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Thermal Cutting Technologies
Thermal cutting processes separate materials by applying heat, with or without a stream of cutting oxygen. The three dominant processes are oxy-fuel, plasma and laser cutting.
The principle behind oxy-fuel (flame) cutting is simple: When hydrocarbons are oxidized, they generate heat. As with other flame processes, oxy-fuel cutting does not require expensive equipment, the energy source is easily transported, and most processes do not require electricity or cooling water. A torch and a fuel-gas fuel-gas cylinder are typically sufficient.
Oxy-fuel cutting is the predominant process for cutting heavy-gauge mild and low-alloy steel. After the oxy-fuel flame brings the material to ignition temperature the oxygen jet is turned on and burns through the material. How quickly ignition temperature is reached is a function of the fuel gas. Once the cut is established, the cutting speed depends on the purity of the oxygen and the velocity of the oxygen gas jet. High purity oxygen, optimized nozzle design and correct fuel gas mean high productivity and minimized overall process cost.
Plasma (arc) cutting was developed in the 1950s, to cut metals that could not be flame-cut (e.g., stainless steels, aluminum and copper). With plasma cutting, gas is ionized in the nozzle and focused through the nozzle’s special design. This hot plasma stream alone can be used to cut materials such as plastics (non-transferred arc). In metal cutting, an electrical arc is also ignited between the electrode and the work piece, to increase the energy transfer. A very narrow nozzle orifice focuses the arc and the plasma stream. Additional lacing of the discharge path can be achieved by a secondary gas (shroud gas). Selecting the correct plasma – shroud gas combination can significantly reduce overall process cost.
Laser cutting is the youngest thermal cutting technology and Linde’s LASERLINE product offering serves the unique needs of laser users. Three areas are of interest here:
The laser beam is produced in the resonator cavity of the laser cutting system. While consumption of the resonator gas is low it’s purity and correct composition are critical. Linde’s LASERMIX resonator gases are quality controlled to protect the expensive equipment and optimize cutting performance. Linde offers high purity supply equipment to maintain the purity of the resonator gas from the cylinder all the way into the resonator cavity.
In order to cut the laser beam has to travel from the resonator to the cutting head through the beam path system. This system needs to be kept free of solvents, particles and fumes. Especially on high power systems (>4kW) nitrogen from a liquid source is recommended.
In laser cutting oxygen or nitrogen can serve as cutting gas. Oxygen is used for mild and low alloyed steel and the process is similar to oxy-fuel cutting. The purity of the oxygen is again important. Nitrogen is used for stainless steel, aluminum and nickel alloys to achieve a clean edge and maintain the critical properties of the base material. Here the purity of the nitrogen is important and the elevated pressure requirements require adequate booster technology.
Important gas shielded welding processes can be found in arc welding and laser welding. Within its ARCLINE product offering Linde has developed several families of shielding gas, such as CORGON, CRONIGON, VARIGON and MISON
In all arc processes, the electrode, molten pool and heat-affected zones must be protected from reaction with the ambient air. This is the primary role of the shielding gas. In addition, the correct shielding gas can also positively affect:
- Weld properties such as strength, toughness and corrosion resistance
- Weld porosity, bead shape and size
- Welding speed, arc stability and amount of spatter
Your choice of shielding gas depends on your daily use and your weld specifications. Shielding gas represents only a small portion of the welding cost per inch, but the use of an appropriate gas can significantly reduce labor costs, by increasing weld speed or reducing finishing efforts (e.g., spatter removal or removal of excessive crown).
Special materials such as aluminum or nickel base alloys sometime require a fine-tuned shielding gas to achieve optimal welding results in high volume production environments. Here Linde’s doped shielding gases offer quality gains at reduced production cost.
During high-power laser welding, light energy melts and evaporates the metal. The pressure of the vapor displaces the molten metal so that a keyhole is formed, which guides the laser beam deep into the material. In this way, keyhole welding allows for very deep and narrow welds and is therefore also called “deep penetration welding.” It is a fast welding process with low heat input and low distortion rate.
Shielding gas plays an important role in laser welding and fulfills several tasks:
- Shielding of the weld pool and the HAZ
- Protection of the optics against fumes and spatter
- Plasma control during CO2 laser welding
To control this plasma, pure helium (which has a high ionization potential) can be used as a shielding gas. Helium, however, is expensive. Under the trademark LASGON® Linde has therefore developed several argon-based shielding gases for laser welding that offer performance and quality gains.
LASGON® C are mixtures of helium, argon and carbon dioxide. They are especially suitable for mild and low-alloy steel, including galvanized sheet metal. LASGON® H mixtures have been developed for laser welding of stainless steel components such as pipes, sensor casings, cabinets etc. with CO2 lasers and also diode, Nd:YAG or fiber lasers. Compared to pure inert gases, LASGON® H allows higher welding speeds and produces less oxide build-up, resulting in a clean and shiny surface.