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Part I: The tendency of a commercially drawn cartridge case to crack in the mercury test and the relation of cracking tendency to residual stress retained after drawing were studied. The fourth drawpiece (next to the final) of caliber 0.30 cartridge cases produced by Frankford Arsenal were selected as test pieces. These exhibited an exceptionally high cracking tendency, making a detailed study possible.
To obtain fast cracking in the mercury test, about 0.0015 to 0.003 in. must be removed from the wall thickness of the drawpiece by pickling, and it must be subjected to the mercury test immediately after washing. Removal of the adhering mercury by annealing and subsequent squeezing of the case in a vise was used to reveal definitely the presence or absence of cracks.
Various lots of apparently identically fabricated cases were shown to possess a different cracking tendency.
Machining off or boring a hole in the base gradually reduced and eventually eliminated the cracking tendency. On the contrary, shortening of the wall (body) reduced the cracking tendency only if the wall became so short that the cracking zone was directly affected.
By boring out the base and pickling from the outside and then splitting the resulting rings, it was possible to determine the approximate stress distribution in the base and in the adjacent part of the wall. Small compression stresses were present in the core and the major part of the base, while the surface of the base and the adjacent wall contained high circumferential tensile stresses. On the contrary, the wall when parted from the base contained only insignificant residual stress.
The conception is advanced, that the stress distribution in a drawn cartridge case corresponds fundamentally to that of a tube (wall) shrunk on a disk (base). Such a stress distribution results from the tendency of the base to recover elastically by an increase in diameter from the biaxial compression during drawing while the wall exhibits little tendency to change its diameter after progressing through the die. This is confirmed by measurements which show the base diameter to be slightly larger than the wall diameter.
Part II: The effect of a number of processing (drawing) variables on the cracking tendency was also investigated.
The principal result was that the use of dies with small angles combined with a small reduction in diameter resulted in drawpieces with no tendency to crack. This also applied to large die angles used in combination with large diameter reductions, with a die angle of approximately 10 deg. giving the highest cracking tendency.
It was also found that the larger the grain size, the greater the tendency to crack. Other variables showed no definite effect upon the cracking tendency.
Professor of Physical Metallurgy, Case School of Applied Science, Cleveland, Ohio.
Senior Research Engineer, Case School of Applied Science, Cleveland, Ohio.
Clark, S. M.
Metallurgist, Thompson Aircraft Products Co., Cleveland, Ohio.