Walter J. May

Casehardening, as the term itself implies, means the formation of a hardened surface on otherwise soft metal, and the term is used more generally in regard to iron and steel, although to some extent a form of case-hardening can he applied to copper and some of the bronzes having a high fusing temperature. Really, the process is one of alloying by absorption, and although this process in many cases causes a lot of trouble, when done with a well-defined purpose it has beneficial effects. With iron and steel, carbon is absorbed, and with copper or bronze, either metallic tin, manganese or arsenic will be the hardening material; but tin is the better hardening for general use. As a rule, however, only iron and steel are dealt with, as these have the most commercial use; but it might, perhaps, be well to give the results of surface hardening on copper as done by the writer experimentally.

Ordinary wrought-iron, mild steel and malleable cast-iron will caseharden well, the last being the least satisfactory, but still for some purposes being very successful. Ordinary cast-iron, if treated by the case-hardening process, would rather be softened, owing to its then being annealed with an excess of carbon, part of which would be absorbed.

In some cases only part of an article can be case-hardened, while the other parts are left soft; and although this may involve some trouble, yet for particular purposes the results are worth all the trouble taken. Take, for instance, the steel rings in rice-dressing machines; if these are hard right through, they break very readily; but if the outer part is hardened and the inner parts remain soft, there is little fear of breakage. Mild steel can be used for such rings, and the saving in time and labor in cutting more than compensates for the cost and trouble of hardening. Again, it is possible to harden pinions and such like in whole or in part as is required; and certainly it is a great ad vantage to do machining on soft metal and then to harden the work to a reasonable depth, and possibly in some cases tempering the metal.

Outside what might be called legitimate work the process of casehardening also enables manufacturers to turn out cheap goods for trade purposes which could not otherwise be done at the price. Even knife blades can be made in soft metal, and after being rough ground can be hardened, and in such a way that they will last a considerable time, both the labor and material costs being very considerably reduced, particularly where an annealing furnace can be kept constantly hot, as in such case costs are reduced to a very small amount. Intermittent working causes a large outlay in repairs, for although no very great heat is needed, cooling and heating the furnace will cause the very best brickwork to give way in a comparatively short time, be as careful as oue may. At the most, only a cherry-red heat in the boxes is needed, and as this is only about 1650° Fahr., a maximum temperature of approximately 1700° Fahr. in the oven or furnace is ample as a working heat. Still, this is quite high enough to bring down brick arches with any but careful handling, and having this occur practically means that the furnace has to be rebuilt.

When iron or steel is casehardencd, the articles are packed in annealing-boxes with animal carbon, and the boxes are covered and the joints luted, then heated to just that temperature at which absorption of carbon takes place. The time the heat is continued depends on the depth of hardening required, and usually the work is thrown into water and made dead-hard as soon as it is considered that a sufficient depth is penetrated by the carbon. With the carbon may be used chemicals to assist in its absorption, cyanides being very frequently used, and according to the skill and practice of individual operators success is secured.

Thin, rather than deep boxes should be used, and in no case should boxes be too large; otherwise the hardening will be very unequal through the mass of metal enclosed. Proper boxes are made and sold at a cheap rate, the method of fixing the covers varying somewhat. In all cases small holes are pierced in the covers for the insertion of test wires, these being used to ascertain the state of the heat and the penetration of the carbon. Both circular and rectangular boxes are used, and both forms have their special uses according to the articles to be hardened ; but rectangular boxes usually heat most regularly. Personally, the writer would select malleable cast-iron boxes, but possibly this is more a matter of personal preference than anything, although cost may have some bearing in the matter. The actual size of boxes will depend on the size of the articles to be treated; but unwieldy sizes should be avoided as much as possible.

The packing may be bone or leather cuttings, and these may be used raw or after conversion into charcoal. A layer of the carbonized material is laid at the bottom of the box, then a layer of the articles to be hardened is put in and packed with carbon, and the process is repeated until the box is full. Powdered potassium cyanide or prussiate of potash may be dusted over and among the articles and packing; but in all cases, phosphorus-bearing materials should be avoided, there being quite enough of this in the carbonaceous matters used. When the box is filled a layer of the packing material should be put over the articles and then the box should be closed and the cover luted down, the test wires being inserted at the same time.

Care must always be taken that the packing is thoroughly dry; otherwise the steam will force out the luting, or in some cases force off the cover, in such cases the results not being extremely satisfactory.

In packing articles which have only to be casehard-ened in parts, the soft portions should have a piotec-tive covering; while the parts to be made hard should be left exposed. Perhaps for ordinary purposes a mix-tuie of white ash from the boxes and enough fireclay or pipeclay to bind the ashes would be as good a mixture as could readily be made to save the soft parts from the carbonaceous packing; but usually each operator has his own especial mixture for the purpose. Lime should be avoided unless it is desired to burn out the carbon from the iron or steel; but only in very exceptional cases will this have to be done.