The Great Silver Soldering Foul Up

Hi Andrew,

Silver soldering of steel takes longer. Steel requires more heat to reach brazing temperature than copper. It takes longer. Be patient! Watch the flux. When it goes into a colourless liquid you are at brazing temperature

The answer is to stop heat loss (brazing hearth/insulation blanket) and burn more gas. Fit a bigger burner. The 2941 burner generates 7.7kw heat at 2 bar. It might not burn enough gas to generate the heat required.

Or using the regulator, increase the gas presure or fit a 2942 burner.

Use a long life flux eg HT5 that accomodates the slower heating rate and matches the melting range of your solder. Fluxes have a very long shelf life but I suspect 35 years is pushing it a bit! Replace it.

Borax is fine if you are using brass as the filler metal. It does not start to work until the joint is at 750 deg C. Brass melts at 830 degC. Your silver solder melts at what 650 deg ? The alloy may well melt but it will not flow until the flux works.

A real concern is that if your alloy is 35 years old, then it probably contains cadmium. Getting the alloy to the temperatures required for borax will definitely give rise to the production of harmful metal fume. The alloy will also take on an "orange peel" appearance.

Overheating a joint is one of the most common reasons for poor results.

Do not use borax flux with cadmium bearing solders.

Getting the heating right is the skillful bit associated with a simple process! Get it right and it's simple. Get it wrong and it's frustrating (?)

For more info go to **LINK**

Keith

Keith: Thanks for the advice. I have read the notes on the Cup website. Having bought some HT5 flux and 455 grade silver solder I had another go over the weekend. Another complete failure. The flux did change, but the job never got even dull red and the silver solder didn't melt when touched on the job. So clearly the job wasn't hot enough. Another possible clue is that the job doesn't just get discoloured with thin oxide layers as one would expect. Instead it seems to 'grow' a thick pitted layer, mostly where the flux was, that makes it look like it has been rusting in a field for decades. It comes off when pickled, but looks 'orrid, and clearly isn't going to help the silver solder flow:

To hell with the workshop, I'm going to take up knitting instead.

Andrew