Spark Test to detect metals
Posted: 06 Oct 2010, 08:13
INTRODUCTION
1. The spark 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.
2. This test does not replace chemical analysis, but is a very convenient and fast method of sorting mixed steels whose spark characteristics are known.
3. When held lightly against a grinding wheel, the different kinds of iron and steel produce sparks that vary in length, shape, and color.
4. 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.
5. Grinding wheels should be hard enough to wear for a reasonable length of time, yet soft enough to keep a free-cutting
edge.
6. Spark testing should be done in controlled light, since the color of the spark is important.
7. 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.
8. 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.
CATEGORIES OF METALS
1. The color, shape, length, and activity of the sparks relate to characteristics of the material being tested.
2. The spark stream has specific items which can be identified.
(a) The straight lines are called carrier lines. They are usually solid and continuous.
(b) At the end of the carrier line, they may divide into three short lines, or forks.
(c) 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.
(d) 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.
3. Cast irons have extremely short streams, whereas low–carbon steels and most alloy steels have relatively long streams.
4. Steels usually have white to yellow color sparks, while cast irons are reddish to straw yellow.
5. A 0.15 percent carbon steel sparks are 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 cabon shows brilliant and minute explosions or sparklers.
6. As the carbon content increases, the intensity of bursting increases.
SUMMARY
1. One big advantage of this test is that it can be applied to metal in, all stages, bar stock in racks, machined forgings or finished parts.
2. 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.
3. Wheel pressure against the work is importantr because increasing pressure will raise the temperature of the spark stream and give the appearance of higher carbon content.
4. The sparks near and around the wheel, the middle of the spark stream, and the reaction of glowing particles at the end of the spark stream should be observed.
Sparks produced by various metals are shown in figure.
1. The spark 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.
2. This test does not replace chemical analysis, but is a very convenient and fast method of sorting mixed steels whose spark characteristics are known.
3. When held lightly against a grinding wheel, the different kinds of iron and steel produce sparks that vary in length, shape, and color.
4. 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.
5. Grinding wheels should be hard enough to wear for a reasonable length of time, yet soft enough to keep a free-cutting
edge.
6. Spark testing should be done in controlled light, since the color of the spark is important.
7. 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.
8. 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.
CATEGORIES OF METALS
1. The color, shape, length, and activity of the sparks relate to characteristics of the material being tested.
2. The spark stream has specific items which can be identified.
(a) The straight lines are called carrier lines. They are usually solid and continuous.
(b) At the end of the carrier line, they may divide into three short lines, or forks.
(c) 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.
(d) 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.
3. Cast irons have extremely short streams, whereas low–carbon steels and most alloy steels have relatively long streams.
4. Steels usually have white to yellow color sparks, while cast irons are reddish to straw yellow.
5. A 0.15 percent carbon steel sparks are 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 cabon shows brilliant and minute explosions or sparklers.
6. As the carbon content increases, the intensity of bursting increases.
SUMMARY
1. One big advantage of this test is that it can be applied to metal in, all stages, bar stock in racks, machined forgings or finished parts.
2. 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.
3. Wheel pressure against the work is importantr because increasing pressure will raise the temperature of the spark stream and give the appearance of higher carbon content.
4. The sparks near and around the wheel, the middle of the spark stream, and the reaction of glowing particles at the end of the spark stream should be observed.
Sparks produced by various metals are shown in figure.