Q: Should I be concerned
whether my valve is cast or
A: Just as in politics, everyone has an
opinion on this issue.
The good news is that both types of
valves should be able to provide you
with acceptable performance, although
a perception exists that forged valves
are superior to cast valves. However, if
cast valves are made properly, they can
and have worked equally well in a variety of services and usually at a much
lower cost than forged valves. Also, the
belief that forged components are infallible is not true. Let’s look at an example of a 4-inch diameter wrought valve
stem in N07718 (Figure 1). A crack-like defect was seen on the end of the
stem, so the part was cut in half, and the
large shrink cavity you see here was discovered. This shows that forged material is not without its own problems. But
let’s examine how cast and forged
valves are made and how we can assure
we get a good valve.
What most people don’t realize is
that cast and forged valves start out the
same way—molten metal is poured into
a mold or ingot. As a result, both types
can have defects such as the shrinkage
in the above mentioned N07718 bar.
Other defects associated with forgings
are inclusions, laps, seams, cold shuts
and cracks. Defects with common castings are inclusions, porosity, misrun and
hot tears. As you can see, both have
their potential issues.
An issue with forgings often overlooked is that forgings and wrought
products will have non-uniform mechanical properties. This is because they are
worked or formed more in one direction
than in another. Therefore, the grains
will be elongated more in one direction
than in the other, which has a direct
Figure 1. A shrink cavity was found in this 4-inch
wrought valve stem.
affect on mechanical properties, particularly impact strength. As a result, the
design of forgings needs to take into
account these anisotropic properties
whereas castings have uniform properties no matter what the orientation of
the test coupons.
Another advantage of cast valves is
that they can be produced in more complex designs than forged valves. Certain
valve designs such as a globe valve are
simply difficult or impossible to produce
as forgings. This flexibility of design in
cast valves allows them to be more efficient in controlling flow than a similarly
Something else to consider with
forged valves is that they usually are
made in halves, particularly the larger
sizes. This means there is either an addi-
tional flanged connection that can be a
potential leak path or the halves are
welded together. Welding, however, is
another process for cast metal that can
have its own set of problems.
THOMAS SPENCE is director of materials engineering for Flowserve Corp. ( www.flowserve.com),
Dayton, OH. Reach him at email@example.com.