Stainless Steel Categories

Stainless steel metals are commonly divided into five categories:

• martensitic stainless steels
• ferritic stainless steels
• austenitic stainless steels
• duplex (ferritic-austenitic) stainless steels
• precipitation-hardening stainless steels

1. Martensitic stainless steels, typified by types 410/420/440, contain about 12Cr (Chromium) and 0.1C (Carbon) wt% as the basic composition. They are not considered as corrosion resistant as other stainless steel classes, but are extremely strong and tough as well as highly machineable. They can be hardened by heat treatment. These stainless steels contain 11.5 to 18% chromium and significant amounts of carbon. Some stainless grades include additional alloying elements in small quantities.


2. Ferritic stainless steels contain larger amounts of Cr (Chromium) which stabilizes the ferritic phase. Ferritic stainless steels are highly corrosion resistant, yet much less durable than austenitic grades and cannot be hardened by heat treatment. They contain between 10.5% and 27% chromium and very little nickel, if any. The least expensive stainless steels found in this family are type 409. Typical applications may include: appliances, automotive and architectural trim (i.e., decorative purposes)


3. Austenitic stainless steels, such as type 304 typically contain 18Cr (Chromium) and 8Ni (Nickel) wt% (aka 18/8 stainless). Austenitic stainless steels comprise over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy. Other standard grades have different preferred applications; for example, type 316 which contains up to 3 wt% Mo (Molybdenum), offers an improved general and pitting corrosion resistance, making it the material of choice for marine applications and coastal environments.


4. Duplex stainless steels are two-phase alloys based on the Fe-Cr-Ni system. The specific advantages offered by duplex stainless steels over conventional 300 series stainless steels are strength (approximately twice that of austenitic stainless steels), improved toughness and ductility (compared to ferritic grades), and a superior chloride SCC resistance and pitting resistance. The high yield strength offers designers the use of thin-wall material (which can lead to major reductions in weight) with adequate pressure-containing and load-bearing capacity. Duplex stainless steels have found widespread use in a range of industries, particularly the oil and gas, petrochemical, and pulp and paper industries.


5. Specialist grades include the precipitation hardened or oxide dispersion strengthened alloys.