Ferritic and martensitic stainless steels are magnetic. Austenitic stainless steels, including the popular 304 and 316 grades, are generally not. The difference comes down to how the atoms are arranged inside the metal, which is determined by the alloying elements mixed into the steel.
Why Some Stainless Steels Are Magnetic
All stainless steels contain iron, which is inherently magnetic. But iron atoms can arrange themselves in different crystal structures, and that structure is what determines whether a magnet will stick. In ferritic stainless steels, the atoms sit on a body-centered cubic (bcc) lattice: imagine a cube with an atom at each corner and one in the center. This arrangement allows the electrons to align in a way that produces a strong magnetic response.
In austenitic stainless steels, the atoms form a face-centered cubic (fcc) lattice: a cube with atoms at each corner and one at the center of each face. This denser packing disrupts the electron alignment needed for magnetism. Even though the steel is still mostly iron, the crystal structure makes it effectively non-magnetic.
What pushes the atoms into one arrangement or the other is chemistry. Adding nickel, manganese, carbon, and nitrogen stabilizes the non-magnetic fcc structure. Adding chromium, molybdenum, and silicon favors the magnetic bcc structure. The balance of these elements is what separates magnetic from non-magnetic grades.
Magnetic Grades: Ferritic and Martensitic
Ferritic stainless steels are iron-chromium alloys with 13 to 18 percent chromium and little to no nickel. Without nickel to stabilize the non-magnetic structure, these steels retain their bcc crystal arrangement at room temperature and behave much like a regular carbon steel magnet. Common ferritic grades include 409, 430, and 439. You’ll find them in automotive exhaust systems, kitchen sinks, and architectural trim.
Martensitic stainless steels are also magnetic. They share the ferritic family’s bcc-type crystal structure but can be hardened through heat treatment, making them useful for knives, surgical instruments, and turbine blades. Grades like 410, 420, and 440C all attract a magnet firmly.
Non-Magnetic Grades: Austenitic (300 Series)
The most widely used stainless steel in the world, Type 304, contains roughly 18 percent chromium and 8 percent nickel. Thermodynamically, this composition would prefer the magnetic bcc structure at room temperature. But the nickel, along with small amounts of manganese (about 1 percent), carbon (less than 0.08 percent), and nitrogen (about 0.06 percent), locks the atoms into the non-magnetic fcc austenite phase as the steel cools. The result is a steel that a refrigerator magnet won’t stick to.
Type 316, the second most popular grade, works the same way. It adds molybdenum for better corrosion resistance but keeps enough nickel to remain austenitic and non-magnetic. The entire 300 series follows this pattern.
When Austenitic Steel Becomes Partly Magnetic
There’s a catch. If you bend, cold-work, or heavily machine a 304 stainless steel part, some of the austenite transforms into martensite at the deformation site. That localized phase change makes the steel partially magnetic in those areas. This is why a magnet might weakly cling to a stainless steel pot that has been spun or drawn into shape, even though the alloy started out non-magnetic. The degree of this effect depends on how severely the metal was worked and the exact composition of the alloy. Grades with higher nickel content, like 316, are more resistant to this transformation.
Duplex Stainless Steels: A Mix of Both
Duplex stainless steels split the difference. Their microstructure contains roughly equal amounts of ferrite and austenite, typically around 45 to 55 percent of each. Since the ferrite phase is ferromagnetic and the austenite phase is not, duplex grades like 2205 are moderately magnetic. A magnet will stick, but the pull feels noticeably weaker than on a fully ferritic grade. These steels are common in chemical processing, oil and gas pipelines, and marine environments where both strength and corrosion resistance matter.
Why the Magnet Test Can Be Misleading
A common shortcut is to touch a magnet to stainless steel to judge its “quality.” The logic goes: if the magnet sticks, it must be inferior steel. This is unreliable for several reasons.
- Magnetism doesn’t indicate corrosion resistance. Ferritic grade 430 is magnetic and performs well in many environments. Corrosion resistance depends on alloy composition and surface treatment, not crystal structure.
- Cold working creates false positives. A high-quality 304 sheet can become slightly magnetic after forming, even though its chemistry hasn’t changed.
- Both types can be high-performance. Magnetic and non-magnetic grades each offer high strength, durability, and corrosion resistance depending on their specific composition. Surgical instruments are made from magnetic martensitic steel, while chemical tanks use non-magnetic austenitic steel. Neither is inherently better.
If you need to confirm a stainless steel grade, chemical analysis or a handheld alloy analyzer will give you a definitive answer. A magnet can tell you roughly which family a steel belongs to, but it can’t tell you the grade, the quality, or how well it will resist corrosion in your application.
Quick Reference by Family
- Ferritic (400 series: 409, 430, 439) — Magnetic. No nickel, bcc crystal structure.
- Martensitic (410, 420, 440C) — Magnetic. Hardenable, bcc-type structure.
- Austenitic (300 series: 304, 316, 321) — Non-magnetic in the annealed state. Can become weakly magnetic after cold working.
- Duplex (2205, 2507) — Moderately magnetic. Roughly half ferrite, half austenite.
- Precipitation-hardening (17-4 PH, 15-5 PH) — Magnetic. Martensitic base structure despite containing some nickel.