Brazing advice

I had to show some new students in our metal arts program how to braze last Saturday. It's something I've done almost none of, so I spent some time during the prior week practicing and getting advice about correct technique. I consulted several welding instructors, researched it online, and watched YouTube videos. Part of the problem was that I hardly ever got the same information twice.

I thought that brazing was much like soldering (which I've done lots of) except at a different heat range, and, in fact, this turns out to be the actual definition. And, although a reliable description of the process says that the metals being joined should be heated to above the melting temp of the brazing material, and the brazing material melted by touching the base metals, after which it will flow in by capillary action (as sweat soldering is done) almost nobody seemed to do it this way. In almost every case, there was a certain amount of melting the brazing rod directly in the flame.

One student in a welding certification class I dropped in on to get some practice offered to show me the process. He used the oxy-acetylene flame to melt the brazing rod directly. Only incidental heat got onto the 16 gauge mild steel coupons he was joining. It was a beautiful job, in appearance, and allowed for much more buildup than I found possible when I tried melting the rod onto cherry red metal. But was it really brazed? My guess was that it was like a beautiful MIG weld bead that sits on top of the base metal with little to no fusion.

When I demonstrated brazing on Saturday, a large group of interested students gathered around to watch me prove that I knew a lot about the theory, and not so much about the practice. It was good that I had warned them not to expect too much.

Stan

Stan, I'm a back yard brazer,I wanted to pick up the skill when I first started restoring a '35 Ford. I had learned a tiny bit in high school shop, but that was it. Having read what you said,I have always tried to get both pieces of metal cherry red and then,held the rod into the flame and ran it onto the metal,then hit the metal with flame to ensure a good flow. What I learned is that both pieces of metal need to be clean to get a good job,sometimes had to take the sanding disc over it and then it would braze. I always grind off all excess. The flux in brazing rod will ruin any body work on a vehicle if it is not cleaned off. Bondo and brazing flux result in a bubble appearing after a couple of years. A bit of practice can make you a pretty good braze welder. On auto body, don't ry to braze any more than an inch at a time or you will warp a perfectly good panel..patience is the answer..like I said, I'm a backyard braze welder but I did get fairly competent with it.

Michael is right. When the brazing rod melts you play the flame over it until it flows into the metal. I was told by my instructor that cast iron brazed has 70,000 lbs tenstile strength

A recognized authority: <a href="http://www.copper.org/applications/plumbing/techref/cth/cth_7brzjts_heat.html">Brazing</a>

Hi Michael,

You've mentioned most of the critical things I discovered: metal has to be clean---the cleaner the better; heat the metal to dull cherry red but then apply some flame to the brazing rod, too; don't expect to be able to do too much at a time.

I found it difficult to get the base metal hot enough to do most of the melting of the brazing material without burning the zinc out when the flame would hit the rod. I found that I could get the metal hot enough to melt the brazing rod without having to put the flame directly on it only when I did lap joints. It worked almost like sweat soldering copper fittings, but I couldn't get that method to work with butt joints or fillets. I can see that it requires a light touch and a whole lot of practice.

I found that it was hard to do what I always tell the metal arts students to do---practice for a long time after you start to get the feel for a technique. Almost everyone seems to think that all the time they spend trying to do something and not being able to get it right IS the practice. The first time they get it to work, they call it good enough and stop. It's the relief of getting past the frustration, I suppose, and once you get there, you don't want to take a chance on getting frustrated again. I can say for certain that at that point you're not ready to give a demonstration to a group of students who actually want to learn the process.

Thanks for your advice,

Stan

It's been a while Stan but I've done a bunch of brazing in the past. There's a couple acceptable techniques to use. One where you keep the torch and brazing rod on the puddle the whole time. This method requires a more exact temperature, kind of like O/A welding. You can vary the temperature by adjusting the flame distance though. The other method is to melt the brazing rod into the puddle, circle the torch a couple times and then pull the torch away while the puddle cools(solidifies) for a second and then repeat the process. With brazing you generally have to move the torch in circles to form the puddle and the puddle will be quite a bit wider than an O/A weld. Different metals can be brazed but I'll just stick to steel for explanation purposes. With brazing you're opening up the pores in the steel so the brazing will attach. Similar to soldering but higher temperatures. If I remember right, sometimes you can use a slightly carburizing flame but normally a neutral flame is used. The steel is heated till it just starts to get red/orange and then the brazing rod is added. Unlike soldering, the flame has to put directly on the brazing rod to melt it. This can be done with brazing rod in the puddle or just above it depending on what technique you're using. You will know if it's hot enough because it will spread out on the heated steel rather than just look like bird droppings on the piece. It's takes a little practice to figure out the proper heat. You will use more brazing rod for the same joint than using O/A welding because the braze will spread out more. You will know if you're too hot because you will have more smoke and have a lot of the white smoke in the HAZ. There could be some green as well. It's been a long time. If you're practicing on a lap joint, you shouldn't see the the edges of the lap joint under the braze when finished. It should be built up higher without a concave profile over the joint. Ideally it should be slightly convex, just like a weld. This was the most common joint to practice on. Just like welding, you should have nice round ripples in the bead and a uniform width the entire length. You'll often get like a slag on a brazed joint. For practice purposes only, dip the hot piece in a tank of water and most of the slag/silicone like covering will come off so you can see what you've accomplished.

Brazing can be used to fill very wide gaps as well. I had to make a new door for my tractor cab out of 3/4" 16 gauge tubing. There were some extreme angles in the corners and it was very hard to have a perfect fit up. I didn't have a MIG welder at the time, but was easily able to braze the corners and fill in the gaps. Brazing is also great for little small parts that would be a real pain to weld. I hope I made sense. Dave

Now I remember what the white smoke is from. It's from the zinc in the brazing rod. You shouldn't have much smoke at all but still need to do brazing in a well ventilated area. I also recall having to grind the pieces perfectly clean for good fusion when brazing. Thanks for giving my memory a jump start. Dave

People say a lot of things that might seem right to them,but actually are far from true.Get some thin metal to practice on and some torch welding wire and flux and weld a little with that.Its like brazing but faster.You heat the metal up red and burn the rod in with the heat on the rod.That will give you the idea of how to do it.Once you can make a good bead like that,try brazing rod.You heat it up and burn the rod the same way,it just takes a lot more rod because the brass flows a lot more.Your weld looks a lot like welding with a torch welding rod though when its done right.A lot more is going on when you are brazing since you are pushing in more rod,to a point and then you raise the flame so it freezes.You can make a real good looking weld by brazing.It might work better if you have your piece where its inclined some so you build it up.
I mean do whatever you can feel comfortable while doing it,flat,inclined or what you like.
Dont worry about it flowing like solder so much as flowing like brazing rod.Its definitely different,but you get the flowing part quickly,just as it flows move on.
I can braze something,but Im far from an expert at it.A lot of it is the flux.Brazing rod with flux on it might work,but I like the rod you dip in a can of flux yourself.It seems like when you have to dip it,the piece cools off a tad and slows it down to where its easier to do.That way you are always heating it up and then making a small weld,stop and dip the rod and back at it again.Using the flux coated rod you have to do it faster and be better at it.

I dont know if any of this makes sense or not.Like I say Im not the best at brazing anyway,but I can stick stuff together with a brazing rod.I didnt really get it until I watched somebody.Like the guy who demonstrated for you,that sounds a lot more like the way Ive been shown than trying to heat the metal up and try to solder with it.Of course I dont know what they were trying to weld,or where it was at,or anything so there might be cases where that would be a good method.I watched a pro do it like the kid who showed you.More like welding with gas,steel,copper coated, welding rod,making a bead with the braze,only going some slower and using more rod as it flows in then pull back and freeze it,then do it again.

Others might have a better explanation than me.I dont see how the soldering idea works as much as the braze kind of flows like solder when its welding.

There is no penetration hardly.Brazing is a poor way to weld something.About all you can do to make it stronger is build it up a little.So by saying its like a soldered joint,yes,if welded right,the brass flows and sticks.That would be because of the flux doing its job,having it clean and then heating it the right temperature and flowing in the brass.Its all much more complicated than soldering,hotter temperature and way more suseptable to warping and brass falling out or not flowing.The feather of flame has to flow,heat and preheat all at the same time.You have to keep moving the flame.You have to move the brass in with the flame and push the rod in at the right time and move on,or it wont hold.

I hold the flame on the brazing rod mostly,but flow it in with the flame and might even pull the rod out if it gets too cold.

Its complicated is about the best way I can put it.

Thanks, everyone.

"Brazing is a poor way to weld something". Technically brazing isn't welding but if this is the case, why do they make brazing rods in the first place? Thousands of things have been successfully brazed. Brazing is the first choice for many cast iron repairs and joining different metals. Dave

It's more like gluing, isn't it? I learned to braze and torch weld in 9th grade, we made a sign/planter hanger that had some of the joints torch welded and some brazed.

Your right 135. Some cast cant be welded.the weld wuill chip off after you weld it,but brazing will hold it together. Line i said before ,I was told its stronger then the cast was orginal

I enjoyed all the comments on brazing, and have brazed both sheet metal and cast iron. But lets hear from someone about how you braze sheet metal vertically, joining two pieces together.

It's not at all like glueing. The brass goes into the pores in the metal but the base metal isn't melted. When we did cast test pieces in school, it didn't matter if it was O/A welded or brazed. When put in the bender, the cast would break and not the seam. Of course it didn't bend. The bender was just a simple way to break the pieces for the test. If it broke in the braze or the weld, you failed. The pieces were 3/8" thick beveled cast iron and both processes took a lot of heat to do properly. Dave

I'm not sure you can braze in the vertical position. Don't remember ever trying it and think they said it wouldn't work in school. Can't remember that far back though. Dave

Stan,
Its been a long time since I've seen a textbook on brazing, but I believe I remember that brazing was defined as joining two pieces that aren't very far apart (gap) for the greatest strength in the brazing process.

Brass welding with brazing rod is used to join larger gaps and but results in a weaker joint (IIRC).

Comment & question for Dave---Appreciated very much your "lecture" on brazing. What is a "carburizing" flame, and a "neutral" flame? Also, isn't dipping a hot brazing project in cold water detrimental to the well-being of the project? Martin

I thought of it like hot-glue, but you had to heat the stuff getting glued first.

I do know it is possible to weld with a cutting tip. And vertical position, too. Danged if I was going to drag the welder out and hook up the generator when the torch was already there.

Hi Cienna,

I'll answer before this topic goes to the limbo of back pages. A neutral flame is one in which the fuel and the oxygen are both completely consumed in combustion. If there is extra oxygen, it is an oxidizing flame; extra fuel (i.e. acetylene) produces a carburizing flame. There may be situations in which you would want an oxidizing flame, but I don't know what they are. I do know several applications for a carburizing flame: 1) you can slightly case harden mild steel by heating it to a specific color (which I don't remember), then washing the surface with a carbon rich flame; 2) you can use a carburizing flame to reduce the flame temperature when brazing to give you more control.

If you know how to adjust an oxy-fuel torch to a neutral flame, all you need to do to get a carburizing flame is to get a neutral flame, then turn up the fuel or turn down the oxygen.

All the best, Stan

A carburizing flame is sometimes called a carbonizing flame. It's like Stan said, excess fuel. The flame will have an outer cone that is longer than the inside blue flame. Some applications will call for 2 or 3 times carburizing flame. That refers to the length of the outer cone in relation to the length of the blue flame. The only time an oxidizing flame might be useful is with a rosebud to get more heat but they're usually used with a neutral flame too. It isn't quite as critical with a rosebud though unless you start melting the steel. Then the excess oxygen would cause problems.

As far as dipping a brazed piece in water while it's hot, that's why I said only on practice pieces. For an actual project you would let it cool on its own. Dave

I thought an oxidizing flame tended to burn cooler than a neutral flame... but keep in mind that I've been using LP for quite some time and that may be a bit different than acetylene... I know that heats take longer with an oxidizing flame... also with a carburizing flame. I find neutral to be hottest.

Rod

With propane you have an oxidizing flame anyway. With O/A an oxidizing flame is 6300 deg. A neutral flame is around 5750/5800 deg. and a carburizing flame is around 5300 deg. depending. I'm sure there'd be a chart on the net somewhere. Dave