Rustproof steel (stainless steel)
Whether Inox, Cromargan, Nirosta or VA steel – all these names stand for non-rusting or rust-proof steel (generally stainless steel in English). The properties of rust-proof steel are poor thermal conductivity, power electrical conductivity, high toughness, poor machinability and very important, increased resistance to acids and corrosion.
Rustproof steel contains more than 10.5% chromium, which is dissolved in ferritic or austenitic mixed crystals. This constituent causes a dense and therefore protective layer of chromium oxide to form on the surface of the steel. Greater resistance to corrosion is achieved by the constituents nickel, manganese, molybdenum or even niobium in the alloys.
Rustproof steel, without nickel fractions, forms ferritic crystals, which makes it magnetic. If the nickel fractions are higher, the rustproof steels form austenitic structures. The resistance to corrosion therefore increases.
Hardening of the surface
The surface can only be hardened by Kolsterising, plasma nitriding or cold working.
With this process the surface of rustproof steel with an austenitic structure, which has a low wear resistance and hardness, is hardened. With kolsterising, a large quantity of carbon is diffused into the steel. This causes compressive stress, which causes hardening of the surface, however the corrosion resistance does not change.
This process takes place in a chamber in an ionised gas atmosphere. The furnace wall is used as an anode and gases containing nitrogen are used. During plasma nitriding of rustproof steel, positively charged ions from the anode hit the workpiece, used as a cathode, at very high impact speeds. The process takes place in three phases. In the first step this process cleans the surface of the workpiece. In the second step the workpiece is heated and in the third phase the surface is nitrided. This process improves the sliding capability of the metal and corrosion-resistant coatings are formed.
Rustproof steel has good formability properties. This material is therefore used in many areas. Important areas include, for example, the food industry and pharmaceutical industry.