Chromium, nickel and molybdenum are the most important alloying elements steels. They are used in various combinations and receive different categories of alloy steels.: chromium, chromonic, chromium-nickel-molybdenum and similar alloy steels.
Effect of chromium on the properties of steels
The tendency of chromium to form carbides is average among others carbide-forming alloying elements. At a low Cr / C ratio of chromium content in relation to iron, only cementite of the type (Fe,Cr)3C. With an increase in the ratio of the chromium and carbon content in Cr / C steel, chromium carbides of the type (Cr,Fe)7C3 or (Cr,Fe)23C6 or both. Chromium increases the heat-hardening capacity of steels, their resistance to corrosion and oxidation, provides increased strength at elevated temperatures, and also increases the resistance to abrasion of high carbon steels.
Chromium carbides are also wear resistant. It is they who provide the durability of steel blades - it is not for nothing that knife blades are made from chromium steels. Complex chromium-iron carbides enter the solid solution of austenite very slowly - therefore, when heating such steels for hardening, a longer exposure at the heating temperature is required. Chromium is rightfully considered the most important alloying element in steels. Chromium addition to steel induces impurities, such as phosphorus, lead, antimony and arsenic segregate to grain boundaries, what can cause temper brittleness in steels.
The effect of nickel on the properties of steels
Nickel does not form carbides in steels. In steels, it is an element, contributing to education and preservation austenite. Nickel increases the hardenability of steels. In combination with chromium and molybdenum, nickel further increases the heat hardening properties of steels, contributes to an increase in the toughness and fatigue strength of steels. Dissolving into ferrets nickel increases its toughness. Nickel increases the corrosion resistance of chromium-nickel austenitic steels in non-oxidizing acidic solutions.
Effect of molybdenum on the properties of steels
Molybdenum readily forms carbides in steels. It dissolves in cementite only slightly. Molybdenum forms molybdenum carbides, as soon as the carbon content in steel becomes high enough. Molybdenum is able to provide additional thermal hardening during tempering of hardened steels. It increases the creep resistance of low alloy steels at high temperatures.
Molybdenum additives facilitate grain refinement of steels, increase the hardenability of steels by heat treatment, increase the fatigue strength of steels. Alloy steels with molybdenum content 0,20-0,40 % or the same amount of vanadium slows down the onset of temper brittleness, but do not eliminate it completely. Molybdenum increases the corrosion resistance of steels and is therefore widely used in high-alloy ferritic stainless steels and in chromium-nickel austenitic stainless steels. The high molybdenum content reduces the tendency of stainless steel (pitting) corrosion. Molybdenum has a very strong solid solution hardening of austenitic steels, which are used at elevated temperatures.
Source: Steel Heat Treatment: Metallurgy and Technologies, ed. G. E. Totten, 2006