Carbon Steel Motor Shaft with Thread and Step

Hardening is followed by the quenching process. During this step, the heater material is cooled down quickly in a suitable quenching medium. Commonly used media are water, air, and oil. The quenching medium, the temperature, and the speed determine the target microstructure of the material, and its properties.
The maximum rate of cooling using mineral oil is 150-200 °C/s. The speed can be three times as high during the cooling using water.
For sub-eutectoid steels, the quenching temperature range is 30 °C to 50 °C above the AC3 temperature specified in the iron-carbon chart. For hypereutectoid steels, the temperature prior to quenching should be just above the AC1 given in the iron-carbon chart.
The thickness of the component (s) determines the dwell time (tH) in the quenching medium.

Tempering is used to remove the so-called glass hardness after the quenching step. The process can take place in different tempering stages. The first one is best performed immediately after quenching. The tempering temperature during this step is approx. 150 °C.
The needle martensite or the tetragonal martensite microstructure resulting from the hardening process is now transformed into a cubic martensite microstructure, with the precipitation of fine to ultra-fine carbides. The volume of the material decreases, and the grain lattice relaxes.
This prevents dislocations from sliding at high loads, and crack formation resulting from this. The secondary hardness maximum resulting from balancing hardness and toughness is attained.
Additional tempering stages can be carried out at temperatures between 200 °C and 350 °C to increase the hardness of the workpiece even further. When applied to high-alloy steels, a tempering stage above 500 °C can convert the iron carbide to more stable special carbides.