arily alloys of iron and carbon. Impurities and
their "remedies" are always present: sulphur, phosphorus, silicon
and manganese--to say nothing of oxygen, nitrogen and carbon oxide
gases, about which we know very little. It has been found that other
metals, if added to well-made steel, produce definite improvements
in certain directions, and these "alloy steels" have found much
use in the last ten years. Alloy steels, in addition to the
above-mentioned elements, may commonly contain one or more of the
following, in varying amounts: Nickel (Ni), Chromium (Cr), Vanadium
(Va), Tungsten (W), Molybdenum (Mo). These steels will be discussed
at more length in Chapters III and IV.

PROPERTIES OF STEEL

Steels are known by certain tests. Early tests were more or less
crude, and depended upon the ability of the workman to judge the
"grain" exhibited by a freshly broken piece of steel. The cold-bend
test was also very useful--a small bar was bent flat upon itself,
and the stretched fibers examined for any sign of break. Harder
stiff steels were supported at the ends and the amount of central
load they would support before fracture, or the amount of permanent
set they would acquire at a given load noted. Files were also used
to test the hardness of very hard steel.

These tests are still used to a considerable extent, especially in
works where the progress of an operation can be kept under close
watch in this way, the product being periodically examined by more
precise methods. The chief furnace-man, or "melter," in a steel
plant, judges the course of the refining process by casting small
test ingots from time to time, breaking them and examining the
fracture. Cutlery manufacturers use the bend test to judge the
temper of blades. File testing of case-hardened parts is very common.

However there is need of standardized methods which depend less
upon the individual skill of the operator, and which will yield
results comparable to others made by different men at different
places and on different steels. Hence has grown up the art of testing
materials.

TENSILE PROPERTIES

Strength of a metal is usually expressed in the number of pounds
a 1-in. bar will support just before breaking, a term called the
"ultimate strength." It has been found that the shape of the test
bar and its method of loading has some effect upon the results,
so it is now usual to turn a rod 5-1/2 in. long down to 0.505 in.
in diameter for a central length