Some suppliers do not stress relieve gray iron castings, and
some do. As a rule, we stress relieves all our gray iron
castings. Common reasons for presence of stresses in castings
are: different cooling rates in different parts of the casting
after it comes out of the mold (due to complex geometry with
varying thicknesses, thin sections, surface/geometry curvatures,
Over time (periods of months to a year), castings left alone
will relax and stresses will dissipate. During this relaxation
period, the geometry and dimensions of the castings will ‘move’
gradually, and eventually stabilize. However, it is not
practical to cast and ‘season’ (although it used to be the
practice decades ago with large castings). Hence the need for
stress relieved. This is especially important if machining
operations are required post-casting, with sub-thousands (of an
inch) tolerances in relational dimensions between features.
It is not inconceivable that a casting with has not been stress
relieved can meet all dimensional requirements at the factory,
and over time (during transportation, sitting on the shelf), can
move out of specifications. Hence, stress relieving is a
necessary procedure for all castings that need to maintain
dimensional stability through their useful life.
While different options are available (thermal, vibratory,
shot-peen, etc.), the most common (and most effective) choice
for small to medium sized castings is thermal stress relieving.
The following is a typical stress relieving treatment employed
by our foundry for our gray iron castings (24 hours full cycle,
with about 6-8 hours of soak at approximately 550 C).
When to stress relieve:
Typically, it is best to stress relieve after rough machining
and just prior to final machining. Because of logistical and
other cost considerations, it may be tempting to stress relieve
raw castings and then perform machining operations with no
further treatment. However, machining operations can also
introduce varying degrees of stresses into the part. By stress
relieving immediately prior to final machining, both casting
stresses and machining stresses can be removed prior to final
machining, thereby the best possible outcome can be achieved.
For castings with demanding precision requirements, it may be
necessary to stress relieve twice, once after the casting comes
out of the mold and again after rough machining.
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