What Sheet Metal Is Magnetic

Content Menu

● Fundamentals of Magnetism in Sheet Metals

● Magnetic Sheet Metals Commonly Encountered

● Non-Magnetic Sheet Metals Regularly Processed

● How Magnetic Properties Affect Manufacturing Processes

● Practical Ways to Control or Eliminate Unwanted Magnetism

Most of us who work with sheet metal have done the quick magnet test countless times. A small neodymium magnet on the corner of a sheet tells you immediately whether you are dealing with ordinary mild steel or something else entirely. That single yes-or-no response influences how the material will be handled, formed, welded, inspected, and even how the finished part will perform in service. In a busy fabrication shop or on a high-volume stamping line, those magnetic properties are rarely just academic details—they are part of daily decisions that affect cycle time, tooling life, and scrap rate.

This article is written for practicing manufacturing engineers, process planners, and material specialists who need to select, form, join, and finish sheet metal with a clear understanding of its magnetic behavior. The discussion covers the fundamental reasons certain sheet metals are strongly magnetic while others are not, identifies the most common magnetic grades used today, and examines how magnetism interacts with typical production processes. Real plant-floor examples are included throughout, along with findings from peer-reviewed work that explain observed behavior.

Fundamentals of Magnetism in Sheet Metals

Why Some Metals Are Strongly Magnetic

Strong magnetism in sheet metal almost always traces back to iron in its body-centered cubic (BCC) crystal form, known as ferrite or alpha-iron. Below the Curie temperature (768 °C for pure iron), the unpaired 3d electrons in iron can align over large volumes called magnetic domains. When an external field is applied, the domains rotate and grow, producing the familiar attraction we test with a hand magnet.

Cobalt and nickel also show ferromagnetism, but they appear far less often in sheet form because of cost and processing difficulties. Gadolinium is ferromagnetic only below room temperature and is therefore irrelevant for normal sheet-metal work. In practice, more than 95 % of the magnetic sheet metal processed worldwide is iron-based.

Alloying elements and heat treatment change the crystal structure and therefore the magnetic response. Carbon, chromium, nickel, and manganese can stabilize the face-centered cubic (FCC) austenite, which is essentially non-magnetic at room temperature. That single phase change explains why 304 and 316 stainless sheets refuse to stick to a magnet while 430 and 410 grades do.

Measuring Magnetic Response in the Shop

Relative permeability (μr) and saturation induction (Bs) are the two numbers that matter most. Ordinary cold-rolled low-carbon steel typically shows μr values between 200 and 2000 and saturates around 1.9–2.1 T. Non-oriented electrical steel reaches μr > 4000, while grain-oriented transformer steel can exceed 30 000 in the rolling direction. Austenitic stainless steels have μr ≈ 1.003–1.05—effectively non-magnetic for any practical handling or inspection purpose.

Magnetic Sheet Metals Commonly Encountered

Cold-Rolled and Hot-Rolled Low-Carbon Steels

Grades such as DC01, SPCC, AISI 1008–1018, and CS Type B make up the bulk of structural and enclosure sheet. Thicknesses from 0.4 mm to 3.0 mm are fully ferromagnetic in the as-received condition. Magnetic lifting devices, stackers, and de-stackers rely on this behavior every day. In automated press lines feeding 800 mm wide coil, a residual field of only 2–4 Gauss is enough to cause two sheets to stick together and crash the feeder.

High-Strength Low-Alloy (HSLA) Grades

S355MC, HSLAS 340–550, and dual-phase DP600–DP980 remain ferromagnetic because the matrix is still predominantly ferritic. The micro-alloying additions (Nb, V, Ti) do not destroy the BCC structure. Permeability is slightly lower than mild steel, but still high enough for magnetic handling and for magnetic particle inspection after welding.

Silicon contents above ≈3.2 wt% in some advanced HSLA grades begin to reduce permeability noticeably, a point worth remembering when specifying material for electric-vehicle structural parts.

Electrical Steels (Silicon Steels)

Non-oriented grades (M-15 to M-47, 50A400 to 50A1300) and grain-oriented grades (M-4, 27CG120, Hi-B) are deliberately engineered for maximum permeability and minimum core loss. Sheet thickness is usually 0.18–0.50 mm. These materials are strongly magnetic, but the directionality of the magnetism is controlled during mill processing. Stamping houses that produce motor laminations routinely use magnetic conveyors and stackers without difficulty.

Ferritic and Martensitic Stainless Steels

430, 409L, 410, and 420 are BCC at room temperature and therefore magnetic. Relative permeability ranges from 600 to 1800 depending on exact composition and cold reduction. These grades are chosen for automotive exhaust flanges, appliance panels, and cutlery blanks where moderate corrosion resistance and magnetic handling are both required.

Duplex Stainless Steels

2205 and 2304 contain roughly 50/50 ferrite/austenite. They are weakly magnetic (μr ≈ 20–80), enough to stick lightly to a hand magnet but not enough for reliable magnetic conveying.

Non-Magnetic Sheet Metals Regularly Processed

Austenitic Stainless Steels

301, 304/304L, 316/316L, 321, and 904L are fully austenitic when correctly solution-annealed. Cold work above ≈15–20 % reduction can generate strain-induced martensite and raise permeability dramatically. Many food-processing and pharmaceutical tanks stamped from 2 mm 304 sheet have become noticeably magnetic around deep-drawn corners, forcing a switch to liquid penetrant instead of magnetic particle inspection.

Aluminum Alloys

1050, 3003, 5052, 6061, and 7075 are all very weakly paramagnetic or diamagnetic. No production line uses magnetic handling for aluminum sheet.

Copper, Brass, Bronze, Titanium, and Nickel Alloys

All essentially non-magnetic. Titanium grades 2 and 5 are weakly paramagnetic (χ ≈ +180 × 10⁻⁶ cm³/mol), still far too low for any practical magnetic manipulation.

How Magnetic Properties Affect Manufacturing Processes

Blanking and Stamping

Residual magnetism after blanking can cause sheets to cling in the stack. Most modern de-stackers include an air knife plus alternating-field demagnetizer to separate sheets reliably. In high-speed tandem lines running 1008 steel at 120 strokes/min, failure to demagnetize increases double-blank detection events from <1 per 10 000 to >50 per 10 000.

Deep drawing of cylindrical cups from low-carbon steel shows measurable differences in wall thickness when the blank is magnetized parallel versus perpendicular to the rolling direction. The effect is small (≈2–3 % thickness variation) but repeatable and sometimes used deliberately to compensate for earing.

Welding

Arc blow is the most common problem when MIG or TIG welding magnetic steels thicker than ≈4 mm. The self-induced magnetic field around the arc deflects it toward the path of least magnetic reluctance. Remedies include AC welding, trailing demagnetizing coils, or simply wrapping a few turns of grounding cable around the workpiece to create an opposing field.

Resistance spot welding of laminated electrical steel packs can suffer from shunting if inter-sheet magnetism is too high. Thin phosphate or oxide coatings (1–3 μm) are applied precisely to break magnetic contact while preserving electrical insulation.

Machining and Grinding

Steel chips cluster around the tool when machining ferromagnetic material. Flood coolant with good chip flushing or mist coolant plus an in-line chip separator is standard. Surface grinding of grain-oriented silicon steel requires non-magnetic copper or brass backup plates; otherwise the workpiece lifts off the magnetic chuck.

Non-Destructive Testing

Magnetic particle inspection (wet fluorescent or dry powder) remains the fastest and cheapest method for surface and near-surface crack detection in ferritic sheet and plate components. Austenitic parts must use dye penetrant or ultrasonic testing instead, roughly doubling inspection cost.

Practical Ways to Control or Eliminate Unwanted Magnetism

Bulk demagnetizing coils operating at 50/60 Hz with slowly decreasing amplitude
Hand-held alternating-field demagnetizers for individual blanks
Passing the coil or cut blanks through a tunnel demagnetizer immediately after slitting
Heating above the Curie point followed by controlled cooling (rarely practical for thin sheet)
Specifying low-magnetic grades when residual magnetism is truly prohibited (e.g., 316L-Nb or Nitronic 32)

Frequently Asked Questions (FAQ)

Q1: Will galvanizing or painting destroy the magnetism of mild steel sheet?
A1: No. Zinc and most paint coatings are non-magnetic and thin enough that the steel beneath retains full ferromagnetic behavior.

Q2: Why do some 304 stainless parts become magnetic after bending or stretching?
A2: Cold deformation converts a portion of the austenite to alpha-prime martensite, which is strongly ferromagnetic. The effect is reversible by solution annealing at 1050 °C.

Q3: Can I use magnetic chucks when surface grinding 430 stainless?
A3: Yes. 430 has sufficient permeability (μr ≈ 800–1200) for secure holding on standard magnetic chucks.

Q4: Are there any high-strength sheet steels that are truly non-magnetic?
A4: Yes—certain manganese-austenitic grades such as Fe-18Mn-0.6C (similar to Hadfield steel) and some TWIP steels remain non-magnetic even after severe deformation.

Q5: Does laser cutting affect the magnetic properties at the cut edge?
A5: The heat-affected zone is narrow (<0.2 mm) and re-austenitizes only in high-alloy grades. In low-carbon and HSLA steel the cut edge remains fully magnetic.