I can't braze for squat...can ya help...?

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OK, here’s the deal…I suck at brazing, plain and simple. Someone here can no doubt set me straight. I’m using plain 1/8” thick steel and pre-coated 3/32” brazing rod. I heat the steel with a brazing tip (nice blue cone inside the flame) until it’s red but not melting. I apply the tip of the brazing rod to the reddened metal, it doesn’t immediately melt so I move the blue cone in on top of the rod and FLASH, POW I get a puff of white smoke, an angry hiss and the melted rod sorta spits / splatters out onto the base metal…the table…me…the torch, you name it. Sometimes the flash up even snuffs out the torch, so needless to say the joint won’t hold any tension and sorta looks like cold solder joint. What am I doing wrong? Ronnie LyonsMeridian, Idaho
Reply:Are you putting flux on the joint as well?? Also what type of joint are you trying to braze?
Reply:Too hot. You dont want red.
Reply:No, no additional flux added to anything. The rods I'm using have the flux already on them, they're like a welding rod with precoated flux. Hot hot do I want the metal?
Reply:You have to haul *** with the coated brazing rods, the uncoated kind ya dip in the tin of flux are WAY easier to get a handle on
Reply:Originally Posted by ronnielyonsNo, no additional flux added to anything. The rods I'm using have the flux already on them, they're like a welding rod with precoated flux. Hot hot do I want the metal?
Reply:Actually, Ronnie, you can braze.  You just don't know it yet.The most important thing in preparing a joint for brazing is cleanliness!  You don't mention joint preparation.  You have to grind down below any rust and mill scale to bright metal over all faying surfaces and everywhere you want the braze metal to flow.Flux is important.  You should get a good brazing flux and apply it to all faying surfaces BEFORE applying heat so that as the metal comes up to brazing temperature the flux will already be in position (even inside the joint) to melt and flow over all surfaces of the joint.You are approaching proper brazing temperature when the flux melts and coats the surfaces to be brazed with a water-clear, viscous fluid.  It's easy to recognize when you are close to brazing temperature because the steel starts to look shiny under the molten flux, even though you are NOT melting the base metal.  I have to disagree with Tek about the red heat.  I think with most alloys you will be at a faint red heat for brazing.  Definitely not a bright red heat.Depending upon the brazing alloy and the base metal and the size of the work, you may find it easier to control the temparature and avoid overheating by using a MAPP/air torch instead of an oxy-acetylene torch.  They are pretty cheap - maybe around $35 or so.  If you keep using the O-A torch, you have to be very careful to avoid overheating the rod or the base metal by keeping the inner cone off the rod or the work.If you are relying only on the flux coating on the rod, you are probably oxidizing the base metal before any flux coats it.  A skilled operator can probably control heat so the flux on the rod melts and flows into the joint before the rod is melted in the flame or the base metal overheats, but while you are learning, give yourself all the advantages by applying a good flux before applying heat.  You can mix some with water to spread over the joint area.  The water boils off quickly, leaving a thin coating of flux over the joint.  You heat the bare rod gently and dip it into the flux powder.  Some flux will melt and cling to the rod.  It is cheaper than coated rods and you control the type of flux used.You are clearly heating the rod too much if it is reacting the way you describe.  The rod must be melted by the hot base metal, NOT by applying the flame to the rod (although the flame will preheat the rod a little.  Remember the braze metals melt around 800-1200 degrees (if I remember correctly), depending upon the alloy, whereas the torch flame is around three thousand degrees.  That's much too hot for the braze metal.  Application of the O-A flame directly to the rod will boil off the lower melting components of the alloy, ruining the alloy properties.The white smoke you are describing is the zinc boiling out of the alloy and oxidizing.  It may also have some cadmium in it, depending upon the alloy.  You should always know what's in the alloy you are using.  DO NOT BREATHE THE SMOKE -it is very bad for you. Have forced ventilation and/or wear a respirator.  With proper technique you should never see white smoke, but you must still avoid the possibility of breathing cadmium vapor.The flame should be only a very slightly on the side of oxidizing.  If you have a "nice blue cone" you may be too far into an oxidizing flame from neutral.  An oxidizing flame will oxidize the metal before you get the braze metal to flow.  Adjust it so the yellow flame just disappears.  Gradually bring up the Oygen flow until the yellow disappears, and check the flame periodically.Check what alloy you are using.  You might ask your welding supplier to recommend a lower melting alloy to practice with  until you get the hang of the process.Hang in there.  It will seem easy soon.awrightLast edited by awright; 12-08-2006 at 02:37 AM.
Reply:Listen to Awrigt, CLEAN CLEAN CLEAN!!!!Now that your steel is clean and shiny, begin heating it.  I recommend you start without flux if using flux coated rods.  Heat the steel slowly and evenly.  Watch the color.  You will see the steel turn blue, then gold, then it will turn silver again.  Once it turns silver again, you are at the right temperature for brazing.  If the steel turns red, it's too hot.  Scratch the rod against the steel and brush the flame over the rod real quick to put some heat in the brazing wire.  It will melt and flow out in to the joint.  I recomend you learn with flux coated rods or wire dipped in flux and not use flux on the joint because the flux will clean off the oxide layers that generate the color changes.  Once you get an idea for the heat input, then you can start working with flux on the joint to promote better capillary action and faster brazing.
Reply:I braze antique bicycle frames and don't use the precoated rod -- too tricky. If the rod's been sitting for some time the flux becomes useless (in the same manner as SMAW rod) That said, a crusty old shop foreman I worked with always insisted that for brazing, and especially with commercial flux coated rod, we use a slighty rich acetylene (carburizing) flame -- not a neutral flame -- just what I was taught.I flux the tubing, heat it until dark red color and apply the fluxed rod -- flame is "washed" over the joint until the filler material flows.Last edited by goodfellow; 12-08-2006 at 09:26 AM.
Reply:Wow, thanks for all the help. First of all, I'll confess to not cleaning the metal...I just cut it, position it and go for it. It doesn't appear physically dirty as it's all new steel, but I'm assuming theres mill oil on it? (too use to MIG welding I guess.)    I do suspect those pre-fluxed rods are part of the culprit. It seems when the rod does melt off, the flux will burn back sometimes as far as 3/4" away from the tip. Seems like that's a bit far away from the work to get any flux on the material. I do know from sweat soldering, that solder follows the flux and usually doesn't go where the flux hasn't gone. I'll try to find a liquid / paste flux and clean well then brush some flux on before hand. As far as the flame, I understand the less than neutral thing and acetylene rich is a bit "cooler" than oxygen rich. Thanks for all the great help...I guess I just need to slow down, open a cold beer and be patient. Ronnie
Reply:Ronnie, the issue of oxidizing vs. reducing flames is not one of temperature.  Both are way hotter than needed for brazing.  It is the chemistry of the flame and how the excess of oxygen or carbon reacts with the hot base and filler metals and flux.  I wasn't familiar with goodfellow's theory about carburizing (reducing, in chemist's terminology) flames being preferred for brazing, but I would try it out.When you say, "... it's all new steel...," that can cover a lot of different "stuff."  Hot-rolled steel (which includes most structural steel, angles, plates, bars, etc., and stuff you get from the scrap yard) has a hard, black oxide on the surface that will definitely interfere with brazing and has to be ground off.You are going to enjoy the process once you get the technique down.  It's very versatile for many types of projects and very strong with properly designed joints (i.e., using the joint in shear, rather than in tension).  As a hobby welder without a dedicated welding shop, I find brazing/silver soldering very handy for small tasks for which I don't want to drag out all the equipment and cables to set up for welding.  You can do a lot with some brazing rod/silver solder, a can of flux, and a MAPP torch.  You can assist the MAPP torch for slightly larger tasks by setting up one or two propane torches to reduce the wicking of heat away from the joint, then apply the MAPP flame to the joint area.  I can finish the joint in the time it takes me to drag the wire feed welder out from under the pile.Have fun.awright
Reply:There are some excellent points that have been made here about requirements for sucessful brazing.To them I would also add acetylene and oxygen pressure settings. Too much gas pressure results in over heating ( hence oxidising )Three to five pounds of gas pressure is more than adequate.How do you set the oxygen to that pressure as the gauge does not go down to that reading,I hear you ask?Easy answer is that the pressure is set pressure on the acetylene regulator first. Open the acetylene valve on the torch body ( no,don't light it ) and feel that pressure against the skin of your palm of your hand. Use your memory of that pressure to set up the  oxygen cylinder regulator pressure until it is the same pressure (feeling ) as was the acetylene.This is especilally important for silver solder brazing. The white smoke mentioned is the zinc content of the brazing alloy.It oxidises and gassifies at only a few degress over its own  melting temperature. It is nasty stuff to breathe in ,beware!With fluxing,try this. On your cleaned plate braze area) heat first and sprinkle a pinch of flux on the hot plate. The heated area makes the flux stick to the plate. adding while heating only results in flame pressure blowing most of it away.The trick is only to maintain the only enough heat needed to get the flow.That may mean withdrawing the flame for a period long enough for the molten  pool to cool slightly to avoiding flashing.Clean up your brazing flux residue with water .It is corrosive.Summarised the points again are:Clean clean clean.Set pressures 3 to 5 PsiHeat plate hot enough to make flux stick.Flux the area to be brazed .Heat plate until until red and wet shiny looking.Stroke filler wire over the red area do not melt filler rod with torch.Maintain brazing heat only.Clean up with waterI hope this helps youGrahame
Reply:well I don't have nothing to add lol!
Reply:WOW!! This was one COOL THREAD!!
Reply:Good advice on using a slightly reducing flame (acetylene rich).A technique that works well for me is to heat the joint just a bit so that the powdered flux just sticks to surface and doesn't blow around when hit with the flame.  Apply flux to your rod the same way, heat it and dip it.Not sure how this works, but I've heard the idea of laying down border-lines on either side of the joint with soapstone, the idea being that the flux and braze will not cross the line and the braze is more uniform along the edges.Sorry, I didn't read all the posts first, someone else suggested putting flux on heated base metal already.