Stainless steel and it's classifications

1. CATEGORIES OF STAINLESS
STEEL
STAINLESS STEELS HAVE TRADITIONALLY BEEN DIVIDED INTO CATEGORIES
DEPENDING ON THEIR MICROSTRUCTURE AT ROOM TEMPERATURE, WHICH GIVES
THEM DIFFERENT COMPOSITION AND PROPERTIES.
COMPOSITION GRADE MICROSTRUCURE
12% Cr 409 Martensitic
17% Cr 430 Ferritic
18% Cr, 9% Ni 304 Austenitic

2. MARTENSITIC STAINLESS STEELS
 These are primarily straight chromium steels with 11.5%
to 18% C, 1.25% Mn and 1% Si.
 Common examples are 403, 410, 416, 420, 440, 501 and
502 (AISI grades)
 They develop in stainless steel provided the balance of
alloying elements produces a fully austenitic structure at
the solution treatment temperature e.eg at 1050◦C, iron
can accommodate up to 13% Cr and still remain austenitic
at that temperature.
 They are similar in composition to the ferritic group but
contain higher carbon and lower chromium to permit
hardening by heat treatment
 They are used for turbine blades and corrosion resistant
applications
 They occur when the MS – MF temperature range is above
room temperature
 Process annealing 650 - 760 ◦C, to increase ductility and

3. FERRITIC STAINLESS
STEELS
 These are primarily straight chromium steels with 14% to
27% Cr. Carbon is restricted both to contain high
toughness and ductility and to prevent austenite
formation ( as carbon expands the gamma loop)
 They are non hardenable iron chromium alloys
 there can be additions of other elements such as Mo for
pitting corrosion resistance and Ti or Nb for improved
resistance to intergranular corrosion
 These steels can be cold formed easily and hence are
used for deep drawn parts such as vessels for food and
chemical industries and for chemical industries and for
architectural and automotive trim.
 Whereas they generally possess high toughness, the
embrittling effect can be experienced in steels
containing more than 12% Cr when heated to
temperatures in range 400 - 550 ◦C
 The loss of toughness is due to the precipitation of a
chromium content

4. AUSTENITIC STAINLESS STEEL
 They are based on 18% Cr but with relatively large
additions of Ni in order to preserve the austenitic
structure
 However, various compositional modifications are
employed in order to improve the corrosion resistance
of these steels
 Type 304 ( 18%CR, 9% Ni) is the most popular grade in
the series and is used in a wide variety of applications
which require a good combination of corrosion
resistance and formability
 The strength of austenitic stainless steels can be
increased by warm working i.e. by finishing rolling at
temperatures below 950 ◦C
 They are difficult to machine but can be improved by
addition of selenium of sulfur
 Best high temperature strength and scaling resistance.

5. STAINLESS STEEL VS CARBON STEEL (COR-TEN
STEEL)
1.STAINLESS STEEL
• Highly resistant to stain and rust.
• Forms a chromium rich film that protects
it from corrosion
• Minimum chromium content around 12%
with maximum chromium content of
around 30%.
• Good performance in various atmospheric
conditions
• Favored for many applications since it is
passive
• Can be coated with material such as
durason and silicolly for further
protection

6. 2.CARBON STEEL(HSS CORTEX STEEL)
• Allows rust to form on its surface, but corten
steel has the ability to prevent rust from
penetrating into it
• The rust layer that is formed on its surface
serves to protect it from highly corrosive
environments.
• Minimum chromium content is around 11.5%
• Does not perform well in damp areas as well as
in environments of high vegetation.
• They are normally favored by designers for
their rustic look, and not suitable for many
applications.
• Are designed in such a manner that they do not
require any coating or pretreatment.