A few general observations on the technique of casehardening will doubtless be of interest. In the first place, we should emphasize the necessity for cleaning the articles from oil, dirt, etc., before they are packed in the carburising mixture. As regards the mixture employed for carburising, it should be pointed out that there are many of these on the market, but that only reliable ones should be used. It will be clear that the most suitable mixtures will vary with the different steels and with the depths of case required.

Much distortion is caused by careless packing of the articles, and it is imperative that articles are so placed that they may suitably expand in all directions with little resistance. If this point be watched, much worry will be avoided. A good layer of the carburising mixture both above and below is a good plan in order to prevent the admission of air, which burns out the carbon. Whilst referring to the inadvertent admission of air, we might emphasize the necessity for completely sealing the cementing boxes.

Generally speaking, the best temperatures lie between 900° and 950° C, and the time at this temperature will vary with the depth of case and type of steel. For general automobile work six to eight hours will be found to give the requisite depth of carbur-isation. If less thickness of case is required, then a less time should be employed. We recommend that the box be now removed from the furnace, and the articles allowed to cool in the box without being disturbed.

It is always advisable to follow with a refining treatment, which will be found to vary with the different compositions of steel employed. We would point out that if, in refining, the material is kept too long at the refining temperature, the carbon introduced into the outer layers in the case-hardening operation is removed, and so the effect of the carburising is reduced.

From a metallurgical point of view, water-quenching is the ideal method in the refining and hardening operations. Owing, however, to the fact that many case-hardened parts are of unsymmetrical shape, oil is frequently used and is recommended for the alloy steels, in order to minimise any tendency to warping.

The final hardening operation is the one which definitely determines the success or otherwise of case-hardened parts, and it must be done with the utmost care and attention to the temperatures. We cannot emphasise too strongly the importance of the use of pyrometers in this connection. A variation of a few degrees in the final hardening operation will make a great difference in the properties of the core. This is instanced by the following example of the quenching of a nickel case-hardening steel at different quenching temperatures.

We strongly recommend the use of nickel casehardening steels, because, in our opinion, a very strong and very tough core becomes absolutely essential in all modern, high duty mechanisms. (Thos. Firth & Sons Ltd.)

Casehardening Practice

We are indebted to Messrs. David Brown and Sons (Hudds) Ltd., for the following :-

" The object of casehardening is so to alter the composition of the surface of a given piece of soft steel that by subsequent heat treatment it will become extremely hard, thus enabling it to withstand wear or resist deformation of the surface under load.

The alteration consists in raising the carbon content of the exterior of the article from approximately 0.15 per cent, to 1.0 per cent. This is accomplished by packing the articles in boxes with a carburising compound, placing the boxes in a furnace, and maintaining them at a suitable temperature for a period of time dependent on the penetration required. The work is then heat treated.

The result is an article possessing a hard case and a comparatively soft and ductile interior. The hard case supports the load and withstands wear, whilst the core gives the necessary toughness and resistance to fracture by shock.