Many methods are used to
identify a piece of metal. Identification is necessary when selecting a metal for use in
fabrication or in determining its weldability. Some common methods used for field
identification are surface appearance, spark test, chip test, and the use of a magnet.
Sometimes it is possible to
identify metals by their surface appearance. Table 1-3 indicates the surface colors of
some of the more common metals. Referring to the table, you can see that the outside
appearance of a metal helps to identify and classify metal. Newly fractured or freshly
filed surfaces offer additional clues.
A surface examination does
not always provide enough information for identification but should give us enough
information to place the metal into a class. The color of the metal and the distinctive
marks left from manufacturing help in determining the identity of the metal. Cast iron and
malleable iron usually show evi-dence of the sand mold. Low-carbon steel often shows
forging marks, and high-carbon steel shows either forg-ing or rolling marks. Feeling the
surface may provide another clue. Stainless steel is slightly rough in the unfinished
state, and the surfaces of wrought iron, cop-per, brass, bronze, nickel, and Monel are
smooth. Lead also is smooth but has a velvety appearance.
When the surface appearance
of a metal does not give enough information to allow positive identifica-tion, other
identification tests become necessary. Some of these tests are complicated and require
equipment we do not usually have; however, other tests are fairly simple and reliable when
done by a skilled person. Three of these tests areas follows: the spark test, the chip
test, and the magnetic tests.
The spark test is made by
holding a sample of the material against an abrasive wheel. By visually inspect-ing the
spark stream, an experienced metalworker can identify the metals with considerable
accuracy. This test is fast, economical, convenient, and easily accom-plished, and there
is no requirement for special equipment. We can use this test for identifying metal
salvaged from scrap. Identification of scrap is particularly impor-tant when selecting
material for cast iron or cast steel heat treatment.
When you hold a piece of iron
or steel in contact with a high-speed abrasive wheel, small particles of the metal are
torn loose so rapidly that they become red-hot. As these glowing bits of metal leave the
wheel, they follow a path (trajectory) called the carrier line. This carrier line is
easily followed with the eye, especial] y when observed against a dark background.
The sparks given off, or the
lack of sparks, aid in the identification of the metal. The length of the spark stream,
the color, and the form of the sparks are features you should look for. Figure 1-2
illustrates the terms used in referring to various basic spark forms produced in spark
Steels having the same carbon
content but differing alloying elements are difficult to identify because the alloying
elements affect the carrier lines, the bursts, or the forms of characteristic bursts in
the spark picture, The effect of the alloying element may slow or acceler-ate the carbon
spark or make the carrier line lighter or darker in color. Molybdenum, for example,
appears as a detached, orange-colored spearhead on the end of the carrier line. Nickel
appears to suppress the effect of the carbon burst; however, the nickel spark can be
identified by tiny blocks of brilliant white light. Silicon suppresses the carbon burst
even more than nickel. When silicon is present, the carrier line usually ends abruptly in
a white flash of light.
Spark testing may be done
with either a portable or stationary grinder. In either case, the speed on the outer rim
of the wheel should not be less than 4,500 feet per minute. The abrasive wheel should be
rather coarse, very hard, and kept clean to produce a true spark To conduct a spark test
on an abrasive wheel, hold the piece of metal on the wheel in a position that allows the
spark stream to cross your line of vision. By trial and error, you soon discover what
pressure is needed to get a stream of the proper length without reducing the speed of the
grinder. Excessive pressure increases the tem-perature of the spark stream. This, in turn,
increases the temperature of the burst and gives the appearance of a higher carbon content
than actually is present. When making the test, watch a point about one third of the
distance from the tail end of the spark stream. Watch only those sparks that cross your
line of vision and try to forma mental image of the individual spark. Fix this spark image
in your mind and then examine the whole spark picture.
While on the subject of
abrasive wheels, it is a good idea to discuss some of the safety precautions associated
with this tool.
Never use an abrasive wheel
that is cracked or out of balance because the vibration causes the wheel to shatter. When
an abrasive wheel shatters, it can be disastrous for personnel standing in line with the
Always check the wheel for
secure mounting and cracks before putting it to use. When you install a new wheel on a
grinder, be sure that it is the correct size. Remember, as you increase the wheel radius,
the peripheral speed at the rim also increases, even though the driving motor rpm remains
the same. Thus, if you should use an oversized wheel, there is a distinct danger the
peripheral speed (and consequent centrifugal force) can become so great that the wheel may
fly apart. Use wheels that are designed for a specific rpm. Guards are placed on grinders
as protection in case a wheel should shatter.
Never use a grinder when
the guards have been removed. When turning the grinder on, you should stand to one side.
This places you out of line with the wheel in case the wheel should burst.
Never overload a grinder or
put sideways pres-sure against the wheel, unless it is expressly built to withstand such
Always wear appropriate
safety goggles or a face shield while using the grinder. Ensure that the tool rest (the
device that helps the operator hold the work) is adjusted to the minimum clearance for the
wheel. Move the work across the entire face of the wheel to eliminate grooving and to
minimize wheel dressing. Doing this prolongs the life of the wheel.
Keep your fingers clear of
the abrasive surface, and do not allow rags or clothing to become entangled in the wheel.
Do not wear gloves while
using an abrasive wheel.
Never hold metal with tongs
Never grind nonferrous
metals on a wheel in-tended for ferrous metals because such misuse clogs the pores of the
abrasive material. This buildup of metal may cause it to become unbalanced and fly apart.
require frequent reconditioning. Dressing is the term used to describe the process
of cleaning the periphery. This cleaning breaks away dull abrasive grains and smooths the
surface, removing all the grooves. The wheel dresser shown in figure 1-3 is used for
dressing grinding wheels on bench and pedestal grinders.
|Referring now to
figure 1-4, notice that in low-carbon steel (view A), the spark stream is about 70 inches
long and the volume is moderately large. In high-carbon steel (view B), the stream is
shorter (about 55 inches) and the volume larger. The few sparklers that may occur at any
place in low-carbon steel are forked, and
in high-carbon steel, they are small and repeating. these metals must be distinguished
from each other by Both metals produce a spark stream white in color.
Gray cast iron (view C) produces
a stream of sparks about 25 inches in length. The sparklers are small and repeating, and
their volume is rather small. Part of the stream near the wheel is red, and the outer
portion is straw-colored.
Monel and nickel (view D)
form almost identical spark streams. The sparks are small in volume and orange in color.
The sparks form wavy streaks with no sparklers. Because of the similarity of the spark
picture, some other method.
Stainless steel (view E)
produces a spark stream about 50 inches in length, moderate volume, and with few
sparklers. The sparklers are forked. The stream next to the wheel is straw-colored, and at
the end, it is white.
The wrought-iron spark test
(view F) produces a spark stream about 65 inches in length. The stream has a large volume
with few sparklers. The sparks appear near the end of the stream and are forked. The
stream next to the wheel is straw-colored, and the outer end of the stream is a brighter
One way to become proficient
in spark testing fer-rous metals is to gather an assortment of samples of known metals and
test them. Make all of the samples about the same size and shape so their identities are
not revealed simply by the size or shape. Number each sample and prepare a list of names
and corresponding numbers. Then, without looking at the number of the sample, spark test
one sample at a time, calling out its name to someone assigned to check it against the
names and numbers on the list. Repeating this process gives you some of the experience you
need to become profi-cient in identifying individual samples.
Another simple test used to
identify an unknown piece of metal is the chip test. The chip testis made by removing a
small amount of material from the test piece with a sharp, cold chisel. The material
removed varies from small, broken fragments to a continuous strip. The chip may have
smooth, sharp edges; it maybe coarse-grained or fine-grained; or it may have sawlike
edges. The size of the chip is important in identifying the metal. The ease with which the
chipping can be accomplished should also be considered. The information given in table 1-4
can help you identify various metals by the chip test.
The use of a magnet is
another method used to aid in the general identification of metals. Remember that ferrous
metals, being iron-based alloys, normally are magnetic, and nonferrous metals are
nonmagnetic. This test is not 100-percent accurate because some stainless steels are
nonmagnetic. In this instance, there is no substitute for experience.