Alloy Steels

Alloy steels are alloys of iron with the addition of one or more of the following elements; carbon, manganese, silicon, nickel, chromium, molybdenum, and vanadium. The alloy steels cover a wide range of steels, including low-alloy steels, stainless steels, heat-resistant steels, and tool steels. Some alloy steels, such as austenitic stainless steels, do not contain intentional additions of carbon. Silicon, when required, is added as a deoxidizer to the molten steel.
Nickel provides strength and assists in hardening the steel by quenching and tempering heat treatment. This latter effect is called hardenability, which has been described earlier. Chromium is found in stainless steels for corrosion resistance. Chromium and molybdenum also assist in hardenability of the low-alloy steels. Vanadium strengthens the steel by forming precipitates of vanadium carbonitride. Vanadium is also a potent hardenability element.

Low-Alloy Steels
There is an SAE/AISI four-digit classification system for the low-alloy steels. As in the carbon steels, the first two digits are for the alloy class and the last two (or three) digits are for the carbon content. Because of the various combinations of elements, the system is more extensive than that used for the carbon steels. The general SAE/AISI classification system for low-alloy steels is as follows:
Manganese steels 13xx series
Nickel steels 23xx, 25xx series
Nickel–chromium steels 31xx, 32xx, 33xx, and 34xx series
Molybdenum steels 40xx, 44xx series
Chromium–molybdenum steels 41xx series
Nickel–chromium–molybdenum steels 43xx and 47xx series
81xx, 86xx, 87xx, and 88xx series
93xx, 94xx, 97xx, and 98xx series
Nickel–molybdenum steels 46xx and 48xx series
Chromium steels 50xx and 51xx series
50xxx, 51xxx, and 52xxx series
Chromium–vanadium steels 61xx series
Tungsten–chromium steels 71xxx, 72xx series
Silicon–manganese steels 92xx series
Boron steels xxBxx series
Leaded steels xxLxx series
The boron-containing steels are low-alloy steels with boron added in the amount of 0.0005– 0.003%. Boron is a strong hardenability element. The leaded steels contain 0.15–0.35% lead for improved machinability (however, lead is no longer favored as an alloying addition because of health concerns).

A table in the aforementioned SAE HS-30 lists the composition limits of most of the families of the SAE/AISI low-alloy steels listed above. These steels are supplied in the form of bar, plate, and forged products and are usually heat treated to obtain specific mechanical properties, especially high strength and toughness. Mechanical properties of selected SAE/ AISI low-alloy steels in the as-rolled, annealed, normalized and quenched and tempered conditions are listed in the aforementioned ASM table. These properties are average properties and should only be used as a guideline. For example, SAE/AISI 4340 steel is usually heat treated by heating the component to 950–1000 C followed by quenching in oil. The component is then tempered at a temperature between 205 and 650 C. According to the aforementioned ASM table, this nickel–chromium–molybdenum steel in the quenched and tempered condition (tempered at 205 C) can achieve a yield strength of 1675 MPa and a tensile strength of 1875 MPa. Quenched and tempered low-alloy steels are used in a large number of applications requiring high strength and good toughness. Note that in the annealed condition, SAE/AISI 4340 steel has a yield strength of only 745 MPa.

Bruce L. Bramfitt
International Steel Group, Inc.
Research Laboratories
Bethlehem, Pennsylvania

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