First time silver solder what am I doing wrong?

[MoreWellie]

I am trying to silver solder two thin steel tubes together, I made a short overlap and tried to silver solder the joint using silverflo 30 which started ok but then I got to a bit that where the flux had gone hard (blackish stuff) and the solder wouldn't wet the metal, just sat in balls on top

I had advice to flux the joint before I pushed the tubes together and wondered if the excess flux had caused the issue

 

[Bladevane]

Could be you have overheated the joint. You probably appreciate that cleanliness is next to godliness in silver soldering so I'm afraid you will have to clean the joint thoroughly and start again. Could you place a sliver of silver solder in or on the joint so that the heat from the metal melts the sliver rather than apply the solder in rod or stick form?

 

[8ob]

I would tin both bits before you start.

Bob

 

[MoreWellie]

You probably appreciate that cleanliness is next to godliness in silver soldering

I had cleaned everything back to bright shiny metal before starting

Could be you have overheated the joint.

Didn't realise it was possible to do that, I was told I would need it red hot before adding the solder so this is a fair bet

I would tin both bits before you start.

lol, if I was you I wouldn't start from here...

 

[Kram]

The flux is cooked. Either too hot or hot for too long. What flux are you using as SF30 needs 700-770 degrees, which is a little above the working temp for easyflo flux, it will be very difficult to get enough heat without too much.

Drop the part in a pot of freshly boiled water to remove the old flux. Dry it off, new flux - I wouldnt bother starting again, but be very careful with temperature and try to heat with the flame away from the joint.

Also silverflow 30 has poor flow, and the description says it is for thermal spray guns, and not for normal brazing filler. You may want to tin the parts seperately to ensure a good joint with this stuff, but you never need to with a good flow rod.

Silverflow 55 would solve the problems, flows nicely at 650c, just before its starting to glow.

Silver-flo-filler-metals - matthey.com

Silver-flo filler metals

matthey.com

SF 30 is near the bottom of page

 

[MoreWellie]

What flux are you using

easyflo flux which I bought with the rods as a kit 5 years ago and only just got around to using them

Obviously bought the wrong thing

 

[Kram]

Thats odd to be sold as a kit... It may be Silverflo 302, which is 30%. I would try again but be careful with the heat.

 

[JMS1972]

The usual problem with silver soldering is not enough heat. You need to heat the work up quite quickly or the flux will expire.

Your vice isn't going to help acting like a big heatsink I usually get the part surrounded by thermalite blocks which act as insulation and reflect a certain amount of heat back.

You don't say what size torch you used but do you have access to a bigger one?

If not some of the high temperature flux (red label forgotten its name) will give you a longer working time....

Good luck.

 

[optima21]

tenacity no5 flux is better for high temperatures than easyflow flux

 

[JMS1972]

tenacity no5 flux is better for high temperatures than easyflow flux

That's the stuff

 

[Kram]

T5 is commonly used on stainless and doesnt come off easily.

Borax can be used as flux from 740c so might be ideal. I have plenty if you want I can post a small bag. Commonly for jewelry.

I use fibreglass exhaust wrapping as an insulator for brazing. Good for holding stuff in a vice or between items its sitting on. Direct flame will melt it.

Zoro/cromwell sell SF55 1.5x 600mm rods for £9 with the discount. Expensive per rod, but I get a lot of joints out of it. For me, I spend less in gas and can do a lot more with a disposable MAPP, that would otherwise need a big propane cylinder and a much higher gas rate.

 

[optima21]

T5 is commonly used on stainless and doesnt come off easily.

it comes off easily with a suphuric acid pickle, although I think it mentions you should use sodium hydroxide in the instructions. I might try citric acid next time I do some silver soldering as its easier to obtain than sulphuric acid these days

 

[MoreWellie]

I can post a small bag

Thank you for the offer, I may come back to you on this but will try the silverflo 55 first as the rods I have are clearly marked 30 not 302

cromwell sell SF55 1.5x 600mm rods

I notice they also do flux coated rods, are they a good or bad idea?

 

[Kram]

I havent tried flux coated, I prefer to flux the part and dip warm rod in flux, but I dont think theres anything wrong with doing it that way. A pre coated rod is going to be a pain to store as I just keep them back of a shelf and dont want flux falling off. Cost almost double too.