re:chrome plating

["Reid, Warren" <Warren.Reid@xxxxxxxxxxxxxxxxxxx>]

Reading the replies from the last digest there still seemed to be some doubt
as to the reasons to be careful when chrome plating various components,
especially those subject to cyclic (or any) stress.

The chrome plating process generates Hydrogen which finds its way into the
interstitional sites within the steel solid solution. If left like this the
Hydrogen can migrate to the grain boundaries (amongst other things), thus
considerately weakening them and allowing cracks to grow under applied
stresses. To overcome this the component needs to be heat treated at about
150 - 200 C for a few hours to allow the hydrogen to diffuse out of the
solid solution.

The same effect occurs when welding alloy steels with the wrong electrodes
or electrodes with high water content in the flux. The hydrogen diffuses
into the base plate and embrittlement occurs via cold cracking as the
component cools.

You can also get hydrogen embrittlement in other materials such as Titanium,
Vanadium and Zirconium through the formation of metal hydrides which are
very hard, but have little to no fracture toughness.

Warren Reid, BSc (Hons), M.App.Sc (Physical Metallurgy)
Aeronautical and Maritime Research Laboratory
Melbourne, Australia

86 Spider Veloce (de-smogged - Yippee!)
68 1750 GTV
84 Alfetta GCL
58 and 61 Bugeyes

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