Chromium (Cr) content sets stainless steels apart
from other steels.
The unique self-repairing 'passive' surface layer on the steel is a result the
Commercially available grades contain around 11% minimum chromium, and may be either ferritic or martensitic, depending on carbon range control.
More chromium enhances corrosion and
oxidation resistance, so a 17% Cr 430 (1.4016) ferritic would be expected to be
an improvement over the '410S' (1.4000) types.
Similarly martensitic 431 (1.4057) at 15% Cr can be expected to have better
corrosion resistance than the 12% Cr 420 (1.4021 / 1.4028) types.
Chromium levels over 20% provide improved 'aqueous' corrosion resistance for the
duplex and higher alloyed austenitics and also forms the basis of the good
elevated temperature oxidation resistance of ferritic and austenitic heat
resisting grades, such as the quite rare ferritic 446 (25% Cr) or the more
widely used 25 % Cr, 20% nickel (Ni) austenitic 310 (1.4845) grade.
In addition to this basic 'rule', nickel (Ni)
widens the scope of environments that stainless steels can handle.
The 2% Ni addition to the 431 (1.4057) martensitic type improves corrosion
resistance marginally. Additions of between about 4.5% and 6.5% Ni are made in
forming the duplex types. The austenitics have ranges from about 7% to over 20%.
The corrosion resistance is not only related to the nickel level however. It is
incorrect for example, to assume that 304 (1.4301) with its 8% Ni has better
corrosion resistance than 1.4462 duplex with 5% Ni.
More specific alloy additions are also made, to enhance corrosion resistance.
These include molybdenum (Mo) and nitrogen (N) for pitting and crevice corrosion
resistance. The 316 types are the main Mo bearing austenitics. Many of the
currently available duplex grades contain additions of both Mo and N.
Copper is also used to enhance corrosion
resistance in some common, hazardous environments such as intermediate
concentration ranges of sulphuric acid. Grades containing copper include the
austenitic 904L (1.4539) type.
Mechanical and physical properties
Basic mechanical strength
increases with alloy additions, but the atomic structure differences of the
various groups of stainless steels has a more important effect.
Only the martensitic stainless steels are hardenable by heat treatment, like
other alloy steels. Precipitation hardening stainless steels are strengthened by
heat treatment, but use a different mechanism to the martensitic types.
The ferritic, austenitic and duplex types cannot be strengthened or hardened by
heat treatment, but respond to varying degrees to cold working as a
Ferritic types have useful mechanical properties at ambient temperatures, but
have limited ductility, compared to the austenitics. They are not suitable for
cryogenic applications and lose strength at elevated temperatures over about 600
°C, although have been used for applications such as automotive exhaust systems
Austenitic types, with their characteristic face centred cube 'fcc' atomic
arrangement, have quite distinct properties. Mechanically they are more ductile
and impact tough at cryogenic temperatures.
The main physical property difference from the other types of stainless steel is
that they are 'non-magnetic' i.e. have low relative magnetic permeability's,
provided they are fully softened. They also have lower thermal conductivity's
and higher thermal expansion rates than the other stainless steel types.
Duplex types, which have a 'mixed' structure of austenite and ferrite, share
some of the properties of those types, but, fundamentally are mechanically
stronger than either ferritic or austenitic types.
Forming, fabrication and joining techniques
Depending on their type and heat-treated
condition, wrought stainless steels are formable and machinable. Stainless
steels can also be cast or forged into shape.
Most of the available types and grades can be joined by use of appropriate
'thermal' methods including soldering, brazing and welding.
Austenitics are suitable for a wide range of applications involving flat product
forming (pressing, drawing, stretch forming, spinning etc)
Although ferritics and duplex types are also useful for these forming methods,
the excellent ductility and work hardening characteristic of the austenitics
make them a better choice.
Formability of the austenitic types is controlled through the nickel level.
The 301 (1.4310) grade which has a 'low' nickel content, around 7% and so work
hardens when cold worked, enabling it to be use for pressed 'stiffening' panels.
In contrast nickel levels of around 8.5% make the steel ideally suited to deep
drawing operations, for example in the manufacture of stainless steel sinks.
Martensitics are not readily formable, but are used extensively for blanking in
the manufacture of cutting blades.
Most stainless steel types can be machined by conventional methods, provided
allowance is made for their strength and work hardening characteristics.
Techniques involving control of feed and speed to undercut work hardening layers
with good lubrication and cooling systems are usually sufficient.
Where high production volume systems are employed, machining enhanced grades may
In this respect, stainless steels are treated in similar ways to other alloy
steels, sulphur additions being the traditional approach in grades like 303
(1.4305). Controlled cleanness types are now also available for enhanced
Most stainless steels can be soldered or brazed, provided care is taken in
surface preparation and fluxes are selected to avoid the natural surface
oxidising properties being a problem in these thermal processes.
The strength and corrosion resistance of such joints does not match the full
potential of the stainless steel being joined, however.
To optimise joint strength and corrosion resistance, most stainless steels can
be welded using a wide range of techniques.
The weldablity of the ferritic and duplex types is good, whilst the austenitic
types are classed as excellent for welding. The lower carbon martensitics can be
welded with care but grades such as the 17% Cr, 1% carbon, 440 types (1.4125)
are not suitable for welding.
Summary of the main advantages of the
||410S, 430, 446
||Low cost, moderate corrosion resistance
& good formability
||Limited corrosion resistance, formabilty
& elevated temperature strength compared to austenitics
||Widely available, good general corrosion
resistance, good cryogenic toughness. Excellent formability &
||Work hardening can limit formability &
machinability. Limited resistance to stress corrosion cracking
||Good stress corrosion cracking resistance,
good mechanical strength in annealed condition
||Application temperature range more
restricted than austenitics
||Hardenable by heat treatment
||Corrosion resistance compared to
austenitics & formability compared to ferritics limited. Weldability
||Hardenable by heat treatment, but with
better corrosion resistance than martensitics
||Limited availability, corrosion
resistance, formability & weldability restricted compared to
Special grades with enhanced compositions have also been developed
that minimise the short comings of any particular type. These include: Super
Ferritics, Super Austenitics,
Super Duplexes, Low Carbon Weldable Martensitics, Austenitic Precipitation
The information above is reproduced and adapted by EngineerOnLine
Ltd. with the knowledge of BSSA.
For your convenience, the following is adapted and reproduced from Wikipedia
as at December 2nd 2005, complete with active hyperlinks.
Some compositions of stainless steel are prone to
corrosion. When heated to around 700 °C, chromium
carbide forms at the intergranular boundaries, depleting the grain edges of
chromium, impairing their corrosion
resistance. Steel in such condition is called sensitised. Steels with
carbon content 0.06% undergo sensitisation in about 2 minutes, while steels with
carbon content under 0.02% are not sensitive to it.
There is a possibility to reclaim sensitised
steel, by heating it to above 1000 °C and then quenching
it in water. This process dissolves the carbide particles and keeps them in
It is also possible to stabilize the steel to
avoid this effect and make it welding-friendly. Addition of titanium,
serves this purpose; titanium
carbide and tantalum
carbide form preferentially to chromium carbide, protecting the grains from
chromium depletion. Use of extra-low carbon steels is another method.
Light-gauge steel also does not tend to display this behaviour, as the cooling
after welding is too fast to cause effective carbide formation.
When subjected to high concentration of chloride
ions (eg. sea
water) and moderately high temperatures, localized severe corrosion known as
corrosion can occur. Acidic environment worsens the problem. This can cause
localized perforation of the parts. Composition rich in chromium and
particularly molybdenum and nitrogen display higher resistance to this mode of
corrosion. Solution of ferric
chloride is used for laboratory testing of pitting corrosion.
In the presence of reducing acids or exposition
atmosphere, the passivation layer protecting steel from corrosion can break
down. This wear can also depend on the mechanical construction of the parts, eg.
under gaskets, in sharp corners, or in incomplete welds. Such crevices may
promote corrosion, if their size allows penetration of the corroding agent but
not its free movement. The mechanism of crevice
corrosion is similar to pitting corrosion, though it happens at lower
corrosion cracking is a rapid and severe form of stainless steel corrosion.
It forms when the material is subjected to tensile
stress and some kinds of corrosive environments, especially chloride-rich
water) at higher temperatures. The stresses can result of the service loads,
or can be caused by the type of assembly or residual stresses from fabrication
(eg. cold working); the residual
stresses can be relieved by annealing.
This limits the usefulness of stainless steel for containing water with higher
than few ppm content of chlorides at temperatures above 50 °C.
Stress corrosion cracking applies only to
austenitic stainless steels and depends on the nickel
stress cracking is an important failure mode in the oil
industry, where the steel comes into contact with liquids or gases with
sulfide content eg. sour
gas. It is influenced by the tensile stress and is worsened in the presence
of chloride ions. Very high levels of hydrogen sulfide apparently inhibit the
corrosion. Rising temperature increases the influence of chloride ions, but
decreases the effect of sulfide, due to its increased mobility through the
lattice; the worst temperature range for sulphide stress cracking is between 60
corrosion occurs when a galvanic
cell is formed between two dissimilar metals. The resulting electrochemical
potential then leads to formation of an electric current that leads to
electrolytic dissolving of the less noble material. This effect can be prevented
by electrical insulation of the materials, eg. by using rubber or plastic
sleeves or washers, keeping the parts dry so there is no electrolyte to form the
cell, or keeping the size of the less-noble material significantly larger than
the more noble ones (eg. stainless-steel bolts in an aluminium block won't cause
corrosion, but aluminium rivets on stainless steel sheet would rapidly corrode.
steel is a very common contaminant here, coming from nearby grinding of
carbon steel or use of tools contaminated with carbon steel particles. The
particle forms a galvanic cell, and quickly corrodes away, but may leave a pit
in the stainless steel from which pitting corrosion may rapidly progress. Some
workshops therefore have separate areas and separate sets of tools for handling
carbon steel and stainless steel, and care has to be exercised to prevent direct
contact between stainless steel parts and carbon steel storage racks.
Particles of carbon steel can be removed from a
contaminated part by passivation
with dilute nitric
acid, or by pickling
with a mixture of hydrofluoric
acid and nitric acid. However if the stainless has been mirror polished it
will need re-polishing after using those methods.
acid at 35% dilution which is commercialy available by various names like
Naval Jelly, RustX or Rust Kill etc is very effective for removing the
staining caused by carbon steel contamination. The following is extracted
from metalwebnews.com :-
technique for removing rust is etching with Phosphoric Acid. Phosphoric Acid has
a unique property of dissolving iron oxide (rust) quickly while etching iron
very slowly. This means that you can leave metal in Phosphoric Acid for much
longer than necessary with very little damage. The acid will attack bare
metal slowly however and will start the process of hydrogen embrittlement, so
use the minimum etch time that removes all rust.
Another unique advantage of Phosphoric Acid is
that it leaves a fine coating of iron phosphate behind. Iron phosphate prevents
rust. However, the iron phosphate coating is not very thick or durable. Some
additional protection is still required.
Phosphoric Acid etch will leave a hard, bright
metal finish. This is because it will etch the surface slightly, exposing new,
bare metal. Often this is desirable. It leaves an attractive surface and a
surface ready to paint. A common product which contains Phosphoric Acid is Naval
Jelly. The soft drink Coca-Cola also contains Phosphoric Acid, so Coke will etch
rust. But Coke also contains carbonic acid and other nasty things. You're going
to drink that stuff?"
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