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 did like your hints for dealing with it,
and can definitely relate on the hydrochloric acid in the nose bit. Also to
be considered one of the "bad things." g

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

I first twigged to what may be happening when I grabbed some pistons
that had been sitting outside and plopped them into the part filled
crucible to get the volume up for a pour. they were damp.
I thought a furnace at many hundreds of degrees would dry the stuff
pretty well instantly.
the casting turned out to be like aluminium foam. first time it had
happened to me.
the possible explanation is that the water didnt evaporate but
dissolved into the molten aluminium.
what points me to this is another oddity.
you would think that copper with a melting point of 1500 degrees would
be difficult to incorporate in aluminium which is only at 360 degrees
or so but it isnt so. stir the mix with a copper tube or rod and the
rod will absorb readily into the molten aluminium.

I think that the same thing occurs with water believe it or not.
the fluid appears to dissociate into its component parts in the molten
aluminium.
the oxygen causes lots of oxide froth on the top of the crucible.
the hydrogen remains as a dissolved gas until the aluminium starts to
solidify whereupon it comes out of suspension as bubbles.

where does it come from? any source of moisture that gets to the
molten metal. wet or damp oxide coated stuff that you are recycling
has done it to me. personally I've never found it related to humidity
in the air but Mr Ammen mentions it.

the easiest way I've found of preventing it is to store the scrap
inside and keep it clean and dry. melting clean dry stuff has always
resulted in sound castings for me.

It is a pity the hydrogen bubbles formed on cooling couldnt be
controlled because the foam aluminium is quite light. you just cant
control where the bubbles form and thus the structural integrity.

Stealth Pilot