Unalloyed and alloyed heat-treatable steel is structural steel with a carbon content between a mass
fraction of 0.25% and 0.60%. It can be quenched and tempered (quenched and tempered = hardened) in a
temperature range of 806°F. It is very important to choose the correct hardening temperature and
quenching speed for the chosen material grade to reach the required hardness.
What is heat-treatable
When producing steel carbon has a very high importance. A high carbon content makes the steel harder but at the same time more brittle. A low carbon content results in a softer and more ductile steel.
Heat-treatable steels have a medium carbon content to keep a balance between hardness and ductility. This balance can be enhanced by quenching and tempering the material, as this treatment will give the steel a pre-determined hardness and ductility.
The delivery condition of some heat-treatable steels is annealed (for example: C45 Steel) while some come pre-hardened/PH (quenched and tempered/QT) (for example: 4140 PH/4142 PH).
Why is it called heat-treatable steel?The name heat-treatable steel originates from the quenching and tempering process of the steel. Quenching and tempering the steel, the so-called conversion hardening, has a direct impact to the balance of hardness and ductility.
The carbon content pre-determines and gives the steel its hardness.
The first step for quenching and tempering steel is the hardening process. By cooling the steel down quickly (quenching), after heating, the carbon will get “trapped” to build a very hard structure, also referred to as martensite. The following tempering process is the re-heating of the material to a lower temperature which reverts the structure back to a more ductile condition (Ferrite and Perlite).
After the tempering process heat-treatable steels have specific and pre-determined technical properties. This could be a wanted tensile strength combined with a good ductility for example which the grade would not have in its normal condition.
Where is heat-treatable steel used?Heat-treatable steel finds its use in components that need a higher strength. Possible applications for heat-treatable steel are axels, bolts, aircraft landing gear, crankshafts, connecting rods, screws, shafts, and other construction parts that need a higher strength.
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