Rule
No. 9:
"No Quenching!"
Never quench light alloy castings into water (either hot
or cold, since both are extremely bad). The US National Specifications
that specify water quenching as part of the heat treatment
of aluminium alloy castings are therefore not (definitely
not) recommended. Such procedures can effectively reduce the
total strength of castings by half, and constitute a major
reason for casting failures in service. (Click
here to view diagram of a typical quenching-induced failure.)
Polymer quenchants or air quenching are much more reasonable
alternatives. The slight loss of strength (about 5 to 10 %)
as indicated by test bar results is more than compensated
by the avoidance of the 50 % or more loss that the casting
as a whole suffers when quenched into water. This is because
water quenched castings are effectively pre-loaded to approximately
50 % of their failure stress before being put into service.
Water quenching to give a planned residual stress can be
beneficial. However, this is not easy to arrange. It is normally
only possible in round components such as discs with radial
spokes like wheels and housings for turbine engines etc.
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