Distribution of Alloying Elements

Distribution of Alloying Elements

In commercial alloy steels, which are multicomponent systems, alloying elements can be found (i) in the free state; (ii) as intermetallic compound with iron or with each other; (iii) as oxides, sulfides, and other nonmetal inclusions; (iv) in the form of carbides; or (v) as a solution in iron. As to the character of their distribution in steel, alloying elements may be divided into two groups:

1) Elements that do not form carbides in steel (e.g. Ni, Si, Co, Al, Cu and N) 2) Elements that form stable carbides in steel (e.g. Cr, Mn, Mo, W, V, Ti, Zr, and Nb).

The first group elements do not form chemical compounds with iron and carbon, and consequently the only possible form in which they can be present in steel is in solid solutions with iron. The only exceptions are Cu and N. Copper dissolves in -iron at normal temperatures in amounts of up to 1.0%. If the Cu content exceeds 7%, iron will contain copper in the free state as metal inclusions. Nitrogen also has a limited solubility in ferrite. When the N content is higher than 0.015%, nitrogen is found in steel in the form of chemical compounds with iron or some alloying elements (V, Al, Ti, and Cr). These chemical compounds are called nitrides.

Alloying elements whose affinity for oxygen is greater than that of iron are capable of forming oxides and other nonmetal compounds. When added at the very end of the steel melting process, such elements (e.g., Al, Si, V, Ti) deoxidize steel by taking oxygen from iron. The deoxidizing reaction yields Al2O3, TiO2, V2O5 and other oxides. Owing to the fact that deoxidizers are introduced at the final stages of the steel melting process, the majority of oxides have no time to coagulate or to pass to slag, and as a result they are retained in the solid steel as fine nonmetal inclusions. In addition to a great affinity for oxygen, some alloying elements have a greater affinity for sulfur than iron does, and upon being introduced into steel, they form sulfides.

Alloying elements that form stable carbides in steel can be found in the form of chemical compounds with carbon and iron or be present in the solid solution. The distribution of theses elements depends on the carbon contents of steel and the concurrent presence of other carbide-forming elements. If a steel contains a relatively small amount of carbon and a great quantity of an alloying element, then, carbon will be bound to carbides before the carbide-forming elements are completely used. For this reason excess carbide-forming elements will be found in the solid solution. If a steel has a large amount of carbon and little of the alloying elements, the latter will be present in the steel mainly as carbides.

Most alloying elements, except C, N, O, B and metalloids standing far from iron in the periodic table, dissolve in great amount in iron. The elements standing to the right of iron (Co, Ni, Cu, etc.) form solutions in iron only and do not enter into carbides. Thus one can state that alloying elements dissolve predominantly into basic phases (ferrite, austenite, cementite) of iron-carbon alloys or form special carbides.

Reference: Mehran Maalekian, The Effects of Alloying Elements on Steels (I), Technische Universität Graz, Institut für Werkstoffkunde, Schweißtechnik und Spanlose Formgebungsverfahren, Christian Doppler Laboratory for Early Stages of Precipitation, October 2007, pp. 8-10.