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Based
on alloy contents, internal structure, magnetic or
non magnetic properties, and other qualities,
stainless steels can be broadly classified into
under noted five categories.
1.
Martensitic Stainless Steels
Martensitic
Stainless steel contain 11 to 18 % chromium along
with carbon in specific percentage, making it
hardenable by air cooling. These stainless steels
are extensively used for cutlery, knives surgical
instruments, industrial blades, chemical plant
equipments, aerospace applications and other
industrial components. In addition to chromium and
carbon elements like nickel, molybdenum, vanadium,
tungsten etc. are also added in some special purpose
grades.
2.
Ferritic Stainless Steels
These
stainless steels contain chromium in the range of 11
to 30% with no or very little nickel, are
ferromagnetic in nature and generally not hardenable
by heat treatment, provide good corrosion resistance
and good formability at low cost, and are
extensively used for many automotive components.
Inferior weldability of these steel can be improved
by alloying with titanium or columbium A 15430 is
the general purpose stainless steel of this group.
3.
Austenitic Stainless Steel
These
steels contain 16 to 30% chromium 6 to 20 % nickel
and low carbon content, has excellent corrosion
resistance, high ductility and toughness. Work
hardening during cold working helps to get desired
mechanical properties. These can not be hardened by
heat treatment. In AISI 200 series steels, nickel is
partially substituted by manganese and nitrogen is
also added which helps in improvement of corrosion
resistance, strength and stabilisation of austenitic
structure. In some grades, copper is also added to
improve cold forming and corrosion resisting
properties.
Austenitic
grades are most widely used stainless steels.
200 series grades are having higher strength than
300 series stainless steel.
4.
Precipitation Hardening Stainless Steels
These
steels contain titanium, copper, niobium, aluminum,
tantalum, phosphorous which precipitates during
ageing or heat treatment and improve the mechanical
properties of the alloy. Precipitation hardening
steels are of three types viz, austenitic, semi
austenitic and martensitic are corrosion and
oxidation resistant and retain its strength at
elevated temperature. These are mostly used in
aerospace applications.
5.
Duplex Stainless Steels
In
these steels structure is controlled in order to
give equal ferrite and austenite to develop
excellent corrosion resistance property to with
stand chloride steels corrosion cracking. These
steels exhibit higher yield strength than the
austenitic grades and better resistance to chloride
corrosion, of these properties makes it suitable for
fabrication of various industrial equipments for
typical corrosive environments, where austenitic or
ferritic grades are generally found unsuitable. |
