abc® Aalco is a registered trademark of Aalco Metals Ltd
© Copyright: Aalco Metals Ltd, The Hersham Centre, Hersham Green, Hersham, Surrey KT12 4HP
All Data is indicative only and must not be seen as a substitute for the full specification from which it is drawn.
In particular, the mechanical property requirements vary widely with product form and product dimensions.
For more complete details please refer to the relevant specification – The BS EN Specifications for Stainless Steel
are listed on a separate Datasheet.
Stainless Steel - Introduction
Stainless steel is not a single material
but the name for a family of corrosion
resistant steels. Like many scientific
discoveries the origins of stainless
steel lies in a serendipitous accident.
In 1913 Sheffield, England, Harry
Brearley was investigating the
development of new steel alloys for
use in gun barrels. He noticed that
some of his samples didn’t rust and
were difficult to etch. These alloys
contained around 13% chromium.
The first application of these steels
was in cutlery for which Sheffield
subsequently became world famous.
Simultaneous work in France led to the
development of the first austenitic
stainless steels.
Worldwide demand for stainless steel
is increasing at a rate of about 5% per
annum. Annual consumption is now
well over 20 million tonnes and is
rising in areas such as the construction
industry and household appliances.
New uses are being continuously found
for the attractive appearance,
corrosion resistance, low maintenance
and strength of stainless steel.
Stainless steel is more expensive than
standard grades of steel but it has
greater resistance to corrosion, needs
low maintenance and has no need for
painting or other protective coatings.
These factors mean stainless steel can
be more economically viable once
service life and life-cycle costs are
considered.
Properties of Stainless Steels
The advantageous properties of stainless
steels can be seen when compared to
standard plain carbon mild steel. Although
stainless steels have a broad range of
properties, in general, when compared with
mild steel, stainless steels have:
Higher corrosion resistance
Higher cryogenic toughness
Higher work hardening rate
Higher hot strength
Higher ductility
Higher strength and hardness
A more attractive appearance
Lower maintenance
Corrosion Resistance
All stainless steels are iron-based alloys
that contain a minimum of around 10.5%
Chromium. The Chromium in the alloy
forms a self-healing protective clear oxide
layer. This oxide layer gives stainless steels
their corrosion resistance. The self healing
nature of the oxide layer means the
corrosion resistance remains intact
regardless of fabrication methods. Even if
the material surface is cut or damaged, it
will self heal and corrosion resistance will
be maintained.
Conversely, normal carbon steels may be
protected from corrosion by painting or
other coatings like galvanising. Any
modification of the surface exposes the
underlying steel and corrosion can occur.
The corrosion of different grades of
stainless steel will differ with various
environments. Suitable grades will depend
upon the service environment. Even trace
amounts of some elements can markedly
alter the corrosion resistance. Chlorides in
particular can have an adverse effect on the
corrosion resistance of stainless steel.
Grades high in Chromium, Molybdenum and
Nickel are the most resistant to corrosion.
Cryogenic (Low Temp.) Resistance
Cryogenic resistance is measured by the
ductility or toughness at sub zero
temperatures. At cryogenic temperatures
the tensile strengths of austenitic stainless
steels are substantially higher than at
ambient temperatures. They also maintain
excellent toughness.
Ferritic, martensitic and precipitation
hardening steels should not be used at sub-
zero temperatures. The toughness of these
grades drops significantly at low
temperatures. In some cases this drop
occurs close to room temperature.
X
abc® Aalco is a registered trademark of Aalco Metals Ltd
© Copyright: Aalco Metals Ltd, The Hersham Centre, Hersham Green, Hersham, Surrey KT12 4HP
All Data is indicative only and must not be seen as a substitute for the full specification from which it is drawn.
In particular, the mechanical property requirements vary widely with product form and product dimensions.
For more complete details please refer to the relevant specification – The BS EN Specifications for Stainless Steel
are listed on a separate Datasheet.
Work Hardening
Work hardenable grades of stainless steel
have the advantage that significant
increases to the strength of the metal can
be achieved simply through cold working. A
combination of cold working and annealing
stages can be employed to give the
fabricated component a specific strength.
A typical example of this is the drawing of
wire. Wire to be used as springs will be
work hardened to a particular tensile
strength. If the same wire was to be used
as a bendable tie wire, it would be
annealed, resulting in a softer material.
Hot Strength
Austenitic grades retain high strength at
elevated temperatures. This is particularly
so with grades containing high levels of
chromium and/or high silicon, nitrogen and
rare earth elements (e.g. grade 310 and
S30815). High chromium ferritic grades like
446 can also show high hot strength.
The high chromium content of stainless
steels also helps to resist scaling at
elevated temperatures.
Ductility
Ductility tends to be given by the %
elongation during a tensile test. The
elongation for austenitic stainless steels is
quite high. High ductility and high work
hardening rates allows austenitic stainless
steels to be formed using severe processes
such as deep drawing.
High Strength
When compared with mild steels, stainless
steels tend to have higher tensile strength.
The duplex stainless steels have higher
tensile strengths than austenitic steels.
The highest tensile strengths are seen in
the martensitic (431) and precipitation
hardening grades (17-4 PH). These grades
can have strengths double that of TYPES
304 and 316, the most commonly used
stainless steels.
Magnetic Response
Magnetic response is the attraction of steel
to a magnet. Austenitic grades are
generally not magnetic although a magnetic
response can be induced in the low
austenitic grades by cold working. High
nickel grades like 316 and 310 will remain
non-magnetic even with cold working.
All other grades are magnetic.
Stainless Steel Families
Although the corrosion resistance of
stainless comes from the presence of
Chromium, other elements are added to
enhance other properties. These elements
alter the microstructure of the steel.
Stainless steels are grouped into families
based on their metallurgical microstructure.
The microstructure may be composed of
the stable phases austenite or ferrite, a
“duplex” mix of these two, martensite or a
hardened structure containing precipitated
micro-constituents.
Austenitic Stainless Steels
Austenitic stainless steels contain a
minimum of 16% chromium and 6% nickel.
They range from basic grades like 304
through to super austenitics such as 904L
and 6% Molybdenum grades.
By adding elements such as Molybdenum,
Titanium or Copper, the properties of the
steel can be modified. These modifications
can make the steel suited to high
temperature applications or increase
corrosion resistance. Most steels become
brittle at low temperatures but the Nickel in
austenitic stainless makes it suited to low
temperature or cryogenic applications.
Austenitic stainless steels are generally
non-magnetic. They are not able to be
hardened by heat treatment. Austenitic
stainless steels rapidly work-harden with
cold working. Although they work harden,
they are the most readily formed of the
stainless steels.
The principal alloying elements are
sometimes reflected in the name of the
steel. A common name for 304 stainless
steel is 18/8, for 18% chromium and 8%
nickel. X
abc® Aalco is a registered trademark of Aalco Metals Ltd
© Copyright: Aalco Metals Ltd, The Hersham Centre, Hersham Green, Hersham, Surrey KT12 4HP
All Data is indicative only and must not be seen as a substitute for the full specification from which it is drawn.
In particular, the mechanical property requirements vary widely with product form and product dimensions.
For more complete details please refer to the relevant specification – The BS EN Specifications for Stainless Steel
are listed on a separate Datasheet.
Austenitic Stainless Applications
Applications for austenitic stainless steels
include:
Kitchen sinks
Architectural applications such as
roofing and cladding
Roofing and gutters
Doors and Windows
Balustrading
Benches and food preparation areas
Food processing equipment
Heat exchangers
Ovens
Chemical tanks
Ferritic Stainless Steels
Ferritic stainless steels include grades like
430 and contain only chromium as a major
alloying element. The quantity of chromium
present ranges from 10.5 to 18%.
They are known for their moderate
corrosion resistance and poor fabrication
properties. Fabrication properties can be
improved by alloy modifications and are
satisfactory in grades such as 434and 444.
Ferritic stainless steels cannot be hardened
by heat treatment and are always used in
the annealed condition.
Ferritic stainless steels are magnetic. They
are also not susceptible to stress corrosion
cracking. Weldability is acceptable in thin
sections but decreases as section
thicknesses increase.
Ferritic Stainless Applications
Ferritic stainless steels are typically used
in:
Vehicle exhausts
Fuel lines
Cooking utensils
Architectural trim
Domestic appliances
Martensitic Stainless Steels
High carbon and lower chromium content
are the distinguishing features of
martensitic stainless steels when compared
with ferritic stainless.
Martensitic stainless steels include 410 and
416. Hardened martensitic steels cannot be
successfully cold formed. They are
magnetic, have moderate corrosion
resistance and poor weldability.
Martensitic Stainless Applications
Martensitic stainless steels are typically
used for:
Knife blades
Cutlery
Surgical instruments
Fasteners
Shafts
Springs
Duplex Stainless Steels
Duplex stainless steels have high chromium
and low nickel contents. This gives duplex
stainless steels microstructures that include
both austenitic and ferritic phases. They
include alloys like 2304 and 2205. These
alloys are so named due to their respective
compositions - 23% chromium, 4% nickel
and 22% chromium, 5% nickel.
By having both austenite and ferrite in the
microstructure, duplex stainless steels
feature properties of both classes. Although
a compromise between the two ‘pure’
types, duplex grades can offer some unique
property solutions. Duplex grades are
resistant to stress corrosion cracking, but
not to the same level as ferritic grades. The
toughness of duplex grades is superior to
that of the ferritic grades – but inferior to
that of the austenitic grades.
Most importantly, the corrosion resistance
of duplex steels is equal, or superior to 304
and 316 stainless steel. This is particularly
so for chloride attack.
Duplex grades are readily welded. They
also have high tensile strengths.
X
abc® Aalco is a registered trademark of Aalco Metals Ltd
© Copyright: Aalco Metals Ltd, The Hersham Centre, Hersham Green, Hersham, Surrey KT12 4HP
All Data is indicative only and must not be seen as a substitute for the full specification from which it is drawn.
In particular, the mechanical property requirements vary widely with product form and product dimensions.
For more complete details please refer to the relevant specification – The BS EN Specifications for Stainless Steel
are listed on a separate Datasheet.
Duplex Stainless Applications
Duplex stainless steels typically find
application in areas like:
Heat exchangers
Marine applications
Desalination plants
Food pickling plants
Off-shore oil & gas installations
Chemical & petrochemical plant
Precipitation Hardening Grades
Precipitation hardening grades contain both
Chromium and Nickel. They develop very
high tensile strengths with heat treatment.
Precipitation hardening grades are usually
supplied in a “solution treated” condition
that allows the steel to be machined. After
machining or forming, the steel can be
aged in a low temperature heat treatment
process. As the heat treatment is
performed at low temperatures, no
distortion is induced in the work piece.
630 is the most common precipitation
hardening grade This grade is also known
as 17-4 PH due to a composition of 17%
chromium, 4% nickel, 4% copper and 0.3%
niobium.
Precipitation Hardening
Applications
Precipitation hardening stainless steels are
typically used for:
Pulp and paper industry equipment
Aerospace applications
Turbine blades
Nuclear waste casks
Mechanical components
Standard Classifications
The old AISI three digit stainless steel
numbering system (e.g. 304 and 316) is
still commonly used. New grades are
defined under the SAE and ASTM system
that uses a 1-letter + 5-digit UNS number.
An example of this is the new term for 304,
which is S30400. Other designations
include old BS and EN numbers like 304S31
and 58E.
Some grades are not covered by standard
numbers and could be proprietary grades or
be named using standards for specialist
products like welding wire.
Grade Selection
The grade selection process for stainless
steels is a compromise between the desired
properties of the finished product.
When selecting a particular grade of
stainless steel, it is essential to consider the
primary properties required, such as
corrosion resistance and heat resistance.
Important consideration must also be given
to the secondary properties, like physical
and mechanical properties. These
properties will determine other factors such
as the ease of fabrication of any candidate
grades.
If the secondary properties are not
adequate, it may not be possible to viably
and economically produce the required
product.
An example of this is 303 stainless steel. It
has excellent machinability due to an
addition of Sulphur. However, the Sulphur
also gives 303 poor weldability, corrosion
resistance and formability.
Selecting the correct grade will ensure the
product will have a long trouble-free life
combined with cost-effective fabrication and
installation.
This information is based on our present
knowledge and is given in good faith. However,
no liability will be accepted by the Company is
respect of any action taken by any third party in
reliance thereon
As the products detailed may be used for a wide
variety of purposes and as the Company has no
control over their use; the Company specifically
excludes all conditions or warranties expressed
or implied by statute or otherwise as to
dimensions, properties and/or fitness for any
particular purpose. Any advice given by the
Company to any third party is given for that
party’s assistance only and without liability on
the part of the Company.
Any contract between the Company and a
customer will be subject to the Company’s
Conditions of Sale. The extent of the Company’s
liabilities to any customer is clearly set out in
those Conditions; a copy of which is available on
request.