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 What Makes Gray Iron so Special?

 

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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