A case containing over 0-90 per cent, carbon in the exterior is always liable to develop cracks in quenching, especially if, as is usual with plain carbon steel the quenching medium is water. Furthermore if the case is high in carbon, grinding operations have to be conducted with extreme care to prevent hair line cracks occurring, thus rendering commercial grinding impossible. The reason for this is that when the carbon content exceeds 0 90 per cent, a new constituent makes its appearance in the steel, known as free iron carbide or cementite. This is an extremely brittle substance, and the slight extra hardness it induces in the case is out of all proportion to the brittleness of the case attendant on its presence.

The Manufacture Of The Dbs Carburiser

The cost of raw materials has no bearing on the composition of DBS Carburising Compound and the ingredients are selected solely on their merits and purity. DBS Carburising Com -pound does not contain an ounce of material which is not essential for carburising ; it is entirely free from useless adulterants. The whole of the raw materials are bought under specification, and are subject to rigid analytical control. The preparation of the compound is conducted under the supervision of metallurgical staffs and in this way a product of unvarying quality and uniformity is assured.

Carburising Practice

The carburising process proper is usually conducted in gas, oil or solid fuel furnaces of the semi-muffle type. It is essential that the furnaces should be capable of easy and rapid regulation, and in this the gas fired and oil fired furnace hold the advantage, although they are usually more expensive to operate.

Solid fuel fired furnaces, if of good design with regenerative chequers, can give excellent results with careful attention, and the attendant economy on large furnaces is an important item. Whatever type of furnace is employed, it is essential that a uniform heat should be maintained in all parts of the chamber proper.

The temperatures employed for carburising vary between 880 degs. C. and 920 degs. C. It is not advisable to exceed the latter temperature for reasons which will be explained later, and the lower temperature should be adhered to with alloy casehardening steels. Carburising Boxes.-The design of the boxes and the material from which they are constructed will vary with the nature of the work being carburised.

Cast iron boxes are comparatively inexpensive in first cost, but their life is extremely uncertain. With a special grade of cast iron (one low in combined carbon and as low as possible in phosphorus) as many as fifty heats of say six hours duration can be obtained. With ordinary cast iron, twelve similar heats represent a good life whilst six heats are more common.

For work of a repetition character it is more economical to select another material. Cast steel usually gives more economical service than cast iron, but it is again very uncertain, some boxes giving excellent life and others failing after a very few heats.

The use of nickel chromium alloy boxes is now fairly common, and the majority of the metallurgical difficulties connected with the production of such castings having been overcome, their more extended use is certain, particularly as many makers are now prepared to guarantee a minimum life and to purchase the scrap after the boxes are unfit for further use.

The boxes should be provided with lids which can be luted down with clay. A layer of compound about 11/2 in. thick is placed on the bottom, well rammed down, and the pieces to be carburised placed in the box. A good layer of compound should separate the several pieces and a layer 11/2in. to 2in. should be placed on top after which the covers are luted down.

The Use Of Test Pieces

It is always a decided advantage to pack in one or two of the boxes, test pieces consisting of short lengths of 3/8in. or 1/2in. bar of a similar material to that being carburised. One of these should be placed so that it may be withdrawn whilst the box is in the furnace as a check to the process of carburisation. Those packed with the work should be heat treated with the work, afterwards being broken to serve as a check on the whole casehardening process.