The Importance of Carbon In Steel

by Royce Low

Carbon is very important in the context of hardness and strength. But in the broader context of steel metallurgy, hardenability is, overall, just as important, if not more so. This property may be defined, simply, as the ability of a steel to harden to a certain depth. It is increased by higher carbon content and by alloy additions. It increases with all alloy additions except cobalt. 

carbon steel 1020

Carbon Steel 1020

Hardenability of steel is the property that determines the depth and distribution of hardness induced by quenching. Since the primary objective in quenching is to obtain satisfactory hardening, it follows that hardenability is the most important factor in the selection of steel for heat treated parts. Hardenability should not be confused with hardness as such or with maximum hardness. The maximum hardness of any steel depends upon carbon content. There are hardenability tests for every category of steel, and hardenability charts for every grade. These will effectively show the depth to which steels will harden in the various media: water, oil, and air. It should be noted, however, that the maximum hardness values that can be obtained with small test specimens under the fastest rates of water quenching are nearly always higher than those developed under production heat treating conditions because the limitations in quenching larger sizes result in less than 100% martensite formation.

Low And High Alloy Steel

Increased hardenability allows some steels to be hardened by quenching in oil instead of water, in certain alloy steels by air cooling (air-hardening steels). The whole range of low-alloy and alloy steels is effectively based upon many, many years of experience of the effects of alloy additions and the attendant resulting mechanical properties, wear resistance, and corrosion resistance. There are many low and higher alloy steels. The chemical analyses are a function of the effects of the various alloying elements added; for example chromium and molybdenum for strength and corrosion resistance, nickel for toughness, copper for corrosion resistance, and silicon for heat resistance. Their chemical analyses will indicate the austenitizing temperature, and the temperature at which they will be heated for the annealing, normalizing, and hardening operations.

Carbon Steel Forging

Stainless steels type 321 and 347 contain, respectively, small additions of titanium and columbium. The additions are to prevent intergranular carbide precipitation that may sometimes be found in 300 series stainless steels such as type 302, 304, and 316. Carbide precipitation leaves the steel vulnerable to intergranular corrosion, and to its inevitable destruction. As such, types 304 and 316 are very often produced to the “L” (low carbon) grade or 0.03% maximum carbon, where intergranular corrosion will not be found.

The carbon content of a carbon steel or of an alloy steel has a direct bearing on the strength and hardness of the steel, and on its hardenability. As previously noted, alloy additions affect a whole range of properties, with hardenability, strength, toughness, and corrosion resistance being the most important.

All Metals & Forge Group uses carbon, alloy, stainless, nickel, and tool steel to manufacture open die forged parts and seamless rolled rings for customers across heavy industry and machinery building enterprises.