A good cam for an ordinary machine, such as a shear or punch, may be made of Bessemer or open-hearth steel where greater strength and endurance are required than can be had in cast iron; on the other hand, makers of cams for delicately adjusted high-speed machines where intricacy and accuracy are necessary will touch nothing but the very best crucible-steel of fine-tool quality for their work. It is of no use to suggest the greater cheapness of the other steels; they have tried them thoroughly, and they know that in their case the highest priced is the cheapest.
This superiority of crucible-steel has been doubted, because the claim appeared to rest solely upon the statements of steel-makers, and not to have any scientific basis; there is, however, a scientific basis for the fact. Given three samples of steel of say the following composition:
| Crucible. | Open-hearth. | Bessemer. | |
|---|---|---|---|
| Carbon | 1.00 | 1.00 | 1.00 |
| Silicon | .10 | .10 | .10 |
| Phosphorus | .05 | .05 | .05 |
| Sulphur | .02 | .02 | .02 |
| Copper, arsenic, etc. | traces |
Why should there be any difference in the strength of the three? In mere tensile strength in an untempered bar the difference might not be very great, although all experienced persons would expect the crucible to show the highest; but it is not necessary to make the claim, because we have not enough tests of crucible-steel to enable us to establish a mean, and one or two tests are insufficient to establish a rule in any case.
There have been made, however, hundreds of tests of hardened and tempered samples by the most expert persons, with one invariable result: the crucible-steel is incomparably finer and stronger than the others, and the open-hearth is almost invariably stronger and finer than the Bessemer.
Unfortunately for the argument these tests cannot be recorded so as to be intelligible to the non-expert, because we cannot tabulate the result of the touch of the expert hand or the observation of the experienced eye.
For a time it was popular to call these differences mysteries, and so let them pass; this, however, was not satisfactory, and the question was studied carefully for the physical reasons which must exist.
Much thought led to the conclusion that the reason lay with the three elements oxygen, nitrogen, and hydrogen; they are known to exist in greater or less quantity in all iron and steel.
It is known that the presence of oxygen beyond certain small limits produces red-shortness and general weakness; it is probably a much more hurtful element than phosphorus or sulphur, but no quantitative method for its determination has been worked out; there is an effort now being made to develop a simple and expeditious oxygen determination, and it is to be hoped that it will be successful.
In the crucible no more oxygen, hydrogen, or nitrogen can get into the steel than is contained in the material charged and in the atmosphere of the crucible, or than may penetrate the walls of the crucible during melting. In the open hearth the process is an oxidizing one, and besides the charge is swept continuously by hot flames containing all of these elements.