Next try the same thing with a diamond; the diamond will cut easily until the point is reached where the file slipped, then there will be found a great increase of hardness.
From this point to the end of the piece it is observed readily by the action of the diamond that there is a gradual increase of hardness from the hump to the end of the piece to the creamy-heated end. Attempts were made to measure this curve of hardness by putting a load on the diamond and dragging it over the piece; but no diamond obtainable would bear a load heavy enough to produce a groove that could be measured accurately by micrometer. An examination of such a groove, through a strong magnifying-glass revealed the conditions plainly; the groove of hardness may be illustrated on an exaggerated scale; thus:
The next question was, Where does this hump occur, and what is the cause of it?
Careful observation showed that it occurred at the point of recalescence, at the refining-point. This word point must not be taken as space without dimension in this connection; it is used in the common sense of at or adjacent to a given place. There is of course a small allowable range of temperature above any given exact point of recalescence, such as 655° C. or 1211° F.
By superimposing Langley’s curves of cooling and of hardening (see Trans. Am. Soc. Civ. Eng., Vol. XXVII, p. 403), the relation between recalescence and the hardening-hump is obvious.
It is safe to say that experience proves that the refined condition is the best for all cutting-tools of every shape and form.
It seems to be obvious; the steel is then in its strongest condition, and when the grain is finest, the crystals the smallest, a fine edge should be the most enduring, because there is a more intimate contact between the particles. That a steel will refine well, and be strong in that condition is the steel-maker’s final test of quality.
No steel-maker who has a proper regard for the character of his product will accept raw material upon mere analysis; analysis is of the utmost importance, for material for steel-making must be of a quality that will produce a certain quality of steel, or the result will be an inferior product. This applies to acid Bessemer and open-hearth, and to crucible-steel especially; the basic processes admit of a reduction of phosphorus not obtainable in the others.