, 29. November 2015

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What is stainless steel?

Under the term of stainless steel it is colloquially comprehended "noncorrosive steels". However, what is really at the bottom of the term "stainless steel" and where are typical fields of application of noncorrosive stainless steel.

What is stainless steel?
All those kinds of steel are described as stainless steel that are melted in a special procedure, have a high degree of purity and react uniformly to intended heat treatments. Following this definition stainless steels do not exclusively have to be alloyed respectively high-alloyed steels. However, for the further consideration we restrict to the high-alloyed stainless steels with a chromium content of at least 10.5%.

Classification of high-alloyed stainless steels
High-alloyed stainless steels can be divided into the following groups based on their structure:

  • ferritic stainless steels
  • martensitic stainless steels
  • austenitic stainless steels
  • ferritic-austenitic stainless steels (duplex steels)

Ferritic stainless steels
The ferritic stainless steels in turn are divided into two groups:

  • with approx. 11 to 13% chromium
  • with approx. 17% chromium (Cr)

Due to their low content of chromium the 10.5 to 13% chromium steels are only described as "slowly corrosive". They are used in those fields where service life, safety and a low maintenance rate are most important and where no specific demands on the appearance are made. This is the case, for example, in container, waggon and vehicle construction.

Martensitic stainless steels
The martensitic stainless steels with 12 - 18% Cr and with C-contents exceeding 0.1% are austenitic at temperatures exceeding 950 - 1050°C. A quick cooling (quenching) leads to the formation of a martensitic structure. This structure has a high hardness especially in the tempered state which still further increases with increasing C-content. These steels are used e. g. for the manufacture of razor blades, knives or scissors. Precondition for a sufficient corrosion resistance is an appropriate surface finish that can be achieved for instance through grinding.

Austenitic stainless steels
The austenitic stainless steels, also referred to as CrNi steels, with Ni-contents exceeding 8% are the best combination with respect to workability, corrosion resistance and mechanical properties. The high corrosion resistance is the most important characteristic of this kind of stainless steel. For this reason austenitic stainless steels are employed in fields where aggressive media are concerned, for example in case of contact with chloride containing sea water, in the chemical or food industry.

Austenitic-ferritic stainless steels
The austenitic-ferritic stainless steels are often referred to as duplex steels due to their two structure components. The high ductility with concurrent improvement of the corrosion resistance let these steels be used especially in the field of off shore technology.


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