1892.] NEW-YORK MICROSCOPICAL SOCIETY. 71 



SO important that all nations have modified the construction of 

 their ordnance from solid guns to the system of built-up guns, each 

 part not being too large to receive the full benefit of heat treatment. 

 To illustrate the changes in the structure of steel by mechani- 

 cal work and heat treatment, I have specimens of open-hearth 

 compressed steel for ordnance — 



1. From the ingot. 



2. After hydraulic forging. 



3. After heat treatment. 



These are designated as specimens Nos. i, 2, and 3. 



The steel contains about .40 carbon, and after being poured 

 into the cylindrical ingot mould, 30 inches in diameter and 18 feet 

 long, was put under compression of 2,000 pounds per square inch 

 to prevent the formation of blow holes or a pipe in the ingot. 

 The steel was several hours in setting ; many more in cooling, 

 and, notwithstanding the compression, the facets of the imperfect 

 crystals are from one-twentieth to one-tenth of an inch square 

 and distinctly foliated, fracture occurring through this structure 

 instead of through the crystals. In turning such steel under 

 heavy feed the imperfect crystals often tear out instead of cutting 

 through. The tensile strength of such steel rarely reaches 40,000 

 pounds and has but little elongation in a test specimen. The 

 structure, as shown in specimen No. i, is very coarse. 



Specimen No. 2 is a ten<;ile test bar taken from the steel after 

 heating and forging. Though the coarse structure has been 

 somewhat reduced, the exterior of the bar has a reticulated sur- 

 face, showing the elongation of the bar is more decided through 

 the foliated structure than through the crystals, and that the steel 

 is not in its best condition. 



Specimen No. 3 is a tensile test bar after heat treatment. The 

 . tensile strength is over 100,000 pounds per square inch, and the 

 test bar is very smooth on the surface. The crystalline structure 

 has been very completely reduced. The steel has not only high 

 elastic limits, but when they are reached the metal will give many 

 per cent of elongation before fracture occurs. The end of the 

 broken test bar has a decided silky fracture, showing the metal 

 to be very uniform in structure and in excellent condition for 

 severe service, as in ordnance. For projectiles the steel could be 

 much harder. 



