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.





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