For many welding-related jobs, metal cutting is required before a welding arc is started. Two popular metal-cutting processes to choose from are plasma arc and oxy-fuel. Both systems have advantages and disadvantages, so determining which tool is the best fit depends on numerous factors, including the type and thickness of the metal, the power resources available and the cost and location of the job.
However, oxy-fuel torches probably are most suitable for scrap yard applications and are by far the most common means of cutting metals in that environment.
This article touches on the basics of each process and the pros and cons of plasma versus oxy-fuel in metal recycling operations.
Plasma is an ionized gas that conducts electricity. It is created by adding energy to an electrically neutral gas. The energy is electricity, and the gas is typically compressed air. Both of these elements are combined in a chamber between an electrode and a nozzle, causing the gas to become imbalanced, creating plasma gas. Air pressure forces the plasma gas through an orifice in the nozzle, creating a concise constricted flow that is electrically conductive. The more energy added via the plasma cutter, the hotter the plasma arc becomes, providing more cutting capacity and efficiency.
Plasma cutters are used to perform cutting and gouging operations, with the average handheld system capable of cutting a maximum metal thickness of about 1 inch. Plasma typically requires a source for compressed air and a substantial amount of electrical power, which greatly restricts its portability.
One of plasma’s greatest advantages is its ability to cut nonferrous metals, such as aluminum and stainless steel, and cast iron. Speed and precision cutting also are benefits of plasma, which typically cuts with minimal slag and can provide smooth cuts with a narrower kerf (the slit or notch made by a cutting torch) than that produced by an oxy-fuel torch. Plasma does not require the metal to be preheated before cutting, and plasma cutters also outperform oxy-fuel torches when cutting stacked metals. Faster speeds can be achieved on thinner metals with plasma with minimal or no metal distortion. Also, plasma systems are relatively simple to use compared with oxy-fuel systems, with the benefit of minimal cleanup, since the air pressure used in plasma systems blows away the molten cuttings and much of the dross.
Oxy-fuel torches have a leg up on plasma torches when it comes to portability and cutting thicker metals. Oxy-fuel torches also offer versatility, as they are capable of cutting, welding, brazing, soldering, heating and gouging. The average handheld system can cut steel 6 to 12 inches thick. However, oxy-fuel hand torches used for scrap operations typically are able to cut steel more than 20 inches thick.
For jobs that require a high degree of portability, oxy-fuel has an advantage because it’s not dependent on a primary power or compressed air source as plasma is. Some oxy-fuel systems are quite portable, with a small setup weighing about 35 pounds. With oxy-fuel tanks and a torch, it is possible to cut steel almost anywhere.
Oxy-fuel torches are normally used for cutting only ferrous metals and for the most part are not used for cutting cast iron, aluminum or stainless steel.
For thicker steels of more than 1 inch, oxy-fuel torches are capable of greater cutting speeds when compared with typical 100-amp handheld plasma cutting systems. Also, certain operations are exclusive to oxy-fuel systems, including fusion welding of ferrous metals, heat treating, heat shaping, riser cutting, soldering and brazing operations.
With oxy-fuel cutting, an oxygen/fuel gas flame preheats the steel to its ignition temperature. A high-powered oxygen jet is then directed at the metal, creating a chemical reaction between the oxygen and the metal to form iron oxide, also known as slag. The high-powered oxygen jet removes the slag from the kerf.
When using oxy-fuel torches, cut quality, preheating times and metal thicknesses all may be influenced by the type of fuel gas used.
Four basic fuel gases are used most frequently in combination with oxygen for this process: acetylene, propane, propylene and natural gas. Fuel gases are typically chosen according to the cutting application, cost, heat output and oxygen consumption. Propane and propylene are the most widely used gases for scrap metal cutting operations.
Pros and Cons
Oxy-fuel torches are the most common means of cutting metals in the scrap yard for numerous reasons. Torches are available in extended lengths to keep the operator at a distance from the heat, flames and slag produced while cutting.
Most torch hoses are connected to a set of cylinders on a portable cart or, in a few cases, to a stationary manifold header system. The use of long hoses allows greater portability than is afforded by a plasma cutter, which requires electrical power and a compressed air source.
Oxy-fuel torches are most commonly used with 75- to 100-foot hoses, though in some operations torch hoses of up to 200 feet may be used. In scrapping operations, oxy-fuel torches also can be used for rough cutting of cast iron, stainless steel and even aluminum, to reduce larger scrap items into smaller manageable pieces.
For the most part, plasma cutting systems have more limited capabilities in the scrap yard, though they can offer benefits for cutting thinner ferrous and nonferrous metals, including shaped metal, such as angles, channels and tubes, for example, that are often offered for retail metal sale at some locations. Plasma cutters also are better able to cut large volumes of thin sheet metal for recycling and offer the fastest grate-cutting capability.
Cost considerations also likely will come into play when choosing between an oxy-fuel and a plasma arc cutting system. Plasma arc cutting is more expensive to start from scratch. A typical handheld plasma cutting unit costs about $1,500 to $3,500, and replacing plasma tips and electrodes costs about $15 to $20 per set. The cost of electricity also must be considered when using plasma.
An oxy-fuel cutting outfit costs from $250 to $900, with replacement cutting tips costing from $10 to $20. Costs also are associated with refilling the oxygen and fuel cylinders and cylinder handling.
Two important questions to ask when choosing between plasma and oxy-fuel cutting tools are: What needs to be cut on a day-to-day basis and what is the thickest metal that will need to be cut? If the job consistently requires cutting thicker metals, the time and money saved by quickly cutting through thick metal with an oxy-fuel system makes a difference.
On the flip side, if precision cutting of stainless steel and aluminum is important, a plasma arc system is the way to go. (For a quick look at the system comparisons in scrap cutting operations, see the accompanying chart above.)
Both systems have their place in most metal-processing applications. Most recycling operations can benefit from having both systems in their arsenals, though oxy-fuel torches are by far the more common choice in recycling operations.
The author works in metal cutting communications at Miller Electric Mfg. Co., Appleton, Wis. More information is available at www.millerwelds.com.