Hardening heat-treatment process is carried out on different types of metal alloys, especially steels. This treatment is fundamental to increase the hardness of the steel itself.
What are the hardening heat treatment phases?
To be considered hardenable steel must contain a good percentage of carbon and certain alloy elements (e.g. chromium, nickel, manganese etc..). 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, liquid that have the task of cool sharply steel. Depending on the type of steel (and the size of the pieces to temper) you must choose the most suitable extinguishing agent. For example:
- 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 steels undergo variations of their technological properties. What are these variations?
- 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 that will be included in the mesh by not having had the time to get out of it. So, limiting the scrolling of the crystals, it is precisely the carbon atoms to cause both the increase of steel hardness and tensile strength. What is the result of these phases? The result is the martensite that has a different structure compared to the initial one and a metastable polymorphic form of steel.
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