may be compromised (heated above 400°F).
- Cadmium embrittlement
may occur (heated above 450°F with cadmium plating present).
- Chromate conversion
coating may be degraded (heated above 400°F).
- Organic coatings or
sealants may crack or become brittle or discolored (wide range
These situations often
occur when components are
- Inadvertently overheated
in an oven.
- Exposed to elevated
temperatures with some finishes intact or bushings installed.
- Exposed to fire.
Residual cadmium often
is left on a part during overhaul processing to protect it from
corrosion. The part is then stripped of all cadmium and replated
near the end of overhaul. Parts with residual cadmium should
not be heated over 400°F during overhaul.
Bushings should not remain
installed during overhaul unless retained by specific CMM requirements.
Bushings must be removed to permit a thorough inspection of the
base metal and to avoid bushing-to-bore interface degradation during
bake cycles. Design finishes are restored and new bushings with
design interferences and dimensions are installed because bushing
wear limits do not apply during overhaul.
Wheel bearing fractures
or high-energy refused takeoffs often result in high local heat
on an axle. Discoloration of the enamel, primer, or chrome or evidence
of cadmium damage on the inner diameter of the axle may require
the heat-damaged component be removed from service.
Overheating affects components
to various degrees; in some instances, only finish durability is
degraded. This may result in a shorter than planned time between
component overhauls. Contact Boeing for assistance with questions
about repairing or salvaging high-strength alloy steel components
that appear to have been damaged by overheating.
Hydrogen embrittlement occurs when a high-strength alloy steel component
absorbs hydrogen, which is not removed in a timely manner in accordance
with the SOPM (e.g., embrittlement relief baking).
When hydrogen remains
in a component for an extended time, the microstructural damage
that develops significantly degrades the mechanical properties of
the steel. The infused hydrogen migrates to areas of high stress
(e.g., material internal stresses) and creates local microstructural
damage. When the component is installed on an airplane, this internal
damage can lead to crack initiation and propagation, resulting in
The elevated temperatures
reached during hydrogen embrittlement relief baking, which is performed
directly after stripping or plating operations during overhaul,
effectively remove hydrogen generated during these operations. Processes
that must be followed with relief baking include chrome, sulfamate-nickel,
and LHE cadmium plating; stripping operations; and many nital etch
inspections. After hydrogen-generating operations, relief bake delay
time limits must be observed to ensure complete hydrogen removal.
In general, the best practice is to initiate baking as soon as possible
following a plating operation.
The delay time between
plating completion and baking start typically is observed. However,
when thick plating deposits or multiple plating operations are performed
on a single component, the total time between initial plating start
and baking start is a key factor when determining the maximum delay
time allowed. For example, embrittlement relief baking must begin
10 hr after sulfamate-nickel plating is completed or within 24 hr
after plating begins, whichever results in the shortest overall
16 shows a flap track that cracked because of hydrogen embrittlement
149 flight cycles after overhaul. Figure
17 is a scanning electron microscope view of a typical hydrogen
embrittlement crack where separation occurs along grain boundaries.
Typically, hydrogen embrittlement cracks propagate rapidly once
loads are applied to the part. In some cases, internal residual
stresses are sufficiently high to cause cracking even before the
part is installed.
Overheating LHE cadmium or cadmium-titanium plated components causes
embrittlement of high-strength alloy steel by cadmium, resulting
in cadmium diffusion into the steel grain boundaries. Solid-metal
embrittlement by cadmium can occur at temperatures below the cadmium
melting point. These effects on the base metal can begin to occur
at 450°F, whereas the cadmium melting point is generally 610°F.
The microstructural anomalies resulting from cadmium embrittlement
can lead to component fractures in service.
Determining whether cadmium
has migrated into the grain boundaries of cadmium-plated, high-strength
alloy steel components requires destructive testing of the components.
If these components have been overheated, salvage may not be possible.
However, if high-temperature exposure was short and discoloration
of the enamel or primer was minimal, the component may be a candidate
for salvage. Slight or no discoloration of the enamel or primer
may indicate the cadmium plating was not heated to the extent that
cadmium embrittlement would be suspected. Boeing can assist in this
Improper application of protective finishes during manufacture or
overhaul can lead to finish degradation, corrosion, and corrosion
pitting, which can result in component fracture while in service
(figs. 2 and 3.)
Some cleaners and chemicals may accelerate finish degradation and
lead to corrosion. Operators should ensure that cleaners and chemicals
are tested before use in accordance with Boeing document D6 17487,
Evaluation of Airplane Maintenance Materials. Testing to these requirements
will determine whether a cleaner or chemical is detrimental to protective
finishes or base metal. However, long-term exposure to the solution
or material still may adversely affect finishes.
Personnel must ensure
that materials used for activities such as cleaning and deicing
conform to Boeing document D6-17487 requirements and will accomplish
the intended task (verified by the material provider or operator).
Refer to the Aircraft Maintenance Manual for materials specified
for aircraft cleaning and deicing. The CMM specifies the materials
for use in repair.
High-strength alloy steel
components should be stripped completely during overhaul (including
removal of bushings and bearings in all structural components).
This allows a thorough inspection of the base metal (a primary component
overhaul requirement) and ensures that all finishes, including the
LHE cadmium plating and conversion coating, are restored to the
original design requirements. This is addressed in an all-model
Boeing service letter dated April 23, 2002, Overhaul of High
Strength Steel ComponentsCadmium Strip Required
(e.g., 757-SL-20-036-A, 767-SL-20-038-A, 747-SL-20-062-A).
Restoration of the shot-peened
layer during overhaul is important to ensure that the shot-peen
compressive residual stresses are maintained or restored. Removing
or damaging the shot-peened layer can reduce the protection that
this compressive layer provides against fatigue and stress corrosion
crack initiation. Discontinuous shot-peening can lead to crack initiation
at the tensile surface stresses adjacent to edges of abrupt compressive
layer runouts (no fadeout). All reworked surfaces must be shot-peened
after removing material damaged by corrosion, heat, and deformation.
As a rule, if material
removal exceeds 0.0015 in (or 10 percent of the Almen strip intensity),
the surface should then be shot-peened to CMM requirements. Exceeding
shot-peen requirements is better than leaving areas without shotpeening.
All portions of a component that are to be shot-peened should first
be completely stripped; no cadmium residue should remain on the