Stainless steel is a versatile iron-based alloy known for its resistance to rust and corrosion, achieved by adding a minimum of 10.5% chromium. This chromium forms a thin, protective oxide layer that self-heals when exposed to oxygen. Although often considered completely non-magnetic, stainless steel’s magnetic behavior depends entirely on its internal crystal structure. While the majority of stainless steel used is non-magnetic, two specific families retain the magnetic properties inherent to iron. This article identifies these two primary families and explores the structural reasons behind their magnetism.
Why Most Stainless Steel is Non-Magnetic
The most widely used type of stainless steel, accounting for approximately 70% of global production, is the Austenitic family, which includes common grades like 304 and 316. These alloys are generally non-magnetic due to their unique atomic arrangement, known as a Face-Centered Cubic (FCC) crystal structure. In this structure, iron atoms are positioned at the corners and the center of each face of the cube. This configuration prevents the magnetic moments of the individual iron atoms from aligning in a uniform direction.
High concentrations of elements such as nickel and manganese are added to stabilize this non-magnetic FCC structure. Although iron itself is strongly ferromagnetic, these alloying elements lock the iron atoms into an arrangement where their magnetic fields cancel each other out. Consequently, the material does not exhibit ferromagnetism under normal conditions, a property valued in applications like Magnetic Resonance Imaging (MRI) equipment where magnetic interference must be avoided.
The Two Magnetic Stainless Steel Families
The two families of stainless steel that are strongly magnetic are the Ferritic and the Martensitic types, classified in the 400 series of grades. Both retain magnetism because their primary atomic arrangement is a variation of the Body-Centered Cubic (BCC) crystal structure. In the BCC structure, iron atoms sit at the corners of the cube with a single atom located precisely in the center. This atomic configuration facilitates the necessary alignment of magnetic moments.
The BCC arrangement allows the microscopic magnetic domains within the iron to line up when an external magnetic field is applied. This ability to align is the fundamental requirement for a material to be ferromagnetic, resulting in a strong attraction to a magnet. These magnetic steels typically contain low or no nickel, making their magnetic response comparable to that of plain carbon steel.
Key Differences Between Ferritic and Martensitic
Although both Ferritic and Martensitic stainless steels are magnetic due to their BCC-based structures, they are chemically and mechanically distinct.
Ferritic Stainless Steel
Ferritic stainless steels, such as Grade 430, are characterized by a low carbon content, generally less than 0.08%. They cannot be hardened through heat treatment. They offer good corrosion resistance and are often used in applications like automotive exhaust systems, kitchen appliances, and decorative trim. The relative lack of expensive nickel makes them a more economical choice compared to the Austenitic grades.
Martensitic Stainless Steel
Martensitic stainless steels possess a higher carbon content, often exceeding 0.15%. This higher carbon allows the material to be hardened and strengthened through a rapid cooling process known as quenching. After hardening, Martensitic steel often exhibits a slightly distorted crystal structure called Body-Centered Tetragonal (BCT), which is an elongated form of the BCC structure that still maintains its strong magnetic properties. This combination of high hardness, strength, and magnetism makes Martensitic grades, like 410 and 420, suitable for manufacturing cutlery, surgical instruments, and turbine blades.