CHROMIUM
Chromium when alloyed with steel, has the characteristic function of opposing the disintegration and reconstruction of cementite. This is demonstrated by the changes in the critical ranges of this alloy steel taking place slowly; in other words, it has a tendency to raise the Ac range (decalescent points) and lower the Ar range (recalescent points). Chromium steels are therefore capable of great hardness, due to the rapid cooling being able to retard the decomposition of the austenite.
The great hardness of chromium steels is also due to the formation of double carbides of chromium and iron. This condition is not removed when the steel is slightly tempered or drawn. This additional hardness is also obtained without causing undue brittleness such as would be obtained by any increase of carbon. The degree of hardness of the lower-chrome steels is dependent upon the carbon content, as chromium alone will not harden iron.
The toughness so noticeable in this steel is the result of the fineness of structure; in this instance, the action is similar to that of nickel, and the tensile strength and elastic limit is therefore increased without any loss of ductility. We then have the desirable condition of tough hardness, making chrome steels extremely valuable for all purposes requiring great resistance to wear, and in higher-chrome contents resistance to corrosion. All chromium-alloy steels offer great resistance to corrosion and erosion. In view of this, it is surprising that chromium steels are not more largely used for structural steel work and for all purposes where the steel has to withstand the corroding action of air and liquids. Bridges, ships, steel building, etc., would offer greater resistance to deterioration through rust if the chromium-alloy steels were employed.
Prolonged heating and high temperatures have a very bad effect upon chromium steels. In this respect they differ from nickel steels, which are not so affected by prolonged heating, but chromium steels will stand higher temperatures than nickel steels when the period is short.
Chromium steels, due to their admirable property of increased hardness, without the loss of ductility, make very excellent chisels and impact tools of all types, although for die blocks they do not give such good results as can be obtained from other alloy combinations.
For ball bearing steels, where intense hardness with great toughness and ready recovery from temporary deflection is required, chromium as an alloy offers the best solution.
Two per cent chromium steels; due to their very hard tough surface, are largely used for armor-piercing projectiles, cold rolls, crushers, drawing dies, etc.
The normal structure of chromium steels, with a very low carbon content is roughly pearlitic up to 7 per cent, and martensitic from 8 to 20 per cent; therefore, the greatest application is in the pearlitic zone or the lower percentages.