Properties of
Stainless Steels
The advantageous properties of stainless
steel can be seen when
compared to standard plain carbon mild steel. Although stainless
steel have a broad range of
properties, in general, when compared with mild steel, stainless
steel 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 of
Stainless Steel
All stainless steel 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
steel 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 steel
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.
Work Hardening of
Stainless Steel
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
steel also helps to resist
scaling at elevated temperatures.
Ductility of
Stainless Steel
Ductility tends to be given by the % elongation during a
tensile test. The elongation for austenitic stainless
steel is quite high. High
ductility and high work hardening rates allows austenitic stainless
steel to be formed using
severe processes such as deep drawing.
High Strength
When compared with mild steels, stainless
steel tend to have higher
tensile strength. The duplex stainless steel
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 304 and 316, the most commonly used stainless
steel.
Magnetic Response of
Stainless Steel
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
steel 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 steel
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 steel
are generally non-magnetic. They are not able to be hardened by heat
treatment. Austenitic stainless steel
rapidly work-harden with cold working. Although they work harden, they
are the most readily formed of the stainless
steel.
The principal alloying elements are sometimes reflected in
the name of the steel. As an a common name for 304 stainless steel is 18/8, for
18% chromium and 8% nickel.
Austenitic Stainless Applications
Applications for austenitic stainless
steel 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 steel
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 434 and 444. Ferritic stainless
steel cannot be hardened by
heat treatment and are always used in the annealed condition.
Ferritic stainless steel
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 steel
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 steel when compared with ferritic stainless.
Martensitic stainless
steel 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
steel are typically used
for:
·
Knife blades
·
Cutlery
·
Surgical instruments
·
Fasteners
·
Shafts
·
Springs
Duplex Stainless Steels
Duplex stainless steel
have high chromium and low nickel contents. This gives duplex stainless
steel 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 steel
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.
Duplex Stainless Applications
Duplex stainless steel
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
steel are typically used
for:
·
Pulp and paper industry equipment
·
Aerospace applications
·
Turbine blades
·
Nuclear waste casks
·
Mechanical components
Standard Classifications of
Stainless Steel
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 of
Stainless Steel
The grade selection process for stainless
steel 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. |