Surface Hardening of Steel
Surface Hardening of Steel : General. A low carbon steel cannot be hardened to any great extent because of its low carbon content, yet the surface can be hardened by means of case hardening. The hardening is accomplished by increasing the carbon content of the surface only.
Case Hardening. This process produces a hard surface resistant to wear but leaves a soft, tough core. It is accomplished as follows:
(1) Pack carburizing. The work is placed in a metal container and surrounded by a mixture of carburizing materials. The container is sealed and heated from 1 to 16 hours at 1700 to 1800°F (927 to 982°C). The approximate penetration is 7/1000 in. per hour. Carburizing is usually followed by quenching to produce a hardened case.
(2) Gas carburizing. The work is placed in a gas tight retort and heated to 1700 (927°C). Natural or manufactured gas is passed through the retort until proper depth of hardening is obtained.
(3) Nitriding. The work is placed in an atmosphere of ammonia gas at 950°F (510°C) for a period of 10 to 90 hours. The maximum depth of 3/100 in. will be reached at 90 hours. The work is then removed and cooled slowly. Little warpage will result because of the low temperature. The case must then be ground so that it will be corrosion resistant.
Cyanide and cyanide fumes are dangerous poisons; therefore, this process requires expert supervision and adequate ventilation.(4) Cyaniding. The work is preheated and immersed in a cyanide bath at 1550°F (843°C). Time of immersion varies from a few minutes to 2 hours with a resulting penetration of 1/100 in. per hour. Parts should be tempered if toughness is desired.
(5) Forge case hardening. This process, usually used in the field, is accomplished by preheating work in a forge or with a torch up to 1650°F (899°C), then dipping the work in potassium cyanide or Kasenite and applying the flame until the compound melts. Repeat until required depth is attained and then quench.
c. Induction Hardening. This process is accomplished by the use of high frequency current with low voltage and a water spray to quench the work. It is used only on high carbon steels.
d. Flame Hardening. This process is accomplished by heating the surface to be hardened with an oxyacetylene torch and quenching it in water. The steel must be high in carbon.