spark test
ashu
Spark testing is a method of determining the general classification of
ferrous materials. Normally it entails taking a piece of metal,
usually scrap, and applying it to a grinding wheel in order to observe
the sparks emitted.These sparks can be compared to a chart or to
sparks from a known test sample to determine the classification. Spark
testing also can be used to sort ferrous materials, establishing the
difference from one another by noting if the spark is the same or
different.
Spark testing is used because it is quick, easy, and inexpensive.
Moreover, test samples do not have to be prepared in any way, so
often, a piece of scrap is used. The main disadvantage to spark
testing is its inability to identify a material positively; if
positive identification is required, chemical analysis must be used
The spark comparison method also damages the material being tested, at
least slightly.
Spark testing most often is used in tool rooms, machine shops, heat
treating shops, and foundries
Process
Usually a bench grinder is used to create the sparks, but sometimes
this is not convenient so a portable grinder is used. In either case
the grinding wheel must have adequate surface velocity, at least 23 m/
s (4500 surface feet per minute (sfpm)), but should be between 38 and
58 m/s (7500–11,500 sfpm). The wheel should be coarse and hard,
therefore, aluminium oxide or carborundum often are employed. The test
area should be in an area where there is no bright light shining
directly into the observer's eyes. Moreover, the grinding wheel and
surrounding area should be dark so that the sparks can be observed
clearly. The test sample is then touched lightly to the grinding wheel
to produce the sparks
The important spark characteristics are color, volume, nature of the
spark, and length. Note that the length is dependent on the amount of
pressure applied to the grinding wheel, so this can be a poor
comparison tool if the pressure is not exactly the same for the
samples. Also, the grinding wheel must be dressed frequently to remove
metallic build-up.
Compressed air method
Another less common method for creating sparks is heating up the
sample to red heat and then applying compressed air to the sample. The
compressed air supplies enough oxygen to ignite the sample and give
off sparks. This method is more accurate than using a grinder because
it will always give off the same length sparks for the same sample.
The compressed air applies essentially the same "pressure" each time.
This makes observations of the spark length a much more reliable
characteristic for comparison.
Automated spark testing
Automated spark testing has been developed to remove the reliance upon
operator skill and experience, thereby increasing reliability. The
system relies upon spectroscopy, spectrometry, and other methods to
"observe" the spark pattern. It has been found that this system can
determine the difference between two materials that give off sparks
that are indistinguishable to the human eye.
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Wrought iron
Wrought iron sparks flow out in straight lines. The tails of the
sparks widen out near the end, similar to a leaf.
Mild steel
Mild steel sparks are similar to wrought iron's, except they will have
tiny forks and their lengths will vary more. The sparks will be white
in color.
Medium-carbon steel
This steel has more forking than mild steel and a wide variety of
spark lengths, with more near the grinding wheel.
High-carbon steel
High-carbon steel has a bushy spark pattern (lots of forking) that
starts at the grinding wheel. The sparks are not so bright as the
medium-carbon steel ones.
Manganese steel
Manganese steel has medium length sparks that fork twice before
ending.
High-speed steel
High-speed steel has a faint red spark that sparks at the tip.
300-series stainless steel
These sparks are not so dense as the carbon steel sparks, do not fork,
and are orange to straw in color.
310-series stainless steel
These sparks are much shorter and thinner than the 300-series sparks.
They are red to orange in color and do not fork.
400-series stainless steel
400-series sparks are similar to 300-series sparks, but are slightly
longer and have forks at the ends of the sparks.
Cast iron
Cast iron has very short sparks that begin at the grinding wheel.
Nickel and cobalt high-temperature alloys
These sparks are thin and very short, they are dark red in color, and
do not fork.
History
In 1909,Max Bermann, an engineer in Budapest, was the first to
discover that spark testing can be used reliably to classify ferrous
material. He originally claimed to be able to distinguish different
types of ferrous materials based on percent carbon and principal
alloying elements. Moreover, he claimed to achieve an accuracy of
0.01% carbon content.