264- SUMMARY OF CURRENT RESEARCHES RELATING TO 



temperature reached, but also on the time for which the temperature is 

 maintained. As, however, the tempering effect on hardened steel 

 also increases with time of heating as well as with temperature, the 

 correctness of the practice of judging the degree of " letting down " by 

 the colour is confirmed. 



Hardness of Steels at Low Temperatures.* — F. Robin has made 

 Brinell hardness measurements on a large number of steels, at - 20°, 

 -80°, and -185°C. He finds that hardness increases as temperature 

 diminishes, the rate of increase being greater at the lower temperatures. 

 Steels cooled to - 185° C, and allowed to return to the ordinary tempera- 

 ture, showed a slight permanent increase in hardness. Microscopic 

 observations were made on austenitic steels cooled to low temperatures, 

 and, together with low-temperature hardness measurements, confirmed 

 the occurrence of a transformation in the austenite. 



Hardness of Steels at High Temperatures. f — F. Robin has made 

 a large number of careful hardness measurements, by the Brinell 

 method, on steels at temperatures 10°-900° C. At the highest tempera- 

 tures the impression was made by impact, while static loading was 

 employed throughout the greater part of the range. The results are 

 embodied in numerous curves. In general, for carbon steels, the hard- 

 ness fell off with increasing temperature, up to 100°-150° C. It then 

 increased, reaching a maximum at about 250° C, then diminished, more 

 rapidly as the temperature rose. At 850° C. the difference in hardness 

 of steels of different carbon content was small. The form of the 

 temperature-hardness curves of the high-speed steels differed consider- 

 ably from that of the carbon steels, these special steels retaining a much 

 greater proportion of their original hardness at temperatures up to 

 600° C. 



Silicon-silver Alloys. J — G. Arrivaut finds that no compounds are 

 formed in the silicon-silver system. Silicon is not held in solid solution 

 by silver, but at the other end of the diagram there is a series of solid 

 solutions of silver in silicon, having a maximum concentration of 10 p.c. 

 The eutectic contains 5 p.c. silicon, and melts at 820°-830° C. The 

 equilibrium diagram was obtained by thermal methods and confirmed by 

 microscopic examination. 



Micrography of Cement. § — E. Stern has microscopically examined 

 cements at different stages of the hardening process, by reflected light. 

 Polished and etched sections show two constituents, A, alite, present in 

 greater amount than B, which, in hardening slowly, increases at the 

 expense of A. The etching reagents found to yield good results were 

 alkalis, alcoholic hydrochloric acid, alcoholic iodine solution, and 25 p.c. 

 hydrofluoric acid. 



Microscopic Study of Mortar. || — G. Gallo has ascertained by micro- 

 scopic examination of mortar that the calcium carbonate formed is 

 distinctly crystalline ; it is concluded that the lime and carbon dioxide 



* Rev. Metallurgie, vi. (1909) pp. .162-79 (25 figs,), 

 t Op. cit., v. (1908) pp. 893-908 ; and vi. (1909) pp. 180-4 (19 figs.). 

 X Tom cit , pp. 932-4 (1 fig.). 



§ Stahl und Eisen, xxviii. (1908) pp. 1542-6 (12 figs.). 



|| Journ. Chem. Soc, xciv. (1908) pp. 843-4 (from Gazetta Chimica Italiana, 

 xxxviii. (1908) pp. 142-204). 



