From Ed Caffrey's website:
CARBON (C): Has by far the greatest influence of any of the elements. Steel could not exist without carbon. Martensite, along with banite give steel a microstructure of hard, tough carbide. None of the other elements so dramatically alter the strength and hardness as do small changes in carbon content. Carbon iron crystalline structures have the widest number and variety known to exist in metallurgy. They also combine with other elements to furnish steel with an assortment of iron alloy carbide systems.
MANGANESE (Mn): Is normally present in all steel and functions as a deoxidizer. It also imparts strength and responsiveness to heat treatment. It is usually present in quantities of 0.5 to 2.0 percent.
NICKEL (Ni): Increases strength and toughness but is ineffective in increasing hardness. It is generally added in amounts ranging from 1 percent to 4 percent. In some stainless steels it is sometimes as high as 20 percent.
SILICON (Si): Has a beneficial effect upon tensile strength and improves hardenability of an alloy. It has a toughening effect when used in combination with certain other elements. Silicon (Si) is usually added to improve electrical conductivity of an alloy. Its average concentration is between 1.5 and 2.5 percent.
CHROMIUM (Cr): Increases the depth penetration of hardening and also the responsiveness to heat treatment. It is usually added with nickel (Ni) for use in stainless steels. Most of the chromium (Cr) bearing alloys contain 0.50 to 1.50 percent chromium; some stainless steels contain as much as 20 percent or more. It can affect forging, causing a tendency in the steel to crack.
VANADIUM (V): Retards grain growth within steel even after long exposures at high temperatures, and helps to control grain structures while heat treating. It is usually present in small quantities of 0.15 to 0.20 percent. Most tool steels which contain this element seem to absorb shock better that those that do not contain vanadium (V).
MOLYBDENUM (Mo): Adds greatly to the penetration of hardness and increases toughness of an alloy. It causes steel to resist softening at high
temperatures, which defeats the purpose of forging. If the alloy has below 0.020 percent molybdenum (Mo), you should be able to forge this alloy with little difficulty.
TUNGSTEN (W): Also known as wolfram, is used as an alloying element in tool steels, as it tends to impart a tight, small, and dense grain pattern and keen cutting edges when used in relatively small amounts. It will also cause steel to retain its hardness at higher temperatures and hence will have a detrimental effect upon the steel's forgeability (otherwise known as "red hard")
SULFUR (S): Is usually regarded as an impurity in most alloys and its addition to steel is held to a minimum as it is damaging to the hot forming characteristics of steel. It is, however added to increase machinability. A word of caution, some alloys are offered in different forms, an example is E52100. This particular steel can be had in either a "Bearing Quality" or "Machining Quality" the latter having sulfur (S) added to increase machinability.
LEAD (Pb): Increase the machinability of steel and has no effect upon the other properties of the metal. It is usually added to an alloy only upon request and then in quantities of 0.15 to 0.30 percent.
PHOSPHORUS (P): Is present in all steel. It increases yield strength and reduces ductility at low temperatures. It is also believed to increase resistance to atmospheric corrosion. Phosphorus (P) is usually treated as an impurity in most alloys.
