Should Gray Iron Castings be Stress Relieved? – Sandmark Global
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Should Gray Iron Castings be Stress
Relieved? – Sandmark Global
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Some suppliers do not stress relieve gray iron castings, and some do. As a rule,
SANDMARK 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, etc.).
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 relieving. This is especially important if
machining operations are required post-casting, with sub-thousands (of an inch)
tolernaces in relational dimensions between features.
It is not inconcievable 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
SANDMARK for our gray iron castings (24 hours full cycle, with about 6-8 hours of
soak at approximately 550 C).
12-12-2019, 11:54
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Typical Profile for Stress Relieving Certain Gray Iron Castings at
SANDMARK
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.
Reference:
Gray Iron – A Unique Material
Stress Relief of Gray Cast Iron, Naval Research Laboratory
12-12-2019, 11:54