By the addition of alloying elements such as chromium, nickel, molybdenum and vanadium it is possible to increase the hardenability of a carbon steel along with other properties, such as corrosion resistance and fatigue strength.
The general trend of improved response to heat treatment and cold working in proportion to carbon content, common to plain carbon steel, applies equally to alloy steels. However, final properties are sensitive to the alloying additions as well.
The number of alloy steels available makes the choice of a steel for any given application difficult, and in most cases there will be a number of steels that would meet the requirements.
Chromium (Cr) can improve general high temperature properties, and also corrosion and oxidation resistance. It forms carbides with the available carbon in preference to iron which aids carburisation. It also slows down metallurgical reactions, thus increasing hardenability. Chromium results in larger grained structures which can cause problems as a result of the associated poorer mechanical performance.
Nickel (Ni) lowers critical heat treatment temperatures and generally allows for easier conditioning. Nickel strengthens and toughens steel by dissolution into the ferritic matrix. It is particularly valued in low temperature service where impact strengths can be maintained at sub zero temperatures.
Vanadium (V) is a very good carbide former, although it is also useful as a deoxidiser. Vanadium carbides are particularly fine and evenly distributed, and they provide the best grain refining properties, which generally improves mechanical properties. Vanadium carbide is very hard and has a stabilising effect on other carbides (notably chromium carbide) which might otherwise precipitate causing grain growth and brittleness during heat treatment. Vanadium forms nitrides and consequently is often present in nitriding steels.
Molybdenum (Mo), like vanadium, yields a fine grain structure with consequent improvements in overall strength. This fine structure is a result of the stable, even distribution of molybdenum carbide. These carbides also serve to stabilise steels with nickel and chromium additions which can otherwise show temper brittleness due to carbide precipitation. Molybdenum also enhances corrosion resistance in stainless steels.
Tungsten (W) forms carbides which are exceptionally hard. These carbides are beneficial in a similar fashion to molybdenum carbides, although far greater concentrations are required. Tungsten is valued in steels requiring hardness with stability at high temperatures, for example, tool steels.
Boron (B) is able to improve hardenability in concentrations as low as 0.001 %. This particularly sensitive behaviour is only effective with low to medium carbon steels. Despite increasing hardenability, these steels are still easily welded and are often specified where controlled hardenability in the weld is required. It is not known for steel to use boron as the sole alloying element, but it is frequently found in conjunction with other elements such as vanadium, chromium and molybdenum. as it also increases their hardening effect.
Copper (Cu) generally enhances corrosion resistance, although if it is present as a tramp element, possibly due to poorly separated scrap, it can be disastrous, causing grain segregation during hot working.
Cobalt (Co) is never present alone, but always as an addition to alloy steels. It is not a carbide former but dissolves in the ferrite matrix, like nickel and silicon. Additions of up to 30% cobalt to ferrous alloys have a significant effect on the materials magnetic properties. Cobalt can not only strengthen the ferrite, but also appears to stabilise the carbides and maintains their properties to much higher temperatures.
Titanium (Ti) forms very stable carbides combining with carbon in preference to iron and chromium. For the titanium to combine with all the carbon, a minimum of eight times as much titanium as carbon is used, resulting in titanium stabilised weldable austenitic steels.
Aluminium (Al) is a good deoxidiser, but alumina (aluminium oxide) is a brittle material which can be a damaging inclusion in steel. Aluminium can however, increase the ability of the steel to nitride and has some grain refining properties.
Manganese (Mn) is a useful deoxidiser and desulphuriser as the oxides and sulphides are particularly ductile and harmless. It is found in almost every steel for these reasons. In fact, because it is such a common addition, it is often omitted from specifications unless it is present in quantities >2%. Manganese lowers heat treatment temperatures and can give a wholly austenitic steel in concentrations greater than about 15%. These steels are non-magnetic. Manganese also strengthens steel and can yield high carbon steels that are tough and workable. It should be noted that manganese tends to increase the likelihood of quench cracking.
Silicon (Si) is a cheap and harmless deoxidiser found almost without exception in steels. It raises heat treatment temperatures and forms graphites which are useful for decarburising. At levels above about 0.5%, silicon can increase corrosion resistance and fatigue strength, although not to the same extent as other alloying elements.