exactly surprising, not when the parts are heated to 890°C while being exposed to air. Subsequent machining work or surface finishing operations may be required to remove the scale.
In practice, normalizing services are executed faster than comparable annealing operations. Ferrous metal workpieces may require subsequent post-processing, but the tasks required to restore a presentable surface finish can be carried out relatively fast. As for the machining risks, coarse-grained materials won’t cut as smoothly as normalized, fine-grained parts. Again, the benefits far outweigh any possible processing downside. In skipping the normalizing stage, ferrous metals are weaker and possibly loaded with deformability potential because of the internal stresses still contained within their crystalline structures. Therefore, certain ferrous alloys, including tool and carbon steels, require that extra degree of hardness, which comes only from normalizing. By the way, if ductility is preferred over hardness, an annealing service should be used in place of the normalizing process.
A second heat treatment effector exists in an industrial furnace, one that’s sometimes forgotten by the average layperson. Besides the heat source, the gas-fuelled or electrically energized thermal energy flowing evenly around a subject workpiece, there’s the atmosphere inside that sealed chamber to consider. Sometimes, the atmosphere is totally taken out of the heat treatment formula, so the process takes place in a vacuum. At other times, that atmosphere becomes an essential process agitator.
Reviewing Atmospheric Effectors
In a regular heat treatment operation, the air itself functions as a heat load or thermal conductor. The currents convect the thermal energies from the walls of the furnace to the workpiece. Radiated heat sources function differently, without the need for air. Importantly, air can be pumped out of a vacuum-sealed chamber to add more control to the process. Alternatively, a regular atmosphere can be replaced by a second gaseous medium. This medium facilitates the formation of different surface protection finishes. Better than a coating, the gas actually impregnates the outer casing of the alloy and transforms this surface layer into a mechanically and chemically desired finish. Essentially, just like a controlled oxidation operation, the atmospherically pressurized gaseous compounds chemically alter a finite percentage of a workpiece’s surface casing.
The Demand for Controllable Metallurgical Outcomes
Even compared to twenty years ago, heat treatment technology has advanced at an unprecedented rate. Vacuum furnaces are one result of this evolutionary jump, then there are the gas-pumped furnaces. These machines pump in carbon to apply a carburizing transformative finish, which improves wear performance. Nitrogen is another gaseous medium of interest. Nitrogen atmospheres augment the annealing process. Inert argon gas environments also act as an annealing improvement agent. Even carbon dioxide, thanks to an additional oxygen atom in its chemical makeup, has become a popular supplementary furnace atmosphere. Instead of air, which only contains a small percentage of oxygen, CO2 packs a stronger oxygenated punch, so it has become something of an oxidization gas standard.
The above passages of text have barely scraped the surface of what’s possible. Modern heat treatment facilities are now working with a whole palette of different gaseous compounds. Hydrogen gas, which obviously isn’t inert, performs as a reducing agent. It purifies iron and copper oxides. Like carbon monoxide and carbon dioxide, hydrocarbons are also in use here, usually as carbon-rich compounds that divulge their chemical loads at contrasting alloy treatment temperatures. However, and this point is of critical importance, such refined chemical reactions can become corrupted. It’s, therefore, best to use clean gasses, free-flowing atmospheres that have been entirely divested of pollutants, especially water.