Corrosion resistance is achieved by a chromium content of over 10.5 % by a maximum Carbon content of 1.2 % and increases with higher Chromium and Molybdenium contents. Balanced alloying technology results in different property profiles and austenitic, ferritic, semi-martensitic, martensitic or ferritic-austenitic microstructures. Austenitic steels are generally not magnetisable.
By definition, corrosion-resistant steel has a chromium content of over 10.5 % and a maximum carbon content of 1.2 %. The resistance to general corrosion is essentially determined by the Cr and Mo content, while the resistance to pitting and crevice corrosion is determined by the pitting equivalent PREN = %C+3.3%Mo + (16 -30)%N. The resistance to intergranular corrosion is determined by the free Cr content, i.e. the Cr content not bound in the form of Cr carbides.
Coordinated alloying technology results in austenitic, ferritic, martensitic, semi-martensitic or ferritic-austenitic microstructures and thus different property profiles and a wide range of possible applications.
Austenitic Cr-Ni-Mo high-performance steels are particularly resistant to corrosion under aggressive conditions, e.g. in strong acids, alkalis and chloride-containing media such as brackish water, seawater and brine. These types of steel tend to exhibit higher strength and greater resistance to stress corrosion cracking.
Austenitic Cr-Mn-Ni-Mo-N steels form a subgroup of this steel category. They have a higher initial strength and a more pronounced tendency to work hardening and are often regarded as a more economical alternative to Cr-Ni-Mo steels.
Ferritic-austenitic (duplex) steels usually have twice the yield strength of standard austenitic grades and are magnetisable in proportion to the ferrite content. Compared to standard austenitic grades, duplex stainless steels are generally more resistant to stress corrosion cracking due to their lower Ni content.
Ferritic stainless steels have a slightly higher strength and are significantly more resistant to stress corrosion cracking than the austenitic Cr-Ni (Mo) steels.
Martensitic and semi-martensitic stainless steels are ferromagnetic and have a microstructure consisting mainly of martensite, possibly with small amounts of ferrite and carbides. Such steels are tempered to higher strengths by heat treatment, e.g. hardening and tempering. The corrosion resistance is comparatively lower and is usually below that of standard austenitic steels.
The stainless, martensitic-hardenable (PH) steels achieve the martensitic microstructure through higher Ni contents with simultaneously low C and N contents. This is associated with higher toughness, better weldability and also higher corrosion resistance. Alloying elements such as Cu, Ti and Al are used to achieve hardenable alloy systems (PH steels) and increase strength through precipitation hardening in the course of a simple, low-dimensional heat treatment.
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