Stainless steel can exhibit varying levels of magnetism depending on its elemental composition and microstructure. There are four different types of stainless steels as follows: austenitic, ferritic, martensitic and duplex.
Magnetism of Stainless Steel Types
Alongside the stainless steel type, the magnetic behavior of stainless steel is also influenced by factors like cold working and heat treatment. The magnetism of stainless steel types is described as follows:
Austenitic Stainless Steel
Austenitic stainless steel like the 304 or 316 grades is typically considered non-magnetic in its annealed state. This means, in its standard condition, it exhibits only very weak magnetic properties if any.
These steels contain high levels of chromium and nickel, the latter stabilizing the FCC crystal structure thus reducing overall magnetic susceptibility. Nonetheless, the lack of magnetism is primarily due to its face-centered cubic crystal structure.
Small amounts of cold working like drawing, rolling or bending can deform the crystal structure resulting in martensitic or ferritic phases. These can then induce some magnetic properties albeit localized and weak. Additionally, heat treatments succeeded by rapid cooling, can achieve the same result.
Ferritic Stainless Steel
These steels are magnetic due to their relatively high iron content and body-centered cubic crystal structure. The BCC structure accommodates unpaired electrons whose magnetic moments influence magnetic behavior.
Martensitic Stainless Steel
Martensitic stainless steels can display magnetic capabilities finding use where some level of magnetism is desired alongside strength and hardness. These steel’s body-centered tetragonal crystal structure have the atoms configured such that it allows for a net magnetic moment.
Additionally, martensitic stainless steels contain higher carbon levels contributing to the formation of a hard and magnetic phase during heat treatment. Rapid cooling of steel from elevated temperatures can also induce martensitic transformation, making the material magnetic.
Martensitic stainless steels will exhibit different degrees of magnetism depending on grades and composition. Additionally, magnetism induced in these steels by heat treatment may be localized and not as strong as pure ferromagnetic materials.
Duplex Stainless Steel
Duplex stainless steels have microstructures that combine both austenitic and ferritic phases. This results in a balanced combination bearing moderate magnetic capabilities and corrosion resistance. The magnetic properties of these steels will vary with the composition and heat treatment.
The mixed microstructure of duplex stainless steel allows these steels to be magnetic thanks to the ferritic phase’s body-centered cubic crystal structure. Furthermore, duplex stainless steel contains higher molybdenum and chromium levels alongside iron-enhancing magnetic properties.
Heat treatment of these steels can change the relative proportions of ferritic and austenitic phases thus affecting the overall material magnetism. Depending on the specific grade and composition, the degree of magnetism can vary.
Conclusion
The magnetism of stainless steel will depend on its alloy composition, microstructure and processing factors like cold working and heat treatment. When magnetic properties are of concern, ferritic and martensitic stainless steels are useful.
