Caution:Before you read this node and think that you know enough about welding to go out and start doing it, please don't. This node is for information purposes only, and while it is factually correct to the best of my knowledge it does not stress safety as much as a proper course in welding would. Welding can be VERY dangerous. If you wish to learn to weld for fun or profit you should check out a local junior college. They often have classes on welding. Even after that you should always heed your instructor's warnings about safety, there are a lot of horror stories out there about people who didn't (past tense stressed).
The Oxy-Fuel Torch
An oxy-fuel torch is a device used in the heating, welding and cutting of steel and other metals. An oxy-fuel torch setup consists of the torch itself and associated tubing, regulators, and tanks. The torch itself can range in size depending on the amount of heat output desired, but in general is about 12 inches long. The bottom 8 inches or so forms a handle, and is usually the heaviest and thickest part of the torch. The handle has two valves at its base, one for controlling the flow of oxygen, the other for controlling the flow of fuel gas. The top part of the torch is referred to as the tip. Tips are usually replaceable and come in a wide variety of shapes and sizes. Tips can become clogged with carbon deposits and must be cleaned or replaced periodically. Fuel gas and and oxygen are supplied to the torch through a pair of hoses. These hoses are attached to pressure regulators which are attached to the gas supply tanks.
Acetylene is the most common fuel gas used with the oxy-fuel torch today (Acetylene torch or oxy-acetylene torch) though other gases such as MAPP, propane, and butane are also used. acetylene is popular because it produces such a hot flame, typically 5500F to 6000F. An oxy acetylene torch is capable of melting most metals including steel relatively quickly.
Oxy Fuel Welding
Oxy fuel welding involves placing two pieces of metal (usually carbon steel) close together and heating the junction until it melts, and the base metal flows together. Sometimes additional metal, called filler metal is added in the form of a welding rod which is added to the puddle of molten base metal. Filler metal is usually added to make up any gaps in the weld.
Oxy-fuel welding used to be the most common methods for welding steel before the invention of the arc welder. This method of welding has mostly fallen out of favor for manufacturing because the relatively low heating rate (relative to an arc welder) can cause warping and thermal damage to a wide area around the weld. Oxy-fuel welding is still often used in small shops and by artists, but is not generally used in manufactured goods because of the lower weld quality.
Oxy-fuel welding is used almost exclusively for the securing of carbon steel, but is also used frequently by jewelers for welding gold and silver. Some metals such as aluminum and titanium cannot be welded using an oxy-fuel torch and must be welded using GTAW (TIG) or GMAW (MIG) techniques.
Alternatives to Oxy-Fuel Welding
While many metals cannot be welded with an oxy-fuel torch, most can be attached using a torch and a method such as soldering, brazing, or braze welding. All three of these techniques involve the melting of a intermediate metal which bonds to the base metal. In all of these techniques the base metal is not melted. The main difference between the three techniques is the temperature at which they take place and by the action of the filler metal. Soldering is done below 840F and involves a filler metal which flows via capillary action. Soldering is commonly used to secure copper water pipes in homes in a technique called sweating. Brazing and braze welding both take place at temperatures above 840F but differ in the action of the filler metals used. In brazing the filler metal flows between the two pieces of base metal through capillary action, in braze welding it does not.
The three methods described above are not as strong as a true weld but are strong enough for many applications and have some clear advantages over welding. Any of the three methods described above can be used to attach two pieces of dissimilar metal, a feat which is difficult if not impossible to do with a weld. If care is taken, these methods can also be used to secure pieces of heat treated steel without softening them.
Cutting Metal with Fire
By far the most common use for the oxy-fuel torch is in the cutting of steel and iron based alloys. Cutting is done with a modified torch. This torch is similar to a general purpose torch but adds several features. The first is a lever which allows the user to release a continuous jet of pure oxygen out of a set of nozzles at the tip of the torch. These nozzles are usually in the center of the tip, and are separate from the nozzles used to generate the torch flame. The second addition to the cutting torch is a knob for adjusting oxygen flow through these nozzles.
To cut steel the user first lights the torch, the oxygen lever is not depressed at this time. Starting at an edge where the cut is to begin the metal is heated until it starts to turn a cherry red. At this point the oxygen lever is depressed and the fun begins. Strictly speaking the metal is not cut by the fire, the fire is merely there to start things off and in fact once the reaction is started the fuel gas can be shut off with little effect. The only purpose of the fuel gas is to bring the steel up to its kindling temperature (about 1600F) so that it can burn. The addition of pure oxygen causes a rapid oxidation of the steel. This is an exothermic reaction and serves to preheat the adjacent metal so that a continuous cut can be made. Steel can be cut quite rapidly in this manner including steel several inches thick. It is a truly awesome sight to see a piece of 1/2 inch thick steel being cut in two like it wasn't even there and an enlightening experience to actually be the one doing it.
C-Dawg asks "Is filler metal the same thing as flux?" Because he asked, and because he made me realize that I didn't mention these very important chemical I will make mention of it as a sideline. First of all flux is not the same as filler metal. Flux is a chemical (actually there are many chemicals that are used for different processes) which facilitates the joining of two metals or impoves the quality of the joint.
Flux is not used when welding with an Oxy-Fuel torch, but it is used when soldering, brazing, or braze welding. In these processes the flux used performs a cleaning action, removing the oxide layer on the surface of the base and filler metals. This allows the two to adhere to one another more effectively. Flux is also used in certain types of arc welding as well (SMAW and flux core arc welding). In this case the flux used forms a glassy protective coating over the metal to protect it from oxidation while cooling. The flux must then be chipped off with a flux hammer. See also flux.