A material with over 100 years of history

The French geologist and mineralogist Pierre Berthier recognised as early as 1821 that adding chromium to steel makes it rust-resistant. The technological applications and manufacturing capabilities of today were not yet possible back then.

Towards the end of the 19th century, tar, chlorine, corrosive acids and toxic alkalis found their way into the chemical industry. The types of steel used at that time, which rusted even under the influence of moisture, suffered even greater damage from these aggressive substances. The affected companies urgently needed a metal that could withstand both corrosion and high temperatures and pressures. Thus began the search for a rust-free, corrosion-resistant steel alloy.

In 1912, the Austrian engineer Max Mauermann invented the first rust-free and acid-resistant chromium and chromium-nickel steels, which he presented a year later at the Vienna Adria Exhibition. Also in 1912, Benno Strauß, head of the Physics Department at Krupp AG, together with the engineer Eduard Maurer, developed a process for producing a stainless steel. This was based on a nickel-chromium alloy (18% Cr, 8 to 10% Ni) and, thanks to a special heat treatment known as final annealing, remained cold-formable whilst exhibiting exceptional strength. The patent for this was granted in 1918.

Stainless, but not free from Oxidation

Whether steel rusts or not depends primarily on its chromium content. Stainless properties are achieved when the alloy contains more than 10% chromium. Stainless steels labelled as such contain an average of 14% chromium.

The high chromium content causes a dense, protective passive layer of chromium dioxide to form on the surface of the material. This layer can also be used, through a special treatment, to colour the steel surface.

Other alloying elements such as nickel, manganese, molybdenum or niobium provide even better corrosion resistance and more favourable mechanical properties. Due to the high proportion of alloying elements, stainless steels cost significantly more than conventional steel.

Contrary to popular belief, stainless steel can indeed oxidise. Particularly in aggressive environments, some grades of stainless steel offer insufficient protection against corrosion and are therefore unsuitable, for example in:

  • high levels of humidity,
  • high chlorine or salt content,
  • the presence of sulphur dioxide,
  • a very high or very low pH value,
  • high temperatures.

It is therefore important to choose a grade of stainless steel that meets the requirements of the specific application and the environmental conditions. Stainless steel offers particular advantages over conventional steel grades, especially when used outdoors.

Different Grades of Stainless Steel for Different Applications

Stainless steels are classified into ferritic, austenitic and martensitic grades based on their crystal structure. Ferritic steels consist primarily of iron and carbon. Chromium is added as an alloying element, with a content of between 13 and 18 per cent. To optimise the material’s properties, other elements such as titanium, niobium or zirconium may be added. These can, for example, increase hardness or improve weldability. Ferritic structures are slightly magnetic and cannot be hardened.

Austenitic steels contain relatively high proportions of chromium and nickel (6 to 26%). Together, these two elements make up at least a quarter of the total alloy. The best-known grades are V2A (material number: 1.4301) and V4A (material numbers: 1.4401, 1.4404 and 1.4571). V2A is acid-resistant, weldable using all methods and heat-resistant up to 600 °C. It is particularly well-suited to deep drawing, roll bending and edge bending. V4A is additionally alloyed with 2% molybdenum. This makes the steel more resistant to corrosion in chloride-containing media.

Martensitic stainless steels are particularly hard-wearing. In terms of their composition, they are similar to ferritic steel and, like it, are magnetic. Typically, they contain a relatively high proportion of carbon (0.1 to 1.2%), 12 to 16% chromium and a small amount of nickel, and, less commonly, molybdenum. They can be hardened to over 1,000 HV by rapid cooling, but cannot subsequently be welded or plastically deformed. Due to the high carbon content, their corrosion resistance is generally poorer than that of other grades.

Another variant is duplex stainless steel, a combination of austenitic and ferritic crystal structures within a single material. This steel achieves twice the yield strength of austenitic stainless steel. Further advantages include increased hardness, a lower coefficient of thermal expansion, and improved weldability and toughness. Duplex is also more resistant to stress corrosion. However, the temperature range in which these steels can be used is limited to +280 °C.

The influence of alloying elements on steel properties

Desired and undesired alloying elements can significantly influence the properties of steel. Desired elements impart specific mechanical and chemical properties, such as good corrosion resistance, high strength, and good formability and machinability. However, other accompanying elements can adversely affect these properties in unintended ways.

Carbon lowers the melting point of iron and increases hardness and tensile strength. If this element is present in large quantities, the brittleness of the steel increases, whilst formability, weldability and elongation at break decrease. Chromium lowers the critical cooling rate and increases wear resistance and heat resistance. At the same time, it increases tensile strength because it acts as a carbide former. From a mass content of 12.2%, it also significantly increases corrosion resistance and is therefore a key factor in the production of stainless steel.

The addition of nickel (from 8%), phosphorus and titanium also increases corrosion resistance. The same applies to molybdenum, which should account for approximately 1% of the composition.

Working With Stainless Steel: The Options Available

In general, stainless steels are easy to work with. To achieve the best results, machining should be carried out in an area used exclusively for stainless steel. If this is not possible, the work area must be thoroughly cleaned before any work on stainless steel begins.

Various cutting methods are suitable for stainless steel grades. In the industrial sector, laser cutting machines, plasma cutters and waterjet cutting are used for this purpose. Other possible processing methods include:

When welding, it is important to select the correct method and the appropriate welding consumables for the specific grade of stainless steel. Incorrect choices in this regard can promote cracking (stress cracks, crevice corrosion) and thus impair the quality of the weld.

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