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STAINLESS STEEL – THE MATERIAL

History

One of the first "non-rusting stainless steels" was patented in 1912 by the Krupp company in Essen.Alloys which can be grouped under the generic term "stainless steels" developed much earlier (around 1740, crucible steel).

Conceptual classification

For our reflections it is important to differentiate between normal "stainless steels" and a special subgroup of particularly non-corrosive, high-grade, high-chromium steels known in Germany as "Edelstahl Rostfrei" or "stainless steel rustfree" (refered to forewith as stainless steel.

"Stainless Steel" is characterized by a particularly high resistance to many environmental influences that promote natural corrosion.

But there are certain chemical substances and environmental influences that can attack stainless steel, such as hydrochloric acid or a lack of oxygen in the ambient atmosphere. The terms "non-rusting" and "rustfree" must therefore be seen merely as a memorable key value terms.

What is "Stainless Steel" actually made of?

Of course, mainly from the element iron (FE)! This element is, however, generally no longer stated in the material descriptions. In many material descriptions only alloying elements are named, for example, Cr = chromium, Ni = nickel, Mo = molybdenum etc. which are important for their properties.

What makes stainless steel is so resistant to corrosion?

Chromium (Cr) from a proportion of approximately 12% upwards is one of the main alloy elements and substantial in the formation, together with oxygen, of a protective oxide layer on the surface of the steel. This process is called "passivation" and it can take several days. In order to shorten the time in the case of surface damage e.g. by mechanical processing, this process can be significantly accelerated with the use of nitric acid (an oxidizing acid). The addition of further alloying metals, such as Mo = molybdenum, significantly improves corrosion resistance.

What must be paid attention to?

In environmental areas where no oxygen exists, stainless steel can corrode just like any other steel if a protective layer has not been previously formed. For example, in swampy or muddy low-oxygen waters, welding seams can corrode very quickly if they were not post-treated properly. In addition, the influence of heat such as by improper welding or heat treatment (annealing/curing) can alter the structure of the material so much that the stainless steel can corrode and become brittle.

We encounter today many terms such as V4A, V2A or 18/10. Abbreviations such as "V4A" originate from a test series of the Krupp company experimenting laboratories. Derived from this we often the letters "A4" for screw parts made of stainless steel. The numbers 18/10 as seen often on household cutlery stand for the alloy components chromium (18%) and nickel (10%). Terms such as Cromargan, Remanit or Nirosta are in-house brand names. These examples are very general and too imprecise for material designation.

There are many types of stainless steel with very different characteristics and application areas making it important to define the material number for specific applications.

As an example, material no. 1.4301 (gen. V2A or A2). The number "1" before the point stands for steel in this classification system. The number "4" after the point indicates that it is a non-rusting steel. There are also standardized abbreviations in order to be able to better manage chemical compositions.

In our example this is: X5CrNi18 -10. The significance of the individual components of this term:

  • X5 = carbon content in hundredths of a percent by weight (for example 0.05%)
  • Cr = chrome
  • Ni = nickel
  • 18 = 18 % chrome
  • 10 = 10 % nickel

Physical properties

Both the chemical composition of the material and a possible cold forming affect the magnetizability. Stainless steel can then respond magnetically depending on grain-structure.

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