Select a torch tip when using oxyfuel

PRACTICAL WELDING TODAY® JULY/AUGUST 2007

August 8, 2007

There has been much discussion on the value of plasma-arc and laser-beam processes for cutting and welding, but oxyfuel is still the most commonly used method. The products are very robust and easy to use. The cost of entry to use this method is relatively modest – only several hundred dollars for equipment, and rental cylinders are portable and do not require electricity, so you can cut and weld quickly and cheaply.

But how do you get started?

There are four decisions that need to be made so that you can select your tip and get started:

  • Type of gas to be used with oxygen
  • Type of seat (manufacturer and model of torch)
  • Thickness of metal to be cut, gouged, heated, or welded
  • Type of application (automated or manual, gouging or welding, etc.)

To best determine how the decisions are made, and what the effects of those decisions will be, it is important to understand the general process. When metal is moved or removed, as it is in cutting, gouging, and welding processes, it is first necessary to get the steel to its melting temperature. The operator manual that comes with your torch set will explain how to adjust the torch and regulators along with the oxygen and gas (fuel) to achieve the optimal flame and preheating the metal. When the steel becomes cherry red in color, applying the pure oxygen stream will remove the unwanted metal in cutting or gouging operations. Welding and heating equipment does not allow for a pure oxygen stream with the heating flames.

Types of Gas

There are five types of gases that can be used in oxyfuel operations. Tips are designed with orifices of different sizes and shapes for the oxygen/gas mixture to burn.

Acetyleneis the hottest and most versatile of gases. It is most versatile because it is very easy to use for all purposes. Preheat time for cutting and gouging is relatively short. This is because the flame temperature is between 5,600 and 5,800 F. This gas is also the most expensive and is not the most efficient for heating large areas.

MAPP®, or methylacetylene-propadiene, is the second-hottest gas. It is relatively versatile, but has a slightly longer preheat time for cutting and gouging procedures. It can still weld, though the lower flame temperature (5,200 - 5,400 F) can be problematic.

Propyleneis commonly used as a generic name for many types of gases. These range from specific gases, with their own chemical formulae, to gases that are primarily propane mixed with ethylene or other chemicals that generally burn hotter than propane. Cutting, gouging, and heating can all be done with reasonable efficiency. Preheat times will generally be longer, though proper tip design can make this time relatively inconsequential. This gas is not generally used for welding in all but the most extreme of cases, but it will work for brazing. Propylene yields flame temperature between 4,800 and 5,300 F.

Propaneand natural gas both have flame temperatures between 4,500 and 4,600 F. These are the least costly of gases under consideration. Propane is the most efficient for heating because of its high BTU output and the large, heavy heating tips that are available. These gases are not used for welding, but are sometimes used for brazing.

Type of Seat

Once the gas is selected, it is necessary to choose the right model of torch tip. Knowing the manufacturer and model number of the torch that you have, or using the model number of the tip that came with the equipment, most easily determines this. Make sure this step is used with other checks, as most equipment assumes the use of acetylene gas. This step is very important because there are safety concerns. If this step is skipped, damage of equipment and flammable gas leakage can result, threatening the safety of the process.

Thickness of Metal

The thickness of the metal being worked is very important because the center hole of the cutting and gouging tips is sized properly for the amount of oxygen and the proper pressure amount of the metal for its design. The preheat orifices are also sized to handle the proper amounts of mixed gas to heat a given thickness of metal adequately.

Type of Application

Tips are designed specifically for the applications in which they are to be used. There are different tips used for welding, cutting, gouging, and heating. They may be of one (of solid copper) or two-piece (a brass inner piece and a copper outer piece) construction. Additionally, tips are specially designed for use on automated cutting machinery and for high-pressure cutting, which permits quicker, cleaner, more accurate cutting that can be accomplished automatically.

Conclusion

There are several thousand types of tips from which to choose. In addition to standard tips that are relatively straight- forward and in common use, there are also special tips that, for example, cut metals more than 3 ft. thick. There are also special tips to cut off the base of a I-beam, or remove the head of the large rivet. Some tips are made 6, 8, 12 or 24 inches long that extend far from the torch. Some tips are bent more than once to cut at odd angles or into the most awkward holes. Tips with stainless steel inserts in the center hole, and special alloy wear rings on the bottom outside for dragging the tips, are also available.



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