Steel Alloys and Their Classification

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Steel Alloys and their classification: Steel alloys have traditionally been used in applications where strength and stiffness are of much greater importance than weight reduction. Steel alloys can be found anywhere from buildings to bridges to ships to home appliances. The first steel alloys were discovered when iron was refined in such a way as to add carbon into the final product, strengthening it immensely.

Two good examples of the use of steel alloys for structural applications can be seen in this photo taken of the USS Iowa passing under the Newport, R.I. bridge. The Iowa has 15" thick armor in places for protection from fire from enemy ships, as well as a special "belt" of armor under her waterline for protection from depth charges, torpedoes, etc. The suspension bridge uses steel in the cables which suspend the roadway from the support towers, as well as to reinforce the concrete in the bridge. figure
U.S. Navy Photo

Steel alloys can have a wide range of properties from corrosion resistance to high tensile strength. These properties depend on the alloying material, the percentage of carbon, and heat treatment.

Here are two examples of steel alloys:

Low Alloy Steel - Usually iron base alloys, which can be hardened to high strength. A common steel alloy for use in the 180 to 200 ksi (1 ksi = 1000 psi) strength range is AISI 4130. Another type is AISI 4340, with a strength range from 200 ksi to 280 ksi, which is used in the 260 to 280 ksi range. A very high strength alloy in this family is 300M, which can be used up to 290 ksi. It is used for landing gears, and in high strength applications. This family of steels is very corrosion prone, and must be plated in order to avoid corroding in most environments.

AISI 4130 Normalized - thickness <= 0.188" (data from Ref. 1)

Inexpensive steel alloy used in landing gear, especially on general aviation airplanes. Little corrosion resistance.

Temperature (°F) Exposure (hr) e (%) s tu (ksi) s cy (ksi) Ec (106psi) s0.7 (ksi) s0.85 (ksi) n
75 0.5 23 90 70 29 61.5 53 6.8
500 0.5 - 81 61.5 27.3 55 48 7.3
800 0.5 - 68 46.2 23.8 40 32.5 5.2
1000 0.5 - 46 30.8 20.6 28 22 4.7

Martensitic Stainless Steel - Steel alloys with 12% to 18% Chromium, with no nickel and are heat treatable by quench and temper. Maximum strength ranges are 140 ksi - 230 ksi for the 410 and 420 series, and 275 ksi - 285 ksi for 440C series. Some typcal uses of these alloys are in cutlery and in turbine blades. This is the least corrosion resistant of the stainless steels, and should be considered only for mild environments like household use, aerospace usage away from the ocean, et cetera.

17-7 PH (TH1050) Sheet, Strip & Plate 0.01" < thickness < 0.125"(data from Ref. 1)

Precipitation hardened stainless steel used in airframe and marine applications where high strength and corrosion resistance are desirable.

Temperature (°F) Exposure (hr) e (%) stu (ksi) scy (ksi) Ec (106 psi) s0.7 (ksi) s0.85 (ksi) n
75 0.5 6 180 162 29 166 145 7.4
400 0.5 - 169 144 27.8 146 126 6.8
700 0.5 - 144 118 24.9 117 104 8.4
1000 0.5 - 88 61.5 20.3 56 47 6

For more information on material properties, go to


[1] Bruhn, E. F., Analysis and Design of Flight Vehicle Structures,1 972.

Aluminum Alloys and Their Classification  |  Titanium Alloys and Their Classification