One
of the features that
characterize
stainless steels
is a minimum 10.5%
chromium content as
the principal
alloying element.
Four major
categories of
wrought stainless
steel, based on
metallurgical
structure, are
austenitic, ferritic,
martensitic, and
precipitation
hardening. Cast
stainless-steel
grades are generally
designated as either
heat resistant or
corrosion resistant.
Austenitic
wrought stainless
steel are classified
in three groups:
·
The AISI 200 series
(alloys of
iron-chromium-nickel-manganese)
·
The AISI 300 series
(alloys of
iron-chromium-nickel)
·
Nitrogen-strengthened
alloys
Carbon content is
usually low (0.15%
or less), and the
alloys contain a
minimum of 16%
chromium with
sufficient nickel
and manganese to
provide an
austenitic structure
at all temperatures
from the cryogenic
region to the
melting point of the
alloy.
For all type of
stainless steel,
stainless steel 304,
stainless steel 304
price, stainless
steel 316, stainless
steel 316l.Contact
Us.
Nitrogen-strengthened
austenitic
stainless steels
are alloys of
chromium-manganese-nitrogen;
some grades also
contain nickel.
Yield strengths of
these alloys
(annealed) are
typically 50% higher
than those of the
non nitrogen-bearing
grades. They are
nonmagnetic and most
remain so, even
after severe cold
working.
Like carbon,
nitrogen increases
the strength of a
steel. But unlike
carbon, nitrogen
does not combine
significantly with
chromium in a
stainless steel.
This combination,
which forms chromium
carbide, reduces the
strength and
corrosion resistance
of an alloy.
Until recently,
metallurgists had
difficulty adding
controlled amounts
of nitrogen to an
alloy. The
development of the
argon-oxygen
decarburization (AOD)
method has made
possible strength
levels formerly
unattainable in
conventional
annealed
stainless alloys.
Austenitic
stainless steels
are generally used
where corrosion
resistance and
toughness are
primary
requirements.
Typical applications
include shafts,
pumps, fasteners,
and piping in
seawater and
equipment for
processing
chemicals, food, and
dairy products.
Ferritic
wrought alloys (the
AISI 400 series)
contain from 10.5 to
27% chromium. In
addition, the use of
argon-oxygen
decarburization and
vacuum-induction
melting has produced
several new ferritic
grades including
18Cr-2Mo, 26Cr-1Mo,
29Cr-4Mo, and
29Cr-4Mo-2Ni.
Low in carbon
content, but
generally higher in
chromium than the
martensitic grades,
these steels cannot
be hardened by heat
treating and are
only moderately
hardened by cold
working. Ferritic
stainless steels
are magnetic and
retain their basic
microstructure up to
the melting point if
sufficient Cr and Mo
are present. In the
annealed condition,
strength of these
grades is
approximately 50%
higher than that of
carbon steels.
Ferritic
stainless steels
are typically used
where moderate
corrosion resistance
is required and
where toughness is
not a major need.
They are also used
where chloride
stress-corrosion
cracking may be a
problem because they
have high resistance
to this type of
corrosion failure.
In heavy sections,
achieving sufficient
toughness is
difficult with the
higher-alloyed
ferritic grades.
Typical applications
include automotive
trim and exhaust
systems and
heat-transfer
equipment for the
chemical and
petrochemical
industries.
Martensitic
steels are also
in the AISI 400
series. These
wrought,
higher-carbon steels
contain from 11.5 to
18% chromium and may
have small
quantities of
additional alloying
elements. They are
magnetic, can be
hardened by heat
treatment, and have
high strength and
moderate toughness
in the
hardened-and-tempered
condition. Forming
should be done in
the annealed
condition.
Martensitic
stainless steels are
less resistant to
corrosion than the
austenitic or
ferritic grades. Two
types of martensitic
steels -- 416 and
420F -- have been
developed
specifically for
good machinability.
Martensitic
stainless steels
are used where
strength and/or
hardness are of
primary concern and
where the
environment is
relatively mild from
a corrosive
standpoint. These
alloys are typically
used for bearings,
molds, cutlery,
medical instruments,
aircraft structural
parts, and turbine
components. Type 420
is used increasingly
for molds for
plastics and for
industrial
components requiring
hardness and
corrosion
resistance.
Precipitation-hardening
stainless steels
develop very high
strength through a
low-temperature heat
treatment that does
not significantly
distort precision
parts. Compositions
of most
precipitation-hardening
stainless steels are
balanced to produce
hardening by an
aging treatment that
precipitates hard,
inter metallic
compounds and
simultaneously
tempers the
martensite. The
beginning
microstructure of PH
alloys is austenite
or martensite. The
austenitic alloys
must be thermally
treated to transform
austenite to
martensite before
precipitation
hardening can be
accomplished.
These alloys are
used where high
strength, moderate
corrosion
resistance, and good
fabricability are
required. Typical
applications include
shafting,
high-pressure pumps,
aircraft components,
high-temper springs,
and fasteners.
Cast stainless
steels usually
have corresponding
wrought grades that
have similar
compositions and
properties. However,
there are small but
important
differences in
composition between
cast and wrought
grades.
Stainless-steel
castings should be
specified by the
designations
established by the
ACI (Alloy Casting
Institute), and not
by the designation
of similar wrought
alloys.
Service temperature
provides the basis
for a distinction
between
heat-resistant and
corrosion-resistant
cast grades. The C
series of ACI grades
designates the
corrosion-resistant
steels; the H
series designates
the heat-resistant
steels, which can be
used for structural
applications at
service temperatures
between 1,200 and
2,200°F. Carbon and
nickel contents of
the H-series alloys
are considerably
higher than those of
the C series.
H-series steels are
not immune to
corrosion, but they
corrode slowly --
even when exposed to
fuel-combustion
products or
atmospheres prepared
for carburizing and
nitriding. C-series
grades are used in
valve, pumps, and
fittings. H-series
grades are used for
furnace parts and
turbine components.
