ume of the castings. No through-wall
cracks or leak paths were found. LP
testing after grinding confirmed that
all defects in the castings had been
removed.
Twelve castings were subjected to
acoustic emission (AE) testing, which is
a non-destructive technique in which
mechanical loading on a structure
releases energy waves. This energy
release involves crack propagation. An
appropriate detection system is used to
“listen” for the events. The test method
used parallels the method described in
ASME Section V, Article 12. 18.
The 12 castings were AE-tested at
increasing pressures to 1,125 psig ( 7. 8
MPa). These castings were also subjected to a helium leak test. The tests did
not show any leak paths. LP was performed, which found multiple surface
indications in the 1/4 to 1 inch ( 6. 4 to
25 millimeters) range. All of the indica-
Figure 7. In the two-feeder casting method (left), a high level of turbulence was created as the
metal flowed into the neck and foot. The three-feeder casting system resulted in tranquil and
low-velocity fill.
heal defects that are open to the surface, the part must be encapsulated.
Any surface exposed to air at elevated
temperatures either during the casting
process or heat treatment will be oxidized and cannot be healed.
One of the castings was destructively
tested, which did not reveal any internal
In the instance of high-nickel alloy
castings, which are increasingly mandated
by today’s process demands, it’s been
demonstrated that special foundry expertise
and practices are required for producing
quality castings.
tions were removed by grinding and
blending or by minor weld repair.
A destructive test was performed on
the valve body casting that had the
greatest number of indications and the
highest AE cumulative energy. The casting was sectioned and mechanically
deformed. The resulting defects were all
on the surface and could be removed by
grinding and blending or by minor weld
repair.
The final six new castings were sub-
jected to hot isostatic pressing (HIP)
processing. (HIP is widely used in the
aerospace industry to heal internal voids
and shrinkage defects.) The castings
were heated to 2,170° F ( 1,190° C) in
15,000 psig (103 MPa) argon. The
combination of high pressure and high
temperature produces creep and diffu-
sion bonding of defects that are not open
to the surface. Conversely, for HIP to
defects. However, there is no way to
know if any defects were healed by the
HIP process. (Note that re-solution heat
treatment generally would be required
before placing a HIP-processed casting
into service.)
THE RESULT
The through-wall leak path in the neck
area was due to shrinkage and thermal
stresses caused by the two feed path rigging system. The surface indications
were caused by interdendritic cracking
formed during the solidification process
or during the heating portion of solution
heat treatment.
Casting simulation provided a solution that reduced filling turbulence and
promoted directional solidification from
the body out towards the foot. This minimized the highly constrained situation
where the foot solidified first.
AE testing requires considerable
investment in equipment and training. It
can only be performed on finish
machined castings, which limits its
application to the final stages of the
manufacturing process. AE testing did
not provide a conclusive measure for
casting acceptance or rejection in a
timely manner.
The HIP process did not produce verifiable improvement in the castings.
In the instance of high-nickel alloy
castings, which are increasingly mandated by today’s process demands, it’s
been demonstrated that special foundry
expertise and practices are required for
producing quality castings.
Recent collaborative work by materials engineers, metallurgists and foundry
experts identified several foundry best
practices specific to high-nickel alloy
castings. Many key variables such as
raw material sourcing, the sequence of
additions during melting, deoxidation,
pattern and mold preparation (rigging,
mold wash, core making, etc.), cleaning,
heat treatment and welding must be
monitored closely to develop a process
that produces high-quality castings.
Additional requirements are necessary beyond those imposed by the ASTM
casting specifications A743, A744,
A494 and A990. The valve supplier,
foundry and end user must work closely
together to ensure the process requirements are understood and met. VM
JAMES L. GOSSETT is principal materials engineer,
Advanced Technology Group, Emerson Process
Management–Fisher. Reach him at
jim.gossett@Emerson.com.
