Carbon steels are most often specified. The broad range of grades gives the designer wide latitude in selecting one with the optimum combination of mechanical properties, forgeability and minimum cost.

By definition, carbon steel is iron combined with carbon varying from 0.06% to 1.5%. Carbon steels may also contain maximum amounts of manganese of 1.65% and/or silicon of 0.60%. The more common grades are between 1018 and 1060 with nominal 0.18% to 0.60% carbon respectively. There are some maximum limits to the residual elements, typically less than a total 0.80% for such elements as Cr, Mo, V, Cu, and Ni.

Free-machining grades of carbon steels that contain sulphur (1100 series), lead (12Lxx series) or sulphur/phosphorous (1200 series) are not as readily forged as the carbon steel counterparts. The additions produce controlled amounts of inclusions in the steel that assist chip breakage during machining, but the inclusions can also cause the steel to crack during forging, particularly during hot or cold upsetting. High amounts of elements such as sulphur may render a steel totally unforgeable.

Alloy steels have manganese or silicon contents at levels similar to those of carbon steels, and additions such as chromium, nickel, molybdenum or vanadium, or, less often, additions of boron, cobalt, columbium, titanium or tungsten.

Alloying elements are added for their effects on processing during manufacture and on performance during service.
Alloying elements are used primarily to:

1.     Improve selected mechanical properties, through control of the metallurgical structure. For example:
         •     Increased hardenability by the addition of elements such as chromium, molybdenum vanadium or boron.
         •     Increased impact toughness by the additions of elements such as manganese and nickel.
2.     Improve retention of strength and ductility at relatively high service temperatures by the additions of chromium, nickel and cobalt.
3.     Improve retention of mechanical properties, particularly ductility and impact toughness, at relatively low service temperatures by the additions of nickel and silicon.
4.     Achieve increased resistance to atmospheric and chemical corrosion and elevated temperature oxidation by the additions of chromium and nickel.

Again, Sulphur, which is often added to some steel grades for improving machinability, can adversely affect both the forgeability and the property performance of forgings, especially those heat treated to strengths above about 550 MPa (80,000 psi) tensile. If machinability is desirable in the forging steel, it is preferable to use "special order partially resulphurized" grades, with about half the sulphur content of normal resulphurized steels. Furthermore, some mills can use special sulphide shape controlled treatments, which include calcium, tellurium and some rare earth materials that tend to modify manganese sulphide inclusions to a more forgeable condition.

The standard alloy steel grades are found in the AISI and SAE 1300 through 9800 series. They are listed in many AISI publications and in the ASM and SAE Handbooks. Modifications of these types are also used in special applications. As a rule, these standard grades are specified when more strength, ductility and impact toughness are required than can be attained in carbon steels. They are also specified when specific properties such as wear resistance, corrosion resistance, heat resistance and special low temperature impact properties are required. Some alloy steels are formulated for special treatments such as carburizing or carbo-nitriding. Detailed discussions of steel grades is beyond the scope of this generic information. For additional information or more detail on any of the information provided here, please contact:

American Institute of Steel and Iron (AISI)
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