Composition And Properties Of Steel





It is a remarkable fact that one can look through a dozen text

books on metallurgy and not find a definition of the word steel.

Some of them describe the properties of many other irons and then

allow you to guess that everything else is steel. If it was difficult

a hundred years ago to give a good definition of the term when the

metal was made by only one or two processes, it is doubly difficult

now, since the introduction of so many new operations and furnaces.



We are in better shape to know what steel is than our forefathers.

They went through certain operations and they got a soft malleable,

weldable metal which would not harden; this they called iron. Certain

other operations gave them something which looked very much like

iron, but which would harden after quenching from a red heat. This

was steel. Not knowing the essential difference between the two,

they must distinguish by the process of manufacture. To-day we

can make either variety by several methods, and can convert either

into the other at will, back and forth as often as we wish; so

we are able to distinguish between the two more logically.



We know that iron is a chemical element--the chemists write it

Fe for short, after the Latin word ferrum, meaning iron--it is

one of those substances which cannot be separated into anything

else but itself. It can be made to join with other elements; for

instance, it joins with the oxygen in the air and forms scale or

rust, substances known to the chemist as iron oxide. But the same

metal iron can be recovered from that rust by abstracting the oxygen;

having recovered the iron nothing else can be extracted but iron;

iron is elemental.



We can get relatively pure iron from various minerals and artificial

substances, and when we get it we always have a magnetic metal,

almost infusible, ductile, fairly strong, tough, something which

can be hardened slightly by hammering but which cannot be hardened

by quenching. It has certain chemical properties, which need not be

described, which allow a skilled chemist to distinguish it without

difficulty and unerringly from the other known elements--nearly

100 of them.



Carbon is another chemical element, written C for short, which is

widely distributed through nature. Carbon also readily combines

with oxygen and other chemical elements, so that it is rarely found

pure; its most familiar form is soot, although the rarer graphite and

most rare diamond are also forms of quite pure carbon. It can also

be readily separated from its multitude of compounds (vegetation,

coal, limestone, petroleum) by the chemist.



With the rise of knowledge of scientific chemistry, it was quickly

found that the essential difference between iron and steel was that

the latter was iron plus carbon. Consequently it is an alloy,

and the definition which modern metallurgists accept is this:



Steel is an iron-carbon alloy containing less than about 2 per

cent carbon.



Of course there are other elements contained in commercial steel,

and these elements are especially important in modern alloy steels,

but carbon is the element which changes a soft metal into one which

may be hardened, and strengthened by quenching. In fact, carbon,

of itself, without heat treatment, strengthens iron at the expense

of ductility (as noted by the percentage elongation an 8-in. bar

will stretch before breaking). This is shown by the following table:



--------------------------------------------------------------------------

Elastic UltimatePercentage.

Class by use. Class by Per cent limit strengthelongation

hardness. carbon. lb. per lb. per in 8 inches.

sq. in. sq. in.

---------------------------------------------------------------------

Boiler rivet steelDead soft 0.08 to 0.15 25,000 50,000 30

Struc. rivet steelSoft 0.15 to 0.22 30,000 55,000 30

Boiler plate steelSoft 0.08 to 0.10 30,000 60,000 25

Structural steel Medium 0.18 to 0.30 35,000 65,000 25

Machinery steel Hard 0.35 to 0.60 40,000 75,000 20

Rail steel Hard 0.35 to 0.55 40,000 75,000 15

Spring steel High carbon1.00 to 1.50 60,000 125,000 10

Tool steel High carbon0.90 to 1.50 80,000 150,000 5

--------------------------------------------------------------------------



Just why a soft material like carbon (graphite), when added to

another soft material like iron, should make the iron harder, has

been quite a mystery, and one which has caused a tremendous amount

of study. The mutual interactions of these two elements in various

proportions and at various temperatures will be discussed at greater

length later, especially in Chap. VIII, p. 105. But we may anticipate

by saying that some of the iron unites with all the carbon to form a

new substance, very hard, a carbide which has been called cementite.

The compound always contains iron and carbon in the proportions

of three atoms of iron to one atom of carbon; chemists note this

fact in shorthand by the symbol Fe3C (a definite chemical compound

of three atoms of iron to one of carbon). Many of the properties

of steel, as they vary with carbon content, can be linked up with

the increasing amount of this hard carbide cementite, distributed

in very fine particles through the softer iron.





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