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| Stainless Steel Alloy Designation |
| Stainless Steel Description |
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Stainless steel alloys are austenitic,
ferritic, martensitic, precipitation hardened, and duplex metals that are
available in a wide variety of grades, shapes, and sizes. Austenitic stainless
steels have excellent corrosion resistance, unusually good formability,
and increased strength due to cold working. They are non-magnetic or only
slightly magnetic. Two hundred (200) series austenitic stainless steels
contain chromium, nickel, and manganese. Three hundred (300) series austenitic
stainless steels contain chromium and nickel. Ferritic stainless steels
are straight-chromium, 400 series metals that cannot be hardened by heat
treatment, and only moderately hardened by cold working. They are magnetic,
have good ductility, and resist corrosion and oxidation. Martensitic stainless
steels, another type of straight-chromium 400 series metals, are magnetic,
fairly ductile, and resist corrosion in mild environments. Some products
can be heated to tensile strengths that exceed 200,000 psi (1379 MPa).
Precipitation hardened (PH) stainless steels are chromium-nickel metals,
some of which contain alloying elements such as copper or aluminum. They
can be hardened by solution treating and aged to high strength. Duplex
stainless steel alloys have improved mechanical properties and consist
of a combination of ferritic and austenitic phases.
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| Stainless Steel Alloy Types |
| Austenitic |
200-series austenitic steels are stainless steels that contain chromium,
nickel, and manganese. 300-series austenitic steels are stainless steels
that contain chromium and nickel. The stainless steels in each austenitic
group have different compositions and properties, but share many common
characteristics. They can be hardened by cold working but not by heat treatment.
In the annealed condition, all are essentially nonmagnetic; although, some
may become slightly magnetic by cold working. They have excellent corrosion
resistance, unusually good formability, and increased strength due to cold
working.
Type 304 or 18-8 stainless steel is the most widely used alloy in the
300-series austenitic group. It has a nominal composition of 18% chromium
and 8% nickel. Type 316 stainless steel has an 18-8 composition modified
with molybdenum to improve pitting corrosion resistance.
Austenitic grades consist of 201, 301, 301, 303, 304, 304L, 305, 309,
310, 316, 316L, 317, 317L, 321, 347, and 348 as well as specialized or
proprietary austenitic stainless steels.
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| Ferritic |
Ferritic stainless steels are straight-chromium 400-series metals that
cannot be hardened by heat treatment, and only moderately hardened by cold
marketing. They are magnetic, have good ductility, and resistant against
corrosion and oxidation. Ferritic stainless steels have chromium levels
that range from 10.5% to 40% (typically 12% or more) and carbon levels
less than 0.20%. Types 409, 430, 434, 430, 439, 442, and 446 belong in
this category. Type 430 is a general-purpose ferritic stainless steel.
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| Martensitic |
Martensitic stainless steels are straight-chromium 400-series metals
that can be hardened by heat treatment. They are magnetic, resist corrosion
in mild environments, and have fairly good ductility. Some can be heated
to tensile strengths that exceed 200,000 psi (1379 MPa). Type 410 is a
general-purpose alloy. Martensitic stainless steel grades include 410,
440, 440C, 403, 414, 416 and 420 as well as specialty and proprietary alloys.
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Precipitation Hardening
(e.g., PH, 17-4) |
Precipitation hardening alloys can be hardened by solution treating
and aged to high strength. Precipitation hardening (PH) stainless steels
are chromium-nickel metals, some of which contain alloying elements such
as copper or aluminum. PH grades 17-7 (Type 631), 17-4 (Type 630), 13-8,
15-5, 15-7, as well as specialty and proprietary alloys. Many aluminum
alloys are hardened or strengthened through a precipitation hardening process.
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Duplex
(e.g., 329, 2205) |
The structure of duplex stainless steels consists of a combination
of ferritic and austenitic phases. Duplex stainless steels have corrosion
resistance properties that are equivalent to or better than austenitic
stainless steels. Duplex stainless steels also have improved mechanical
properties. AISI 329 and ASTM 2205 are examples of duplex grade stainless
steels.
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| Stainless Steel Finishes |
| 1 |
A rough, dull surface that results from hot rolling to the specified
thickness followed by annealing and descaling.
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| 2D |
A dull finish produced by cold rolling to gauge, then annealing and
pickling in acid to remove scale and oxide from an open air anneal.
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| 2B |
A reflective cold tolled finish produced in the same manner at a 2D
sheet finish, except that a light temper pass on polished rolls is performed
on the annealed and pickled product. This is the general purpose
cold rolled finish that can be used as is, or as a preliminary step to
polishing.
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| Alloying Elements and their Functions |
| Chromium |
Forms a surface film of chromium oxide to make the stainless steel
corrosion resistant. It also increases the scaling resistance at
elevated temperatures.
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| Nickel |
Stabilizes the austenitic structure and increases ductility, making
stainless steel easier to form. It increases high temperature strength
and corrosion resistance, particularly in industrial and marine atmospheres,
chemical, food and textile processing industries.
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| Silicon |
Increases scaling resistance by forming a tight initial scale, which
will withstand cyclic temperature changes. It resists carburizing
at high temperatures and slightly increases tensile strength and hardness.
Small amounts of silicon are added to all grades of stainless for deoxidizing.
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| Manganese |
Promotes the stability of austenite, at or near room temperature and
improves hot working properties. Addition of up to 2% manganese has
no effect on strength, ductility and toughness. Manganese is important
as a partial replacement of nickel in 200 series stainless grades.
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| Molybdenum |
Increases corrosion resistance, strength at elevated temperatures,
and creep resistance. It expands the range of passivity and counteracts
tendency to pit especially in chloride environments.
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| Aluminum |
Is a very strong ferrite former and lowers the hardenability of stainless
steel. It improves scaling resistance.
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| Carbon |
Strengthens stainless steel but promotes the formation of precipitates
harmful to corrosion resistance.
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| Columbium |
Combines with carbon to reduce susceptibility to intergranular corrosion.
It acts as a grain refiner and promotes the formation of ferrite.
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| Copper |
Is added to stainless steels to increase their resistance to certain
corrosive environments. It also decreases susceptibility to stress
corrosion cracking and provides age-hardening effects.
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| Titanium |
Combines with carbon to reduce susceptibility to intergranular corrosion.
It acts as a grain refiner and promotes the formation of ferrite. |
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