ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 669 



Brittleness of Cemented Mild Steels.* — To determine the cause of 

 the brittleness resulting from the cementation of mild steel, J. Lecarme 

 worked on steels of the following composition : — 



1. 2. 



Carbon 0100 p.c. 0-090 p.c. 



Manganese 0-300 „ 0-623 „ 



Phosphorus 0-031 „ 0-065 „ 



Silicon 0-750 „ 0*152 „ 



Four groups, each made up of ten pieces of each steel, were packed 

 (a) in neutral matter, (b), (c), and (d) in carburising material of different 

 degrees of activity, and heated at 1000° C, the different pieces in each 

 group being maintained at this temperature for varying periods. The 

 object of this series of experiments was to determine whether the 

 brittleness is due to heating at a high temperature, or is influenced by 

 the composition of the carburising material. After treatment the pieces 

 were submitted to mechanical tests and microscopically examined. The 

 changes in microstructure are shown by photomicrographs. The author 

 concludes that the thermal treatment necessarily accompanying cement- 

 ation does not induce brittleness, this fragility being caused by some 

 chemical change in the soft core taking place simultaneously with the 

 superficial cementation. Widely differing degrees of brittleness result 

 when steels obtained from different sources, though of similar chemical 

 composition, are submitted to the same treatment. It is usually 

 possible by suitable treatment to remove the brittleness resulting from 

 cementation. 



H. le Chatelierf iputs forward some criticisms of J. Lecarme's 

 inferences, and remarks that the chief object of their publication is to 

 induce other workers to investigate the subject more fully. The presence 

 of nitrogen may influence the results. 



Technique of Microscopic Metallography.? — H. le Chatelier de- 

 scribes the improvements in the details of polishing, etching, etc., 

 effected in his laboratory since the publication of his former article on 

 the same subject. § 



Grinding. — A rapidly revolving emery wheel, against which the 

 section is lightly pressed, gives the best results. For quenched steels 

 which surface-heating might let down, a wheel flooded with water and 

 revolving at slower speeds should be used. It has been stated that if 

 the section does not become too hot to hold with the fingers, the 

 temperature cannot rise sufficiently to have any effect on the metal. 

 This is not the case, as the surface pressed against the emery wheel may 

 be considerably hotter than the mass of the piece. To remove the 

 modified skin which appears to be the unavoidable result of grinding on 

 emery iwheels, the section should be rubbed by hand on moderately 

 coarse emery paper. Moistening emery paper with oil of turpentine 



• Rev. Metallurgie, ii. (1905) pp. 516-25 (6 figs.), 

 t Tom. cit.. pp. 526-7. 



I Rev. Metallurgie, ii. (1905) pp. 528-37 (3 figs.) 



§ Bull. Soc. d'Enc. ; see also " Contribution a l'e'tude des alliages," pp. 421-40, 

 and Metallograpbist, iv. (1901) pp. 1-22. 



Oct. 18th, 1905 2 Y 



