Metal Quenching

Hot metal parts (gears, plates, slabs, billets, etc) are quenched using air, water, oil, or liquid polymers to obtain certain hardness and mechanical properties requirements. Quenching is generally done by cooling at a sufficiently high rate to avoid undesirable internal microstructure as well as to ensure uniform mechanical properties, minimize residual stresses, and avoid warpage. Quenching is a complex process and involves three distinct stages: formation of a vapor film around the parts, boiling phase involving nucleate boiling, and convection phase. The most critical of these stages is the vapor phase as a vapor blanket around the parts reduces the efficiency. This vapor blanket can be broken by sufficient agitation near the parts.

The main challenges face by the industry is to maintain a uniform quench rate together with a required agitation near all the surfaces of all the parts. Non-uniform cooling would lead to residual stresses and then to warpage of the parts.

The quench rate is a function of the quenchant (liquid used for quenching) and degree of agitation. A simple flow modeling inside the quench tank including the parts quickly reveals non-uniform flow on certain parts, and hence non-uniform quenching. By changing the geometry used in the simulation, i.e. by modifying the layout of the tank uniform cooling can be obtained. Small adjustments or tests made in the virtual world of the simulation can then greatly improve the quality of the real by avoiding defect in the end products and improving their quality.

More advanced and detailed models could also include the heat transfer during the quenching. The thermal profiles can then be used to study residual stresses.

Flow Path lines inside a quench tank with (a) single impeller (b) two impellers [path lines starting from only one impeller shown for clarity].