More Experienced Brazing Techniques

[meech151]

Just wanted to say thanks for all the advice. Working in opposing quadrants does make a big difference.

[MrkiMedo]

Nick- Then how do you explain the distortion left over after heating one side of a bar of steel? No brazing needed just the torch in one spot on one side. I might not know the underlying science but I do know what I've seen.

There is explanation in "Metals and how to weld them", I hope I will relay it correctly. If you heat one side of the bar 2 things are at work:

1) heated metal wants to expand
2) heated part has less strength than non heated side

As result while hot, that part goes under plastic deformation by compression - because it has less strength than non heated side and it is kept in the same lenght by the stronger cooler side. Now when the hot part cools it wants to contract and pulls the bar.

I do not remember who recommended "Metals and how to weld them" on this forum, but I would like to thank him. Great little book.

I would also add that this example explains why it is not good idea to braze the frame in the rigid jig. It would not let parts the expand, and they would as result undergo plastic deformation, and wrap when cooled down. It was completely counter intuitive for me until I read the explanation of heating the bar in the MAHTWT

[EricKeller]

that's an interesting theory, but it seems that "witch wanding" works at relatively low temperatures.

[MrkiMedo]

that's an interesting theory, but it seems that "witch wanding" works at relatively low temperatures.

Hmm, but why do you think this is not valid explanation for relatively low temperature?

A few data points they use:

- if 1" piece of low carbon steel is fully restricted from expanding longitudinally increase of only 200 F will cause it to be stressed beyond its yield point

- yield temperature of steel decreases with temperature

But maybe it is best that I do not try to transfer their words, it would be best to consult the book directly; the discussion of witch wanding is in chapter "welding low carbon steel" and in my ancient second edition it is on page 165