Microstructures
by
George
Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA,
1966
Copyright©2005 by George Langford
Non Ferrous Alloys - Lesson 2 -
Fifth specimen
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"Season cracking" occurs in copper alloys containing more
than 15% zinc. It is a form of stress corrosion cracking (SCC)
and it is more likely, the higher the stress, whether applied in
service or residual stress from the prior deformation history.
This specimen is an alpha brass Bourdon tube, the active element in a
pressure gauge. The tube is slightly flattened and bent into an
arc so that the overall curve stretches out and moves a pointer when
the internal pressure on the tube is increased.
The first photomicrograph is at 100x ...
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... and the second is at 500X. Here you can see that
the tube was used in the cold worked condition, as there are
deformation markings and bent annealing twins.
The cracks started where the tensile stress was highest - at the minor
radius on the inside of the tube.
Therefore, this was probably a pressure gauge (not vacuum) and the
corrosive atmosphere was definitely on the inside of the tube.
Ammonia is a common culprit for SCC in brasses.
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