The Working of Steel | Page 6

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
|Ultimate|Percentage. Class by use. | Class by | Per cent | limit
|strength|elongation | hardness. | carbon. |lb. per |lb. per |in 8 inches. | |
|sq. in. |sq. in. |
------------------|-----------|------------|--------|--------|------------ Boiler
rivet steel|Dead soft |0.08 to 0.15| 25,000 | 50,000 | 30 Struc. rivet
steel|Soft |0.15 to 0.22| 30,000 | 55,000 | 30 Boiler plate steel|Soft |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 carbon|1.00 to 1.50| 60,000 |125,000 | 10 Tool steel |High
carbon|0.90 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.

SULPHUR is another element (symbol S) which is always found in
steel in small quantities. Some sulphur is contained in the ore from
which the iron is smelted; more sulphur is introduced by the coke and
fuel used. Sulphur is very difficult to get rid of in steel making; in fact
the resulting metal usually contains a little more than the raw materials
used. Only the electric furnace is able to produce the necessary heat and
slags required to eliminate sulphur, and as a matter of fact the sulphur
does not go until several other impurities have been eliminated.
Consequently, an electric steel with extremely low sulphur (0.02 per
cent) is by that same token a well-made metal.
Sulphur is of most trouble to rolling and forging operations when
conducted at a red heat. It makes steel tender and brittle at that
temperature--a condition known to the workmen as "red-short." It
seems to have little or no effect upon the physical properties of cold
steel--at least as revealed by the ordinary testing
machines--consequently