Reasons Why Ferrous Metals Need Normalizing Heat Treatment29 January 2020
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.
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