Page 70 - Shigley's Mechanical Engineering Design

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Materials 45
Heating due to electric and gas welding also changes the mechanical properties.
Such changes may be due to clamping during the welding process, as well as heating;
the resulting stresses then remain when the parts have cooled and the clamps have been
removed. Hardness tests can be used to learn whether the strength has been changed by
welding, but such tests will not reveal the presence of residual stresses.

2–7 Numbering Systems
The Society of Automotive Engineers (SAE) was the first to recognize the need, and to
adopt a system, for the numbering of steels. Later the American Iron and Steel Institute
(AISI) adopted a similar system. In 1975 the SAE published the Unified Numbering
System for Metals and Alloys (UNS); this system also contains cross-reference num-
7
bers for other material specifications. The UNS uses a letter prefix to designate the
material, as, for example, G for the carbon and alloy steels, A for the aluminum alloys,
C for the copper-base alloys, and S for the stainless or corrosion-resistant steels. For
some materials, not enough agreement has as yet developed in the industry to warrant
the establishment of a designation.
For the steels, the first two numbers following the letter prefix indicate the compo-
sition, excluding the carbon content. The various compositions used are as follows:

G10 Plain carbon G46 Nickel-molybdenum
G11 Free-cutting carbon steel with G48 Nickel-molybdenum
more sulfur or phosphorus
G50 Chromium
G13 Manganese
G51 Chromium
G23 Nickel
G52 Chromium
G25 Nickel
G61 Chromium-vanadium
G31 Nickel-chromium
G86 Chromium-nickel-molybdenum
G33 Nickel-chromium
G87 Chromium-nickel-molybdenum
G40 Molybdenum
G92 Manganese-silicon
G41 Chromium-molybdenum
G94 Nickel-chromium-molybdenum
G43 Nickel-chromium-molybdenum
The second number pair refers to the approximate carbon content. Thus, G10400 is a
plain carbon steel with a nominal carbon content of 0.40 percent (0.37 to 0.44 percent).
The fifth number following the prefix is used for special situations. For example, the old
designation AISI 52100 represents a chromium alloy with about 100 points of carbon.
The UNS designation is G52986.
The UNS designations for the stainless steels, prefix S, utilize the older AISI des-
ignations for the first three numbers following the prefix. The next two numbers are
reserved for special purposes. The first number of the group indicates the approximate
composition. Thus 2 is a chromium-nickel-manganese steel, 3 is a chromium-nickel
steel, and 4 is a chromium alloy steel. Sometimes stainless steels are referred to by their
alloy content. Thus S30200 is often called an 18-8 stainless steel, meaning 18 percent
chromium and 8 percent nickel.

7 Many of the materials discussed in the balance of this chapter are listed in the Appendix tables. Be sure to
review these.