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Engineering Materials - Ferrous Metals - Stainless Steel

Materials: Ferrous Metals


Ferrous Metals

 

Stainless Steel

 

Stainless steel is the generic name for a number of different steels used primarily for their resistance to corrosion. The one key element they all share is a certain minimum percentage (by mass) of chromium: 10.5%. Although other elements, particularly nickel and molybdenum, are added to improve corrosion resistance, chromium is always the deciding factor. The vast majority of steel produced in the world is carbon and alloy steel, with the more expensive stainless steels representing a small, but valuable niche market.

Stainless steels are commonly divided into five groups:

  1. martensitic stainless steels
  2. ferritic stainless steels
  3. austenitic stainless steels,
  4. duplex (ferritic-austenitic) stainless steels
  5. precipitation-hardening stainless steels

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


Ferritic stainless steels contain larger amounts of Cr which stabilizes the ferritic phase. Ferritic stainless steels are highly corrosion resistant, but far 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. Typical applications may include appliances, automotive and architectural trim (i.e., decorative purposes), as the cheapest stainless steels are found in this family (type 409).


Austenitic stainless steels, such as type 304 typically contain 18Cr and 8Ni 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, offers an improved general and pitting corrosion resistance, making it the material of choice for marine applications and coastal environments.


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.

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

See Reference Table - Properties of Stainless Steels

See Reference Table - AISI Stainless Steel Alloy Designations




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