A designation is the specific identification of each grade, type,
or class of steel by a number, letter, symbol, name, or suitable
combination. Unique to a particular steel grade, type and class
are terms used to classify steel products. Within the steel
industry, they have very specific uses: grade is used to denote
chemical composition; type is used to indicate deoxidation practice;
and class is used to describe some other attribute, such as strength
level or surface smoothness.
In ASTM specifications, however, these terms are used somewhat
interchangeably. In ASTM A 533, for example, type denotes chemical
composition, while class indicates strength level. In ASTM A 515,
grade identifies strength level; the maximum carbon content permitted
by this specification depends on both plate thickness and strength
level. In ASTM A 302 grade denotes requirements for both chemical
composition and mechanical properties. ASTM A 514 and A 5117 are
specifications for high-strength quenched and tempered plate for
structural and pressure vessel applications, respectively, each
contains several compositions that can provide the required
mechanical properties. However, A 514 type A has the identical
composition limits as A 517 grade.
Chemical composition is by far the most widely used basis for
classification and/or designation of steels. The most commonly
used system of designation in the United States is that of the
Society of Automotive Engineers (SAE) and the American
Iron and Steel Institute (AISI). The Unified Numbering System (UNS)
is also being used with increasing frequency.
As stated above, the most widely used system for designating carbon
and alloy steels is the SAE-AISI system. As a point of technicality,
there are two separate systems, but they are nearly identical and
have been carefully coordinated by the two groups. It should be
noted, however, that AISI has discontinued the practice of designating
The SAE-AISI system is applied to semi-finished forgings, hot-rolled
and cold-finished bars, wire rod and seamless tubular goods, structural
shapes, plates, sheet, strip, and welded tubing.
Carbon steels contain less than 1.65% Mn, 0.60% Si, and 0.60%
Cu; they comprise the lxxx groups in the SAE-AISI system and are
subdivided into four distinct series as a result of the difference
in certain fundamental properties among them.
Designations for merchant quality steels include the prefix M. A
carbon steel designation with the letter B inserted between the
second and third digits indicates the steel contains 0.0005 to
0.003% B. Likewise, the letter L inserted between the second and
third digits indicates that the steel contains 0.15 to 0.35% Pb for
enhanced machinability. Resulfurized carbon steels of the 11xx group
and resulfurized and rephosphorized carbon steels of the 12xx group
are produced for applications requiring good machinability. Steels
that having nominal manganese contents of between 0.9 and 1.5% but
no other alloying additions now have 15xx designations in place of
the 10xx designations formerly used.
Alloy steels contain manganese, silicon, or copper in quantities
greater than those listed for the carbon steels, or they have
specified ranges or minimums for one or more of the other alloying
elements. In the AISI-SAE system of designations, the major alloying
elements are indicated by the first two digits of the designation.
The amount of carbon, in hundredths of a percent, is indicated by
the last two (or three) digits.
For alloy steels that have specific hardenability requirements,
the suffix H is used to distinguish these steels from corresponding
grades that have no hardenability requirement. As with carbon steels,
the letter B inserted between the second and third digits indicates
that the steel contains boron. The prefix E signifies that the steel
was produced by the electric furnace process.
HSLA Steels. Several grades of HSLA steel are described in
SAE Recommended Practice J410. These steels have been developed as
a compromise between the convenient fabrication characteristics and
low cost of plain carbon steels and the high strength of heat-treated
alloy steels. These steels have excellent strength and ductility
The Unified Numbering System (UNS) has been developed by ASTM and
SAE and several other technical societies, trade associations, and
United States government agencies.
A UNS number, which is a designation of chemical composition and
not a specification, is assigned to each chemical composition of
a metallic alloy. The UNS designation of an alloy consists of a
letter and five numerals. The letters indicate the broad class of
alloys; the numerals define specific alloys within that class.
Existing designation system, such as the AISI-SAE system for steels,
have been incorporated into UNS designations. UNS is described in
greater detail in SAE J1086 and ASTM E 527.
Aerospace Materials Specifications (AMS), published by SAE, are
complete specifications that are generally adequate for procurement
purposes. Most of the AMS designations pertain to materials intended
for aerospace applications; the specifications may include mechanical
property requirements significantly more severe than those for grades
of steel having similar compositions but intended for other
applications. Processing requirements, such as for consumable
electrode remelting, are common in AMS steels.
ASTM (ASME) Specifications
The most widely used standard specifications for steel products in
the United States are those published by ASTM. These are complete
specifications, generally adequate for procurement purposes. Many
ASTM specifications apply to specific products, such as A 574 for
alloy steel socket head cap screws. These specifications are
generally oriented toward performance of the fabricated end
product, with considerable latitude in chemical composition
of the steel used to make the end product.
ASTM specifications represent a consensus among producers,
specifiers, fabricators, and users of steel mill products. In many
cases, the dimensions, tolerances, limits, and restrictions in the
ASTM specifications are similar to or the same as the corresponding
items of the standard practices in the AISI Steel Products Manuals.
Many of the ASTM specifications have been adopted by the
American Society of Mechanical Engineers (ASME) with little or no
modification; ASME uses the prefix S and the ASTM designation for
these specifications. For example, ASME-SA213 and ASTM A 213 are
Steel products can be identified by the number of the ASTM specification
to which they are made. The number consists of the letter
A (for ferrous materials) and an arbitrary, serially assigned number.
Citing the specification number, however, is not always adequate to
completely describe a steel product. For example, A 434 is the
specification for heat-treated (hardened and tempered) alloy steel
bars. To completely describe steel bars indicated by this
specification, the grade (SAE-AISI designation in this case)
and class (required strength level) must also be indicated.
The ASTM specification A 434 also incorporates, by reference,
two standards for test methods (A 370 for mechanical testing
and E 112 for grain size determination) and A 29, which specifies
the general requirements for bar products.
SAE-AISI designations for the compositions of carbon and alloy steels
are sometimes incorporated into the ASTM specifications for bars,
wires, and billets for forging. Some ASTM specifications for sheet
products include SAE-AISI designations for composition. The ASTM
specifications for plates and structural shapes generally specify
the limits and ranges of chemical composition directly, without
the SAE.AISI designations.
General Specifications. Several ASTM specifications, such as A 20
covering steel plate used for pressure vessels, contain the general
requirements common to each member of a broad family of steel
products. These general specifications are often supplemented by
additional specifications describing a different mill form or
intermediate fabricated product.
European and Japanese Designation Systems
Below some basics of European and Japanese designation systems are
explained. Please refer to articles about corresponding national
and international standards for more details.
DIN standards are developed by Deutsches Institut fur Normung in the
Federal Republic of Germany. All West German steel specifications
are preceded by the uppercase letters DIN followed an alphanumeric or
numeric code. The latter method, known as the Werkstoff number,
uses numbers only with a decimal point after the first digit.
JIS standards are developed by the Japanese Industrial Standards
Committee, which is part of the Ministry of International Trade and
Industry in Tokyo. The JIS steel specifications begin with the
uppercase letters JIS and are followed by an uppercase letter
(G in the case of carbon and low-alloy steels) designating the
division (product form) of the standard. This letter is followed by
a series of numbers and letters that indicate the specific steel.
British standards (BS) are developed by the British Standards
Institute in London, England. Similar to the JIS standards, each
British designation includes a product form and an alloy code.
AFNOR standards are developed by the Association Francaise de
Normalisation in Paris, France. The correct format for reporting
AFNOR standards is as follows. An uppercase NF is placed to the
left of the alphanumeric code. This code consists of an uppercase
letter followed by a series of digits, which are subsequently
followed by an alphanumeric sequence.
UNI standards are developed by the Ente Nazionale Italiano di
Unificazione in Milan, Italy. Italian standards are preceded by
the uppercase letter UNI followed by a four-digit product form
code subsequently followed by an alphanumeric alloy identification.
Swedish standards (SS) are prepared by the Swedish Standards
Institution in Stockholm. Designations begin with the letters SS
followed by the number 14 (all Swedish carbon and low-alloy steels
are covered by SS14). What subsequently follows is a four digit
numerical sequence similar to the German Werkstoff number.