380 CARNEGIE INSTITUTION OF WASHINGTON. 



that the wind effects are probably cumulative to an appreciable extent, 

 and that the exponent in question is slightly greater than 2. 



Stream-flow has not yet been studied in this investigation, as the 

 most favorable time is not yet here. 



Howe, Henry M., Columbia University, New York, New York. Research asso- 

 ciate in metallurgy. (For previous reports see Year Books Nos. 6-14.) 



The following are the most important items of the 1915-16 work, 

 which has been chiefly on eutectoid carbon steel. This was chosen 

 because it has the simplest constitution. 



(1) Detennination of the influence of time as distinguished from 

 temperature, in the tempering of hardened steel, on the hardness and 

 micro-structure, showing approximately what longer time at a lower 

 tempering temperature is equivalent to given time at a higher one. 



(2) Determination of the influence of the quenching temperature 

 on the hardness, density, constitution, and micro-structure of hardened 

 steel. 



(3) The temperature of the transfonnation, Ari, is lowered at 

 least from about 725° to 625°, if not to 520°, by successive accelera- 

 tions of the rate of coohng, and it is lowered about 5° by raising from 

 800° to 900° the temperature reached in the heating. 



(4) The rise of temperature during the transformation increases 

 with the rapidity of cooling to a maximum which may reach 18°, 

 and with further acceleration of the cooling in turn decreases. 



(5) Within wide limits the rate of cooling from 650° down affects 

 neither the micro-structure, hardness, nor tensile properties, unless 

 the cooling from above the transformation range to 650° has been at 

 least relatively rapid. 



(6) Lamellar pearlite is present when the cooling is at an inter- 

 mediate rate, slow enough to allow it to form, apparently through 

 under-cooling, yet not slow enough to allow it in turn to dissociate. 

 This intermediate rate is much faster when the temperature reached in 

 the heating is 800° than when it is 900°, every rise of the heating temper- 

 ature apparently increasing the stability of the lamellar structure. 



(7) Every increment of the rate of cooling increases the tensile 

 strength and lessens the ductiUty. 



(8) Unless the rate of cooling is rather rapid, heating to 900° leads 

 to much greater tensile strength but much less ductility in the steel 

 after cooling at given rate than heating to 800°, probably in part 

 because of the greater stabihty and coarseness of the lamellar pearlite 

 formed in cooling from 900°. 



(9) Other conditions being constant, the micro-structure of the 

 cooled state is determined primarily by the conditions of the last heat- 

 ing and cooling past the transformation range, the influence of prior 

 excursions past this range being in large part effaced by this latest one. 



