Environmental Effects of a Stearate Coating on the Fracture Behavior of Gun Steel
WATERVLIET ARSENAL NY
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A stearate coating has been proposed for lead as a lubricant in the swage autofrettage process for cannon tubes. In this study, the environmental effects of this coating and the various chemical compounds used in its formation on the fracture behavior of gun steel have been evaluated at various temperatures up to 675F. No environmental embrittlement effects were observed on crack initiation in smooth specimens tested in tension monotonically or in constant load delayed failure type tests. Similarly, minimal effects were observed on crack propagation in single edge notched flat bar specimens which were tested to failure in fatigue. Thus, from the standpoint of environmental embrittlement, the stearate coating is superior to lead as a lubricant in the swage autofrettage process. X-ray diffraction studies of the stearate coating revealed that it consists of beta-sodium stearate rather than zinc stearate as reported in the literature. beta-sodium stearate does not react with gun steel and forms a loosely adhering surface coating as compared to a good intimate coating formed with lead. Thus, the use of stearate coating as an effective lubricant may have some limitations in the swage autofrettage processing for cannon tubes.
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