We continue our discussion from last time.
In order for the surface metallurgy to be correct and for the successful performance of the gear, the core metallurgy and mechanical properties demand equal consideration. It is the core hardness and tensile strength that will give support to the carburized case. If the core hardness is too soft, it is likely that the core will collapse under case loading. The recommended core hardness is usually around 38-42 HRC (please note, this is a suggested value). The ability of a specific steel to attain core hardness will be dependent on the steel’s alloy chemistry.
The usual complaint is that the material is too hard to machine. Remember that core hardness is determined during carburize, austenitize and quench. It is the austenitize temperature that will harden the case surface, but it will also manipulate the core hardness of the steel.
The selection of the material will be made from the analysis of the material in relation to carbon and other alloying elements, such as the carbide formers of chromium, manganese, molybdenum, tungsten, vanadium and others.
Careful consideration needs to be given to the nickel content. In order to accomplish toughness in the steel, nickel must be present. Too much nickel, however, will begin to promote the problem of retained austenite and also increase the core-hardness value. Be aware that nickel is not a carbide former.
It goes without saying that process temperature control is critical. Remember that both temperature and time at temperature will promote grain growth. All carburizing procedures necessitate time at temperature for the diffusion of the surface carbon content. Thus, good temperature control and good temperature uniformity within the process are mandatory requirements. It will be necessary to perform temperature uniformity surveys (TUS) on a regular basis as well as regular calibration of the furnace’s temperature-control instruments. A good guide to TUS is, of course, AMS 2750 (latest revision).