Stainless steel’s most notable attribute is its
ability to resist corrosive attack. Like aluminium it is a relatively
new material but it has found many applications. It is a basic alloy of
steel that contains chromium but other alloying elements are added to
provide specific attributes. Stainless steel finds many applications
where a hard material is required that will resist corrosive attack.
The basics
It is the addition of a minimum of 11% of chromium that gives stainless
steel its ability to withstand corrosive attack. Chromium has the effect
of creating a naturally protective oxide film on the surface of the
metal – a film that ‘self-heals’ almost directly after removal of the
metal surface. This is very similar to the action found in aluminium,
which makes it such an atmospheric resistant metal.
There are four main alloy groups, known as
“Austenitic”, “Ferritic”, “Martensitic” and “Duplex/Super Duplex”. All
are produced in a variety of shapes and sizes.
Of course, no metal is totally “corrosion resistant”.
In the right circumstances all metals will corrode including stainless
steel, but as long as the passive surface layer is maintained,
“rusting”, and other forms of corrosion such as pitting, crevice,
galvanic, intergranular, stress etc. are avoided.
Alloy groups
Ferritic stainless steels contain a minimum of 11% chromium. They are
relatively inexpensive as they contain no nickel. Ferritic stainless
steels are magnetic.
Austenitic stainless steels contain a minimum of 18%
chromium, along with a minimum of 8% nickel. They are non-magnetic,
which is a great advantage in many applications, especially marine
applications where unwarranted magnetic influence can affect compasses
and other direction finding equipment.
They also possess improved formability (e.g. deep
drawing or stretching) compared to ferritic stainless steels and are
tougher, maintaining their strength better at high temperatures.
Weldability is improved as is corrosion resistance, and, with the
addition of less that 2% molybdenum, corrosion resistance is
significantly improved.
Martensitic stainless steels contain a minimum of
11.5% chromium, generally with no nickel content but an addition of
0.15% - 0.4% carbon. These steels give very high strength in the
hardened condition. Tempering at 150°C to 170°C increases corrosion
resistance, and the abrasion and wear resistance of these alloys is well
known. Some variants containing high carbon content are not, however,
recommended for welding.
Duplex and Super Duplex stainless steels typically
contain approx. 22% chromium, 5% nickel and around 0.1% minimum of
nitrogen.
Effectively, they are of a mixed ferritic/austenitic
structure, and have high strength combined with good corrosion
resistance, good weldability, formability and resistance to crevice and
pitting corrosion in
chloride solutions.
Alloy selection
| 1.4005 (416) |
1.4305 (303) |
1.4541 (321) |
| 1.4057 (431) |
1.4307 (304L) |
|
| 1.4301 (304) |
1.4404 (316) |
|
|
