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Types of Alloy Steels
In ANSI and SAE, the alloy types are classed by the use of a four-digit
numbering system. The first digit indicates the type of alloy steel, the second
digit is an approximation of the major alloy percent, and the last two digits
represent the carbon content of an alloy. As an example, AISI 2320 represents
nickel (2xxx), in the amount of 3.5 % (x3xx), with a carbon content of 0.20%
(xx20).
The table below provides a summary of selected types of alloy steels and
their characteristics. High alloy steels, such as stainless steels and tool
steels, are discussed separately.
| Type |
ANSI Number |
Description |
Application |
| Mn Steels |
(13xx) |
Mn is one of the least costly
alloying elements to aids in fine grain structur. Its effects in steels
depends upon the C content of the steel. When C is unavailable, Mn dissolves
in the gamma or alpha iron matrix, adding considerable strength. A high Mn
content changes the steel from pearlite to martensite, with some increase in
hardness and brittleness. An alloy containing about 12% Mn is called
Hadfield steel. With the proper heat treatment, this alloy develops high
strength, good ductility and resistance to wear. |
Primarily for making axles,
forgings, gears, shafts, and gun barrels. Hadfield steel is used extensively
for crushing, grinding and powershovel machinery, and for railroad tracks. |
| Ni Steels |
2xxx |
Ni is one of the oldest and most
common of alloy elements. It has high solubility in gamma iron, and is
soluble in ferrite. It aids in corrosion resistance, impact strength and
toughness, and slows down grain growth at high temperatures. |
Small amounts of nickel
(1.5-3.0%) are used for high-strength structural steel castings and
forgings. The 3.5% nickel, low carbon alloys are used for such products as
carburized connecting rods, gears, and kingpins. The 5% nickel alloys are
used in heavy equipment, truck cams, crankshafts, and other heavy-duty
products. |
| Ni-Cr Steels |
3xxx |
The Ni increases ductility and
toughness while the Cr aids hardenability and wear resistance. Low Ni-Cr
alloys, usually in a ratio of 2.5 to 1, add the combined properties of each
element with greater effect than when each is added separately. |
Carburized low carbon parts such
as cams, connecting rods, drive shafts, gears, and piston pins. |
| Mo Steels |
4xxx |
Mo is a costly alloy element
having limited solubility in alpha and gamma iron. It has high chemical
affinity for carbon and, therefore, increases alloy hardness. |
Molybdenum is often used in
combination with Cr and/or Ni to make carburized structural parts for
aircraft. |
| Low Cr Steels |
5xxx |
Cr is an inexpensive
carbide-forming alloy having fair solubility in gamma iron and limited
solubility in ferrite. |
Low C-Cr alloys are usually
carburized and case hardened to provide combined wear resistance and
toughness. Medium C-Cr alloys are commonly used in axles, bolts, springs,
and studs for automobiles. They are also used in tool steel for chisels,
dies, drills, files, knives, and shears. |
| Cr-V Steels |
6xxx |
Cr-V alloys are noted for
fine-grain size, excellent hardenability, high strength, and toughness. V is
one of the most expensive alloys, yet it is one of the most important in
terms of grain growth and carbide formation. |
Cr-V steels are widely used in
heavy forgings, axles, springs, and tools, depending upon their carbon
content. Low C alloys are case hardened, as necessary. |
| Ni-Cr-Mo Steels |
8xxx and 9xxx |
Meet critical needs for
hardness, strength, and corrosion resistance. Special emphasis has been
placed upon end-quench hardness tests for these alloys. With this added
hardenability data, these alloys are identified and sold with the letter H,
as follows: 4140H, 5140H, 8640H, as well as others. |
Ni-Cr-Mo alloys are used in
their carburized state for heavy equipment and aircraft parts. |
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