Are stainless steels magnetic? Grades, cold work and what it means
Ferritic and martensitic grades are magnetic; austenitic 304/316L are usually non-magnetic but become slightly magnetic after cold working. Magnetism is not a quality test.
A common buyer question: why is my "stainless" magnetic? The answer depends on the family.
• Ferritic (430, 409, 439) and martensitic (410, 420, 440C) grades are magnetic by nature because of their body-centred crystal structure.
• Austenitic grades (304, 316L, 321) are austenitic and normally non-magnetic in the annealed condition.
• Cold working (bending, drawing, rolling, cutting) can transform some austenite to martensite, making 304 or 316L slightly magnetic — especially at bends and sheared edges. Annealing restores the non-magnetic state.
• Duplex grades are magnetic because of their ferrite content.
Practical implications:
1) A magnet is not a grade test: a magnetic 304 sheet can be genuine, and a non-magnetic 201 can be passed off as 304. Use chemical analysis (PMI) to verify grade.
2) Some applications require low permeability (instruments, MRI environments) — specify magnetic permeability limits and use annealed austenitic grades.
3) Welding can also produce small amounts of ferrite in austenitic welds (intentionally, for crack resistance), which is magnetic.
If a customer insists on non-magnetic, clarify the condition: annealed, stress-relieved, or as-fabricated.
Conclusion: magnetism tells you about structure and processing history, not about whether the material is good.
