Why is my stainless steel magnetic?

There are two main reasons why stainless steel could be magnetic, but generally Stainless Steel that falls into the most common 200 or 300 series is non-magnetic, while 400 series is (see below for composition breakdowns). Austenitic stainless (200 & 300 series) has a primarily crystalline structure is face-centered that makes them essentially non-magnetic. That is in large part due to the large percentage of Nickel in the mix. Martensitic stainless steel on the other hand (400 series) has a martensite crystal structure is Body-Centered. This has the crystalized iron in a cubic form which is highly magnetic.

Stainless Steel Austenitic cell structure vs Martensitic

- The metal composition of Stainless Steel

Stainless steel is common term for iron (Fe) alloy that contains at least 10.5% chromium (Cr). The most common Stainless used in Fasteners is 18-8 for US (SAE) fasteners (A2 when used with Metric fasteners). 18-8 refers to 18% Chromium to 8% nickel (Ni) and is part of the 300 grade steel family (304 actually). A2 stainless steel is non-magnetic in nature. 
- 200 and 300 series Stainless Steel (most common forms of stainless steel) - Austenitic Stainless Steel (non magnetic - notice the large Nickel amount)

  • 66 - 75% Iron
  • 5 - 20% Chromium
  • 8 - 12% Nickel
  • 0 - 2% Molybdenum (316 / A4 Marine Grade)
  • <1% (Trace) Carbon, Manganese, Phosphorus, Sulphur and Silicon 
- 400 Series Stainless Steel - Martensitic stainless steel (magnetic - much less Nickel)
  • 66 - 75% Iron
  • 5 - 17% Chromium
  • 0 - 4% Nickel
  • 0 - 1.1% Carbon
  • 0 - 1.1% Molybdenum

- How the Stainless Steel is worked after formed

Cold working stainless steel can increase the amount of martensite in it. This is especially common in the process used to make fasteners. So while a material may have been non-magnetic when produced, some amount of magnetism may be transferred through cold working the material.

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