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Letter 1747
Chrome plating
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After chrome plating, how important is it to get a part into an
oven for embrittlement relief bake? If the time until baking extends
too long, will the hydrogen tie up and be unable to diffuse out of
the steel. Also, what time is sufficient to bake a high strength
steel that has just been chrome plated? What benefit does shot
peening provide during chrome plating? Does chrome plating induce a
stress on the substrate during plating and how large? What causes
hydrogen embrittlement?
Drake Clarke
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Hydrogen Embrittlement is caused by the Hydrogen evolved during
electroysis being, to some extent, absorbed into the crystal
structure of the steel at its' surface. Normally baking at 200 deg. C
will remove the atomic Hydrogen before it has had chance to migrate
and become molecular. Documented tests carried out by the Spring
Research Institute in the UK have suggested that if baking is carried
out within 24 hrs. of plating the Hydrogen can be removed. However it
appears that many specifications request that the treatment be
carried out within a much tighter timescale. I have heard evidence
that suggests that this may, in some cases, be due to the
desirability to ensure that the operation is carried out and not
forgotten!
If the baking operation is not carried out on High Strength Steels
then almost certainly it will be to late after the 24 hr. period and
the Hydrogen will become molecular, being tied up with an inability
to remove and provide a high degree of risk to failure.
The degree of baking of High Stregth Steel would be suficient at
24 hrs at not less than 200 deg. C. Consultation with UK Defence
Standard DEF STAN 03-4/2 would provide more specific info (sorry
don't have one to hand to quote at present).
sorry no details on the third and second last points.
Regards
Martyn Grimes
Overton Plating Company - Farnham, Surrey, England
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The present aircraft standards and
AMS-2406 [link is to spec at TechStreet] , average 3 hours
post plating bake at 190 centigrade . Dependent on the weight of the
part . Hard Chrome does have a tendency to increase crack propagation
sites in steel , due to the micro cracked structure of the coating .
There is also a drop in the fatigue strength of the material which
can be as much as fifty percent . Shotpeening will increase the
fatigue life of some steels as much as 200 percent . Normal treatment
of high strength steel parts would be : pre heat treat 190 c for 2
hrs after grind , shotpeen 12A2 , Chromium plate , heat treat 190 c
for 3 hrs , final grind , heat treat 1 hr at 190 c to relieve
grinding stress , magnetic particle inspect . As for the
embrittlement problem it is to be taken very seriously when dealing
with Chrome due to the high ammount of hydrogen evolved at the
cathode surface , in the process .
Dave Brackenridge
aerospace plating - Germiston, South Africa
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I am interested to know if there is any conclusive data to support
or oppose the use of sodium (alkaline) based cleaning solutions on
T22 and if there is potential for caustic embrittlement. Any data
appreciated.
SQUADZRT
Nederland, TX
Ed. note: Real name next time please, Squadzrt.
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Hello, This is not really relevant to the discussion, but I was
wondering if you could help me. I am doing some research for a
mini-project at uni on the chromium electroplating industry and I
need some information on plating baths : what they are, what
chemicals are used in them, how long the electroplating process
actually takes, etc...I have not been able to find out these basic
facts, although there is a lot of really technical stuff on the net.
Any help anyone can offer will be greatly appreciated. Thank you.
Bethany Mason Bangor, Gwynedd, UK
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Bethany,
Please see our 'must have'
booklist. On that page you will see a good selection of
electroplating introductions. Probably the Garden State AESF 'Basic
Practical Electroplating' would be the most responsive to your
situation. You are not going to find the info you seek on the web for
a few more years, and you'll probably never find it on the web for
free.
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Ted Mooney, P.E.
finishing.com
Brick, New Jersey
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