ned 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 sulph