350 SUMMAllY OF CURRENT RESEARCHES RELATING TO 



Metallography, etc. 



Notes on Pearlite.* — H. M. Howe and A. G. Levy have made a 

 detailed study of the effects of different conditions of cooling from 

 temperatures of 800° and 900° C. on the microstructure and mechanical 

 properties, and on the position and length of the Ar 1 change point, of 

 a eutectoid steel containing 0*92 p.c. carbon. 



The sequence and genesis of different microstructures and the exact 

 conditions of cooling favouring their development are set forth. Two 

 additional stages in the series of microstructures representing the 

 transition from sorbite to lamellar pearlite, termed sorbitic pearlite and 

 sub-lamellar pearlite, are described and illustrated. Sorbite is considered 

 to be the first stage in which the transformation is complete, and to 

 consist of an irresoluble mixture of ferrite and cementite ; sorbitic 

 pearlite is the next stage in which the ferrite and cementite have 

 sufficiently segregated to cause distinct lighter and darker microscopic 

 masses ; sub-lamellar pearlite occurs when the segregation has proceeded 

 a stage further so as to give strong suggestions of lamellarity yet 

 without showing distinct lamellte. Lamellar pearlite in specimens 

 cooled from 900° C. is coarser than in those cooled from 800"" C, and 

 shows much smaller tendency to break up into granular pearlite. The 

 microstructures obtained are correlated with the results of the mechanical 

 tests to show the great deterioration of strength which accompanies 

 granulation of the pearlite. The temperature of the Ar 1 transformation 

 is progressively lowered as the rate of cooling increases, the lowering 

 for certain rates of cooling amounting to as much as 100° below the 

 equilibrium temperature of transformation. Raising the initial tem- 

 perature of heating has also a slight lowering effect on the temperature 

 of Ar 1. The explanation of these observations is associated with 

 undercooling phenomena. The rate of cooling which is just sufficient 

 to inhibit the Ar 1 change appears to be intermediate between that 

 obtained by an air-blast cooling and that obtained by quenching in 

 molten lead. 



Formation of Nickel Steel Scale.! — The reason for the strong 

 adherence of the scale formed by heating nickel steel to high tempera- 

 tures for any length of time is explained by J. E. Stead, by the 

 interpenetration of metal and oxide at the junction between scale and 

 metal, which arises through preferential oxidation of the iron. Use is 

 made of Stead's cupric re-agent, which does not deposit copper so readily 

 on alloys rich in nickel. Photomicrographs of cross-sections through 



* Journ. Iron and Steel Inst., xciv. (1916, 2) pp. 210-32 (7 figs.). 



* Journ. Iron and Steel Inst., xciv. (1916, 2) pp. 243-5 (4 figs.). 



