Most metals are characterised by their stability and strength and are therefore a perfect material for structures, which have to withstand extreme loads. These include, among other things, bridges and steel reinforced skyscrapers. Yet iron and steel have an invisible enemy, which slowly but surely attacks its victims.
It is the oxygen in our air, which we so urgently need to live, which attacks the metals. Ever since man has been able to work metals he has been infuriated by rust. This occurs, in case of poor care and maintenance, on all iron parts and in chemical terms it is iron oxide.
The problem with oxygen
With more than 20% share by volume, oxygen, is the second most frequently occurring gas in our atmosphere and on our planet and by far the most frequently occurring element. Oxygen is highly reactive and forms compounds with most of the other elements. Its decomposing effect however does not start until it combines with water. Through diffusion processes, water removes ions from the iron, which then enables oxygen to attack iron effectively.
Oxygen cannot attack iron easily without water. In metalworking methods such as forging and welding, so much energy is added to the material that the oxygen is given another opportunity to form compounds with the material. During welding a thin scale layer forms on the surface of the glowing iron, which then spalls off during the forming process.
During welding a scale is also produced, which mainly deposits on the weld joints and impairs the quality of the weld.
Shielding gas or inert gas is the name given to all elements that either do not form chemical compounds with other substances or are very non-reactive (inert). The gases helium and argon are particularly suitable for this; they only exist in atomic form and due to their chemical properties they are not able to react with other materials.
Inert gas has a double effect:
- Because of their non-reactiveness they do not attack metals.
- Shielding gases displace breathable air from the machining area and therefore and prevent oxygen from attacking the metal.
If welding is carried out without shielding gas, not only does scale form but atomic oxygen accumulates in the weld pool. Over time, this causes the weld to be far more susceptible to rust than a weld made under shielding gas. With the shielding gas welding procedures MIG, MAG and WIG, a shielding gas mixture of argon and helium is blown directly into the area to be welded through the welding gun. This prevents the reaction with oxygen in the weld pool and the weld is far more resistant, stable and durable.
Nitrogen in laser cutting
Nitrogen also has inert properties and is ideally suited for oxidation-free cutting of metal sheets and plates. Theoretically, an argon-helium mixture could also be used; however, the price of nitrogen is around 40% lower than that of inert gas made with helium. The decision to use nitrogen is therefore also an economic choice, in order to be able to offer laser cutting at reasonable prices.
At Rime, fusion cutting is used, in which a highly focussed laser beam liquefies the material. At the same time, a nozzle blows nitrogen into the cutting gap (clearance), which in turn blows out the liquid metal and protects the cooling material edges from scale formation and slag deposits.
As the nitrogen, like the inert gases, does nor form chemical compounds with the metal, the workpiece edges are very clean and rarely have to be reworked. This fact makes laser machining an economic sheet metalworking method, despite the high nitrogen consumption.