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Stainless Steel 101

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  • Harry
    Stainless Steels are iron-base alloys containing Chromium. Stainless steels usually contain less than 30% Chromium and more than 50% Iron. They attain their
    Message 1 of 1 , Feb 28, 2005
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      Stainless Steels are iron-base alloys containing Chromium. Stainless
      steels usually contain less than 30% Chromium and more than 50%
      Iron. They attain their stainless characteristics because of the
      formation of an invisible and adherent chromium-rich oxide surface
      film. This oxide establishes on the surface and heals itself in the
      presence of oxygen.

      Some other alloying elements added to enhance specific
      characteristics include nickel, molybdenum, copper, titanium,
      aluminum, silicon, niobium, and nitrogen. Carbon is usually present
      in amounts ranging from less than 0.03% to over 1.0% in certain
      martensitic grades.

      Corrosion resistance and mechanical properties are commonly the
      principal factors in selecting a grade of stainless steel for a
      given application.

      Stainless steels are commonly divided into five groups:
      • Martensitic stainless steels
      • Ferritic stainless steels
      • Austenitic stainless steels
      • Duplex (ferritic-austenitic) stainless steels
      • Precipitation-hardening stainless steels.

      Martensitic stainless steels
      Martensitic stainless steels are essentially alloys of chromium and
      carbon that possess a martensitic crystal structure in the hardened
      condition. They are ferromagnetic, hardenable by heat treatments,
      and are usually less resistant to corrosion than some other grades
      of stainless steel. Chromium content usually does not exceed 18%,
      while carbon content may exceed 1.0 %.

      Ferritic stainless steels
      Ferritic stainless steels are chromium containing alloys with
      Ferritic, body centered cubic (bcc) crystal structures. Chromium
      content is typically less than 30%. The ferritic stainless steels
      are ferromagnetic.

      Austenitic stainless steels (300 series)
      Austenitic stainless steels have a austenitic, face centered cubic
      (fcc) crystal structure. Austenite is formed through the generous
      use of austenitizing elements such as nickel, manganese, and
      nitrogen. Austenitic stainless steels are effectively nonmagnetic in
      the annealed condition and can be hardened only by cold working.

      Duplex stainless steels
      Duplex stainless steels are a mixture of bcc ferrite and fcc
      austenite crystal structures. The percentage each phase is a
      dependent on the composition and heat treatment. Most Duplex
      stainless steels are intended to contain around equal amounts of
      ferrite and austenite phases in the annealed condition. The primary
      alloying elements are chromium and nickel. Duplex stainless steels
      generally have similar corrosion resistance to austenitic alloys
      except they typically have better stress corrosion cracking
      resistance. Duplex stainless steels also generally have greater
      tensile and yield strengths, but poorer toughness than austenitic
      stainless steels.

      Precipitation hardening stainless steels
      Precipitation hardening stainless steels are chromium-nickel alloys.
      Precipitation-hardening stainless steels may be either austenitic or
      martensitic in the annealed condition. In most cases, precipitation
      hardening stainless steels attain high strength by precipitation
      hardening of the martensitic structure.

      Stainless steel and Magnetism
      Magnetic permeability is the ability of a material to carry
      magnetism, indicated by the degree to which it is attracted to a

      All stainless steels, with the exception of the austenitic group,
      are strongly attracted to a magnet.

      Austenitic Grades
      All austenitic grades have very low magnetic permeabilities and
      hence show almost no response to a magnet when in the annealed
      condition; the situation is, however, far less clear when these
      steels have been cold worked by wire drawing, rolling or even
      centreless grinding, shot blasting or heavy polishing.

      After substantial cold working Grade 304 may exhibit quite strong
      response to a magnet, whereas Grades 310 and 316 will in most
      instances still be almost totally non-responsive.

      The change in magnetic response is due to atomic lattice straining
      and formation of martensite.

      Stress Relieving
      Any austenitic (300 series) stainless steel which has developed
      magnetic response due to cold work can be returned to a non-magnetic
      condition by stress relieving. In general this can be readily
      achieved by briefly heating to approximately 700 - 800°C (this can
      be conveniently carried out by careful use of an oxy-acetylene

      Note, however, unless the steel is a stabilised grade it could
      become sensitised to carbide precipitation. Full solution treatment
      at 1000 - 1150°C will remove all magnetic response without danger of
      reduced corrosion resistance due to carbides.

      regards Harry
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