On Sun, 11 Aug 2013 13:21:24 -0700, wrote:

On Sun, 11 Aug 2013 13:57:46 -0400, Bob Engelhardt
wrote:

I don't know why everybody says to vee the crack. The depth of the vee
will be so much less than the frame thickness that it can't be
significant. And the braze will wick into the crack without needing a
funnel.

The link Ed posted in the "Joining Stainless Wire" thread about silver
brazing (http://www.aws.org/wj/amwelder/9-00/fundamentals.html) had a
chart showing joint strength versus joint width. The maximum was for a
joint .0015 thick!! That thickness joint had a much higher tensile
strength than the silver itself (3X). Joint strength fell off
dramatically with increasing thickness. Thus a vee will be much weaker
than a close joint.

Bob
Greetings Bob,
It is very hard to get brazing to wick into cast iron. Even silver
brazing alloys. I thick it is because of all the free carbon
(graphite) present at the surface. I have brazed a lot of cast iron
and it is always a bit of a chore to get the brazing alloy to wet the
cast iron. Brazing rod typically has at least as high tensile strength
as cast iron, and typically more, in thick sections.
Eric

Wetting is a big issue, and graphite flakes or smeared graphite on the
surface is a major cause of trouble.

But it's only one issue of several, when it comes to brazing cast
iron. Here's an article from the British journal _Welding and Metal
Fabrication_ from 1980 that I remember using as a source for an
article I wrote a year or two later:

http://tinyurl.com/k8fzynj

It covers it well, although it doesn't address repairs. You probably
could find an alloy available in the US that has the same properties
as the one discussed in the article.

A few notes: This casting is almost certainly gray iron. Where the
article discusses nodular iron, that's what we mostly call "ductile
iron" in the US.

And, finally, I think this particular job is a waste of time and
money, as others have commented. g It could be a worthwhile learning
experiment, however. But don't count on finding any brazing alloy or
flux that could allow the braze to wick into that crack without
opening it up first.

--
Ed Huntress