What is Stainless Steel?

Stainless steel is a family of iron-based alloys containing about 10.5% chromium or more, plus other alloying elements such as nickel, manganese, molybedenum, sulfur, selenium, titanium, etc. The chromium is chiefly responsible for corrosion and heat resistance; the other alloying elements are present in stainless steel to enhance corrosion resistance and to impart certain characteristics with respect to strength and fabricability.

Stainless Steel Fasteners:  The stainless steel fastener materials are identified as the B8 class of alloys and are identified in the ASTM Specification A193/193M. The corresponding nut specification is ASTM Specification A194/194M.

Most AISI stainless steels are identified by a system of numbers in either 200, 300 or 400 series. There are five primary classifications of stainless steel:

  • Austenitic
  • Martensitic
  • Ferritic
  • Precipitation hardening
  • Duplex

Austenitic:

When nickel is added to stainless steel in sufficient amounts the crystal structure changes to "austenite". The basic composition of austenitic stainless steels is 18% chromium and 8% nickel. Austenitic grades are the most commonly used stainless steels accounting for more than 70% of production (type 304 is the most commonly specified grade by far).

- Basic properties -

  • excellent corrosion resistance
  • excellent weldability (all processes)
  • excellent formability, fabricability and ductility
  • excellent cleanability, and hygiene characteristics
  • good high and excellent low temperature properties
  • non magnetic (if annealed)
  • hardenable by cold work only
  • - Common uses -

    • computer floppy disk shutters (304)
    • computer keyboard key springs (301)
    • kitchen sinks (304D)
    • food processing equipment
    • architectural applications
    • chemical plant and equipment

Martensitic:

Martensitic stainless steels were the first stainless steels commercially developed (as cutlery) and have relatively high carbon content (0.1 - 1.2%) compared to other stainless steels. They are plain chromium steels containing between 12 and 18%  chromium.

- Basic properties -

  • moderate corrosion resistance
  • can be hardened by heat treatment and therefore high strength and hardness levels can be achieved
  • poor weldability
  • magnetic

- Common uses -

  • knife blades
  • surgical instruments
  • shafts
  • spindles
  • pins

Ferritic:

These are plain chromium stainless steels with varying chromium content between 12 and 18%, but with low carbon content.

- Basic properties -

  • moderate to good corrosion resistance increasing with chromium content
  • not hardenable by heat treatment and always used in the annealed condition magnetic
  • weldability is poor
  • formability not as good as the austenitics

- Common uses -

  • computer floppy disk hubs (430)
  • automotive trim (430)
  • automotive exhausts (409)
  • colliery equipment (3CR12)
  • hot water tanks (444)

Precipitation hardening:

Precipitation hardening stainless steels have been formulated so that they can be supplied in a solution treated condition, (in which they are machineable) and can be hardened, after fabrication, in a single low temperature "ageing" process.

- Basic properties -

  • moderate to good corrosion resistance
  • very high strength
  • good weldability
  • magnetic

- Common uses -

Shafts for pumps and valves

Duplex:

These are stainless steels containing relatively high chromium (between 18 and 28%) and moderate amounts of nickel (between 4.5 and 8%). The nickel content is insufficient to generate a fully austenitic structure and the resulting combination of ferritic and austenitic structures is called duplex. Most duplex steels contain molybdenum in a range of 2.5 - 4%.

- Basic properties -

  • high resistance to stress corrosion cracking
  • increased resistance to chloride ion attack
  • higher tensile and yield strength than austenitic or ferritic steels
  • good weldability and formability

- Common uses -

  • marine applications, particularly at slightly elevated temperatures
  • desalination plant
  • heat exchangers
  • petrochemical plant

 

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