Stainless steels - Selection criteria and structure

Stainless steels form a vast family of metal alloys, all of which have iron and chromium in common. Thanks to their chromium content, they are able to develop a thin surface barrier, or passive film, which gives them a primary quality: resistance to corrosion in a wide range of environments.

The higher the chromium content of the alloy, the more stable the passive layer. The first threshold is around 11% chromium. This is followed by a second threshold, at around 17% chromium, at which point the passive layer is more stable and, consequently, more resistant to corrosion. For other reasons (brittleness of continuous casting slabs), the upper chromium limit of these alloys is around 30%.

This fundamental property of corrosion resistance is reflected in the name of these steels. In French, we say "inoxydable", in English "stainless steel" (acier sans tache) and in German "rostfreï stahl" (acier qui ne rouille pas).

Depending on their chemical composition, and after heat treatment (annealing, quenching, tempering, hyperquenching), stainless steels can be classified according to their crystalline structure.

The first family are martensitic steels. Their chromium content is generally between 12% and 17%, while carbon content can be as high as 0.6%. In addition to these two basic alloying elements, molybdenum, nickel and even nitrogen may be added in small quantities. Carbon plays a fundamental role in these alloys. With a chromium content of around 13%, the crystalline structure is cubic-centered up to 0.18% carbon, and tetragonal above that.

The second family is that of ferritic steels, whose chromium content can reach 30%, while carbon content is kept at very low levels, below 0.02%. They may contain molybdenum, and may be stabilized with titanium or niobium when welded. Their crystalline structure is cubic-centered. They are ferromagnetic. The most representative grade is 17% chromium, which covers over 20% of market needs.

The third family are austenitic steels. These are iron-chromium-nickel alloys with a very low carbon content. They may contain molybdenum (up to 8%), to improve corrosion resistance. Some grades contain aluminum or rare earths, to enhance their performance at very high temperatures. Their crystalline structure is face-centered cubic. These steels are said to be non-magnetic (or paramagnetic). The most representative grade in this family is that containing 18% chromium and 9% nickel (commonly known as 18-8), which covers around 80% of market needs.

The fourth family is that of austenitic-ferritic stainless steels (also known as duplex), whose structure is composed of two phases, ferritic and austenitic, in roughly equal proportions....