What is the difference between Stainless steel A2 and A4

A couple of early items that need to be addressed. Just like your old friend Elizabeth*, Stainless Steel has a ton of confusing nicknames (isn’t that right Liz). In the fastener universe, generally 304 Stainless steel is called 18-8 for US (Inch) products, and A2 for metric fasteners. 316 Stainless Steel on the other hand tends to be called A4 for Metric products and 316 for US… but can also have words like ‘Marine Grade’ thrown in there for good measure.

So..., A2 is the same as 304 and 18-8..., and A4 is the same as 316 and Marine Grade.

  • How are A2 and A4 different?

The chemical properties that make up the steel differ with the most notable difference being the addition of molybdenum used when making A4. Molybdenum is highly corrosion resistant and passes that trait to stainless steel when added to it.  A4 tends to be more expensive than A2.

  • A4 is a great choice for…

Marine environments (by the sea), photographic chemicals, surgical implants as well as solutions of sulfuric acid, chlorides, bromides and iodides, most notably resistance to pitting corrosion in chloride environments where traditional A2 stainless steel falls short.

  • What is the most common stainless steel for fasteners?

A2 is by far the most common stainless steel fastener used worldwide. Beyond fasteners, it is the most versatile and most widely used stainless steel used period having excellent forming and welding characteristics. Reaching out to Brighton-Best (largest fastener distributor in the US) they estimated that its about a 80/20 split between A2 and A4 sold.

  • Is A4 stronger than A2 stainless steel?

No. A2 and A4 come in property classes that relate to their strengths, not the material itself. This includes 50, 70, 80 and 100 classes. You will typically see them given as A2-70 (A2 material, 70 property class). A2-70 and A4-70 both have the same tensile strengths. For the tensile strengths of each class, you just need to add a zero behind the class and that gives your tensile strength (A2-70 has a 700 tensile strength, A2-80 has a 800 MPa tensile strength, etc…). Below is a link that shows the different strength grades of stainless steel.

  • Chemical Composition of Both A2 and A4 Stainless Steel

A2 / 18-8 / 304 - 18% Chromium (Cr), 8% Nickel (Ni)

C, ≤

Si, ≤

Mn, ≤

P, ≤

S, ≤











A4 / 316 / 18-10 / Marine Grade - 16% Chromium (Cr), 10% Nickel (Ni), 2% Molybdenum (Mo)

% Cr

% Ni

% C

% Mn

% Si

% P

% S

% N

% Mo











Links to other stainless steel articles 

*Google ‘nicknames for Elizabeth’ and you will find there are over 100 nicknames for her. Crazy right? Small sample – Elise, Isabella, Lisa, Betty, Liz, Beth, Billie, Izzie, Birdie, Buffy, Tess…. It just goes on and on!

Difference between a2 and a4 stainless

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