Oxygen purity level in metal cutting
Oxygen is the most abundant element on earth. It is a colorless, odorless and tasteless gas that supports life and makes combustion possible.
Oxygen makes up about a fifth of the Earth’s atmosphere (20.99% by volume). This clear, blue liquid is lighter than water at temperatures below -300 degrees Fahrenheit.
All elements except the inert gases combine with oxygen and are usually oxidized. Oxygen purity is critical for efficient production. For example, 99.5% pure oxygen can produce high-quality edges and cutting flames.
A 1% decrease in oxygen purity reduces the cutting speed by 25%. With 95% oxygen purity, an oxygen-fueled cutting torch will not cut steel at all.
Cylinder oxygen is produced from vaporized liquid oxygen. Its minimum guaranteed purity is 99.5%. Liquid oxygen is classified as Type II by the Compressed Gas Association (CGA).
Other grades of CGA oxygen are available by special order. It ranges from CGA Grade C, also 99.5% oxygen content to 99.995% oxygen. ULSI and VLSI ultra-high purity grades of 99.9999% pure liquid oxygen are also available for customers who require exacting purity standards.
How much oxygen is used in cutting?
Cutting speed and cutting edge quality are primarily determined by the purity of the oxygen flow. Therefore, the design of the nozzle plays an important role in protecting the flow of oxygen from the air bubble.
The oxygen used for cutting must have a purity of 99.5% or higher. Less purity reduces the cutting efficiency. For example, a 1% reduction in oxygen purity to 98.5% results in a reduction in cutting speed of approximately 15% and an increase in cutting oxygen consumption of approximately 25%.
Oxy-fuel cutting is a thermal cutting process that uses oxygen and fuel gases (such as acetylene, propane, MAPP, propylene and natural gas) to cut materials.
The oxyfuel process is the most widely used industrial thermal cutting process. Because it can cut thicknesses from 0.5 mm to 250 mm. The equipment is inexpensive and can be used manually or mechanized. There are several fuel gas and nozzle design options that can significantly increase performance in terms of cut quality and cutting speed.
Principles of oxygen process in metal cutting
Basically, a mixture of oxygen and fuel gas is used to preheat the metal to its “ignition” temperature, which for steel is 700-900°C (bright red heat). But it is much lower than its melting point. A jet of pure oxygen is then directed into the preheated area, stimulating an intense exothermic chemical reaction between the oxygen and the metal to form iron oxide or slag. The oxygen jet destroys the slag, enabling the jet to pass through the material and continue to cut the material.
Four basic conditions for Oxy Cut There is fuel:
- The ignition temperature of the material must be lower than its melting point, otherwise the material will melt and be destroyed before cutting.
- The melting point of the oxide must be lower than the melting point of the surrounding material so that it can be removed mechanically by the oxygen jet.
- The oxidation reaction between the oxygen jet and the metal must be sufficient to maintain the combustion temperature.
- At least gaseous reaction products should be produced so that oxygen is not diluted.
Since stainless steel, cast iron and non-ferrous metals form refractory oxides, that is, the melting point of the oxide is higher than that of the material, the powder must be injected into the flame to form the low melting point, fluid slag.
What is the purity of the industrial oxygen used to cut the sheet?
Industrial oxygen is usually supplied in cylinders with a minimum purity of 99.5%. In general, this is not sufficient for laser cutting, as it does not result in the best cut quality or the fastest cutting speed.
With 95% oxygen purity, an oxygen-fueled cutting torch will not cut steel at all. Cylinder oxygen is produced from vaporized liquid oxygen. Its minimum guaranteed purity is 99.5%. Liquid oxygen is classified as Type II by the Compressed Gas Association (CGA).
The cut oxygen stream is ignited when the metal reaches an orange color, which is its ignition temperature (1600-1800°F).
The flow of high purity oxygen causes the steel to oxidize quickly and this reaction is very exothermic. An exothermic reaction is a chemical reaction that releases energy by light or heat.
Carbon steel and stainless steel can be cut with either oxygen or nitrogen, while aluminum must be cut with nitrogen.
Basically, a mixture of oxygen and fuel gas is used to preheat the metal to its “ignition” temperature, which for steel is 700-900°C (bright red heat), but well below its melting point. The oxygen jet destroys the slag, enabling the jet to pass through the material and continue to cut the material.
Why is oxygen needed in the steel industry?
The lance blows 99% pure oxygen onto the hot metal, igniting the dissolved carbon in the steel. As a result, it forms carbon monoxide and carbon dioxide and causes the temperature to rise to about 1700 degrees Celsius. It melts waste, reduces the carbon content of molten iron and helps remove unwanted chemical elements.
Safety in working with oxygen
Oxygen makes things very easy to catch fire. Even materials that do not normally burn, such as wool or steel, will burn in an oxygen atmosphere. Only equipment “cleared for oxygen service” should be used for gaseous or liquid oxygen.
Never use oil or organic lubricants on oxygen valves and regulators. Pure oxygen reacts almost five times faster in oxidizing organic matter than in air, which is why spontaneous combustion is more likely to occur.
Care must be taken in housekeeping and when transporting the product so as not to allow oxygen to come into contact with organic or flammable materials.
Paints, thinners, and cleaning solvents should also be kept away from exposure to oxygen, and no sources of ignition should be available in areas where oxygen is stored or used. Smoking is also prohibited in areas where oxygen is consumed and stored.
When working with oxygen equipment, use clean gloves or hands that have been washed and free of any oil or grease.
Liquid oxygen is very cold and causes cryogenic “burns”. Eyes and lungs are especially sensitive to cold vapors. Protect eyes and skin from exposure to low-temperature materials with safety glasses, loose-fitting gloves, and protective clothing.
Oxygen is non-toxic, but high concentrations may damage the respiratory system over time.
Clothing that has absorbed liquid oxygen or oxygen gas must be removed and aired for at least 30 minutes before it is considered safe to wear.
