790 SUMMARY OF CURRENT RESEARCHES RELATING TO 



affected by heating in nitrogen, some " 7 p.c. temper-carbon was changed 

 to combined carbon by heating in hydrogen. Microscopical examination 

 of the specimens confirmed the chemical analyses. 



A2 Point in Chromium Steel.*— H. Moore has observed that in a 

 series of steels containing up to 6 • 4 p.c. chromium, the position of Ac x 

 was progressively raised by increase of chromium-content. When more 

 than 3 p.c. chromium was present, a critical point occurring below A^ 

 was observed. After rejecting several possible explanations, the author 

 concluded that this apparently new critical point was Ac 2 , and, to test this 

 conclusion, devised a method for determining accurately the tempera- 

 ture at which steel loses or regains its magnetic properties on heating or 

 cooling. The identity of the critical point in question with the magnetic 

 change-point proved it to be Ac 2 . The addition of chromium to steel, 

 raising Ac x while not affecting the position of Ac 2 , causes a reversal in 

 the relative positions of Ac 2 and Ac 2 when 3 p.c. or more chromium is 

 present. The author holds that the occurrence of Ac 2 below Ac x de- 

 monstrates the insolubility of carbide of iron in /?-iron. 



Chromium Steel.f — A. McWilliam and E. J. Barnes have made 

 tensile and alternating- stress tests upon six steels containing 2 p.c. 

 chromium, about • 2 p.c. manganese, the carbon varying from • 2 to 

 • 85 p.c. The steels were tested after treatments similar to those given in 

 a previous investigation.! Heating curves showed a critical point, assumed 

 to be Ac 2 , below Ac r Cooling curves were also taken. The thermal 

 analysis, and microscopical examination of the steels after different heat- 

 treatments, would indicate that the carbon content of pearlite, when 2 p.c. 

 chromium is present, is decidedly lower than that of pearlite in steels 

 containing no chromium. 



*6 



Case-hardening.§ — S. A. Grayson has investigated the case-hardening 

 efficiency of four commercial materials. Turned test-pieces of mild steel 

 were heated in these materials, at different temperatures, and for varying 

 lengths of time. Carbon was then estimated in thin layers successively 

 turned off, and sections, the edges of which were protected from round- 

 ing in polishing by electro-deposited copper, were examined microscopic- 

 ally. The best temperature appears to be 950° to 1000° C. Sulphur 

 may be absorbed by the steel from case-hardening compositions, with 

 deleterious effects. Large quantities of the sulphides of manganese and 

 iron were observed at the edges of sections which had been case-hardened 

 in compositions containing notable amounts of sulphur. 



Constitution of Cast Irons and Carbon Steels. || — D.M.Levy suggests, 

 as a simplified view of the iron-carbon system, that the alloys may be 

 considered as a series of alloys of- iron with iron-carbide. A " consti- 

 tutional diagram " based on this view is given, and the changes which 

 occur in the solidification and cooling of alloys of different carbon content 

 are discussed in detail, being regarded as the separation of carbide of iron 



* Joura. Iron and Steel Inst., lxxxi. (1910) pp. 268-75 (2 figs.). 



t Tom. cit., pp. 246-67 (15 figs.). J See this Journal, 1909, p. 787. 



§ Journ. Iron and Steel Inst., lxxxi. (1910) pp. 287-303 (17 figs.). 



i| Tom. cit., pp. 403-30 (2 figs.). 



