The common process of heat treatment of ferrous metals include hardening, tempering, annealing, normalizing, and case hardening. In my previous article, I have explained The 3 Stages of Heat Treatment Process.
Process of Heat Treatment of ferrous metals
Hardening is done by subjecting a metal under a required temperature and then quickly cooled through a medium of quenching such as oil or water. Hardening is done in order to increase the hardness and strength of metal which is achieved by the rapid cooling. The metal becomes brittle after it is rapidly cooled.
metal is annealed when heated to a specific temperature and then allowing it to cool slowly back to room temperature. It helps in relieving internal stresses of metals, make them more ductile, and refine their grain structures. During the cooling process, the metal is covered under the arches created when heating or if heated with furnace, it is expected to shut it off leaving the metal inside allowing the furnace and metal to slowly cool together.
I believed you can clearly see the difference between the two processes, both are subjected to heat but their cooling process differs. In the case of hardening, it is done to increase the hardness of metals while annealing is done to soften metals.
Normalizing is done on ferrous metal that undergoes forging, machining, or welding to relieve the internal stresses and make it harder and stronger. It is done by subjecting the ferrous metals or alloys to heat at a specific temperature, soaking it until it is uniformly heated and then cooled in open air. This technique helps to provide uniformity in their grain size and composition. Temperature used in normalizing is higher than of hardening and annealing as it give maximum toughness to the metal and resistance to external stresses.
as steel properties are too brittle and very hard to make it applicable for most items, tempering is done to reduce the internal stresses and brittleness, and help to impact toughness to the steel. It is achieved by heating of steel below the lower critical temperature usually from 400 to 1105 degree Fahrenheit or 205 to 595 degree Celsius depending on the result needed and at higher temperature usually from 1,300 degree Fahrenheit or 700 degree Celsius depending on the alloy and application. The high temperature helps in increasing the level of ductility but loses some strength. In this heat treatment, the subjected heat is lower than of hardening and the cooling is done in still air. The rate of cooling determines if is to produce soft or hard metal. High speed steel is obtained from tampered heat treatment.
Case hardening is done in order to harden the surface of a metal and leaving the inside soft and tough. It is a diffusion process using a low carbon steel, usually straight carbon steel or low carbon alloy steel, exposing it to a specific heat temperature and adding an alloying element, usually nitrogen or carbon, leaving more of it to the metal surface. It is rapidly cooled giving the outer surface hard and inner part soft but tough.
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Naturalizing, tempering and case hardening seems to be a completely different process, most especially case hardening being a thermo-chemical process, adding alloying element to the surface during the heating process which brings about the two effects on the metal. And as for tempering, it uses a very high temperature of a specific amount in heating steel in order to reduce the brittleness and internal stresses. While, tempering heat treating can give either hard and soft to the metal due to the low heat applied and the rate of cooling.
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Question: Explain any one of the heat treating process.