Machinery's Handbook, 31st Edition
Heat Treatment of Steel 483 and alloy nitrides in the case. White layer formation, familiar in conventional gas nitrid - ing, is readily controlled by this process. Liquid Carburizing.— Activated liquid salt baths are now used extensively for carburiz ing. Sodium cyanide and other salt baths are used. The salt bath is heated by electrodes immersed in it, the bath itself acting as the conductor and resistor. One or more groups of electrodes, with two or more electrodes per group, may be used. The heating is accompa nied by a stirring action to ensure uniform temperature and carburizing activity through - out the bath. The temperature may be controlled by a thermocouple immersed in the bath and connecting with a pyrometer designed to provide automatic regulation. The advan - tages of liquid baths include rapid action, uniform carburization, minimum distortion, and elimination of the packing and unpacking required when carbonaceous mixtures are used. In selective carburizing, the portions of the work that are not to be carburized are copperplated, and the entire piece is then immersed in an activated cyanide bath. The cop - per inhibits any carburizing action on the plated parts, and this method offers a practical solution for selectively carburizing any portion of a steel part. Gas Carburizing.— When carburizing gases are used, the mixture varies with the type of case and quality of product desired. The gaseous hydrocarbons most widely used are methane (natural gas), propane, and butane. These carbon-bearing gases are mixed with air, with manufactured gases of several types, with flue gas, or with other specially pre - pared “diluent” gases. It is necessary to maintain a continuous fresh stream of carburizing gases to the carburizing retort or muffle as well as to continuously remove the spent gases from the muffle in order to obtain the correct mixture of gases inside the muffle. A slight pressure is maintained on the muffle to exclude unwanted gases. The horizontal rotary type of gas carburizing furnace has a retort or muffle that revolves slowly. This type of furnace is adapted to small parts, such as ball and roller bearings, chain links, small axles, bolts, etc. With this type of furnace, very large pieces, such as gears, for example, may be injured by successive shocks due to tumbling within the rotor. The vertical pit type of gas carburizer has a stationary workholder that is placed vertically in a pit. The work, instead of circulating in the gases as with the rotary type, is stationary, and the gases circulate around it. This type is applicable to long, large shafts or other parts or shapes that cannot be rolled in a rotary type of furnace. There are three types of continuous gas furnaces that may be designated as 1) direct quench and manually operated 2) direct quench and mechanically operated 3) cooling-zone type Where production does not warrant using a large continuous-type furnace, a horizontal muffle furnace of the batch type may be used, especially if the quantities of work are varied and the production not continuous. Vacuum Carburizing.— Vacuum carburizing is a high-temperature gas carburizing process that is performed at pressures below atmospheric. The furnace atmosphere usu- ally consists solely of an enriching gas, such as natural gas, pure methane, or propane; ni- trogen is sometimes used as a carrier gas. Vacuum carburizing offers several advantages, such as combining of processing operations and reduced total processing time. Carburizing Steels.— A low-carbon steel containing, say, from 0.10 to 0.20 percent of carbon is suitable for carburized case hardening. In addition to straight-carbon steels, the low-carbon alloy steels are employed. The alloys add to case-hardened parts the same advantageous properties they give to other classes of steel. Various steels suitable for case hardening will be found in the section on SAE steels. To Clean Work after Case Hardening.— To clean work, especially if knurled, or if dirt is likely to stick into crevices after case hardening, wash it in caustic soda (1 part soda to 10 parts water). In making the solution, the soda should be put into hot water gradually, and
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