fuel filter fittings (long)

[Joe Boucher]

Eric Schumacher wrote:

[snip]

> 
> I have a question regarding using these techniques on brake lines. Perhaps a
> metallurgist out there can offer some insight. I worry, just a little bit,
> that heating the steel brake lines to silver-soldering temperatures anneals
> them, making them subject to the kinds of problems you get into if copper
> lines are used in a hydraulic system. The copper work hardens and
> crystallizes. Insights requested.
> Lotsa Luck Eric
> 85 GTI with VR6 power

You're braver than I am.  Having any cut and repair in a brake line
would make me nervous but appears to be working for you.  I don't think
this would rupture and should give you some warning by leaking.

The purpose of annealing is to relieve stress induced into metal from
cold working or deformation.  By heating the metal the crystal structure
is allowed to reform and relax so to speak.  Tempering is a reheating
process to relieve stress induced into the metal from rapid cooling to
obtain heat treatments.  Annealing is done at a much higher
temmperature, usually into the red glowing stages.  I doubt your getting
it that hot.

Both processes lower the strength of the metal, but improve the
toughness, especially tempering.  Brake line steel is not heat treated
and the temperature is not high enough to do any real changes.  Burning
off any corrosion resistant chemicals is another story.

What you're really worried about is vibration induced failure called
fatique.  If the stress from a vibrating load is below a level called
the endurance limit, then steel will never fail from that load. 
Aluminum and copper do not have an endurance limit and will eventually
fail from fatique.  May take awhile but it will.  A steel piece at the
same stress will last much longer.  This is why copper can't take hyd
loads.  It isn't very strong and therefore fatiques quickly.

Which brings up a pet peave of mine.  Metals are crystaline structures. 
Metals don't crystalize.  They're already there.  When you look at a
fatique broken part, you're looking at the side of billions and billions
(where's Carl?) of tiny crystals all jammed together.