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Get It Plated Right
This fact sheet series is produced
by the Minnesota Association of Metal Finishers &
Minnesota Technical Assistance Program for metal fabricators
and their platers.
High-Temperature Processing Burns
on Soils
Fact Sheet #5
Oils, coolants and many organic liquids
form a varnish or paint-like coating on part surfaces
when heated or aged. These coatings can form at room
temperature over a period of days or weeks. They form
much faster as temperatures are elevated in heat treating,
buffing, drawing and some drying operations. In extreme
cases, a tough carbon-based smut is formed on surfaces.
Removing these coatings requires chemical or mechanical
stripping rather than a milder standard cleaning operation.
Faster Processing Degrades the Surface
True Story
A medium-sized metal finisher plates
a large quantity of heat-treated parts every day. Most
jobs plate relatively trouble free but some are a problem
every time they are processed. One particular job started
easy and then became difficult.
Shafts were machined, carburized (57-62
Hc with a case depth of .015-.025 inches), finish ground,
and then plated with electroless nickel. This job ran
trouble free for the first six months. Then, black particles
appeared on part surfaces that had not been finish ground,
even after a strong alkaline bath cleaning was done
according to the developed process. Parts were salvaged
by polishing the areas which had not been ground before
cleaning. This cost the customer an extra $0.75 per
shaft.
Upon investigation, the only process
change was the manufacturer had moved the carburizing
to an outside heat treater. After working with three
different heat treaters to resolve the surface particulate
problem, the manufacturer brought the heat treating
back in house. The black particulate did not recur.
Problem Sources and Solutions
The problem encountered was probably
due to:
1) machine fluids remaining on parts
while in the oven, 2) the aging of these fluids before
outside processing, and 3) perhaps in part, the conditions
used by the commercial heat treater.
High-temperature processes can bake
soils onto surfaces, forming a tough organic film or
an equally tough inorganic smut or scale. These processes
can include abrasive finishing processes like buffing,
polishing and some types of fine, dry sanding or grinding;
oven processes like heat treating or stress relief;
and joining processes like welding, brazing and soldering.
Buffing
& Polishing - Solutions
Avoid overheating to prevent metal discoloring,
distorting and warping.
- Control the feed rate and pressure
used to buff and polish.
- Lubricate wheels and operate to
avoid over heating. This is especially important when
using synthetic fiber buffing media which can melt
onto parts.
- Use an abrasive belt rather than
a wheel to keep surface temperatures down by spreading
work over a larger surface area.
Use liquid and waterbased
buffing compounds
which tend to be easier to remove and less susceptible
to burning on than pastes and solids or oil-based compounds.
Consider electropolishing as
an alternative to buffing before plating stainless steel.
Electropolishing does not add surface contaminants,
improving the bond of subsequent plated deposits. In
some cases, electropolishing can also eliminate a deburring
step.
Heat Treating - Solutions
Clean parts
before heat treating.
Filter quench baths to keep
them clean.
Use inert atmospheres or a vacuum
for parts made of tool steel or stainless steel to avoid
forming oxides and scales.
Remove scale before plating
when inert atmospheres cannot be used. Scale removal
involves at least one extra step and can damage parts.
Abrasive removal of scale generally roughens surfaces.
Chemical scale removal (acid pickling) tends to change
dimensions and surface properties (removes carbon),
and can cause hydrogen embrittlement.*
Maintain consistent
batch characteristics.
- Pack ovens to assure even treatment.
- Use consistent treatment times
and procedures.
Use the lowest temperature
for adequate heat treating. Higher temperatures can
increase production rates but also create soils that
are more difficult to remove.
Avoid polyvinyl alcohol (PVA)
quench baths on parts-to-be-plated to prevent coating
them with a resin that is hard to remove.
Welding, Brazing
and Flame Processing - Solutions
Clean parts before joining. Good cleaning
prevents burning on organic soils that can interfere
with plating. It also yields higher quality welds by
either removing or preventing smut and scale layers
that interfere with metal to metal bonding.
Use the proper welding rod
for the alloy being joined.
Use the lowest temperature and
voltage for adequate bonding. Higher temperatures
can increase production rates, but they also create
soils that are more difficult to remove. Make sure operators,
especially on night shifts, run equipment according
to design specifications rather than pushing the process
during part of the shift to generate slack time later.
Use an inert shield gas where
possible to avoid forming oxides. Shield gas should
be at least 97% inert for stainless steel brazing.
Avoid the over-application of brazing
fluxes and use acid flux where possible to avoid
burning on rosin.
Avoid brazing steels with copper-zinc
filler. Copper-based fillers will stain steels during
plating.
Avoid tin-lead soldering of
parts to be plated. Lead alloys require special plating
processes to give adequate bonding of the plated coating.
It also contaminates process baths.
Plating and metal finishing are extra
steps which enhance the decorative or mechanical properties
of surfaces. If the end-use of parts justifies the extra
metal finishing, care during part fabricating is also
warranted.
| * |
Hydrogen embrittlement can be
corrected or eliminated by treating parts within
one hour of acid pickling/chemical descaling. Soak
parts in a neutral to alkaline bath or bake in an
oven according to ASTM or military specifications. |
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