
Stainless steel, also known as stainless steel, stainless steel, stainless steel or simply stainless steel and inox, accounts for approximately 3% of the total amount of steel used in almost all industrial sectors and household use.
Stainless steel and rustproof steel are both correct. However, stainless steel is the most common and well-known in common parlance.
Stainless steel is often used in places where good corrosion and/or sanitary resistance is a primary requirement. Just look around you, stainless steel is used almost everywhere. Cutlery, knives, scissors, handles, sinks, hot plates, (sewing) needles, various parts of machines and/or utensils, taps, valves, facade panels, lifts, jewellery, tools, complete process components, piping and constructions are just some of the examples that we encounter every day. The technical and economic importance of stainless steel is therefore much greater than indicated by the above percentage. Stainless steel is more expensive than ordinary carbon steel types. In contrast to many other materials, however, stainless steel can generally be used without a coating and its lifespan is also longer. This is therefore cost-saving and results in lower maintenance costs. It is also less harmful to the environment.
The specific properties of stainless steel differ greatly from those of unalloyed steel. An example of this is corrosion damage, which is caused by the application of certain processing and treatment methods. These methods are suitable for ordinary types of steel, but not for stainless steel. Corrosion damage can occur, among other things, due to the attack of the chromium oxide skin. The chromium forms a bond with the carbon on the surface, so that a chromium oxide skin is formed. This skin is colourless, flexible, dense and adheres very well to the substrate. All this therefore provides extra corrosion resistance. If this chromium oxide skin is therefore attacked by, among other things, mechanical and/or chemical processing methods and/or damage (scratches), then local corrosion damage can occur. You should also pay close attention to the storage and installation of stainless steel. Direct contact with other materials can also cause corrosion problems. Consider, for example, the storage of stainless steel in carbon steel racks and grinding, sawing, milling and welding work. Furthermore, the quality of the oxide layer is strongly dependent on the chemical composition of, among other things, the chromium oxides, iron oxide and nickel oxides, surface finish and the conditions under which the oxide layer is formed. For example, if the layer is formed at a high temperature in combination with oxygen, it is more porous and therefore less corrosion resistant than at lower temperatures.
The removal and/or restoration of the (old) oxide layer can be done by means of mechanical processes such as brushing, grinding, polishing, tumbling, blasting, electrolytic polishing or by means of pickling with a mixture of various acids. HOLLIMEX also has various products in stock for this. Think of grinding, polishing, pickling and cleaning agents, tools, acid-resistant brushes and protective aids such as (acid-protective) gloves, goggles, etc.
The transition from rusting to non-rusting is quite abrupt. For chrome steel, the limit is around 11%. Chrome also has the nice property that it ensures that the material is more resistant to oxidation at higher temperatures. In order to obtain certain specific properties, other elements can be added to the material in addition to chrome. Think of improvements to the material for the purpose of corrosion and wear resistance and the strength, processing and temperature properties.
Some of the additional elements are:
nickel, this gives better resistance to high temperature oxidation and stress corrosion cracking
molybdenum, this provides better resistance to pitting and crevice corrosion
carbon, when lowered it gives better resistance to inter-crystalline corrosion
silicon, this gives better resistance to high temperature oxidation
titanium, this gives better resistance to inter-crystalline corrosion
sulphur and selenium, these provide better machinability.
There are dozens of stainless steel types. To distinguish between them, there are various classification systems. The best-known classification systems in the Netherlands are the American Iron and Steel Institute (AISI) and the German Material Number. The material numbers are generally more precisely defined than the AISI numbers, which is why there are usually several German material numbers that correspond to one AISI number. As an example, we take the material numbers: 1.4404, 1.4408, 1.4435 and 1.4435BN2. These all fall under the AISI type 316L. Manufacturers who only work with AISI types therefore have it much easier and can also often produce more cheaply.
The austenitic stainless steel or the AISI 300 series, represents the largest group of stainless steel types. This group is derived from 18/8 composition. 18 stands for 18% Cr (chromium) and 8 stands for 8% Ni (nickel). The material is non-magnetic, has good corrosion resistance, a high coefficient of expansion and is a poor conductor of heat and electricity. The mechanical strength is moderate, but increases strongly during cold deformation. The material then becomes weakly magnetic.
The most commonly used types in the AISI 300 series are: 304, 304L, 321, 316, 316L and 316Ti. In the material numbers these are the types: 1.4301, 1.4306, 1.4541, 1.4401, 1.4404 and 1.4571. Most of the products that HOLLIMEX has in stock fall under one of these types. The 316 types are generally somewhat more resistant to corrosion and are therefore more suitable for an acidic or maritime climate. The L and Ti types are better suited for welding.