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There should be no quenching of light alloy castings into water following solution treatment. (Boiling water is also not permitted, but polymer quenchant or forced air quench may be acceptable if casting stress is shown to be negligible.)
Following high-temperature heat treatment, if the castings are
quenched, the cooling of the outer sections of castings may be too fast to allow time for heat to diffuse out from interior sections. In this case the internal sections will cool and contract after the outer parts of the casting have cooled to form a rigid, unyielding frame. Thus the interior sections go into tension. It is a fact, therefore, that quenching into water causes high residual stresses in large and complex castings. The stresses will be tensile in some regions, mainly in the centre of the casting volume, and compressive in others, mainly the outer walls. The use of a boiling water quench has been demonstrated to be of insignificant assistance in reducing the stresses introduced by water quenching. Furthermore, the stresses are not significantly reduced by the subsequent ageing treatment.
Immediately following the quench, the residual stress in aluminium
alloy castings solution treated and quenched into water is well above the yield point of the alloy. Even after the strengthening during the ageing treatment, the stress remains usually around 50 ± 20% of the yield stress. Thus the useful strength of the alloy is reduced from its unstressed state of 100%, down to 70, 50 or even 30%. This massive loss of effective strength makes it inevitable that residual tensile stresses are a significant cause of casting failure in service.
Many national standards for heat treatment specify water quenching.
This situation needs to be remedied. In the meantime such national standards are recommended to be avoided.
The reduced mechanical strength when using polymer or forced air
quenching is more than compensated by the benefit of increased reliability from putting unstressed castings into service. Thus somewhat reduced mechanical strength requirements should be specified by the casting designer and/or customer. The reductions are expected to be in the range 5-10% for strength and hardness. Although the strength of the material will therefore be lowered by the slower quench, the strength of the casting (acting as a whole,
as a l
component in service) will effectively be increased.
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