304, 316, 430... the logic behind the three-digit codes: the austenitic, ferritic and martensitic family system.
Although stainless steel grade codes (such as 304, 316, 430) may look random at first glance, they actually follow a classification system defined by AISI/ASTM. The first digit of the code indicates the metallurgical family the steel belongs to: those starting with 2 and 3 are austenitic, while those starting with 4 are ferritic or martensitic.
This is the most widely used group. Thanks to the combination of chromium and nickel, it is non-magnetic, easy to form and welds well. 304, 316, 321, 309S and 310S are examples of this family.
Contains chromium but little or no nickel. It is magnetic, much lower in nickel than the austenitic group and cannot be hardened by heat treatment. 430, 409, 439 and 441 belong to this family.
Like ferritic, it is chromium-based but contains more carbon, which allows it to be hardened by heat treatment. It is magnetic. 420 and 410 are examples of this family, used in cutting tools and parts requiring high strength.
The letter L (304L, 316L) means low carbon and improves strength after welding. The letters Ti (316Ti, 321) indicate stabilization with titanium, protecting against carbide precipitation at high temperature. Codes ending in S (309S, 310S) are standard variants defined by ASTM and optimized for high-temperature applications.
The ferritic (430, 409, 439, 441) and martensitic (420, 410) families are magnetic. The austenitic family (304, 316, 321, 309S, 310S) is non-magnetic under normal conditions; a quick family test can be done with a magnet.
Only the carbon content: in 304L carbon is limited to max 0.03%, in 304 it is max 0.08%. Low carbon preserves corrosion resistance after welding.
304 is the most widely produced and used stainless steel grade worldwide; its general-purpose durability and wide workability are the main reasons.
