Gas Torch Welding

Small portable combination welding/cutting setups are available that will perform a variety of welding/cutting operations on steel up to around 1" thick. These are made in a variety of types and styles that can be used economically around the home or shop. The right setup will allow you to weld, braze, solder, or cut many different kinds of common metals found around the typical home or shop.

There are two basic types of gas torches: Air-Fuel and Oxy-Fuel (oxygen-fuel). A propane torch is an example of an Air-Fuel torch. As the fuel gas moves through the torch body enough air or oxygen is mixed with it to enable it to burn with a clean flame.

Oxy-Acetelyne, Oxy-Mapp Gas, and Oxy-Propane are examples of Oxy-Fuel torches. In them pure oxygen is used instead of air. This is done to create a high temperature flame. Oxy-Acetelyne produces the hottest flame at around 6,500 degrees F. Oxy-Mapp Gas produces the second hottest flame at about 6,200 degrees F., with Oxy-Propane producing a flame of about 5,000 degrees F. Newer design propane-air torches can produce a flame of around 3,500 degrees F. A simple propane torch can handle most soldering and light brazing operations, but for welding or cutting anything heavier than sheet metal, a system with oxygen is needed.

Since air is composed of about 70% nitrogen and 30% oxygen and since all gasses involved in the flame are heated to a high temperature it is apparent that using part of the heat of the flame to heat up nitrogen (which contributes nothing to the production of heat) is wasteful. Even though high temperatures can be achieved with an air torch, you can heat much hotter and faster with an oxygen torch and there is no waste of fuel to heat nitrogen.

There are three adjustments that are used on a torch: fuel flow, oxygen flow and tip size. Fuel flow is used to light the torch, and on a torch with only one adjustment knob, that knob will control fuel flow. If there are two knobs then there will be two hoses. The red hose will control the fuel while the green hose feeds the oxygen. Check valves are highly recommended on both the gas and oxygen lines to prevent the flame from burning back to the tanks and causing an explosion. Check valves are relatively inexpensive one way valves that are a safety requirement in many work environments. Crack the fuel knob slightly on the torch before lighting.

Be sure to use a spark lighter, not a butane lighter. Butane lighters are highly explosive when subjected to a penetrating molten metal droplet. The tip size is not usually thought of as an adjustment but using a range of tips with different size orifices or gas hole openings allows you to use the torch for a much wider range of welding, cutting, or brazing than if you had only one size of tip. Use a smaller size orifice on thinner material and a larger one on thicker material.

Once you have your torch lit open the fuel and oxygen valves a little at a time alternately until you get the size flame you need. If you have too much fuel, then the flame will burn with a yellowish color due to oxygen starvation. Too much oxygen is harder to see but should be avoided if possible. Start with too much fuel, a yellowish flame appears, then open the oxygen slowly. You will see a bluish cone start to form in the center of the flame. As you open the oxygen valve the cone will become smaller and more distinct. Just as it's boundary becomes solidly defined, stop adjusting. An even hissing sound should be heard at this point. This is the hottest flame and does not waste either gas or oxygen. Too much gas or oxygen cools the flame and will waste gases. Too much oxygen can also oxidize or rust your workpiece. This oxidation if not cleaned off will adversely affect the quality of the the workpiece.

Torch manipulation is best learned by experience, but there are tips to help you with the learning curve. If you move the torch too slowly you risk melting holes in your workpiece. Too fast and your workpiece may never get hot enough. The object is to apply just enough heat to do the job. Even if you are working on a small localized area on anything thicker than sheetmetal, it is best to preheat a larger surrounding area first in a slow circling motion if possible.

Stainless steel is the exception to this preheating. In fact, torch welding is not recommended for stainless at all. Silver brazing is the preferred joining method. High heat ruins the chemical and mechanical properties of stainless steel. It also warps too easily from heat, so less is better on stainless. To heat a localized area fast, touch the tip of the inner blue flame to the piece. Be careful on thin material as this can create a rapid burn-through. I suggest that you practice on a piece of similar scrap metal first if possible.

You will have to choose a filler metal for your project. Your local welding supply store can be of great assistance in that choice if you are having trouble identifying the base metal. Welding carbon based steel does not usually require any flux, but soldering and brazing usually do require a flux on most types of metals.

Flux can be in the form of a paste or powder and helps to prevent oxidation of the brazed or soldered joint. Oxidation is caused by oxygen combining with other substances at a slow rate. As the temperature increases the oxidation happens at a faster rate. Rapid heating and cooling of most metals also accelerates oxidation.

Oxides that are present in a weld zone can cause a weak joint or even prevent you from making a joint at all. To use a specific example let's use aluminum. Aluminum oxidizes into aluminum oxide which is very hard and melts at a much higher temperature than the original aluminum base metal. Because it melts at such a high temperature it does not bond well to the aluminum metal and represents a weak spot in a joint. A flux will combine with an oxide and cause it to melt at a lower temperature thus combining with the rest of the melt zone and allowing a stronger weld.

You have to be careful when welding aluminum with an oxy-fuel process. Aluminum does not change color like steel and other metals when it reaches it's melting point. These other metals will start to glow red. When aluminum gets near it's melting point, it will darken slightly. When it hits the melting point it will run like water.

A way to see the effects of fluxing is to use copper and solder. Find an old piece of copper such as an old water pipe, one that has a brown or green surface from oxidation. Now try to melt some unfluxed solder onto it and make it stick. It won't work no matter how hard you try. The corrosion won't melt and combine with the solder. By the time you have got the oxides hot enough to melt the underlying copper base metal will be a pool of molten copper. The oxides have a higher melting point than the copper base metal.

As you can see the oxides have to go. You should remove the oxides mechanically. Use a file, steelwool, sandpaper, steel brush, etc. to remove them. If you try to solder, now things will start out just fine but you will soon have problems again as excess oxygen in the torch flame, and atmospheric oxygen combine with the hot copper. If you now switch to a flux core solder you will see how the flux seems to eat through the oxide layer so that the solder can bond to the copper underneath. This is a good example of what flux does for welding, soldering and brazing.

Different metals have different chemical properties and thus require different fluxes. It used to be a matter of fact that it was impossible to solder or weld aluminum with anything short of an atmosphere controlled process like heliarc, gas metal arc welding, or neutral atmosphere soldering and brazing much like titanium. Now flux manufacterers have discovered fluxes that will allow aluminum welding without the need for shielding gases.

What you can do with a good torch setup is limited only by your imagination. I have owned an oxy-propane setup for many years and have used it for many repair and fabrication projects. For the rare times that I have needed extra heat for welding on a heavier piece of metal, I simply switch to Mapp Gas. There is no need to make any tip or regulator changes for the switch from one to the other. Propane is about four times cheaper than Mapp Gas and works fine for about 90% of what you will encounter for general repairs. I also own an electric arc welder, but use the torch setup much more often. Why? It's simply more versatile and faster to setup for most small projects. It paid for itself within the first year that I owned it.

I have two last important safety tips. If you should try to repair any tank that has held gasoline, fill it up with water before attempting to weld on it or purge it with an inert gas such as argon or nitrogen. Any empty gas tank is highly explosive no matter how long it has been sitting around. Never attempt to use Mapp Gas on copper. Mapp Gas and copper are another potentially highly explosive mix.

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