There seems to be dozens of products for the welder to use when brazing
cast iron.  In 10 minutes, I found dozens.  What I have discovered is
that both the flame selected, as well as the rod chosen, is important
for a secure bond.

Basically, the type of rod used by one company, at least, is called a
non-fuming bronze.  One manufacturer's product number is #529 general
purpose torch rod.  It has a 65000 lb/sq.in strength.  There are also
other alloys, however.

The reason braze seems to be recommended for grey cast iron and cast
steel where high strength is concerned is because the temperatures
required are much lower than arc welding, and will not oxidize or
otherwise degrade the amorphous structure of the casting as long as
other proper techniques are observed, and an oxidizing flame is not
used.  They recommend a "clear flame with orange tips."  It creates
a superior bond by flowing into the mechanical structure, as well
as creating a metallurgical/chemical bond.  Its tensile strength
(stretching) is over 30 tons/sq.in.

One of the many sites suggesting this use of brazing rod is
http://www.usalloysweldtech.com/copper.htm

Years ago, broken cast iron manhole covers were always repaired with
braze, but engine blocks were always arc welded.  I don't doubt Larry's
experience with engine blocks at all, since that's how I remember it,
too.  But, I also see now that knowledgeable welders even today seem
to know the difference when it's primarily strength-related.

Here is a letter just received from a welding professional:

  Dear Mr. Brougher

  Cast iron can be brazed with more strength than welding.  I alloy
  I recommend using is called Welco 17.  This is a nickel silver
  alloy.  Use this product with Welco 17 flux.  Welco 17 has a
  tensile strength of up to 95,000 psi.  The strongest weld rod
  would give a tensile strength up to 72,000 psi.  I am attaching a
  product specification sheet for you on the Welco 17.  If you have
  any questions, please contact me.

  Thank you,
  Jef Heuerman
  Technical Representative
  jheuerman@jwharris.com
  800-733-5956 x-610

Craig Brougher