It is impossible in the short space at our disposal to discuss fully the various heat treatments and methods adopted for different articles and different steels. The most that can be done is to enumerate generalised principles, from which the reader may select and modify to suit his own particular requirements.

It should be always remembered in heat treating a carburised article, that one is dealing with a duplex steel, i.e., the case and the core, and that with any type of casehardening steel a duplex heat treatment is necessary to secure the best results.

The effect of carburising, consisting as it does of prolonged heating of the steel at high temperatures, generally much above the critical temperature, of the steel, is to increase the size of the crystals of the steel. Such large crystal size, or grain size, as it is more commonly termed, is always associated with weakness and brittleness in a steel, though, paradoxical as it may be, the ductility is generally highest in such steels.

Heat Treatment Of Carbon Case-Hardening Steels

The first operation, therefore, when the steel has cooled down after carburising should be the refining of the coarse grain of the core. In low carbon steels this is accomplished by heating to 880 degs. C-900 degs. C. and cooling either in air, oil or water, the last-named being the most common method.

It is often thought that the same object can be achieved by quenching the pieces straight from the box after carburising at 880 degs. C.- 900 degs. C. without previously allowing the pieces to cool down. This is entirely erroneous and such treatment whilst hardening the steel, preserves the coarse structure developed by the carburising operation. The first refining operation of the core must be followed by a further reheating to refine the grain of the case and quenching after reheating hardens this case.

When the material is carbon case-hardening steel, this second reheat is performed at 750 degs. G.-760 degs. C. and the quenching is usually done in water. For very light and delicate articles for which the water quenching might be too severe, the first and second quench-ings are better done in oil which gives a softer but tougher case and core.

Modifications With Alloy Case-Hardening Steels

Where the steel used contains in addition to carbon and the usual elements appreciable amounts of nickel or chromium, the treatment subsequent to carburisation is somewhat modified.

As previously stated, the duplex heat treatment gives the best results, but where the steel contains above 3 per cent, of nickel a single heat treatment very often suffices to develop mechanical properties which are perfectly satisfactory for all but very exceptional circumstances. This is due to the fact that steels containing appreciable amounts of nickel do not stufer from abnormal grain growth at temperatures above the critical point to anything like the same extent as plain carbon steels. Thus if the carburising of such alloy steels is conducted at 860 -880 degs. C, a single quenching from 760-770 degs. C. will give very good mechanical properties in both case and core.

This single quenching, coupled with the fact that an oil quenching will generally give satisfactory hardness, present decided advantages due to the fact that much less distortion takes place than occurs with plain carbon steels with the standard treatment.

Low Temperature Tempering

It is a decided advantage, from all points of view, that all casehardened work should be subject, after final hardening, to a low temperature tempering treatment. Such treatment is best conducted in a liquid bath of either oil or tallow. A satisfactory temperature for all steels is 130-150 degs. C. and the time of treatment is varied from one hour upwards, according to the size of the work.

This treatment effectively removes all normal hardening stresses, and, in addition, gives a case which is considerably tougher though little or no softer than if the tempering were omitted. In fact, in certain circumstances, where the final quenching has been very drastic the hardening is slightly increased. Such treatment is specially beneficial where the parts have to be subsequently ground."