Creating a metal casting is a highly detailed and intricate
process and has so many different variables that no two castings
are the same. When the variables are not controlled tightly
enough that is when the differences occur, usually in the form
of defects. Even castings produced from the same pattern have
their differences, those differences may be hard to spot but
they are there. The difference from casting to casting may be so
small that you can only spot the difference under a microscope,
making it crucial to control the variables as much as possible.
Defects are inevitable in the metal casting industry, the goal
is to keep the amount of defects to a minimum. With so many
different variables going into each casting, there is bound to
be at least one casting per production run that has some sort of
defect. Some defects are obvious and can be seen with just a
glance at the casting, such as the one pictured above. Other
defects may require a microscopic or chemical analysis to
detect, but both processes take up valuable time and resources.
The first diagnosis that should be performed on a casting is
fracturing... yes I said fracturing. Fracturing involves taking
that brand new casting and putting a saw to it. For smaller and
thin castings, the suspect area is cut to reveal any porosity
issues, grain size, oxides, and inclusions. Depending on the
application in which the casting will be used, further
inspection under a microscope may be required. For large
castings, fracturing may not be possible, so a similar process
is used. Sectioning is the process of cutting out multiple cross
sections of the defected or suspect area. Taking multiple cross
sections allows the inspector to create three-dimensional views
of the suspect area to expose any or all evidence.
If these tests do not produce any defects within the casting and
you still have your suspicions, further testing such as image
and chemical analysis may be performed. But fracturing should
still remain the first step in the casting defect and failure
analysis.
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