Stainless Steel Grade Designations

The grade designations for seamless stainless steel tubing are the same as those used for stainless steel bar. Three general classifications are used to identify stainless steels. They are:

1. Metallurgical structure.

2. The AISI numbering system: namely 200, 300 and 400 Series numbers. *

3. Unified Numbering Systems (UNS).

Following is a description of these classifications. All AISI-numbered stainless steels are classified according to metallurgical structure; martensitic, ferritic, austenitic, and precipitation hardening.

Martensitic stainless steels are straight - chromium AISI 400 Series types that are hardenable by heat treatment. They have fairly good ductility, and some can be heat treated to tensile strengths exceeding 200,000 psi. Type 410 is the general-purpose alloy of the martensitic group.

Ferritic stainless steels are straight-chromium AISI 400 Series types that cannot be hardened by heat treatment, and only moderately hardened by cold working. They are magnetic, have good ductility and resistance to corrosion and oxidation. Type 430 is the general-purpose stainless of the ferritic group.

Austenitic stainless steels containing chromium and nickel are identified as AISI 300 Series types. Alloys containing chromium, nickel and manganese, are identified as AISI 200
Series types.