On the Tempering of Iron hardened by Overstrain. 465 



to above. The " steps " tabulated in the last column are the amounts 

 by which the yield-points were raised in consequence of overstrain and 

 recovery from overstrain : 



In order to show the tempering of steel hardened by tensile over- 

 strain, a specimen of annealed steel was overstrained in a manner 

 analogous to that illustrated by Curves 1, 5, and 6 of Diagram 2. 

 The material, after recovery from overstrain, had thus been brought 

 into a condition of hardness, which enabled the specimen to be loaded 

 to 50 tons per square inch without a yield-point being reached. The 

 specimen was then subjected to a series of tests after being heated 

 successively to various temperatures, the result being to show that 

 310 C. produced no softening of the material, 360 C. lowered the 

 yield-point to 47 tons ; 500, 600, and 700 C. lowered the yield-point 

 to about 40, 35, and 30 tons per square inch respectively. 



It was further shown that the same temperature brought the yield- 

 point to approximately the same stress, no matter what might be the 

 original hardness of the specimen under test ; and that the harder the 

 material was made by tensile overstrain that is, the higher the yield- 

 point was raised by permanent stretching the lower was the tempera- 

 ture which could be shown to produce a slight tempering effect. Thus 

 in the above instance had the material been made harder (by further 

 overstraining) than was shown by the elastic range of from zero to 

 50 tons per square inch, then possibly the temperature of 310 C. 

 would have produced a slight softening of the hardened material ; a 

 temperature of about 300 C. was, however, found to be the minimum 

 temperature which had a tempering effect on the hardest condition of 

 steel tested. 



The tempering effects which have been ascribed above solely to tem- 

 perature, were found to be influenced to some extent by time. Thus 

 it was found that by baking a hardened specimen for several hours at 

 any temperature a greater effect was produced than by simply raising 

 the specimen for a few minutes to that temperature. The effect of 

 time was, however, small compared with that produced by increase of 

 temperature. 



VOL. LXVII. 



2 L 



