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How do you identify head markings on Stainless Steel bolts?

How do you identify head markings on Stainless Steel bolts?

How do you identify head markings on Stainless Steel bolts?

Last Updated: 16/2/24 at 9:02am

What are some common head markings on stainless steel bolts and screws?

If you look at the head of a stainless steel fastener, you might see a marking stamped onto the surface. These marks hold valuable information about the fastener and learning how to read them can help you decide if it’s the correct product for the job.

In the chart below are the most common head marks found on Anzor's fasteners.

How to identify Stainless Steel Bolts and Screws

Markings What they mean
A2 304 Stainless
A4 316 Stainless
A2 or A4 with 70 or 80 e.g. A4-80 Class 70 or 80 (700/800mpa tensile strength)
18-8 304 (usually on imperial sized fasteners)
18-10 316 (usually on imperial sized fasteners)
304 304 SS
316 316 SS
316L 316 SS low carbon
Manufacturer markings e.g THE, Bumax
70 700 class Mpa
80 800 class Mpa

Is Stainless Steel magnetic?


No. Austenitic grades of stainless such as 304 and 316 are NOT magnetic in their raw, annealed state.

However when fasteners are manufactured there are processes such as drawing, cold forming and thread rolling which cold-work the material and can magnetize it. This is also the case with lost wax casting of fittings made from molten stainless steel. Therefore some stainless steel fasteners and fittings made of grade 304 and 316 are slightly magnetic, hence using a magnet to determine if it is stainless steel is not a suitable test.

Stainless Steel Fasteners can be Magnetic

Which types of stainless are magnetic?


The following types of stainless are typically magnetic in any form:

  • Ferritic - such as grades 409, 430 and 439.
  • Martensitic - such as grades 410, 420, 440.
  • Duplex - such as grade 2205 and 2507 Super Duplex.

The following types of stainless are NOT magnetic in their raw, annealed state but can be made magnetic in fastener and fitting manufacturing processes and cold-working:

  • Austenitic - such as grades 304 and 316.

How can I tell if my stainless steel is 304 or 316?


The head marking 'A2' identifies 304 grad and 'A4' identifies 316 grade. However testing the stainless chemically is the only sure way of knowing the grade.


How do I guarantee that my fastener or fitting is stainless?


The only true way to ensure specification compliance is a material elementary analysis with an X-Ray Spectrometer. To be able to be classed as stainless it will have a chemical composition of at least 10.5% Chromium, and meet other requirements depending on what stainless steel grade it should be.

A moly drop test is a chemical test showing the presence of Molybdenum which is present in grade 316 but not 304. However this test is only indicative and will not ensure the material meets the minimum 2% Molybdenum required in the Stainless Grade 316 specification.

Stainless Steel Fastener Grade Testing

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