Ferritic steels with higher Chromium and Molybdenum contents are mostly used for thin-walled products in corrosive media in all industrial sectors. Martensitic and semi-martensitic steels with Chromium contents of approx. 12 to 18% and Molybdenum and Nickel achieve high strength and good wear resistance through heat treatment. The main areas of application are cutlery and mechanical engineering.
The most important alloying elements of such steels are Cr and Mo. A ferritic, body-centred cubic solid solution structure can be achieved through a special balance of the alloying elements. Cr, Mo and others act as strong ferrite-stabilising elements. The elements stabilising the austenite, such as Ni, Mn, C, N, are kept very low. The Cr content of ferritic stainless steels is usually between 12 and 28%. In the case of high-performance steels in particular, so-called superferrites, extremely low C and N contents are aimed for, which has a positive effect on the toughness characteristics. Ni is added in small quantities as a toughness-enhancing alloying element. Ferritic steels are highly ferromagnetic.
Corrosion resistance is essentially determined by the Cr and Mo content. Resistance to intergranular corrosion is determined by the free Cr content, i.e. the Cr content in the solid solution that is not bound in the form of Cr carbides.
In general, ferritic stainless steels have a slightly higher strength and are significantly more resistant to stress corrosion cracking than austenitic Cr-Ni-(Mo) steels. Formability, on the other hand, is comparatively poorer, as is toughness, which also depends heavily on the cross-section. Ferritic stainless steels also show a pronounced transition from ductile to brittle fracture behaviour with decreasing temperature.
Due to these limitations, their use is severely restricted to thin-walled products (sheets, strips, pipes) in all industrial sectors and in corrosive media, e.g. for household goods, chemical process technology, magnetic applications and many more.
Martensitic and semi-martensitic stainless steels are ferromagnetic and have a microstructure consisting mainly of a martensitic phase (carbon martensite), possibly with small amounts of secondary phases, mainly ferrite and carbides. Such steels are tempered to higher strengths by heat treatment, e.g. hardening and tempering. The Cr content is approx. 12 to 18%, with proportions of Mo and Ni. Depending on the C and N content and the proportion of ferrite, high strengths and good wear resistance can be achieved with these steels.
Martensitic and semi-martensitic stainless steels have lower toughness and a relatively high transition temperature, are difficult to weld and generally require subsequent heat treatment, which means that their use is usually limited to non-welded parts. In order to achieve a martensitic or semi-martensitic microstructure, the Cr content tends to be in the low range and some of the chromium is bound in carbide precipitates. As a result, the corrosion resistance is comparatively lower and is usually below that of standard austenitic steels. For this reason and also because of their limited toughness, martensitic stainless steels are used where strength and hardness are primarily required, e.g. for cutting tools, general mechanical engineering, shafts and fasteners.