High carbon steels include those with a carbon content exceeding 0.55 percent. The unfinished surface of high carbon steels is dark gray and similar to other steels. High carbon steels usually produce a very fine grained fracture, whiter than low carbon steels. Tool steel is harder and more brittle than plate steel or other low carbon material. High carbon steel can be hardened by heating to a good red and quenching in water. Low carbon steel, wrought iron, and steel castings cannot be hardened. Molten high carbon steel is brighter than low carbon steel, and the melting surface has a cellular appearance. It sparks more freely than low carbon (mild) steel, and the sparks are whiter. These steels are used to manufacture tools which are heat treated after fabrication to develop the hard structure necessary to withstand high shear stress and wear. They are manufactured in bar, sheet, and wire forms, and in the annealed or normalized and annealed condition in order to be suitable for machining before heat treatment. The high carbon steels are difficult to weld because of the hardening effect of heat at the welded joint. Because of the high carbon content and the heat treatment usually given to these steels, their basic properties are impaired by arc welding.
The welding heat changes the properties of high carbon steel in the vicinity of the weld. To restore the original properties, heat treatment is necessary.
High carbon steels should be preheated from 500 to 800°F (260 to 427°C) before welding. The preheating temperature can be checked with a pine stick, which will char at these temperatures.
Since high carbon steels melt at lower temperatures than low and medium carbon steels, care should be taken not to overheat the weld or base metal. Overheating is indicated by excessive sparking of the molten metal. Welding should be completed as soon as possible and the amount of sparking should be used as a check on the welding heat. The flame should be adjusted to carburizing. This type of flame tends to produce sound welds.
Either a medium or high carbon welding rod should be used to make the weld. After welding, the entire piece should be stress relieved by heating to between 1200 and 1450°F (649 and 788°C) for one hour per inch (25.4 mm) of thickness, and then slowly cooling. If the parts can easily be softened before welding, a high carbon welding rod should be used to make the joint. The entire piece should then be heat treated to restore the original properties of the base metal.
In some cases, minor repairs to these steels can be made by brazing. This process does not require temperatures as high as those used for welding, so the properties of the base metal are not seriously affected. Brazing should only be used in special cases, because the strength of the joint is not as high as the original base metal.
Either mild or stainless steel electrodes can be used with high carbon steels.
Metal-arc welding in high carbon steels requires critical control of the weld heat. The following techniques should be kept in mind:
(3) Small, high carbon steel parts are sometimes repaired by building up worn surfaces. When this is done, the piece should be annealed or softened by heating to a red heat and cooling slowly. The piece should then be welded or built up with medium carbon or high strength electrodes, and heat treated after welding to restore its original properties.