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).
Continued microscopic examination and comparison of nitrided test bars and a liner, focusing on surface layers and brittleness.
| Identifier | ExFiles\Box 154a\1\ scan0145 | |
| Date | 15th March 1929 guessed | |
| -2- contd: Under the microscope it was found that the liner differed from the Firth - Derihon bars in possessing no white layer on the surface of the case, but on nitriding a sample ourselves, it was found to be present, which confirms your remarks in this connection. The case on the liner differs in another respect from that on the Firth - Derihon bars. In the latter the junction of the case and core showed always as a very sharply defined line, and in several of the mechanical tests, there was a distinct tendency for the case to break away along this plane, whereas in the liner the case merged in a very gradual and satisfactory manner into the core. The third sample consisted of a number of nitrided test bars from Messrs. Aubert and Duval. These were not of suitable dimensions for any of our standard mechanical tests, and were used for hardness tests of the core, micro work, etc. The bars had been polished only, a small patch being left unpolished to indicate the amount of material removed from the surface, which was negligible. The case on these bars had only a slight tendency to spall, as compared with that on the Firth - Derihon bars. A number of Rockwell impressions were examined microscopically and found to have small cracks radiating from them. Micro-examination showed that a white layer was present, but that this contained no grey layer, and was much less in depth than that formed on the Firth - Derihon bars. (See table.) We consider that this confirms our previous remarks. The complete absence of the white layer, as in the liner, results in a case free from brittleness. The presence of a thin white layer as in the last sample of test bars, results in a case with slight tendency to surface brittleness under certain conditions. The presence of a white layer of the type found in the Firth - Derihon bars results in a case with a very brittle surface. The presence of the thin grey layer in this case may also have some connection with its brittleness. Its presence in the Firth - Derihon bars we have repeatedly confirmed, but we have not found a similar effect in any of the other nitrogen - treated steel we have dealt with up to the present. To sum up, the type of case found on the liner is the nearest to the ideal that has yet been submitted, but it apparently depends for its absolute freedom from surface brittleness on the slight grinding it has received. The type of case on the Aubert Duval test bars, however, which have been polished only, approaches in quality very closely to that on the liner, and we think should prove satisfactory, without grinding, on most parts which are at present carburised, always excepting, of course, such parts as tappets, which however could usually be ground without difficulty. It is quite definitely superior to the case on the Firth - Derihon bars in regard to toughness and hardness "taper". continued. | ||