On Sun, 01 Feb 2009 09:13:53 -0600, Charles Vincent
wrote:

Stealth Pilot wrote:
On Tue, 27 Jan 2009 16:47:51 -0500, "Morgans"
wrote:

"Stealth Pilot" wrote

hydrogen embrittlement was a big bogey man in home castings but it is
easily understood and conquered.
OK, I understand hydrogen embrittlement is a "bad thing" but my question
is, where does it come from, start, or what do you do to prevent it from
happening in the first place.

I'm not an industrial chemist. this comes from watching what happens
in my castings and reading some of the references mentioned.


What you are describing is not Hydrogen Embrittlement. Hydrogen
Embrittlement is usually more of an issue for high carbon steels I
believe, though it effects aluminum as well. You can find information
on it under the heading of stress corrosion cracking and it is more of
an environmental issue than casting, as it is a problem for forgings and
weldments as well. It is the mechanism of failure I am pretty sure for
the old VW cases, as magnesium alloys are very susceptable to SCC.

Your description of the problem of the casting problem though is spot
on, if misnamed. Here is a good link on this and other alumminum
casting issues:

http://www.keytometals.com/Article83.htm

Charles

not a bad article.
as long as we understand what each other means the semantics isnt too
much of a problem.

hydrogen bubbles through a casting make it more brittle. they
significantly reduce the impact strength which is useful when you want
to smash it up for a retry at a melt. both I and a retired mechanical
engineer I know refer to this as hydrogen embrittlement (of the
casting) if industry has diverged off and use the term in a more
specialised manner good on 'em.

a hydrogen bubbled casting is seldom porous though.it is porous in the
sense that it has voids through it but they arent continuous. if it is
cup shaped you can fill one with water and it wont leak.
as long as we understand what we intend by the words then a little
error in semantics here or there is of no importance.

what I'm describing should be spot on because I'm describing exactly
what I've done. ...in my driveway.

my casting rejection rate would be about 1 in 20, if that.
Stealth Pilot