A plain carbon steel is one in which carbon is the only alloying element. The amount of carbon in the steel controls its hardness, strength, and ductility. The higher the carbon content, the harder the steel. Conversely, the less the carbon the greater the ductility of the steel.

Carbon steels are classified according to the percentage of carbon they contain. They are referred to low, medium, high, and very-high-carbon steels.

Low-carbon steels. Steels with a low-carbon range of 0.05 to 0.30 percent are called low-carbon steels. Steels in this class are tough, ductile, and easily machined, formed, and welded. Most of them do not respond to any heat treating process except case hardening. Low-carbon steel, when subjected to the spark test, will throw off long, white-colored streamers with very little or no sparklers.

Medium-carbon steels. These steels have a carbon range from 0.30 to 0.45 percent. They are strong and hard but cannot be worked or welded as easily as low-carbon steels. Because of their higher carbon content, they can be heat treated. Successful welding of these steels often requires special electrodes, but even then greater care must be taken to prevent formation of cracks around the weld area.

The spark test will show more numerous sparklers, beginning closer to the wheel, with the streamers much lighter in color.

High and very-high-carbon steels. Steels with a carbon range of 0.45 to 0.75 percent are classified as high-carbon and those with 0.75 to 1.7 percent carbon as very-high-carbon steels. Both of these steels respond well to heat treatment. As a rule, steels up to 0.65 percent carbon can be welded with special electrodes, although preheating and stress relieving techniques must often be used after the welding is completed. Usually it is not practical to weld steels in the very-high-carbon range.

The spark test for high-carbon steels can easily be recognized by the numerous explosions or sparklers given off, which are practically white in color.

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