Plasma cutting for pipe, tube

Technology advancements improve edge quality, minimize dross

By Jim Colt

December 2, 2008

Plasma cutting has come a long way since it was developed in the1950s. Includes a sidebar that reflects some tube and pipe trends and a maintenance sidebar.

Plasma cutting of metal plate, tube, and pipe is nothing new—it has been in commercial use since the late 1950s. The process evolved from the gas tungsten arc welding (GTAW) process. Its development was based on implementing various combinations of gas flows and power levels to sever electrically conductive metals.

Plasma relies on an electric arc that superheats an ionized gas stream to create a high-velocity, high-temperature jet that melts and expels molten metal. Some plasma systems use a shielding gas to assist with cooling consumables and constricting the arc.

The shielding gas also improves the workpiece's edge metallurgy.

When it was first devised, plasma cutting was a complement and competitor to oxyfuel cutting. Oxyfuel uses fuel to heat the metal and a jet of oxygen to oxidize it and blow it out of the kerf. The oxidizing process performs the cutting action, so this process is suitable for carbon steel. Plasma filled a niche that oxyfuel couldn't because plasma cuts corrosion-resistant materials such as aluminum and stainless steel. At the same time, plasma competes against oxyfuel because of its speed.

The Early Years

Early plasma cutting systems—those manufactured from 1960 to approximately 1982—used nitrogen or a mixture of argon and hydrogen as the cutting gas and water or carbon dioxide as a shield or secondary gas. These systems were strictly thermal cutting processes, using the extremely hot plasma jet to melt and eject molten material from the workpiece. It was especially useful for stainless steels and aluminum because oxyfuel doesn't cut these materials.

Although plasma cutting brought numerous advantages—nonferrous cutting capability, cutting rates up to six times faster than oxyfuel, and adaptability to automated contour cutting machines—the early machines had several disadvantages. Two chief drawbacks were poor edge squareness and dross (resolidified metal) on the bottom edge of the cut (see Figure 1). The process also was known for high power consumption, high consumable parts usage (nozzles and electrodes), and edge hardening on some materials.

Because of its drawbacks in cutting steel, plasma generally was reserved for cutting stainless steel and aluminum. For cutting carbon steel, manufacturers limited plasma use to applications in which the process's high cutting speed outweighed the poor edge quality.

Two Decades of Progress

From the mid-1980s until 2004 or so, the plasma process was improved through better edge squareness and less dross formation. Power consumption also dropped and consumable life increased. Major technology developments included:

• Oxygen as the plasma-forming gas which, compared to nitrogen, decreases power consumption, improves cut edge metallurgy, and nearly eliminates dross formation on carbon steel.
• Microprocessor controls for power supplies and improved gas flow technology that helps extend the service lives of nozzles and electrodes.
• Solid-state power supplies that control the current more accurately and improve power efficiency.
• Torch shield technology that minimizes consumable damage in the event that the torch contacts the workpiece.
• Automation that shortens the plasma system operator's learning curve and reduces the likelihood of process setup errors.
• High-definition plasma machines, which develop a higher energy density, thereby resulting in a narrower kerf and better edge angularity on material up to 2 in. thick.

The result of this progress is the ability to cutting thicker materials with better edge quality and less dross (see Figure 2).

Modern Developments

Modern plasma cutting systems have embraced PC-based CNC technology, so one controller governs the machine's cutting processes and motions. In addition, programming is less prone to errors than it used to be.

In the past the operators used floppy disks to transfer part programs from a computer to the plasma machine. The operator would turn on the plasma system; set the plasma-forming gas flow and shielding gas flow; adjust the arc voltage, pierce delay, and cut speed; and set a few other parameters.

Today many of these machine controllers have wireless networking capability, making the process of downloading cutting programs seamless. The operator uses the CNC's touchscreen to initiate a wizard and download the cut program. The wizard asks the operator a few more questions, such as material type and material thickness. In a couple of seconds the gas flows and all other process parameters are set automatically.