Why can't you weld with propane?

[Stan in Oly, WA]

The reason I've heard given that you can't weld with propane---only cut, heat, or braze with it---in spite of that fact that its neutral flame temperature is well above the melting temperature of mild steel, is that it's hot enough, but it doesn't deliver the heat fast enough. Can somebody explain to me what that means?

With oxy/acetylene welding (which I never do except to show students how to do it) when you need to weld heavier metal you use a larger tip and maybe turn up the gas pressure. That delivers more heat, right? Why won't the same thing work for propane (or natural gas, MAPP, etc., for that matter)?

Thanks, Stan

 

[RobMD]

Actually, the inner cone of propane torches are not really hot enough to weld. By using an injector nozzle tip, you are able to cut with it, but to try to weld with it is kind of a far shot.

 

[Anonymous-0]

I think I remember being told that you can't get a fine enough pin point flame with propane. I just thought of another more important reason. Propane uses an oxidizing flame to get enough heat and you don't want excess oxygen in the weld puddle as it would make the weld brittle. A propane flame might not keep other impurities out of the weld as well. A neutral flame with propane isn't hot enough. Acetylene uses a neutral flame so there is no excess oxygen to get in the weld. I think that's also part of the reason that acetylene cutting tips are one piece and don't need a whole bunch of flames for cutting. Acetylene cutting tips can use anywhere from 1 to 6 preheat holes. Propane cytting tips have about 12 preheat holes. When cutting, extra oxygen doesn't hurt anything. The whole concept of cutting is rapid oxidation(rusting).I wish I had my old apprentice book from the tech. school. It had a lot of details and reasons for things in it. Anyway, I hope I made sense. Dave

 

[Anonymous-0]

Sorry Stan. You did mention a neutral flame. I read that propane uses 3 times as much oxygen as acetylene. Maybe when equal propane and oxygen are used, there is still excess oxygen and it's not a true neutral flame? I wish I could remember. Burning out impurities might take more heat than than just melting the steel. Kind of like the oxides when TIG welding aluminum. Hopefully someone will come along and be able to clarify why only acetylene works for welding. Dave

 

[Bus Driver]

Oxy-acetylene has temperatures of 5900- 6100 F. Propane is probably not more than 4000 F. The metal (and the atmosphere) transfers the heat away from the weld area very quickly. I suspect that the heat transfer rate just outstrips the ability of the propane to maintain the proper temperature in the weld area.

 

[Ken Macfarlane]

I've never tried but I'd bet my propane cutting nozzle could make a butt ugly weld in an emergency. The small welding tip is no good with propane. I've used it for precise heating a few times and it doesn't get up to welding temps except maybe on sheet metal.

 

[trucker40]

The thing to do would be try and weld with it.You might need a way bigger tip or something.Lots of people say lots of things,but it doesnt mean its always true.It might be real hard to do,the weld might be poor and look ugly,but it might be possible.I watched 2 old men nearly get in a fist fight when one of them said a cutting torch would not cut under water.Finally the old guy that said it would,After lots of cussing,got a bucket of water in a steel bucket,set the guages a little different,lit it,put the burning torch in the water and cut a hole in the side of the bucket,then he put a piece of thick angle iron in the water and cut it too.I read in books about WW2 that they welded under water and patched up battleships and stuff so I knew it would work.That wasnt acetalene it was knap(spelling)gas,so I dont know if propane would work or not.The thing I dont like about propane is that its not as hot,but I preheat before I cut and that makes it much better.Try that,preheat the heck out of it,maybe until its about to glow,then try and weld,I have an idea it will weld if you do.If you manage to weld something,put it in a vise and beat on it with a hammer until you break it.If you have to bend it 3 0r 4 times before it breaks,it might work.If it breaks real easy experiment a little and see if you can get it stronger.

 

[Stan in Oly, WA]

Hi trucker40,

Pretty interesting info. Thanks. In general, my attitude when someone says something can't be done and I can't see any reason why not is to try it. In this case I'll probably have to wait until I see that the track cutting torch at the college is set up with oxy/propane and then put on a welding tip and try it. I've got propane and all the equipment for oxy/propane cutting here at home, but it would take me longer than I want to spend to drag everything out and set it up.

In Googling to try to find the answer to my question I learned a few interesting things. One is that, unlike acetylene, the hottest part of a neutral propane flame is at the outter flame. This seems good to know for preheating steel when oxy/propane cutting, but also may be part of the reason it doesn't work well for welding---the hottest part of the flame is out where the heat is too dispersed to reach the temperature needed to melt the steel. Several sources confirm that propane has plenty of heat---almost twice as much per unit volume as acetylene---but can't deliver it fast enough for welding. So I keep coming back to that statement which I still can't make sense of.

All the best, Stan

 

[Bus Driver]

Interesting discussion. My knowledge of the chemistry and physics of fuels is very limited. Methane is CH4. Natural gas may be mostly or totally methane. Acetylene is C2H2. Propane is C3H8. Oxidation (burning) of carbon produces much more heat than does the burning of hydrogen. Acetylene is a small molecule, propane is a large molecule. I suspect that the large molecule of propane and the fact that it is about 73% hydrogen is the factor which slows burning and limits the heat output (BTU) per unit time.

 

[Anonymous-0]

I can't remember the exact reason from class 40 years ago but it had to do with the chemical reaction of the propane flame with the steel while the acetylene flame didn't react with the steel when adjusted properly. I think you can stick things together but the steel will be compromised. joe-

 

[Stan in Oly, WA]

Hi Bus Driver,

I agree that it's interesting. I think that your idea that the size and composition of the propane molecule is what determines its BTU output is probably the key to the situation. It's easy to think of fuels that burn hotter than other fuels. But there's still a piece missing for me. Why can't you make up for a cooler burning fuel by burning more of it?

If you needed to heat a certain area with a stove and your available fuel was wood, you would choose a stove with a high enough BTUH output to do the job. A smaller device that burned a fuel with a higher BTUH output per unit volume than wood might be an alternate choice, but the point is, you could do it with a larger (or more efficient) wood burning device. What I'm still struggling to understand is what it is about propane that prevents you from being able to use it as a welding gas by simply using more of it.

All the best, Stan

 

[T_Bone]

Hi Stan,

Here's some typical flame temps:
LP/Oxy======== 4380º
Mapp/Oxy====== 5300º
Propylene/Oxy 5900º
Acetylene/Oxy==5900º

Propylene is a mix of LP and Xylene. It comes premixed or DIYS mix with a cylinder adapter. Costs about twice of straight LP and 3/4 the cost of acetylene.

Acet/Oxy has a huge welding/cutting advantage as it requires less flame spread for the same BTU requirement thus will cost less per-unit(cft). That is too say LP & Mapp gas has about 1/2 the total usable heat content when compared to acetylene, thus the required use of a larger orifice for LP use.

Acetylene has a gravity index (the density of a gas) of 0.92 where LP & Mapp is about 1.50 thus a acet/oxy flame would contain 61% more heat content under ALL circumstances.

Another consideration is both LP & Mapp like too swirl as the gas is being delivered thru a orifice where as acetylene is more of a linear gas flow. This would be of a concern as then LP would require a different type of distributing orifice and mixing chamber too yield 100% of the LP heat content.

Before Propylene came onto the market, LP just had too many limited uses when compared too acetylene for a small shop use and it still may have even with Propylene.

T_Bone

 

[Anonymous-0]

Propane has more BTU's than acetylene. That's why it's such a good heating fuel. Acetylene is a hotter flame though. Temperature and BTU's are not interchangable. Max temperature with an oxidizing Acetylene flame is 6300 deg.. For welding it's around 5850/5900 deg. if I remember right. Maybe another reason acetylene works is because you need very low pressures and higher pressures would blow the puddle around. I still think that an O/A flame is the only one that keeps impurities out of the weld. Dave

 

[Anonymous-0]

The analogy with the wood stove isn't quite on point.

Welding requires more than bulk heat. The advantage of acetylene is that the high flame temp gives you CONCENTRATED heat. Welding requires adding heat to the weld area faster than the surrounding steel can carry it away, in sufficient amount to exceed the melting point of steel. The small, hot flame of an acetylene torch can do this. You can melt the steel in the weld joint area without trying to melt the steel 1/2 inch on either side.

If you crank up the propane (and oxygen) to deliver an equal number of BTU per hour as the acetylene flame, you must necessarily get a bigger flame. The bigger flame impinges on a bigger area of steel, so there is more steel carrying the heat away, and it is harder/impossible to melt the steel in the weld area.

So fine, you say. Crank up the propane even more, to get MORE BTU per hour than the acetylene flame. Eventually, you may be able to melt steel with a big enough flame. But the resulting flame is so large, and the heat affected zone is so large, that it becomes nearly impossible to get a serviceable weld.