Temperature of Iron and Steel during/ Recalescence. 183 



The relation of the critical point D' to the hardening of 

 Steel. — Fig. 12 shows the connexion between the critical 

 point D' and the hardening-capacity of steel. 



An annealed steel rod of Bofors wire containing 0'9 per 

 cent, of carbon was heated as usual, and then cold water was 

 poured over it while it was still lying between the arms of 

 the dilatometer ; this was done first immediately before, and a 

 second time towards the end of the expansion at IX, after 

 which it was again heated. The phenomena of longitudinal 

 change here presenting themselves are represented by curves 

 1, 2, and 3 (fig. 12). Curve 2 shows the re-tempering points 

 characteristic of hardened steel, while these are lacking in 

 curve 3. Thus the steel rod has been hardened at the first, but 

 not at the second cooling. 



These, and several other curves not given here, show: — 



(ix.) In order that steel may be hardened, it is necessary to 

 heat it to a degree of temperature not lower than the corre- 

 sponding point D, and then to cool it suddenly when it is at 

 a degree of temperature which is higher than that at which 

 the anomalous change of length at D' begins. 



OCT CT 



The connexion between the anomalous changes in length and 

 temperature of Iron and Steel. — Figs. 13-15 reproduce some 

 simultaneously photographed length and temperature curves 

 of Bofors wire with different percentages of carbon. All of 

 the temperature-curves show, in congruity to the corre- 

 sponding longitudinal curves, the presence of critical points. 

 These critical temperature-points have been more closely 

 studied, and some of the results thereby obtained ought to be 

 of interest in ascertaining the connexion between the anoma- 

 lous length and temperature changes of iron and steel ; they 

 agree rather well with the results which have already been 

 obtained by M. Osmond. 



During the heating of the different specimens, I always 

 observed at about 725° a decrease of temperature, or an 

 intermission in the speed of heating, which appeared more 

 strikingly the higher the percentage of carbon was. In hard 

 steel this decrease of temperature even reached 5°. 



During the cooling of the specimen which then took place, 

 I observed at about 600° either a recalescence, which in hard 

 steel ran up as far as to 20°, or an intermission in the speed 

 of cooling, which could only be discovered with difficulty in 

 very soft iron. During the cooling of soft steel and iron I 

 also discovered a second intermission in the speed of the 

 cooling process at 710° and 800° respectively. 



A comparison between the length and temperature curves 

 reproduced in figs. 13-15 shows : — 



