Guide to Metal Composition

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Guide to Metal Composition

Summary:

It is necessary to know the metal composition before welding in
order to produce a successful weld. Welders and metal workers must be
able to identify various metal products so that proper work methods may
be applied. Any engineering drawing should be examined in order to determine the metal to be used and its
heat treatment, if required.

After some practice, the welder will learn
that certain parts of machines or equipment are always cast iron, other
parts are usually forgings, and so on.

Summary of Metal Composition and Identification Tests - Tables 7-4

Metal Composition Tests

There are seven tests that can be performed in the shop to identify metals.

Six of the different tests are summarized in table 7-4. These should be supplemented by tables 7-1 and 7-2, which present physical and mechanical properties of metal, and table 7-3, which presents hardness data.

These tests are as follows:

Appearance Test

The appearance metal composition test includes such things as color and appearance of
machined as well as un-machined surfaces. Form and shape give definite
clues as to the identity of the metal. The shape can be descriptive; for
example, shape includes such things as cast engine blocks, automobile
bumpers, reinforcing rods, I beams or angle irons, pipes, and pipe
fittings. Form should be considered and may show how the part was rode,
such as a casting with its obvious surface appearance and parting mold
lines, or hot rolled wrought material, extruded or cold rolled with a
smooth surface.

For example, pipe can be cast, in which case it would be
cast iron, or wrought, which would normally be steel. Color provides a
very strong clue in metal identification. It can distinguish many metals
such as copper, brass, aluminum, magnesium, and the precious metals. If
metals are oxidized, the oxidation can be scraped off to determine the
color of the un-oxidized metal. This helps to identify lead, magnesium,
and even copper. The oxidation on steel, or rust, is usually a clue that
can be used to separate plain carbon steels from the
corrosion-resisting steels.

Fracture Test

Some metal can be quickly identified by looking at
the surface of the broken part or by studying the chips produced with a
hammer and chisel. The surface will show the color of the base metal
without oxidation. This will be true of copper, lead, and magnesium. In
other cases, the coarseness or roughness of the broken surface is an
indication of its structure. The ease of breaking the part is also an
indication of its ductility of lack of ductility. If the piece bends
easily without breaking, it is one of the more ductile metals. If it
breaks easily with little or no bending, it is one of the brittle
metals.

Spark Test

The spark metal composition test is a method of classifying steels and iron according to
their composition by observing the sparks formed when the metal is held
against a high speed grinding wheel. This test does not replace chemical
analysis, but is a very convenient and fast method of sorting mixed
steels whose spark characteristics are known. When held lightly against a
grinding wheel, the different kinds of iron and steel produce sparks
that vary in length, shape, and color. The grinding wheel should be run
to give a surface speed of at least 5000 ft (1525 m) per minute to get a
good spark stream. Grinding wheels should be hard enough to wear for a
reasonable length of time, yet soft enough to keep a free-cutting edge.
Spark testing should be done in subdued light, since the color of the
spark is important. In all cases, it is best to use standard samples of
metal for the purpose of comparing their sparks with that of the test
sample.

Limitations

Spark metal composition testing is not of much use on nonferrous metals such as
coppers, aluminums, and nickel-base alloys, since they do not exhibit
spark streams of any significance. However, this is one way to separate
ferrous and nonferrous metals.

Spark Test Results

Spark testing is not of much use on nonferrous metals such as
coppers, aluminums, and nickel-base alloys, since they do not exhibit
spark streams of any significance. However, this is one way to separate
ferrous and nonferrous metals.

The spark resulting from the metal composition test should be directed downward and
studied. The color, shape, length, and activity of the sparks relate to
characteristics of the material being tested. The spark stream has
specific items which can be identified. The straight lines are called
carrier lines. They are usually solid and continuous. At the end of the
carrier line, they may divide into three short lines, or forks. If the
spark stream divides into more lines at the end, it is called a sprig.
Sprigs also occur at different places along the carrier line. These are
called either star or fan bursts. In some cases, the carrier line will
enlarge slightly for a very short length, continue, and perhaps enlarge
again for a short length. When these heavier portions occur at the end
of the carrier line, they are called spear points or buds. High sulfur
creates these thicker spots in carrier lines and the spearheads. Cast
irons have extremely short streams, whereas low-carbon steels and most
alloy steels have relatively long streams. Steels usually have white to
yellow color sparks, while cast irons are reddish to straw yellow. A
0.15 percent carbon steel shows sparks in long streaks with some
tendency to burst with a sparkler effect; a carbon tool steel exhibits
pronounced bursting; and a steel with 1.00 percent carbon shows
brilliant and minute explosions or sparklers. As the carbon content
increases, the intensity of bursting increases.

Summary of Spark Test - Tables 7-5, Pictures a - c below

Advantages

One big advantage of this metal composition test is that it can be applied to metal in,
all stages, bar stock in racks, machined forgings or finished parts. The
spark test is best conducted by holding the steel stationary and
touching a high speed portable grinder to the specimen with sufficient
pressure to throw a horizontal spark stream about 12.00 in. (30.48 cm)
long and at right angles to the line of vision. Wheel pressure against
the work is important because increasing pressure will raise the
temperature of the spark stream and give the appearance of higher carbon
content. The sparks near and around the wheel, the middle of the spark
stream, and the reaction of incandescent particles at the end of the
spark stream should be observed. Sparks produced by various metals are
shown in figure 7-4.

Characteristics of Sparks Generated by the Grinding of Metals - Figure 7-4

Caution: The torch metal composition test should be used with discretion, as it may damage teh part being tested. Additionally, magnesium may ignite when heated in the open atmosphere.

Torch Test

With the oxyacetylene torch, the welder can identify various metals by
studying how fast the metal melts and how the puddle of molten metal and
slag looks, as well as color changes during heating. When a sharp
corner of a white metal part is heated, the rate of melting can be an
indication of its identity. If the material is aluminum, it will not
melt until sufficient heat has been used because its high conductivity.
If the part is zinc, the sharp corner will melt quickly, since zinc is
not a good conductor. In the case of copper, if the sharp comer melts,
it is normally de-oxidized copper. If it does not melt until much heat
has been applied, it is electrolytic copper. Copper alloys, if composed
of lead, will boil. To distinguish aluminum from magnesium, apply the
torch to filings. Magnesium will burn with a sparkling white flame.
Steel will show characteristic colors before melting.

Magnetic Test

The magnetic metal composition test can be quickly performed using a small pocket magnet.
With experience, it is possible to judge a strongly magnetic material
from a slightly magnetic material. The nonmagnetic materials are easily
recognized. Strongly magnetic materials include the carbon and low-alloy
steels, iron alloys, pure nickel, and martensitic stainless steels. A
slightly magnetic reaction is obtained from Monel and high-nickel alloys
and the stainless steel of the 18 chrome 8
nickel type when cold worked, such as in a seamless tube. Nonmagnetic
materials include copper-base alloys, aluminum-base alloys, zinc-base
alloys, annealed 18 chrome 8 nickel stainless, the magnesium, and the
precious metals.

Chisel Test

The chip test or chisel metal composition test may also be used to identify metals. The
only tools required are a banner and a cold chisel. Use the cold chisel
to hammer on the edge or corner of the material being examined. The ease
of producing a chip is an indication of the hardness of the metal. If
the chip is continuous, it is indicative of a ductile metal, whereas if
chips break apart, it indicates a brittle material. On such materials as
aluminum, mild steel and malleable iron, the chips are continuous. They
are easily chipped and the chips do not tend to break apart. The chips
for gray cast iron are so brittle that they become small, broken
fragments. On high-carbon steel, the chips are hard to obtain because of
the hardness of the material, but can be continuous.

Hardness Test

Refer to table 7-3 for hardness values of the various metals, and to the above information on the three hardness tests that are commonly used.

A less precise
hardness test is the file test. A summary of the reaction to filing, the
approximate Brinell hardness, and the possible type of steel is shown
in table 7-6. A sharp mill file must be used. It is assumed that the part is steel and the file test will help identify the type of steel.

Approximate Hardness of Steel by the File Test - Table 7-6

Chemical Test

There are numerous chemical metal composition tests than can be made in the shop to
identify some material. Monel can be distinguished form Inconel by one
drop of nitric acid applied to the surface. It will turn blue-green on
Monel, but will show no reaction on Inconel. A few drops of a 45 percent
phosphoric acid will bubble on low-chromium stainless steels. Magnesium
can be distinguished from aluminum using silver nitrate, which will
leave a black deposit on magnesium, but not on aluminum. These tests can
become complicated, and for this reason are not detailed further here.

Color Code for Marking Steel Bars

SAE Metal Grading and Classifying System

The Bureau of Standards of the United States Department of Commerce has a
color code for making steel bars. The color markings provided in the
code may be applied by painting the ends of bars. Solid colors usually
mean carbon steel, while twin colors designate alloy and free-cutting
steel.

For Additional Reading and Reference

Largest Metals Online Database (free trial access)

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