Standard test specimens of zinc bronze (Cu88, Sn10, Zn2)--Parts I and II . sons it is not compar-able in the physical properties developed under stress to the sand-cast specimens. In considering the physical properties of the chill-bar specimenit is well to remember that at rather low and very high pouringtemperatures the chill bar represents the utmost tensile strengthof which the metal is capable, and only on account of its suddensolidification is its corresponding percentage of elongation andreduction of area greatly lowered over what is expected of the sand- Standard Zinc-Bronze Test Bars
Standard test specimens of zinc bronze (Cu88, Sn10, Zn2)--Parts I and II . sons it is not compar-able in the physical properties developed under stress to the sand-cast specimens. In considering the physical properties of the chill-bar specimenit is well to remember that at rather low and very high pouringtemperatures the chill bar represents the utmost tensile strengthof which the metal is capable, and only on account of its suddensolidification is its corresponding percentage of elongation andreduction of area greatly lowered over what is expected of the sand- Standard Zinc-Bronze Test Bars 25 cast specimen. This initial chill creates internal stresses andprevents the flow of the metal imder continuous tensile other words, the internal core of the bar is in an abnormalstate; now if this state be normalized by restoring its internalstructure to a condition of equilibrium at the surface with respectto stresses, by submitting such a chill bar to a proper heat treat-ment, either annealing or quenching after heating to a temperature --N- -*, -(•k-. f^ ^7/6^ m Fig. 3.—Patterns of test bars of from 700° to 800° C (1292° to 1472° F) for 30 minutes, it will befound either in the properties of elongation or reduction of areaor in the metallographic examination, that the microstructurehas been normalized, and that the elongation and reduction ofarea have been increased by nearly 400 per cent, giving valuesequivalent to those of the best sand-cast specimen that can beobtained. In investigating a chill-bar specimen it would be advis- 26 Technologic Papers of the Bureau of Standards able, therefore, to submit it to a proper heat treatment in orderto restore the metal to its normal state and then to test it for itstrue physical properties, because the heat treatment recommendeddoes not deform the specimen in any way nor does it alter itschemical composition. The general results show that there is no proportionate ratiobetween elastic limit and tensile strengt
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