Comprehensive Guide to Stainless Steel: Understanding Grades 304, 316, 317, 321, 347, 904L and Beyond

Stainless steel refers to steel that is resistant to corrosion by air, steam, water and other weak corrosive media, as well as chemical immersions such as acids, alkalis and salts. It is also called corrosion-resistant acid steel. In actual use, steel that resists weak corrosive media is commonly called stainless steel, whereas steel that resists chemical medium corrosion is called acid-resistant steel. Because the two differ in chemical composition, the former does not necessarily resist chemical medium corrosion, while the latter generally has stainless behaviour.

The corrosion resistance of Stainless Steel depends on the alloying elements contained in the steel.

Common classifications:

Generally speaking, stainless steels are classified by metallographic structure into three basic types: austenitic, ferritic and martensitic stainless steels. On that basis and for specific demands or purposes, there are further derivatives such as duplex stainless steels, precipitation-hardening stainless steels and high-alloy steels with iron content less than 50%.

1. Austenitic stainless steels
   Their matrix is predominantly austenite (face centred cubic structure, γ phase). They are non-magnetic in the annealed condition and are typically strengthened by cold working (which may also induce some magnetism). The U.S. Steel Association uses the 200 and 300 series numbers, e.g. 304. 

2. Ferritic stainless steels
   Their matrix is mainly ferrite (body-centred cubic structure, α phase). They are magnetic, and generally cannot be hardened by heat treatment but can be somewhat strengthened by cold working. The U.S. Steel Association marks them in the 400 series, such as 430 and 446. 

3. Martensitic stainless steels
   Their matrix is martensite (body-centred tetragonal or body-centred cubic depending on carbon and cooling), they are magnetic, and their mechanical properties can be adjusted by heat treatment (quenching & tempering). The U.S. Steel Association uses numbers like 410, 420, 440.  Martensite forms when austenite (at high temperature) cools at a suitable rate to room temperature and transforms. 

4. Austenitic-ferritic (Duplex) stainless steels
   Their matrix consists of both austenite and ferrite phases. Typically the minor phase content is greater than about 15%. They are magnetic, strengthenable by cold work, and compared with purely austenitic stainless steels, duplex steels have higher strength, better resistance to intergranular corrosion, better resistance to chloride stress corrosion and pitting corrosion. An example is grade 329. (In other sources duplex grades are often 2205, etc.) 

5. Precipitation-hardening stainless steels
   Their matrix is either austenite or martensite, and they can be hardened by a precipitation-hardening treatment. In U.S. steel numbering they are often in the 600 series, such as 630 (17-4PH). 

In general, aside from alloying elements, austenitic stainless steels exhibit comparatively superior corrosion resistance. In environments of low corrosivity you can use ferritic stainless steels; in mildly corrosive environments, if high strength or high hardness is required then martensitic stainless steels or precipitation-hardening stainless steels may be adopted.