Color test
Density or specific gravity test
Ring or sound of the metal upon impacting
with some other metal
Magnetic test
Chip test
Spark Test
The spark test is made by holding a sample of the
material against a power grinder. The sparks given off,
or the lack of sparks, assist in identifying the metal.
The length of the spark stream, its color, and the type
of sparks are the features for which you should look.
There are four fundamental spark forms produced
by holding a sample of metal against a power grinder.
(See fig. 6-10.) View A shows shafts, buds, breaks,
and arrows. The arrow or spearhead is characteristic
of molybdenum, an alloying element in steel. When
Figure 6-10.--Fundamental spark forms.
swelling or buds are present in the spark line, nickel
is also present as an alloying element with
molybdenum. View B shows shafts and sprigs, or
to suppress the effect of the carbon burst. However,
sparklets, which indicate a high carbon content. View
the nickel spark can be identified by tiny blocks of
C shows shafts, forks, and sprigs that indicate a
brilliant white light. Silicon suppresses the carbon
medium carbon content. View D shows shafts and
burst even more than nickel. When silicon is present,
forks that indicate a low carbon content.
the carrier line usually ends abruptly in a white flash
The greater the amount of carbon present in the
of light.
steel, the greater the intensity of bursting that will take
To make the spark test, hold the piece of metal
place in the spark stream. To understand the cause of
the bursts, remember that while the spark is glowing
against the wheel in such a manner as to throw the spark
and in contact with the oxygen of the air, the carbon
stream about 12 inches at a right angle to your line of
present in the particle is burned to carbon dioxide. As
vision. You will need to spend a little time to discover
the solid carbon combines with oxygen to form carbon
at just what pressure you must hold the sample to get a
dioxide in the gaseous state, the increase in volume
stream of this length without reducing the speed of the
builds up a pressure that is relieved by an explosion of
grinder. It is important that you do not press too hard
the particle. An examination of the small steel
because the pressure will increase the temperature of the
particles under a microscope when they are cold
spark stream and the burst. It will also give the appear-
reveals a hollow sphere with one end completely
ance of a higher carbon content than that of the metal
blown away.
actually being tested. After practicing to get the feel of
correct pressure on the wheel until you're sure you have
Steels having the same carbon content but
it, select a couple of samples of metal with widely
differing alloying elements are not always easily
varying characteristics; for example, low-carbon steel
identified because alloying elements affect the carrier
and high-carbon steel. Hold first one then the other
lines, the bursts, or the forms of characteristic bursts
against the wheel, always being careful to strike the
in the spark picture. The effect of the alloying element
same portion of the wheel with each piece. With your
may retard or accelerate the carbon spark or make the
eyes focused at a point about one-third of the distance
carrier line lighter or darker in color. Molybdenum,
for example, appears as a detached, orange-colored,
from the tail end of the stream of sparks, watch only
spearhead on the end of the carrier line. Nickel seems
those sparks that cross the line of vision. You will find
6-15