Demystifying Stainless Steel Magnetism: Austenitic vs Martensitic

• Not all stainless steel is "non-magnetic." Whether it attracts a magnet depends on its internal microstructure. Austenitic stainless steels are generally non-magnetic, while martensitic and ferritic grades are magnetic. This article explains the differences, the cause of magnetism, and how heat treatment can alter this property.

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• 1. Why Are Some Stainless Steels Magnetic?

  Magnetism in stainless steel is directly linked to its crystal structure:

  · Austenitic Stainless Steels: Face-centered cubic (FCC) structure, chromium-nickel alloys (e.g., 304, 316). Typically non-magnetic.

  · Martensitic Stainless Steels: Body-centered tetragonal (BCT) structure, high carbon & chromium (e.g., 410, 420). Magnetic.

  · Ferritic Stainless Steels: Body-centered cubic (BCC) structure (e.g., 430, 443). Magnetic.

  
  2. Why is Austenitic Stainless Steel Usually Non-Magnetic?

  Austenitic grades (300 series) are metastable at room temperature. Their lack of magnetism is due to:

  · The FCC crystal structure, which impedes the formation of magnetic domains.

  · High nickel content (>8%) stabilizes the austenite phase.

  · Cold working (e.g., bending, cutting) can induce partial transformation to martensite, creating slight magnetism.

  
  3. Controlling Magnetism Through Heat Treatment

  3.1 Demagnetizing Treatments (for Martensitic/Ferritic Grades)

  Solution Annealing: Heat to 1040-1150°C, then quench rapidly.

  Dissolves carbides, forms homogeneous austenite.

  Used for work-hardened components.

  Tempering Process Control:

  Medium-temperature tempering (400-600°C) reduces residual stress.

  Combined with controlled cooling to manage phase transformation.

  3.2 Making Steel More Magnetic

  Cold Rolling/Drawing: Induces austenite-to-martensite transformation.

  Cryogenic Treatment: Deep freezing below -70°C increases martensite content.

  
  

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