Metallurgical Failure Analysis
- Forensic Engineering
Metallurgical Technologies,
Inc. routinely performs failure analysis services
for the top NASCAR racing teams. Race teams count
on MTi's failure analysis to help determine causes
of failure so the problem can be corrected before
the next race. We also test new components before
they are installed to ensure that the best possible
component available is used.
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will finish up front. MTi's expert metallurgists
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Failure Analysis of a Pitted Automotive Racing
Valve |
| A titanium exhaust valve
that exhibited unusual pitting in the molybdenum-coated
stem was analyzed to characterize the condition
of the valve. An arrow points to a pitted area on
the stem. The transition edge of the molybdenum
coating was approximately 1.5-in. from the head
end of the valve. Most valves previously examined
exhibited transitions approximately 1.75-in. or
greater from the head end of the valve. A companion
exhaust valve from the same engine failed; however,
the failed component was destroyed and was not available
for failure analysis. |
 |
| |
A closer view of the pitted region
near the end of the molybdenum coating is presented
in this figure. Pitting is most pronounced at the
molybdenum coating transition area and is progressively
less severe towards the keeper end. The pitted area
extends for approximately 0.5-in. from the edge
of the coating. |
| The pitted region was removed from
the valve and prepared for high magnification examination
using a Scanning Electron Microscope (SEM).
The pitted surface is detailed in this SEM image.
The pits are spalls in the molybdenum coating.
(SEM Photo, Mag: 10X) |
 |
| |
Several pits are presented at a
higher magnification in this SEM image. Arrows point
to transverse cracks noted in the bottom of some
of the pits. (SEM Photo, Mag: 100X) |
| A longitudinal cross-section was
taken through some of the pits and prepared for
metallographic examination. One of the pits in the
molybdenum coating cross-section is presented in
this photomicrograph using an optical microscope.
The pit is a spall in the molybdenum coating. A
layer of corrosion product is found in the spall.
Corrosion has penetrated into the titanium stem
to a depth of approximately 0.001-in. Thermal fatigue
cracks are noted under the corrosion layer in the
pit. A thin white alpha case is noted under the
corrosion layer and around the cracks. Alpha case
is the formation of a layer of oxygen-rich brittle
alpha phase. It is a detrimental condition in titanium,
resulting from high temperatures in an oxygen-rich
atmosphere. (Mag: 250X) |
 |
| |
This image displays the transition
edge of the molybdenum coating. The coating has
spalled and an oxidation corrosion layer has filled
in the spall. A shallow crack is noted under the
corrosion layer. (Mag: 400X) |
| CONCLUSIONS:
Failure
analysis reveals that pits in the valve stem are
spalled areas of the molybdenum coating. Oxidation
corrosion of the titanium base metal was found
at the spall locations. A thin layer of alpha
case was noted under the oxidation layer. Thermal
fatigue cracks and alpha case formation were observed
at many of the spalled and oxidized locations.
These features indicate unusually high temperatures
and increased oxidation potential in the environment.
The molybdenum coating is farther towards the
head end than found on previously examined exhaust
valves. Differences in thermal expansion characteristics
are more pronounced at the higher temperatures
near the head of the valve and lead to thermal
fatigue cracking.
Forensic examination of a failed component can
prevent recurring failures, help with preventive
detection, and suggest remedies. If you have a
component failure that requires a failure analysis,
please take a moment to fill in our information
request form and we'll gladly give you an
estimate. |
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