Does flux on flux-coated brazing rod go bad?

I am doing an overhaul on my Monarch lathe. One of the iron end covers had been cracked, probably for decades. I ground a vee along the cracks then I tried oxy-acetylene brazing it with some Uniweld silicon bronze rod (UNI 1800FC and UNI 1000, tried both), and had problems getting it to wet the surface of the iron. The rods have different fluxes, one is green blue the other dark grey. The rod is 15 years old, but is stored in the original plastic tubes in a dry shop. So then I tried fluxing the rod with standard Borate flux, and got it to wet the iron quite well. So, I am wondering if the flux was bad, or I just needed a different flux for the iron cover. I have used both brazing rods for other projects (brazing steel and brazing bronze) recently with no issues, so I am leaning towards the second possibility, but there are smarter welders than I am reading this, so I would appreciate some advice.

cast iron is such a shitbasket of different actual materials, and so full of graphite, which is a near perfect "anti-flux". I doubt that your rod is at fault regardless of its age, unless its got the coating blowing off in chunks. there is basically no way just the coating on the rod is enough to really adequately flux a stick braze in cast iron.

are you giving it a decent pre-heat of at least 300-350F?

Some projects always need more flux so I keep a can nearby.

I agree that more flux like from a can is the way to go. I also found that when you vee prep the joint. 90 degree opening is recommended. If you grind the sides you tend to wipe the cast iron. Finish the surfaces with a file of a carbide burr so that the surface is cut rather than smeared with the grinding disc. The metal flows better and the flux cleans better on the filed surface. I wasn't sure if this was an old wives tale until on one job I left one side with grinding marks. I had more difficulty getting the surface to wet with the brazing alloy.

Pre heat and shield with argon if you have to.

Sent from my ASUS_X017DA using Tapatalk

Can you recomend a flux powder to have on hand. I haven’t had any in years and need to order some, amazon has many listed not sure what to order.

true temper said:

Can you recomend a flux powder to have on hand. I haven’t had any in years and need to order some, amazon has many listed not sure what to order.

Click to expand...

The flux I have came from a welding supply store - many years ago.
ESAB........Allstate Welding Products for cast and malleable iron, steel, copper, bronze, copper alloys except aluminum bronze.
Heat the rod and dip, also recommend sprinkling in a V-groove or joint as needed. 1,400 to 2,200 F heat range.

I have both coated and uncoated rod. My beef with the coated rod has always been that some of the coating pops off so having a can of flux open for the job means no interruptions or reheating in the middle of the work.

I have a Smith torch with a selection of tips for large or small repairs.
John

true temper said:

Can you recomend a flux powder to have on hand. I haven’t had any in years and need to order some, amazon has many listed not sure what to order.

Click to expand...

handy flux, black B=1 for iron and steel is a good, tho not specifically for cast iron. one key to understand is the need to actually OXIDIZE the graphite during preheat. use a slightly O2 heavy flame if using the oxy-fuel torch (but DONT actually melt the surface), or "play" the torch on and off the surface if you are using a fuel-air like a "burnz-o-matic" or a plumbers acetylene air rig.

clean after the pre-heat wire brush, file, scraper. THEN flux.

Yeah, the pre-heat was adequate, but without the added flux the braze would just ball up and sit on the surface of the iron, although some would stick. The flux I used was an old can of Victor general brazing flux, which is a yellow borate compound.