Case Hardening Practices Part II
Page 45
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THE second of the case hardening methods is really a series of processes having but one common requirement, the need for a furnace not only for the initial heat but also for heat treatment in the final stages. Probably the oldest known form of hardening, which is -carburizing" or "pack hardening", falls into this group.
I spoke of the carbon content of the steel in the first article on hardening; for this second method we add or make carbon. The early practice was to seal the item to be hardened inside a box packed with leather scraps or charcoal. The sealed boxes were then heated for a number of hours at a temperature in the region of 900°C, then finally
heat treated to obtain the surface hardness. Considerable knowledge and skill is required to obtain the best results, together with fine temperature control.
Additional compounds now in use are barium carbonate and sodium carbonate, with charcoal still in demand. In the motor vehicle industry there is a need to harden Certain parts of an item yet leave other sections in their original "soft" state, and one of the methods used to protect the soft areas is to copper-plate them. Where toothed gearing is to be hardened, it is quite usual for the gear bore hole to be plugged with clay, although there is also an anti-carburizing paint which can protect the non-hard section.
Now for a more recent method or process, nitriding, that has caused many an argument in road transport garages, where it will crop up in connection with crankshaft journals. It is applied to something rather special in steel; usually an aluminium-chromemolybdenum type. These steels are heat treated to give the right physical properties in the core; the nitriding process then follows, the crankshaft or other item being brought up to around 580°C in an atmosphere of nitrogen and maintained at that temperature for the time needed to obtain the required depth of hardness; this can be 40-60 and up to 100 hours.
This is a gas hardening process and the nitrogen is obtained by "cracking" ammonia. The surface is by now fully hardened and ready for work, and a further advantage, particularly in connection with crankshafts, is that a depth of hardness is obtainable up to 0'030 in., and a crank regrind can be achieved whilst still inside the hardened layer.