The stainless steel coding system consists of a three digit number. The first number indicates the material grade; the other two numbers serve only to designate a specific alloy within that grade.
The austenitic and ferritic grades cannot be hardened by heating and quenching; only the martensitic grades are hardenable by standard heat-treatment methods. The PH grades are hardenable by a special type of heat-treatment known as artifical aging.
Note that the austenitic grades are not magnetic; the ferritic and martensitic grades are magnetic at room temperature. The PH grades may or may not be magnetic, depending on the underlying microstructure.
The high chromium content in stainless steel makes most grades difficult to machine. Free machining grades of stainless steel are made by adding sulfur or selenium; if selenium is used the chemical symbol Se is added as a suffix.
Although most grades of stainless steel can be welded, the high chromium content can cause carbide formation and the consequent cracking, leaking, and loss of strength in welded joints. Extremely low carbon weldable grades of stainless steel are indicated by an L added as a suffix, as in 304L.
4XX............Chromium, ferritic (grades 405,430,442,446)
Chromium, martensitic (all other 4XX grades)
5XX............Low chromium, martensitic
PHXX-YY........Precipitation hardening grades, where XX stands for
the percent of chromium and YY stands for the percent
of nickel in the alloy. An example would be PH18-8
which has approximately 18% Cr and 8% Ni.