Stainless steel: Classification of Stainless Steel, Martensitic stainless steels, Ferritic stainless steels, Austenitic stainless steels

Stainless steel

Stainless steel contains
chromium together with nickel as alloy and rest is iron. It
has been defined as that steel which when
correctly heat treated and finished, resists oxidation and corrosive attack
from most corrosive media. Stainless steel surface is responsible
for corrosion resistance. Minimum chromium
content of 12% is required for the film’s formation, and
18% is sufficient to resist the most severe atmospheric corrosive conditions. Their principal alloying element is chromium
while some other elements like nickel, manganese etc. can also be present in
small
amounts. Addition of nickel
improves ductility and imparts strength. Corrosion resistance to stainless
steels increases with increase in nickel content against neutral chloride
solution and weakly oxidizing
acids. Addition
of molybdenum improves
its resistance
to
sulphuric, sulphurous and organic
acids. Addition of manganese
increases
hot workability of these steels.

Steels having 15 to 20% Ni and about
0.1 % carbon possesses great strength and toughness
and
extremely good resistance to corrosion. Such steels
are called stainless
steels. Another
type of stainless steel containing 11 to 14% chromium and about 0.35%
carbon is used for
cutlery, surgical and dental instruments and other purposes
where hard edges are required.
Maximum resistance to corrosion is obtained when this steel is ground
and polished after
heat-treating.

 A steel containing 18% chromium and  8% 
nickel  is  widely 
used
and is commonly referred to as 18/8 steel.
Stainless steel is highly resistance to corrosion and
oxidation. It can
be classified into three
major categories according to the type of micro structures.

General Properties of Stainless Steels

It possesses wide range of strength
and hardness, high ductility, formability, high corrosion
resistance, good creep resistance, good
thermal
conductivity, good machinability,
good weldability, high hot, cold workability,
high resistance to scaling and oxidation at elevated temperatures, excellent surface appearance and finish.

Classification of Stainless Steel

On basis of their structure, stainless steels are
classified as follow:

1. Martensitic stainless steels

 2. Ferritic
stainless steels

3. Austenitic stainless steels

These types of stainless steel are discussed as under.

1. Martensitic Stainless Steels

These steels contain 12 to 16%
chromium and 0.1 to 1.2 per cent carbon. The structure consists of hard
martensite phase after hardening. The general utility chromium stainless steel
with 12% chromium and 0.15% carbon are ferromagnetic and air hardening. It is
very hard and possesses high strain and high corrosion resistance properties.

Applications of Martensitic Stainless Steels

Stainless steels containing 12
to 14% chromium and 0.3% carbon are extensively used for table
cutlery, tools and equipments etc.
Stainless steels containing 16-18% chromium
and
0.2% carbon are used as springs, ball bearing, valves, knife blades and
instruments under high
temperature and corrosive conditions. These steels are generally used for making
utensils, surgical and dental instruments, and springs of high
temperature operations, ball valves
and toilet seats.

2. Ferritic Stainless Steels

Ferritic stainless steels are
non hardenable and contain 16 to 30% chromium and 0.08
  to 0.2 per cent carbon. Structure of these
steel consists of ferrite phase which cannot be hardened by heat treatment.
They have very low carbon and possess considerable
ductility, ability to be worked hot or cold, excellent corrosion
resistance and are relatively in expensive. They
are always magnetic
and retain their basic microstructure up to the melting point.

Applications

These are extensively used for
kitchen equipment, diary machinery interior decorative work, automobile
trimmings, chemical engineering industry, stainless steel sinks, food
containers, refrigerator parts, beer barrels, automobile trimming etc. These
are also used as high temperature furnace parts when chromium content is high.

3. Austenitic Stainless Steel

Addition of substantial
quantities of Ni to high Cr alloys gives rise to, austenitic steel.   It
has good resistance to many acids (even hot or cold nitric acid). Slight amount
of W and 
Mo are added in such steels to increase its strength at elevated temperatures. This steel 
contains 16 to 24% Cr, 8 to 22% Ni and less than 0.2% C. Addition of nickel stabilizes austenite, and hence the structure
of these steels consists of austenite at room
temperature.
A steel containing 18% Cr and 8% Ni is very widely used and is commonly
referred to as
18/ 8 stainless steel. These steels do not
harden by heat treatment but can be rolled hard. These steels possess a
brilliant luster when polished. These are highly resistant to many acids even
nitric
acids. The heat conductivity
of steel is
low, about 5% that of copper. Tungsten and molybdenum are added
to increase the strength
at elevated temperatures, silicon and aluminium
to improve the resistance to scaling and selenium and sulphur are added
to improve
machinability. This steel
is easily weldable. After
welding, it
is susceptible to corrosive attack in
the area adjacent to the weld.

Applications

It is used for making heat exchangers, conveyors chains, furnaces, spokes, brewery, dairy and chemical industrial components, cutlery parts, surgical
and dental instruments, household appliances such as kitchen utensils, sinks and saucepans.
These are also used in
making components
in power stations, especially in nuclear power stations, steam pipes, boiler
tubes, radiator and super heater tubes.




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