From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Extract from 'The American Machinist' discussing methods for testing the hardness of nitrided steel cases.
| Identifier | ExFiles\Box 154a\1\ scan0120 | |
| Date | 24th November 1928 | |
| Extract from "The American Machinist" - Nov. 24th. 1928. METHODS OF APPROXIMATING CERTAIN PHYSICAL CHARACTERISTICS OF NITRIDED STEEL CASES. By G.M.Eaton (Molybdenum Corporation of America). This paper takes up the problem of finding out whether a nitrided product possesses the characteristics which justify the expectation that it will perform satisfactorily in a given service. The file test fails completely to define the degree of hardness. Furthermore, it is utterly erratic because the first pass of any file we have been able to find, over a case with a hardness of say 700 to 800 Brinell, ruins the keen cutting edge of the file, so that it will refuse to cut a materially softer case on subsequent passes. The ease and quickness of the Scleroscope test make it tempting, but the varying hardness of the nitrided case prohibits its use, as under the best conditions of set-up, it is entirely possible to secure the same reading on two cases of radically different surface hardness, and internal hardness gradation. The hardest steel Brinell ball is far too soft for use with nitrided steel. The diamond ball offers some possibilities, particularly with very small balls, but here again varying hardness is hostile. Unfortunately we were not successful, in the time available, in locating a Baby Brinell machine equipped with a diamond ball. A comparison of impressions showed an insufficient difference to permit distinguishing between a ductile and a brittle case when using a hard steel Baby Brinell ball. It is possible that a diamond ball would give more satisfactory results, although we doubt this. The Rockwell C diamond offers a better approach to the characteristics of the nitrided case although here again the varying hardness is inimical. Also, the stability of the diamond in this severe service remains to be proven. It seems logical to believe that some of the recent diamond scratch instruments might offer a very good research tool for studying hardness penetration. This method, however, does not appear to lend itself to production work, as micro-polish is essential. The ability of the diamond to stand up properly for any length of time against the nitrided case remains to be proven. The Herbert pendulum with a diamond ball probably offers the most accurate means now available for determining the hardness of the nitrided case. There are, however, very serious objections to its adoption for commercial work, as follows: The harder the case the more important it is that the surface be smooth, and micro-polish is really necessary for good results; this instrument requires more skill on the part of the operator than it is customary to expect of men doing routine testing; with the Herbert pendulum we failed to detect an adequate distinction between brittle and ductile cases; we were not successful in devising a practical way of placing the instrument in accurately spaced locations for exploring the graduation of hardness of the case, and the failure of the instrument to give some measure of ductility is of itself sufficient reason for considering it impractical as a means for testing nitrided cases. | ||