Hardening heat-treatment process is carried out on different types of metal alloys, especially steel. This treatment is fundamental to increase the hardness of the steel itself.
What are the hardening heat treatment phases?
To be considered hardenable, the steel must contain a good percentage of carbon and certain alloy elements, such as chromium, nickel and manganese. If the carbon content is not enough it will be necessary to enrich the workpiece surface with carbon through carburizing, nitriding or carbonitriding.
So, what is the hardening heat-treatment process? This is a sudden cooling of the metal, brought in earlier to high temperatures. It’s the cooling rate to give the steel a high mechanical resistance. Hardening heat-treatment, therefore, is the result of 3 different phases:
- reheating of steel
- maintenance of an austenilic temperature
- rapid cooling (in order to obtain the martensite at room temperature).
This last phase is done by tempering medium, a liquid which immeditaely cools the steel. Depending on the type of steel (and the size of the pieces to temper), there are different extinguishing agents, such as
- oil (pay attention to use non-flammable mineral oils)
- water (with the addition of dissolved salts)
- forced or calm air
- polymer (water with additives).
Steel subjected to hardening heat-treatment process. Many changes in technological properties.
After hardening, the steel undergoes variations of their technological properties.
- hardness increase depending on the degree of steel carburization
- tensile strength increase (about 50%) and the consequent increase in Rm
- elastic limit increase (about 100%) and the consequent increase of yield Rs.
The martensite
After hardening, the structure of the steel will contain an excess of carbon atoms, which will remain in the mesh. Therefore, limiting the scrolling of the crystals, the carbon atoms will cause the increase in steel hardness and tensile strength. What is the final result? The martensite will have a different structure compared to his initial one and a metastable polymorphic form of steel.
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