TIG brazing with silicone bronze, so much soot

[DennisCA]

So I was working on this part for my small emco planer/thicknesser and the thing was rusted in place, I tried tapping it with the hammer and whoops a cast iron part broke off an "ear"... Fortunately I have a pack of silicone bronze filler rod so I cleaned it as well as I could, grooved the break lines and finally wiped it with acetone. Stil the part went black and dirty when I started welding. I got it done but had to stop many times and clean the surfaces and then grind a lot afterwards.

Not sure what's going on, I am theorizing the old cast iron part (many decades old) was impregnated with dirt and fat so it wasn't possible to clean it good enough. Perhaps with AC and aluminium bronze it would have worked better. But I have a DC only machine.

 

[WorkshopChris]

What brand or number are the rods? Gas brazing rods are a different combination of elements compared to tig brazing rods.
If the contamination is from the cast can you heat then clean then repeat. Sometimes it takes a few heating cycles for any oil soaked into the casting to leach out.
However i wouldn't expect planer/thicknesses parts to have oil contamination normally the sort of thing you find with a gearbox casting.

 

[anto44]

I've had this problem a few times doing repairs on cast car parts. Sometimes it's very sooty and the braze has a dirty look to it, a grey colour bead instead of the nice gold colour you'd expect. Swapped over to a piece of fresh mild steel and everything is perfect so i put it down to contaminants in the cast burning out of it.

 

[DennisCA]

What brand or number are the rods? Gas brazing rods are a different combination of elements compared to tig brazing rods.
If the contamination is from the cast can you heat then clean then repeat. Sometimes it takes a few heating cycles for any oil soaked into the casting to leach out.
However i wouldn't expect planer/thicknesses parts to have oil contamination normally the sort of thing you find with a gearbox casting.

Don't remember the brand but they were sold as TIG brazing rods (CuSi, think it was 97-3% ratio), I've used them on clean pieces of steel just for training purpose and they performed normally there.

This was an exterior part used to lift and lower the cutter protection and I tried lots of oil and WD40 to loosen it up beforehand. Only did the one heating cycle.

 

[DennisCA]

I've had this problem a few times doing repairs on cast car parts. Sometimes it's very sooty and the braze has a dirty look to it, a grey colour bead instead of the nice gold colour you'd expect. Swapped over to a piece of fresh mild steel and everything is perfect so i put it down to contaminants in the cast burning out of it.

Yes this was exactly how it behaved for me too.

 

[anto44]

Clean uncontaminated metal is very important to get a nice braze. Unfortunately that's the opposite of what cast is known for. I clean it then heat it to bring any contaminants to the surface then clean it again and repeat this several times to try get all the contamination out. If it starts going sooty or dirty looking as I'm brazing i stop and give it another clean.