Stainless steels are iron-based alloys containing at least 10.5% Cr.
Few stainless steels contain more than 30% Cr or less than 50% Fe.
They achieve their stainless characteristics through the formation
of an invisible and adherent chromium-rich oxide surface film.
This oxide forms itself in the presence of oxygen.
Other elements added to improve characteristics include nickel,
molybdenum, copper, titanium, aluminum, silicon, niobium, nitrogen,
sulfur, and selenium. Carbon is normally present in amounts ranging
from less than 0.03% to over 1.0% in certain martensitic grades.
The selection of stainless steels may be based on corrosion resistance,
fabrication characteristics, availability, mechanical properties in
specific temperature ranges and product cost. However, corrosion
resistance and mechanical properties are usually the most important
factors in selecting a grade for a given application.
Stainless steels are commonly divided into five groups: martensitic
stainless steels, ferritic stainless steels, austenitic stainless
steels, duplex (ferritic-austenitic) stainless steels, and
precipitation-hardening stainless steels.
The development of precipitation-hardenable stainless steels was
spearheaded by the successful production of Stainless W by U.S. Steel
in 1945. The problem of obtaining raw materials has been a real one,
particularly in regard to nickel during 1950s when civil wars raged
in Africa and Asia, prime sources of nickel, and Cold War politics
played a role because Eastern-bloc nations were also prime sources
of the element. This led to the development of a series of alloys
(AISI 200 type) in which manganese and nitrogen are partially
substituted for nickel. These stainless steels are still produced
Over the years, stainless steels have become firmly established as
materials for cooking utensils, fasteners, cutlery, flatware,
decorative architectural hardware, and equipment for use in chemical
plants, dairy and food-processing plants, health and sanitation
applications, petroleum and petrochemical plants, textile plants,
and the pharmaceutical and transportation industries. Some of these
applications involve exposure to either elevated or cryogenic
temperatures; austenitic stainless steels are well suited to
either type of service.
Modifications in composition are sometimes made to facilitate
production. For instance, basic compositions are altered to make
it easier to produce stainless steel tubing and casting. Similar
modifications are made for the manufacture of stainless steel
welding electrodes; here combinations of electrode coating and
wire composition are used to produce desired compositions deposited
Martensitic stainless steels are essentially alloys of chromium and
carbon that possess a distorted body-centered cubic (bcc) crystal
structure (martensitic) in the hardened condition. They are
ferromagnetic, hardenable by heat treatments, and are generally
resistant to corrosion only to relatively mild environments.
Chromium content is generally in the range of 10.5 to 18%, and
carbon content may exceed 1.2%. The chromium and carbon contents
are balanced to ensure a martensitic structure after hardening.
General corrosion is often much less serious than localized forms
such as stress corrosion cracking, crevice corrosion in tight spaces
or under deposits, pitting attack, and intergranular attack in
sensitized material such as weld heat-affected zones (HAZ). Such
localized corrosion can cause unexpected and sometimes catastrophic
failure while most of the structure remains unaffected, and therefore
must be considered carefully in the design and selection of the proper
grade of stainless steel.
Corrosive attack can also be increased dramatically by seemingly
minor impurities in the medium that may be difficult to anticipate
but that can have major effects, even when present in only
part-per-million concentrations; by heat transfer through the
steel to or from the corrosive medium; by contact trimmed only on
Stainless steels are available in the form of plate, sheet, strip,
foil, bar, wire, semi-finished products, pipes, tubes, and tubing.
Sheet is a flat-rolled product in coils or cut lengths at least
610 mm wide and less than 4.76 mm thick. Stainless steel sheet is
produced in nearly all types except the free machining and certain
martensitic grades. Sheet from the conventional grades is almost
exclusively produced on continuous mills. Hand mill production is
usually confined to alloys that cannot be produced economically on
continuous mills, such as certain high-temperature alloys.
The steel is cast in ingots, and the ingots are rolled on a slabbing
mill or a blooming mill into slabs or sheet bars. The slabs or sheet
bars are then conditioned prior to being hot rolled on a finishing
mill. Alternatively, the steel may be continuous cast directly into
slabs that are ready for hot rolling on a finishing mill. The current
trend worldwide is toward greater production from continuous cast
Sheet produced from slabs on continuous rolling mills is coiled
directly off the mill. After they are descaled, these hot bands
are cold rolled to the required thickness and coils off the cold
mill are either annealed and descaled or bright annealed. Belt
grinding to remove surface defects is frequently required at hot
bands or at an intermediate stage of processing. Full coils or
lengths cut from coils may then be lightly cold rolled on either
dull or bright rolls to produce the required finish. Sheet may be
shipped in coils, or cut sheets may be produced by shearing lengths
from a coil and flattening them by roller leveling or stretcher
Strip is a flat-rolled product, in coils or cut lengths, less than
610 mm wide and 0.13 to 4.76 mm thick. Cold finished material 0.13 mm
thick and less than 610 mm wide fits the definitions of both strip
and foil and may be referred to by either term.
Cold-rolled stainless steel strip is manufactured from hot-rolled,
annealed, and pickled strip (or from slit sheet) by rolling between
polished rolls. Depending on the desired thickness, various numbers
of cold rolling passes through the mill are required for effecting
the necessary reduction and securing the desired surface
characteristics and mechanical properties.
Hot-rolled stainless steel strip is a semi-finished product obtained
by hot-rolling slabs or billets and is produced for conversion to
finished strip by cold rolling.
Heat Treatment. Strip of all types of stainless steel is usually
either annealed or annealed and skin passed, depending on requirements.
When severe forming, bending, and drawing operations are involved,
it is recommended that such requirements be indicated so that the
producer will have all the information necessary to ensure that
he supplies the proper type and condition. When stretcher strains
are objectionable in ferritic stainless steels such as type 430,
they can be minimized by specifying a No 2 finish. Cold-rolled
strip in types 410, 414, 416, 420, 431, 440A, 440B, and 440C
can be produced in the hardened and tempered condition.
Experience in the use of stainless steels indicates that many
factors can affect their corrosion resistance. Some of the more
prominent factors are:
Surface Finish. Other characteristics in the stainless steel
selection checklist are vital for some specialized applications
but of little concern for many applications. Among these
characteristics, surface finish is important more often
than any other except corrosion resistance. Stainless steels are
sometimes selected because they are available in a variety of
attractive finishes. Surface finish selection may be made on
the basis of appearance, frictional characteristics, or sanitation.
- Chemical composition of the corrosive medium including impurities
- Physical state of the medium-liquid, gaseous, solid, or combinations thereof
- Temperature variations
- Aeration of the medium
- Oxygen content of the medium
- Bacteria content of the medium
- Ionization of the medium
- Repeated formation and collapse of bubbles in the medium
- Relative motion of the medium with respect to the steel
- Chemical composition of the metal
- Nature and distribution of microstruc-tural constituents etc.
Plate is a flat-rolled or forged product more than 250 mm (10 in.)
in width and at least 4.76 mm (0.1875 in.) in thickness. Exceptions
include highly alloyed ferritic stainless steels, some of the
martensitic stainless steels, and a few of the free-machining
grades. Plate is usually produced by hot rolling from slabs that
have been directly cast or rolled from ingots and that usually
have been conditioned to improve plat surface. Some plate may
be produced by direct rolling from ingot.
For strip, edge condition is often more important than it usually
is for sheet. Strip can be furnished with various edge specifications:
- Mill edge (as produced, condition unspecified)
- No.1 edge (edge rolled, rounded, or square)
- No.3 edge (as slit)
- No.5 edge (square edge produced by rolling or filing after slitting)
Foil is a flat-rolled product, in coil form, up to 0.13 mm thick
and less than 610 mm wide. Foil is produced in slit widths with
edge conditions corresponding to No.3 and No.5 edge conditions
for strip. Foil is made from types 201, 202, 301, 302, 304, 304L,
305, 316, 316L, 321, 347, 430, and 442, as well as from certain
The finishes, tolerances, and mechanical properties of foil differ
from those of strip because of limitations associated with the way
in which foil is manufactured. Nomenclature for finishes, and for
width and thickness tolerances, vary among producers.
Mechanical Properties. In general, mechanical properties of foil
vary with thickness. Tensile strength is increased somewhat,
and ductility is lowered, by a decrease in thickness.
Bar is a product supplied in straight lengths; it is either hot
or cold finished and is available in various shapes, sizes, and
surface finishes. This category includes small shapes whose
dimensions do not exceed 75 mm and, second, hot-rolled flat
stock at least 3.2 mm thick and up to 250 mm wide.
Hot-finished bar is commonly produced by hot rolling, forging,
or pressing ingots to blooms or billets of intermediate size,
which are subsequently hot rolled, forged, or extruded to final