An overview of oxyfuel cutting

An oxyfuel torch is still a very effective way to cut plate thicker than 0.75 in. Images: ESAB Welding & Cutting Products

If you are a reader of The Fabricator magazine, you know that the metal fabricating industry is in a kind of arms race as it relates to laser cutting. Just how much power is enough?

At this writing, 30-kW laser cutting machines are being delivered to metal fabricating companies in the U.S. Some of those same companies have indicated a willingness to purchase even more powerful machines when they become commercialized. These shops have an insatiable appetite to have the latest in laser cutting technology.

They feel that such cutting power puts them in a better competitive position. They can cut run-of-the-mill gauge materials at much faster rates than nearby metal fabricators, and the extra power also gives them the opportunity to cut even thicker metals at speeds that can’t be matched by more conventional plate cutting technologies.

Does that mean that a process like mechanized oxyfuel cutting is an endangered species? No. In fact, given the considerable investment needed for these high-powered laser cutting machines, a tried-and-true technology like oxyfuel cutting is showing no signs of being banished from the shop floor.

A Description of the Oxyfuel Cutting Process

John Henderson, a sales director at ESAB, recalled a conversation he had with an engineer in an automotive manufacturing facility a few years back. Henderson told the young man that the company likely had oxyfuel cutting capability and he didn’t realize it. The engineer scoffed at the suggestion as he looked around at the robots and semiautomated equipment surrounding him. Henderson suggested they take a walk to the maintenance shop, and sure enough, an oxyfuel system was easy to find.

“From large fab shops to small fab operations to those guys working out of the back of a truck, oxyfuel cutting is still prevalent,” he said.

The process, also referred to as torch or flame cutting, has been around for more than 120 years. Two French engineers, Edmond Fouché and Charles Picard, are credited with coming up with the combination of oxygen and acetylene for welding purposes. Even though it is still taught in some schools, oxyfuel welding has been supplanted by arc welding processes in most instances. But whenever it might prove difficult to get a welding power source to a particular location, oxyfuel welding still makes sense.

Oxyfuel cutting, on the other hand, remains a useful tool for heavy-duty fabricating shops all over the world. It’s a cost-effective cutting tool, and it can produce cut edges that display no signs of a bevel. Henderson said that an experienced operator of a mechanized oxyfuel table can deliver edges that look like they came off a machining center.

Oxyfuel cutting relies on preheating of the steel material and the subsequent use of oxygen to begin a rapid oxidation process, which deteriorates the steel and makes it susceptible to being blown through the kerf. Henderson called it “accelerated rusting.”

“People see the flame that’s burning and think that’s what does the cutting, but that’s not the case,” he said. “The flame preheats the material to get it to the point where it can reach the kindling temperature. At that point, when you add pure oxygen to it, the oxygen is removing the material. So it’s technically oxygen that does your cutting.”

Although oxyfuel cutting is sometimes referred to as flame cutting, an oxygen stream does the actual removal of material. The flame actually preheats the steel, which allows the oxygen stream to begin the cutting action.

A fuel gas (typically acetylene, but also propane, natural gas, or even proprietary mixes) is combined with oxygen inside the cutting torch and is ignited outside of the nozzle to deliver the flame used to preheat the metal. Dialing in the correct fuel-to-oxygen ratio enables operators to produce the highest possible temperature with the most efficient flame, which concentrates the heat in a small area.

When that kindling temperature reaches the temperature at which the material starts to turn a red color (approximately 1,600 degrees F), a high-pressure stream of pure oxygen is directed toward the area to be pierced. That begins the rapid oxidation process, and the oxidized steel changes into molten slag.

Henderson said if the proper gas flow is being used, it should provide the impetus to continue the pierce through the material and blow the molten slag out of the pierced hole.

Once the piercing is completed, the torch can be moved in any direction to commence the cutting action. The flame continues even as the torch moves because the metal needs to be oxidized as it was during the initial pierce.

The oxyfuel cutting process is effective only on those materials with oxides that have lower melting points than the base metal itself. If oxyfuel cutting is used on a metal that has an oxide with a higher melting point than the actual base metal, a protective crust will form as soon as the oxygen jet is introduced, effectively ending the cutting process. That’s why oxyfuel cutting is typically limited to low-carbon steels and some low alloys.

Technology Advances

Oxyfuel cutting might be more than 100 years old, but that doesn’t mean it hasn’t improved over the years, particularly mechanized cutting. In metal fabricating, the push for better production results occurs every day, which forces technology providers to investigate better ways to deliver the expected results.

“When you get into automated or mechanized cutting, we always look to increase cutting speeds,” Henderson said. “In order to do that, you have to find ways to deliver performance greater than what conventional tip technology can provide.”

That’s where divergent tip design makes a difference. It provides a restriction of the gas flow, which accelerates the oxygen velocity exiting the nozzle. Think of it like someone placing a thumb over the open end of a garden hose. As that opening is partially covered with a thumb, the water stream exiting the hose picks up speed.

Conventional thinking suggests that larger or oversized nozzles might be better for most oxyfuel cutting applications, but that’s not true. In thin-gauge material, the oversized nozzle is going to deliver more gas flow, but it’s also going to result in reduced cut quality. When cutting thick material, an oversized nozzle also results in wasted gas, and in some cases where several oversized nozzles are used, the squareness of the cut can be affected, with the bottom of the kerf wider than the top of the kerf.

“A lot of engineering went into the divergent tip design to be able to cut at greater speeds,” Henderson said.

The divergent tips are especially helpful when cutting thicker materials, say, anything thicker than 0.75 in. The higher pressure required to operate a divergent tip successfully, around 75 to 100 PSIG, helps to deliver the gas flow to and through the bottom of the cut. Cutting with a conventional tip can be challenging when working with thick materials because the velocity of the gas stream decreases as the distance from the nozzle increases. Conventional tips operate best at about 40 PSIG. (ESAB officials report that oxyfuel cutting tips typically operate satisfactorily at pressures +/- 10 PSIG.)

Even though the divergent tips call for higher operating pressure than conventional tips and can cut faster, they actually consume less oxygen per foot of cut than typical oxyfuel cutting approaches. Also, the cut speeds shouldn’t be underplayed. Because oxyfuel cutting is traditionally limited to cutting speeds of less than 20 IPM, an increase of even 1 IPM is significant.

Henderson added that manufacturers are constantly striving to make the equipment more user-friendly. Nozzles and tips are designed to be easily removed and replaced. Torch design also allows for more effective cooling, which in turn helps to extend the life span of oxyfuel cutting consumables.

When asked what metal fabricators will be asking of oxyfuel equipment suppliers in the coming years, Henderson said, “Let me control the elements that are there, like gas flow and cutting speed, but give me tools that give me better consistency in everything.”

Putting Oxyfuel Cutting to Use

Running a mechanized oxyfuel table isn’t the most difficult assignment, but it takes someone with experience to get the most out of the equipment.

For example, an operator might be able to get an oxyfuel table up and running, but that doesn’t translate into a consistent cut. Preheating of the material has to remain consistent so that when the oxygen stream reaches the preheated material, it can deliver the desired edge. If the torch hits an improperly preheated portion of the plate, a smooth edge might be impossible to deliver.

Henderson shared another instance where an operator can make a difference, this time an all-too-familiar cutting application in large plate fabricating operations. To maximize throughput, shops often have multiple torches on their flame-cutting machines. If the torches aren’t balanced—performing in the same manner—cut quality can be negatively affected.

“For instance, if I had one torch that was dialed in perfectly and I had another one that preheats hotter than it needs to, I’m not going to have a smooth edge on some of the parts because I’m overheating the top of the material,” Henderson said.

The operator’s influence can make a really big difference in some of these large-scale cutting operations. Henderson described one shop that had 20 torches on a 400-ft.-long table. That’s a lot to keep track of, but when the oxyfuel table is working as it should, it’s producing quality parts that likely won’t require postcutting processing.

Cutting-edge technology has its place in metal fabricating, but at the end of the day, cost-effective manufacturing practices tend to dominate where they make the most sense. For heavy-duty fab shops processing a lot of plate, oxyfuel cutting still makes a lot of sense.