Stainless steel is a family of iron based alloys that must contain
at least 10.5% CHROMIUM (Cr). The presence of chromium creates
an invisible surface film that resists oxidation and makes the
material "passive" or corrosion resistant (i.e. "stainless").
This family can be simply and logically grouped into five (5)
branches. Each of these branches has specific properties and a
basic grade or "type." In addition, further alloy modifications
can be made to "tailored" the chemical composition to
meet the needs of different corrosion conditions, temperature
ranges, strength requirements, or to improve weldability, machinability,
work hardening and formability.
As we mentioned, to be a stainless, the iron base must contain
at least 10.5% Cr. and the carbon content is less that 1%. These
two things made stainless "Steel" totally different
from mild "Steel."
The basic stainless with 12 to 18% chromium
are called "Martensitic" (based on the structure) and
have the following characteristics:
* Are magnetic
* Can be hardened by "heat treatment"
* Have "poor" welding characteristics
Common Uses:
* Knife blades
* Surgical instruments
* Fasteners
* Shafts
* Springs
Common Grades:
Stainless is designated by three different systems
1. Metallurgical structure - Martensitic
2. Grade: 410 (most used), 420 (cutlery), 440C (for very high
hardness)
3. Unified Numbering System UNS: S41000, S42000, S44004 (Note
chemistry and properties given for 410)
CHROMIUM CONTAINING
The second branch also contain 12 to 18% chromium BUT has a LOWER
carbon level (less than 0.2%). Since the carbon is low, these
grades have a different metallurgical structure and are called
"Ferritic" stainless steels. They have the following
characteristics:
* Are magnetic
* CANNOT be hardened by "heat treatment" (always used
in the annealed or softened condition)
* Weldability is still poor
Common Uses:
* Automotive exhaust and fuel lines
* Architectural trim
* Cooking utensils
* Bank vaults
Common grades:
Stainless is designated by three different systems
1. Metallurgical structure - Ferritic
2. Grade: 409 (high temperature), 430 (most used)
3. Unified Numbering System UNS: S40900, S43000 (Note chemistry
and properties given for 430)
Forging:
Stainless steel forging are also available in several shapes.
For information, contact the "Forging Industry Association"
(216-781-0102).
Rolled Products:
Angles are available as a hot rolled product. Contact "Slater
Steel Corporation (219-432-2561).
Hot rolled and cold rolled shapes, as well
as cold-drawn "shapes" can also be made from stainless
steel as detailed in the Designer Handbook "The Selection
and Use of Stainless Steel."
Tubular Products:
Tubular products are commonly used for structural applications
either in round or square sections. Further information is available
for the "Specialty Tubular Group" (202-342-8450).
When nickel is added and the chromium level is increased, the
structure changes again and it is called "Austenitic"
and they have the following characteristics:
* Are NOT magnetic
* CANNOT be hardened by "heat treatment" BUT CAN be
hardened by cold working
* Have the "BEST" corrosion resistance
* Can be easily welded
* Have excellent cleanability and hygiene characteristics
* Have exceptional resistance to both high and low temperature
Common Uses:
* Kitchen sinks
* Architectural applications such as roofs and gutters, doors
and windows, tubular frames
* Food processing equipment
* Restaurant food preparation areas
* Chemical vessels
* Ovens
* Heat exchangers
Common Grades:
Stainless is designated by three different systems
1. Metallurgical structure - Austenitic
2. Grade: 304 (most used), 310 (for high temperature), 316 (for
better corrosion resistance), 317 (for best corrosion resistance)
3. Unified Numbering System UNS: S30400, S31000, S31600, S31700
(Note chemistry and properties given for 304 and 316)
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Some Limitations:
Austenitic stainless steels have some limitations:
* The maximum temperature under oxidizing conditions is 925†C
(see heat resisting stainless steels)
* They are suitable only for low concentrations of reducing acid
(Super Austenitics are available for higher acid levels)
* In cervices and shielded areas, there might not be enough oxygen
to maintain the passive oxide film and crevice corrosion might
occur (Super Austenitics, Duplex and Super Ferritic are available
in these situations)
* Very high levels of halide ions, especially the chloride ion
can also breakdown the passive surface film (Super Austenitics
and Duplex are available to withstand these conditions)
Information on Super Ferritic and Super Austenitic
material is available from the list of Stainless Producers.
When the chromium content is high (18 to 26%) and the nickel content
is low (4 to 7%), the resulting structure is called Duplex. In
addition most grades contain 2 to 3% molybdenum. This results
in a structure that is a combination of both Ferritic and Austenitic
(hence the name Duplex), and have the following characteristics:
* High resistance to stress corrosion cracking
* Increased resistance to chloride ion attack
* Very weldable
* Have higher tensile and yield strengths than Ausenitic or Ferritic
stainless steels
Common Uses:
* Sea water applications
* Heat exchangers
* Desalination plants
* Food pickling plants
Common Grades:
1. Metallurgical structure - Duplex
2. Grade: 2205
3. Unified Numbering System UNS: S31803
Stainless steel is produced in an electric arc furnace where carbon
electrodes contact recycled stainless scrap and various alloys
of chromium (and nickel, molybdenum etc. depending on the stainless
type). A current is passed through the electrode and the temperature
increases to a point where the scrap and alloys melt. The molten
material from the electric furnace is then transferred into an
AOD (Argon Oxygen Decarbonization) vessel, where the carbon levels
are reduced (remember stainless has a much lower carbon level
than mild steel) and the final alloy additions are made to make
the exact chemistry. It is then melted and cast either into ingots
or continually cast into a slab or billet form. Then the material
is hot rolled or forged into its final form. Some material receives
cold rolling to further reduce the thickness as in sheets or drawn
into smaller diameters as in rods and wire.
Most stainless steels receive a final annealing
(a heat treatment that softens the structure) and pickling (an
acid wash that removes furnace scale from annealing and helps
promote the passive surface film that naturally occurs).
The fact that stainless steel has a great resistance to corrosion
means that using stainless will result in a very long life compared
to mild steel. Structures made from stainless steel will last
many times the normal life (well over 100 years in most cases).
So, while stainless steel is probably more expensive to buy in
the beginning -- because it lasts a long time, it is usually cheaper
in the long run because there is little or no maintenance and
repair costs. AND, once the useful life is over, stainless steel
is 100% RECYCLABLE. Scrap stainless steel is recharged into the
electric furnaces for re-melting back into stainless steel. Stainless
steel is a true "full life cycle" material.
Mechanical Properties
(Annealed condition)
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Tensile Strength Yield Strength Elongation
Hardness
Stainless ksi MPa ksi MPa
410 70 483 45 310 25 B80
430 75 517 50 345 25 B85
304 84 579 42 290 55 B80
316 84 579 42 290 50 B79
Elongation in 2" (50.80 mm)
Hardness in Rockwell B
Corrosion resistance
--lower alloyed grades resist corrosion in atmospheric and pure
water environments, while high-alloyed grades can resist corrosion
in most acids, alkaline solutions, and chlorine bearing environments,
properties which are utilized in process plants.
Fire & heat resistance
--special high chromium and nickel-alloyed grades resist scaling
and retain strength at high temperatures.
Hygiene
--the easy cleaning ability of stainless makes it the first choice
for strict hygiene conditions, such as hospitals, kitchens, abattoirs
and other food processing plants.
Aesthetic appearance
--the bright, easily maintained surface of stainless steel provides
a modern and attractive appearance.
Strength-to-weight advantage
--the work-hardening property of austenitic grades, that results
in a significant strengthening of the material from cold-working
alone, and the high strength duplex grades, allow reduced material
thickness over conventional grades, therefore cost savings.
Ease of fabrication
--modern steel-making techniques mean that stainless can be cut,
welded, formed, machined, and fabricated as readily as traditional
steels.
Impact resistance
--the austenitic microstructure of the 300 series provides high
toughness, from elevated temperatures to far below freezing, making
these steels particularly suited to cryogenic applications.
Long term value
--when the total life cycle costs are considered, stainless is
often the least expensive material option.
For further information on carbon steels search
the Welding Abstract Database type in keyword "stainless
steel "
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