Welders, plumbers, and many other building and fabrication trades frequently use oxy-fuel cutting, a process that uses a fuel gas and oxygen to cut metals. A torch with a specially designed tip is connected to a fuel tank, typically acetylene or one of three popular fuel alternatives, and mixes the fuel with oxygen in order to produce a high-temperature flame cone to cut through metals.
Oxy-cutting with fuels such as acetylene provide simple ways to cut metals rapidly without the wear and tear of a saw. This cutting application is very portable and requires a minimal upfront investment that is especially appealing to the occasional user who doesn't want to invest in an expensive plasma cutting unit.
The following resource article will introduce the basic fuels, cutting tips, usage, and safety precautions for oxy-cutting.
Types of Cutting Fuels
There are four main kinds of gas used for oxy-cutting: acetylene, propane, propylene, and MAPP (methyl acetylene propadiene).
Acetylene: Acetylene has been the cutting fuel of choice for oxy-welding, brazing, and cutting because it reaches the highest possible temperature, provides clean, efficient cuts, and offers versatility. The heat in acetylene's flame is concentrated on the inside of the flame's cone.
The high cost of acetylene and its recent shortage have led many welders to consider alternative fuels that don't necessarily burn quite as hot as acetylene (though the HGX propane additive may change that) but offer their own advantages.
Propane for Oxy-cutting
Propane and other propane-based fuels are frequently used as substitutes for acetylene. In fact, many cutting operations have switched over to propane in order to save on fuel costs. Propane's outer cone offers a high number of BTUs, as opposed to the flame for acetylene which concentrates the heat inside the cone. Welders used to acetylene will need to adapt their techniques by preheating and cutting with the outer cone of a propane flame in order to achieve the appropriate cutting temperature.
Welders who use propane instead of acetylene may need to wait a little longer in order to heat the metal, but if they use the outer edge of the heat cone, they’ll find that preheating won’t take much longer than acetylene. Switching from acetylene to propane or any other alternative fuel will require equipment changes for the torch tip, torch itself, and hose depending on the fuel.
HGX-3 is a new product on the market that can be added to propane in order to increase its performance. This mixture, referred to commonly as HGX propane, cuts metal at temperatures comparable to acetylene. One gallon of HGX-3 can be added to 1,000 gallons of propane.
HGX-3 increases the flame temperature of propane gas by 15%, reaching flame temperatures of 5400°F and uses less oxygen than acetylene. This makes HGX propane a viable alternative for welders used to the heat output and speed achieved with acetylene for flame cutting. Much like other alternative fuels, HGX propane reduces slag and makes for a smoother, cleaner cut.
One of the biggest advantages alternative fuel gases such as HGX propane have over acetylene is its supply and storage. Acetylene must be supplied in individual cylinders with a maximum capacity of approximately 400 cubic feet per cylinder, while the alternative fuel gases can be supplied in either cylinders, bulk stations, and even from a pipeline.
Propylene: Much like propane, propylene is often misunderstood as ineffective as a fuel for cutting since it requires an injector torch in order to achieve optimal heat flow and cuts and concentrates the heat on the outer edges of the heat cone. The tips for propylene rarely need cleaning and provide eight holes for effective pre-heating. In other words, effectively using propylene rests entirely on having the correct torch and tip setup.
Welding Tips and Tricks recommends propylene as a great alternative since it offers no 15 psi limit on working pressures, no soot, more BTU's of heat for heating thick metal, and quick cuts on metal.
MAPP gas is commonly used instead of acetylene because it can be used above 15 psi and is therefore far less dangerous while cutting steel up to 12 inches thick. MAPP burns at a lower temperature than acetylene and is a liquefied petroleum gas that can be stored more easily since it compresses easily.
MAPP gas is more commonly used as a substitute for acetylene in Oxy-welding. While other gases such as propane, propylene, and HGX Propane are more widely used for cutting and brazing, MAPP can be used in place of acetylene, even though MAPP doesn’t burn quite as hot and can be expensive for large-scale operations.
Tips on Cutting Torches
The tips on torches are designed for each specific kind of fuel and will determine how effectively you can cut. Using the wrong tip for a particular kind of fuel will prevent you from reaching the optimal temperature and will limit the torch's effectiveness for cutting. For example, an acetylene torch tip does not have the correct number of holes for cutting with propane, and therefore propane with an acetylene tip won't reach optimal heat and will be extremely ineffective for cutting.
Torch cutting tips come in two styles: one piece and two-piece.
One-piece tips are made from copper alloy and are used with acetylene. They are machined with either 4 or 6 preheated holes and can handle light, medium, and heavy preheats. There are different 1-piece torch cutting tips that perform different functions from gouging out metal to cutting sheet metal and other specialty functions. One-piece tips use methyl acetylene propadiene (MAPP), acetylene, and propylene, though each fuel has a different number of holes that match a particular kind of fuel, so be sure to take note of the number of holes in each torch tip.
Two-piece tips require cooler and slower burning fuel gases such as propane. In the case of propane, it's also critical to use the right kind of torch. An injector torch allows welders to make cuts that some claim to be cleaner and faster than acetylene. Propane also offers more versatility for bending and heating.
Finding the right tip for a job can be confusing. The American Welding Society (AWS) issued a Uniform Designation System for Oxy-Fuel Nozzles back in 2000. In it they asked that all standard tips have the manufacturer's name stamped on them as well as the identifying fuel symbol, maximum material thickness, and part number for data and reference; however many manufacturers still do not follow these designations due to the extra manufacturing costs entailed. When shopping for tips, check the oxygen bore size, orifice size, and fuel gas required.
Torch Cutting Basics
When oxy-cutting, managing the oxygen flow rate will determine what kind of cut is made. Torch cutting with too much oxygen both adds to the overall cost and results in a wide cut with curved edges, rather than the sharp, clean edges that are desired when cutting with a torch. Too little oxygen produces a slow, uneven cut that will be frustrating. The flow rate of the oxygen should match the manufacturer's specifications for the torch tip used for cutting.
Cutting with a torch is performed by preheating the metal and then cutting at a high temperature. This process will melt the metal being cut. Therefore set up a grate or other object that can catch the melted metal.
Oxy-cutting is primarily used for cutting steel. It is almost always twice as fast as a grinder and can cut large sections with very little effort or noise. Ferrous metals up to two inches thick can be effectively cut with oxy-cutting fuels such as acetylene.
Torch Cutting Safety
Oxy-cutting fuels are among the most flammable and require proper safety precautions for storage and usage. Acetylene is hazardous above 15 psi pressure, as it becomes unstable and explosively decomposes.
One should never use concrete as base for cutting since concrete holds water. When the cutting torches heat makes contact with the concrete, the heat will cause the water in the concrete to expand and the concrete to explode.
Take extra caution and time when cleaning the tips of cutting torches. Make sure the orifice is always free of any debris and smooth. Use protective welding gear at all times and welding screens. Make sure you are using the right size tip for the job and that the tip matches the fuel use.
Hoses should regularly be checked for fuel leaks. In addition, using the wrong kind of hose for a particular fuel will deteriorate your hose over time. While some welders have found they can occasionally use a fuel such as propane with an acetylene hose, the best long term solution is to use the proper hose with each type of fuel.
If your hose leaks, fuel gases that are denser than air (Propane, Propylene, MAPP, Butane, etc...) will most likely collect in lower areas and present a flame hazard, especially in basements, sinks, storm drains, and other enclosed areas. Cutting fuels are made to burn, so take proper precautions with your work clothing, location of fire extinguishers, ventilation, and equipment maintenance.