The high oxidation-resistance in
stainless steel in air at ambient temperatures, is normally achieved with additions
of 13% to 26% ( 26% is used for the harsher environments) chromium by
weight to the steel. Because of its chemical properties, the chromium in
stainless steel forms a passivation layer of chromium oxide (Cr2O3) when
exposed to oxygen. This layer is too thin to be visible, and the metal
remains lustrous. It is impervious to water and air, protecting the
stainless steel beneath. Also, this layer quickly reforms when the surface
is scratched. This phenomenon is called passivization and is seen in other
metals, such as aluminum and titanium. Corrosion resistance can however be
adversely affected if the component is used in a non-oxygenated
environment. When stainless steel parts such as nuts and bolts are forced
together, the oxide layer can be scraped off causing the parts to weld
together. When disassembled, the welded material may be torn and pitted, an
effect that is known as galling. This destructive galling can be best
avoided by the use of dissimilar materials, e.g. bronze to stainless steel,
or even different types of stainless steels (martensitic against
austenitic, etc.), when metal-to-metal wear is a concern. In addition,
Nitronic alloys (trademark of Armco, Inc.) reduce the tendency to gall
through selective alloying with manganese and nitrogen.Nickel also
contributes to passivization, as do other less commonly used ingredients
such as molybdenum and vanadium. A picture of the In
metallurgy, stainless steel is defined as a steel alloy with a minimum of
11% chromium content by mass. Stainless steel does not stain, corrode, or
rust as easily as ordinary steel (it stains less), but it is not
stain-proof It is also called corrosion-resistant steel, when the alloy
type and grade are not detailed, particularly in the aviation industry.
There are different grades and surface finishes of stainless steel to suit
the environment to which the material will be subjected in its lifetime.
Common uses of stainless steel are cutlery and watch straps.Stainless steel
differs from carbon steel by the amount of chromium present which changes
the chemical properties of the steel. Carbon steel rusts when exposed to
air and moisture. This iron oxide film is active and accelerates corrosion
by forming more iron oxide. Stainless steels have sufficient amounts of
chromium present so that a passive film of chromium oxide forms which
prevents further surface corrosion and blocks corrosion from spreading into
the metal's internal structure. The
corrosion resistance of iron-chromium alloys was first recognized in 1821
by the French metallurgist Pierre Berthier, who noted their resistance
against attack by some acids and suggested their use in cutlery.
Metallurgists of the 19th century, however, were unable to produce the
combination of low carbon and high chromium found in most modern stainless
steels, and the high-chromium alloys they could produce were too brittle to
be practical.In the late 1890s, Hans Goldschmidt of Stainless
steel’s resistance to corrosion and staining, low maintenance,
relatively low cost, and familiar luster make it an ideal base material for
a host of commercial applications. There are over 150 grades of stainless
steel, of which fifteen are most common. The alloy is
milled into coils, sheets, plates, bars, wire, and tubing to be used in
cookware, cutlery, hardware, surgical instruments, major appliances,
industrial equipment, and as an automotive and aerospace structural alloy
and construction material in large buildings. Storage tanks and tankers
used to transport orange juice and other food are often made of stainless
steel, due to its corrosion resistance and antibacterial properties. This
also influences its use in commercial kitchens and food processing plants,
as it can be steam cleaned, sterilized, and does not need painting or
application of other surface finishes.Stainless steel is also used for
jewelery and watches. The most common stainless steel alloy used for this
is 316L. It can be re-finished by any jeweler and will not oxidize or turn
black. Some firearms incorporate stainless steel components as an
alternative to blued or parkerized steel. A few, more expensive revolvers
(like the Smith and Wesson Model 60) and pistols (like versions of the Colt
M1911) are milled entirely from stainless steel. This gives a high-luster
finish similar in appearance to nickel plating; but, unlike plating, the
finish is not subject to flaking, peeling, wear-off due to rubbing (as when
repeatedly removed from a holster over the course of time), or rust when
scratched. Stainless
steel like most other steels are 100% recyclable, in fact, an average
stainless steel object is composed of about 60% recycled material, 25% originating
from end-of-life products and 35% coming from manufacturing processes. There are
different types of stainless steels: when nickel is added, for instance,
the austenite structure of iron is stabilized.
This crystal structure makes such steels non-magnetic and less brittle at
low temperatures. For greater hardness and strength, carbon is added. When
subjected to adequate heat treatment, these steels are used as razor
blades, cutlery, tools, etc.Significant quantities of manganese have been
used in many stainless steel compositions. Manganese preserves an
austenitic structure in the steel as does nickel, but at a lower
cost.Stainless steels are also classified by their crystalline structure: • 100 Series—austenitic chromium-nickel-manganese
alloys The information found on this page is free from
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Information and Chemical Properties of Stainless Steels
Chemical
Properties
Stainless
Steel General Information
History
Pipes and fittings made of
stainless steel
Recycling
& reuse
Types of stainless steel
• Austenitic, or 300 series, stainless steels comprise over 70% of
total stainless steel production. They contain a maximum of 0.15% carbon, a
minimum of 16% chromium and sufficient nickel and/or manganese to retain an
austenitic structure at all temperatures from the cryogenic region to the
melting point of the alloy. A typical composition of 18% chromium and 10%
nickel, commonly known as 18/10 stainless, is often used in flatware.
Similarly, 18/0 and 18/8 are also available. Superaustenitic stainless
steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to
chloride pitting and crevice corrosion due to high molybdenum content
(>6%) and nitrogen additions, and the higher nickel content ensures
better resistance to stress-corrosion cracking versus the 300 series. The
higher alloy content of superaustenitic steels makes them more expensive.
Other steels can offer similar performance at lower cost and are preferred
in certain applications. The low carbon version of the Austenitic Stainless
Steel, for example 316L or 304L, are used to avoid corrosion problem caused
by welding. The "L" means that the carbon content of the
Stainless Steel is below 0.03%, this will reduce the sensitization effect,
precipitation of Chromium Carbides at grain boundaries, due to the high
temperature produced by welding operation.
• Ferritic stainless steels are highly corrosion-resistant, but less
durable than austenitic grades. They contain between 10.5% and 27% chromium
and very little nickel, if any, but some types can contain lead. Most
compositions include molybdenum; some, aluminium or titanium. Common
ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni.
These alloys can be degraded by the presence of σ chromium, a
intermetallic phase which can precipitate upon welding.
• Martensitic stainless steels are not as corrosion-resistant as the
other two classes but are extremely strong and tough, as well as highly
machineable, and can be hardened by heat treatment. Martensitic stainless
steel contains chromium (12-14%), molybdenum (0.2-1%), nickel (0-<2%),
and carbon (about 0.1-1%) (giving it more hardness but making the material
a bit more brittle). It is quenched and magnetic.
• Precipitation-hardening martensitic stainless steels have corrosion
resistance comparable to austenitic varieties, but can be precipitation
hardened to even higher strengths than the other martensitic grades. The
most common, 17-4PH, uses about 17% chromium and 4% nickel. There is a
rising trend in defense budgets to opt for an ultra-high-strength stainless
steel when possible in new projects, as it is estimated that 2% of the US
GDP is spent dealing with corrosion. The Lockheed-Martin Joint Strike
Fighter is the first aircraft to use a precipitation-hardenable stainless
steel—Carpenter Custom 465—in its airframe.
• Duplex stainless steels have a mixed microstructure of austenite
and ferrite, the aim being to produce a 50/50 mix, although in commercial
alloys, the mix may be 40/60 respectively. Duplex steels have improved
strength over austenitic stainless steels and also improved resistance to
localised corrosion, particularly pitting, crevice corrosion and stress
corrosion cracking. They are characterised by high chromium (19–28%)
and molybdenum (up to 5%) and lower nickel contents than austenitic stainless
steels. The most used Duplex Stainless Steel are the 2205 (22% Chromium, 5%
Nickel) and 2507 (25% Chromium, 7% Nickel); the 2507 is also known as
"SuperDuplex" due to its higher corrosion resistance. Stainless
grades and specs
o Type 101—austenitic that is hardenable through cold working
for furniture
o Type 102—austenitic general purpose stainless steel working
for furniture
• 200 Series—austenitic chromium-nickel-manganese alloys
o Type 201—austenitic that is hardenable through cold working
o Type 202—austenitic general purpose stainless steel
• 300 Series—austenitic chromium-nickel alloys
o Type 301—highly ductile, for formed products. Also
hardens rapidly during mechanical working. Good weldability. Better wear
resistance and fatigue strength than 304.
o Type 302—same corrosion resistance as 304, with slightly
higher strength due to additional carbon.
o Type 303—free machining version of 304 via addition of
sulfur and phosphorus. Also referred to as "A1" in accordance
with.
o Type 304—the most common grade; the classic 18/8 stainless
steel. Also referred to as "A2" in accordance with ISO 3506.
o Type 304L— same as the 304 grade but contains less carbon to
increase weldability. Is slightly weaker than 304.
o Type 304LN—same as 304L, but also nitrogen is added to
obtain a much higher yield and tensile strength than 304L.
o Type 308—used as the filler metal when welding 304
o Type 309—better temperature resistance than 304, also
sometimes used as filler metal when welding dissimilar steels, along with
inconel.
o Type 316—the second most common grade (after 304); for food
and surgical stainless steel uses; alloy addition of molybdenum prevents
specific forms of corrosion. It is also known as marine grade stainless
steel due to its increased resistance to chloride corrosion compared to
type 304. 316 is often used for building nuclear reprocessing plants.
o Type 316L—extra low carbon grade of 316, generally used in
stainless steel watches and marine applications due to its high resistance
to corrosion. Also referred to as "A4" in accordance with ISO
3506.
o Type 316Ti—includes titanium for heat resistance, therefore
it is used in flexible chimney liners.
o Type 321—similar to 304 but lower risk of weld decay due to
addition of titanium. See also 347 with addition of niobium for
desensitization during welding.
• 400 Series—ferritic and martensitic chromium alloys
o Type 405— ferritic for welding applications
o Type 408—heat-resistant; poor corrosion resistance; 11% chromium,
8% nickel.
o Type 409—cheapest type; used for automobile exhausts;
ferritic (iron/chromium only). o Type 410—martensitic
(high-strength iron/chromium). Wear-resistant, but less
corrosion-resistant.
o Type 416—easy to machine due to additional sulfur
o Type 420—Cutlery Grade martensitic; similar to the
Brearley's original rustless steel. Excellent polishability.
o Type 430—decorative, e.g., for automotive trim; ferritic.
Good formability, but with reduced temperature and corrosion resistance.
o Type 439—ferritic grade, a higher grade version of 409 used
for catalytic converter exhaust sections. Increased chromium for improved
high temperature corrosion/oxidation resistance.
o Type 440—a higher grade of cutlery steel, with more carbon,
allowing for much better edge retention when properly heat-treated. It can
be hardened to approximately Rockwell 58 hardness, making it one of the
hardest stainless steels. Due to its toughness and relatively low cost,
most display-only and replica swords or knives are made of 440 stainless.
Also known as razor blade steel. Available in four grades: 440A, 440B,
440C, and the uncommon 440F (free machinable). 440A, having the least
amount of carbon in it, is the most stain-resistant; 440C, having the most,
is the strongest and is usually considered more desirable in knifemaking
than 440A, except for diving or other salt-water applications.
o Type 446—For elevated temperature service
• 500 Series—heat-resisting chromium alloys
• 600 Series—martensitic precipitation hardening alloys
o 601 through 604: Martensitic low-alloy steels.
o 610 through 613: Martensitic secondary hardening steels.
o 614 through 619: Martensitic chromium steels.
o 630 through 635: Semiaustenitic and martensitic precipitation-hardening
stainless steels.
Type 630 is most common PH stainless, better known as 17-4; 17%
chromium, 4% nickel.
o 650 through 653: Austenitic steels strengthened by hot/cold work.
o 660 through 665: Austenitic superalloys; all grades except alloy 661
are strengthened by second-phase precipitation.
• Type 2205— the most widely used duplex
(ferritic/austenitic) stainless steel grade. It has both excellent
corrosion resistance and high strength.
See also the Different Stainless Steel Grades
and their uses.
