 Today, I wanted to share 3 important thoughts on Gray Iron
Castings that could help in a number of situations. These topics
include: Gray Iron’s Shrinkage, Gray Iron Stress-Related
Problems, and Different Uses of Gray Iron Castings. Shrinkage
and Stress consideration must be regarded by all engineers no
matter the alloy, but I wanted to give insight on what makes
Gray Iron so special!
Shrinking of Gray Iron Stands Tall
One aspect of Designing with Gray Iron Castings that is often an
after-thought is the use of Shrinkage while designing. Compared
to Designing with Ductile Iron or Steel for castings, Gray Iron
(or Cast Iron) can be much simpler, easier to work with. This is
due to:
Solidification Shrinkage Rate is at a minimum comparably to DI
or Steel
Since Gray Iron does not require as much heat to achieve melting
point, therefore it is easier to work with and has greater
Fluidity in the mold
With a few exceptions such as very large castings or unusual
wall thickness changes, far less concern is given to the problem
of feeding metal to heavier sections
So what does this mean? The low shrinkage characteristics
contribute to larger freedom from hot tears encountered with
some of the other foundry metals, such as Ductile Iron or Steel.
This allows for GREATER FREEDON TO DESIGN of complexity and
thickness/size of features in comparison.
Stressing Gray Iron Castings
Ever have stress problems with your Gray Iron Casting? This is a
common problem in almost all sand casting alloys due to a lack
of experience of designing sand castings. Commonly, a casting
will come from a fabrication where the designer has been unduly
influenced by the characteristics of flat plates and other
wrought shapes. The use of these surfaces and shapes will be
indicated by a lack of tapered sections, long radius fillets,
and variable thickness sections which are easily obtained in a
casting. Instead of a clean design, the casting is a
conglomeration of plates, ribs, bosses, and small radii. All of
these characteristics can lead to poor castings that crack under
stress.
Why?
The low level of elongation value for Gray Iron Sand Casting
causes large amounts of stress on the poorly designed casting.
How is can Stress be measured?
The only satisfactory method of determining stress levels in a
casting under load is through the use of SR-4 strain gages.
What happens next?
Without proper stress analysis, the first tendency is to "beef"
up the section in which failure has occurred. This approach does
not result in the best design and often makes the condition
worse. It is important to properly design castings with the
proper wall thickness, radii and fillets.
Well, Who Else Uses Gray Iron Castings?
Gray Iron Castings are used in both a plethora of different
industries and purposes. For instance, Gray Iron also has an
excellent track record performance in internal combustion
engines.
Another common place Gray Iron Castings will be used is in
applications involving sliding surfaces such as machine tool
ways, cylinder bores, and piston rings. Many explanations have
been given for this behavior, such as the lubricating effect of
the graphite flakes and retention of oil in the graphite areas.
This is very likely true, but it is also possible that the
graphite flakes allow some minor accommodation of the pearlite
matrix at areas of contact between mating surfaces. It is seldom
possible to obtain perfect fits, and, ordinarily, high spots in
mating metal surfaces may result in high unit pressures causing
seizing.
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